Thrust, Flap and Speed Optimisation - ATPL 032 — Performance

Flex and derate

Two different ideas hide behind reduced take-off thrust. Flex (reduced thrust by the assumed temperature method) pretends the day is hotter than it is: the crew finds the highest temperature at which the take-off would still work at the actual mass, and sets the thrust the full rating would give on that imaginary day. It is not a new rating: the full thrust remains one lever-push away, and every control speed, VMCG and VMCA included, stays that of the full rating. A derate is a genuine re-certification downward: a lower rating with its own limits and its own lower VMCG and VMCA, and precisely because the control speeds were built on the lower thrust, pushing the levers back up during the take-off is forbidden.

The trade both share: less thrust means less engine wear, longer engine life and lower cost, against longer distances and shallower climbs with thinner margins. The scheduled speeds move with the calculation (flex speeds are computed at the assumed temperature), and one comfort rides with flex: the true airspeed behind each IAS on the real, cooler day is lower than the assumed-day calculation used, so the actual margins are slightly kinder than the paper ones. The permissions differ where the runway is slippery: flex is not permitted on a contaminated runway (nor when conditions like windshear warnings demand full thrust), though a merely wet runway still admits flex with the appropriate wet data, while a derate remains available even on contamination, its lower VMCG unlocking the lower V1 the previous lesson wanted.

The flap trade, optimised

The flap setting bargains between the two great limits. More flap lowers the lift-off and screen speeds, so the field-limited mass rises; the same flap drags on the second segment, so the climb- and obstacle-limited mass falls. Plot both against flap and the optimum flap setting sits where the binding limits cross: at any less flap the field is the cap, at any more the climb is, and the performance-limited mass peaks at the crossing. Given a table of limits per flap setting, the drill is mechanical: for each setting take the lowest limit, then choose the setting whose lowest limit is highest.

Improved climb: the increased-V2 procedure

When the runway is long but the WAT or obstacle limit bites, the surplus field can be converted into climb. The increased-V2 procedure accelerates further along the runway to a V2 above the minimum schedule: the second segment is then flown closer to the speed the gradient actually prefers, and the climb- and obstacle-limited mass rises. The price is paid on the ground: higher V1, VR and VLOF, longer take-off and accelerate-stop distances, and the procedure is only usable where that field-length surplus genuinely exists. Its hazards are the fast speeds themselves: V1 pressed toward the brake-energy ceiling with a rejected take-off that arrives at maximum energy, and VLOF pressed toward the tyre-speed limit; both are managed by checking the VMBE and tyre margins in the analysis and by the crew treating the high-speed reject with the go-minded discipline it deserves.

The hard ceilings

Brake energy and tyre speed close the module the way they close the envelope. The VMBE cap on V1 and the tyre-speed cap on VLOF can each become the binding take-off limit, shaving the allowed mass when the scheduled speeds press against them. They are most likely to bite when the true speeds over the ground are highest: hot, high, heavy, with a tailwind or a downslope, and whenever the increased-V2 procedure has deliberately pushed the schedule upward. When one of them binds, the remedies run backwards through this lesson: less mass, a different flap, or giving the improved climb back.

Flex and derate

Check you understood

The flap trade, optimised

Improved climb: the increased-V2 procedure

The hard ceilings