"karl" wrote in message
...
| *****but it will never suffer an electrical or mechanical failure.*****
|
| Never say never. I've personally seen the failure of just the type of
gauge
| you are talking about. In a Super Cub. Showed 3/4 actual zero.

John Denver's LongEZ also had a sight gauge. What he did not know was that
with the nose wheel retracted on the ground the gauge would always look more
full than it really was, even when the tank was almost empty.

The problem with sight gauges is that they are not linear, meaning that when
the gauge is 1/4 full it does not necessarily indicate that the tank is 1/4
full. This is true of fuel sticks, too. Fuel tanks tend to be rounded at the
top and bottom, so an inch of fuel in the bottom is less gas than an inch of
fuel in the middle. The gauge should be calibrated and marked with the fuel
quantity at several different levels, but no one had done this on Denver's
plane.

NTSB report he
http://www.ntsb.gov/ntsb/brief.asp?e...08X09045&key=1