What is QNH and how do you set it in a flight simulator?
In aviation and flight simulation, QNH is the local atmospheric pressure reduced to mean sea level. Set it on the aircraft’s altimeter with the BARO control, in hectopascals or inches of mercury. With the correct QNH selected, an aircraft on the ground should indicate the airfield’s elevation, within normal reporting and instrument tolerances.
What does QNH mean on an altimeter?
QNH makes an altimeter indicate approximate altitude above mean sea level rather than height above the ground. It is a three-letter aviation Q-code, not initials that expand into a useful modern phrase.
The pressure measured at an airport is adjusted mathematically to a mean-sea-level value. Aircraft in the same area can then use a common pressure reference even when operating from airfields at different elevations. QNH does not make indicated altitude identical to true geometric or GPS altitude, especially in unusually cold or warm air.
QNH, QFE and standard pressure
| Setting | Typical value | What the altimeter indicates | Normal use |
|---|---|---|---|
| QNH | Local pressure, such as 1016 hPa or 30.00 inHg | Approximate altitude above mean sea level | Departure, arrival and flight below the applicable transition level |
| QFE | Pressure at a selected airfield datum | Zero at that datum | Used only where local procedures call for it |
| Standard | 1013.25 hPa or 29.92 inHg | Pressure altitude expressed as a flight level | Flight above the published transition altitude |
How do you set QNH in a flight simulator?
Set QNH by obtaining the applicable pressure, checking its unit and entering the value through the simulated altimeter or BARO control.
- Obtain the pressure setting. Use the simulator’s ATIS or automated weather report, an ATC instruction or the METAR associated with the weather being simulated.
- Check the unit. A value such as
Q1017means 1017 hectopascals. A value such asA2998means 29.98 inches of mercury. Hectopascals and millibars are numerically equivalent for altimeter settings. - Select QNH rather than STD or QFE. Glass cockpits normally show the selected mode beside the pressure value. On a conventional altimeter, the pressure appears in the Kollsman window.
- Turn the BARO or altimeter-setting knob. Enter the exact reported value. Mouse-wheel, drag and hardware-axis behaviour depends on the aircraft and simulator.
- Set every required instrument. Captain, first-officer and standby altimeters may have separate controls. Some aircraft synchronise them; others do not.
- Cross-check the indication. On the ground, the altimeter should be close to the elevation of your actual parking position or runway threshold. It may not exactly match the published airport reference elevation.
Should you use the automatic altimeter shortcut?
An automatic altimeter-setting command is convenient, but manual entry is better when practising real procedures. Automatic commands may use the simulator’s nearest weather station, which can differ from a pressure value supplied by online ATC or an external weather report.
Where do you find the correct QNH?
The best QNH is the pressure supplied by the weather and ATC environment in which the simulated aircraft is operating. ATIS, automated airport weather, ATC and METAR reports are the usual sources.
In a METAR, look for a group beginning with Q for hectopascals or A for inches of mercury. Our guide to decoding METAR pressure groups and weather fields explains how those formats fit into the complete report.
Real-world METAR data and a simulator’s weather engine can disagree because of reporting times, interpolation or a custom weather preset. If the aircraft is using simulated live weather, its in-sim ATIS usually provides the pressure that best matches the simulated atmosphere. When flying under online ATC, use the controller’s pressure setting so your indicated altitude agrees with the altitude reference used for separation.
When should you change from QNH to standard pressure?
On a climb, change from QNH to standard pressure when passing the published transition altitude; on descent, restore the issued QNH at the transition level or when local procedure directs.
Do not assume the transition altitude is the same worldwide. It varies by country and sometimes by airport. Above it, aircraft use 1013.25 hPa or 29.92 inHg and report flight levels rather than altitudes. Our practical IFR flying workflow covers how this change fits into clearances, descent preparation and instrument checks.
Why is the simulated altitude wrong after setting QNH?
A large altitude error usually comes from the wrong pressure unit, STD mode, mismatched weather data or an altimeter that was not set independently.
- Wrong units: confirm that a four-digit value near 1000 is hPa, while a value near 30 is inHg.
- STD still selected: many glass displays show
STDprominently while hiding or replacing the local pressure value. - Different weather sources: an external METAR may not match the pressure generated by the simulator at the aircraft’s position.
- Only one instrument was changed: check both primary altimeters and the standby instrument where fitted.
- Scenery elevation differs: a parking stand, sloping runway or inaccurate airport mesh may not match the published airport elevation.
- GPS and pressure altitude are being compared: GPS altitude is geometric, while an altimeter derives altitude from atmospheric pressure.
- Non-standard temperature: QNH corrects the pressure reference, not temperature-related altimetry error. In cold air, true altitude can be lower than indicated; simulation fidelity varies by aircraft and weather engine.
For a quick diagnostic near sea level, a one-hectopascal setting error changes indicated altitude by roughly 27 to 30 feet. An error approaching 1,000 feet suggests a pressure difference of about 34 hPa or 1 inHg rather than a minor reporting discrepancy.