Berikut merupakan kutipan ilmiah tentang aerodynamic yang bermanfaat sehingga disusun dan digunakan sebagai referensi pribadi.
Cruise is the level portion of aircraft travel where flight is most fuel efficient. It occurs between ascent and descentphases and is usually the majority of a journey. Technically, cruising consists of heading (direction of flight) changes only at a constant airspeed and altitude. It ends as the aircraft approaches the destination where the descent phase of flight commences in preparation for landing.
For most commercial passenger aircraft, the cruise phase of flight consumes the majority of fuel. As this lightens the aircraft considerably, higher altitudes are more efficient for additional fuel economy. However, for operational and air traffic control reasons it is necessary to stay at the cleared flight level. On long haul flights, the pilot may climb from one flight level to a higher one as clearance is requested and given from air traffic control. This maneuver is called a step climb.
Commercial or passenger aircraft are usually designed for optimum performance at their cruise speed or VC. There is also an optimum cruise altitude for a particular aircraft type and conditions including payload weight, center of gravity, air temperature, humidity, and speed. This altitude is usually where the higher ground speeds, the increase in drag power, and the decrease in engine power and efficiency at higher altitudes are balanced.
Typical cruising air speed for long-distance commercial passenger flights is 475–500 knots (878-926 km/h; 546–575 mph).
The service ceiling is the maximum usable altitude of an aircraft. Specifically, it is the density altitude at which flying in a clean configuration, at the best rate of climb airspeed for that altitude and with all engines operating and producing maximum continuous power, will produce a given rate of climb (a typical value might be 100 feet per minute climb or 30 metres per minute, or on the order of 500 feet per minute climb for jet aircraft). Margin to stall at service ceiling is 1.5 g.
The one engine inoperative (OEI) service ceiling of a twin-engine, fixed-wing aircraft is the density altitude at which flying in a clean configuration, at the best rate of climb airspeed for that altitude with one engine producing maximum continuous power and the other engine shut down and feathered, will produce a given rate of climb (usually 50 feet per minute).
However some performance charts will define the service ceiling as the pressure altitude at which the aircraft will have the capability of climbing at 50 ft/min with onepropeller feathered.
The absolute ceiling, also known as coffin corner, is the highest altitude at which an aircraft can sustain level flight, which means the altitude at which the thrust of the engines at full power is equal to the total drag at minimum drag speed. In other words, it is the altitude where maximum thrust available equals minimum thrust required, so the altitude where the maximum sustained (with no decreasing airspeed) rate of climb is zero. Most commercial jetliners have a service (or certificated) ceiling of about 42,000 feet (12,802 m) and some business jets about 51,000 feet (15,545 m). While these aircraft's absolute ceiling is much higher than standard operational purposes, it is impossible to reach (because of the vertical speed asymptotically approaching zero) without afterburners or other devices temporarily increasing thrust. Flight at the absolute ceiling is also not economically advantageous due to the low indicated airspeed which can be sustained: although the true airspeed (TAS) at an altitude is typically greater than indicated airspeed (IAS), the difference is not enough to compensate for the fact that IAS at which minimum drag is achieved is usually low, so a flight at an absolute ceiling altitude results in a low TAS as well, and hence in a high fuel burn rate per distance traveled. The absolute ceiling varies with the air temperature and, overall, the aircraft weight (usually calculated at MTOW).
A large number of modern jet aircraft, of all sizes and including Very Light Jets (VLJs)s, routinely cruise at high altitudes.
The record of Accidents and Serious Incidents which have accompanied this increase in high altitude flight has suggested that pilot understanding of the aerodynamic principles which apply to safe high-altitude flight may not always have been sufficient. This applies particularly to attempts to recover from an unexpected loss of control. The subject is introduced in this article and covered in comprehensive detail in the references provided.
From a practical point of view, ‘high altitude’ operations are taken to be those above FL250, which is the altitude at above which aircraft certification requires that a passenger cabin overhead panel oxygen mask drop-down system has to be installed. Above this altitude a number of features begin to take on progressively more significance as altitude continues to increase:
- There is a continued reduction in the range of airspeed over which an aircraft remains controllable;
- True airspeed (TAS) (and therefore aircraft momentum) increases with altitude. However, the effectiveness of the aerodynamic controls and natural aerodynamic damping are both dependant upon indicated airspeed (IAS) and remain largely unchanged. Therefore, the ability of the aerodynamic flight controls to influence flight path or to recover from an upset is progressively reduced as altitude increases;
- In the event of depressurisation, the time of useful consciousness for occupants deprived of oxygen reduces dramatically - see the separate articles on Emergency Depressurisation, andHypoxia.
- At very high altitude, occupants are exposed to slightly increased cosmic radiation. This is covered by the separate article "Cosmic Radiation".
This article focuses on aerodynamics and aircraft handling.
The key to an understanding of the practical implications of high altitude flight is an understanding of the Total Drag curve and the relationship between its two primary components, Induced Drag and Parasitic Drag. Induced drag is directly related to lift production and is greatest at low speeds and high angle of attack. Conversely, parasitic drag increases in proportion to the square of the aircraft speed. Total drag, at any given speed, is the sum of its two components and, as can be demonstrated graphically, its minimum value occurs where the induced drag and parasitic drag curves intersect. The speed corresponding to the point of minimum total drag is known as the minimum drag speed or Vimd (sometimes Vmd) and will vary as a function of aircraft weight. Obviously, level flight at speeds greater than or less than Vimd is possible.
To fly slower than Vimd, a greater angle of attack is necessary and an increase in thrust is required to compensate for the increase in induced drag caused by the increased angle of attack. Angle of attack can be increased to the point that there is insufficient thrust available to maintain level flight or until reaching the wing's stall angle of attack, whichever occurs first. Conversely, increasing speed above Vimd requires a reduction in the angle of attack to maintain level flight. Additional thrust will be required to offset the increase in parasitic drag produced due to the additional speed that is required to enable the airfoil to generate the equivalent amount of lift at a lower angle of attack. At speeds above Vimd, the aeroplane is in stable flight whereas at speeds below Vimd, an aeroplane is in unstable flight.
The question of stability is illustrated by the effect of any encounter with air turbulence. If this occurs when an aircraft is in the stable flight regime and the power/ thrust setting is not altered, it will result in increased drag and reduced aircraft airspeed; this reduces drag so that airspeed eventually returns to the previous value. If an aircraft is in the unstable fight regime, a similar disturbance would cause a decrease in airspeed and so increased drag; this would result in a further decrease in airspeed unless the power/thrust setting were increased; the lower speeds would mean increased drag which would result in a further decrease in airspeed. This is referred to as being on the "back side of the drag curve". Since the applicable TAS for a ‘low speed’ stall increases as altitude increases, and the reference speed for higher altitude flight is Mach Number rather than IAS, the minimum cruise speed as altitude increases begins to approach the Mmo (the maximum operating Mach Number).
Mmo and Risk of Mach Stall
Certification of aircraft types includes the setting of Mmo. This is based upon setting a suitable margin from the Critical Mach Number (Mcrit), at which airflow over a wing becomes transonic, that is, reaches the local speed of sound, and forms shock waves. These shock waves induce Wave Drag and disturb previously smooth airflow leading to a loss of lift and. potentially, to Mach stall. The margin between this upper limit and the prevailing TAS for a low speed stall, which increases as altitude increases and air density decreases, narrows with an increase in altitude resulting in a flight regime often referred to as Coffin Corner. It also means that the normal cruise speed at high altitude will be nearer to Mmo. Any exceedence of Mmo at high altitude will bring the aircraft closer to the critical Mach Number and to the risk of a Mach stall.
Variation in Cruise Speed
Slower cruising speeds are often used as a means to save fuel, but this will mean routinely flying closer to the minimum drag speed (Vimd); this gives less time to recognise and respond to any speed loss and eventual risk of a stalled wing condition.
Small changes in either ‘External Factors’, such as variable winds, increased drag in turns, turbulence from any source, ice accretion or ‘Internal Factors’ such as use of anti-icing, un-commanded thrust rollback or engine malfunction can lead to loss of airspeed. Heavily damped autothrottles, designed for passenger comfort, may not always apply thrust aggressively enough to prevent a slowdown which places the aircraft on the back side of the drag curve. Close monitoring is essential.
Optimum Cruise Altitude
The optimum cruise altitude is that at which a given thrust setting results in the corresponding maximum range speed. The optimum altitude is not constant and changes over the period of a long flight as atmospheric conditions and the weight of the aircraft change. A large change in temperature will significantly alter the optimum altitude with a decrease in temperature corresponding to an increase in altitude. At the optimum altitude, operating costs will be minimum when operating in the most economical (ECON) mode; it is also the cruise altitude for minimum fuel burn when in the Long Range Cruise (LRC) mode. In both cases, optimum altitude increases with reducing aircraft weight. If the aircraft is at its maximum certified level or the altitude is operationally capped, speed reduction as weight decreases will help to maintain a minimum fuel burn profile.
Maximum operating altitude is determined by reference to three basic characteristics which are unique to each aircraft type. It is the lowest of:
- Maximum Certified Altitude as stated in the Aircraft Flight Manual (AFM) (This is usually structural and is most often defined by pressurisation load limits on the fuselage. However, a component maximum operation envelope may impose lower limits, especially when dispatching under Minimum Equipment List (MEL) relief).
- Thrust Limited Altitude at the prevailing aircraft operating weight and environmental conditions - the altitude at which sufficient thrust is available to provide a specific minimum rate of climb (this is the usual controlling limit especially when turning and available thrust may be very small).
- Buffet limited maximum altitude at the prevailing aircraft operating weight and environmental conditions - the altitude at which a 1.3g loading from turning, manoeuvring or turbulence may be experienced without encountering buffet associated with either low speed stall or Mach stall (low speed pre-stall buffet occurs at increasingly high IAS as altitude increases whereas the pre-Mach stall buffet occurs at a decreasing IAS so that the margin between the two is progressively reduced with increase in altitude).
Mass and Balance Effects on Handling Characteristics
For conventional airplanes, a C of G towards the aft limit of the mass and balance envelope means less longitudinal stability whereas an aircraft with a C of G near the forward limit means greater longitudinal stability. Since an airplane is dependent on the elevator to provide pitch control, the forward C of G limit occurs at a point where the increase in stability will not exceed the ability of the elevator to provide this control. If the C of G moves forward, additional force is required on the elevator to raise the nose up causing the stall speed to increase. The relative longitudinal instability which comes when the C of G is near the aft limit means that the inherent susceptibility to loss of control is greater. Less effort is required by the tailplane to counteract the nose down pitch moment of the wing and this results in less induced drag on the entire aircraft and thus maximises efficient flight.
The wing can be stalled at any airspeed, true or indicated, and at any altitude, and aircraft attitude has no absolute relationship to the onset of an aerodynamic stall. If the wing angle of attack exceeds the stalling angle of attack, the wing will stall. Successful recovery from a full stall often involves a very different technique to that required for the recovery from the approach to one. Training for recovery from an incipient or near-stalled condition has in the past often emphasised minimum altitude loss as a goal by focusing on the application of a rapid increase in thrust without consideration of the overiding imperative to reduce aerofoil angle of attack to ensure that a fully stalled condition is avoided. For the less frequently trained fully stalled condition, the overriding imperative is to un-stall the wing by reducing the angle of attack and this is likely to involve a reduction in aircraft pitch well into the negative with the concomitant loss of altitude that this implies. Engine thrust may be significantly lost during an aerodynamic full stall of the wing for as long as the air intake to the engines is disrupted by the high angle of attack which is implied by the condition. In all cases, it is vital that pilots understand the differences between the actions required to recover from an incipient stall and those required to recover from a full stall and that they are able to apply them correctly.
High Altitude Handling Considerations
- Stay alert! The high altitude environment in not a place for complacency. The available flight envelope between low and high speed buffet may be quite limited and manoeuvering induced loads or external forces such as windshear or turbulence can result in an exceedence or, potentially, an upset.
- Avoid flight at speeds at or below minimum drag speed (Vimd). Speeds below Vimd are referred to as on the "back side of the drag curve" and are inherrently unstable. In this regime, an external force, such as turbulence, which causes a speed reduction will result in further speed decay if additional thrust is not immediately applied. If not immediately arrested, the speed decay will continue and could result in stall or the necessity to initiate a descent to regain airspeed.
- Use of V/S mode in climb can lead to a critical loss of airspeed and potentially to a stall. Be aware of the climb capabilities of the aircraft - trying to comply with a restrictive Air Traffic clearance such as "be level in two minutes" could also result in a critical reduction in airspeed. This is because in V/S mode, the vertical speed selected has priority over speed. If too high a rate of climb is selected, when the aircraft thrust limit (usually CLIMB _CL, CL1 or CL2) is reached, rate of climb will continue to take priority and speed will decay as the autopilot attempts to sustain climb rate, running the risk of a stall.
- Monitor the outside air temperature (OAT). Optimum altitude is reduced by an increase in OAT. If an increase in temperature is encountered while in level flight, buffet margins will be reduced and a descent may be necessary.
- Be aware that although an aft centre of gravity improves fuel economy, it also reduces aircraft longitudinal stability and increases susceptibility to upset.
- Be smooth. When assuming manual control at high altitude be aware that there is less aerodynamic flight control damping due to the thinner air. Avoid over controlling as it could potentially lead to an upset. Avoid bank angles beyond 15 degrees as the additional drag could exceed the thrust available and lead to a speed decay. Conversely, be aware of the fact that in a confirmed upset or stall situation, full control deflection may be required to regain control of the aircraft.
- Understand and be able to recognise the difference between an incipient (approaching) stall and a full stall.
- Know the difference in the actions required to recover from an incipient stall as compared to those required to recover from a full stall.
- Recognise that, in the event of a full stall, stall recovery is the priority. Altitude recovery is secondary and can only be achieved after successful stall recovery.
Inadvertent loss of control of aircraft continues to occur. Such losses of control usually involve a full stall or an approach to a stall at some stage in the event sequence, whether as the initiating factor or as a later consequence. This article briefly reviews the awareness and avoidance of aerodynamic wing stalls with particular reference to modern multi-engine public transport aircraft. It also touches on the subjects of stall detection and recognition as well as the generic requirements for stall recovery. It does not consider the subject from the perspective of light aircraft or address the special case of tailplane stalls, although in both cases, the underlying principles which govern the stall of any aerofoil remain the same.
The wing is mounted on the fuselage at an angle of incidence so that in level cruising flight, with the fuselage close to horizontal, the wing has a small, positive angle of attack relative to the airflow. As airspeed is reduced, the angle of attack needs to increase to maintain the value of the lift force being produced by the wing. This will continue until the critical angle is reached at which point the wing stalls. The value of the critical angle will be dependent on the cross-section of the aerofoil, the configuration of the wing (flaps/slats/spoilers) and the design planform of the wing.
The indicated airspeed (IAS) at which the critical angle is reached and the wing stalls will vary with changes in aircraft weight, centre of gravity (CG) and load factor (or G):
- In 1G flight, the critical angle is reached at increasing IAS as aircraft weight increases.
- The critical angle will be reached at a higher IAS as CG moves forwards.
- When compared to a wings level (1G) stall, the IAS at which critical angle is reached in a level stabilised, 15 degree angle of bank turn is 3.5% higher; in a 45 degree angle of bank turn it is 19% higher. These increases are a result of the increased G needed to keep the aircraft in a stabilized turn as bank is increased.
- When manoeuvring hard at 2G, the IAS at which the critical angle will be reached – and stall will occur – will be 41% higher than when flying at 1G. Although this is still a high incidence stall in terms of airflow behavior, it is often referred to as a High Speed stall
When flying below transonic speeds, at any given aircraft weight, the indicated airspeed (IAS) at which the wing will stall is the same at all altitudes because both the stalling angle of attack (critical angle), and indicated airspeed, change relative to ambient air density. This type of stall is sometimes described as a high incidence stall. While the IAS at the stall remains constant for any given condition of weight, CG and G loading, the true airspeed (TAS) at which a wing stalls will increase with altitude.
As altitude increases and air density decreases, the gap between IAS and TAS increases, until the TAS becomes a significant proportion of the speed of sound. Eventually the airspeed over the upper surface exceeds the local speed of sound, and shock waves form toward the trailing edge. These shocks will eventually cause a high speed buffet but at Mach numbers well above Mmo. Shock waves can also form near the leading edge at a high angle of incidence and high altitude and these will progressively limit the achievable incidence, so the stalling speed, IAS, will increase. So if buffet occurs at high altitude it could be due to either under- or over-speed, the clue is the angle of incidence - lower than normal cruise incidence = high speed buffet, higher than normal cruise incidence = low speed buffet (but at an IAS rather larger than normal low speed stall occurs). It follows that pilots must be aware of their normal operating conditions in order to correctly diagnose any anomaly. Note that while some modern aircraft have stall-warning systems that adjust for Mach number, others do not, and a stall as just described can occur without an accompanying stall warning
When Stalls Most Often Occur
Accident and incident reports show that most full or near-full stalls of transport aircraft occur in one of five situations, as for other paths to loss of control, and usually in IMC or where there is no natural visual horizon:
- During inappropriate response to an un-commanded autopilot disconnect at high altitudes. (Uncommanded AP Disconnect due to malfunction of other systems)
- At low altitudes when the indicated airspeed is unintentionally allowed to deviate significantly from the intended and necessary target (Airspeed Awareness)
- At low altitudes in the presence of frozen deposits on the wings (Airframe Icing)
- During a mishandled go around (Aircraft management and Flying Skills)
- Because of insufficient understanding of automation as it affects flight envelope protection systems.
- Improper slats/flaps configuration (Aircraft Configuration)
Autopilot (AP) Disconnect due to malfunction of other systems is liable to create a significant ‘startle factor’ for both pilots and remove some of the flight envelope protections commonly provided by Fly-By-Wire (FBW) flight control systems. Manual flying at high altitude is rarely practiced and there is not always sufficient awareness of the different ‘feel’ of the flight controls at high altitude compared to those experienced lower altitudes. Simultaneous AP disconnect and automatic reversion to a lower FBW Control Law, makes it important to keep the aircraft within a safe envelope. It is also important that crews are completely familiar with which levels of protection are (or are not) available under all normal, non-normal and emergency situations operative at all times.
Loss of Airspeed Awareness arises because of a fundamental failure to prioritise the flying and management of an aircraft over other things, especially in the presence of distractions of any type. Typical distractions are minor technical malfunctions and their secondary effects. However, such failures involve the roles of both PF and PM and may arise as an indication of much wider issues for individuals, CRM between them or evidence flight standards problems in the Operator.
Unanticipated Airframe Icing typically leads to unexpected stalling at relatively low altitudes:
- Just after take off when ground de/anti icing has not been properly carried out (or carried out at all) or when the effective hold over time since the commencement of treatment has been exceeded. Contamination of one or both wings with frozen deposits reduces the critical angle of attack so that the stall warning/protection systems do not operate before an actual stall occurs.
- During approach as configuration is changed, accumulated wing surface ice changes the stall angle of attack and compromises the degree of warning provided by the stall protection system
A stall or near stall during a go around is usually the result of poor Aircraft management and Flying Skills, including ineffective CRM and failure to follow standard operating procedures, and may arise from poor manual handling or because of mismanagement of the automatic flight control systems/autopilot.
A stall that occurs as a consequence of insufficient understanding of automation usually involves the pilot making flight control inputs without adequately monitoring parameters (e.g. IAS) that the pilot thought to be under automatic control; these can occur through the automation acting as designed but not as the pilot expected or following automation failure (see B738, vicinity Amsterdam Netherlands, 2009 (HF LOC)).
Stall warning systems are designed to identify an impending stall at the incipient stage so pilot intervention can occur in time to prevent the full aerodynamic stall of the aircraft. However, pilot overload due to an emergency or an unreliable speed situation, the failure of stall warning equipment or an aircraft upset can all mask the effectiveness of the warning systems and a fully developed stall may occur without pilot awareness.
Generic indicators of an aerodynamic stall can include:
- Activation of artificial stall warnings
- Aircraft buffet
- Reduced flight control authority, especially reduced or loss of roll control
- Significant aft control column displacement
- High rate of descent
- A nose down pitching tendency at the point the stall occurs
However, not all aircraft react in the same manner and the visual and tactile cues found in many aircraft may be absent in others. As examples:
- Not all aircraft have an audio stall warning
- Not all aircraft experience significant buffet during a stall
- In some Fly-By-Wire aircraft, such as the Airbus A320, there is no aft flight deck control (side stick) displacement
- Some swept wing aircraft experience a nose up pitching tendency at stall onset vice the nose down pitching moment associated with a "conventional" stall
For these reasons, it is critical that pilots thoroughly understand the stall characteristic of their aircraft type as well as the aircraft stall warning, prevention and/or recovery systems inclusive of their limitations, and the possible ramifications of any potential system failures (e.g. failure modes).
Pilots must be able to recognise the stall characteristics of their aircraft type and, if specified by the manufacturer, know and be able to correctly apply the recommended recovery technique. In the absence of a manufacturer specified recovery procedure, the following guidance is offered by the FAA and endorsed by EASA.
In late 2012, the FAA released AC 120-109 Stall and Stick Pusher Training which acknowledges "Reduction of Angle of Attack (AOA) is the most important response when confronted with a stall event." It further outlines the following stall recovery template:
- Autopilot and autothrottle ... Disconnect - While maintaining the attitude of the airplane, disconnect the autopilot and autothrottle. Ensure the pitch attitude does not increase when disconnecting the autopilot. This may be very important in out-of-trim situations. Manual control is essential to recovery in all situations. Leaving the autopilot or autothrottle connected may result in inadvertent changes or adjustments that may not be easily recognized or appropriate, especially during high workload situations.
- Nose down pitch control ... Apply until stall warning is eliminated or, when required, Nose down pitch trim ... As Needed - Reducing the angle of attack is crucial for recovery. This will also address autopilot-induced excessive nose up trim. If the control column does not provide sufficient response, pitch trim may be necessary. However, excessive use of pitch trim may aggravate the condition, or may result in loss of control or high structural loads.
- Bank ... Wings Level - roll wings level if the stall is in the turn. This orients the lift vector for recovery.
- Thrust ... As Needed - During a stall recovery, maximum thrust is not always needed. A stall can occur at high thrust or at idle thrust. Therefore, the thrust is to be adjusted accordingly during the recovery. For airplanes with engines installed below the wing, applying maximum thrust may create a strong nose-up pitching moment if airspeed is low. For airplanes with engines mounted above the wings, thrust application creates a helpful pitch-down tendency. For propeller-driven airplanes, thrust application increases the airflow around the wing, assisting in stall recovery.
- Speed brakes/Spoilers ... Retract - This will improve lift and stall margin.
- Return to the desired flight path - Apply gentle action for recovery to avoid secondary stalls, then return to desired flight path. Having recovered from the stall, the priority is to ensure clearance from terrain.
Managing the Risk
The best way to avoid inadvertent stall is to first be aware of the conditions where stall is likely and then to avoid those conditions or to concentrate on stall avoidance when operating deliberately close to the stall.
Technical mitigations to avoid inadvertent stall include stall warning devices and flight envelope protection systems, though under some circumstances as described above, these may be ineffective. Therefore, the actions available for reducing any heightened risk of a stall and consequent loss of control generally lie in the area of flight training - in the classroom and the full flight simulator - and in the application of, and adherence to, appropriate SOPs.
Establishing whether training and SOPs adequately address the risk depends largely on the effective assessment of individual pilot competency. However, the widespread adoption of OFDM now provides an opportunity to analyse a range of lesser ‘precursor’ events where there has been an abnormal deviation from an expected flight path towards a stalled condition followed by a successful recovery. Such occurrences can be tracked back to particular pilots and to their training histories, enabling gaps in pilot training, knowledge or SOPs to be addressed throughout an operator’s fleet.
Some Examples of Stall Accidents and Incidents
- A332, en-route, Atlantic Ocean, 2009 (LOC HF AW) - (Uncommanded AP Disconnect due to malfunction of other systems): On 1 June 2009, an Air France Airbus A330-203disappeared over the Atlantic Ocean while transiting the ITCZ, a belt of thunderstorm activity. The accident is the subject of an on-going investigation by the French BEA.
- B738, vicinity Amsterdam Netherlands, 2009 (HF LOC) - (Airspeed Awareness): On 25 February 2009, a Boeing 737-800 being operated by Turkish Airlines crashed 1.5 kilometres short of the threshold of Runway 18R at Schiphol airport, Amsterdam following a loss of control during a daylight coupled ILS approach to that runway.
- CL60, Birmingham UK, 2002 (GND LOC HF FIRE) - (Airframe Icing): On 4 January 2002, a Challenger 604 operated by Epps Air Service, crashed on takeoff from Birmingham, UK, following loss of control due to airframe icing.
- B733, vicinity Bournemouth UK, 2007 (LOC HF) - (Aircraft management and Flying Skills): On 23 September 2007, a Boeing 737-300 operated by Thomsonfly, on routine ILS approach at night to Bournemouth Airport, experienced a stall during early stage of the approach. The auto-throttle disengaged with the thrust levers in the idle thrust position. The disengagement was neither commanded nor recognised by the crew and the thrust levers remained at idle throughout the approach. As result of the stall, the commander took control and initiated a go-around. During the go-around the aircraft pitched up excessively; flight crew attempts to reduce the aircraft’s pitch were largely ineffective. The aircraft reached a maximum pitch of 44° nose-up and the indicated airspeed reduced to 82 kt. The flight crew, however, were able to recover control of the aircraft and complete a subsequent approach and landing at without further incident.
- A30B, vicinity Nagoya Japan, 1994 (LOC HF) - (insufficient understanding of automation as it affects flight envelope protection systems ): On 26 April 1994, a China AirlinesAirbus A300 flying an ILS approach to Runway 34 at Nagoya Airport, Japan, under manual control, stalled and crashed after mishandling by the pilots caused by inadvertent selection of GO AROUND mode, failure to recognise the developing abnormal out of trim situation, and a lack of understanding of the Flight Director and Autopilot.
The cruise phase of flight starts after aircraft has leveled off from the climb and it ends when the descent for landing is initiated by the crew. This phase involves level flight most of the time and very few level changes. There are two main options for cruise: maximum possible range (also known as best range) and maximum possible speed (also known as best speed). Sometimes due to operational reasons (holding for example) best endurance cruise is used instead of the two previous. Some of the factors that affect the cruise phase and their relations are already explained in the flight envelope; however the following paragraphs are written from a different (operational) point of view.
Factors affecting range
Range is the distance traveled with the fuel available. It is usually required to fly so that the maximum range is achieved, that is, to cover the greatest distance for the fuel carried, or to use the least fuel for the distance which is required to travel. The maximum range will depend on the amount of fuel carried and the number of miles flown per kilogram of fuel (also known as specific range). The following factors will have effect on the maximum possible range:
Increased aircraft mass increases the drag due to increased induced drag (greater lift required) and increased profile drag (higher speed at the same angle of attack). This requires greater thrust to balance the drag, which increases the fuel flow and reduces the specific range (number of miles flown per kilogram of fuel).
Approximately 10% increase in mass will require 10% increase in thrust and fuel flow and 5% decrease in range.
Air density (altitude)
Increasing altitude (decreasing air density) increases the range up to an optimum altitude, and then decreases again. The range is increased with altitude because of increased jet engine efficiency. However, above the optimum altitude, the effects of air compressibility cause the drag to increase resulting with reduced specific range.
This optimum altitude for best range increases as weight decreases. The procedure to give maximum range would therefore be to allow the aircraft to climb as the weight decreases during the flight. This is achieved with few step climbs during the cruise phase of the flight.
The speed which gives the maximum range for a given aircraft weight and altitude is called best range speed. Flying at higher speeds than the best range speed increases the drag and the fuel flow, and therefore reduces the range. Lower speeds than the best range speed reduce the drag and the fuel flow, but they also reduce the distance traveled per time which is more dominant, and therefore reduce the range.
Best range speed is considered in reference to the air. If the air mass is moving (wind), the speed in reference to the ground is different. The best range will be reduced in a headwind condition and the best range speed would be higher. The opposite occurs in a tailwind condition, the best range will be increased and the best range speed would be lower.
The required change of speed in headwind conditions is relatively small unless the wind is very strong (jetstream), and in this case a change of altitude where the wind strength is less would be preferable. The change of altitude is desirable if the gain for more favorable wind exceeds the loss from the change of altitude. This is also known as wind-altitude trade.
Factors limiting the speed (TAS)
The second cruise concept (maximum possible speed) is usually used on short or mid range flights when airlines are running behind schedule. It allows greatest distance travelled per given time or use minimum time to travel the required distance. The maximum speed is achieved when the maximum thrust available is balanced by the drag and it depends on the thrust available and the maneuver capability.
The speed achieved with maximum cruise thrust will vary with aircraft mass, altitude and temperature: − increased aircraft mass increases the drag, and therefore reduce the excess thrust required for acceleration; − thrust decreases with altitude, however the speed (TAS) will increase until a certain altitude and then decrease. For a given weight there is an optimum altitude that gives the best speed. This altitude is different from the optimum altitude for the best range; − increased temperature decreases the maximum thrust that a jet engine can produce;
As an aircraft increases speed (TAS) the airflow over some part of the fuselage or wings may be accelerated up to the speed of sound and a shock wave will form. These shock waves cause more drag, less lift, turbulent flow, and a reduction in effectiveness or even a reversal of control reactions. So, the maximum speed is limited by the critical Mach number.
Factors affecting endurance
Endurance is the time that aircraft can remain airborne with the fuel available. It will be greatest when the fuel is used at the lowest possible rate, that is, the fuel flow is minimum. The fuel flow depends on the thrust. Minimum thrust is required when an aircraft is flying with minimum possible drag. The following factors will have effect on the maximum possible endurance:
Increased aircraft mass increases the drag due to increased induced drag (greater lift required) and increased profile drag (higher speed at the same angle of attack). This requires greater thrust to balance the drag, which increases the fuel flow and reduces the endurance.
Air density (altitude)
Increasing altitude (decreasing air density) increases the endurance due to the increase in jet engine efficiency. However, at a very high altitude, compressibility will increase the drag, causing increased fuel flow, and therefore reducing the endurance.
At the optimum altitude, operating costs will be minimum when operating in the most economical (ECON) mode; it is also the cruise altitude for minimum fuel burn when in the Long Range Cruise (LRC) mode. In both cases, optimum altitude increases with reducing aircraft weight. In addition, in ECON Mode the optimum altitude increases with a reduction in cost index; in LRC Mode, it increases as speed reduces. Speed
The speed which gives the minimum drag for a given aircraft weight and altitude is called best endurance speed. Flying at higher speeds then the best endurance speed increases the drag and the fuel flow, and therefore reduces the endurance.
Also slower cruising speeds, often used as a means to save fuel, but may mean routinely flying closer to L/D max (Lift/Drag maximum); this gives less time to recognise and respond to any speed loss and eventual risk of a stalled wing condition. Small changes in either ‘External Factors’, such as variable winds, increased drag in turns, turbulence from any source, ice accretion or ‘Internal Factors’ such as use of anti-icing, un-commanded thrust rollback or engine malfunction can lead to loss of airspeed. Heavily damped autothrottles, designed for passenger comfort, may not always apply thrust aggressively enough to prevent a slowdown below L/D max. Close monitoring is essential.
Temperature (jet engine)
The jet engine efficiency reduces with increase of temperature, giving increased fuel flow and reduced endurance.
To achieve the best possible range it is necessary to fly at an optimum altitude and at an optimum speed for the given aircraft mass. However, small deviations from these optimum conditions do not make great difference to the range. On the other hand large deviations from the optimum, may cause significant loss in range and may even require aircraft to divert to an enroute alternative airport for an intermediate stop and refueling, which is time consuming and expensive. The following table gives an overview to the loss in range depending on the deviation in cruising level:
Deviation from optimum % in loss in range 2000 feet above 1 Optimum 0 2000 feet below 1 4000 feet below 4 8000 feet below 10 12000 feet below 15
To extract the maximum cruise performance from any airplane, the power setting tables provided by the manufacturer should be closely followed (either calculated by the pilot or by aircraft systems).
Flying at higher speeds than optimum requires more thrust and burns more fuel; while flying at lower speeds than optimum requires longer flying time and again more fuel. Therefore speed control (speed assigned to an aircraft) has an adverse effect to the range. It shall be used when absolutely necessary for long periods of time and the deviations from the optimum shall be as small as possible.
Airplanes are a very convenient way to travel, but not every individual is delighted to travel 37,000 feet high up in the air. It is natural for people to feel a little insecure about traveling in airplanes but there are many ways by which you can help control that overwhelming feeling. Having the right information can help you understand why airplanes need to travel that high up in the air and that this is not a thing to get anxious about.
A cruising altitude refers to the altitude at which the aircraft shall spend most of its flight. This is exactly the altitude where the plane shall level out after taking off. Not only does this altitude allow the plane to fly more efficiently but it also prevents the chances of meeting other aircraft in the air.
Although airplanes cruise at a wide range of levels and there are several factors that determine the cruising altitude of a plane. Cruising altitude for different airlines may vary between 25,000 feet and 40,000 feet. It is also not uncommon for an aircraft to change cruising altitude several times if it is a long journey flight. Every flight has a certain optimum cruising altitude which would depend on the weight of the aircraft.
In this article we help you understand the various factors which determine the cruising altitude to give you a better idea about it. And also why it is more beneficial for the plane to fly high up in the air rather than, down below. It can get scary for people to realize that they are up among the clouds but there are certain reasons why planes need to fly high in the air.
Here below are the factors and reasons why the aircraft has to travel high up in the air:
• The air in the earth’s atmosphere becomes thinner as the altitude increases. When the air becomes thinner, it offers less resistance to objects flying through; this is why less thrust is required to move the aircraft. This in turn, helps the aircraft fly more efficiently.
• Altitudes are defined in relation to number of feet above sea level. Pilots need to aware of the terrain over which they are flying. For example, twenty thousand feet may be suitable for southern Florida but it may not be the same if the aircraft was flying over mountains.
• Cruising altitude would also depend on weather. Pilots receive weather reports and request alternate cruising altitude from air traffic control to avoid air turbulence.
• The length of the flight also plays an important role. The cruising altitudes for short flights are usually less as compared to the cruising altitude for longer flights.
• Flying too close to the clouds may make it hard for the pilot to see and cross winds may lead to air turbulence.
• Flying low means more bug and insects on the wind shield which again can make it hard for the pilot to see in front and reduce visibility.
• There is less friction at higher altitudes, less friction and higher air speed causes engines to burn less fuel therefore it improve fuel efficiency allowing the airplane to travel further.
• Also, at higher altitudes, the air is less dense, therefore the aircraft can run more effectively
• Best fuel efficiency occurs when the plane gets into the mid 30’s +
• High altitude means less power is needed to propel the plane and at 40,000 feet the engines are just above idle power.
These were just some reasons why aircrafts travel high up in the air. So you know now, that this is nothing to be worried about; good airlines take all safety measures to guarantee you have a safe and pleasurable trip. And even though you may be high up in the air, do not forget that the pilots operating the plane are experienced and know everything there is to be known in order to help you reach your destination at the earliest without any inconvenience. Enjoy your flight!
The pilot uses a gauge called an altimeter, which measures air pressure, to determine how high the plane is flying at any given time. The altimeter is shaped like a soup can with a dial on one end, and inside the “can” are a series of aneroid wafers, which act as a barometer, that expand or contract depending on the air pressure. As a plane climbs, the air becomes less dense, and the aneroids expand. When a plane loses altitude, or comes closer to the earth’s surface, the air pressure increases, and the aneroids contract. The movement of the aneroids is transferred to hands, like those on a clock, on the altimeter gauge. A separate knob allows the pilot to adjust the altimeter settings to reflect the terrain in the flight path.
The pilot controls the altitude of the plane by using the yoke, the airplane’s equivalent of a steering wheel. The yoke controls the elevators, two horizontal wing-like pieces that are on the tail of the plane. If the pilot wants to decrease the altitude, he pushes the yoke forward and points the nose of the plane down, which causes the elevators to point down. To climb, the pilot pulls back on the yoke to bring the nose of the plane and the elevators up. If the pilot wants to hold a steady altitude, the yoke is held in a neutral position, causing the elevators to point straight back.
Sumber : Internet
In aviation, V-speeds are standard terms used to define airspeeds important or useful to the operation of all aircraft.These speeds are derived from data obtained by aircraft designers and manufacturers during flight testing and verified in most countries by government flight inspectors during aircraft type-certification testing. Using them is considered abest practice to maximize aviation safety, aircraft performance or both.
The actual speeds represented by these designators are specific to a particular model of aircraft, and are expressed in terms of the aircraft's indicated airspeed, so that pilots may use them directly, without having to apply correction factors. "V" stands for velocity.
In general aviation aircraft, the most commonly used and most safety-critical airspeeds are displayed as color-coded arcs and lines located on the face of an aircraft's airspeed indicator. The lower ends of the green arc and the white arc are the stalling speed with wing flaps retracted, and stalling speed with wing flaps fully extended, respectively. These are the stalling speeds for the aircraft at its maximum weight. The yellow range is the range in which the aircraft may be operated in smooth air, and then only with caution to avoid abrupt control movement, and the red line is the Vne, the never exceed speed.
Proper display of V speeds is an airworthiness requirement for type-certificated aircraft in most countries.
These V-speeds are defined by regulations.
|V1||Engine failure recognition speed. (See V1 definitions below)|
|V2||Takeoff safety speed. The speed at which the aircraft may safely become airborne with one engine inoperative.|
|V2min||Minimum takeoff safety speed.|
|V3||Flap retraction speed.|
|V4||Steady initial climb speed. The all engines operating take-off climb speed used to the point where acceleration to flap retraction speed is initiated. Should be attained by a gross height of 400 feet.|
|VA||Design maneuvering speed. This is the speed above which it is unwise to make full application of any single flight control (or "pull to the stops") as it may generate a force greater than the aircraft's structural limitations.|
|Vat||Indicated airspeed at threshold, which is equal to the stall speed VS0 multiplied by 1.3 or stall speed VS1g multiplied by 1.23 in the landing configuration at the maximum certificated landing mass. If both VS0 and VS1g are available, the higher resulting Vat shall be applied. Also called "approach speed".|
|VB||Design speed for maximum gust intensity.|
|VC||Design cruise speed, used to show compliance with gust intensity loading.|
|Vcef||See V1; generally used in documentation of military aircraft performance.|
|VD||Design diving speed.|
|VDF||Demonstrated flight diving speed.|
|VEF||The speed at which the Critical engine is assumed to fail during takeoff.|
|VF||Designed flap speed.|
|VFC||Maximum speed for stability characteristics.|
|VFE||Maximum flap extended speed.|
|VFTO||Final takeoff speed.|
|VH||Maximum speed in level flight at maximum continuous power.|
|VLE||Maximum landing gear extended speed. This is the maximum speed at which it is safe to fly a retractable gear aircraft with the landing gear extended.|
|VLO||Maximum landing gear operating speed. This is the maximum speed at which it is safe to extend or retract the landing gear on a retractable gear aircraft.|
|VMC||Minimum control speed. Mostly used as the minimum control speed for the takeoff configuration (takeoff flaps). Several VMC's exist for different flight phases and airplane configurations: VMCG, VMCA, VMCA1, VMCA2, VMCL, VMCL1, VMCL2. Refer to the minimum control speed article for a thorough explanation.|
|VMCA||Minimum control speed in the air (or airborne). The minimum speed at which steady straight flight can be maintained when an engine fails or is inoperative and with the corresponding opposite engine set to provide maximum thrust, provided a small (3° - 5°) bank angle is being maintained away from the inoperative engine and the rudder is used up to maximum to maintain straight flight. The exact required bank angle for VMCA to be valid should be provided by the manufacturer with VMC(A) data; any other bank angle results in a higher actual VMC(A). Refer to the minimum control speedarticle for a description of (pilot-induced) factors that have influence on VMCA. VMCA is also presented as VMC in many manuals.|
|VMCG||Minimum control speed on the ground is the lowest speed at which the takeoff may be safely continued following an engine failure during the takeoff run. Below VMCG, the throttles need to be closed at once when an engine fails, to avoid veering off the runway.|
|VMCL||Minimum control speed in the landing configuration with one engine inoperative.|
|VMO||Maximum operating limit speed.|
|VMU||Minimum unstick speed.|
|VNE||Never exceed speed.|
|VNO||Maximum structural cruising speed or maximum speed for normal operations.|
|VO||Maximum operating maneuvering speed.|
|VR||Rotation speed. The speed at which the aircraft's nosewheel leaves the ground. Also see note on Vref below.|
|Vrot||Used instead of VR (in discussions of the takeoff performance of military aircraft) to denote rotation speed in conjunction with the term Vref (refusal speed).|
|VRef||Landing reference speed or threshold crossing speed.|
(In discussions of the takeoff performance of military aircraft, the term Vref stands for refusal speed. Refusal speed is the maximum speed during takeoff from which the air vehicle can stop within the available remaining runway length for a specified altitude, weight, and configuration. ) Incorrectly, or as an abbreviation, some documentation refers to Vref and/or Vrot speeds as "Vr."
|VS||Stall speed or minimum steady flight speed for which the aircraft is still controllable.|
|VS0||Stall speed or minimum flight speed in landing configuration.|
|VS1||Stall speed or minimum steady flight speed for which the aircraft is still controllable in a specific configuration.|
|VSR||Reference stall speed.|
|VSR0||Reference stall speed in landing configuration.|
|VSR1||Reference stall speed in a specific configuration.|
|VSW||Speed at which the stall warning will occur.|
|VTOSS||Category A rotorcraft takeoff safety speed.|
|VX||Speed that will allow for best angle of climb.|
|VY||Speed that will allow for the best rate of climb.|
Some of these V-speeds are specific to particular types of aircraft and are not defined by regulations.
|VBE||Best endurance speed – the speed that gives the greatest airborne time for fuel consumed.|
|VBG||Best power-off glide speed – the speed that provides maximum lift-to-drag ratio and thus the greatest gliding distance available.|
|VBR||Best range speed – the speed that gives the greatest range for fuel consumed – often identical to Vmd.|
|VFS||Final segment of a departure with one powerplant failed.|
|VLLO||Maximum landing light operating speed – for aircraft with retractable landing lights.|
|Vmbe||Maximum brake energy speed|
|Vmd||Minimum drag (per lift) – often identical to VBR. (alternatively same as Vimd)|
|Vmin||Minimum speed for instrument flight (IFR) for helicopters|
|VPD||Maximum speed at which whole-aircraft parachute deployment has been demonstrated|
|Vra||Rough air speed (turbulence penetration speed).|
|VSL||stall speed in a specific configuration|
|Vs1g||stall speed at 1g load factor|
|Vsse||Safe single engine speed|
|VTO||Take-off speed. (see also VLOF)|
|Vtocs||Take-off climbout speed (helicopters)|
|Vtos||Minimum speed for a positive rate of climb with one engine inoperative|
|Vtmax||Max threshold speed|
|Vwo||Maximum window or canopy open operating speed|
|VXSE||Best angle of climb speed with a single operating engine in a light, twin-engine aircraft – the speed that provides the most altitude gain per unit of horizontal distance following an engine failure, while maintaining a small bank angle that should be presented with the engine-out climb performance data.|
|VYSE||Best rate of climb speed with a single operating engine in a light, twin-engine aircraft – the speed that provides the most altitude gain per unit of time following an engine failure, while maintaining a small bank angle that should be presented with the engine-out climb performance data.|
|VZRC||Zero rate of climb speed in a twin-engine aircraft|
Whenever a limiting speed is expressed in terms of Mach number, it is expressed relative to the speed of sound, e.g. VMO: Maximum operating Mach Number, MMO: Maximum operating limit Mach.
V1 is the critical engine failure recognition speed or takeoff decision speed. It is the decision speed nominated by the pilot which satisfies all safety rules, and above which the takeoff will continue even if an engine fails. The speed will vary among aircraft types and varies according to factors such as aircraft weight, runway length, wing flap setting, engine thrust used and runway surface contamination.
V1 is defined differently in different jurisdictions:
- The US Federal Aviation Administration defines it as: V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V1 also means the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.
- Transport Canada defines it as: Critical engine failure recognition speed and adds: This definition is not restrictive. An operator may adopt any other definition outlined in the aircraft flight manual (AFM) of TC type-approved aircraft as long as such definition does not compromise operational safety of the aircraft.
Sumber : Internet