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D6.2.3 Stirring the fuel
If the fuel in the aircraft tanks could be continuously agitated, it would remain fluid to
much lower temperatures, owing to the nature of hydrocarbon fuels.
Laboratory tests have shown that booster pump re-circulation, used in conjunction with
very slight tank rocking (less than 1 degree at a frequency of 6 cycles per minute),
lowered the pumpability temperature limit by 8°C to 11°C.

142

INDICATION

 

E1 GENERAL

The pressure (barometric) altimeters installed on the aircraft are calibrated to indicate
true altitude under International Standard Atmosphere (ISA) conditions.

This means that the pressure altimeter indicates the elevation above the pressure
reference by following the standard atmospheric profile.
Any deviation from ISA will, therefore, result in an incorrect reading, whereby the
indicated altitude differs from the true altitude.

Figure E1
Temperature greatly influences the isobaric surface spacing which affects altimeter indications.
When the temperature is lower than ISA, the true altitude of the aircraft will be lower than the figure indicated by the altimeter.
Specifically, this occurs in cold weather conditions, where the temperature may be considerably lower than the temperature of the standard atmosphere and may lead to a significant altimeter error.
A low temperature may decrease terrain clearance and may create a potential terrain clearance hazard. It may also be the origin of an altitude/position error.
144

E2 CORRECTIONS

Various methods are available to correct indicated altitude, when the temperature is lower than ISA. In all cases, the correction has to be applied on the height above the elevation of the altimeter setting source. The altimeter setting source is generally the atmosphere pressure at an airport, and the correction on the height above the airport has to be applied on the indicated altitude. The same correction value is applied when flying at either QFE or at QNH.
The choice of a method depends on of the amount of precision needed for the correction.
E2.1 Low altitude temperature corrections
● Approximate correction
Increase obstacle elevation by 4% per 10°C below ISA of the height above the
elevation of the altimeter setting source, or,
decrease aircraft indicated altitude by 4% per 10°C below ISA of the height above the
elevation of the altimeter setting source.

This method is generally used to adjust minimum safe altitudes and may be applied for all
altimeters setting source altitudes for temperatures above -15°C.

Figure E2
Example:
Let’s assume an airport elevation of 1000 ft. The airport elevation is the same as altimeter
setting source altitudes elevation = 1000 ft.
The ISA temperature at 1000 ft is 13°C.
　
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