Recovery from Unusual Aircraft Attitudes
From SKYbrary Wiki
|Category:||Loss of Control|
|Content control:||Air Pilots|
In large transport aircraft, an unusual attitude is usually defined as a nose up pitch attitude greater than 25 degs, a nose down pitch attitude greater than 10 degs, a bank angle greater than 45 degs or flight within these parameters but with airspeeds inappropriate for the conditions.
- The first priority is to ensure that these parameters are never exceeded.
- The second priority is to correct any exceedence as rapidly as possible.
- The third is to understand the principles involved in recovering from an ‘upset’.
- The fourth priority is never to stall the aircraft. A fully stalled aircraft is out of control – recovery from the stall must be achieved before initiating recovery from any unusual attitude.
Different types of aircraft have different flying characteristics and may need different recovery techniques. For example, swept wing aircraft behave differently to straight wing, T-tailed aircraft can exhibit different stall characteristics to those with other configurations. It is important always to heed the manufacturers’ and operators’ advice relevant to the specific type.
Some modern types have fly-by-wire primary control systems with built in protections which prevent the exceedence of critical flight parameters and which can ensure recovery without manual response from the flight crew. However, flight crews must clearly understand that certain system failures can degrade these protections such that the aircraft is left with reduced or no protections and become, in effect, a ‘basic’ aircraft.
Successful recovery from any significant loss of control is likely to be a challenging aircraft handling experience for which both theoretical understanding and simulator practice will be needed. However, pilots should be aware that even the most modern simulators cannot fully simulate the aircraft behaviour or its handling characteristics beyond the point of stall.
NOTE: High performance/aerobatic capable aircraft will have a more rigorous and dynamic set of rules and recovery actions for any upset recovery, often also known as 'Unusual Positions'. These are not covered within this article.
This article only provides a basic introduction. The reader should take this important subject further by following the references below (See: Further Reading).
An upset is not necessarily a departure from controlled flight (i.e. a stall/spin) but it does include abnormal attitudes and gross over/under-speed conditions as defined in the Preface above. Avoidance and Recognition are as important as recovery, if not more so. Effective monitoring by both pilots of the flight path and of each other cannot be over emphasized. Even on those aircraft with fly-by-wire primary controls and envelope protections, pilots must remain aware that certain system failures can degrade these protections in which case the recovery actions outlined in this and related SKYbrary articles will apply.
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. Thus the ‘startle factor’ may confuse the crew’s initial reaction. It is most important to work out what is really happening before reacting on the controls.
The actions required to correct an upset must be practised in the full flight simulator for the specific aircraft type, and on one which is approved for this type of training. All flight crew (both line and training pilots) must be aware of the limitations of the simulator’s fidelity. It can only reproduce the behaviour of the real aircraft within the parameters of the data package used in the simulator. Test pilots do not deliberately lose control of their aircraft just to get data for the simulator manufacturer. For example, no simulator can reproduce the behaviour of the real aircraft in extreme attitudes when fully stalled.
Within the aviation industry, some divergent views still exist over appropriate recovery techniques. In the event of an upset, flight crews should use only those techniques recommended by the aircraft’s manufacturer and the operator. However, an understanding of the principles involved can provide a helpful background and this is the objective of this short note on the subject.
Whatever the cause of an upset, recovery must begin as soon as possible, using the correct flight control inputs – i.e. those which will bring about recovery without causing structural damage to the aircraft, that might then make it difficult or impossible to control properly. Mismanaged 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 (Attitude Director Indicator). Most passenger and cargo aircraft have a restricted field of view. For example, at over 25 degrees nose up, the view is probably of the sky only. In the nose down case, greater than 10 degrees pitch down, the view is likely to be restricted to the ground. It may also be Instrument Meteorological Conditions (IMC) - or dark - outside. Therefore it may be difficult or even impossible to see the natural horizon and to use it as a reference datum. In this case the only references are the ADIs (they should be cross-checked), supported by the other flight instruments.
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
In both cases then:
- Cross Check the other attitude indicators - the SBY AH/ADI and the PNF ADI
- Assess the aircraft energy state
- Disengage the autopilot and auto throttle
- Retract speedbrakes (if extended)
- Apply flight and engine control inputs to recover following the manufacturers’ and operators advice relevant to aircraft type.
Things to Expect!
- The ‘startle factor’ - beware of reacting too quickly or inappropriately 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.
- Unusual trim forces - as the upset was developing, the autopilot may have been applying large and unexpected trim offsets in an attempt to maintain normal flight path that will only become evident at the point of autopilot disengagement. In some aircraft, large trim offsets reduce the amount of control authority available to the pilot until the trim offset has been removed. Understand your aircraft systems and always heed the manufacturers’ and operators’ advice relevant to the specific type.
- 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 use full down elevator control or to reduce thrust when recovering from a high angle of attack (AoA), 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.
- Positive ‘g’ - when pulling ‘g’, either when recovering from a dive or when in a steeply banked turn, the AoA is increased and the associated stall speed increases. In both cases pulling 2g increases the stall speed by 41%. Recovery action should be achieved by first rolling the wings level and then apply ‘g’ (In some fly-by-wire aircraft, roll control authority may be inhibited or limited while pulling ‘g’ to respect structural limits). The average airline pilot will have little experience of flight with increased ‘g’ loadings and may well feel the load is well in excess of what it actually is. This can impede appropriate control input under ‘g’ loads.
- Negative ‘g’- aggressive action may be necessary to unload ‘g’ forces to less than 1 g by pushing the control column forward without delay. Having said that, reducing ‘g’ beyond zero 'g' should not be necessary. Negative 'g' creates an unfamiliar environment in which the 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.
- Under-wing mounted engines- pitch attitude may change with thrust. Reducing thrust creates a nose-down pitching moment; increasing thrust creates a nose-up pitching moment. With under-wing mounted engines, adding high thrust when already in a nose-high unusual attitude may aggravate the situation and even prevent recovery. Although counter-intuitive, it may be necessary to reduce thrust to prevent the angle of attack from increasing.
- Unusual engine effects - engine performance may be affected by some unusual aircraft attitudes. Large angles of attack or sideslip may reduce airflow into the engines and precipitate engine surge or compressor stall.
- 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.
- Flight Instrument Presentation of Aircraft Attitude
- Bank Angle Awareness
- Airplane Upset Prevention & Recovery Training Aid (AUPRTA) for Transport Category Airplanes 3rd revision, ICAO, February 2017
- IATA Guidance Material and Best Practice for the Implementation of Upset Prevention and Recovery Training, 1st edition, June 2015.
- IATA LOC-I Prevention: Beyond the Control of Pilots, 1st Edition, 2015
- RAeS Aeroplane Upset Recovery Training History, Core Concepts & Mitigation
- 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
- CAP 1038 - CAA Check Flight Handbook, Issue 2, December 2016.
- ICAO Amendment No.3 to PANS-TRG (Doc 9868) - Chapter 7, Upset Prevention and Recovery Training, April 2014.
- Aviation Safety and Pilot Control: Understanding and Preventing Unfavorable Pilot-Vehicle Interactions, by the US National Academy of Sciences, 1997
- SAFO 17009: Airman Certification Standards (ACS): Slow Flight and Stalls, US FAA, May 2017
- FAA AC 120-111 Upset Prevention and Recovery Training, January 2017.
- Stall warnings in high capacity aircraft: The Australian context, ATSB Australia, Nov 2013
- Monitoring Matters: Guidance on the development of Pilot Monitoring Skills, CAA Paper 2013/02.