On Mon, 07 May 2007 20:41:13 GMT, "Jim Carter"
wrote:

I think what you've cited is for bench testing during certification. Once
installed in the aircraft then comparison against the pressure as recorded
by the official weather observation is +/- 75'.

An interesting discussion point came up recently that exposed the difference
in teaching over the past 30 years. 30 years ago we were taught to set the
altimeter to the known field elevation and record the difference between the
official pressure and the indicated pressure, then apply that difference to
every setting you received along your route of flight.

Today they teach to set your altimeter to the official pressure and that's
it.

So my question becomes, when executing a precision approach to a minimum DH
of 200' and then executing the missed procedure, the aircraft is allowed to
descend slightly below the DH as things spool up. If you are already 75'
lower than you think because of altimeter error, and you descend only 20'
more (one gradient on the altimeter) aren't you really only 105' off the
deck?

I did cite the bench testing numbers, as they are the ones of which I am
aware.

I've heard of the 75' "allowance" for a field measurement, but I've not
seen a regulatory justification for that value. As someone who flies
instrument approaches to minimums, I, personally, would not be happy with a
75' error, and would have the system rechecked.

So far as you being only 105' off the deck in your hypothetical instance,
that is obviously the case.

By the way, for Category II operations, which may have a DH as low as
100'AGL, it is a requirement to have readily available the altimeter
calibration information from the last certification, (and to apply it
appropriately).
--ron