Altimeter

The altimeter is a sensitive aneroid barometer: a hollow capsule set changes volume with static outside pressure; the geared mechanism translates volume change into altitude.

Sensor — aneroid capsule or electronic

The sensor for indication of a barometric altimeter is:

• In mechanical instruments: an aneroid capsule — an airtight, evacuated metal capsule that changes volume with static outside pressure.
• In modern electronic instruments (glass cockpit, PFD): an electronic pressure sensor — semiconductor sensor that digitises the pressure signal.

Both sensor types connect to undisturbed outside air via the static port.

Vertical distance from reference level

The barometric altimeter always indicates the vertical distance from the selected reference pressure level (QNH, QNE, QFE) — depending on what is set in the subscale.

Q-code settings

Higher QNH setting = higher altitude indication

A higher QNH setting in the subscale will cause a higher altitude indication on the dial:

• Example: QNH from 1013 hPa raised to 1023 hPa → subscale 10 hPa higher → indication +280 ft more (rule: 1 hPa ≈ 28 ft).
• Reason: the instrument compares outside pressure with subscale setting. With higher subscale, a given outside pressure is interpreted as "further below" the reference → higher altitude.

→ Higher QNH in subscale = higher indicated altitude.

QNH subscale — when to reset?

A current QNH must be set regularly, before every flight and during cross-country flights:

• Before every flight: from ATIS or tower/AFIS.
• During cross-country: every 50-100 NM, at weather changes, or when entering a new FIR/CTR.

The altimeter subscale must be reset before every flight and on cross-country flights — otherwise altitude indications can deviate by several hundred feet, with risk for terrain conflict and altitude separation.

True altitude — definition

True altitude is the actual altitude above mean sea level (AMSL), corrected for non-standard temperature:

• Indicated altitude is based on ISA standard atmosphere (15°C MSL, lapse rate 1.98°C/1000 ft).
• In air colder than standard the true altitude is lower than indicated (see temperature error below).
• True altitude = indicated altitude × correction factor (table or CRP-5).

Altimeter errors

Temperature error — mechanism

The temperature error is caused by the tendency of the capsule to contract in low temperature and expand in high temperature:

• Low temperature: capsule contracts → mechanism interprets as "less outside pressure" → altimeter shows a higher altitude than actual.
• High temperature: capsule expands → "more outside pressure" → altimeter shows lower altitude than actual.

Consequence in cold weather: low temperatures may lead to altitude indications that are too high — i.e. the aircraft is lower than indicated → critical for terrain clearance in winter and mountain flying.

→ Mnemonic: "FROM HOT TO COLD, DON'T BE BOLD" — from warm into cold air → altimeter reads too high.

Pressure error — mnemonics

"HIGH to LOW, LOOK OUT BELOW" — flying from high to low pressure (without updating altimeter), the altimeter shows more than actual altitude — you are lower than indicated.

Flight toward low pressure — altitude decreases

During a flight with constant altimeter indication toward a low-pressure area, the actual altitude above MSL reduces:

• Pilot keeps indication constant by descending (follows the pressure surface that drops with low pressure).
• Without updating the subscale he unknowingly loses true altitude.
• Consequence: risk of terrain conflict, especially in mountain flying.

→ Standard: at transition into a new FIR/CTR, or at most every 50-100 NM, update QNH.

Altimeter check

The altimeter can be checked on the ground by setting the subscale so that the indication matches the airfield elevation — and then comparing the subscale to the actual QNH:

• If at correct elevation indication the subscale is within ±2 hPa of actual QNH: altimeter is accurate.
• Otherwise: altimeter has an error — contact maintenance.

Behaviour with constant altimeter indication during descent

If after initiating a descent the altimeter reading remains constant (altimeter still shows the entry altitude), the most likely cause is a blocked static port. Recommended steps:

1. Engage alternate static air if installed (resolves the static blockage).
2. If no alternate static: altitude must be estimated and the pilot must be prepared for a possibly excessive airspeed indication (see ASI errors with static blockage).
3. As a last resort emergency: break the glass of the vertical speed indicator (VSI) — this opens the static path to cabin pressure and reactivates altimeter, ASI, and VSI (with a small position error from cabin pressure).

ICAO Standard Atmosphere — key values