Altimeter tolerance range

[Pilotjohan]

Dear,


The ICAO document 8168 volume 1 Flight Procedures states the following note related to "Pre-flight operational test":


Note 2.— The tolerance of 20 m or 60 ft for altimeters with a test range of 0 to 9 000 m (0 to 30 000 ft) is considered acceptable for aerodromes having elevations up to 1 100 m (3 500 ft) (Standard atmospheric pressure). Table III-1-3-1 indicates the permissible range for aerodromes at different elevations when the atmospheric pressure at an aerodrome is lower than the standard, i.e. when the QNH setting is as low as 950 hPa.




When do we use the tolerance ranges in table III-1-3-1? Is it only when the elevation of the aerodrome is higher OR is it when the elevation is higher and the QNH is equal to or lower than 950 hPa?


Kind regards.

[Singapore ATPL]

From 0 to 30 000 ft, tolerance of 60 ft for altimeters is considered acceptable for aerodromes having elevations from 0 ft (Standard atmospheric pressure = 1013.25 hPa) up to 3 500 ft (Standard atmospheric pressure = 657.80 hPa)

You use the tolerance range when the atmospheric pressure at an aerodrome is lower than the standard. For example, the aerodrome elevation is 4 000 ft (Standard atmospheric pressure = 616.60 hPa), but the actual atmospheric pressure is lower than the standard 616.60 hPa (i.e. 608 hPa), that's when you have to refer to the Table.

The Table states that aerodrome elevation of 4 000 ft, permissible range is 3 925 ft to 4 075 ft. Therefore, when your altimeter reads within this range at an aerodrome elevation of 4 000 ft, [(atmospheric pressure (i.e 608 hPa) < Standard atmospheric pressure (616.60 hPa)], it is considered acceptable.

Hope this helps.

[Pilotjohan]

Dear,


So if the aerodrome elevation is 4000 ft and the actual atmospheric pressure is equal to the standard pressure of 616.60 hPa what is then the permissible range? Is it +/- 60 ft or +/- 75 ft?


Kind regards, Johan.