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High Altitude Flight Operations
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| Article Information | ||
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| Category: | Loss of Control | 
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| Content source: | SKYbrary | 
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| Content control: | EUROCONTROL | 
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Description
A large number of modern jet aircraft, of all sizes and including Very Light Jets (VLJs)s, routinely operate at high cruise altitudes.
The record of Accidents and Serious Incidents which have accompanied this increase in high altitude flight has suggested that the 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 if view, ‘high altitude’ operations are taken to be those above FL250. Above this altitude a number of features begin to take on progressively more significance as altitude continues to increase:
- There is a reduction in the range of airspeed over which an aircraft remains controllable;
- In the event of depressurisation, the time of useful consciousness for occupants deprived of oxygen reduces dramatically - see the separate articles on Emergency Depressurisation, and Hypoxia.
- At high altitude, occupants are exposed to increased cosmic radiation. This is covered by the separate article "Cosmic Radiation".
This article focuses on aerodynamics and aircraft handling.
Lift Versus Drag and Going Too Slowly
The key to an understanding of the practical implications of high altitude flight is an understanding of the theoretical relationship of Lift and Drag. The highest point on the lift/drag curve, L/D Max, corresponds to the most efficient angle of attack. The L/D ratio increases very rapidly up to around 3-4 degrees and then declines. The angle of attack at which L/D Max occurs therefore produces the optimum, or most efficient wing performance for level flight. Above L/D Max, an aeroplane is in stable flight - until of course the stalling angle of attack is reached. Below L/D Max, efficiency of flight falls off rapidly and it is inherently unstable. Lower angles of attack could provide the required lift if a higher forward airspeed were maintained. Greater angles could also provide the required lift but would do so at a slower speed.
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. 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).
High Speed Stall risk and Mmo
Certification of aircraft types includes the setting of Mmo[1]. This is based upon setting a suitable margin from the Critical Mach Number, at which airflow over a wing begins to break down and lift is lost in a high speed stall. The gap between the TAS for a low speed stall, which increases as altitude increases and air density decreases, means that cruise speed at very 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 the risk of a high speed stall.
Variation in Cruise Speed
Slower cruising speeds are often used as a means to save fuel, but may mean routinely flying closer to L/D max; 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.
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. 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.
Maximum Altitude
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 AFM (usually structural and determined pressurisation load limits on the fuselage).
- Thrust Limited Altitude - 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 or Manoeuvre limited altitude - the altitude at which a stated manoeuvre margin exists ahead of buffet onset (low speed pre-stall buffet occurs at increasingly high speeds as altitude increases whereas the high speed pre-stall buffet occurs at a decreasing speed so that the margin between the two is progressively reduced).
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.
Stalls
The wing can be stalled at any true airspeed and at any altitude, and aircraft attitude has no absolute relationship to the onset of an aerodynamic stall. Even if the aeroplane is descending at what looks like adequate airspeed, the wing surface can still be stalled. If the wing angle of attack exceeds the stalling angle of attack, the wing will stall. Successful recovery from a full stall involves a very different technique to recovering from the approach to one. Training for recovery from an incipient or near-stalled condition has often emphasised minimum altitude loss as a goal by focusing on the application of a rapid increase in thrust without consideration of the imperative to reduce aerofoil angle of attack for recovery from a full stall. In the case of a fully stalled aircraft at high altitude, which is less often regularly trained than the approach to the stall, this technique is now being increasingly recognised as potentially dangerous because it fails to prioritise reduction of the angle of attack when there is no obstacle to some loss of altitude. In any case, there may be little additional thrust available or a significant delay if thrust setting is at or near idle because of engine spool up times.
Related Articles
Note
- ^ Whenever a limiting speed is expressed in terms of Mach number, it is expressed as an "M speed", e.g. Vmo: Maximum operating limit speed (in knots), Mmo: Maximum operating limit Mach.