An altimeter does not measure height; it measures pressure and calls it height. Which height it reports depends entirely on the datum wound into its subscale, and the altimeter-setting procedures of Doc 8168 exist to make sure every aircraft in the same piece of sky is using the right datum at the right time. The rules of the air module touched the boundary between altitudes and flight levels only in passing; this lesson is where the whole system is taught: QNH and QFE, the flight level, and the transition altitude, layer and level that join the two worlds.
Two objectives, two settings
The altimeter-setting procedures serve two main objectives: to provide adequate vertical separation between aircraft, and to provide adequate terrain clearance during all phases of a flight. The two pull in different directions. Terrain is fixed to the earth, so clearing it needs an altimeter that reads true relative to the ground below: a local, current pressure setting. Other aircraft only need to be using the same setting as each other, however arbitrary: if every altimeter in the cruise carries an identical datum, identical indications mean identical levels, and separation works without anyone knowing the true height at all.
Hence the two settings:
QNH — the setting that makes the altimeter read aerodrome elevation when the aircraft is on the ground; in flight it therefore indicates altitude: vertical distance above mean sea level.
QFE — the setting that makes the altimeter read zero on the aerodrome; in flight it indicates height above the aerodrome (or runway threshold) datum.
QNH is the working setting of the system this lesson describes; QFE survives mainly in local flying and some military use.
The flight level
Above the terrain's reach, the system switches every aircraft to one shared, arbitrary datum: the standard pressure of 1,013.2 hPa. A flight level is defined as a surface of constant atmospheric pressure, related to the standard datum of 1,013.2 hPa, and separated from other such surfaces by specific pressure intervals. Three facts complete the definition set:
Where FL zero lies — at the level where the atmospheric pressure is exactly 1,013.2 hPa. On a low-pressure day that surface sits below sea level; on a high-pressure day, above it. FL zero is wherever the standard datum happens to be.
The interval — consecutive flight levels are separated by pressure intervals corresponding to at least 500 ft (152.4 m) in the standard atmosphere.
How they are numbered — a flight level carries the number of hundreds of feet it lies above the standard datum in the standard atmosphere: the 1,013.2 hPa surface plus 5,000 standard feet is FL 50.
Transition altitude, layer and level
The changeover between the QNH world and the standard world is built from three terms, and the figure stacks them. The transition altitude (TA) is the altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes — the top of the QNH world. It is specified normally in multiples of 300 m (1,000 ft); its height above the aerodrome is kept as low as possible, but normally not less than 900 m (3,000 ft), the calculated value being rounded up to the next full 300 m (1,000 ft). Transition altitudes are published in the Aeronautical Information Publication (AIP) and shown on the relevant charts.
The transition level (TRL) is the lowest flight level available for use above the transition altitude — the floor of the standard world. Unlike the TA it is not fixed: as the QNH falls, the standard-datum surfaces sink toward the ground, and the lowest flight level available for use must rise to keep clear of the TA; the TRL therefore varies with the day's pressure, which is why it is determined by ATC and passed to the aircraft in due time before it is reached in descent (in practice with the approach or descent clearance, or by the ATIS broadcast). Between the two lies the transition layer: the airspace between the transition altitude and the transition level, a buffer never used for level flight.