Re: Peruvian 757 crash -- possible cause reported

Date:         11 Nov 96 01:50:36 
From:         rdd@netcom.com (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
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In article <airliners.1996.2329@ohare.Chicago.COM> Keith Barr <barr@nilenet.com> writes:
>In article <airliners.1996.2279@ohare.Chicago.COM>,
>Karl Swartz <kls@ohare.Chicago.COM> wrote:
>>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.)
>
>A pitot-static system failure in a large aircraft should not be an
>occurance that causes an accident.

Run it through a simulator some time and get back with us.


>You can live without your airspeed indicator by knowing what power settings
>and approximate pitch attitudes will give you what you are looking for
>(i.e.: set 2800 pounds fuel flow, 5 degrees nose up, flaps 15, and you get
>170 knots and level flight--these are for a 737-200 in the low 90,000 pound
>range at 7000 feet MSL).

That's a fine theory.  It'll work for a while.  In this case, they were
flying at night, no ground contact.  Let's assume that the static ports
were all stuck over. They'd be able to fly pitch and power for a while.
After a certain point, though, they'll reach the apex during a climb, either
because of maximum performance or overspeed conditions.  Back-of-envelope
calculations indicate the airplane would have reached MMo around 10,000',
if they were showing 250knots IAS, with a blocked static port.

>In a 757 you have a radar altimeter, which should be enough to keep you out
>of the water anyway.  RA's generally don't work very high (although the one
>on the DC-8 I have spent a lot of time on was accurate into the 30,000 foot
>range).

At ~10,000', therefore, they get to lower the nose.  This starts some major
uncertainty.  After a certain point, the "pitch vs. thrust" theory will
fail, and they will lose altitude.  Jet aircraft can lose a lot of altitude
in a very short period of time.

So after fighting this situation (at night, in the dark) for 30 minutes,
unable to make any sense of or able to stabilize the situation, they pass
under 2500', when the radio altimeter should theoretically kick in.  The
initial (horrible) reports indicated that they heard the GPWS at least twice.
That indicates that they were unable to do what you suggested, and may have
been in a long-term PIO.

If this static report is true, then the actions of the crew were remarkable--
remarkable because they lasted so long.  The Birgenair didn't do nearly as
well, with likely much more info available.




--
Robert Dorsett                         Moderator, sci.aeronautics.simulation
rdd@netcom.com                         aero-simulation@wilbur.pr.erau.edu
                                       ftp://wilbur.pr.erau.edu/pub/av