A transport aircraft does not require maximum takeoff thrust for every departure. When approved performance calculations demonstrate that less thrust is sufficient, a lower thrust setting may reduce engine stress while preserving every applicable takeoff-performance requirement.
On a long runway, with a relatively light aircraft and favorable environmental conditions, the engines may be capable of producing considerably more thrust than is necessary. Using reduced thrust does not mean accepting a lower safety standard. The aircraft must still satisfy the applicable runway, climb, obstacle and controllability requirements for the conditions included in the calculation.
Reduced thrust is not less performance than required. It is less thrust than the maximum available—used only when the required performance remains protected.
01 — WHY REDUCE THRUST?
Maximum thrust is not always necessary
Takeoff performance changes from one departure to another. Aircraft weight, runway length and slope, airport elevation, temperature, wind, runway condition, obstacles, configuration, systems in use and inoperative equipment may all influence the thrust required.
Two aircraft of the same type can depart from the same runway with different thrust requirements. An aircraft close to its limiting weight may require maximum takeoff thrust. A considerably lighter aircraft may satisfy every applicable requirement with less.
Applying maximum thrust when it is not required exposes the engines to higher temperature and mechanical load without providing a necessary operational benefit. Reduced thrust uses available performance margin through an approved calculation.
Reduced thrust may be used only when the calculated thrust still provides the performance required for that specific takeoff.
02 — TWO DIFFERENT CONCEPTS
Reduced thrust and derated thrust
Two concepts are commonly associated with takeoff thrust below the maximum available level. They may produce similar engine indications, but they are not the same certification or performance concept.
The terminology, selections and limitations shown in the cockpit vary among aircraft, engines and operators. The important question is not only how much thrust is displayed, but which certified rating forms the basis of the performance calculation.
03 — ASSUMED TEMPERATURE
Why a higher temperature produces less thrust
The assumed-temperature method calculates takeoff performance as though the outside temperature were higher than the temperature that actually exists. Jet-engine thrust capability generally decreases as ambient temperature rises. Performance based on a higher assumed temperature can therefore establish a lower takeoff thrust setting.
The aircraft is not being told that the assumed value is the real weather condition. Actual environmental temperature remains real and relevant. The assumed temperature is part of the approved method used to establish the thrust setting.
Manufacturers may use terms such as FLEX temperature, assumed temperature, selected temperature or ATM—Assumed Temperature Method. These expressions can describe the same general principle, but the implementation remains aircraft-specific.
If the actual temperature is 20°C and the approved calculation establishes an assumed temperature of 50°C, the higher value provides the calculated reduced-thrust setting. The 50°C value does not describe the actual weather.
A higher assumed temperature generally produces a lower calculated thrust setting, but only within the approved limits of the aircraft and performance data.
04 — FIXED DERATE
A lower certified takeoff rating
A derated takeoff uses a lower certified takeoff thrust rating. For performance purposes, that rating becomes the takeoff thrust basis. Separate or clearly distinguishable limitations and performance data support it in the approved aircraft documentation.
Some aircraft identify multiple thrust ratings with designations such as TO, TO 1 or TO 2. Exact names and capabilities vary. A fixed derate is not simply another temperature entry; it changes the certified rating forming the basis of the takeoff calculation.
Where the aircraft and operator approve a combined method, the derated rating becomes the starting point and an additional assumed-temperature reduction may be calculated from it.
05 — ENGINE BENEFIT
Protecting engine condition across many cycles
Turbine engines experience high thermal and mechanical demand during takeoff. High thrust is associated with elevated turbine temperatures, internal pressure, rotational load and thermal expansion. These conditions are expected and remain within the engine's certified capability.
Maximum takeoff thrust is not harmful when required and used according to approved procedures. Repeatedly applying it when unnecessary, however, increases cumulative exposure to demanding conditions.
The takeoff segment is brief. Any immediate fuel-flow difference is not the main reason for the technique. Its principal value is engine preservation over hundreds or thousands of cycles.
06 — PERFORMANCE FIRST
One calculation, one connected solution
Reduced thrust cannot be selected merely because the runway looks long, the aircraft feels light, another crew used FLEX earlier, or the same assumed temperature was used yesterday. Every departure is a new performance problem.
The thrust setting, takeoff speeds, weight, configuration, runway, takeoff position and environmental conditions form one integrated solution. A value from one calculation must not be combined with speeds or configuration data from another.
Thrust, takeoff speeds, aircraft weight, configuration and runway condition must belong to the same approved performance calculation.
The calculation must continue to protect runway distance, climb capability, obstacle clearance, aircraft control, system effects, brake energy, tire speed and other applicable limitations.
07 — THRUST AVAILABILITY
Can greater thrust be selected?
During an assumed-temperature takeoff, the approved method is generally developed so that the applicable takeoff rating—or derated rating when that is the performance basis—can be selected within the certified framework. The circumstances and method remain governed by aircraft procedures and operator SOPs.
During a derated takeoff, the derated rating is itself the selected takeoff rating and operating limit. Availability and approved use of thrust above that rating are aircraft-specific.
Universal statements such as “TOGA is always immediately available” or “thrust can never be increased after selecting a derate” can therefore be misleading. Pilots must know which rating was selected, whether assumed temperature was added and which approved procedure governs any increase.
08 — LIMITATIONS
Why there is no universal prohibition list
Conditions that may restrict reduced-thrust operation include insufficient runway or climb performance, limiting obstacles, adverse runway conditions, windshear precautions, inoperative equipment, system penalties, anti-ice use and operator-specific limitations.
The effect is not identical across all fleets. Wet and contaminated runways are not interchangeable terms, and an approved method may be available under one surface condition while prohibited under another.
A statement that takeoff thrust can never be reduced by more than 25 percent should also not be presented as a universal pilot rule. FAA and EASA guidance must be understood within their complete certification frameworks. The applicable limit comes from aircraft-specific approved data.
Reduced thrust is available only when the approved performance calculation and applicable procedures explicitly support its use.
09 — COMMON MISUNDERSTANDINGS
Seven ideas pilots should correct
FLEX and derate are the same thing
Both can produce thrust below maximum, but they are different certification and performance concepts.
The assumed temperature becomes the real temperature
The actual environmental temperature remains real. The assumed value belongs to the approved thrust-calculation method.
Reduced thrust removes normal takeoff requirements
Every applicable requirement included in the approved calculation must still be satisfied.
There is always a universal 25 percent limit
The permissible reduction depends on the applicable certification basis and approved aircraft data.
Maximum thrust damages the engine
Maximum thrust is a certified capability and must be used whenever performance, conditions or procedures require it.
A long runway automatically permits FLEX
Runway length is only one element. Weight, temperature, wind, elevation, obstacles, surface condition and systems also matter.
Reduced thrust is mainly used to save takeoff fuel
Its principal benefit is lower cumulative engine stress and maintenance exposure.
PROFESSIONAL KNOWLEDGE REVIEW
The pilot's mental model
- 01
What is the applicable takeoff thrust rating?
- 02
Are we using that rating, a derated rating or an assumed-temperature reduction?
- 03
Do the thrust setting, speeds and configuration come from the same calculation?
- 04
Do the present runway, weather and aircraft conditions still match the calculation?
- 05
What do the aircraft procedures say about increasing thrust if required?
KEY TAKEAWAYS
What the professional pilot should retain
- Maximum takeoff thrust is not required for every departure.
- Reduced thrust is primarily used to reduce engine stress and maintenance-related deterioration.
- It may be used only when all applicable performance requirements remain satisfied.
- Assumed temperature/FLEX and fixed derate are different concepts.
- Assumed-temperature reduction is variable; fixed derate uses a lower certified rating.
- Some aircraft may combine the methods when approved.
- Indications, limitations and thrust-increase logic are aircraft-specific.
- Thrust, speeds and configuration must originate from the same approved calculation.
- Maximum thrust remains appropriate whenever required.
OFFICIAL REFERENCE FRAMEWORK
Traceable primary sources
These official sources establish the certification concepts and engine-life context used in this educational synthesis. Current version, status and aircraft applicability must always be verified.
FAA — AC 25-13: Reduced and Derated Takeoff Thrust (Power) Procedures↗EASA — CS-25, AMC 25-13: Reduced and Derated Take-off Thrust (Power) Procedures↗EASA — CS 25.1591 and AMC 25.1591: Slippery Wet and Contaminated Runways↗Airbus Safety First — Prevention of EGT Overlimit Events↗This NFA article develops professional understanding. It does not establish takeoff-performance procedures or provide aircraft-specific performance data. It does not supersede ICAO provisions; FAA, EASA, ANAC or other applicable regulations; AFM, FCOM, FCTM, QRH or MEL/CDL material; approved performance applications; operator manuals; or company SOPs. Operational decisions must always use current authoritative documentation and approved procedures applicable to the aircraft, operator and operation.