Altimetry

050-01-06Copilot, Pablo Asensio Martínez2026-04-014 min

Key Definitions

Altimeter Settings (QNH, QFE, QNE)

  • True Altitude: The actual height of the aircraft above mean sea level (MSL).
  • Indicated Altitude: The altimeter reading when set to the local QNH.
  • Pressure Altitude: The altimeter reading when set to standard pressure (1013.2 hPa). Used for flying at Flight Levels (FL).
  • QNH: Atmospheric pressure reduced to sea level according to the standard atmosphere. When setting QNH on the ground, the altimeter indicates the aerodrome elevation.
  • QFE: Atmospheric pressure measured at the aerodrome. When setting QFE on the ground, the altimeter indicates zero.
  • QFF: Atmospheric pressure reduced to sea level considering the actual temperature.

Altimeter Errors

The altimeter is essentially a barometer that measures pressure and converts it to altitude assuming standard conditions (ISA). If conditions are not standard, the indication will be erroneous.

Temperature's influence on aircraft altimeters

Pressure Error

  • High to Low: If you fly towards an area of lower pressure without adjusting the altimeter (keeping the previous higher setting), the altimeter will indicate a higher altitude than you actually have.
    • "High to Low, Look out below" -> True Altitude < Indicated Altitude. (Dangerous).
  • Low to High: If you fly towards an area of higher pressure, the altimeter will indicate a lower altitude than actual.
    • True Altitude > Indicated Altitude.

Temperature Error

  • Cold Air (Colder than ISA): The air column contracts. For the same pressure, the actual altitude is lower.
    • "Warm to Cold, Don't be bold" -> True Altitude < Indicated Altitude. (Dangerous).
  • Warm Air (Warmer than ISA): The air column expands.
    • True Altitude > Indicated Altitude.

How to Solve Altimetry Exercises

1. Pressure Correction

A barometric gradient of 30 ft per hPa (or 27 ft/hPa if specified) is assumed.

$Correction = (QNH - 1013) \times 30$

  • If QNH > 1013: Indicated Altitude > Pressure Altitude.
  • If QNH < 1013: Indicated Altitude < Pressure Altitude.

2. Temperature Correction (4% Rule)

True altitude changes approximately 4% for every 10°C of ISA deviation.

$Temp\_Correction = 4 \times (ISA\_Deviation) \times \frac{Indicated\_Altitude}{1000}$

  • Warmer than ISA: Add correction (True Alt > Indicated Alt).
  • Colder than ISA: Subtract correction (True Alt < Indicated Alt).

Note: The temperature correction applies to the air column between the station (aerodrome) and the aircraft. If calculating true altitude above the ground, use (Indicated Altitude - Station Elevation) in the formula.

Pilot Perspective in the Aircraft: Worked Example

Here is how these corrections play out in the cockpit. Let's go through a complete exercise:

Situation: You are on approach to an aerodrome on a cold winter day. You have the following information:

  • Destination QNH: 1003 hPa
  • Current Indicated Altitude: 5,000 ft (altimeter set to QNH)
  • OAT (Outside Air Temperature): -5°C
  • Aerodrome elevation: 500 ft

Step 1 – Pressure Correction (converting Indicated Altitude to Pressure Altitude):

$Correction = (1003 - 1013) \times 30 = -10 \times 30 = -300 \text{ ft}$

The QNH is below 1013, so your Pressure Altitude (with 1013 set) would be 5,300 ft. In other words, your altimeter is understating how high you are in standard pressure terms.

Step 2 – Temperature Correction (finding True Altitude):

ISA temperature at 5,000 ft = 15 − (5 × 2) = +5°C
ISA deviation = OAT − ISA = −5 − 5 = −10°C (colder than ISA)

$Temp\_Correction = 4 \times (-10) \times \frac{5{,}000}{1{,}000} = -200 \text{ ft}$

True Altitude = 5,000 − 200 = 4,800 ft

Cockpit interpretation: Your altimeter reads 5,000 ft with QNH set, but you are actually at 4,800 ft above sea level. The aerodrome is at 500 ft, so your actual clearance above terrain is only 4,300 ft — 200 ft less than the altimeter shows. On cold days like this, always add extra safety margins above terrain.

💡 Practical rule: In low pressure and/or very cold conditions, your actual altitude is always lower than the indicated altitude. The altimeter "lies in your favour" on the exam, but can be dangerous in real flight.

Transition Procedures

  • Transition Altitude (TA): The altitude at or below which vertical position is controlled by reference to altitudes (QNH). When climbing and crossing the TA, change to 1013 hPa.
  • Transition Level (TL): The lowest flight level available for use above the transition altitude. When descending and crossing the TL, change to QNH.
  • Transition Layer: The airspace between the TA and the TL.