Introduction
One of the most insidious hazards in winter instrument flying is the effect of cold air on altimeter accuracy. A standard altimeter is calibrated to the International Standard Atmosphere (ISA), which assumes a sea-level temperature of 15°C and a lapse rate of approximately 2°C per 1,000 feet. When the actual air temperature is colder than ISA, the air is denser and shallower than the model predicts, meaning the aircraft is physically lower than the altimeter indicates. In extreme cold, the difference between indicated and true altitude can be significant — enough to erode obstacle clearance margins on an instrument approach.
To address this hazard, the FAA established the Cold Temperature Restricted Airport (CTRA) program, codified in the Aeronautical Information Manual (AIM) Section 7-3. Designated airports on the CTRA list require pilots to apply cold temperature altitude corrections to certain segments of instrument approach procedures when the surface temperature falls at or below a published threshold. Understanding when, where, and how to apply these corrections is an essential competency for Airline Transport Pilot (ATP) certificate applicants and professional instrument pilots.
How Cold Temperature Affects Altimetry
The aneroid altimeter measures static pressure and converts it to an altitude reading based on a standardized pressure-altitude relationship. That relationship assumes standard atmospheric temperatures at every level. When the actual temperature is colder than standard, a given pressure level sits physically closer to the ground than the ISA model predicts. The result: the altimeter over-reads. The aircraft believes it is at, say, 3,000 feet MSL when it is actually lower. The colder the temperature relative to ISA, and the higher the indicated altitude above the elevation where the altimeter is set, the larger the error grows.
A useful rule of thumb widely cited in FAA and ICAO cold temperature error guidance is: roughly 4 feet of altitude error per 1,000 feet of height above the elevation of the temperature source for every 10°C the surface temperature is below ISA standard temperature at that elevation. This approximation illustrates how the error compounds with height above the airport and with colder temperatures — an approach at a high-elevation airport in mountainous terrain, flown at very cold temperatures, can produce errors that meaningfully erode obstacle clearance if uncorrected.
The Cold Temperature Restricted Airport (CTRA) Program
The FAA publishes the Cold Temperature Restricted Airports list, which identifies specific airports where cold temperature altitude corrections are mandatory during instrument approaches. Each airport on the list has two pieces of published information: a restricting temperature (the surface OAT at or below which corrections are required) and the affected altitude segments of the approach procedure (for example, the final approach fix altitude, the missed approach holding altitude, or intermediate segment altitudes).
Airports are added to the CTRA list when a risk analysis determines that the temperature-induced altimeter error, combined with local terrain and obstacle environments, creates unacceptable safety margins on published instrument approaches. The list is maintained and updated in the Chart Supplement (formerly Airport/Facility Directory) and is also referenced in NOTAM format. Pilots operating at non-CTRA airports are not required by FAA regulation to apply corrections, but the AIM strongly encourages pilots to apply corrections at any airport when conditions warrant it.
Where to Find the CTRA List
The CTRA list is published in the U.S. Chart Supplement (available in FAA digital products) and is updated on the standard 56-day charting cycle. Each entry includes the airport identifier, the restricting temperature threshold, and notation of which approach segment altitudes require correction. Applicable cold temperature correction information may also appear on individual approach charts published by authorized chart providers, particularly in the notes or margins of affected procedures.
When Corrections Are Required
At a CTRA airport, corrections are mandatory when the reported surface OAT is at or below the published restricting temperature for that airport. The surface temperature is obtained from the current METAR or ATIS for the destination airport. The pilot — or, in a Part 121/135 operation, the flight crew — must apply the appropriate correction to the altitudes on the approach procedure that the airport's CTRA designation specifies.
Corrections apply to the specific published altitudes identified in the airport's CTRA listing, such as the FAF crossing altitude, intermediate and step-down fix altitudes, MDA, and missed approach altitudes. The corrections increase these altitudes to be flown, adding a buffer that compensates for the altimeter's tendency to over-read in the cold. These higher corrected altitudes are what the crew must fly; they do not change the altimeter setting — the crew continues to set the current altimeter setting in the Kollsman window and simply adds the calculated correction to the published altitude, then flies that corrected altitude.
Calculating Cold Temperature Corrections
The FAA provides a Cold Temperature Error Table (AIM 7-3-3, Table 7-3-2) that pilots use to determine the correction. The table is entered with two variables:
- Airport OAT — the reported surface temperature at the destination airport in degrees Celsius.
- Height of the minimum altitude above the airport elevation — the published minimum altitude (in feet MSL) minus the airport elevation (in feet MSL), giving the height above the altimeter setting source.
The intersection of these two parameters in the table yields a correction value in feet. That value is added to the published minimum altitude to obtain the corrected altitude to fly. For example: suppose a final approach fix (FAF) minimum altitude is 3,400 feet MSL and the airport elevation is 1,200 feet MSL, giving a height above airport of 2,200 feet. If the surface OAT is −20°C, the table might indicate a correction of approximately 300–400 feet (exact values depend on the table). The crew would then fly the FAF crossing at roughly 3,700–3,800 feet MSL, even though the chart shows 3,400 feet.
For ATP and professional flight crew training, many operators publish approved cold temperature correction tables or electronic EFB tools that automate this lookup. In all cases, the underlying data source is the FAA Cold Temperature Error Table found in AIM 7-3-3, Table 7-3-2.
Which Altitudes Are Corrected
Not every altitude on an approach requires correction at every CTRA airport. The specific altitude segments requiring correction are published in the airport's CTRA listing. Commonly corrected segments include:
- Initial approach fix (IAF) crossing altitudes
- Intermediate segment minimum altitudes
- Final approach fix (FAF) crossing altitude
- Step-down fix altitudes on non-precision approaches
- Missed approach holding minimum altitudes
- Circling minimum descent altitudes (MDA)
The Decision Altitude (DA) or Decision Height (DH) on a precision approach or approach with vertical guidance (APV) is generally not corrected at most CTRA airports, because the glidepath itself will keep the aircraft on the correct geometric path. However, pilots should verify this for each specific airport and approach, as some CTRA designations may address this differently. Missed approach segment altitudes, including holding pattern minimum altitudes at or after the missed approach point, are typically corrected.
Why It Matters: Safety and Regulatory Context
The consequences of failing to apply cold temperature corrections can be fatal. In documented accidents and incidents involving cold temperature altimetry errors, aircraft have descended below obstruction clearance surfaces during instrument approaches, with some events resulting in controlled flight into terrain (CFIT). The CTRA program is the FAA's structured response: by mandating corrections at the highest-risk locations, the program ensures that professional flight crews maintain the required obstacle clearance margins regardless of atmospheric temperature.
From a regulatory standpoint, operating at a CTRA airport without applying the required corrections when the OAT is at or below the threshold means the aircraft would actually be flying below the effective altitude needed for obstacle clearance, even though the charted altitude is being flown. This undermines the obstacle clearance margins built into the procedure and the operating minimums framework applicable under Parts 91, 121, and 135, and is treated by the FAA and operators as a serious safety-of-flight issue requiring strict compliance with the CTRA correction requirement.
Key Numbers and Rules
- Error direction: Cold temperatures always cause the altimeter to over-read — you are lower than indicated. Never the reverse.
- Correction direction: Always add the correction to the published altitude. Never subtract.
- Triggering condition: Corrections are mandatory at CTRA airports when surface OAT is at or below the published restricting temperature.
- Table inputs: Surface OAT (°C) and height of the altitude above airport elevation (feet).
- Altimeter setting unchanged: The Kollsman window still receives the current local altimeter setting; only the target altitude changes.
- Corrected segments: Cold temperature corrections apply to the specific published altitude segments identified in the airport's CTRA listing (e.g., FAF, step-down fixes, MDA, missed approach altitudes).
- CTRA list location: U.S. Chart Supplement, updated every 56 days.
- Non-CTRA airports: Corrections are not required by the FAA, but the AIM strongly recommends applying them whenever significant cold temperature errors are possible.
Common Test Traps
- Thinking cold means you are higher than indicated. The opposite is true — cold air is denser, pressure surfaces are closer together, and the altimeter over-reads. You are always lower than the altimeter shows in cold temperatures.
- Subtracting the correction instead of adding it. Because you are lower than indicated and must fly higher to compensate, corrections are always additive.
- Applying corrections to altitude segments not identified in the CTRA listing. Only the specific published altitude segments identified for that airport receive the cold temperature correction.
- Assuming the DA/DH always needs correction. On approaches with vertical guidance, the glidepath itself provides geometric guidance; the DA typically does not require a cold temperature correction, though you must verify for the specific CTRA airport and approach.
- Forgetting that the altimeter setting stays the same. Some students mistakenly alter the Kollsman setting. The altimeter setting remains the published current setting; only the target altitude to be flown changes.