Reduced and Derated Takeoff Thrust

Most airline take-offs do not need all the thrust the engines can give. When the actual mass is below the maximum the runway, climb and obstacle limits allow for the day, the surplus can be used to take off with less thrust. Doing so lowers turbine temperatures and engine wear and reduces noise, while every certified take-off requirement is still met.

There are two ways to reduce take-off thrust. The assumed temperature method, called FLEX on Airbus types, sets the thrust the engine would give on a hotter day. A derate selects a lower take-off rating, certified with its own data. The FAA's guidance on both is Advisory Circular 25-13. The two methods differ in ways that matter when something goes wrong.

Thrust ratings: TOGA, MCT and climb

A rated thrust is the maximum thrust an engine is certified to deliver in a given phase, with its own limits on temperature and time. Three ratings dominate a jet's operation:

• Take-off/go-around thrust (TOGA), the highest rating, limited in time. On the A320 it may be used for 5 minutes with all engines operating and 10 minutes with one engine inoperative, with an EGT limit of 950 °C on the CFM56-5B.
• Maximum continuous thrust (MCT), slightly lower, available without a time limit. It is the rating used with an engine inoperative once the take-off is complete, in the final take-off segment and en route.
• Climb thrust (CLB), the maximum for normal climb, slightly less than MCT.

On the A320 the thrust levers have detents for TO/GA, FLX/MCT, CL and idle, and the FADEC applies the rating that matches the detent. The Embraer E-Jets offer a choice of take-off ratings, TO-1 being the highest, with TO-2 and, on the E190-E2, TO-3 below it; several have a higher one-engine-inoperative value. On the E190-E2, with the thrust levers at TO/GA and the automatic take-off thrust control system (ATTCS) on, the FADEC raises the remaining engine to this reserve thrust when the N1 difference between the engines exceeds 15 per cent, as after an engine failure.

Modern turbofans are flat rated: they deliver the same maximum thrust from a cold day up to a flat-rating temperature, called TREF on Airbus types, above which the available thrust falls as the temperature rises. The assumed temperature method exploits this behaviour. The engine and its ratings are described in gas turbine principles, thrust and ratings.

Why reduce take-off thrust

The margin comes from the regulated take-off mass. When the actual mass is below it, the aeroplane could meet every requirement with less thrust (see take-off mass limitations). Using less thrust lowers exhaust gas temperature and the stress on the hot section, which extends the engine's life on the wing and reduces maintenance cost, and it reduces noise.

A reduced-thrust take-off gives up only the unused margin. The calculation still shows that the aeroplane meets the field length, climb gradient and obstacle clearance requirements at its actual mass with the reduced thrust. The maximum reduction is 25 per cent below the certified take-off rating: Boeing's manuals give it as 25 per cent below any certified rating, and EASA exam texts use the same figure.

Assumed temperature method

In the assumed temperature method (ATM), also called assumed temperature thrust reduction, the performance calculation finds the highest temperature at which the actual mass would still be the maximum permissible mass for the runway. That temperature is the assumed temperature. The crew enters it, and the engine control sets the take-off thrust it would give on a day that hot. Because the real air is cooler than assumed, the thrust set is less than the engine could deliver.

In the chart method taught for EASA exams, the V1, VR and V2 used are those for the assumed temperature. The real air is also colder and denser than assumed, so the true airspeed at each indicated speed is lower and the aeroplane reaches its speeds in a shorter distance than calculated. This adds an unscheduled margin to the reduced-thrust take-off.

The engine's rating does not change. Full take-off thrust remains available, and the minimum control speeds VMCG and VMCA must therefore still be those for full rated thrust, since the crew may apply it (see minimum control speeds). Boeing's terms are the assumed temperature method or reduced thrust; Airbus calls it the flexible take-off, or FLEX.

FLEX take-off thrust

On Airbus types the crew enters the flexible temperature, FLEX TO TEMP, on the MCDU PERF TAKE OFF page together with the take-off speeds. For take-off the thrust levers are set first to about 50 per cent N1, then to the FLX/MCT detent, and the pilot monitoring checks that the N1 has reached the rating limit before 80 kt. The thrust limit mode and the FLEX temperature appear in blue on the engine display.

The flexible temperature must respect three limits on the A320:

• it may not be higher than TMAXFLEX, which is ISA + 53 °C, about 68 °C at sea level;
• it may not be lower than the flat-rating temperature TREF;
• it may not be lower than the actual outside air temperature.

FLEX is not permitted on a contaminated runway, but it is permitted on a wet runway using wet-runway performance data. Changing the FLEX temperature during the take-off roll has no effect; the rating set when the roll began is kept. Pushing the thrust levers to TO/GA gives full take-off thrust at any time.

If an engine fails at or after V1 during a FLEX take-off, no thrust lever action is required. FLEX performance is computed for an engine failure at V1 with FLEX thrust on the remaining engine, so the FADEC keeps the FLEX rating, although the crew may still select TOGA if more thrust is needed. The rating changes from FLEX to MCT only when the thrust lever is moved to TO/GA or CL and then back to FLX/MCT, normally at green dot speed when LVR MCT flashes on the flight mode annunciator. Once that has happened, FLEX is no longer available for the rest of the flight.

Derated take-off thrust

A derated take-off thrust is a lower take-off rating certified for the engine, with its own flight manual performance data. For performance purposes the aeroplane behaves as if it had smaller engines. On the E-Jets the ratings below TO-1 fill this role, and on the 737 a take-off thrust derate can be selected in the FMC.

Because the derated rating is certified, its minimum control speeds are determined at the lower thrust, and they are lower than those for full thrust. On a short, wet or slippery runway, where V1 would otherwise be held up by VMCG, a derate can therefore allow a lower V1 and a higher permissible mass. The same property imposes the key restriction: if thrust were increased above the derated rating at low speed, the aeroplane could be below the minimum control speed for the higher thrust. FAA guidance notes that full thrust cannot always be restored during a derated take-off, and the A320 does not permit TOGA during a derated take-off except when an abnormal or emergency procedure calls for it.

Derated thrust is also treated differently on contaminated surfaces. The A320 limitations permit a derated take-off whatever the runway condition, dry, wet or contaminated, while FLEX is excluded from contaminated runways.

Assumed temperature versus derate

On the 737 the thrust mode display shows R-TO whenever the take-off N1 limit is reduced, whether by an assumed temperature, a derate or both, so the annunciation alone does not tell the crew which method is in use.

Reduced and derated climb thrust

Thrust can also be reduced in the climb. On the 737 the crew can select two fixed climb derates on the FMC N1 limit page: CLB-1 lowers the climb N1 limit by 3 per cent, about 10 per cent less thrust, and CLB-2 by 6 per cent, about 20 per cent. The reduction is phased out gradually, reaching full climb thrust by 15,000 ft, and the thrust mode display shows R-CLB. Derated climb thrust reduces engine maintenance costs but increases the total trip fuel, so operators weigh one against the other.

Matching the climb rating to the take-off thrust also avoids a thrust increase at the thrust reduction altitude after a heavily reduced take-off. The E190-E2 applies that logic directly: CLB-1 is the default climb rating unless it would be higher than the selected take-off thrust, in which case CLB-2 becomes the default.

Restrictions on reduced thrust

Reduced take-off thrust uses up margin that full thrust would keep in hand, so it is not used where that margin may be needed. Across manufacturers' limitations and exam texts, the main restrictions on the assumed temperature method are:

• contaminated runways, with standing water, slush, snow or ice, where the contamination already cuts V1 and the permissible mass;
• anti-skid inoperative, a limitation in the 737 manual and in exam lists;
• windshear, reported, forecast or suspected, because an escape needs full thrust at once;
• the increased V2 or improved climb procedure, which uses the same runway margin;
• the limits of the method itself: no more than 25 per cent below the rating, and on Airbus types a flexible temperature no lower than TREF or the actual temperature and no higher than TMAXFLEX.

Exam texts also list reverse thrust inoperative and an inoperative power management computer on some types, and minimum equipment list items can prohibit FLEX. Operators often add their own policy on top, and the crew can always choose full thrust. A reduced-thrust take-off is a saving on engine wear, never a reason to accept a smaller margin than the rules require (see wet and contaminated runways).

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.