Mode C Altitude Reporting
A Mode C/S transponder reports pressure altitude referenced to 1013.25 hPa; dividing by 100 gives the flight level, independent of the local altimeter setting.
Formula
Variables
Example
Pressure altitude 35000 ft → FL350
How it works
In Mode C or Mode S altitude reporting, the transponder sends pressure altitude, which is altitude measured against the standard pressure setting of 1013.25 hPa rather than the local weather-based altimeter setting. Dividing that pressure altitude in feet by 100 converts it into the flight level number used by ATC, so 29,000 ft becomes FL290. This works because flight levels are defined on the standard pressure datum, making them comparable across all aircraft regardless of local pressure differences.
When to use it
Use this when converting a transponder-reported pressure altitude into the equivalent flight level, especially in high-altitude operations and air traffic control communications. It is also useful when checking whether an aircraft’s reported altitude should be spoken as a flight level rather than a height above mean sea level.
Worked example, step by step
Pressure altitude = 18,400 ft FL = PA / 100 FL = 18,400 / 100 FL = 184 Final answer: FL184
Common mistakes to avoid
- Using the local altimeter setting instead of the standard 1013.25 hPa reference
- Forgetting that the division is by 100, not by 1,000
- Treating the result as a true geometric altitude instead of a flight level
Did You Know?
Every transponder transmits altitude at the standard 1013.25 hPa datum, so ATC always sees consistent levels even when different regions set wildly different altimeter pressures.
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