Accident du Boeing 737 immatriculé ET-AVJ et exploité par Ethiopian Airlines survenu le 10/03/2019 près de Bishoftu [Enquête menée par EAIB / Ethiopie]
Collision avec le sol peu après le décollage
Ethiopie - AIB
Résumé issu du rapport d'enquête des autorités éthiopiennes (EAIB):
On March 10, 2019, at 05:38 UTC, Ethiopian Airlines flight 302, Boeing 737-8(MAX), ET-AVJ, took off from Addis Ababa Bole International Airport bound to Nairobi, Kenya Jomo Kenyatta International Airport.
ET302 was being operated under the provisions of the Ethiopian Civil Aviation Regulations (ECARAS) as a scheduled international flight between Addis Ababa Bole International Airport (HAAB), Ethiopia and Jomo Kenyatta Int. (HKJK) Nairobi, Kenya. It departed Addis Ababa with 157 persons on board: 2 flight crew (a Captain and a First Officer), 5 cabin crew and one IFSO, 149 regular passengers.
At 05:36:12 the Airplane lined up on runway 07R at field elevation of 7,656 ft with flap setting of 5 degrees and a stabilizer trim setting of 5.6 units1. Both flight directors (F/D) were ON with LNAV and VNAV modes armed. At 05:37:17 the F/O reported to Tower ready for takeoff and at 05:37:36 ATC issued take off clearance to ET-302 and advised to contact radar on 119.7MHz.
The takeoff roll and lift-off was normal, including normal values of left and right angle-of-attack (AOA). During takeoff roll, the engines stabilized at about 94% N1. Shortly after liftoff, the left Angle of Attack sensor recorded value became erroneous and the left stick shaker activated and remained active until near the end of the recording. In addition, the airspeed and altitude values from the left air data system began deviating from the corresponding right side values. The left and right recorded AOA values began deviating. Left AOA decreased to 11.1° then increased to 35.7° while the right AOA indicated 14.94°. Then after, the left AOA valuereached 74.5° in ¾ seconds while the right AOA reached a Maximum value of 15.3°, the difference between LH and RH AOA was 59°and near the end of the recording it was 49°
At 05:39:30, the radar controller identified ET-302 and advised to climb FL 340 and when able to turn right direct to RUDOL. At 5:39:51, the selected heading increased from 072° to 197°.
After the flaps were fully retracted the1st automatic nose-down trim activated and engaged for 9 secondspositioning the stabilizer trim to 2.1 units. The pilot flying pulled to pitch up the Airplane with a force more than 90lbs. He then applied electric trim-up inputs. Five seconds after the end of these inputs a second automatic nose-down trim activated.
At 5:40:22, the second automatic nose-down trim activated. Following nose-down trim activation GPWS DON’T SINK sounded for 3 seconds and “PULL UP” also displayed on PFD for 3 seconds. At 05:40:43, approximately five seconds after the end of the crew manual electrical trim up inputs, a third automatic trim nose-down was recorded but with no associated movement of the stabilizer.
At 05:40:50, the captain told the F/O: “advise we would like to maintain one four thousand. We have a flight control problem”. The F/O complied and the request was approved by ATC. Following the approval of the ATC, the new target altitude of 14,000ft was set on the MCP. The Captain was unable to maintain the flight path and requested to return back to the departure airport. At 05:43:21, approximately five seconds after the last main electric trim up input, an automatic nose-down trimactivated for about 5s. The stabilizer moved from 2.3 to 1 unit. The rate of climb decreased followed by a descent in 3s after the automatic trim activation.
One second before the end of the automatic trim activation, the average force applied by the crew decreased from 100 lbs to 78 lbs in 3.5 seconds. In these 3.5 seconds, the pitch angle dropped from 0.5° nose up to -7.8° nose down and the descent rate increased from -100 ft/min to more than -5,000 ft/min.
Following the last automatic trim activation and despite calculated column force of up to 110lbs, the pitch continued decreasing. The descent rate and the airspeed continued increasing between the triggering of the 4th automatic trim activation and the last recorded parameter value. At the end of the flight, Computed airspeed values reached 500Kt, Pitch values were greater than 40° nose down and descent rate values were greater than 33,000 ft/min. Finally, both recorders stopped recording at around 05 h 43 min 44s.
At 05:44 The Airplane impacted terrain 28 NM South East of Addis Ababa near Ejere (located 8.8770 N, 39.2516 E.) village at a farm field and created a crater approximately 10 meters deep (last Airplane part found) with a hole of about 28 meters width and 40 meters length. Most of the wreckage was found buried in the ground; small fragments of the Airplane were found scattered around the site in an area by about 200 meters width and 300 meters long. The damages to the Airplane were consistent with a high energy impact. All157 persons on board: 2 flight crew (a Captain and a First Officer), 5 cabin crew and one IFSO, 149 regular passengers were fatally injured.
Communiqué de Presse du 03/01/23
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- PUBLICATION DU RAPPORT FINAL PAR LES AUTORITES ETHIOPIENNES -
Le Bureau d’Enquêtes Éthiopien sur les accidents d’aviation (EAIB) a publié, vendredi 23 décembre 2022, le rapport final de l’enquête sur cet accident.
Les autorités éthiopiennes avaient demandé l’assistance du BEA pour l’analyse des FDR et CVR qui étaient endommagés. À la suite de cette demande, conformément aux dispositions de l’Annexe 13 à la Convention relative à l’aviation civile internationale (Annexe 13 de l’OACI), le BEA avait nommé un enquêteur représentant accrédité afin de participer à l’enquête menée par l’Éthiopie, État d’occurrence.
Les dispositions de l’OACI prévoient que l’État qui mène l’enquête envoie un projet du rapport final aux États participant (dans le cas présent au représentant accrédité du BEA pour la France et à celui du NTSB pour les États-Unis). Ceux-ci sont alors invités à présenter aussitôt que possible toutes observations dignes d’intérêt qu’ils souhaiteraient formuler sur le rapport. Si l’État qui a mené l’enquête reçoit des observations dans les 60 jours, il amende le projet de rapport final de façon à y inclure la teneur des observations reçues ou, en cas de désaccord sur celles-ci, et si l’État qui a formulé les observations le souhaite, présente ces observations en annexe au rapport.
À partir de janvier 2021, le NTSB et le BEA ont été consultés sur un projet de rapport final élaboré par l’EAIB. Le NTSB et le BEA ont notamment demandé que les aspects relatifs à la performance de l’équipage soient mieux exposés et analysés. Ces échanges n’ont pas permis d’aboutir à des modifications satisfaisantes et ont conduit le NTSB et le BEA à demander la mise en annexe de leurs observations au rapport final.
Le 23 décembre 2022, l’EAIB a publié sur son site Internet le rapport final sans y intégrer directement les observations du BEA en annexe. Le rapport de l’EAIB contient un lien vers un document BEA qui ne représente pas les observations que le BEA avait finalement demandé d’annexer.
Le BEA partage l’analyse et les conclusions du rapport de l’EAIB en ce qui concerne la contribution du système MCAS du 737 Max à l’accident. Les commentaires du BEA portent principalement sur l’analyse de la performance de l’équipage et sa contribution au scénario de l’accident, notamment pendant la première partie du vol (entre la rupture de la palette de la sonde d’incidence et l’activation du système MCAS). Le BEA considère que cette analyse permettrait de dégager des enseignements de sécurité au-delà de ceux relatifs au système MCAS.
Le rapport final publié par l’EAIB, ainsi que les observations que le BEA a demandé à l’EAIB d’annexer à ce rapport, est disponible dans l'onglet "OBSERVATIONS DU BEA".
The BEA wishes to thank the EAIB for being consulted on the draft final report concerning the aforementioned accident.
From the very start of the investigation, the BEA has, under the authority of the Ethiopian AIB and in cooperation with the NTSB and their technical advisers, actively participated in the understanding and the analysis of the aircraft systems, the operational aspects of the occurrence and the crew performance issues.
The BEA considers that the operational and crew performance aspects are insufficiently addressed in the EAIB final report, in particular with regard to the sequence of events that occurred before the activation of the 1st MCAS. Consequently this prevents the reader from having a precise and complete understanding of the event.
Shortcomings relating to the crew’s actions, particularly in the first phase of the flight, is not accompanied by a thorough analysis of the reasons for the behaviours observed, in relation with their training, their experience and the company organization with regard to the training and knowledge acquisition principles.
The following comments represent important points that the BEA considers necessary to take into account in order to have a comprehensive and balanced report.
In accordance with paragraph 6.3 of ICAO Annex 13, the BEA requests that this document be appended to the Final Report.
Sequence of events
1. From take-off to the 1st MCAS
Shortly after take-off, as a result of the erroneous left AOA value, the left stick shaker was activated.
On the activation of the stick shaker, the flight crew must apply the Approach to Stall or Stall recovery procedure, which is a memory item. As described in the FCOM/QRH, the first steps in the Approach to Stall or Stall Recovery procedure are to hold the control column firmly, disengage the autopilot and authrottle and then smoothly apply a nose down input.
Only the nose down input was performed by the flight crew. The autothrottle remained engaged and the pilot later insisted on engaging the A/P.
Wind tunnel testing, modelling and AOA rupture mode analysis performed by the AOA sensor manufacturer concluded that the only possible scenario for the AOA sensor erroneous data was a foreign object impact, most likely a bird, causing separation of the vane at the hub and breaking of the vane heater wires.These results were presented to the EAIB in Addis Ababa in September 2019.
Five seconds after the activation of the stick shaker, an IAS DISAGREE message appeared on both PFDs. It was followed two seconds later by an ALT DISAGREE message. Although these messages were not recorded on the FDR, it has been established, by computations and later confirmed at the Boeing engineering simulator (eCAB), that they appeared on both PFDs.
In the case of the IAS DISAGREE, the flight crew has to apply the Airspeed Unreliable Non-Normal Checklist. This checklist states to first disengage the AP, then the Autothrottle and to put the F/D switches on OFF, before setting 10° pitch attitude and 80% N1 (when flaps are extended).
The captain tried, in vain, to engage the AP. He did not disconnect the autothrottle. There were no crew exchanges on the CVR regarding the IAS DISAGREE or ALT DISAGREE messages, which supports the conclusion that the IAS DISAGREE and ALT DISAGREE messages were most probably not seen by the crew throughout the flight.
At a height of approximately 350 ft, the captain again tried to engage the A/P although the airline policy requests pilots to wait until they reach 500 ft AGL to engage the autopilot. This premature action, although not appropriate in stick shaker conditions, may be symptomatic of a state of stress that had been rapidly developping following the activation of the stick shaker and Master Caution immediately after take-off.
A second attempt was made six seconds later, above 500 ft. The captain’s reaction after this second engagement attempt ("What's going on ?") was probably related to the inability to engage the A/P. The Captain’s question remained unanswered and did not trigger any process of information acquisition, cross-check or crew decision making.
Passing 1000 ft/radio altitude, at the third attempt, the A/P was successfully engaged. MCP speed was set to 238 kt. However, the A/P airspeed target was not the MCP speed but that of the erroneous LH minimum operational speed, which was above VMO (340 kt). The A/P therefore commanded a nose down to accelerate towards the erroneous minimum operational speed. The stabilizer trim value decreased from 5.6 to 4.6 units of trim. The aircraft descended at -1,400 ft/min.
Thirty-three seconds after being engaged, the A/P disconnected.
The increase of speed beyond the target speed was not mentioned by the crew. Moreover, the autothrottle remained engaged and, due to the AOA foreign object impact resulting in partial vane separation, failed to change to N1 mode. It remained in ARM mode with take-off thrust. The ARM mode was never verbalized. It is highly probable that it was not identified by the crew.
The lack of thrust reduction aggravated the difficulties encountered by the crew to control the aircraft throughout the reminder of the flight.
During this phase, besides the destabilizing cockpit environment linked to the activation of the stick shaker and a Master Caution immediately after take-off, the coordination and the communication between the captain and the F/O were very limited and insufficient. There was no discussion nor diagnosis with respect to the nature of the events on board. The situational awareness, problem solving and decision making were therefore deeply impacted. The F/O’s lack of proactivity, which comes out from the CVR transcripts, seems to show that he was overwhelmed by the events on board from the moment the stick shaker triggered. His low flight experience (300 hours total) may have accounted for this situation.
The BEA regrets that the parts of the CVR transcript which allow to show the difficulties encountered by the F/O have been removed from the extracts of the CVR transcript published in the report.
It is regrettable that the report does not include a thorough analysis of the reasons for the behaviours observed, in relation with their training, their experience and the company organization with regard to the training and knowledge acquisition principles.
2. From the 1st MCAS until the end of the flight
As the flaps reached the retracted position, the MCAS was activated and the stabilizer trim position decreased from 4.6 to 2.1 units . Although the Captain was applying an increasing nose up force (between 100 and 125 Lbs), only a brief electric trim up input of 2 seconds was recorded, which was insufficient to trim out the MCAS inputs and to relieve the aerodynamic loads. The stabilizer remained at 2.1 units of trim.
The force applied by the Captain on the control column during this phase only kept the airplane almost level. During the ECAB simulator sessions, the simulator crews felt it was instinctual to use as much electric trim as needed to reduce the column forces in response to MCAS inputs while recognizing that it was not very common in a normal flight.
During the second MCAS activation, the Captain applied a 9-second electric trim-up input. This interrupted the MCAS two seconds before its expected end. The effect of this second trim input fully countered the 2nd MCAS. However the aircraft was not returned to a fully trimmed condition as the first MCAS input had never been trimmed out.
During the 9 seconds of electric trimming, the stab trim cut-out switches were put in the cut-out position. At that moment, the stabilizer was at 2.3 units of trim and the Captain was pulling on the control column with a force of 80 Lbs. The altitude was 9,100ft, IAS 332 kt, pitch 2.5°, and vertical speed + 350 ft/min.
When the speed exceeded VMO 340 kt (varying between 360 and 375 kt), the overspeed warning triggered. The crew expressed their surprise. This may indicate that after retracting the flaps, the crew lost track of the IAS values. No thrust reduction was however performed.
During this phase, the physical efforts applied by the crew on the column probably impacted their situational awareness and their cognitive resources and did not allow them to undertake the proper actions.
Probable cause of the accident (paragraph 3.2 of the report)
The BEA notes that the only probable cause retained in the EAIB report is related to the activation of the MCAS system.
The BEA believes that the crew’s inadequate actions and the insufficient Cockpit Resource Management (CRM) played a role in the chain of events that led to the accident, in particular during the first phase of the flight, before the first MCAS activation.
Contributing factors (paragraph 3.3 of the report)
The BEA notes that the contributing factors identified by the EAIB are only related to the MCAS system. The following contributing factors, that come out of the analysis of the event, should also be stated in the report:
Flight crew’s failure to apply, immediately after take-off and before the first MCAS activation, the Approach to Stall or Stall Recovery Maneuver and the Airspeed Unreliable Non-Normal Check-list;
Captain’s insistence on engaging the A/P, contrary to the Approach to Stall or Stall Recovery maneuver procedure;
Insufficient use of the electric trim to relieve the high control column forces after the MCAS nose down orders;
Captain’s lack of thrust reduction when the speed became excessive, which in combination with insufficient trim, caused an increase of the forces which became unmanageable on both the control column and the manual trim wheel.
The use of the Logipad system by the airline as the sole means to disseminate information on new systems and/or procedures, which doesn’t allow the evaluation the crews’ understanding and knowledge acquisition on new systems and procedures. This system was used to disseminate the information related to the MCAS system issued following the previous 737 Max accident and did not allow the airline to ensure that the crews had read and correctly understood this information.