Corrected Altitude (Altitude Adjusted for Errors) – Aviation Glossary

Definition

Corrected Altitude is the value used by pilots after adjusting the barometric altimeter reading to account for deviations from International Standard Atmosphere (ISA) conditions, specifically temperature (and, less commonly, pressure) anomalies. It ensures the aircraft’s true height above mean sea level (MSL) does not fall below published minimums, maintaining obstacle and terrain clearance—especially during instrument approaches in cold weather. The adjustment is critical for preventing controlled flight into terrain (CFIT) and is mandated by global aviation authorities such as ICAO, FAA, and EUROCONTROL.

Corrected altitude is also known as temperature-corrected altitude or altimeter temperature error correction, and is not achieved by changing the altimeter setting, but by adding a correction to published minimum altitudes and flying at or above the adjusted values using the current QNH.

Altimeter Fundamentals and Standard Atmosphere

The barometric altimeter measures static air pressure, converting it to altitude based on the ISA, which assumes:

• Sea-level pressure: 1013.25 hPa (29.92 inHg)
• Sea-level temperature: +15°C
• Temperature lapse rate: -2°C per 1,000 ft

Key altimetry terms:

• Indicated Altitude: Altimeter reading with QNH set
• True Altitude: Actual height above MSL (after corrections)
• Pressure Altitude: Altitude above standard datum plane (set to 1013.25 hPa)
• Density Altitude: Pressure altitude adjusted for temperature/humidity (impacts performance)
• QNH: Local MSL pressure setting
• QFE: Field elevation setting (altimeter reads zero on ground)
• QNE: Standard pressure setting for flight levels

Instrument procedures define minimum altitudes (MSA, MVA, MDA, DA) to guarantee clearance from obstacles and terrain, but these are valid only under ISA conditions. Deviations—especially cold temperatures—can make indicated altitude unreliable, necessitating corrections.

Why Corrections Are Needed

Altimeters assume ISA conditions. In reality, atmospheric temperature and pressure vary. The most significant operational error source is temperature deviation:

• Cold Weather: Air is denser, pressure decreases faster with altitude. The altimeter overreads—aircraft is lower than indicated. This is hazardous near terrain.
• Warm Weather: Air is less dense, pressure decreases slower. The altimeter underreads—aircraft is higher than indicated. This rarely poses a terrain hazard but can affect airspace separation.

Rule of thumb: For every degree Celsius below ISA, true altitude is about 0.4% less than indicated per 1,000 ft. Operationally:
4 ft per °C per 1,000 ft above reporting station.

Example: At -30°C (45°C below ISA) and 4,000 ft above the reporting station:
4 ft × 45 × 4 = 720 ft error (aircraft is 720 ft lower than indicated).

Barometric pressure errors are corrected by updating QNH; temperature errors require altitude corrections.

Procedures for Applying Corrected Altitude

When to Apply Corrections

Corrections are required when surface temperatures fall below published thresholds (typically 0°C or as specified on approach charts/NOTAMs). Airports with special cold temperature requirements are marked as Cold Temperature Airports (CTAs).

Corrections are mandatory:

• For all instrument approach segments in cold weather
• In mountainous areas or segments with high terrain/obstacle proximity
• When charted or NOTAMed

ATC may provide temperature-corrected minimums in some regions, but pilots remain responsible for corrections unless explicitly stated.

How to Apply Corrections

1. Obtain surface temperature at the airport/reporting station.
2. Determine height above reporting station for each relevant segment (published altitude minus airport elevation).
3. Use official correction tables (ICAO, FAA, EUROCONTROL, operator’s manual) or the 4 ft/°C/1,000 ft rule.
4. Add correction to the published minimum altitude for each segment.
5. Keep QNH unchanged on the altimeter.
6. Notify ATC if corrections affect critical approach segments.

Example:

• Temp: -20°C (35°C below ISA at sea level)
• Segment: 2,000 ft above airport
• Correction: 4 × 35 × 2 = 280 ft (add to the minimum altitude)

Automation

Modern FMS/VNAV systems can automate these corrections if properly certified. Pilots must enter correct data, confirm corrections, and ensure autopilots use the adjusted values.

Responsibilities

Pilots are always responsible for applying corrections and maintaining obstacle/terrain clearance. ATC may vector at corrected altitudes in some airspaces, but the final responsibility lies with the pilot.

Practical Implications and Safety

Obstacle and Terrain Clearance

Failure to apply corrections in cold weather can erase or reverse designed safety margins, especially in mountainous terrain or where minimum altitudes are close to terrain height.

Example:
Approach segment at 4,026 ft MSL, peak at 4,088 ft MSL, -30°C:
Without correction, aircraft could be up to 800 ft lower than indicated—below the terrain.

Common Errors

• Not getting up-to-date weather/temperature data
• Misapplying correction tables
• Failing to apply corrections to all required segments
• Changing the altimeter setting instead of adjusting the minimum altitude
• Not notifying ATC when required

Regulatory Guidance

• FAA AIM 7-3-3: Procedures and lists of restricted airports
• ICAO PANS OPS Doc 8168: Global standard for cold temperature operations
• EUROCONTROL Guidance: Harmonized procedures for Europe

Examples and Use Cases

Manual Correction

Aircraft approaches at an airport with a published minimum of 4,026 ft, surface temp -30°C, segment 4,500 ft above field.
Correction: 4 × 45 × 4.5 = 810 ft (add to minimum altitude).

FMS Automation

Modern FMS (e.g., Garmin G3000) can correct approach altitudes automatically when pilots input the current surface temperature. Always verify system certification and data entry.

Minimum Sector Altitude (MSA)

Operators may require adding a standard buffer (e.g., 1,000 ft) to MSA in extreme cold if not already corrected on charts.

Example: ICAO Cold Temperature Correction Table (Excerpt)

Reference: ICAO PANS OPS Doc 8168; use full tables for precise corrections.

Checklist for Applying Corrected Altitude

1. Identify segments requiring correction (approach charts, NOTAMs, AIP).
2. Obtain current surface temperature (ATIS, METAR).
3. Calculate height above reporting station for each segment.
4. Consult official correction table/tool.
5. Apply correction to minimum altitude (do not change QNH).
6. Notify ATC if corrections affect controlled segments.
7. Verify FMS/autopilot corrections if using automation.

Regulatory References

• FAA AIM 7-3-3
• ICAO PANS OPS Doc 8168
• EUROCONTROL Cold Temperature Correction Guidance
• Transport Canada eCAPGEN
• SKYbrary, AOPA, Airhead ATPL

Glossary of Related Terms

Indicated Altitude: Altimeter reading with QNH set.
True Altitude: Actual height above MSL after applying corrections.
Pressure Altitude: Altitude with altimeter set to 1013.25 hPa.
Density Altitude: Pressure altitude adjusted for temperature/humidity (affects performance).
QNH: Altimeter setting for airport MSL.
QNE: 1013.25 hPa standard setting for flight levels.
QFE: Altimeter reads zero at field elevation.
Minimum Descent/Decision Altitude (MDA/DA): Lowest authorized approach altitude.
Minimum Vectoring Altitude (MVA): ATC vectoring minimum, may/may not be corrected.
Minimum Sector Altitude (MSA): Sector-wide minimum for obstacle clearance.
Cold Temperature Airports (CTA): Require corrections per chart/NOTAM.

Frequently Asked Questions: Corrected Altitude

Q: Why is corrected altitude essential in cold weather operations?
A: Cold air causes the altimeter to overread, putting the aircraft lower than indicated. Corrected altitude ensures terrain and obstacle clearance is maintained, preventing CFIT.

Q: How do pilots calculate the corrected altitude?
A: By finding the difference between actual and standard temperature, multiplying by height above the reporting station, and referencing official tables or using 4 ft/°C/1,000 ft. The correction is added to the minimum altitude.

Q: Do pilots change the altimeter setting when applying corrected altitude?
A: No. The QNH remains unchanged; the correction is added to the published minimum altitude.

Q: Who is responsible for applying corrected altitude—the pilot or ATC?
A: The pilot is responsible. ATC may provide corrected altitudes in some cases, but the pilot must verify and apply corrections as needed.

Q: Are there automated systems for corrected altitude?
A: Yes, many modern FMS/autopilots can automate corrections. Pilots must check certification, input correct data, and verify system output.

Corrected altitude procedures are critical for safe flight in non-standard conditions, especially during instrument approaches in cold weather, mountainous terrain, or both. Proper application of corrections preserves safety margins and complies with international aviation regulations.