HC Deb 23 February 1938 vol 332 c344
32. Lieut.-Commander Fletcher

asked the Under-Secretary of State for Air whether he is aware that the Air Ministry instructions directing pilots to fly during bad visibility at specified heights for specified courses, with a view to preventing collisions, fail to take account of the fact that as between say England and Germany there may be in certain weather conditions a difference of atmospheric pressure on the ground equivalent to a difference of height in the air of as much as 600 feet, so that converging aircraft within the three-mile limit off the British coast might in fact be flying at the same height although their altimeters showed a difference in height, and that the instructions are therefore dangerous; and whether he will cause them to be withdrawn and re-issued with a table of corrections to a given datum for a given district?

The Under-Secretary of State for Air (Lieut.-Colonel Muirhead)

Adjustments to the altimeter, which is fitted in the standard instrument panel of service aircraft and used by the majority of civil aircraft, to allow for variations of barometric pressure can and should be made by pilots while in flight, and constitute an adequate safeguard in the circumstances visualised. Information as to barometric pressure can be obtained from all local meteorological stations before starting the flight or in Britain from the Borough Hill broadcast system every hour or on request from any Air Ministry Control Station while in the air.