Immediately after this, the first officer, who was the
pilot flying, tried to deploy the thrust reversers; when
they did not initially deploy, the captain took over the
thrust reverser controls and they deployed about 18
seconds after touchdown. Subsequently, the airplane
continued off the departure end of the runway, coming to
a stop in deep snow off the end of the paved surface.
American Airlines training and procedures require the
pilot monitoring (in this case, the captain) to observe
and call out the position of the speedbrake lever after
landing; if the speedbrakes do not deploy automatically,
the captain is to manually deploy them. Although the
pilots could have manually deployed the speedbrakes at
any time during the landing roll, neither pilot
recognized that the speedbrakes had not automatically
deployed because they were both trying to resolve the
thrust reverser issue.
The landing performance analysis showed that under
similar runway conditions, even without thrust reverser
deployment, the airplane would have stopped about 4500
feet down the 6300-foot runway had the speedbrakes been
promptly deployed.
The investigation revealed that the speedbrakes did not
automatically deploy because of a latent assembly defect
in one of the speedbrakes mechanisms. In addition, the
NTSB determined that the thrust reversers did not
initially deploy because of a rare mechanical/hydraulic
interaction that occurred in the thrust reverser system
as a result of an unloading event at the precise instant
that the first officer commanded their deployment
immediately after touchdown.
As a result of the investigation, the NTSB made the
following new safety recommendations to the Federal
Aviation Administration (FAA): require that all
transport category air carriers develop and incorporate
training to address recognition of a situation in which
the speedbrakes do not deploy as expected after landing;
require all newly type-certificated air transport
category airplanes to have an aural, or otherwise
unique, alert that warns pilots that the speedbrakes
have not deployed during the landing roll; and 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.
The NTSB also reiterated to the FAA three safety
recommendations related to multiple emergency
situation flight training and monitoring skills and
workload management that it had made during
investigations of previous accidents.
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