ATM 005

Altimetry

Pressure and height

An altimeter measures air pressure, and the pressure falls as you climb. What it shows depends on the setting in its subscale window.

About 1 hPa for every 30 ftThe working figure behind every calculation in this lesson: it turns a difference in pressure into a difference in height.

Three settings, three zero levels

Zero at mean sea levelQNH

Reads altitude, measured from mean sea level.

Zero at the field elevationQFE

Reads height, measured from the aerodrome.

Zero at the standard pressure1013.25 hPa

The standard setting, the ISA’s pressure at mean sea level. Reads flight levels.

Levels, and the words for them

Where a vertical distance is measured from, and what the word a pilot uses tells you.

MSLMean sea level

AMSLAbove mean sea level

GNDGround

AGLAbove ground level

SFCSurface

FLFlight level

LevelA generic term relating to the vertical position of an aircraft in flight, and meaning variously height, altitude or flight level.

The pilot’s word tells you the setting

“Height”

QFEHeight above the aerodrome.

“Flight level”

1013.25 hPaA flight level.

“Altitude”, or no word at all

QNHAltitude above mean sea level.

Altitude, height and flight level

Each setting measures from a different level.

Here the QNH is below 1013.25, so the 1013.25 hPa level lies below the sea. Flight levels are measured from it, so the flight level reads higher than the altitude.

AltitudeMeasured from mean sea level, on QNH.

HeightMeasured from the aerodrome, on QFE.

Flight levelMeasured from the 1013.25 hPa level, on the standard setting, in hundreds of feet: FL 50 is 5 000 ft.

ElevationHow high the aerodrome sits above mean sea level.

The same three are in LAW 005, and the settings they come from in MET 002.

The transition

Below the transition altitude, levels are altitudes on QNH. From the transition level up, they are flight levels on 1013.25 hPa. Between the two is the transition layer.

The colour is the setting in use: blue is QNH, orange 1013.25 hPa. A climbing aircraft changes at the transition altitude, a descending one at the transition level.

ClimbingOn QNH, or QFE, up to the transition altitude. Passing it, the altimeter goes to 1013.25 hPa and levels become flight levels.

DescendingOn flight levels down to the transition level. Passing it, the altimeter goes to QNH, or QFE, and levels become altitudes.

So inside the transition layer, a climbing aircraft is on flight levels and a descending one on altitudes (ICAO Doc 8168).

TATransition altitude

The altitude at or below which the vertical position of an aircraft is controlled by reference to altitude. Fixed and published for every aerodrome.

TRLTransition level

The lowest flight level available for use above the transition altitude. Determined by ATS for every aerodrome.

Transition layer

The airspace between the transition altitude and the transition level. At least 1 000 ft deep: the transition level shall be at least 300 m (1 000 ft) above the transition altitude, so both can be used in cruising flight with vertical separation ensured.

Transition altitudes

Denmark3 000 ft

Denmark · Copenhagen area5 000 ft

Sweden5 000 ft

Transition level tables

Which transition level goes with which QNH. The lower the QNH, the lower the 1013.25 hPa level sits, and the flight levels sink with it, so the transition level has to be a higher one.

Transition altitude 5 000 ftAIP Sweden, ENR 1.7

QNHTransition level

1051 and aboveFL 50

1032–1050FL 55

1014–1031FL 60

996–1013FL 65

978–995FL 70

960–977FL 75

943–959FL 80

942 and belowFL 85

The table in the notes. 5 000 ft is the transition altitude in the Copenhagen area too.

Transition altitude 3 000 ftAIP Denmark, ENR 1.7

QNHTransition level

1051 and aboveFL 30

1014–1050FL 40

978–1013FL 50

943–977FL 60

942 and belowFL 70

Denmark’s own, for its 3 000 ft transition altitude. It goes in steps of 1 000 ft.

Working it out

Every calculation here is the same move: find the difference in hPa between two settings, and multiply it by 30 to turn it into feet. The standard setting is 1013.25 hPa. The transition altitude is fixed and published, so there is nothing to work out for it. Pick what you want to find, then change the numbers.

What do you want to find?

An aircraft is at FL 50, and the QNH is 1003. What is its altitude?

Flight level

QNH

hPa

Mean sea level, QNH 1003The 1013.25 hPa level

FL 50 is 5 000 ft above the 1013.25 hPa level.
The QNH, 1003, is 10.25 hPa below 1013.25. At 30 ft per hPa that is 307.5 ft, so the 1013.25 hPa level lies 307.5 ft below the sea.
So the aircraft is 307.5 ft lower than its flight level says: 5 000 − 307.5 = 4 692.5 ft above the sea.

Altitude4 692.5 ftabout 4 690 ft

In your head, use 1013: 10 hPa × 30 = 300 ft, so about 4 700 ft. Lower QNH, lower aircraft.

In one line: Altitude = FL × 100 − (1013.25 − QNH) × 30

Close to a band’s edge, the 30 ft rule and the tables can be one level apart. The tables are worked out with the exact standard atmosphere, in which 1 hPa is about 27 ft near sea level, not 30. With a 5 000 ft transition altitude, at QNH 1030 the rule gives FL 55 and the table FL 60; at QNH 996 the rule gives FL 70 and the table FL 65. The transition level is determined by ATS, so where there is a table, the table is right.

QNH and QFE are given in whole hPa, rounded down. Searching the page puts the example numbers back.

Flight level allocation

Which flight levels go with which magnetic track. Some countries split the compass north and south, instead of east and west.

The same rule turned a quarter of the compass: in these four countries, southbound is odd and northbound even.

Odd flight levelsMagnetic track 090° to 269°: southbound.

Even flight levelsMagnetic track 270° to 089°: northbound.

Used in Italy, France, Spain and Portugal. SERA Appendix 3 allows the turn where most traffic flows north and south. The usual split, east and west, is in LAW 005.

Vertical limits on a chart

Charts can write the vertical limits of an area in lots of different ways. The upper limit goes above the line and the lower limit under it, and each can be measured from a different level.

19500 AMSLMSL

Up to 19 500 ft above mean sea level. Down to mean sea level.

16500 AGLGND

Up to 16 500 ft above the ground, so the top follows the ground. Down to the ground.

FL0951500 AMSL

Up to FL 95, on the standard setting. Down to 1 500 ft above mean sea level, on QNH. Two references in one area.

Reading material

Air Traffic Management

ATM 005 Altimetry PowerPoint / PDF

Commission Implementing Regulation (EU) 2017/373 (Rules for ATM-ANS), ATS.TR.140 with GM1 and GM2, ATS.TR.125, and ATS.TR.130 with GM1 Reading list, free

Commission Implementing Regulation (EU) No 923/2012 (SERA), definitions (applicable parts), GM1 Article 2(39), GM1 Article 2(78), GM1 Article 2(84), SERA.3110 and Appendix 3 (Table of cruising levels) Reading list, free

European Union Aviation Safety Agency, Easy Access Rules for Air Traffic Management/Air Navigation Services: Provision of Services (Regulation (EU) 2017/373), March 2025 Reference, free

European Union Aviation Safety Agency, Easy Access Rules for Standardised European Rules of the Air (SERA), May 2025 Reference, free

International Civil Aviation Organization, Annex 4, Aeronautical Charts, 11th edition, July 2009 Reference, part free

International Civil Aviation Organization, Procedures for Air Navigation Services: ICAO Abbreviations and Codes, Doc 8400, 9th edition, 2016 Reference, paid

SKYbrary, Altimeter Setting Procedures Reference, free