Altimeter Setting Procedures (PANS-OPS Doc 8168)

Altimeter Setting Procedures (PANS-OPS Doc 8168)

An altimeter is essentially a barometer with a scale in feet: it can only show a meaningful altitude if the correct reference pressure is set on its subscale. Altimeter setting procedures — standardised in ICAO Doc 8168 (PANS-OPS) — make sure that all aircraft in the same volume of airspace measure their vertical position from the same datum, which is what makes vertical separation and terrain clearance work. For the PPL exam you must know the three pressure references, the transition altitude/level system, and the VFR setting rules applied in Germany.

The three pressure references

QNH is the aerodrome pressure reduced to mean sea level. With QNH set, the altimeter indicates altitude above mean sea level (AMSL) and shows the aerodrome elevation when the aircraft is on the runway. QFE is the actual pressure at aerodrome level; with QFE set, the altimeter indicates height above the aerodrome and reads approximately zero on the runway — it is rarely used today. The standard setting 1013.25 hPa (also called QNE) makes the altimeter indicate pressure altitude, which is expressed as a flight level (FL) — FL 80 corresponds to a pressure altitude of 8000 ft.

Transition altitude, transition layer, transition level

The transition altitude (TA) is the altitude at or below which vertical position is expressed as an altitude (QNH set). The transition level (TL) is the lowest usable flight level above the transition altitude. The airspace between the two is the transition layer. Because the transition level must always lie above the transition altitude, its number varies with the current QNH: on a low-pressure day the transition level is higher.

• Climbing through the transition altitude: set 1013.25 hPa and express vertical position as a flight level.
• Descending through the transition level: set QNH — in German practice the QNH of the nearest aerodrome with an ATC unit — and express vertical position as an altitude.

A practical consequence: during a flight at FL 80 the altimeter setting must be 1013.25 hPa. A flight level exists only with the standard setting; the same needle position with QNH set would be an altitude, not a flight level.

VFR altimeter setting rules (German practice)

On VFR flights at or below 5000 ft AMSL — and also above 5000 ft AMSL as long as the height does not exceed 2000 ft AGL — the altimeter must be set to the QNH of the nearest aerodrome with an air traffic control unit. The 2000 ft AGL branch matters over high terrain, where an altitude above 5000 ft AMSL can still be close to the ground.

When cruising above 5000 ft AMSL and more than 2000 ft AGL, the altimeter is set to the standard pressure of 1013.2 hPa and the flight is conducted at flight levels instead of QNH-based altitudes.

During descent, when passing the transition level / 5000 ft AMSL, set the altimeter to the QNH of the nearest aerodrome with an ATC unit. (Transponder operation is a separate subject — see the transponder lesson.)

Germany

Germany: transition altitude 5000 ft AMSL nationwide (AIP Germany ENR 1.7); the transition level varies with QNH and is published by ATC/ATIS. Above the transition level, the standard setting 1013.25 hPa applies.

Exam essentials

• QNH indicates altitude AMSL, QFE indicates height above the aerodrome, and the standard setting 1013.25 hPa indicates pressure altitude (flight level).
• During a flight at FL 80 the altimeter setting must be 1013.25 hPa.
• Climbing, the setting changes to standard at the transition altitude; descending, it changes to QNH at the transition level.
• The transition altitude in Germany is 5000 ft AMSL nationwide; the transition level varies with the current QNH.
• On VFR flights at or below 5000 ft AMSL, or above 5000 ft AMSL when not more than 2000 ft AGL, the altimeter is set to the QNH of the nearest aerodrome with an ATC unit.
• On VFR flights above 5000 ft AMSL and more than 2000 ft AGL, the altimeter is set to 1013.2 hPa and cruising is by flight levels.
• During descent, on passing the transition level, the altimeter is set to the QNH of the nearest aerodrome with an ATC unit.