I am a system engineer dealing with Thales ATC product. In my system there is a server called sensor data processor, one of its function is to receive QNH data to do altitude correction. I can't understand this meaning since the system receives the Flight level from the Mode C which is calculated base on the QNH value given to the pilot from the ATC.

  • $\begingroup$ Above the transition level there is no need to correct the Mode C flight level, but below the transition level there is a need. Are you certain that your system will never ever handle any aircraft below the transition level? $\endgroup$
    – randomhead
    Sep 1, 2023 at 13:05
  • $\begingroup$ My system handle the QNH. But I am confusing and have miss understanding about the meaning and significance of altitude correction.. I still confused. May you kindly discribe the meaning? $\endgroup$ Sep 1, 2023 at 20:59

1 Answer 1


"Flight level" and "altitude" are not synonymous. They are two different concepts.

When aircraft are flying fast and flying high above the earth, pilots do not want to be constantly fiddling with the altimeter. So above a certain height, every aircraft has its altimeter set to the standard altimeter setting of 1013 hPa (29.92 inHg). When an aircraft is using this standard altimeter setting we say that it is flying at some flight level. Because the true altimeter setting (QNH) varies from day to day, any given flight level will move up and down relative to the surface of the earth. But that's okay because everyone is using the same standard and it doesn't matter if FL290 happens to be 28,600' or 29,300'.

But when aircraft are lower, it matters how high they are above sea level. In that case they set their altimeter to the local QNH, and the altimeter shows their altitude.

The transponder in the aircraft does not know or care what setting the pilot is using. It simply reads the outside air pressure and encodes that air pressure into an altitude assuming that the local QNH is standard. In other words, the transponder will always transmit the aircraft's flight level even if the aircraft should be using altitude instead of flight level. This means that your radar system must take the transmitted flight level and convert that into an altitude for display to ATC. In order to convert the flight level into an altitude, it must know what the correct QNH is.

  • $\begingroup$ It's called Mode C correction factor. $\endgroup$
    – RetiredATC
    Sep 2, 2023 at 19:10

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