Q-Codes Reference

Pressure Setting Q-Codes

The pressure-setting Q-codes are among the most important concepts in aviation. They determine what your altimeter displays and are critical for terrain clearance, traffic separation, and safe approaches.

QNH — Altitude Above Mean Sea Level

QNH is the atmospheric pressure adjusted to mean sea level (MSL) using the International Standard Atmosphere (ISA). When QNH is set on the altimeter subscale, the instrument indicates altitude above mean sea level (AMSL). This is the most commonly used pressure setting in aviation worldwide.

• ATC provides the regional or aerodrome QNH to pilots.
• With QNH set, your altimeter reads aerodrome elevation when on the ground.
• Used for all flight below the transition altitude (typically 3,000–18,000 ft depending on the country).
• Ensures all aircraft in the same area share a common altitude reference, providing safe vertical separation from terrain and other traffic.

QFE — Height Above Aerodrome

QFE is the atmospheric pressure at the aerodrome reference point (runway threshold elevation). When QFE is set on the altimeter subscale, the instrument reads zero on the ground at that aerodrome and indicates height above aerodrome level during flight.

• Still used at some aerodromes in the United Kingdom, Russia, and several other countries.
• Convenient for circuit flying — pattern altitudes are read directly as height above the aerodrome.
• The main disadvantage is that QFE changes between aerodromes, so it cannot be used for en-route navigation or terrain clearance relative to sea level.

QNE — Standard Pressure (Flight Levels)

QNE refers to the indication on the altimeter when the standard pressure setting of 1013.25 hPa (29.92 inHg) is applied. The altimeter then indicates pressure altitude, and aircraft positions are expressed as flight levels (e.g., FL350 = 35,000 ft pressure altitude).

• Used above the transition altitude — the altitude at which pilots change from QNH to standard pressure.
• The transition level is the lowest flight level available above the transition altitude; the band between the two is the transition layer.
• Because all aircraft above the transition altitude use the same standard setting, vertical separation is maintained even though the true altitude may differ from the flight level.
• Transition altitude varies by country: 3,000 ft in parts of Europe, 5,000 ft in the UK, 18,000 ft in the USA and Canada.

QNH vs QFE vs QNE Comparison

The Four Q-Codes You Will Actually Use

Of the several hundred Q-codes in the ITU list, four turn up in a normal flying week: QNH and QFE on every departure and arrival, QNE whenever you climb through the transition altitude, and QDM if you ask a VDF-equipped unit for a steer. The rest are covered further down for completeness, but they are exam material rather than radio practice.

Worked Scenario: Joining the Circuit at a 550 ft Airfield

Popham in Hampshire sits at 550 ft AMSL, which makes it a useful example because the difference between QNH and QFE is large enough to matter. You are inbound at 2,400 ft on the regional QNH and the circuit height is 1,000 ft.

The arithmetic works because near sea level the atmosphere loses roughly 1 hPa for every 27 ft of climb. That factor stretches as you go higher (nearer 30 ft per hPa at 5,000 ft, and around 90 ft per hPa in the mid-thirties), which is why the 27 ft figure is only used for aerodrome-level conversions and why flight-level separation is defined by pressure rather than by feet.

QDM and QDR: One Bearing, Two Directions

QDM and QDR describe the same line between you and a station. QDM is the magnetic heading that would take you to it in still air; QDR is the magnetic bearing of your aircraft measured outward from it. They differ by 180°, and that is the entire relationship.

A QDM is a homing instruction, not a track. Ask for a second QDM a few minutes later; if the number has drifted from 042 to 048, the wind is pushing you left of the line and simply chasing each new QDM will curve you into the station the long way. Apply a drift correction of about twice the change and re-check. The same trap catches students homing an NDB with the ADF needle centred.

Navigation Q-Codes

Navigation Q-codes are used in direction-finding (DF) operations. Although modern navigation relies on VOR, DME, and GNSS, these codes still appear in exam syllabi and are occasionally used in ATC communications.

Communication, Meteorological & Other Q-Codes

A handful survive in HF operations, VOLMET services, and written traffic such as NOTAMs and flight plans. The table lists the codes still referenced in examinations and documentation, excluding the pressure-setting and navigation codes covered in their own sections above. Those marked Limited see occasional use (e.g., HF comms, written messages). Codes marked Obsolete are no longer used in routine voice communications but may appear in exam questions or historical references.

Regional Usage

Although ICAO standards provide a universal framework, the practical use of Q-codes — particularly pressure settings — varies significantly between countries and regions.

Exam Tips — QNH, QFE & QNE

Q-codes relating to pressure settings are heavily tested in PPL, CPL, and ATPL examinations worldwide. Here are the key points examiners focus on:

• QNH vs QFE: You must know that QNH gives altitude (above MSL) while QFE gives height (above aerodrome). A favourite exam question asks what the altimeter reads on the ground with each setting — QNH shows aerodrome elevation; QFE shows zero.
• Transition altitude / transition level: Know that the transition altitude is where you change FROM QNH TO standard pressure (1013.25 hPa), and the transition level is the lowest usable flight level above it. The gap between them is the transition layer, which is never used as a cruising level.
• “From high to low” problems: Exams commonly present scenarios where an aircraft flies from a high-pressure area to a low-pressure area without updating QNH. The correct answer is that the true altitude is lower than indicated — a collision risk with terrain.
• QDM / QDR relationship: QDM and QDR differ by 180°. If your QDR (bearing FROM the station) is 090°, your QDM (heading TO the station) is 270°. Exams test this reciprocal relationship frequently.
• QTE vs QDR: The difference between QTE (true) and QDR (magnetic) is the magnetic variation at the station. Apply variation correctly: “Variation west, magnetic best (add); variation east, magnetic least (subtract).”
• Temperature corrections: In extremely cold conditions, QNH-based altimeter readings overestimate your true altitude. Some exams test whether you know to apply temperature corrections in cold weather operations (per ICAO Doc 8168).