§ Lieut.-Commander Fletcherasked the Under-Secretary of State for Air what types of altimeters are in use in commercial aeroplanes and by private owners; what is the degree of accuracy of such altimeters; and whether the directional system necessitates the employment in all such aircraft of sensitive altimeters?
§ Lieut.-Colonel MuirheadAny type of altimeter may be used on commercial and private aircraft, but in practice an aneroid type is carried. The error of the aneroid type varies with the height of the aircraft, but in general this error is about 300 feet, which is equivalent to one layer of the directional system. This is an instrumental error which cannot be eliminated. The sensitive altimeter was introduced with the object of providing a more accurate height indication when approaching an aerodrome to land, and under these conditions the error may be as much as 50 feet. At greater heights the error increases, in common with other types of altimeter, but the improvement in accuracy of the sensitive type is not such as to necessitate its employment on aircraft unless blind approaches to earth are envisaged.