TAROM A310 Incident (was Airbus/Boeing pilot-aircraft interface)

From:         bareynol@cca.rockwell.com (Brian A. Reynolds)
Organization: Rockwell Avionics - Collins
Date:         14 Nov 95 14:46:20 
References:   1 2 3
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AvWeek 17 April 1995
The Romanian team investigating the probable cause of the March 31 crash
of a Tarom A310 transport is focusing on the combination of an engine
autothrottle system failure that generated asymetrical power setting and
on the pilots' apparent failure to react quickly to the developing
emergency.

When the autothrottle was set to climb thrust, the Digiatl Flight Data
Recorder (DFDR) showed the right engine remaining at takeoff thrust, while
the left engine's thust reduced to idle over a 42-second period.
Investigators speculated that the right throttle may have been
mechanically jammed and the authrottle electric motor continued applying
force through a slipping clutch to the jammed right throttle, while the
left clutch was drawing its throttle to idle.

"We are amazed.  We do not understand (what happened); the roots of the
accident could be somewhere else.  The asymmetrical thrust situation
developed slowly, giving the pilots enough time to identify the failure
and act accordinly" an A310 French pilot said.  The pilot added the A310
is not equipped with fly-by-wire controls and that the aircraft has a
conventional yoke - two features that contribute to giving the pilots the
tactile feel of developing asymetcial engine power.

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Commentary.
I have in this forum and others used throttle movement as an example of
good human factors design as it provides a clear indication of what
the automatic system is attempting to do.  But alas, here is a case
of where, apparently, the left throttle was at flight idle but the right
remained at takeoff thrust setting.  Because of other factors, the
flight crew failed (or was not provided the  correct information) to
recognize what was happening in time to break the chain. (The weather was
also a factor, "due to poor visibility and heavy snow, the pilots had no
visual horizon reference.")  This accident could have happened in anyones
flight deck (assuing that the mechanics of how throttle lever position
is conveyed to the engines is similar).  I appologize for making the
generalization from an A310 incident to the later fully automated
flight deck design of the modern Airbus aircraft.

Brian

p.s.  In the Douglas implementation of autothrottles (and I think also
for Boeing), it is the actual position of the throttle lever which is
used as the throttle command.  A slip clutch is provided so that the
flight crew can override a run-away servo motor.  In addition, the
autothrottle system also has to release a brake in order to drive the
throttle levers.  The drawing show in the AvWeek article is generally consistent with this scheme.  So increase friction in the mechanical
connection between the throttle levers and the engines could have been
overridden by the action of the flight crew, but still been too high
for the servo system to drive against.  "Seconds before the aircraft
impacted the ground, both engines were at idle thrust, indicating the
pilots then were acting to eliminate asymmetrical poser condition and were
trying to restore a normal flight pattern."

I hope that this has cleared up any confusion my original posting may
have caused.