Re: Peruvian 757 crash -- possible cause reported

From:         Praxis - Bjxrn Erling Flxtten <praxis@online.no>
Organization: Telenor Online Public Access
Date:         08 Nov 96 05:24:23 
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Karl Swartz wrote:
> ...
>
> Reportedly, maintenance crews placed duct tape over the static ports
> (which feed the airspeed, altitude indicators, and verticle speed
> indicators) to protect them during "polishing" of the aircraft, then
> forgot to remove the tape.
>
> This would explain the complete instrumentation failure, though it
> fails to explain why the pilots did not recognize a problem with the
> airspeed indicator during takeoff and either abort the takeoff or
> immediately return to the airport.  (The pilots first reported
> problems and requested a return to Lima about five minuts into the
> flight.)
>
> ...

An obstructed static port would not be noticed during takeoff because
the surrounding static pressure doesn't change until the aircraft starts
climbing into thinner air.

The effect of an obstructed static port during climb would be a lower
indicated airspeed than correct (the airspeed indicator is continually
feeded with the static pressure that existed on the ground, instead of
the gradually lower static pressure that exists higher up in thinner
air. Indicated airspeed would then be equal to the sum of correct static
and dynamic pressure feeded by the presumed unobstructed dynamic port
minus the incorrect too high static pressure feeded by the obstructed
static port).

If the pilots are unaware of the problem then they would lower to nose
to increase what they believe is a too low airspeed. This could possibly
lead to an overspeed condition, with a resulting airframe breakup.

If the dynamic port is obstructed then the problem would be the
opposite (as I believe it was with the 757 accident in the Caribbean).
The airspeed indicator would register zero airspeed as the aircraft
takes off, because no dynamic pressure reaches the airspeed indicator.
Then as the aircraft starts climbing, the airspeed indicator compares
the pressure from the obstructed dynamic port (which remains the same),
with an ever-decreasing pressure from the static port. This results in
an ever-increasing indicated airspeed irregardless of the actual
airspeed, as long as the aircraft continues to climb (the airspeed
indicator has in fact become an altimeter). The crew would raise the
nose to combat what they believe is a too high airspeed, eventually
stalling the aircraft. If they do not become aware of the problem, they
would stall the aircraft all the way to the ground (which is what I
believe happened in the Caribbean).

However, theory aside, isn't it amazing that modern aircraft with all
this technology is not equipped with a sensor to check for obstructed
static and / or dynamic ports? It would be fairly simple to sound an
alarm if for instance increasing wheel-speed on take-off doesn't
correspond with an increasing dynamic pressure. Or to sound an alarm if
engine-power and climb-attitude doesn't correspond with a decreasing
static pressure. I believe all the sensors are already in place, so all
you would need is some additional software logic. This could be of
assistance to the pilots, because a situation with obstructed static or
 dynamic ports is extremely confusing since you can loose both the
airspeed indicator, the altimeter and the climb indicator.


Bjørn Erling Fløtten
praxis@online.no