Re: How do fuel guages work?

From:         pluymers@pi.net (Rene Pluijmers)
Organization: Planet Internet
Date:         14 Aug 95 03:43:36 
References:   1
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kward@apple.com (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
>starvation?

On most modern planes, all fuel tanks contain several fuel probes.
Basically, a fuel probe is a capacitor with two plates. The amount of
fuel between the plates determines the capacity. This capacity is
translated into a fuel quantity reading in the cockpit.

The system is fail-safe: a circuit in parallel with the fuel probes
carries a small current. When all systems are operating normally, the
currents from the fuel probes predominate. If there IS something
wrong, the current from the parallel circuit lead to an 'Empty'
reading in the cockpit.

In computer controlled systems, self-test routines are carried out by
the software on a regular basis. Faults are displayed by an
appropriate message, e.g. 'FUEL QTY'.

So,  it is very unlikely, if not impossible that a situation you
described will occur.

I can recommend the book 'Aircraft Instruments & Integrated Systems'
by E.H.J. Pallet (Longman Scientific & Technical,  1992, ISBN
0-582-08627-2)  to you. Chapter 14 (20 pages) deals completely with
fuel quantity indicating systems.

Best regards,
Rene Pluijmers
(Private Pilot, highly interested in Avionics)