The setting determines the altimeter reference
A barometric altimeter converts pressure into an indication only after a reference setting is selected. With QNH, it indicates altitude above mean sea level and should show approximately aerodrome elevation on the ground. With standard pressure 1013.25 hPa, it indicates a flight level.
QNH and standard pressure are not interchangeable at will. The correct setting depends on flight phase, transition procedures and ATS instructions.
QNH, QFE and standard pressure
| Setting | Reference | Typical use |
|---|---|---|
| QNH | Altitude above mean sea level | Normally below transition altitude |
| QFE | Height relative to the selected datum | Only where required by local procedures |
| Standard | Pressure level based on 1013.25 hPa | Flight levels at or above transition level |
Transition altitude and transition level
The transition altitude is the altitude at or below which vertical position is normally expressed using QNH. The transition level is the lowest usable flight level above it. The intervening space is the transition layer.
Do not invent a fixed transition level: it can vary with pressure and local procedures. Check the AIP, charts, ATIS and ATS information.
Correct climb and descent sequence
- Before take-off, set current QNH and compare the indication with aerodrome elevation.
- In the climb, crossing transition altitude, set standard pressure and use flight levels.
- Before descent, obtain current QNH and transition level.
- Crossing transition level, set QNH and use altitudes.
- Cross-check the indication and required vertical clearance.
Pre-flight altimeter check
Part-NCO requires the pilot in command to ensure that the aircraft is fit for the intended operation. Applicable AMC material describes a practical check against a known aerodrome reference; an efficient altimeter should indicate that reference plus instrument height within the applicable tolerance, commonly ±60 ft for this check.
This does not authorize an internal correction or replace maintenance. Do not rely on an improvised adjustment if the indication is outside the applicable limit.
QNH does not provide terrain clearance by itself
Indicated altitude alone does not guarantee clearance from terrain, obstacles or controlled airspace. Use current charts, minimum altitudes, airspace limits and suitable allowances for pressure, temperature and instrument error. Pressure altitude and density altitude serve different performance purposes.