Q Codes Explained

Matthew Stibbe posted a comment asking about Q Codes:

Now if only someone could explain the origin of the Q-codes (e.g. QNH, QFE and the ever-mysterious QSY which means ‘I’m changing frequency en route’ but why?)

Q codes were developed in 1907 in order to facilitate communication with other nations for which there was no common language. This was primarily used in Morse Code in order to get some basic understanding of vital information. For example, if you wanted to know someone’s true bearing, you could just send the “QRC” code.

Using Q codes provides two main benefits:

1. Frequency usage remained to a minimum as you only had to transmit three characters
2. The user on the other end didn’t need to speak English to understand the question.

This was mainly used for shipping, however when aviation became popular it was being used by Aviators to communicate with one another. Back in the day a sample communication would look like the following, remember in morse code:

GJX GJX DE GADHL GADHL GADHL – GM – TEST – QRK – QSA – K

Translated means, “Eastleigh from G-ADHL – Good morning – Testing – Do you receive me well? – Are my signals good? What is the strength of my signals? – Please reply” The from and to call signs were repeated to ensure that users knew who the message was intended for.

Notice the QRK and QSA codes are used.

A reply would look like:

GHL GHL GHL DE GJX – GM – QRK – QSA 5 – K

Translated: “G-ADHL [abbreviated] from Eastleigh – Good morning – Receiving you well – The strength of your signal is 5 – Reply”

Once voice communication became prevelant and there were no need for Morse Code anymore the Q codes got dropped. Well, most of them did. There are some still in use today:

QNH – Known in North America as the altimeter setting. You could hear “QNH 1013.2” instead of “altimeter setting: 29.92”.

QFE – Not used in North America at all. It is the altimeter setting at the station where the indicated altitude would show 0′ ASL.

As far as I can tell, there are no order to Q codes and seem to be randomly generated for each phrase.

Question to Aussie/Brit pilots out there, other than QNH, QFE, and QNE, do you use any other ones?

For example would ATC say:

“”callsign” QMI your current position”.

For which you would reply:

“QMI is scattered at 4000′ and broken at 8000′”.