Learn why a Prepar3D altimeter is inaccurate at high altitude, when to set 29.92/1013, and how to diagnose pressure, weather and gauge errors.
In Prepar3D, an altimeter can appear inaccurate at high altitude because it converts static pressure into altitude using the standard atmosphere; it does not measure geometric or GPS height. Above the applicable transition altitude, set 29.92 inHg/1013.25 hPa. Non-standard temperature and weather pressure can still make true altitude differ.
Why do barometric and GPS altitudes disagree?
Because each display may use a different altitude reference, disagreement at cruise altitude is often normal. A pressure altimeter is calibrated against a standard atmospheric model, while a GPS, map or utility may report the aircraft’s geometric height above mean sea level.
| Altitude reference | What it represents | How it is used |
|---|---|---|
| Indicated altitude | Static pressure interpreted using the selected barometric setting | Assigned altitudes below the transition level |
| Pressure altitude | Altitude with 29.92 inHg or 1013.25 hPa selected | Flight levels and aircraft performance calculations |
| Geometric or GPS altitude | The aircraft’s simulated physical height above mean sea level | Position displays and some add-on utilities |
| Height above ground | Vertical distance from the terrain beneath the aircraft | Terrain clearance; it is not an MSL altitude |
Prepar3D also exposes different simulation variables for these references. A gauge may use INDICATED ALTITUDE, while another display or utility uses PLANE ALTITUDE. Those values are not required to match in a non-standard atmosphere.
Which altimeter setting should I use at high altitude?
At flight levels, use STD pressure: 29.92 inHg or 1013.25 hPa. This makes every aircraft use the same pressure datum for vertical separation, even though its true geometric altitude may be higher or lower than the nominal flight level.
During the climb, change from local QNH to STD at the applicable transition altitude. During descent, change back at the transition level or when instructed. The transition altitude is not universally 18,000 feet; it depends on the country and procedure. Our explanation of QNH, standard pressure and transition levels covers the underlying pressure references.
A mistake we see constantly is leaving the departure QNH selected throughout the climb. As a useful approximation, each 0.01 inHg of setting error changes the indication by about 10 feet, while 1 hPa represents roughly 27 feet. A setting 0.50 inHg away from standard can therefore create an error of about 500 feet.
Use the aircraft’s BARO or STD control and verify the actual display. In more detailed add-on aircraft, the captain’s, first officer’s and standby instruments may have separate settings. Prepar3D’s general altimeter-reset command may also produce an unsuitable local setting at cruise altitude, so do not rely on it blindly.
How much altitude difference is normal?
There is no fixed acceptable difference between barometric and geometric altitude because it depends on the complete pressure and temperature profile. A discrepancy of several hundred feet at cruise altitude can be entirely plausible without indicating a faulty instrument.
Temperature is a major cause. Cold air compresses the pressure levels, making true altitude lower than indicated; warm air generally produces the opposite effect. The difference becomes more noticeable high above the pressure datum. This is the basis of the familiar warning that, when flying from high temperature to low temperature, true altitude reduces unless the procedure accounts for it.
Prepar3D weather can add another variable. Default weather, custom themes and weather add-ons may inject different temperatures or pressures, and a transition between weather areas can move the simulated pressure surfaces. Under altitude hold, the aircraft may climb or descend geometrically while maintaining the same barometric flight level.
Should I adjust the altimeter to match the GPS?
No—do not turn the barometric setting until the altimeter agrees with a GPS or map display. At cruise altitude, fly the assigned barometric altitude or flight level using the correct QNH or STD setting.
Changing the setting to force agreement removes the common pressure reference used by ATC and other aircraft. Conventional altitude hold also follows a barometric or air-data reference, so chasing the geometric reading can put the aircraft away from its assigned level.
How do I troubleshoot a genuine Prepar3D altimeter error?
A repeatable disagreement under controlled conditions is the quickest way to separate an expected atmospheric difference from a gauge, weather or aircraft fault.
- Identify the comparison. Record whether the conflicting value comes from the primary altimeter, standby instrument, GPS, map, ATC or an add-on utility. Do not compare MSL altitude with height above terrain.
- Set the correct pressure reference. Use local QNH below the transition altitude and 29.92 inHg/1013.25 hPa at flight levels. Check that the cockpit is displaying the expected units.
- Compare all barometric instruments. Primary and standby altimeters should be close when they share the same pressure setting. A large difference between them points towards an aircraft-specific gauge or air-data problem.
- Stabilise the weather. Repeat the flight with a simple weather setup using standard pressure and temperature, with external weather injection disabled. This removes changing pressure layers from the test.
- Try a default Prepar3D aircraft. If the default aircraft behaves correctly but one add-on does not, check that aircraft’s BARO synchronisation, instrument options and Prepar3D-version compatibility.
- Check failures and the static system. An altimeter that freezes or responds sluggishly may be using a blocked or failed static source. Pitot heat does not necessarily heat the static ports; the exact behaviour depends on the aircraft’s systems modelling.
- Observe ATC and altitude hold. If Prepar3D ATC repeatedly reports the aircraft high or low with STD correctly selected, test the same situation using a default aircraft and controlled weather. ATC, an add-on gauge and the autopilot may be consuming different altitude variables.
If the error also appears on the ground or below the transition altitude, our broader altimeter error diagnostic guide covers pressure units, airport elevation, weather data, temperature and scenery-related mismatches.