Every takeoff on a transport-category aircraft involves a deliberate decision about how much thrust to use. Using full rated thrust on every departure accelerates engine wear, increases maintenance costs, and shortens the time between overhauls. To address this, manufacturers and operators have developed two distinct, FAA-accepted methods of reducing takeoff thrust: derate and assumed temperature (flex) thrust reduction. Both methods are addressed in Advisory Circular 25-13, which provides guidance on the airworthiness and performance accountability requirements for these procedures. An ATP candidate must understand not only what each method does, but how they differ mechanically, how they interact with performance guarantees, and when one is preferable over the other.
It is important to recognize from the outset that these are not interchangeable terms for the same process. They operate through fundamentally different mechanisms, carry different certification implications, and can even be combined on aircraft whose flight management systems permit it. Confusing the two is one of the most common errors on ATP written examinations and oral evaluations.
How Derate Works
A derate is a certified, named reduction in the rated thrust of an engine for a specific phase of flight. When an engine is derated, the manufacturer certifies a new, lower thrust level — commonly labeled with a designation such as TO-1 or TO-2 — and the aircraft's entire performance data package (accelerate-stop distance, climb gradients, V-speeds) is re-derived at that lower thrust setting. In regulatory terms, the derated thrust level becomes a new rating in its own right, complete with its own approved takeoff field length, obstacle clearance, and second-segment climb gradient data.
Because the performance data are fully recalculated from the ground up at the lower thrust, a derate provides a genuine, contractual performance guarantee. The published V1, VR, and V2 speeds, as well as the accelerate-stop and accelerate-go distances, are all valid at the derate level. If an engine fails at V1 during a derated takeoff, the aircraft will still meet all Part 25 climb gradient requirements — 2.4% for a two-engine aircraft in the second segment, for example — because those requirements were computed at the derate thrust. This is the defining safety feature of a derate: performance integrity is fully preserved.
Typical derate levels reduce thrust by approximately 10% (TO-1) or 25% (TO-2) below the full rated value, though the exact percentages are type-specific and appear in the approved Airplane Flight Manual (AFM). Because these values are FAA-approved ratings, an operator cannot simply invent a derate level; the levels available are those certified during the original type certification or via subsequent supplemental type certificate.
How Assumed Temperature Works
The assumed temperature method — often called flex thrust in Airbus terminology or reduced thrust in Boeing documentation — works through a completely different principle. Instead of establishing a new lower certified thrust rating, the pilot or flight management system programs a temperature that is higher than the actual ambient temperature. The engine's FADEC or fuel control unit then limits thrust to the level it would produce at that higher, fictitious temperature. Because turbine engines produce less thrust as outside air temperature rises (hot air is less dense), programming a higher temperature causes the engine to hold back, producing less than full rated thrust.
The performance accountability for an assumed temperature reduction is more nuanced. The V-speeds, field lengths, and obstacle margins used for a flex takeoff are still computed at the actual ambient conditions, but the runway analysis must confirm that sufficient performance margins exist even with the reduced thrust. Critically, AC 25-13 and the AFM impose a floor on how much assumed temperature reduction is permitted: the reduced thrust level must not be less than 75% of the full rated takeoff thrust (a 25% maximum reduction). Additionally, the assumed temperature must not exceed the flat-rated temperature for the engine (often ISA+15°C or the engine's maximum flat-rated temperature, whichever is applicable) and must not exceed the actual ambient temperature — you cannot assume a temperature colder than reality.
Unlike a derate, an assumed temperature reduction does not result in a fully recertified performance package at the reduced thrust level. If an engine fails, the crew can advance thrust to the full rated level (or the rated derate level if a derate is also applied), and the published full-rated or derate performance data then governs. This ability to recover thrust in a failure scenario is a key operational distinction.
Combining Derate and Assumed Temperature
Some modern aircraft and FMS installations allow operators to use both methods simultaneously — sometimes called a derate plus flex combination. In this case, the baseline is a certified derate level (e.g., TO-1), and an assumed temperature reduction is applied on top of that lower baseline. The 75% floor and the temperature ceiling limits apply relative to the derate rating being used, not the full rated thrust. This approach can provide meaningful fuel and engine-wear savings while still maintaining the structural performance guarantee of the underlying derate. Operators must have this combination explicitly authorized in their OpSpecs and the AFM; it cannot be improvised.
Why It Matters: Safety and Operational Tradeoffs
The practical safety significance of these distinctions comes into focus when an engine actually fails on takeoff. On a pure assumed temperature (flex) takeoff, the remaining engine can spool up to full rated thrust. The crew then operates under the full-rated performance data, which typically gives them more runway and climb margin than they planned for — a conservative outcome. On a derated takeoff, if an engine fails, the operating engine spools up only to the derate level unless the crew manually advances to full rated thrust. However, because the entire performance schedule was computed at the derate, the aircraft still meets all regulatory gradient requirements at that derate level, so there is no safety shortfall.
From an engine-life perspective, derate is generally the more aggressive engine-preservation strategy because the thrust reduction is absolute and cannot be inadvertently overridden during a normal takeoff roll. Assumed temperature reductions, by contrast, are easily overridden by advancing the thrust levers to the full rated detent — a useful option in a performance emergency, but one that also means the engine is only one crew action away from full stress.
Key Numbers and Rules
- Maximum assumed temperature reduction: Reduced thrust must be no less than 75% of full rated takeoff thrust (i.e., a maximum 25% reduction via assumed temperature alone).
- Temperature ceiling: The assumed temperature must not exceed the engine's flat-rated temperature limit and must be greater than the actual ambient OAT.
- Derate levels: Only FAA-approved, named derate levels from the AFM may be used; operators cannot interpolate or create their own.
- Second-segment gradient (Part 25): 2.4% net for two-engine aircraft; derate performance data must satisfy this at the derate thrust level.
- Combined derate + flex: The 75% floor applies relative to the derate rating in use, not full rated thrust — requires specific AFM and OpSpec authorization.
- Contaminated runways: Many operators prohibit or restrict assumed temperature reductions on contaminated runways; derate use policies vary by company OpSpecs.
Common Test Traps
- Assuming the 75% rule applies to both methods equally. The 25% maximum reduction cap applies specifically to the assumed temperature method. Derate reductions can be larger (up to ~25-30% for TO-2 on some engines) because they come with fully recertified performance data.
- Thinking flex/assumed temperature is a certified thrust rating. It is not. Only derate levels are new certified ratings. Flex operates within the existing certification envelope using a fictitious reference temperature.
- Confusing recovery thrust in failure scenarios. On a flex takeoff, recovery to full rated thrust is available and expected. On a derate, recovery is to the derate level for performance accountability purposes unless the crew deliberately selects full rated — which may be necessary in an emergency but takes the aircraft outside the planned performance schedule.
- Ignoring the temperature direction. The assumed temperature must always be higher than actual OAT to reduce thrust. Entering a temperature lower than OAT would command more than rated thrust — which is not a valid or safe operation.
- Overlooking AFM and OpSpec authority. Neither method can be used simply because the FMS supports it. The AFM must authorize the procedure, and for Part 121/135 operators, the OpSpecs must explicitly permit it, including any combination of derate plus flex.