Re: B 737-500 Depressurization Incident

Date:         17 Mar 99 01:24:59 
From:         Seth Dillon <bdillon1@mindspring.com>
Organization: MindSpring Enterprises
References:   1
Followups:    1
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Puranjai Dass wrote:
> " The aircraft was cleared by Approach Control to descend from FL 370 to
> FL 350 then re-cleared  to descend to 10,000 ft. At FL 360 the F/O
> called "one to go". Just below FL 360 the Master Warning sounded and the
> Left Bleed Trip-Off light came on. The F/O  reached for the QRH. 15 to
> 20 seconds later the Right Bleed Trip-Off Light illuminated. The F/O
> operated the Trip Reset button, then turned to the Captain who was
> putting on his oxygen mask. The F/O donned his mask. The cabin altitude
> warning (an intermittent horn) sounded at 10,000 cabin altitude and this
> was silenced by the F/O operating the horn cutout. He then armed the
> passenger oxygen switch. The aircraft cabin altitude continued to climb.
> Approaching a cabin altitude of 14,000 ft the Captain ordered manual
> deployment of the passenger oxygen masks.
>
> At about FL340 in the descent both bleed lights extinguished and the
> duct pressures returned to a normal reading. An emergency descent was
> initiated. The F/O selected manual mode of the pressure controller and
> manually operated the outflow valves to the closed position. A duct
> overheat illuminated at about the halfway point during the descent. This
> was extinguished after reference to the checklist.
...
> As a result  of this incident a passenger suffered permanent ear damage.
> 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.
> If we assume the pressure differential reached 8.65 psi when the a/c was
> at say 31,000 ft then the cabin altitude would be 4,000 ft (using
> standard atmosphere calculations).

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.  In a perfect world that would not happen
but  there are a bunch of little leaks everywhere so the cabin will
climb. If the cabin is climbing (psid being reduced) why would the
saftey valves even be in the picture?  They only open when the psid
exceeds a set value.  If they didn't open at altitude with both packs
running, the max psid in this scenario, then they would not open during
the aircraft descent and cabin climb (a reducing psid situation).  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.

In all probability the aircraft experienced a negative pressure
situation during the descent with the outflow valve cranked closed.
This is when the pressure inside the aircraft is less than the pressure
outside the aircraft and is concievable in an aggressive descent.  The
aircraft is not designed to handle this negative pressure and has a
means of alleviating it.  The doors.  Any negative pressure will push
the doors in, the upper and lower gates of the cabin doors and the whole
door in the case of the cargo doors, a little, but enough to releive
negative pressure.  In addition many aircraft have negative pressure
releife valves which open with the slightest amount of negative
pressure.

In my opinion, during the descent inside and out side pressure equalized
and the cabin descended at the same rate as the aircraft.  By my
calculations the descent was at about 1600 fpm (30,000 to 5,000 in 15
minutes).  I don't know and cannot speculate  if that descent rate is
enough to cause permanent ear damage.

-Seth