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In a message dated 5/25/2002 2:32:14 PM Central Daylight Time, George Braly wrote:
One day, the main vac pump failed. Within 30 minutes, the attitude
indicator quit, even though the backup pump was working. Why? carbon crud
sent through the attitude indicator by a poorly thought out filter/switch
over system.
George,
In vacuum systems for many high-speed aircraft,
cabin air->filter->instrument->regulator->pump->engine compartment
with the pump failure not taking into account that the cabin is low pressure and the air filled cowl is at a higher pressure. Thus, we get a reversal of air flow carrying the pump trash back into the dainty instrument. I guess we are lucky that the filter at least keeps the dirt off the pilot.
Knowing that "why" questions have no answer, why did the FAA not require a simple check valve be placed between the instrument and the pump? Or, after many such instrument destructions, why not an AD to have one installed? Maybe the instrument overhaulers had undue influence. Maybe the narrow minded FAA only saw the pump failure and not consequential damage other than the occasional crash.
Scott Krueger
N92EX
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