Cold temperature corrections for IAPs

[BeechBoy]

In my Jepps I have a table labeled "Cold Weather Corrections". Basically, it's the practical application of the "high to low, look out below" phenomena as it applies to temperature. You can also work it out on your whiz wheel.

I was surprised at the magnitude of the error that below-freezing temperatures can create. For example, suppose we're doing the DME arc into Rhinelander, Wisconsin with a published altitude of 3500 feet (2000 AGL) and a temperature at the surface of -20C. Even though your indicated altitude is 3500 feet your TRUE altitude is 3220 feet. The same thing applies to your DA. In this case if you flew down to the published DA you would really be at 170 above TDZE rather than 200.

From doing some informal questioning it appears that most pilots do not make a habit of applying these corrections when they fly an approach in below freezing temperatures.

I'm curious if there are any airlines that specifically train their pilots to do so.

 

[Flysher]

Good post. I always wondered this as well. You would think they would make a notation such as "increase DH/MDA 50 feet for temperatures below XX".

The only other thing that I could speculate is that perhaps TERPS already builds in a buffer with the current DH/MDAs.

 

[ananoman]

The only other thing that I could speculate is that perhaps TERPS already builds in a buffer with the current DH/MDAs.

No, they don't. In the lower 48, it is usually not much of an issue. At 200' AGL on an ILS, it has to be pretty cold to make a big difference. Same on a non-precision at 400-500 AGL.

If you are curious, there is also a correction table in the AIM. Look at Table 7-2-3. You can see that at 200 AGL when it is -15C, you will be about 25' low. For a non-precision at 500 AGL, you would be about 50' low.

The one note you will commonly see on US approach plates is a notice on GPS approaches that have LNAV/VNAV minimums. You will see a note that 'Baro VNAV is not authorized below -15C'. These approaches use your FMS and Air Data Computer to generate a glideslope. So, when it is cold, it affects all of it's calculations and it can't hold the correct angle for the glideslope.

For WAAS approaches with LPV minimums that use GPS to generate glide slope information, this does not apply.

 

[seagull]

The Canadians and Russians had these charts for years. You would believe how much of a fight it was to get FAA to publish them at all, let alone actually spread the word on the issue!

Take a whiz-wheel and play with these corrections sometime. It gets a LOT worse than that chart goes!

 

[Foxcow]

Excellent post. I have to admit, I am one of the many that have not applied those corretions.

 

[Tommay85]

None of this was even mentioned to me during my intrument training and I'm up in the frozen tundra. Granted, I probably should have noticed it on my own I think it's something my flight or ground instructor should have pointed out. Then again, what the heck am I going to hit in North Dakota. Approach minimums could probably be 5 feet and you'd be alright.

 

[fish314]

In the USAF, we are required to correct all altitudes inside the FAF or GS intercept altitude, as well as some others, depending on the conditions. Depending on where you fly, though, you may or may not get much practice at it. I quoted the applicable regulation below. I don't think the FAR/AIM is quite this regulatory, so some (or all) of this may not apply to civilians, but here's what we do in the Air Force:

(AFI 11-217Vol1, chapter 8):

...8.1.4. Cold Weather Altimeter Corrections.​

​

Pressure altimeters are calibrated to indicate true altitude under International Standard Atmospheric (ISA) conditions. Any deviation from these standard conditions will result in an erroneous reading on the altimeter. This error becomes important when considering obstacle clearances in temperatures lower than standard since the aircraft’s altitude is below the figure indicated by the altimeter. The error is proportional to the difference between actual and ISA temperature and the height of the aircraft above the altimeter setting source. The amount of error is approximately 4 feet per thousand feet for each degree Celsius of difference. Corrections will only be made for Decision Heights (DHs), Minimum Descent Altitudes (MDAs), and other altitudes inside, but not including, the Final Approach Fix (FAF). The same correction made to DHs and MDAs can be applied to other altitudes inside the FAF. For thecurrent cold weather altimeter correction procedure, you must refer to the Flight Information Handbook (FIH)...​

8.1.4.1.​

​

To ensure adequate obstacle clearance the values derived from the chart below will be:​

−​

Added to the published DH or MDA and step-down fixes inside the FAF whenever the outside air temperature is less than 0° Celsius​

−​

Added to ALL altitudes in the procedure in Designated Mountainous Regions whenever the outside air temperature is 0° Celsius or less​

−​

Added to ALL altitudes in the procedure whenever the outside air temperature is -30° Celsius or less, and/or procedure turn, intermediate approach altitude HATs/HAA are 3000 feet or more above the altimeter setting source​

−​

ATC will continue to apply correction to Minimum Vectoring Altitudes...

...Example: Published MDA 1180’ MSL
HAT 402
Temp -30° C
Correction 60’​

MDA to use: 1180 + 60 = 1240​

’ MSL​

NOTE:​

​

Pilots should advise ATC of corrections in excess of 80 feet.​

 

[Foxcow]

None of this was even mentioned to me during my intrument training and I'm up in the frozen tundra. Granted, I probably should have noticed it on my own I think it's something my flight or ground instructor should have pointed out. Then again, what the heck am I going to hit in North Dakota. Approach minimums could probably be 5 feet and you'd be alright.

You know the same is true for me when I was up there. I don't even recall seeing anything like that in the NOS chart pack. I am looking through my jepps now.

 

[B767Driver]

In my Jepps I have a table labeled "Cold Weather Corrections". Basically, it's the practical application of the "high to low, look out below" phenomena as it applies to temperature. You can also work it out on your whiz wheel.

I was surprised at the magnitude of the error that below-freezing temperatures can create. For example, suppose we're doing the DME arc into Rhinelander, Wisconsin with a published altitude of 3500 feet (2000 AGL) and a temperature at the surface of -20C. Even though your indicated altitude is 3500 feet your TRUE altitude is 3220 feet. The same thing applies to your DA. In this case if you flew down to the published DA you would really be at 170 above TDZE rather than 200.

From doing some informal questioning it appears that most pilots do not make a habit of applying these corrections when they fly an approach in below freezing temperatures.

I'm curious if there are any airlines that specifically train their pilots to do so.

Delta has cold temperature correction cards that correct for altitude on each segment of an instrument approach. I have a set in my bag...but never use them. I think they are only recommended for temps under -20C...and don't see that too often.

 

[BeechBoy]

The thing that puzzles me is that we have to learn this stuff in instrument ground school, it's on the written test, it's frequently covered in the oral portion of the test (which ARE good ideas, don't get me wrong), but when it comes to the practical application (keeping the airplane out of trees, power lines, and other non-airplane-friendly objects) the civilian system is, for the most part, silent.

According to B767 Driver, Delta trains its pilots on the procedure. They are the only ones outside of the military that I've heard do this. Granted, at DA/MDA we're only talking about 20-30 feet but we shouldn't be going below minimums at all unless the runway environment is in sight. Also, as I previously pointed out, when we do a DME arc we can be tooling around 300' below the minimum altitude. Imagine doing that on a checkride and just telling the examiner that "they have a buffer built into these altitudes anyway". (Then wonder why you got one of those pink certificates).

Anyway, I really appreciate all the input everyone provided.

 

[seagull]

I think that other airlines do teach this, actually.