Altimeter 3066

"M" wrote in message
oups.com...
Jim Macklin wrote:
That is the highest pressure you can set on the Kollsman
window.



I can hardly see how it should affect VFR flights. Additionally if the
pressure is higher than 31 inch mercury and the altimeter is set to
3100, the true altitude would be higher than what's indicated by the
altimeter. For VFR flights this would almost never introduce any
problems. (Even for IFR terrain separation this allows a bigger margin
for terrain clearance).

The only problem I could think of is the mode C requirement for
altitude above 10,000 MSL, if someone flying without a mode C x-ponder,
set their altimeter to 3100, on day with pressure higher than 31"
mercury, and flying right near 10,000 feet indicated on their altimeter
:-)


I thought the Mode C transponder reports Pressure Altitude and is basically
fixed at 29.92?

ATC computer converts the pressure altitude with local altimeter
setting. If ATC knows everyone is using 3100 as their altimeter setting
(can be by regulation in high pressure 31" situation, such as in
Canada), the computer can do similar converson, so the IFR separation
would not be unaffected.

Wade Hasbrouck wrote:


I thought the Mode C transponder reports Pressure Altitude and is basically
fixed at 29.92?

M wrote:

ATC computer converts the pressure altitude with local altimeter
setting. If ATC knows everyone is using 3100 as their altimeter setting
(can be by regulation in high pressure 31" situation, such as in
Canada), the computer can do similar converson, so the IFR separation
would not be unaffected.

If the pressure gets above the limit IFR flight would not be allowed,
simple as that. That happened 10 or 15 years ago in Alaska. Very rare.