Date: 07 Dec 98 23:19:07
From: MJones <rmjones@cyberhighway.net>
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Stephan Stephany LAC-CC wrote:
>The crew had no clue of altitude or airspeed (also the "backup" system
>was inoperative), but just out of curiosity: could some pressure
>indicator (if there is any) of the pressurization system had given to
>the crew an idea of the outside pressure and (in consequence) of the
>real altitude?
There was most likely a static source available: Break the glass of the
(stand-by, in this case) ASI. You would then have AS and altitude. Yes,
the pilots would have to depressurize the cabin for the instruments to give
meaningful numbers. Also, the radar altimeter, which operates below 2500
ft AGL operated fine.
Even if they did not do this (or I am mistaken and this is not an option on
the 757 for whatever reason), somewhat understandable perhaps with all of
the conflicting info they were receiving and the numerous alarms adding to
the confusion, it was still a manageable situation by ignoring the ASI and
airdata altimeter, setting up low-cruise power on the engines (they
certainly had looked at the power instruments often enough over the years
to know what approximate power setting would give a reasonable AS) and
flying the horizon, either real or gyro-artificial. They even did this for
a while, until the pilot again attempted to go to auto control. IFR would
add some complications, but AFAIK, it was VFR.
The pilot (from the cvr transcript) was hopelessly confused, the copilot
apparently less so but unable to overcome the pilot's mind-set. The pilot
kept insisting on enabling the autopilot and auto throttles, even though
that was exactly the wrong thing to do since the data feeding the autopilot
was erroneous. Likewise by asking ATC for altitude info, since that
altitude info would come from the aircraft's (erroneous) Mode-C
transponder, the crew showed a lack of understanding of very basic
precepts. By all accounts, enough information was available to allow a
safe, if white-knuckled, return.
My opinion only, of course. Judge for yourself. The transcript is
available at http://www.avweb.com/other/peru603.html
Mike Jones
