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Aerodynamic Stall Awareness and Avoidance
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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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Contents |
Description
Inadvertent loss of control of aircraft continues to occur. Such losses of control usually involve a full stall or an approach to one 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 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).
Theory
A clean wing stalls at an angle of attack which is determined by its cross section and configuration. Since all wings have upper surfaces with a convex camber, straight and level flight for a clean aircraft will occur with the aircraft pitched very slightly up from the horizontal. The indicated airspeed at which a wing will stall in a given condition is the same at all altitudes because both the stalling angle of attack, and the indicated airspeed, are derived from ambient air density. Of course the TAS at which a wing will become stalled will increase with altitude. This type of stall is sometimes described as a high incidence stall in order to distinguish it from another way in which a wing can become stalled without a high angle of attack. This is the shock or high speed stall, which results from the turbulence at the wing surface which begins to occur as a shock wave develops near to the speed of sound. At the high subsonic cruise speeds at which modern transport aircraft routinely operate, the TAS at which a high incidence stall occurs will be much greater than at lower altitudes and, during manoeuvre of the aircraft, may be quite close to the speed at which the beginnings of a shock stall may be encountered - at the critical mach number. For the diagrammatic Flight Envelope, the intersection of the boundary defined by a high incidence stall and that defined by the critical mach number has attracted the epithet "coffin corner".
When Stalls Most Often Occur
Circumstantial evidence shows that most full or near-full stalls of transport aircraft occur in one of five situations, as for other paths to loss of control, often but not always when the aircraft is either in IMC or during ‘dark night’ conditions clear of cloud so that no natural horizon is available:
- 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)
Uncommanded AP Disconnect due to malfunction of other systems is not only liable to create a significant ‘startle factor’ for both pilots but is also likely to remove some of the high level flight envelope protections commonly provided by Fly-By-Wire (FRW) flight control systems. Flying manually at high altitude is not a feature of normal operations and there is not always sufficient awareness of the different ‘feel’ of the flight controls in the high altitude case compared to the routinely- experienced low altitude case. The simultaneous removal of some or all automated flight envelope protections at the same time, often because of an automatic reversion to a lower FBW Control Law, creates a heightened imperative to retain control within that envelope in a situation where a full understanding of the different degree of protection provided by Control Laws, other than "Normal", may not be fully understood or appropriately recalled.
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 alters the stall angle of attack to the extent that the stall protection system is often ineffective.
- 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 may arise during manual handling or because of mismanagement of the automatics.
A stall which occurs as a consequence of insufficient understanding of automation usually involves the pilot making flight control inputs which are contrary to the effect of the protection system.
Managing the Risk
It is generally accepted that any risk management process must be able to confirm that the level of risk arising from any particular aspect of a flight operations is similar.
Technical mitigations include stall warning devices and highly reliable flight envelope protection systems. However, 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 both the classroom and the full flight simulator - and in the application of appropriate SOPs.
Establishing whether the training input and SOPs have adequately addressed the risk has always depended largely on an effective routine assessment of the competency if individual pilots. However, the widespread adoption of OFDM now provides an opportunity to configure data analysis software for a range of lesser ‘precursor’ events in which there has been an abnormal deviation from an expected flight path towards a stalled condition from which a successful recovery has been made. Such detected occurrences can easily be tracked back to particular pilots and their training histories as a means to understand how the ‘excursion’ came about and ensure that the risk management of stall outcomes is as effective as it was believed to be - and is modified if necessary.
Some Examples of Stall Accidents and Incidents
- A332, en-route, Atlantic Ocean, 2009 (WX AW LOC) - (Uncommanded AP Disconnect due to malfunction of other systems): On 1 June 2009, an Air France Airbus A330-203 disappeared 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 Schiphol 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 AW 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 (HF LOC) - (insufficient understanding of automation as it affects flight envelope protection systems ): On 26 April 1994, a China Airlines Airbus 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.