Definition
The highest indicated airspeed at which an aircraft may be safely operated, which on certain high-performance airplanes varies with altitude. It is shown on the airspeed indicator by a movable pointer (often called a barber pole) that automatically adjusts as altitude changes, reflecting the lower of the structural limit (Vne or Vmo) or the Mach limit (Mmo) at any given altitude.
Plain English
The fastest speed the airplane is allowed to fly right now. In some airplanes this limit changes with altitude, so a moving pointer on the airspeed indicator shows the current limit at all times.
Context Anchor
Seen in airspeed indicator markings, aircraft operating limitations, and instrument flying discussions about different types of airspeed.
Derivation
‘Maximum’ comes from the Latin maximus meaning ‘greatest.’ ‘Allowable’ comes from the Old French alouer, ‘to permit.’ Together the term simply means the greatest speed permitted — with the important twist that in some aircraft, what is permitted depends on where you are in the atmosphere.
Why Pilots Care
Pilots must respect this limit to avoid damaging the aircraft or losing control.
Analogy
It is like a speed limit for the airplane. The aircraft may be capable of going faster, but that does not mean it is approved or safe to do so.
Intuition Check
Do not read “maximum” as “best performance speed.” Here it means the upper approved limit the pilot must stay below.
Example Sentence 1
As the jet climbed through the flight levels, the barber pole crept down the dial, showing the maximum allowable airspeed dropping with altitude.
Example Sentence 2
In the handbook, maximum allowable airspeed is listed among other airspeed types like indicated and true airspeed.