Definition
The amount of thrust an airplane's engine must produce to exactly overcome total drag and maintain steady, level flight at a given airspeed, weight, and configuration.
Plain English
The push the engine has to make to keep the airplane flying level at a chosen speed. If thrust matches drag, the airplane holds its speed and altitude. If it falls short, the airplane slows or descends.
Context Anchor
Seen in takeoff and ground effect discussions, especially when comparing how easily an airplane can lift off near the runway with how well it can climb after leaving ground effect.
Derivation
Thrust comes from an older word meaning to push. Required means needed. In aviation, the phrase points to the push the airplane needs, not necessarily the push the engine is able to give.
Why Pilots Care
In ground effect the drop in induced drag reduces thrust required, allowing shorter takeoffs; the increase as the airplane climbs out of ground effect must be anticipated to avoid settling.
Grounding Statement
Close to the runway, the airplane may need less forward push to stay airborne than it will need a few feet higher.
Intuition Check
Do not read required as what the engine is producing. It means what the airplane needs to overcome drag in that flight condition.
Example Sentence 1
In ground effect, induced drag is reduced, so the thrust required to maintain level flight is lower than it will be once the airplane climbs above one wingspan.
Example Sentence 2
Once the airplane climbs above one wingspan the thrust required rises and the pilot may need to add power to maintain climb speed.