Finally! After wishing the FAA would follow the lead of our friends to the north concerning errors associated with the altimeter and colder than standard weather we have a listing of airports that are required to temperature compensate their altitudes when the temperature is below a certain threshold.
Please read the notice to airman concerning the change. Here.
Background: In response to aviation industry concerns over cold weather altimetry errors, the FAA conducted a risk analysis to determine if current 14 CFR Part 97 instrument approach procedures, in the United States National Airspace System, place aircraft at risk during cold temperature operations. This study applied the coldest recorded temperature at the given airports in the last five years and specifically determined if there was a probability that during these nonstandard day operations, anticipated altitude errors in a barometric altimetry system could exceed the Required Obstacle Clearance (ROC) used on procedure segment altitudes. If a probability, of the ROC being exceeded, went above one percent on a segment of the approach, a temperature restriction was applied to that segment. In addition to the low probability that these procedures will be required, the probability of the ROC being exceeded precisely at an obstacle position is extremely low, providing an even greater safety margin.
For most professional pilots compensating for temperature is as easy as turning the feature on in the flight management system (FMS). For general aviation pilots, we must use the chart provided in the AIM 7-2-3.
To use this chart, you must know the temperature at the airfield through a weather report and you must know the AGL altitude for each of the segments you are going to use to fly the approach. For example, on a standard ILS with a 5NM final approach fix the AGL height leading up to the FAF is 1500 feet. If the temperature was
-20°C it is likely the aircraft is 210 feet lower than where it should be. To comply with the new notice to airman, the pilot should elect to fly 1,710 feet AGL instead of 1,500 AGL.
Pilots must report cold temperature corrected altitudes to Air Traffic Control (ATC) whenever applying a cold temperature correction on an intermediate segment and/or a published missed approach final altitude. This should be done on initial radio contact with the ATC issuing approach clearance. ATC requires this information in order to ensure appropriate vertical separation between known traffic. Pilots must not apply cold temperature compensation to ATC assigned altitudes or when flying on radar vectors in lieu of a published missed approach procedure. Pilots should query ATC when vectors to an intermediate segment are lower than the requested intermediate segment altitude corrected for temperature. Pilots are encouraged to self-announce corrected altitude when flying into uncontrolled airfields.
The FAA has listed 283 airfields as cold weather airfields with 307 different segments. The segments that must be corrected are intermediate, final and missed approach. There are 24 airports that have multiple segments listed.
The picture above shows the number of segments that must be temperature corrected in each state. No surprise that Alaska has 87 different segments. Another interesting fact is that all airports are above the 35° latitude line.
There are quite a few airports where the temperature compensation is required and a current listing can be located on the Terminal Procedures Basic Search page on the Aeronav website. A link directly to the PDF.
In future changes to the approach plates a “snowflake” symbol will be used to identify airports where temperature compensation must take place. It will be the pilots responsibility to apply the correct compensation and inform ATC of the new altitude that are going to be used.