Circling Approach Obstacle Clearance Basis

[Nick]

Before I skim through TERPS looking for this I want to check if anyone already knows the answer to my question. Is sea level, standard temperature used to determine the 300 foot obstacle clearance for a circling approach?

I'm curious because on a very cold day it gets to be less than 300 feet and the type of conditions it is based on would determine just how close you are to obstacles.

[SteveC]

My understanding is that altimeters are ISA based. You should correct for temperature when it's really cold out.

Here's some stuff from the AIM:
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AIM 7.2.3

c. Temperature also has an effect on the accuracy of altimeters and your altitude. The crucial values to consider are standard temperature versus the ambient (at altitude) temperature. It is this "difference" that causes the error in indicated altitude. When the air is warmer than standard, you are higher than your altimeter indicates. Subsequently, when the air is colder than standard you are lower than indicated. It is the magnitude of this "difference" that determines the magnitude of the error. When flying into a cooler air mass while maintaining a constant indicated altitude, you are losing true altitude. However, flying into a cooler air mass does not necessarily mean you will be lower than indicated if the difference is still on the plus side. For example, while flying at 10,000 feet (where STANDARD temperature is -5 degrees Celsius (C)), the outside air temperature cools from +5 degrees C to 0 degrees C, the temperature error will nevertheless cause the aircraft to be HIGHER than indicated. It is the extreme "cold" difference that normally would be of concern to the pilot. Also, when flying in cold conditions over mountainous country, the pilot should exercise caution in flight planning both in regard to route and altitude to ensure adequate en route and terminal area terrain clearance.
d. TBL 7-2-3, derived from ICAO formulas, indicates how much error can exist when the temperature is extremely cold. To use the table, find the reported temperature in the left column, then read across the top row to locate the height above the airport/reporting station (i.e., subtract the airport/reporting elevation from the intended flight altitude). The intersection of the column and row is how much lower the aircraft may actually be as a result of the possible cold temperature induced error.
e. The possible result of the above example should be obvious, particularly if operating at the minimum altitude or when conducting an instrument approach. When operating in extreme cold temperatures, pilots may wish to compensate for the reduction in terrain clearance by adding a cold temperature correction.

TBL 7-2-3
ICAO COLD TEMPERATURE ERROR TABLE



EXAMPLE
Temperature -10 degrees Celsius, and the aircraft altitude is 1,000 feet above the airport elevation. The chart shows that the reported current altimeter setting may place the aircraft as much as 100 feet below the altitude indicated by the altimeter.

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If I remember correctly there are (or were at one time) some restrictions against airlines flying into Alaska with certain altimeter/temperature combinations that were "off-the-chart". Heard that somewhere, but can't find the source. Northwest maybe?

[BrettInLJ]

Definitely a good idea to raise your minimums appropriately as far as indicated goes when it gets that cold out. I believe its common place up in Alaska.