Re: How do fuel guages work?

From: (NADIR)
Organization: Megaweb
Date:         14 Aug 95 03:43:35 
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  or MIME structure (Ken E. Ward) wrote:

>Or more to the point, could an airliner experience a failure in the fuel
>lines, which resulted in the loss of fuel, combined with a failure in the
>fuel guage (or whatever fuel-available monitoring system exists), such that
>everything would appear normal until the engines quit due to fuel

Not sure how all fuel gage systems work, but generally they use a
combination of optical probes and capacitance probes.  The optical
probes are usually placed at either full or half full positions and
are used to signal when to stop fuelling when using single point
refueling (at least if only a half or full tank is required) Depending
on the type of aircraft, these probes may also be hooked to idiot
lights in the cockpit.  The capacitance probes measure the diffence in
conductivity of the air and fuel.  Using these readings, the gage (and
associated amplifiers, etc) determine an average from the probes in
the system, and give a total fuel reading.  It is possible for the
probes to misread, or the system to be miscalibrated.  The only time I
heard of a major airliner with a like problem was that Air Canada 767
which was misfueled and subsequently made an emergency landing at a
dragstrip!!..Wonder if they tried clocking his 100-0mph reaction
Loco Hombre