HC Deb 09 March 1938 vol 332 c1881
40. Lieut.-Commander Fletcher

asked the Under-Secretary of State for Air what is his Department's definition of an aerodrome obstruction; and in what Air Ministry publication the definition is to be found?

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

The definition of an aerodrome obstruction is given in paragraph 5 of Air Ministry Pamphlet No. 55, and is as follows: Any object beyond the perimeter and within half a mile of the aerodrome which subtends a vertical angle of more than 3° 45 measured from the nearest point on the perimeter.