Definition
The amount of engine power, measured in horsepower, that the powerplant can actually deliver at a given moment under the current conditions of altitude, temperature, throttle setting, and engine condition. It is the power on hand to overcome drag and produce thrust, as opposed to the power that would be required for a given flight condition.
Plain English
The power the engine can actually produce right now, given the conditions you are flying in. It is what the engine has to give, not what you wish it had.
Context Anchor
Seen in performance and drag-curve discussions, especially when comparing the power the airplane needs with the power it can provide.
Derivation
Available' comes from the Latin valere, meaning 'to be strong' or 'to be of worth.' In this context it simply means 'on hand' or 'ready to be used' — the horsepower the engine can supply at this moment, not a theoretical maximum.
Why Pilots Care
It determines whether the aircraft can climb or maintain altitude, especially critical during instrument flight at higher altitudes where engine output falls.
Grounding Statement
If the airplane needs more horsepower than it has available, it cannot maintain that flight condition.
Intuition Check
Available horsepower does not mean the engine’s advertised maximum horsepower in all situations. It means the usable power the aircraft can actually produce under the conditions at that moment.
Example Sentence 1
At higher altitudes, available horsepower drops because the air is thinner, which is why climb rate decreases as the aircraft gets higher.
Example Sentence 2
The pilot checked the power-available curve before deciding whether to continue the instrument approach.