And really finally this time …
When I started this thread over a year ago I assumed that mph was the standard unit of speed during the early years of WW2, but that this changed to knots during the latter stages. I was attempting to find out when this change took place.
However, from the many replies it became clear that both knots and mph were in use at the same time – which seemed a strange state of affairs to a non-aviator.
It now transpires there was no standard procedure for much of the war. This from AP1234 (“Air Navigation”), Chapter 1, paragraph 62:
“It is immaterial whether the ground speed is measured in mph or knots, though it is customary to adopt the units in which the airspeed indicator happens to be calibrated. Many navigators prefer to use knots at all times, since the nautical mile is more convenient than the statute mile as a unit of distance when employing astronomical methods of navigation.”
Followed by paragraph 63:
“As a general rule the wind speed given by meteorologists is in mph unless specifically asked for in other units. The navigator may, however, measure the wind speed in whatever unit is most convenient to him.”
It wasn’t until 12 April 1945, and the issue of Air Ministry Order A.365/45, that mph was consigned to history with the adoption of the knot as the unit of speed (see my post No 24).
Brian
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