IFR in mountainous areas requires 2,000 above obstacles with
5 miles of the intended route. Considering that an
altimeter setting may be from a location nearly 100 miles
away and the temperature may be very much below standard,
that 2,000 foot terrain clearance may be reduced to 0 by
temperature, pressure and venturi effects in the mountains.
Try the calculations on your flight computer for true
altitude. VFR and you should see the granite, but IFR or at
night, you depend on the altimeter.
Of course, now that you can get a GPS altitude that has non
of the errors, you can see the altitude errors on the
pressure altimeter.

Altimeters have other errors besides temperature and as a
purely mechanical device they can fail with no indication at
all. The electric altimeters in the high dollars airplanes
(King Airs and jets) use an air data computer that processes
the raw pressure data and sends the result to a display.
These will flag when there is an error, but the cheap
altimeter used in most general aviation piston airplanes has
no such warning. Thirty years ago one manufacturer decide
to save a few bucks in construction and used Teflon tape for
bearings in the housing. When this tape broke loose and
jammed the gears, the altimeter would just freeze. I once
had a student out in central Illinois doing hood work on a
MVFR day. He seemed to be doing a good job as we flew
around under a 1500-2000 foot overcast. When he took the
hood off, his first remark was "How did we get so low?"
Since I'd been watching in and outside, the slow loss of
altitude did not reach the alarm stage, we were below the
clouds and above the ground. But when he went under the
hood he had a picture of what it looked like and when the
hood came off it was very different and quickly.


--
James H. Macklin
ATP,CFI,A&P

--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm



"Peter Duniho" wrote in
message ...
| "Rich Lemert" wrote in message
|
ink.net...
| While temperature may play a role in altimeter
"errors" I believe
| it's extraneous to the discussion at hand.
|
| Why would you believe that? People can and do use that
very same mnemonic
| to remember the effect of temperature on the altimeter
indication.
|
| To pretend otherwise is silly. It happens, and so given
that it happens, it
| makes perfect sense to explain the error and why the
phrase fits (albeit
| imperfectly, AS I ALREADY POINTED OUT, before the post
that claimed
| temperature was irrelevant).
|
| The phrase in question says nothing about why the
pressure is changing,
|
| The phrase in question says nothing about pressure at all.
The word
| pressure is not even used.
|
| only that it does.
|
| The only thing the phrase say that anything "does", is
that the altimeter
| reads too high in certain circumstances.
|
| The
| effect of this pressure change on altimeter readings
can - and has
| been - adequately explained without discussion of
temperature.
|
| The effect of this temperature change on altimeter
readings can - and has
| been -adequately explained without discussion of
temperature.
|
| We can always discuss temperature effects once the
pressure effects
| are understood. Trying to get someone to grasp two
separate concepts at
| the same time when it's not necessary does only serve to
confuse the
| student.
|
| As long as the exact same phrase gets used for two
different concepts, you
| are stuck teaching two different concepts at the same
time.
|
| Now, maybe that really means people ought to stop using
the mnemonic. But
| you and I have no control over it. Pretending that
temperature is
| irrelevant is dumb, and is likely to get someone killed.
|
| Pete
|
|