We learn about the effects of temperature on our altimeters in Private Pilot training, but it's usually an aside, or something memorized for the test - "High to Low or Hot to Cold, look out below..." But if it gets really cold like it has this winter in much of the country, there can be some really big, and dangerous, effects!
I got an e-mail from a listener to my interview on the Stuck Mic AvCast. He is a pilot in Vermont, and asked the following question (I have edited for length and content):
"What kind of temperature spread does TERPS use to calculate required obstacle clearance for altitudes on stepdown fixes for an ILS? (i.e -10C to +20C etc...) At BTV [Burlington, VT] we have an ILS 33... East of the airport is the green mountain range and so the glideslope is at 3.2 degree with stepdown fixes leading up to the FAF.
The crossing altitude at NIDUQ is 5500' but if you notice the highest prominent obstruction in the area is a mountain named "Camel's Hump" about a half mile north of NIDUQ. Camel's Hump peak is 4088'.
As you'd figure, in the winter time it gets very cold here in Vermont. The question is does 5500' crossing at NIDUQ meet required obstacle clearance when its say -20C?"
This is a GREAT question. We learn in our pilot training that temperatures below standard (15 C at sea level) cause the altimeter to read higher than we really are - or to turn it around, the plane is lower than the altimeter reads. This is discussed some in the Aeronautical Information Manual, section 7-2-3, and in the Instrument Flying Handbook,starting on page 5-5. It is typically described as the pressure levels "bunching up" in cold weather. See Figure 5-6 from the IFH below:
So the airplane is lower than we think. But how much? The error is increased by two factors - 1) the current temperature at the "reporting point" (usually the airport) and 2) the height above that reporting point. There's even a chart! Figure 5-7 from the IFH:
Important - notice again that the temperature is that recorded at the airport, not at your current altitude.
You can see that for mildly cold temperatures and altitudes close to the station, there is only a little difference. But for even colder temperatures and higher altitudes above the station, the error can be in the hundreds of feet! This, of course, is only an estimate, and relies on certain assumption about lapse rate - nonstandard lapse rate or temperature inversions will affect this calculation.
Let's go back to the approach in question, KBTV ILS RWY 33. This approach has a segment that passes over the Camel's Hump mountain. Now, the writer asked if 5500 MSL to NIDUQ offered sufficient clearance, but the real question is does the 4800 MSL to HONIB clear the obstacle? Let's work it out!
Camel's Hump is 4088 MSL and is shown on the chart between NUDIQ and HONIG. Minimum altitude on this segment is 4800. Let's say the temperature at BTV is -20C. Does it ever get to -20C (-4F) in Burlington, VT? Absolutely, at least several times a year (source - aviationweather.gov).
KBTV is at 335 MSL. The segment altitude, therefore, is 4800-335 = 4465 ft above the station elevation. Using the chart above, find where -20C intersects 4465 ft. There is no entry for 4465 ft, but that's about halfway between 4000 and 5000 ft, so I'd say the altitude error is about halfway between them, or 640 ft. And remember, this is in the WRONG direction - so you and your altimeter think you're at 4800 MSL, but you're really at 4800-640=4160 MSL. That 4088 MSL mountain is looking pretty close, isn't it? How accurate is your altimeter? The general rule on preflight is within 75 feet, so you might be at 4160-75=4085. Now you're 3 feet below the peak of that mountain. And that's only if you're maintaining altitude right on 4800 on the altimeter - have you ever been 20 or 40 feet below an assigned altitude? Of course, everyone has.
So what do you do?
Back to one of the original questions - what consideration does TERPS (the approach procedure design rules) make for low temperatures? The answer, in the vast majority of cases (except RNAV (RNP) procedures and Baro-VNAV minimums on an RNAV (GPS) procedure) is "none"!
This correction is left up to the pilot. Some more advanced avionics systems may compensate for this, but that is on a case-by-case basis, and you'd have to read your specific flight manual. Back to the IFH:
Under extremely cold conditions, pilots may need to add an appropriate temperature correction determined from the chart in Figure 5-7 to charted IFR altitudes to ensure terrain and obstacle clearance with the following restrictions:
* Altitudes specifically assigned by Air Traffic Control (ATC), such as "maintain 5,000 feet" shall not be corrected. Assigned altitudes may be rejected if the pilot decides that low temperatures pose a risk of inadequate terrain or obstacle clearance.
* If temperature corrections are applied to charted IFR altitudes (such as procedure turn altitudes, final approach fix crossing altitudes, etc.), the pilot must advise ATC of the applied correction.
For me, any time the airport is reporting around -10C or colder, I'll be paying much closer attention to that correction!