On 29 December 2010 a Boeing 757-200 being operated by American AL on a scheduled passenger flight from Chicago O’Hare to Jackson Hole failed to stop before the end of landing runway 19 at destination after a daylight landing in normal ground visibility with light snow falling and finally stopped in deep snow 220 metres beyond it. The aircraft incurred only minor damage and with none of the 185 occupants injured, the aircraft commander decided that the best option was for everyone to remain on the aircraft and await ground assistance. Passengers were eventually disembarked using steps and bussed to the Terminal.
An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB). FDR and CVR data was available for the Investigation. It was noted the aircraft commander, who was acting as PM for the flight, had considerable experience of winter operations into Jackson Hole and its relatively short (1920 metre) runway and both pilots were familiar with it and in compliance with American Airlines special category airport requirements for operations into it.
The aircraft in its final stopped position viewed from the end of the runway reproduced from the Official Report
Evidence available to the Investigation showed that the preparation made by the flight crew for the arrival was thorough and had considered all the relevant issues. The most up to date measured runway friction reports were obtained and gave ‘good’ for the first two thirds of the runway and then very slightly less for the final third. The speed brakes were armed and auto brakes set to ‘MAX’. The approach to the runway was normal and the touchdown was as intended firm and occurred about 600 feet beyond the runway threshold. The PF reported that when he tried to deploy the thrust reversers promptly after touchdown, they did not properly deploy and after several more attempts, the aircraft commander took over and eventually succeeded in deploying them 18 seconds after touchdown with about 640 metres of runway remaining. It was noted that the PM had erroneously made the callout ‘speed brakes deployed’ at touchdown and considered that this was “likely made in anticipation (not in confirmation) of speed brake deployment after he observed the speed brake handle‘s initial movement”. Since both pilots stated that they had been unaware until after the aircraft came to a stop that the speed brakes, which they had armed for automatic deployment, had not automatically deployed, it can be presumed that once the erroneous callout had been made, both pilots had assumed that the normally reliable automatic speed brakes were functioning correctly and focused attention on the (relatively less important) thrust reverser problem. The Investigation noted that had they recognised the failure to deploy, the pilots could have manually extended the speed brakes at any time during the landing roll. The Investigation found that if the speed brakes and spoilers had been deployed as armed/selected, the aircraft would have stopped in the runway distance available following touchdown. It was also calculated that if the speed brakes had been deployed normally and the thrust reversers had not deployed until the time they eventually did, the aircraft would also have stopped on the runway. Attention was therefore focussed on reasons why the two unrelated malfunctions occurred and the response of the crew to the circumstances they faced.
FDR data indicated that the signal from the air/ground sensing system transitioned from ‘air’ to ‘ground’ at touchdown but then about 1 second later reverted momentarily to ‘air’ before returning to and remaining in ‘ground’. It was considered that such a brief cycling of the air/ground signal during a touchdown was ‘not uncommon’. However, in this case, it coincided with the First Officer‘s attempt to promptly deploy the thrust reversers immediately after touchdown. Because of the precise timing of these events, it was concluded that “a rare mechanical/hydraulic interaction occurred in the thrust reverser system, and the thrust reversers were locked in transit instead of continuing to deploy”. It was noted that although the pilots reported multiple movements of the reverse thrust levers after the air/ground sensing system had not initially been effective, this was because it was necessary to move the reverse thrust levers forward of their interlock position before reselection in order to overcome the locked-in-transit status, an action that was recorded on the FDR about 10 seconds after touchdown. During interviews, both pilots indicated that they were unaware of a circumstance in which the thrust reversers could be locked in transit and were unaware of the actions needed to correct the situation. It was noted that “American Airlines’ personnel in general, including the Company’s 757/767 Fleet Manager, were unaware of this rare event or its resolution”.
In respect of the speed brakes, it was noted that American Airlines‘ procedures require that the PM called out the position of the speed brake lever after landing and that, in the event that the speed brakes do not automatically deploy, the Captain should manually deploy them regardless of which pilot is [PM]. The type Operations Manual was noted to specifically state that “pilot awareness of the speed brake lever during the landing phase is important in the prevention of overrun” and that “without speed brakes deployed after touchdown, braking effectiveness may be reduced initially by as much as 60%”
In respect of the failure of the speed brakes to deploy even though they had been correctly armed, removal and careful examination of the system and its components initially revealed no evidence of a malfunction. Only after the same aircraft experienced another automatic speed brake system non-deployment on 31 March 2011 was a further examination made which disclosed “a latent assembly defect” in the no-back clutch mechanism which intermittently prevented the speed brake actuator from automatically driving the speed brake lever beyond its armed detent to extend the speed brakes. This defect was the improper securing of one of the four speed brake lever braking pins which had allowed it to intermittently rotate within its assembly and prevent the no-back clutch from transmitting torque from the automatic speed brake actuator to the speed brake lever. Additional testing showed that this condition would only occur when the actuator was attempting to drive the speed brake lever towards the speed brakes extended position. It was also noted that this defect only affected automatic deployment - manual deployment would always have been successful.
In respect of the importance of adhering to pilot monitoring responsibilities, it was considered that their more effective inclusion in pilot training might have meant that the PM would have been less likely to assume control of the reverse thrust levers (which is a PF responsibility) when problems arose with their deployment and, as a consequence, remained focused on his PM duties so more likely to have noticed the failure of the armed speed brakes to deploy.
The Investigation identified a number of ‘Safety Issues’ as a result of the its findings including the following:
- Inadequate pilot training for recognition of a situation in which the speed brakes do not automatically deploy as expected after landing.
- Lack of an alert to warn pilots when speed brakes have not automatically deployed during the landing roll.
- Lack of guidance for pilots of certain Boeing airplanes to follow when an unintended thrust reverser lockout occurs.
- Lack of pilot training for multiple emergency and abnormal situations.
- Lack of pilot training emphasizing monitoring skills and workload management.
The Investigation noted that readout of the incident aircraft FDR had disclosed that several parameters including a mandatory one were not functioning properly. More generally, issues had also been identified with American Airlines’ FDR maintenance and related documentation and the effectiveness of the corresponding FAA regulatory oversight. Specifically, it was noted that American Airlines’ personnel could not provide appropriate documentation and/or wiring diagrams for the downloaded FDR, had improperly performed the two tests which are intended to validate correct operation of the FDR system and had allowed numerous FDR discrepancies to go undetected, all of which “hampered FDR data collection and evaluation during this Investigation”. It was observed that although not directly related to the causes of the investigated event, these issues had, as in many previous Investigations, affected the ability of the Board to evaluate the available data.
The Probable Cause of the event determined by the NTSB as:
A manufacturing defect in a clutch mechanism that prevented the speed brakes from automatically deploying after touchdown and the Captain’s failure to monitor and extend the speed brakes manually. Also causal was the failure of the thrust reversers to deploy when initially commanded. Contributing to the incident was the Captain’s failure to confirm speed brake extension before announcing their deployment and his distraction caused by the thrust reversers’ failure to initially deploy after landing.
Three New Safety Recommendations were made as a result of the Investigation:
- that the Federal Aviation Administration require all operators of existing speed brake-equipped transport-category airplanes to develop and incorporate training to specifically address recognition of a situation in which the speed brakes do not deploy as expected after landing.
- that the Federal Aviation Administration require all newly type-certificated 14 Code of Federal Regulations Part 25 airplanes to have a clearly distinguishable and intelligible alert that warns pilots when the speed brakes have not deployed during the landing roll.
- that the Federal Aviation Administration require Boeing to establish guidance for pilots of all relevant airplanes to follow when an unintended thrust reverser lockout occurs and to provide that guidance to all operators of those airplanes.
In addition, three Safety Recommendations previously made were reiterated:
- that the Federal Aviation Administration establish best practices for conducting both single and multiple emergency and abnormal situations training.
- that once the best practices for both single and multiple emergency and abnormal situations training asked for in Safety Recommendation A-09-24 have been established, the Federal Aviation Administration require that these best practices be incorporated into all operators’ approved training programs.
- that the Federal Aviation Administration require that all pilot training programs be modified to contain modules that teach and emphasise monitoring skills and workload management and include opportunities to practice and demonstrate proficiency in these areas.
The Final Report of the Investigation was adopted by the NTSB on 18 June 2012.