Description

On 24 July 2014, radio contact with a McDonnell Douglas MD83 (EC-LTV) being operated by Spanish ACMI operator Swiftair on a scheduled international passenger flight for Air Algérie from Ouagadougou, Burkina Faso to Algiers as AH5017 was lost while it was in the cruise at night in Instrument Meteorological Conditions (IMC) over central Mali. The wreckage of the aircraft was located later the same day in the vicinity of Gossi near to the Burkina Faso / Mali border and it was apparent that ground impact had led to the complete destruction of the aircraft and a post crash fire. There were no survivors from the 110 passengers and six crew members on board.

Investigation

An Investigation was carried out by a Commission of Inquiry established by the Republic of Mali with technical assistance provided by the BEA France. The FDR was recovered and the data were successfully downloaded but the recovered CVR yielded no useful data because the erase mechanism was found to have been faulty prior to the flight so that older data were not being properly erased before further recording. The Investigation was unable to determine the origin of this malfunction. This Investigation noted that the absence of a usable CVR recording meant that the behaviour of the crew during the accident flight could not be analysed and in particular "it was not possible to study CRM aspects or to evaluate the possible contribution of the employment context and the experience of the crew members". No signal was received from the ELT and it was found damaged at the crash site.

An Interim Report on the progress of the Investigation was approved and released on 20 September 2014. Its content is incorporated in the Final Report.

It was noted that Swiftair had an ACMI contract for the provision of one MD83 to carry out wet lease for operations on behalf of Air Algérie during the period from 20 June 2014 to 23 September 2014.

It was found that both pilots had joined Spanair as First Officers on the accident aircraft type in the late 1990s and, after that airline had closed down, had joined Swiftair in 2012. It was noted that both had gained experience in operations in North and West Africa during their time at Spanair and that since the start of Swiftair operations for Air Algérie, they had been one of three crews deployed to Algiers to support operation of the aircraft. Each of them had operated over 40 flights from this temporary base including to Ouagadougou and return on 21/22 July. The 47 year old Captain had progressed to command on type whilst at Spanair and he had subsequently accumulated 6,445 hours on type at that rank out of 10,007 hours on type in total. His total flying hours were 12,988. The 42 year old First Officer had spent 14 years as a First Officer on the MD80 at Spanair and she had accumulated 7,016 total flying hours which included 6,180 hours as an MD80 First Officer. Recorded data and other circumstantial evidence indicate that the aircraft commander had been PF for the accident flight.

Contrary to the filed FPL, the flight was cleared to depart Ouagadougou for Algiers on a northbound clearance via GAO and EPEPO, the same route as had been used on the inbound flight (see diagram below). The A/T was active from take off and the left side AP was engaged as the aircraft passed approximately FL 105 on the climb to FL 310. In the absence of any contrary information, this latter action was taken to indicate that it was "likely" that the Captain had been acting as PF. Above FL 200, the crew made three successive heading changes to the left for weather avoidance before turning right to take up a track almost parallel to the initial one. The evidence indicated that the aircraft had probably been in |cloud above FL240. The SAT at FL310 was -32° C, equivalent to ISA + 11° C and the tops of the most active cloud which was avoided by the track deviation were estimated to have been "well above FL 400" - typical of the ITCZ.

Some 22 minutes after take off and at about the time the weather deviation left was followed by a turn to the right onto a northerly track almost parallel to the one initially flown, the aircraft reached FL 310. In the first two minutes after leveling, the airspeed increased to a maximum of M 0.775 and the EPRs stabilized at around 1.92. Then, a slow but steady decrease in airspeed had begun and the normal cruise A/T MACH Mode began to alternate with the MACH ATL Mode, which indicated that the thrust needed to maintain the Mach target displayed on the FGCP was greater than the thrust being produced at the displayed EPR limit value. Soon afterwards, with the speed back to M 0.752, MACH ATL Mode became continuous. For the next four minutes, altitude was maintained but speed continued to decrease and pitch attitude started to increase. Engine N1 remained stable but EPR progressively increased. Then, a 45 second period of fluctuation in the EPR and N1 of both engines occurred. A 20 second period in which these fluctuations continued accompanied by small oscillations in roll. Near the end of the period, the speed had reduced to M 0.561 (203 KCAS) and the A/T was disconnected. With the angle of attack recorded as 9º, the aircraft started to descend in a stalled condition. After a further 20 seconds, during which the EPR and N1 of both engines "started to decrease to values corresponding to idle", the AP was disengaged with the aircraft still stalled. It was seen from the FDR data that "when the crew took over manual control of the aeroplane the angle of attack was 25° and the aircraft was trimmed pitch up".

Recovery from the stall was not achieved. With airspeed down to 162 KCAS, pitch attitude began to decrease but an increasing bank to the left developed. DFDR data showed that both parameters were thereafter "subject to significant changes" respectively reaching 80° nose-down and 140° left bank to the left although the aircraft remained "pitched nose-down and banked to the left until it struck the ground" with the flight control surfaces remaining mainly deflected pitch-up and in the direction of a (corrective) bank to the right. Impact occurred just over half an hour after take off and just over 3 minutes after the beginning of EPR fluctuations in both engines. There was no evidence that the engine anti-icing system had been activated by the crew at any point in the flight but “the Investigation was not able to determine if […] non-activation […] was due to an oversight or to a decision by the crew (to the effect) that the environmental situation did not require it". However it was concluded that during the weather deviation, the aircraft would have passed through an area where ice crystals were likely to have been present.

It was concluded from comparison of recorded N1 speeds and EPR values that very soon after levelling at FL 310 the indicated EPR values of the right engine had become invalid "probably due to the obstruction of the Pt2 (inlet) pressure sensor of this engine by ice crystals". The A/T had then adjusted the thrust to prevent the erroneous values from exceeding the EPR LIMIT in cruise setting but the thrust being delivered by the engines was lower than the thrust required for level flight, so the airspeed continued to decrease. The left engine EPR was also affected in this way just under a minute after the right one. It appeared that, possibly in part because of concurrent difficulties in communicating with ATC, the crew had not been aware of the various indications that a serious aircraft control problem was developing – something which it was noted had also been found during the investigation of previous similar MD80 events. As the AP attempted to keep the aircraft at FL 310 with insufficient engine thrust, these indications included:

• abnormal movements of the Trimmable Horizontal Stabiliser (THS)
• a steadily increasing pitch attitude
• a continuously decreasing indicated airspeed - in particular it was considered that when the speed reached 210 KCAS, the pointer on the airspeed indicator would have been "close to vertical (and) at that moment a quick reading of the instrument could have made it possible to detect inadequate speed".

As the airspeed reached 203 KCAS, the conditions for a "SPD LOW" alert were met and this "might have been displayed on the two FMAs". There was no crew reaction to this alert even though "the buffet phenomenon likely also appeared at the same time". When the speed reached 200 KCAS, "the stickshaker triggered, followed three seconds later by the triggering of the stall warning". No crew action followed and the Investigation concluded that, in addition to failing to detect the various prior indications of an abnormal cruise condition, "presumably they did not identify this critical situation". Disconnection of the AP did not occur until 25 seconds after the triggering of the stickshaker and the crew's continued response was "mainly nose up inputs, contrary to the inputs required to recover the stall". It was found that around twenty seconds prior to impact, engine N1s had begun to increase, quickly reaching values close to maximum thrust. This was attributed to the disappearance of ice from the Pt2 (inlet) pressure sensors which was likely to have been the effect of the significant increase in temperature during the descent.

Noting that stall buffet had occurred at 203 KCAS, 7 seconds before the stall occurred and that stickshaker activation had occurred at 200 KCAS, 3 seconds before the stall, the Investigation found "no (aircraft type) certification document which would explain why a speed margin lower than (the normally required) 7% would have been accepted for the certification of the MD80".

It was noted that the Swiftair Operations Manual did not indicate any minimum lateral distance for circumventing storm systems. It was also noted that the FCOM detail on the selection of engine anti-icing stated that icing conditions can exist when the TAT is less than 6°C and visible moisture is present or if ice build-up occurs on the windshield wipers or edges of the windshield. It was considered that these conditions had existed prior to the occurrence of inlet probe icing but noted that "reports from two major airlines with wide MD80 experience" had indicated that "some crews did not activate anti-icing systems in visible moisture conditions in the absence of visible signs of icing". It was also acknowledged that there were indications that Pt2 icing can occur without visible moisture - and that flight at night adds a complication to maintenance of crew awareness of atmospheric conditions.

The formally documented Conclusion of the Investigation was that the accident was the result of a combination of the following events:

• the non-activation of the engine anti-icing systems
• the obstruction of the Pt2 pressure sensors, probably by ice crystals, generating erroneous EPR values that caused the autothrottle to limit the thrust produced by the engines to a level below that required to maintain the aeroplane at FL310
• the crew's late reaction to the decrease in speed and to the erroneous EPR values, possibly linked to the work load associated with avoiding the convective zone and communication difficulties with air traffic control
• the crew's lack of reaction to the appearance of buffet, the stickshaker and the stall warning
• the lack of appropriate inputs on the flight controls to recover from a stall situation.

It was further concluded that these events could be explained by a combination of the following Contributory Factors:

• The FCOM procedure relating to the activation of the anti-icing systems that was not adapted to Pt2 pressure sensor obstruction by ice crystals
• Insufficient information for operators on the consequences of a blockage of the Pt2 pressure sensor by icing
• The stickshaker and the stall warning triggering logic that led these devices to be triggered belatedly in relation to the aeroplane stall in cruise
• the autopilot logic that enables it to continue to give pitch-up commands beyond the stall angle, thereby aggravating the stall situation and increasing the crew's difficulties in recovery.

Three Interim Safety Recommendations were made during the Investigation in duplicate by both the Malian authorities and the French BEA (and in some cases also in duplicate to the FAA and EASA) as follows:

• that the FAA, as primary certification authority, or if not EASA, require a modification MD80 type Flight Manual to draw crews' attention to the risks linked to possible icing of the Pt2 pressure sensor at cruise altitude including where there are no visible signs of icing, specifically when the engine anti-icing system is not activated. [MLI-2015-002] and [MLI-2015-003]; [FRAN-2015-014] and [FRAN-2015-015]
• that the FAA, as primary certification authority, or if not EASA, provide pilots with the means to rapidly detect an erroneous EPR indication and to remedy it. [MLI-2015-004] and [MLI-2015-005]; [FRAN-2015-016] and [FRAN-2015-017]
• that the FAA and EASA study the need for similar action for other aeroplanes equipped with engines using the same thrust management principles. [MLI-2015-006] and [MLI-2015-007]; [FRAN-2015-018] and [FRAN-2015-019]

Upon completion of the Investigation, a further 10 Safety Recommendations were made, again in duplicate by both the Malian authorities and the French BEA (and in some cases also in duplicate to the FAA and EASA) as follows:

• that the FAA ask the manufacturer to study the feasibility of instalment of a permanent anti-icing system for the Pt2 sensors, independent of any activation by the crew of existing anti-icing systems for the engines or the airframe. [MLI-2016-001] and [FRAN-2016-022]
• that the FAA require, pending the introduction of a permanent anti icing system for the Pt2 sensors that the manufacturer's FCOM procedures related to "Engine Anti-Ice on Ground and in Flight" mention the difficulties of ice crystals detection in particular at night and define clearly the associated criteria for the anti-icing systems activation. This (should) require the systematic activation of these systems in flight as soon as the Total (Air) Temperature is below 6°C, without (there necessarily being any) visible humidity or signs of ice accretion on the windshield wipers. [MLI-2016-002] and [FRAN-2016-023]
• that the FAA require that the manufacturer integrate into the documentation provided to operators the specific features of a stall in cruise on MD 80 type aeroplanes, linked to the late appearance of buffet, of the stick shaker and of the stall warning and with the non-automatic disengagement of the autopilot after the stall warning. [MLI-2016-003] and [FRAN-2016-024]
• that the FAA and EASA require that (the specific features of a stall in cruise) on MD 80 type aeroplanes (should) be taught during type rating and recurrent crew training. [MLI-2016-004] and [MLI-2016-005]; [FRAN-2016-025] and [FRAN-2016-026]
• that the FAA ensure that the data made available by the manufacturer Boeing for the design of MD80 simulators be representative in respect of the triggering of approach to stall warnings and the non-disengagement of the autopilot after the stall, at low altitude and at cruise level. [MLI-2016-006] and [FRAN-2016-027]
• that the FAA and EASA ensure the simulators used for training MD80 crews (are) representative of the triggering of approach to stall warnings, and the non-disengagement of the autopilot after the stall, at low altitude and at cruise level. [MLI-2016-007] and [MLI-2016-008]; [FRAN-2016-028] and [FRAN-2016-029]
• that the FAA require that the manufacturer Boeing studies the feasibility of a modification of the autopilot's engagement logic to allow automatic disengagement when the approach to stall is detected by the MD80 systems. [MLI-2016-009] and [FRAN-2016-030]
• that the FAA ensure that the maintenance check procedure for CVRs on MD80 type aeroplanes is modified by the manufacturer in order to ensure that all of the recording tracks are tested, including the CAM track. [MLI-2016-010] and [FRAN-2016-031]
• that the FAA and EASA ensure that the modification recommended to the CVR maintenance check procedure is implemented by the operators concerned. [MLI-2016-011] and [MLI- 2016-012]; [FRAN-2016-032] and [FRAN-2016-033]
• that the National Authorities of Niger, Burkina-Faso and Mali put in place (SAR) coordination plans and ensure that their effectiveness is validated by regular exercises. [MLI-2016-013/014/015]; [FRAN-2016-034/035/036]

It was also considered that certain Safety Recommendations which had been made as a result of previous Investigations into crew loss of control events after stalls during cruise were relevant to the Swiftair accident. The events quoted were:

• a 2002 Serious Incident to a Spirit Airlines MD82 in which the prelude to a stall in the cruise was also the unrecognised consequences of ice crystal icing which occurred with engine anti-ice not selected on.
• the 2005 fatal Accident to a West Caribbean Airways MD82 in which the stall was due to an attempt to cruise at too high an altitude and there was no recovery from the stall that followed.
• the 2009 fatal Accident to the Air France A330-200 in which there was no recovery from a stall created by the crew whilst in the cruise.

The Final Report of the Investigation was approved on 22 April 2016 and published in both English and French the same day.

Related Articles

• Loss of Control
• Aerodynamic Stall Awareness and Avoidance
• High Level Ice Crystal Icing: Effects on Engines
• Ice Formation on Aircraft
• Aircraft and In Flight Icing Risks
• Crew Resource Management
• Unreliable Airspeed Indications

Further Reading

• "Ice Particle Threat to Engines in Flight" Mason JG Strapp JW & Chow P Paper presented to 44th AIAA Meeting 2006.