DGCA CPL Aviation Meteorology: Pressure, Altimetry and the Standard Atmosphere

Why altimetry matters

An altimeter is a barometer calibrated to the ISA. It measures pressure and displays a height assuming standard pressure and temperature. Errors arise whenever the real atmosphere departs from ISA.

QNH, QFE, QNE

• QNH: pressure reduced to mean sea level; altimeter reads elevation (height above MSL) on the ground.
• QFE: station-level pressure; altimeter reads height above the aerodrome.
• QNE / standard setting 1013.25 hPa: used above transition altitude; altimeter reads flight level.

Error types

1. Pressure error: fly from high to low pressure without resetting → altimeter over-reads, aircraft is lower than indicated. "High to low, look out below."
2. Temperature error: in air colder than ISA, the column is denser and the aircraft is lower than indicated. Cold-temperature altimetry is a classic hazard in mountainous terrain.
3. Position/terrain clearance: always check the lowest safe altitude.

Worked example

Aerodrome elevation 500 ft, QNH 1003 hPa. Pressure is 10 hPa below standard; 1 hPa ≈ 8 ft near the surface, so the altimeter set to 1013 would read about 80 ft too high. Setting QNH 1003 makes it read 500 ft correctly. If you then flew toward a region of 993 hPa without resetting, you'd be ~80 ft lower than indicated - dangerous near terrain.

Traps

Don't confuse QFE (reads zero on the ground) with QNH (reads elevation). Remember 1 hPa ≈ 8 ft (or ~27 ft per inch of mercury).