Q-Codes, Abbreviations & Definitions

Part I (ii) General & Aeronautical 'Q' Code signals and other abbreviations (ICAO Annex 10, Vol I & II)

Q-codes were invented for Morse telegraphy, where every character costs time. A three-letter group beginning with Q was agreed to stand for a complete phrase, so a long sentence could be sent as three letters. Cleverly, the same code can be a question or its answer depending on context: "QFE?" asks the field pressure; "QFE 1004" answers it. Voice radio later made full sentences cheap, so most Q-codes faded — but a handful proved so compact and unambiguous (and impossible to mishear as ordinary words) that aviation kept them: above all the pressure settings and a few bearings.

A standardised three-letter code, each group beginning with Q, representing a fixed phrase, listed in ICAO documentation (Annex 10 and related). It is international and language-independent, and may be used as a question or an answer.

Atmospheric pressure falls with height — roughly 1 hPa per 27–30 ft near sea level. A pressure altimeter is simply an aneroid barometer with its scale marked in feet: it measures the outside static pressure and displays the corresponding altitude. But "the corresponding altitude" depends entirely on what pressure you call the datum. The subscale (Kollsman window) lets the pilot set that datum. Setting a different datum slides the whole scale up or down — which is exactly what the pressure Q-codes select.

≈ 1 hPa = 27 ft near sea level (often rounded to 30 ft). So a 1 hPa error in the subscale setting puts the altimeter out by ≈ 27 ft; a 10 hPa error ≈ 270 ft. Always set the correct datum — this small number is why a wrong setting is so dangerous near the ground.

QNH is 1013 hPa but you mistakenly set 993 hPa (20 hPa low).
Error ≈ 20 hPa × 27 ft = 540 ft. With the lower pressure set, the altimeter over-reads — it shows you higher than you are, so you are actually ~540 ft lower than indicated. On an approach in cloud, that is the difference between clearing terrain and not.
Memory aid: "High to Low, look out below." Flying from high pressure to low (or setting too low a value) makes you lower than indicated.

Low down, near terrain, everyone wants altitude above the actual sea-level pressure (QNH). High up, for traffic separation, everyone wants a common datum so two aircraft's altimeters agree — so all switch to the standard 1013.2 (QNE) and call levels "flight levels". The switch happens at a defined boundary.

Transition Altitude — the altitude at or below which vertical position is expressed as altitude on QNH. Transition Level — the lowest flight level (on 1013.2) available above the transition altitude. Transition Layer — the airspace between the two. Climbing: set 1013 (QNE) passing the transition altitude. Descending: set QNH passing the transition level.

These four answer "which way?" relative to a station. Two are magnetic (QDM, QDR), two are true (QUJ, QTE); two point to the station, two point from it.

QDR = QDM ± 180° (the bearing from is the reciprocal of the heading to). Likewise QTE = QUJ ± 180°. And magnetic ↔ true differs by the local variation: True = Magnetic ± Variation (East least, West most → "Variation East, magnetic least").

An ADF gives QDM 090° to an NDB.
QDR (radial you are on) = 090 ± 180 = 270°.
So you are on the 270° radial (west of the station) and would steer 090° in nil wind to reach it.

QDM is 090° and the local magnetic variation is 5° West.
True bearing to the station (QUJ): with West variation, True = Magnetic − Variation = 090 − 5 = 085°.
QTE (true from) = 085 ± 180 = 265°. ("Variation West, magnetic best" — the magnetic figure is the larger.)

Altitude — the vertical distance of a level, point or object measured from mean sea level (read on QNH).
Height — the vertical distance of a level, point or object measured from a specified datum, e.g. the aerodrome (read on QFE).
Elevation — the vertical distance of a point on or affixed to the surface above mean sea level.
Flight Level (FL) — a surface of constant atmospheric pressure related to the standard datum 1013.2 hPa (read on QNE); e.g. FL100 = 10,000 ft pressure altitude.

On departure you fly on QNH reading altitude; passing the transition altitude you wind the subscale to 1013 (QNE) and read flight levels; arriving at a small field you might use QFE so the altimeter reads height above the runway. Three codes, three readings — set the wrong one and your "1,000 ft" may be 1,000 ft lower than you think (recall 4.2).