Cold Temperature Correction

Cold Temperature Correction refers to the essential process of adjusting published instrument flight altitudes to mitigate the effects of barometric altimeter errors in colder-than-standard atmospheric conditions. This is crucial for maintaining obstacle clearance and flight safety during instrument approaches, particularly in regions with severe winter climates.

Why Is Cold Temperature Correction Needed?

Barometric altimeters, the primary altitude reference in most aircraft, assume the International Standard Atmosphere (ISA) temperature and pressure lapse rates. When the outside air is much colder than ISA, the altimeter overstates the aircraft’s true altitude above ground. This error can be several hundred feet on approach, especially at airports surrounded by high terrain or obstacles.

The risk is summarized in aviation with the phrase:
“From Hot to Cold – Look Out Below.”

When uncorrected, these errors can result in aircraft descending below published minimum altitudes, increasing the risk of Controlled Flight Into Terrain (CFIT) or obstacle collision.

How Does the Error Occur?

The pressure levels in the atmosphere compress closer to the ground when temperatures are lower than standard. The altimeter, using the ISA model, interprets the same pressure as a higher altitude than it actually is.

• For every 10°C below ISA, the altimeter error increases by about 4% of the height above the airport.
• At −20°C, flying an approach segment 3,000 feet above a cold airport, the indicated altitude can be up to 240–320 feet higher than the true altitude.

Where Is Cold Temperature Correction Required?

Cold temperature correction is required:

• At airports listed on national or regional Cold Temperature Airports (CTA) lists.
• On approach procedures marked with the ❄ snowflake symbol (found in the notes section of approach plates).
• When the reported airport temperature is at or below the published threshold for that airport and procedure.

Regions with frequent cold temperature operations include North America, Northern Europe, and parts of Asia.

Regulatory Criteria and Triggers

Cold Temperature Airports (CTA) Lists

Authorities like the FAA and ICAO maintain lists of airports and procedures requiring cold temperature correction. These specify:

• The airport ICAO code.
• The temperature threshold (in °C/°F).
• Which approach segments need correction (initial, intermediate, final, missed approach).

Pilots must check these lists during flight planning.

The Snowflake Symbol

A snowflake (❄) on a U.S. Government approach chart signals that cold temperature correction is required on one or more segments of that procedure when the temperature is at or below the published threshold.

Affected Segments

Depending on the procedure and airport, correction may be required for:

• Initial segment (from Initial Approach Fix to Intermediate Fix)
• Intermediate segment (Intermediate Fix to Final Approach Fix)
• Final segment (Final Approach Fix to DA/MDA or step-down fixes)
• Missed approach segment (to holding altitude or intermediate fixes)

Only the segments specified on the CTA list or approach plate require correction.

The Correction Process

Manual Correction Using Official Tables

Official correction tables (found in the FAA’s Aeronautical Information Manual or ICAO PANS-OPS) provide the number of feet to add to published segment altitudes, based on temperature and height above the airport.

Step-by-Step:

1. Identify the Segment: Determine which approach segment(s) require correction.
2. Calculate Height Above Airport: Subtract airport elevation from the segment’s published altitude.
3. Select the Table Row and Column: Find the row for the reported temperature and the column for the height above airport.
4. Read the Correction: The intersection gives the number of feet to add.
5. Apply the Correction: Add this number to the published approach segment altitude. Do not adjust the altimeter setting.
6. Round Up: For operational safety, round corrections up to the next 10 or 100 feet as appropriate.

Example Table (FAA AIM):

Important: For altitudes above the highest column, use the highest value.

Automated Correction (FMS/Avionics)

Many modern avionics (e.g., Garmin G1000, Honeywell, Collins) allow pilots to input the surface temperature, then auto-calculate and display corrected altitudes for each fix. This minimizes workload and error risk.

• Enter the reported temperature into the system.
• The system adjusts approach altitudes and step-down fixes.
• Verify which segments are compensated; manually adjust if needed.
• Corrections for intermediate or missed approach segments must be reported to ATC.

Segment-Specific Application

Corrections are only applied to the approach segments specified by the CTA list or approach chart notes. For multi-segment procedures, each correction is calculated separately based on height above the airport at each fix.

Do not apply corrections to ATC-assigned vectoring altitudes unless coordinated with ATC.

Compliance Notes

• Altimeter remains set to QNH/ATIS value.
• Corrections are additive—always add the correction to the published altitude.
• Report to ATC when correcting intermediate or missed approach segments.
• No report is needed for final segment corrections (inside the FAF).

Practical Examples

Example 1: Missoula, MT (KMSO)

Conditions:

• Approach: RNAV (GPS) Y RWY 12
• CTA threshold: −12°C
• Airport elevation: 3,206 ft
• FAF (6,200 ft), MDA (4,520 ft)
• Reported temp: −12°C

FAF Correction:

• Height above airport: 6,200 − 3,206 = 2,994 ft (~3,000 ft)
• Table at −10°C: 180 ft; at −20°C: 240 ft
• Interpolate for −12°C: 180 + (2/10)(240−180) = 192 ft (round to 200 ft)
• Corrected FAF: 6,200 + 200 = 6,400 ft

MDA Correction:

• Height above airport: 4,520 − 3,206 = 1,314 ft (~1,500 ft)
• Table at −10°C: 90 ft; at −20°C: 120 ft
• Interpolate for −12°C: 90 + (2/10)(120−90) = 96 ft (round to 100 ft)
• Corrected MDA: 4,520 + 100 = 4,620 ft

ATC Report:
“Request 6,400 feet for cold temperature operations from ODIRE to SUPPY.”

Example 2: Jackson Hole, WY (KJAC)

Conditions:

• Approach: S-19
• CTA threshold: −26°C
• Airport elevation: 6,451 ft
• FAF (8,500 ft), MDA (7,840 ft), Missed Approach (14,000 ft)
• Reported temp: −28°C

FAF Correction: 8,500 − 6,451 = 2,049 ft (~2,000 ft). Table at −30°C: 210 ft → 8,710 ft
MDA Correction: 7,840 − 6,451 = 1,389 ft (~1,500 ft). Table at −30°C: 160 ft → 8,000 ft
Missed Approach Correction: 14,000 − 6,451 = 7,549 ft (use 5,000 ft column). Table at −30°C: 540 ft → 14,540 ft

ATC Report:
“Require 8,710 feet from LAGIC to KARCE for cold temperature operations.”
“Require final holding altitude 14,540 feet on missed approach for cold temperature operations.”

Automated Example

For aircraft with FMS compensation, simply:

• Load the approach.
• Enter the reported temperature.
• The system adjusts all required segment altitudes.
• Verify that corrections are applied to all necessary fixes.
• Advise ATC as needed.

Meteorological Basis

ISA assumes a sea-level temperature of 15°C and a lapse rate of −2°C/1,000 ft. When the real temperature is colder, pressure levels compress closer to the surface, so the altimeter overstates altitude. The greater the cold deviation and the higher the segment above the airport, the larger the correction needed.

Always use current AWOS, ASOS, ATIS, or validated online sources for temperature.

ATC Communication and Reporting

• Intermediate/missed approach corrections: Must be reported to ATC, e.g., “Require 6,520 feet for cold temperature operations.”
• Final segment corrections: No ATC notification needed.
• Uncontrolled airports: Self-announce on CTAF, e.g., “Missoula traffic, Cessna 12345, inbound RNAV Y RWY 12, applying cold temperature corrections.”

Operational Best Practices

• Plan for cold temperature corrections during flight preparation in winter climates.
• Use up-to-date CTA lists and official correction tables.
• Double-check segment heights and corrections.
• Use automated FMS features where available, but always verify.
• Communicate corrections clearly to ATC when required.
• Do not adjust the altimeter setting—correct the target altitude only.

Summary

Cold Temperature Correction is a vital safety procedure for instrument approaches in cold climates. By understanding and applying these corrections, flight crews ensure compliance with regulations and, most importantly, maintain critical obstacle clearance. Whether using manual calculations or modern avionics, proper cold temperature correction is a hallmark of safe, professional airmanship.

For the latest regulatory guidance and tables, always consult the FAA Aeronautical Information Manual, ICAO PANS-OPS, and your regional authority publications.