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Recovery from Unusual Aircraft Attitudes
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Revision as of 09:31, 25 March 2010 by Timo.Kouwenhoven
|Category:||Loss of Control|
When it comes to recovery from a loss of control or 'upset', an understanding of the principles involved is valuable to any pilot. However, current transport aircraft types can be divided into two contrasting groups in this respect. Those modern aircraft types which have fly-by-wire primary control systems also have built-in protection which will ensure that recovery occurs without a manual response from flight crew. For all other aircraft, successful recovery from any significant loss of control is likely to be a challenging aircraft handling experience for which both some theoretical understanding and simulator practice will be needed. This article is just a start. The (See: Further Reading) references below take this important subject further.
The onset of an unusual aircraft attitude, in a commercial transport aircraft, operating a normal passenger or cargo flight, is usually quite slow but the flight crew realisation that this circumstance exists is usually quite sudden. What to do to re-gain control must be practiced in the full flight simulator for the specific aircraft type, which must perfectly replicate the main flight instrument display. However, an understanding of the principles involved can provide a helpful background and this is the objective of this short note on the subject.
It is probably self evident that recovery must begin as soon as possible and use the correct flight control inputs – ones which will bring about recovery without causing structural damage to the aircraft which might then be difficult or impossible to control properly. There is plenty of evidence that mis-managed recovery from one excursion to the edge of or beyond controlled flight can lead quite easily into another different excursion from fully controlled flight which may be more serious than the previous ‘event’.
The key to recovery is the ADI. It is normally difficult or impossible to see the natural horizon and use it as a reference datum. Most large commercial aircraft have a restricted field of view – for example at over 25 degrees nose up, the view is probably sky only and for the nose down case, the view is likely to be restricted to the ground at greater than 10 degrees pitch down. In any case, it may be Instrument Meteorological Conditions (IMC) – or dark - outside anyway.
The Basic Sequence of Actions
- Check bank and pitch on the ADI. For pitch use the ‘pitch ladder’ and remember that most ADIs have blue for sky and brown for ground. Even in extreme aircraft attitudes, some part of the sky or ground indication is usually visible
- Compare ADI information with Airspeed/VSI/Altimeter before starting the recovery.
For a nose-low case:
- speed is probably increasing
- altitude is probably decreasing
- VSI is probably indicating a descent
For a nose-high case:
- Airspeed is probably decreasing
- Altitude is probably increasing
- VSI is probably indicating a climb
- Cross Check the other attitude indicators – the SBY AH/ADI and the PNF ADI
- Assess the aircraft energy state
Things to Expect!
- The ‘startle factor’ – beware of reacting on the controls before working out what is happening
- The need for full control inputs – flight control forces become less effective when an aircraft is at or near its critical angle of attack or stall. Full control authority may well need to be used even though such action will induce a very unfamiliar sensation.
- Counter-intuitive responses – after much routine training emphasising the recovery from the approach to the stall, which usually requires an increase in thrust and a relatively small reduction in pitch attitude, it may well be counter intuitive to initiate much greater unloading of the pitch control force or to reduce thrust when recovering from a high angle of attack, especially at low altitudes. Note that should the aircraft be stalled when already in a nose down attitude, the nose must still be further lowered in order to reduce angle of attack. In these circumstances, altitude cannot be maintained and must temporarily be accorded secondary importance.
- Negative ‘g’ – it may be necessary to aggressively unload ‘g’ forces to less than 1 g by pushing the control column forward without delay. Having said that, more than 0 ‘g’ should not be necessary. An unfamiliar environment will accompany negative ‘g’ – pilots will be ‘floating’ up against their harnesses and it may become difficult to reach the rudder pedals if they were not previously correctly adjusted. Unsecured objects such as approach plates, meal trays or drink containers may be flying around.
- Unusual engine effects – engine performance may be affected by some unusual aircraft attitudes. Large angles of attack may reduce airflow into the engines and precipitate engine surge or compressor stall. Also, significant sideslip angles and/or rapid change in sideslip angle may create abnormal internal engine loads and lead to engine damage.
- Loss of Control
- Stall - Recovery techniques assume that the airplane is not stalled. If the airplane is stalled, it is imperative to first recover from the stalled condition before initiating any upset recovery technique.
- A practical look at stall recovery for ice-loaded turboprops: Understanding the Stall Recovery Procedure for Turboprop Airplanes in Icing Conditions Flight Safety Digest , April 2005, pps 3-19.
- Airplane Upset Recovery - A Test Pilot's View: an article by Captain William Wainwright, Airbus Chief Test Pilot
- FAA "Airplane Upset Recovery Training Aid" Revision 2, published November 2008. The training package consists of: