Take Off Stall
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|Category:||Loss of Control|
A stall which occurs immediately after an aircraft attempts to get airborne. When it occurs following rotation at or near the applicable Vr, it may be attributable to an unintended attempt to take off without the appropriate wing configuration set or because the necessary thrust has not been set.
Other possible origins of a take off stall are an attempt to get airborne with frozen deposits on the airframe, especially the wings - which is covered in the separate article "Aircraft Ground De/Anti-Icing", where an aircraft is not actually loaded in the manner described on the certified and accepted load and trim sheet, when an encounter with negative wind shear occurs or due to aircraft system malfunctions
Prevention and Recovery
The main means of preventing such occurrences is the effective use of Normal Checklists. The primary means of alerting pilots to an aircraft status, including but not limited to a wing configuration, which is not within the range approved for take off and stated in the aircraft AFM limitations, is the Takeoff Warning System (TOWS), sometimes called the Takeoff Configuration Warning System (TOCWS). This system often forms part of a wider system of crew alerting on modern aircraft but will always include an audible alert.
In some cases, it may be possible for a pilot to retain control of an aircraft which gets airborne with an incorrect configuration, but neither ground awareness nor simulator training for such a scenario is a regulatory requirement. Whilst the Stall Warning System is likely to be activated as soon as the aircraft is sensed as being in flight mode, the QRH memory drills applicable to recovery from an incipient stall are not immediately applicable to the circumstance where at least a modest rate of climb is likely to be required.
Relevant Regulatory Requirements
Aircraft design certification currently detailed under FAR 25.703 and EASA CS 25.703 is based upon the principles first brought into effect in FAA Part 25 on 1 March 1978 in AL25-42 and in JAR 25 on 1 January 1979 in AL5. Prior to these standards, there was no such requirement. The requirements were and remain the same and the background to their current version is described in FAA AC 25.703-1.
The key change from the initial specification, in which TOWS were acceptable if designed as single channel systems with only limited built in monitoring, was the 1993 recognition that the safety level of these systems should be increased so as to classify them as essential. This was achieved by requiring system design in accordance with FAA AC 25.1309-1A and its EASA counterpart, AMC 25.1309 because of a recognition that a inoperative TOWS should be regarded as having a severe effect on safety, TOWS activation on western built aircraft currently in service is thereby rarely false and can be generated only during the initial part of the take off roll. System design is also required to include immediate annunciation to the flight crew should a system failure be identified or if an interruption to necessary electrical power occurs.
The current certification requirement, which extends the provisions of paragraph 1309 to certification of TOWS under paragraph 703, covers the majority of aircraft types or has been voluntarily accepted in the case of some older types still in production based on type certificates which predate the more stringent design criteria. However, ‘grandfather rights’ still affect the TOWS design installed on some older aircraft, including most notably in terms of the world fleet size, the Douglas DC9 and MacDonnell Douglas MD series derivatives.
In operational terms, the original exclusion of TOWS from both FAA and JAR / EASA MMELs has been sustained so that release to service / despatch with the TOWS inoperative is not permitted. However, there is currently no universal regulatory requirement for a TOWS operational check to be carried out before every flight even though such a practice is quite common.
It should also be noted that in respect of the content of Crew Checklists, there is no requirement under EU OPS for them to be specifically approved, although the Operations Manual which contains checklists are subject to approval as a whole. The overall approval of an Operation will include an acceptance of the way checklists are used as well as focusing particular attention paid to any differences that exist between the ones used and the equivalent aircraft manufacturer’s standard versions. Under the FAA System, checklists must be approved under FAR 121.315and are expected to take available guidance material into account to obtain that approval, so that the effect is similar to the European approach.
According to the 1978 preamble to AL25-42 to the FAR which introduced TOWS, the system was originally introduced to serve as a “back up for the checklist, particularly in unusual situations, e.g. where the checklist is interrupted or the takeoff delayed”. In many documented investigations of accidents and serious incidents relating to take off stall, the TOWS has, for various reasons, been inoperative at the same time as crew discipline in relation to checklists has been poor.
In many instances, the actual crew response to a stall protection system activation at Vr has been to add thrust rather than reject the take off. Since Vr can never be less than V1 for Performance ‘A’ aeroplanes and is almost always quite a lot higher, this “instinctive” response is one which needs to be at least discussed during training. This response is of particular concern given that in almost all documented cases, the Stall Protection System has generated a warning as soon as the aircraft has sensed flight mode, which can be sensed from the nose landing gear raised at rotation.
Normal checklists used with sufficient discipline and in accordance with effective SOPs can ensure that pilots remain appropriately focussed on their prioritised tasks by removing the risks associated with divided attention and with any effects arising from stress, whether of self-generated or external origin, or fatigue. Most checklists are read from a hard copy or a screen and require a specific response from either PF or the PM but on screen checklists with an aural readout based on manual sequencing by the PM do exist.
Take-off stall events attributed to attempts to take off without setting the flaps / slats to an approved take off position invariably involve the omission of the checklist item(s) relating to that action, usually due to the interruption of a checklist prior to its completion. This problem has been partially attributed to a general absence of effective CRM and flight crew discipline but has also been indirectly related to variation in the SOP for the selection of take off flap depending on the earliest and latest times they should be selected in relation to push back and/or taxi out towards the runway. An additional complication may exist in respect of any on-stand or remotely-sited ground de/anti-icing, which may require a delay in wing configuration compared to normal procedures. This may justify an additional checklist to follow completion of such a de/anti-icing treatment or rely on the subsequent pre take off checklist.
All on-screen checklists have the advantage that any deferred items can be highlighted which, given the variable SOPs for deployment and checking of flaps/slats sometime between the completion of engine start and arrival at the departure runway, has proved extremely useful.
Effective CRM is obviously a major factor in the effective use of Check Lists.
Stall Awareness Training
In the absence of any regulatory requirement, most operators limit any attention given to crew awareness of this issue to ground school and consider that the primary focus should be on avoiding the situation by creating the right attitude to SOP compliance.
Relevant Precursor Events detectable by suitably configured OFDM programmes include the absence of a prescribed pre fight test of the TOWS system, all recorded activations of the TOWS system and any change to flap settings when a departing aircraft is lined up on the take off runway. In the absence of OFDM data, a suitable safety culture may facilitate the raising of ASRs after instances of late configuration which can contain - or allow the gathering of - useful information about how lapses in the application of SOP have arisen
Relevant Precursor Observations may include LOSA observations relating to pre flight checking of the TOWS system and use of checklists and especially with respect to deferred or temporarily overlooked items. They may also be available from recurrent training records if relevant trends in respect of checklist use and standards of crew cooperation are available and appropriately collated. Any unserviceability of TOWS systems should also be tracked to ensure that functional reliability is high and recognised by flight crew as such.
Accident and Serious Incident Examples
Over a long period, a number of fatal accidents and ‘near misses’ involving take off stall have occurred. The following is a list of events that involve incorrect aircraft configuration for the phase of flight, which includes events that resulted in loss of control on take off:
- MD82, Madrid Barajas Spain, 2008 (On 20 August 2008, an MD82 aircraft operated by Spanair took off from Madrid Barajas Airport with flaps and slats retracted; the incorrect configuration resulted in loss of control, collision with the ground, and the destruction of the aircraft.)
- A321, Charlotte NC USA, 2015 (On 15 August 2015, an Airbus A321 on approach to Charlotte commenced a go around but following a temporary loss of control as it did so then struck approach and runway lighting and the undershoot area sustaining a tail strike before climbing away. The Investigation noted that the 2.1g impact caused substantial structural damage to the aircraft and attributed the loss of control to a small microburst and the crew’s failure to follow appropriate and recommended risk mitigations despite clear evidence of risk given by the aircraft when it went around and available visually.)
- A321, vicinity Singapore, 2010 (On 27 May 2010 an Airbus A321-200 being operated by Australian operator JetStar on a passenger flight from Darwin to Singapore continued an initial approach at destination in day VMC with the aircraft inappropriately configured before a late go around was commenced which was also flown in a configuration contrary to prescribed SOPs. A subsequent second approach proceeded to an uneventful landing. There were no unusual or sudden manoeuvres during the event and no injuries to the occupants.)
- B738, en-route, near Sydney Australia 2018 (On 12 July 2018, a Boeing 737-800 was climbing through FL135 soon after takeoff from Sydney with First Officer line training in progress when the cabin altitude warning horn sounded because both air conditioning packs had not been switched on. The Captain took control and descended the aircraft to FL100 until the situation had been normalised and the intended flight was completed. The Investigation noted that although both pilots were experienced in command on other aircraft types, both had limited time on the 737 and concluded that incorrect system configuration was consequent on procedures and checklists not being managed appropriately.)
- B77L, Paris CDG France, 2015 (On 22 May 2015, a Boeing 777F augmented crew attempted a reduced thrust daylight takeoff from Paris CDG using a thrust setting based on a weight 100 tonnes below the actual weight after an undetected crew error. The tailstrike protection system prevented fuselage runway contact after rotation attempts but only after a call from an augmenting crew member was full thrust set with the aircraft becoming airborne near the runway end. The Investigation noted poor crew performance but concluded that operator management of the risk involved and the corresponding regulatory oversight had been inadequate in a number of ways.)
- AT75, Port Vila Vanuatu, 2018 (On 28 July 2018, a right engine compressor stall on an ATR72-500 bound for Port Vila followed by smoke in the passenger cabin led to a MAYDAY declaration and shutdown of the malfunctioning engine. The subsequent single engine landing at destination ended in a veer-off and collision with two unoccupied parked aircraft. The Investigation noted the disorganised manner in which abnormal/emergency and normal checklists had been actioned and found that the ‘Before Landing’ Checklist had not been run which resulted in the rudder limiter being left in high speed mode making single engine directional control on the ground effectively impossible.)
- FAA AC 25-22 (2000) "Certification of Transport Airplane Mechanical Systems" includes the current rule text and a summary of some aspects of the AMC guidance provided in AC 25.703-1, the current version of TOWS design requirements, on pps 8-9.
- FAA AC 25.703-1 "Take Off Configuration Warning Systems (1993)" provides AMC guidance material.
- Airbus FOBN on Takeoff and Departure Operations: "Response to Stall Warning Activation at Takeoff"
- NASA Ames Research Center: "Human Factors of Flight-Deck Checklists: The Normal Checklist" - Degani & Wiener (1990)
- Aircraft Loss-of-Control Accident Analysis, C. Belcastro and J. Foster, NASA, 2010.