Takeoff Climb Segments

When an engine fails during the take-off of a large aeroplane, the runway is only the first hurdle. The aeroplane must then climb away on the remaining engines, clean up and reach a height from which it can safely continue or return. Certification divides that climb into take-off climb segments, each with its own configuration, thrust and speed, and sets a minimum climb gradient for each. Failing to meet any of them at the planned mass means the mass must come down.

The requirements behind the segments are set in CS 25.111 and CS 25.121, harmonised with the same sections of 14 CFR Part 25, and the operating rules of Performance Class A apply them to every departure. They describe a deliberately pessimistic climb: one engine inoperative from VEF, the crew flying the certified speeds, and the gradients checked against minimums rather than against what the aeroplane usually achieves.

Take-off path overview

The take-off path extends from a standing start to the point at which the aeroplane is 1,500 ft above the take-off surface, or at which the transition to the en-route configuration is complete and the final take-off speed is reached, whichever is higher. The airborne part of it, the take-off flight path, begins at the screen height at the end of the take-off distance: 35 ft normally, and 15 ft on a contaminated runway in EASA exam texts. Above the end of the take-off path the en-route phase begins.

Throughout, the critical engine is assumed to have failed at VEF. The speed is held at V2 until at least 400 ft, and apart from retracting the landing gear the configuration may not be changed below that height. The gradients below are gross gradients, the aeroplane's certified capability; the obstacle analysis uses net gradients, reduced by 0.8 per cent for a twin, 0.9 per cent for a three-engined and 1.0 per cent for a four-engined aeroplane (see take-off flight path and obstacle clearance).

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The requirements rise with the number of engines because losing one of four engines removes a smaller share of the thrust than losing one of two, so a four-engined aeroplane is expected to keep more climb capability.

First segment

The first segment begins at the 35 ft screen height at the end of the take-off distance, where the take-off flight path starts, and ends when the landing gear is fully retracted. The aeroplane is in the take-off configuration with take-off flap, the remaining engines at take-off thrust, and the speed is held at V2, which must be reached by the screen height.

The gear is retracted as soon as a positive rate of climb is established, but it adds drag until it is stowed. The segment is short in height, yet the distance it covers can be considerable. A twin needs only a positive gradient, a three-engined aeroplane 0.3 per cent and a four-engined one 0.5 per cent.

Second segment

The second segment runs from the point where the gear is fully up to at least 400 ft above the take-off surface. The flaps are still at the take-off setting, the remaining engines at take-off thrust, and the speed is V2. The minimum gross gradient is 2.4 per cent for a twin, 2.7 per cent for a three-engined and 3.0 per cent for a four-engined aeroplane. For a twin that means a net gradient of 1.6 per cent for obstacle clearance.

It is usually the segment that limits the take-off mass. Its gradient requirement is the strictest of the four, the take-off flap still adds drag, the speed is fixed at V2, and it is flown on the thrust of the remaining engines. Thrust falls with temperature and pressure altitude, so hot and high aerodromes make the second segment the binding requirement, and the resulting limit is the climb-limited mass (see take-off mass limitations).

Accurate flying matters here. With one engine inoperative the aeroplane is flown with a small bank towards the live engine, which reduces the sideslip and its drag. On Airbus types the sideslip indicator on the PFD turns into a blue target in this situation. If the engine fails after V2 has already been passed, Airbus take-off guidance holds the speed at the moment of the failure, within the limits of V2 and V2 + 15 kt.

Extended second segment

The 400 ft level-off is a minimum, not a standard. When an obstacle lies further out along the departure track, the acceleration is delayed and the aeroplane keeps climbing in the second-segment configuration until it has cleared it. This is an extended second segment.

The extension has a limit: take-off thrust may be used only for a set time, normally 5 minutes, extended to 10 minutes for one-engine-inoperative operation where that has been demonstrated and approved. The A320's take-off thrust limit, for example, is 5 minutes with all engines and 10 minutes with one engine inoperative. The climb, the acceleration and the flap retraction must be completed within that time, because the final segment is flown at maximum continuous thrust.

Third (acceleration) segment

In the third segment the aeroplane levels off, or nearly so, at the acceleration height and uses its excess thrust to accelerate from V2 to the final take-off speed, retracting the flaps on schedule as the speed builds. Take-off thrust is maintained. A climb and an acceleration at the same time are impractical on the thrust of the remaining engines, so height gain is traded for speed.

Exam texts give this segment no climb gradient requirement, since the aeroplane is not climbing. The certification rule for the take-off path, 14 CFR 25.111 and its harmonised twin CS 25.111, still requires an available gradient of at least 1.2 per cent for a twin, 1.5 per cent for a three-engined and 1.7 per cent for a four-engined aeroplane at every point from 400 ft; in level flight that capability goes into acceleration instead of climb. The horizontal distance flown while accelerating still counts in the take-off flight path, so a distant obstacle must also be cleared by the level segment.

Final segment and VFTO

The final segment, also called the fourth segment, begins when the flaps are fully retracted and the aeroplane has reached the final take-off speed (VFTO). It is flown in the clean configuration, with gear and flaps up, at maximum continuous thrust (MCT), which unlike take-off thrust may be used without a time limit. It continues to at least 1,500 ft above the take-off surface, higher if obstacles require. The minimum gross gradient is 1.2 per cent for a twin, 1.5 per cent for a three-engined and 1.7 per cent for a four-engined aeroplane, a net gradient of 0.4 per cent for a twin.

VFTO must be at least 1.18 times the reference stall speed VSR in the en-route configuration, and high enough to give the manoeuvring capability the certification rules require. On Airbus types the corresponding clean-configuration target is green dot speed, the speed for the best climb gradient. On reaching it after an engine failure, LVR MCT flashes on the flight mode annunciator and the pilot selects MCT on the live engine. After a FLEX take-off this means moving the thrust lever to the CL or TOGA detent and back to FLX/MCT, since the FLEX rating otherwise stays active.

Acceleration and thrust reduction heights

The acceleration height is the height at which the pitch is reduced so that the aeroplane accelerates and the flaps can be retracted. The rules set a minimum of 400 ft above the take-off surface. With an engine inoperative, operators set an engine-out acceleration altitude, entered on Airbus types as EO ACC on the take-off performance page and cross-checked by both pilots. Airbus requires it to be at least 400 ft above the aerodrome, high enough for the net flight path to clear obstacles by 35 ft, and low enough that the maximum time at take-off thrust is not exceeded.

With all engines operating, two further heights shape the departure. At the thrust reduction height, or thrust reduction altitude, take-off thrust is reduced to climb thrust. At the all-engines acceleration altitude the aeroplane accelerates and cleans up. On the A320 both default to 1,500 ft unless airline policy sets other values, and at thrust reduction LVR CLB flashes on the flight mode annunciator and the thrust levers are moved to the CL detent. The 737 FMC accepts a thrust reduction altitude between 800 and 9,999 ft. Airline all-engines procedures commonly accelerate at 1,000 ft above the aerodrome, or at 3,000 ft under the close-in noise abatement departure procedure, NADP 1 (see noise abatement procedures).

Flap retraction schedule

Retracting flap at a constant angle of attack and speed reduces the lift coefficient, and the aeroplane sinks. Each stage of retraction is therefore made only once the aeroplane has accelerated past a minimum speed for the next configuration, which keeps an adequate margin above the stall.

Airbus defines these speeds on the speed scale. At F speed, with the speed increasing, the flaps are retracted to 1; at S speed, still accelerating, to 0. The lowest selectable speed VLS is 1.13 VS in the take-off configuration, 1.23 VS after the first step of retraction and 1.28 VS when clean. With CONF 1+F set, the flaps retract automatically to CONF 1 at 210 kt.

Boeing publishes a flap retraction schedule for the 737 that, for most take-off flap settings, begins at V2 + 15 kt:

The bank angle is limited to 15° until the speed reaches V2 + 15 kt. With one engine inoperative, the same schedule is flown after the engine-out acceleration height, and once clean the aeroplane continues at maximum continuous thrust into the final segment, leading into the en-route one-engine-inoperative performance phase.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.