Cold-temperature altimeter errors

Introduction Altimeter Error Calculator Instrument Approach Dangers

The barometric altimeter is like the magnetic compass, an instrument of unquestioned reliability. All of the other stuff in the panel serves one purpose: to tell you if you are flying in the right direction at the right altitude.

But both of these instruments are subject to error. The major source of error in the altimeter is non-standard temperature. The altimeter is a pressure instrument alone, but the relationship between pressure and height changes when the temperature changes. When the air is cold, each foot of altitude gained has more effect on the pressure than it does when the air is warm. Hence "high to low, look out below!".

Some countries, notably Canada, require pilots and controllers to add a correction to charted altitudes in cold weather conditions. Below is an excerpt from the altitude correction chart used in Canada. It shows how much you must add to the charted altitude in order to be at the proper altitude. You use the surface temperature (in Celsius) to choose the row and the height above the altimeter source to choose the column.

0 0204080220

For example, when the surface temperature is -40C and you want to fly 2000' above the airport, your altimeter should read 2400' above airport elevation.

In the USA, the FAA does not require cold-temperature corrections, but a table is published in the Airman's Information Manual, paragraph 7-2. It is a little different from the Canadian table: the FAA table assumes a sea level airport, while the Canadian table assumes that the airport is at 2000MSL. The FAA is the more conservative. Here is an excerpt.

0 203060120280
-20 3070140280710
-40 501202404801210

Temperatures of -20C are not unusual in Northern-tier states. At these temperatures your altimeter reads 30' high at decision height on an ILS approach; in other words, you are 30' lower than you think. Imagine noting in your pre-takeoff checks that the altimeter is reading 50' above field elevation, a perfectly legal error. Thus, on your approach, you are really at 150' when the altimeter says that you are at decision altitude. Now, on a cold morning you get a little low on the glideslope; this could put you at 110', and the cold temperature error could put you another 50' lower, at sixty feet!

Worse, imagine flying a procedure turn 5000' AGL when the temp is -40: you are really at 4000'AGL, in other words, all of your required obstacle clearance has been used up by the cold weather effect.

This script calculates the altimeter error caused by cold temperatures. Standard lapse rate (-2C/1000ft) is assumed.

Airport elevation (ft)
Surface Temperature (Celsius)
Indicated Altitude

For information purposes only. Not for navigation.

The basic formula is that (p/p_0) = (T/T_0)^5.25608 where p is pressure and T is absolute temperature. We use this to determine standard atmosphere (p_0 = 29.92"Hg) and to determine pressure readings when p_0 is standard pressure at the airport and T_0 is actual temperature.

What about instrument approaches?

As you may know, the FAA is implementing the Wide Area Augmentation System (WAAS) and has already commissioned LNAV/VNAV approaches which use Vertical Navigation to implement a pseudo-glideslope. Decision Heights for these approaches are 200' and 250', respectively. FAA approach design criteria (as specified in FAA Orders 8260.47 and 8260.48) adjust the required obstacle clearance area on the final approach course in relation to the coldest temperature one can expect at the airport: at cold-weather airports, more obstacle clearance is required. This has the disadvatage that the approach will then have a minimum allowable temperature. When this is the case there will be a note on the plate that reads something like "Proc NA when temp below -15C".

But, as far as I can tell, they do not adjust altitudes or obstacle clearance on the intermediate or initial segments of an instrument approach. The TERPS manual (FAA order 8260.3) specifies 1000' of obstacle clearance during initial approach segments, and 500' of obstacle clearance during intermediate segments. Yet, this is where the cold temperature errors are the largest and most dangerous. The combination of high terrain and low temperature can easily erode these margins. Try this on the calculator: airport elevation 2000', intermediate approach segment altitude 6000', and temperature -30C: the cold temperature error is over 500'! In other words, you have no terrain clearance if you fly the charted altitudes.

So, be careful out there, and remember, "high to low, look out below!"