Re: B 737-500 Depressurization Incident

Date:         11 Mar 99 03:54:09 
From:         "K Hall" <kevin.hall@cwcom.net>
Organization: Pavilion Internet USENET Server
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Negative pressure altitudes do exist.  The pressure law can be extended
below sea level to give an equivalent altitude.  Most mechanical altimeters
will read down to negative 10,000 feet.  Negative pressure altitude
represents a static pressure in excess of 1013 millibars (ISA sea level
pressure) and therefore a height below mean sea level.

Assuming the cabin differential of 8.65 psi you quote, at 31,000 feet the
cabin pressure would have been 12.82 psi. This represents a cabin altitude
of 3,730 feet.  By the time the aircraft had reached 5,000 feet the cabin
pressure would have increased to 20.88 psi.  This is equivalent to
approximately 10,000 feet below mean sea level.

The high pressure would not in itself cause ear damage, but the sudden
pressure reduction resulting from opening the outflow valves at 5,000 feet
might.

In summary, if you have difficulty in expressing negative cabin altitudes,
try expressing them in terms of cabin pressure instead.  The cabin altitude
is only an equivalent altitude after all, not an actual one.

Hope this helps

Kevin Hall