Thunderbirds and Altimeters

"Dudley Henriques" wrote...

The Kollsman range,
which is also the basic baro range in the Viper's CADC which drives the
Viper's altimeter in both ELECT and PNEU backup, is aprox 22.00 inches on
the low side and 32.00 inches on the high side.

This gives you, even figuring the 1 inch per thousand rule, an elevation
reset capability to a zero reset before takeoff of 10, 000 feet. This
basically allows a zero reset anywhere in the U.S. at least, allowing for a
standard atmosphere.

Not quite...

You have a 10,000' nominal range using that rule of thumb, but from the 29.92"
standard you only have 7,920' of 'low' correction available at sea level, and
2080' of 'high' correction. That gives you QFE capability up to an airport
elevation of 7,920'. Still, as you say, adequate for most US airports in most
conditions.

It also illustrates the complexity in attempting to fly IFR using QFE (0' runway
altitude) at high-altitude airports. If you use the wrong setting, misinterpret
a controller's QFE / QNH call, or use the wrong minimums on a chart, it can be
deadly. Those errors are all too easy to make, especially if you are not used
to using QFE at all.