Cold Wx Minima Correction?

[FoxHunter]

When the temperature at a reporting airport is cold altimeters always read higher than the true altitude. The accepted correction is 4% per 10 deg below ISA. If the reporting is -25C it is ISA-40 which yields a 16% error.

In this case if you set the ILS Mins to the published 200' HAT you would actually be 168' HAT when your altimeter read 200'. If you were flying a VOR approach with MDA of 500' HAT you would actually be 420' HAT when your altimeter read 500'. Circle at 1000' HAT, your actual altitude is actually 840'.

Legal, not legal to not make the correction for the cold wx altimeter error???????

 

[MooseTrax]

This is what the military uses. There is a chart that accompanys this with temperatures on one axis and HAA/HAT on the other axis which leads you to the appropriate correction. Here is the link to an FAA draft I found on the net. It says you are responsible and gives some charts for guidance. I would say based on all of this it is not only legal, but expected.

http://www.faa.gov/regulations_policies/rulemaking/historical_documents/1999/media/ac91-xx.pdf


3. TEMPERATURE ERRORS​

a. Pressure altimeteres are calibrated to indicate true altitudes 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.

b. The error is proportional to the difference between actual and ISA temperature and the height of the aircraft above the altimeter setting source. Height above altimeter source is considered to be published HAT or HAA for the approach. The amount of error is approximately 4 feet per thousand feet for each degree Celsius of difference.

c. Corrections should be made to all altitude inside the FAF to include Decision Heights (DHs), Decision Altitude (DAs), Minimum Descent Altitude(MDAs) as well as any step down fix altitudes. See service regulations for corrections to the FAF altitude and other altitudes outside the FAF.

​

 

[nosehair]

!!!aaaaaarrrrrrrrrrrrrrrrrrrrrrrrggggggggggggggghhhhhhhhh!!!!!

 

[FoxHunter]

This is what the military uses. There is a chart that accompanys this with temperatures on one axis and HAA/HAT on the other axis which leads you to the appropriate correction. Here is the link to an FAA draft I found on the net. It says you are responsible and gives some charts for guidance. I would say based on all of this it is not only legal, but expected.

http://www.faa.gov/regulations_policies/rulemaking/historical_documents/1999/media/ac91-xx.pdf


3. TEMPERATURE ERRORS

​

a. Pressure altimeteres are calibrated to indicate true altitudes 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.

b. The error is proportional to the difference between actual and ISA temperature and the height of the aircraft above the altimeter setting source. Height above altimeter source is considered to be published HAT or HAA for the approach. The amount of error is approximately 4 feet per thousand feet for each degree Celsius of difference.

c. Corrections should be made to all altitude inside the FAF to include Decision Heights (DHs), Decision Altitude (DAs), Minimum Descent Altitude(MDAs) as well as any step down fix altitudes. See service regulations for corrections to the FAF altitude and other altitudes outside the FAF.



​

I agree that the correction should be made. That FAA draft AC has been around for some time never acted upon. My guess is that a large percentage of pilots trained in the USA have no idea of the problem. The FAA seems to be in the "Don't ask, Don't tell" mode on this one. ICAO says the PIC is responsible. You can be sure if there is ever a problem the FAA would be right there to help.

 

[Flyboy6420]

There are some approaches out there that do have temperature restrictions, reading something like "Procedure N/A when temperature is -15C or less".

So far I've only seen this on RNAV approaches, but it would indicate that the FAA is aware of the issue. I would think that there are TERPS criteria which require such a restriction, but I am not personally aware of such a standard.

I would never fault a pilot for not descending to the published MDA if they had a good reason for doing so. If you have a reasonable concern, which I believe that you do, than it would certainly be prudent to err on the converative side.

Very good question.

 

[ERJdca]

There are some approaches out there that do have temperature restrictions, reading something like "Procedure N/A when temperature is -15C or less".

As far as i know you may still fly the approach to the LNAV MDA and not LNAV/VNAV DA. I don't see how it can be illegal to add height to your DA with respect to the intitial question. I admit i never gave it much of a thought till i started to fly out of the US. We have a Temp/HAT correction minima in our company manual.

Good Ques.

 

[Prof. ATP]

[...]My guess is that a large percentage of pilots trained in the USA have no idea of the problem. The FAA seems to be in the "Don't ask, Don't tell" mode on this one. ICAO says the PIC is responsible

This has been a big issue with me for quite a long time. The danger should be more widely publicized.

The error is most dangerous when you are high, not when you are low. Look at some IAPs to mountainous airports (Jackson Hole, WY; Montrose, CO, etc). Some initial approach altitudes are more than 5000' above the altimeter source. Assume a surface temp of -30C (not unheard-of), and apply the table AIM 7-2-2. There is roughly 1000' of altimeter error. On these segments, the terrain clearance is 1000', so a little "flight technical error" or instrument error could leave you sharing the cockpit with a lot of rocks and snow.

Under the right circumstances it is possible to hit terrain at FL180.

Hot-and-high with perfect technique is OK; cold-and-high with perfect technique kills you.

It is legal to make a correction, as long as you inform ATC. The corrections are required in Canada.

 

[MTpilot]

"!!aaaaaarrrrrrrrrrrrrrrrrrrrrrrrggggggggggggggghh hhhhhhh!!!!!"

Could you maybe be a little more specific?