Re: FYI, B757 crash at Bermuda.

From:         bareynol@cca.rockwell.com (Brian A. Reynolds)
Organization: Rockwell Avionics - Collins
Date:         03 Jul 96 01:23:52 
References:   1 2
Followups:    1 2
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Boeing has a useful mind-set of considering that accidents are
a chain of events which terminate in something nasty.  Brake the
chain anywhere prior to the termination, and the something nasty
becomes much more benign.  It is easy to blame EITHER the equipment
or the pilots; it is more difficult to understand the interelationships.

Consider what we have been told/know about this incident:
1)	The pitot probe was left uncovered for several days prior to the
	flight.
2)	Standard practice is to cover the pitot probe if the aircraft is
	not to be flown for 'some' time.
3)	The Dominician Republic has a humid atmosphere; which MAY result in
	condensation forming inside of the pitot probe.
4)	The pilot observed anomolous airspeed indications during the takeoff
	roll
5)	Differential pressure between the pitot tube and static port forms
	the basis of airspeed calculation
6)	If the pitot tube is obstructed, then the differential pressue
	will increase with altitude resulting in an indication of higher
	airspeed.
7)	Airplanes come apart at high speeds; and the overspeed warning is
	designed to call the pilots attention to this in a clear fashion
8)	The overspeed alert cannot be silenced in the usual manner (by
	acknowledging the alert).  It can only be silenced by correcting
	the situation (slowing down).
9)	A stall is caused by disrution of airflow over the wings.  If
	the aircraft's nose is excessively high, the airflow will be
	disrupted.  The angle of attack (the angle of the aircraft
	centerline relative to actual direction of travel) is deteced by
	an Angle-of-Attack sensor (AOA for short) which is a vane like
	device which 'flies' in the airstream.  If airspeed is sufficient,
	then the AOA will stay within limits.  If the angle of attack
	vs airspeed is insufficient, then the AOA (as measured by the sensor)
	will be come excessive and generate a stall warning.  For western
	aircraft, this is a mechanical vibrator located on the control
	columns (this is obviously not relivent to the A320 and later
	aircraft which do not have columns).
10)	Failure of any instrument or system necessary for flight is brought
	to the attention of the flight crew through 'flight deck indications.'
	The 'flight deck indications' for a blocked pitot tube do not, at
	first analysis, appear to be related to the fundamental failure.
11)	ALL Part 25 aircraft are required to have a totally seperate
	means of measuring and displaying critical flight data to the flight
	crew.  This includes airspeed and altitude.  In the event of TOTAL
	failure of the electronic diplays, this information from the 'standby
	instruments' is still available.  If there is ANY difference between
	the data displayed between the Captain and First Officer, the
	information from the standby instruments is intended to assist in
	determining which display is actually correct.
12)	The flight crew experienced simultaneous stall and overspeed
	indications.  In reacting to the overspeed warning, the nose
	was pitched up to decrease the airspeed.  This increased the
	angle of attack, deepening the stall.
13)	At some point the aircraft ceased to have sufficient airflow over
	the wings and stopped flying.


The role of the NTSB is to look at the aircraft design, maintenance
practices, flight crew training, regulator oversight, and any other
factors which they consider relivent.  From this they will prepare their
conclusions as to the sequence of events which ended when a flyable
aircraft found its way to the bottom of the ocean.  Any attempts to
appoint blame, or exonerate any participent prior to the release of the
report is speculation, opinion, conjecture, blowing smoke, etc.

Speculation on how this chain could have been broken is a normal part
of the industry retrospection which occurs following any accident.
Establishing blame is best left to the legal system.

Brian