Why altimeter settings matter
Altimeter-setting procedures have two safety objectives: preserve terrain clearance and keep aircraft vertically separated. Both depend on crews using the correct pressure reference at the correct phase of flight.
Altitude, height and flight level
| Expression | Reference | Normal setting |
|---|---|---|
| Altitude | Vertical distance above mean sea level | QNH. |
| Height | Vertical distance above a stated datum, commonly the aerodrome or runway | QFE. |
| Flight level | Surface of constant atmospheric pressure related to 1013.2 hPa | Standard pressure setting. |
Terrain and common reference
Obstacle elevations, minimum flight altitudes and instrument-procedure altitudes are referenced to mean sea level. Near terrain, a current local QNH makes the altimeter use the same datum. At cruising levels, all aircraft use 1013.2 hPa so vertical separation does not change with local sea-level pressure.
Who remains responsible
ATC passes settings and levels, but the pilot must set, cross-check and read back the correct value. A correctly cleared altitude flown with the wrong subscale setting is not a safe altitude.
QNH, QFE, QNE and standard pressure
The three Q expressions answer different operational questions. Standard pressure is a setting, while QNE is the touchdown indication produced when standard pressure must be used for landing.
| Term | What is set or supplied | Indication at the aerodrome reference point | Use |
|---|---|---|---|
| QNH | Aerodrome pressure reduced to mean sea level using the standard lapse relationship | Aerodrome elevation | Altitude and terrain clearance. |
| QFE | Pressure at the aerodrome or runway datum | Zero at that datum | Height above the QFE datum. |
| Standard pressure | 1013.2 hPa, normally spoken as 1013 | A pressure altitude, not necessarily aerodrome elevation or zero | Flight levels. |
| QNE | The calculated altimeter reading at touchdown with standard pressure set | The notified QNE value | Landing when actual pressure is outside the altimeter subscale range. |
QNE worked example
Suppose aerodrome elevation is 100 ft, the actual QNH is 930 hPa and 1013 hPa is set. The pressure difference is 83 hPa. Using 27 ft per hPa, 83 multiplied by 27 gives 2,241 ft. Add the 100 ft aerodrome elevation, giving a QNE of about 2,341 ft. ATC could instruct the aircraft to land with QNE 2,340.
Transition altitude, level and layer
Transition allows aircraft to change safely between local-pressure altitudes and standard-pressure flight levels without destroying vertical separation.
The three terms
| Term | Definition | Setting change |
|---|---|---|
| Transition altitude | Fixed altitude at or below which vertical position is expressed as altitude | On climb, set standard pressure when passing it. |
| Transition level | Lowest available flight level above the transition altitude | On descent, set QNH when passing it. |
| Transition layer | Airspace between transition altitude and transition level | Do not use it for level cruising unless specifically coordinated. |
Indian transition-altitude values
A State specifies a transition altitude for each aerodrome. Bali states that the lowest transition altitude in India is 4,000 ft. Its height above the aerodrome should be as low as possible but normally not less than 900 m, 3,000 ft. The calculated height is rounded up to the next full 300 m, 1,000 ft.
Transition level changes
Unlike transition altitude, transition level varies with QNH. Approach control or aerodrome control determines and supplies it before descent, in the approach clearance or when requested. A lower QNH normally requires a higher transition level to preserve the layer.
Flight levels and the lowest usable level
A flight level is not a fixed true altitude. It is a pressure surface related to 1013.2 hPa, so its height above mean sea level changes as atmospheric pressure changes.
System of flight levels
Flight level zero lies at the 1013.2 hPa pressure surface. Consecutive levels are separated by a pressure interval corresponding to at least 500 ft, 152.4 m, in the standard atmosphere. Intermediate reports may be made in 30 m, 100 ft, increments.
Lowest usable flight level
The lowest usable flight level is the flight level that corresponds to, or lies immediately above, the established minimum flight altitude. It must also remain above the transition altitude and preserve any required transition layer. Low QNH raises the lowest usable flight level because a standard-pressure surface lies lower above mean sea level.
Minimum flight altitude
The State determines and publishes minimum flight altitudes for ATS routes and control areas in the AIP. A cruising clearance does not remove the pilot's obligation to ensure terrain clearance. Where no transition altitude has been established, an en-route aircraft is flown at a flight level.
| Situation | Vertical-position expression |
|---|---|
| At or below transition altitude | Altitude with QNH set. |
| Above transition altitude after climb changeover | Flight level with 1013.2 hPa set. |
| Descending below transition level | Altitude with QNH set. |
| Approach using QFE under specified circumstances | Height above the QFE datum. |
Settings through each phase of flight
The required setting changes with the task. Close to the surface, the priority is terrain clearance. At cruising levels, the priority is a common vertical reference for all aircraft.
| Phase | Normal setting and report | Operational point |
|---|---|---|
| Before departure | Current QNH, or QFE for a local circuit where authorised | Verify against known elevation or height. |
| Climb below transition altitude | QNH, report altitude | Set standard pressure when passing transition altitude. |
| En route above transition altitude | 1013.2 hPa, report flight level | All aircraft share the same pressure datum. |
| Descent | Set current destination QNH when passing transition level | Report altitude after the change. |
| Final approach using QFE | QFE, report height | Use only under the stated local or procedural circumstances. |
Altimeter-setting regions
An altimeter-setting region is an area in which a common pressure value or an organised network of QNH reports supports terrain clearance below the transition altitude. Update to the applicable setting when moving between regions or closer reporting stations. The value and boundary published for the route or area control.
When current QNH is limited
- Use current QNH reports from an adequate reporting network.
- If reports are sparse, combine available QNH with forecast lowest mean sea-level pressure for the route section.
- If current information is unavailable, use the published lowest altitude or flight level derived from climatological data.
Approach and landing
Obtain the transition level before starting the approach and obtain the latest aerodrome QNH before descending below it. Charts may show an altitude followed by a height in parentheses, such as 2,000 (1,485), with the mean-sea-level altitude first and height above the stated datum second.
Pre-flight checks and pressure errors
A correct setting cannot make an unserviceable altimeter reliable. The operational check confirms both the subscale response and the displayed elevation or height.
Pre-flight operational test
- Position the aircraft at a known elevation on the aerodrome.
- Set the current QNH or QFE.
- Vibrate the instrument gently as prescribed so the mechanism settles. Do not strike the glass.
- Compare the indication with known elevation for QNH or known height above the QFE datum.
- Cross-check all installed altimeters and record or resolve an unacceptable difference.
| Altimeter test range | Maximum pre-flight error in Bali |
|---|---|
| 0 to 9,000 m, 0 to 30,000 ft | Plus or minus 20 m, 60 ft. |
| 0 to 15,000 m, 0 to 50,000 ft | Plus or minus 25 m, 80 ft. |
Pressure-setting error
Near sea level, one hectopascal represents roughly 27 ft. If the pressure set is higher than the pressure that should be set, the altimeter over-reads and the aircraft is lower than indicated. Moving from high pressure to low pressure without resetting creates the same hazard. The memory aid is, from high to low, look out below.
Static and instrument errors
Position error comes from disturbed airflow at the static source. Instrument error comes from mechanical imperfections. Lag and friction can delay response. Apply only the corrections and limitations published for the aircraft and instruments.
Cold-temperature altitude correction
An altimeter assumes the standard temperature profile. In air colder than standard, pressure levels are closer together, so true altitude and obstacle clearance are lower than the indicated values.
The cold-air hazard
When temperature is below ISA, an aircraft maintaining a published indicated altitude is physically lower than the charted value suggests. The error increases with the height above the altimeter-setting source and with the amount by which temperature is below standard. From hot to cold, the indication becomes bold: the aircraft is lower than indicated.
Applying a correction
Use the approved cold-temperature correction table or procedure. Enter with aerodrome temperature and height above the QNH source, then add the tabulated positive correction to the relevant published altitude or height. Do not reduce an altitude because of cold temperature.
| Item | Cold-temperature action |
|---|---|
| Minimum sector or procedure altitude | Add the required correction when the published procedure or State requires it. |
| Obstacle-clearance altitude | Ensure the corrected indicated altitude preserves the required true clearance. |
| ATC-assigned altitude | Coordinate any correction that changes compliance with the clearance. |
| Final approach vertical guidance | Follow approved procedure limitations and any temperature compensation. |
A quick mental approximation sometimes used for awareness is about 4 ft per 1,000 ft of height above the setting source for each degree Celsius below ISA. It is not a substitute for the approved table or aircraft system.
Cruising levels and the semicircular system
Direction-based cruising levels reduce head-on conflict. The magnetic track selects the level family, while the pressure reference decides whether that level is expressed as an altitude or a flight level.
Indian semicircular families
| Magnetic track | IFR family | VFR family where permitted | Examples below FL 290 |
|---|---|---|---|
| 000 degrees to 179 degrees | Odd thousands | Odd thousands plus 500 ft | IFR FL 050, 070, 090; VFR FL 055, 075, 095. |
| 180 degrees to 359 degrees | Even thousands | Even thousands plus 500 ft | IFR FL 040, 060, 080; VFR FL 045, 065, 085. |
RVSM and higher levels
Within RVSM airspace from FL 290 to FL 410 inclusive, the directional sequence uses 1,000 ft level increments. For tracks 000 to 179 degrees, IFR examples are FL 290, 310, 330, 350, 370, 390 and 410. For tracks 180 to 359 degrees, examples are FL 300, 320, 340, 360, 380 and 400. Above FL 410, use the modified published table and the applicable larger vertical separation.
Setting in use
At or above the lowest usable flight level, or above transition altitude as applicable, express the level as a flight level with 1013.2 hPa set. Below that boundary, express it as an altitude with QNH. VFR flights in India are not normally operated above FL 150 unless authorised, and any ATC clearance or published regional procedure takes precedence over the general directional table.