Learn what the 29.92 altimeter setting means, when pilots select standard pressure, how it relates to 1013 hPa, and when to restore local QNH.
An altimeter setting of 29.92 inHg is standard atmospheric pressure at mean sea level, equivalent to 1013.25 hPa. Pilots select it above the applicable transition altitude so every aircraft uses the same pressure reference and reports altitude as a flight level; below the transition level, they normally set local QNH.
In our Aviation & Real-World Flying coverage, the distinction we stress is that 29.92 is a common datum, not a report of the pressure surrounding an aircraft at cruise altitude. It lets pilots and air traffic control separate aircraft using standardised pressure surfaces.
What does 29.92 inHg do to an altimeter?
Setting 29.92 makes the altimeter display pressure altitude relative to the standard-pressure datum. A barometric altimeter senses static air pressure and converts it into an indicated altitude; changing the pressure setting changes the reference from which that altitude is calculated.
With standard pressure selected, altitude is expressed as a flight level in hundreds of feet. For example, FL240 represents a nominal pressure altitude of 24,000 feet. It does not guarantee that the aircraft is exactly 24,000 feet above mean sea level, because non-standard pressure and temperature affect true altitude.
Altimeters calibrated in hectopascals use 1013.25 hPa, normally displayed or spoken as 1013. Instruments using inches of mercury show 29.92 inHg. Many glass cockpits and airliner panels simply display STD.
When do pilots set 29.92?
Pilots select 29.92 or STD when climbing through the published transition altitude and restore the local altimeter setting when descending through the applicable transition level.
| Phase | Pressure reference | Vertical position reported as |
|---|---|---|
| Below the transition altitude | Local QNH or altimeter setting | Altitude in feet above mean sea level |
| Climbing through the transition altitude | Select 29.92 inHg, 1013 hPa or STD | Flight level |
| Descending through the transition level | Restore the current local setting | Altitude in feet above mean sea level |
The transition altitude is the published changeover altitude used during a climb. The transition level is the lowest usable flight level above it and is used for the descent changeover. The airspace between them is the transition layer.
What is the rule in the United States?
In US-controlled airspace, aircraft use 29.92 inHg at and above 18,000 feet MSL and operate using flight levels. On descent below FL180, pilots change to the local altimeter setting; low atmospheric pressure can make FL180 or higher levels unavailable as cruising levels.
Is the transition altitude always 18,000 feet?
Outside the United States, the transition altitude may be much lower and can differ between airports or regions. The transition level may also change with local pressure, so pilots use the applicable chart, ATIS information and ATC instructions rather than applying the US 18,000-foot rule everywhere.
A clearance to a flight level is not necessarily an instruction to select STD immediately. The pressure change is made at the locally prescribed changeover point.
Is 29.92 the same as QNH?
29.92 is the fixed standard setting, while QNH is a local pressure setting that changes with weather and location. With QNH selected, an altimeter should indicate the aerodrome elevation while on the ground, within normal instrument and atmospheric tolerances.
Local QNH can coincidentally be 29.92 inHg. If an ATIS reports “altimeter two niner niner two” while the aircraft is below the transition altitude, the same number is entered, but it remains the local setting; it does not turn the aircraft's altitude into a flight level. Our explanation of how QNH differs from standard pressure covers that change of reference in more detail.
What happens if the wrong altimeter setting is left in?
The altimeter displays an incorrect altitude for the phase of flight, creating a risk of a level deviation or reduced terrain clearance.
- Local pressure retained after climbing: the cockpit indication no longer uses the same standard datum as other aircraft flying at assigned flight levels.
- STD retained during descent: the indicated altitude does not correspond to the local QNH reference used for approach and terrain clearance. If QNH is 28.92 but 29.92 remains set, the altimeter reads roughly 1,000 feet too high, leaving the aircraft lower than indicated.
- Wrong pressure units: 29.92 belongs in an inHg scale; the equivalent for an hPa scale is 1013. Mixing the units should produce an obviously invalid entry or indication.
- Only one instrument changed: aircraft may have separate captain, first-officer and standby altimeters. Each required instrument must show the correct reference.
How do you set standard pressure in a flight simulator?
Select STD on aircraft that provide a standard-pressure mode, or turn the barometric control until it shows 29.92 inHg or 1013 hPa. The exact control and display differ between analogue panels, glass cockpits and add-on aircraft.
- Confirm the units: identify whether the instrument is displaying inHg or hPa.
- Use local pressure below transition altitude: enter the reported QNH rather than automatically using 29.92.
- Change at the transition altitude: select STD, 29.92 or 1013 while climbing through the applicable altitude.
- Reset during descent: obtain the destination pressure and enter it when passing the transition level, then check the main and standby indications.
Automatic altimeter commands can replace STD with local pressure on some simulator-aircraft combinations, so verify the displayed mode after using one. For the Microsoft simulator's pressure sources and controls, see our walkthrough for entering QNH in Microsoft Flight Simulator 2024.