Indicated Airspeed - IAS and V Speeds

IAS and V Speeds

The aircraft's pilot manual usually gives critical V speeds as IAS, those speeds indicated by the airspeed indicator. This is because the aircraft behaves similarly at the same IAS no matter what the TAS is: E.g. A pilot landing at a hot and high airfield will use the same IAS to fly the aircraft at the correct approach and landing speeds as she would when landing at a cold sea level airfield even though the TAS must differ considerably between the two landings.

Whereas IAS can be reliably used for monitoring critical speeds well below the speed of sound this is not so at faster speeds. An example: Because (1) the compressibility of air changes considerably approaching the speed of sound, and (2) the speed of sound varies considerably with density and therefore altitude; the maximum speed at which an aircraft structure is safe, the never exceed speed (abbreviated V_NE), is specified at several differing altitudes in faster aircraft's operating manuals, as shown in the sample table below.

Ref: Pilot's Notes for Tempest V Sabre IIA Engine - Air Ministry A.P.2458C-PN