Altimeter-setting procedures

QNH, QFE and standard pressure

An indication has meaning only with its setting.

Definitions and purpose

QNH gives an altitude referenced to mean sea level under the pressure-setting convention; on the aerodrome a correctly set altimeter approximately indicates aerodrome elevation. QFE makes the altimeter approximately read zero at its specified datum. A flight level is a constant-pressure surface referenced to 1013.2 hPa; FL zero is that standard-pressure reference surface, not permanently mean sea level.

The main purposes of setting procedures are common vertical references for separation and suitable terrain/obstacle awareness. Consecutive numbered flight levels represent nominal 100-ft steps, but that definition is not an authorisation to separate aircraft by 100 ft. Operational separation uses the applicable much larger minima.

Reference versus true height

Indicated altitude is not necessarily true geometric altitude. Pressure changes shift flight-level heights; temperature changes layer thickness. Sufficient QNH reporting locations make local references available along a route, but a regional setting does not remove cold-temperature corrections or obstacle considerations.

A common datum enables aircraft to compare indicated levels. It does not make either altimeter a terrain-warning system. Read the dedicated meteorology explanation for pressure and temperature errors; the regulatory procedure here establishes which reference is used and communicated.

Climb and descend through the transition layer

Use the transition altitude in climb and transition level in descent.

TA, TL and layer

Transition altitude is the altitude at or below which vertical position is controlled by reference to altitudes. It is normally published in the AIP and charts as an altitude above mean sea level, rounded as specified for the design. Transition level is the lowest available flight level above transition altitude; it is selected and passed by the appropriate ATS unit under the published system. The transition layer lies between them.

Climbing through TA, change QNH to standard and express position as flight levels. Descending through TL, change standard to QNH and express position as altitude. During the layer crossing use the climb/descend reference respectively. Obtain the actual QNH and TL in sufficient time; ATIS and approach information commonly provide them. A published TA does not tell you today’s TL.

Before take-off and during approach

Before take-off set at least one altimeter to the applicable QNH, except where the specified QFE/local arrangements apply, and check the indicated value. QNH is provided before departure and made available to approaching aircraft in the approach/landing information. The pilot can request missing transition information.

When cleared to an altitude before descending below TL, the applicable procedure can permit reference to altitude above TL where level flight at or above TL is not anticipated. This does not mean the pilot should independently change every instrument early without coordination. Follow the published and cleared procedure with a consistent reference.

Can you apply the rules?

Four original questions. Choose from the facts, then review the explanation.