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.
• Normally used for altitude reference at or below the applicable transition altitude; follow published local procedures.
• Provides a common altitude reference. QNH alone does not ensure terrain clearance or traffic separation; temperature, procedures and other factors still matter.

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.

Standard pressure / 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 is location-specific. Check the current AIP and chart for the area; there is no single 5,000 ft value for the whole UK. Change from standard pressure on descent according to the applicable transition level and procedure.

QNH vs QFE vs QNE Comparison

Terminology note: UK CAA guidance uses “standard pressure”; QNE is sometimes used informally for this setting. With QFE, height is relative to its reference datum, not necessarily the terrain directly below. Even with correct QNH, non-standard temperature can make true altitude differ from indicated altitude.

Pressure settings in a worked example

The examples below distinguish altitude, height and bearing. Which settings and codes you use depends on your location and the applicable procedures.

Worked Scenario: Joining the Circuit at a 550 ft Airfield

Consider a fictional aerodrome at 550 ft AMSL with a training circuit height of 1,000 ft. The following exchange illustrates the numbers only; it is not current aerodrome information or a joining procedure.

The 27 ft per hPa relationship is a near-sea-level approximation. The relationship changes with altitude and atmospheric conditions, so this calculation is a teaching example, not a substitute for an issued pressure setting.

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.

Check the procedure for your location

Pressure-setting practice, units and transition altitudes vary. Use the current AIP, chart and applicable instructions for the place you are flying; broad country tables can hide local exceptions.

• Identify the published transition altitude and the applicable transition level.
• Confirm whether a quoted level is altitude, height or a flight level.
• Read back the setting with its reference and confirm the units.

For UK context, see the CAA altimetry tutorial. For pressure and temperature errors, see the FAA altimeter guidance. Targeted corrections made 1 October 2026.

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).