Definition
The highest weight at which an aircraft is permitted to begin the takeoff roll on a given runway under the existing conditions. It is determined by the most limiting of several factors: structural takeoff weight limits, runway length available, climb performance requirements (including one-engine-inoperative obstacle clearance), tire speed and brake energy limits, and field elevation, temperature, wind, and runway slope.
Plain English
The heaviest the aircraft is allowed to be when it starts the takeoff roll, given today's runway, weather, and obstacles. The lowest weight that any single rule produces is the one you must respect.
Context Anchor
Seen in takeoff planning, especially when checking whether a multi-engine aircraft can clear obstacles if one engine fails during the departure.
Derivation
Maximum comes from a Latin word meaning “greatest.” Allowable means “permitted.” In this term, the key idea is not simply what the aircraft can lift, but what weight is permitted after the conditions and safety requirements are applied.
Why Pilots Care
It limits how much fuel, passengers, or cargo can be carried and determines whether a safe takeoff is possible from a given runway.
Intuition Check
Do not assume maximum allowable takeoff weight always means the aircraft’s published maximum takeoff weight. Here, it means the highest weight allowed for this specific takeoff after performance limits, conditions, and obstacles are considered.
Example Sentence 1
After running the performance numbers for the high-elevation airport on a hot afternoon, the crew found the maximum allowable takeoff weight was 4,000 pounds below the aircraft's structural limit, so they offloaded fuel before departure.
Example Sentence 2
The OEI performance chart showed the maximum allowable takeoff weight dropped further when an obstacle lay just beyond the departure end.