The Altimeter
The Altimeter is a cockpit instrument that displays our altitude in feet. It works by measuring changes in air pressure as altitude changes.
The altimeter is calibrated for the international standard atmosphere (ISA) conditions, so any deviations from ISA will lead to errors (more on this later).
An altimeter indicating 2,000ft
Pressure Settings
The pilot enters a pressure setting in hectopascals (hPa) on the altimeter subscale. This is usually one of two settings:
QNH
QNH is the calculated air pressure at mean sea-level. This is done by measuring the air pressure at an airfield, then using ISA conditions to calculate what the pressure would be at mean sea-level (i.e. adding approx. 30hPa for every 1000ft that the airfield is above sea level.)
With QNH set in the subscale, the altimeter will read altitude above mean sea level (AMSL).
QFE
QFE is the actual pressure measured at an airfield.
If the pilot sets QFE on the subscale, the altimeter will display height above that airfield.
For an aircraft sitting on the ground, the altimeter will read 0 feet when QFE is set.
Which QNH to use?
When flying from one airport to another, the pressure changes and so the QNH setting we use must also change for the altimeter to continue to be accurate. A pilot gets the QNH setting from one of a few sources:
Aerodrome QNH
Aerodrome QNH is the QNH calculated for a specific airport, and it is provided to the pilot by that airport over the radio. This can either be from an air traffic controller or an automated radio broadcast.
Regional QNH
The country is divided into Altimeter Setting Regions (ASRs) and the Regional QNH is the lowest forecast QNH in a region for the next hour. Since the regions are large, this is a less accurate method than using the aerodrome QNH.
Standard Pressure Setting (SPS)
The standard pressure setting of 1013hPa is used when flying above the transition altitude. When using the standard pressure setting, altitudes are referred to as Flight Levels. For example, 8,000ft is FL80 when flying above the transition altitude (“Flight Level Eight-Zero”). Altitudes and Flight Levels are covered more in the next lesson.