(eg: an aircraft) considered as a point, measured from a specified datum (eg: the ground).

Altitude: The vertical distance of a level, a point or an
object considered as a point, measured from mean sea level (MSL). Flight level: A surface of constant atmospheric pressure which is related to a specific pressure datum (1013.2 hPa) and is separated from other such surfaces by specific pressure intervals.

495 hPa
500 hPa
flight level
995 hPa 1000 hPa 1005 hPa 1010 hPa 1013 hPa 1015 hPa 1020 hPa
altitude
height
Definitions
The pressure established at the aerodrome level is called: “QFE” The pressure computed at the sea level is called: “QNH” The vertical distance between those two pressures is the aerodrome altitude (Zt).
..pointers turn anticlockwise . . .
. . . and pressures increase When turning the knob to the left . . .
Pressure on the ground is 1012 hPa
, we display the ground pressure in the window…. If, by turning the knob



495 hPa
500 hPa
995 hPa 1000 hPa 1005 hPa 1010 hPa 1013 hPa 1015 hPa 1020 hPa
QFE Zt QNH

Standard ICAO atmosphere Definitions Altimeter
Altimeter setting Flight Level and RVSM



A face
Two pointers
A window showing pressure settin A setting knob
..pointers turn clockwise . . .
. . . and pressures decrease When turning the knob to the right . . .
atmospheric pressure - 1hPa / 28ft
Standard ICAO atmosphere

Standard ICAO atmosphere is an “ideal” atmosphere.

Actually, isobars are not horizontal or equidistant . . .
. . . The pointers will point to zero (height = 0ft)
0 9
1
8
2
The aircraft is on the ground
7
1012
6
5 4
3
Pressure on the ground is 1012 hPa
Conversely, if with the knob, we move the pointers to zero (height of the aircraft) . . . . . . the window will indicate the ground pressure
0 9
1
The aircraft is still on the ground8 Nhomakorabea2
7
1012
6
5 4
3
Pressure on the ground is 1012 hPa
If,with the knob , we display the ground pressure in the window... The aircraft is 1000ft above the aerodrome
0 9 1
8
2
The aircraft is 1000ft above the aerodrome
7
1022
6 5 4
3
The altimeter reads 1280ft
1000m + 280ft
The main use of the QNH setting is approximately between 1000ft and 6000ft to avoid obstacles
0 9 1
8
2
The aircraft is 1000ft above the aerodrome
7
1012
6 4
3
The altimeter reads 1000ft
5
The main use of the QFE setting is during the landing and taking-off phases (near the ground)
Flight Level and RVSM


Standard ICAO atmosphere Definitions
Altimeter Altimeter setting Flight Level and RVSM
Standard ICAO atmosphere

Air is dry and its chemical composition (oxygen, azot, …) is constant; At the sea level, temperature is 15°C;
Standard ICAO atmosphere Definitions Altimeter Altimeter setting Flight Level and RVSM
Definitions

Height: The vertical distance of a level, a point or an object
1 2
0
9
8
. . . The pointers will indicate 1000ft
7
1012
6 5 4
3
The sea level pressure is 1022 hPa
If, with the knob, we display the sea level pressure in the window...


The temperature decreases of 2°C every 1000ft (300m)
approximately down to minus 56°C.
05°C 07°C
Temperature -2°C / 1000ft
09°C 11°C 13°C 15°C
Standard ICAO atmosphere

At sea level, atmospheric pressure is 1013,25 hPa; The atmospheric pressure decreases by 1hPa every 28ft (8,5m).
1008 hPa 1009 hPa 1010 hPa 1011 hPa 1012 hPa 1013 hPa
The altimeter is set to the QFE (1012hPa)
0
9 1
8
2
7
1012
6 4 5
3
The aircraft is on the ground
The altimeter reads zero
The altimeter is set to the QFE (1012hPa)
(or water)
596hPa 36hPa ~ 1000ft
560hPa
Standard ICAO atmosphere

Furthermore, we have to deal with the 1013hPa isobar that varies :
above .
sea level (anticyclonic pressures)
Standard ICAO atmosphere

Nevertheless, we’ll use this model (1hPa=28ft) to calculate distances:
Between an aircraft and the ground Between two aircraft in flight
. . or below sea level (cyclonic pressures)
1009 hPa 1010 hPa 1011 hPa 1012 hPa 1013 hPa 1014 hPa
1011 hPa 1012 hPa 1013 hPa 1014 hPa 1015 hPa 1016 hPa