Altimeter
An altimeter is a flight instrument that measures atmospheric pressure and displays the result as altitude above mean sea level. The standard barometric altimeter uses an aneroid capsule connected to a mechanical display calibrated in feet. The pilot sets the local altimeter setting (QNH) using the Kollsman window knob.
Setting the Altimeter
- Obtain the current QNH from ATIS, AWOS, or ATC
- Set the value in the Kollsman window
- Verify: the altimeter should read field elevation ± 75 ft
- If the error exceeds 75 ft, the altimeter is suspect — do not depart IFR
Transition Altitude
Below 18,000 ft ASL in Canada, pilots set QNH. At and above FL180, all aircraft set 29.92 inHg (1013.2 hPa) — called QNE — and report altitude as a flight level. The transition is coordinated with ATC.
Altimeter Errors
- Temperature error: Cold air makes the altimeter over-read — the aircraft is lower than indicated. Cold temperature corrections are required for IFR approaches at Canadian airports in cold weather.
- Position error: Static port placement causes minor errors at high angles of attack
- Hysteresis: Lag in response during rapid altitude changes
On the TC Written Exam
One of the most tested instruments. Know: QNH vs QNE; the ±75 ft ground check; transition altitude in Canada (18,000 ft); cold temperature effects on indicated altitude.
Related Terms
See also: Altimeter Setting, Altitude, Pressure Altitude, Density Altitude.