Re: Peru 757

Date:         08 Dec 96 13:07:43 
From:         "Mark E. Ingram" <>
Followups:    1
Next article
View raw article
  or MIME structure

The November 1996 issue of NASA ASRS's "CALLBACK" (Number 209)
matter-of-factly reports the following:

Waxing Eloquent

Last month we reported on how an insect nest in a fuel tank vent caused
the implosion of the fuel tank.  Here, we look at a case of malfunctioning
instruments caused by the human touch.  The Captain of a corporate jet

"During takeoff roll, passing through 100 knots, airspeed difference was
noted between Captain's and First Officer's airspeed indicators.  As
airspeed increased, the difference between the two systems became larger.
As the ADC [Air Data Computer] sensed the airspeed difference, the ADC
miscompare alert illuminated, the yaw damper disengaged, the elevator trim
inop alert illuminated, and the elevator trim disengaged, along with the
autothrottles disengaging.  With all these cautions and their associated
chimes, there was a great deal of activity, including IMC almost
immediately after takeoff.

"The SID calls for level-off at 2000 feet and 200 knots airspeed.  Well,
we missed both of those.  The airspeed difference remained for
approximately 20 minutes, then went away, restoring all systems.

"Cause:  the aircraft had been waxed the day before, and burned wax
residue was found in the Captain's pitot tube on postflight."

[The "CALLBACK" editor helpfully adds:]  Meticulous pre-flights are as
important after a wash-and-wax job as after maintenance work.

Sheesh!  All this because of one blocked pitot tube?  (Remember, this was
a *corporate* aircraft, but presumably certified under FAR Part 25.)

Mark E. Ingram

MarkT@Mo-Net.Com (also