Air Mileage Unit

Air Mileage Unit Mk.4A.
Ref. No. 6B/554
Kelvin & Hughes Ltd.
Basingstoke.
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Serial No. 3265/51
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Modifications
Inst./B/84
Inst./B/107
Inst./B/112
Inst./B/203
Inst./B/212
Inst./B/215
Inst./B/232
Inst./B/262
Inst./B/263
Inst./B/264
Inst./B/265
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Clamp ring:
Ref. No. 6B/2577
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Ink markings:
STI. Inst. 194
REM/R/2/71
408

In the Air Mileage Unit air pressure from the pitot head is directly balanced against an artificial pressure produced by a motor-driven fan, so that the average speed of the motor provides either the true airspeed at any height within reason, or, if required, the, true air mileage. This is a Mk4 AMU and was fitted to the Canberra and located in the port wheel well. The air mileage is fed into a Position Indicator, placed directly in front of the navigator via a flexible drive, and the latitude and longitude can be derived and displayed on a pair of counters. These counters are periodically re-set manually when definite visual or other fixes are obtained, so that the result at any particular moment is accurate enough for normal navigational purposes. Accuracy was not up to modern GPS standards.