High to low, look out below: the altimeter error

What the altimeter measures

At its core the altimeter is a barometer: it measures air pressure and displays it as altitude. Its scale follows the standard atmosphere. An altimeter calibrated to it and set to a QNH altimeter setting indicates altitude above mean sea level. With the setting knob you adjust for surface pressure: the altimeter setting is the value at which the altimeter indicates true altitude at field elevation.

True altitude is the actual vertical distance above mean sea level. Indicated altitude is the altitude above mean sea level that the altimeter shows when set to the local altimeter setting.

High to low: the pressure error

A setting is accurate only in the vicinity of the weather station it applies to. You therefore have to adjust the altimeter as the flight progresses from one station to the next.

If you fly from high pressure to low pressure without adjusting, the altimeter displays a constant altitude, but your actual height above the ground is lower than the indicated altitude. The FAA handbook sums up the rule: when the actual pressure is lower than the pressure set in the altimeter window, the actual altitude is lower than the indicated altitude. A fall in pressure therefore makes the altimeter indicate a higher altitude, and a rise in pressure a lower one.

There is an old mnemonic for it: “Going from a high to a low, look out below.” Conversely, if you fly from a low into a high without adjusting, your actual altitude is higher than the indicated altitude.

How big is the error?

The EASA syllabus gives an approximation near mean sea level: a pressure difference of 1 hPa corresponds to 30 ft (9 m) of altitude. The FAA handbook arrives at the same result via the unit inHg: a pressure difference of 1 inHg is equal to approximately 1,000 ft of altitude, and standard sea level pressure is 29.92 inHg, or 1013.2 hPa. That gives about 33.9 hPa per inHg and roughly 30 ft per hPa.

Cold air has the same effect

The pressure setting does not compensate for a temperature deviation. With the altimeter you can adjust for surface pressure, but not for the mean temperature of the column of air below you. Cold air is denser than warm air: in air that is colder than the standard atmosphere, your actual altitude is lower than the altimeter indication. In warmer air it is the other way round. The handbook has a second mnemonic for it: “From hot to cold, look out below.”

A table in the handbook, derived from ICAO formulas, shows how large the error can become. At −10 °C and 1,000 ft above the airport, the aircraft may be as much as 100 ft below the indicated altitude, even with the current altimeter setting. In cold weather over mountainous terrain you must take the difference between indicated and true altitude into account, and a higher indicated altitude may be required to keep adequate terrain clearance. Over high mountains, certain atmospheric conditions can make the altimeter indicate an altitude of 1,000 ft or more above the actual altitude. The handbook advises a generous margin.

What you can do

• Keep the setting current: the FAA handbook advises adjusting it frequently in flight according to the nearest weather reporting station and obtaining current altimeter settings, so that terrain and obstruction clearance is assured.
• Leave extra room above terrain on cold days and in the mountains.
• Expect fast pressure changes near thunderstorms: pressure usually falls rapidly as one approaches. Without a corrected altimeter setting, the altimeter may be more than 100 ft in error.