Cracking the ‘Q’ Code

by Fly News 

No, this has nothing to do with barmy, dangerous conspiracy theories, but rather with the rarefied language of aircraft communications. These days airline pilots can make contact almost instantaneously with any part of the world, via voice or data link, thanks to satellites in orbit. But there was of course a time when they relied on radio, Morse code, and even light signals from the ground.

But of course, during long-haul and even medium-haul flights, pilots commonly fly over countries which speak a language foreign to their own, so that can naturally complicate communications. So English has been established as the world’s lingua franca of aviation since the Chicago Convention on International Civil Aviation in 1944 and the 1951 ruling of the United NationsInternational Civil Aviation Organisation; it was not, however, till 2008 that pilots were required to pass English proficiency tests as a condition of receiving a licence to fly.

Just to back up and provide a little history here, in 1909 the British government instituted a series of three-letter codes for ship-to-shore communications, all starting with the letter Q (for question), and three years later 45 of these codes were adopted by the International Radiotelegraph Convention. Over the years, the number of Q codes was expanded; their use extended to aviation; and some codes (QAA to QNZ) were added specifically for use by aircraft. So apart from the universal use of English, this is another means of communications that all pilots can use regardless of what languages they speak.

How does it work? For example, here’s an exchange you might hear from between a pilot and control tower in Spain: “QAF Madrid 11:30 15000, QAM? Burgos QBK 12000”.  Meaning that the aircraft passed Madrid at 11:30 am at 15,000 feet and the pilot requests the latest weather data for Burgos, to which the flight is currently headed at 12,000 without clouds in the vicinity. And of course if the code is followed by a question mark, it’s a query – so for example “QAF? Madrid” means “when will you be or were you over Madrid?” Or “QBK?”is asking the pilot if he or she is flying near cloud formations.  Other cloud-related codes include QBG (flying above), QBF (flying amidst), and QBH (flying below).

Now, while it’s true that with the increasing prevalence of voice communications and more recently of data links, Q codes have been used less in aviation, but there are still certain codes which remain in use, although usually not with their original meanings. So when pilots receive weather data whilst taking off or landing, in addition to wind speed, temperature, dew point (the temperature to which air must be cooled to become saturated with water vapor), height to the base of any cloud cover, and vertical/horizontal visibility, the code QNHXXX is added, meaning as a statement “if you set your altimetre subscale to XXXX millibars, the instrument will mark your elevation if you were on the ground at this airport at that time”, or as a question, “What should I set my altimetre subscale to indicate my elevation if I were on the ground at your airport”?

FYI, aircraft altimetres by the way, measure atmospheric pressure according to a standard reference, and based on that reference they indicate an aircraft’s position above or below a point – at sea level or at a specific point on the ground such as an airport. Thus indicating the QNH means that the altimetre will mark the altitude of the runway in relation to sea level.

Nonetheless, the altimetre can also be adjusted by the QFE, which is the setting at which the altimetre will read zero when on the runway. These days the QFE is little used, since the aeronautical charts used by pilots are guided indicate each airport’s altitude above sea level.

Atmospheric pressure also depends on air temperature. As this varies, in aviation a standard atmosphere is defined, and used as a reference in all aircraft that fly above the so-called transition altitude. By having all their altimetres set to 1,013.2 millibars or 29.92 inches – regardless of whether the temperature and pressure are higher or lower than the standard atmosphere – all aircraft in the area will have altimeters that will indicate the same flight altitude, which may be above or below the actual distance above the ground. But they will all have the same error, so this one is cancelled out. Finally, if they tell us about the QFE, they are indicating the height above the pressure layer of those 1,013.2 millibars. This, depending on the existing conditions, may even be below the median sea level.

Another code still in use is QMS, which refers to the axis of a runway, and more specifically its extension beyond its physical limits. So an aircraft cleared to cross the QNS of runway XX at 1,500 feet is being told that it can pass from one side of the runway to the other at 1,500 ft. This makes it possible to keep the axis of the runway free of other aircraft so that another plane taking off or landing on said runway knows that there isn’t another one in its way.

Finally, “QEG, QEJ? QEK” means “May I leave parking position, taxi, and enter the runway? I’m ready to take off”.

(By the way, although as we noted above most Q codes have largely been discontinued in aviation (and for that matter, maritime) communications, there is one community in which they’re still in common use: ham radio operators.)

And one last aside: the military – especially in NATO – utilise have their own code, only the first letter is Z instead of Q.

That’s all, folks – we hope you’ve found this avgeekily interesting!