QNH

Details
Category: Altimetry & Vertical References

QNH is the local barometric pressure setting that, when set on an aircraft’s altimeter, causes it to indicate altitude above Mean Sea Level (MSL) while the aircraft is on the ground at a specific airfield.

In simple terms:

  • Set QNH → altimeter reads field elevation on the ground
  • In flight → altimeter shows altitude referenced to sea level

What QNH represents

  • Atmospheric pressure reduced to mean sea level
  • Varies with weather systems and location
  • Expressed in:
    • hPa / millibars (most of the world)
    • inHg (United States)

Example:

  • QNH 1013 hPa is the ICAO standard pressure
  • A lower QNH usually indicates low pressure / bad weather
  • A higher QNH usually indicates high pressure / fair weather

Operational significance

  • Ensures all aircraft in the area reference the same vertical datum
  • Allows ATC to maintain vertical separation
  • Aligns altimeter readings with:
    • Terrain elevations
    • Obstacle heights
    • Published approach altitudes

Incorrect QNH setting can result in:

  • Altitude errors
  • Terrain clearance issues
  • Incorrect approach minima

Rule of thumb:

Low QNH = altimeter reads higher than true altitude
High QNH = altimeter reads lower than true altitude

QNH vs related settings

  • QNH: Altitude above mean sea level (standard for enroute, approach, landing)
  • QFE: Altimeter reads zero on the runway threshold (used rarely)
  • Standard (1013 hPa / 29.92 inHg): Used above transition altitude for flight levels

Use in approaches

  • All published altitudes on charts are based on QNH
  • Decision Height (DH) and Minimum Descent Altitude (MDA) depend on correct QNH
  • ATIS and ATC always provide the current QNH

Application in DCS World

  • DCS models QNH correctly through weather settings
  • Altimeters respond realistically when QNH is changed
  • ATC and ATIS usually provide a single pressure value (implicitly QNH)
  • Transition altitude and flight levels are not strictly enforced

Cadets must:

  • Set QNH before takeoff
  • Recheck QNH before approach
  • Cross-check indicated altitude against known field elevation

Training focus

Cadets should practice:

  • Recognizing altitude discrepancies caused by wrong QNH
  • Switching between standard pressure and QNH
  • Understanding the relationship between QNH, MSL, and AGL

QNH discipline is foundational. Every navigation error starts here.