Re: B 737-500 Depressurization Incident

Date:         30 Mar 99 01:54:02 
From: (Neil Wagner)
References:   1 2
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On 17 Mar 99 01:24:59 , Seth Dillon <> wrote:
>Something doesn't add up here.  If the aircraft was cruising along and
>both packs tripped off the cabin would start to climb, even with the
>outflow valve full closed.
>If the cabin is climbing (psid being reduced) why would the
>saftey valves even be in the picture?

I think you missed one key statement in the original posting, viz:

>> At about FL340 in the descent both bleed lights extinguished and the
>> duct pressures returned to a normal reading.

I read that as meaning the packs came back on.

Then, at the beginning of the emergency descent, "The F/O selected
manual mode of the pressure controller and manually operated the
outflow valves to the closed position."  If the outflow valve stayed
fully closed from FL340 all the way down to 5000' with the packs
running, you bet the overpressure relief valves are going to open.

Seth Dillon <> also wrote:
>If the cabin reached 14,000 ft then the flight crew is full of crap in
>their assertion that a psid of 8.65 was maintained.

You'll note that it wasn't the flight crew that made any
reference to the relief valves, viz: (again from the original posting)

>> The senior investigators believe that the pressure differential was
>> maintained at  8.65 psi (by the safety relief valves) some time after
>> the F/O manually closed the outflow valve (passing 34,000 ft) until the
>> a/c reached the 5,000 ft altitude when the outflow valve was re-opened.

I think things add up just fine.

Neil -