What is the Assumed Temperature Method for takeoff, and how does it reduce thrust?

The Assumed Temperature Method (ATM) is an FAA-approved technique in which the flight crew enters an outside air temperature higher than the actual OAT into the aircraft's performance or thrust management system. The system then commands the engines to produce only the thrust they would naturally generate at that warmer temperature, resulting in a legal reduction from full rated thrust. Because the actual ambient air is cooler and denser, the engines retain a reserve of available thrust that can be called upon in an emergency by advancing the throttles to full rated power.

Does using the Assumed Temperature Method change the climb gradient requirements under 14 CFR Part 25?

No. All four takeoff flight path segment gradient requirements under Part 25 remain exactly the same regardless of whether ATM is used. The second-segment one-engine-inoperative gradient minimums—2.4% for two-engine, 2.7% for three-engine, and 3.0% for four-engine transport-category airplanes—must be met at actual ambient conditions, not at the assumed temperature. ATM only determines the thrust setting used; it never relaxes the regulatory performance standards the airplane must satisfy.

When is the Assumed Temperature Method not allowed for a reduced-thrust takeoff?

ATM may not be used in several situations, including operations on contaminated runways (wet, slush, ice, or snow) unless the operator has specific approved contaminated-runway performance data authorizing reduced thrust, when Minimum Equipment List items explicitly prohibit it, and when the departure requires an obstacle departure procedure or SID gradient that consumes all available performance margin at full rated thrust. Additionally, the assumed temperature can never exceed the engine's flat-rated temperature (T-flat), and the operator must hold FAA Operations Specifications authorizing reduced-thrust takeoffs under Part 121.

Reduced Thrust Takeoff (Assumed Temperature Method) and Performance Accountability

Every revenue departure places two competing pressures on a flight crew: maximize engine longevity by reducing thermal wear, and guarantee that the airplane can meet every obstacle-clearance and climb-gradient requirement the regulations demand. The Assumed Temperature Method (ATM)—sometimes called flex thrust or the derate-via-temperature method—resolves that tension with engineering elegance. By entering a temperature higher than the actual outside air temperature (OAT) into the performance management system, the autothrottle or thrust management computer commands less than full rated thrust during the takeoff roll, because the engine would naturally produce that reduced thrust level at the warmer assumed temperature. The result is a legal, precisely calculated thrust reduction that meaningfully extends hot-section life without compromising safety margins. For the Airline Transport Pilot (ATP) written test and oral examination, a thorough understanding of ATM—including its limits, its interaction with Part 25 performance segments, and the conditions that prohibit its use—is essential.

How the Assumed Temperature Method Works

Jet engine thrust decreases as ambient temperature rises, up to the engine's flat-rated temperature (often called T-flat or T-ref). Below T-flat, the engine control system actively limits thrust to maintain a constant rated value regardless of temperature. Above T-flat, no further limiting is possible and thrust begins to fall naturally with rising temperature. ATM exploits the region below T-flat: the crew selects an assumed OAT higher than the actual OAT, and the thrust management system reduces thrust to the level that would be produced at that higher temperature—without actually being at that temperature. Because actual conditions are cooler and denser than the assumed temperature, the engine is in fact capable of producing more thrust; it is simply being commanded not to. The airplane thus departs with a defined, repeatable thrust reduction while retaining a cushion of available power if the crew needs to advance throttles to maximum in an emergency.

The assumed temperature must always remain at or below T-flat. If an operator were to enter a temperature above T-flat, the engine's rated thrust would already be declining in that regime, making the performance calculation circular and invalid. Aircraft Flight Manuals (AFMs) and approved performance software incorporate this limit automatically, but the pilot-in-command must understand why it exists. Beyond T-flat, the mathematical relationship between temperature and thrust breaks down in the context of the certified data, and no regulatory performance guarantee can be made.

The Critical Distinction: Thrust Is Reduced, Standards Are Not

The single most important concept about ATM—and the one most frequently tested—is what ATM does not do. It does not relax any performance standard. Every climb gradient, accelerate-stop distance, and obstacle clearance requirement imposed by 14 CFR Part 25 (airworthiness) and Part 121 (air carrier operations) must still be satisfied. Those requirements are evaluated against actual conditions: actual OAT, actual pressure altitude, actual runway length and slope, actual aircraft weight, actual wind, and actual flap configuration. The assumed temperature is solely a mechanism for selecting the thrust setting; the performance guarantee that protects the flight remains anchored in physical reality.

In practice, an airline's performance engineering department—or an approved performance tool such as an electronic flight bag application—computes the maximum assumed temperature permissible for a given departure. That temperature represents the threshold above which one or more regulatory requirements (climb gradient, accelerate-stop distance, obstacle clearance corridor) would be violated. The crew may select any assumed temperature up to that limit. Selecting the maximum permitted assumed temperature yields the largest thrust reduction and the greatest engine-preservation benefit; selecting a lower assumed temperature yields a more conservative reduction.

The Four Takeoff Flight Path Segments

Transport-category aircraft are certificated under Part 25 to demonstrate compliance with a four-segment takeoff flight path, all evaluated one-engine-inoperative (OEI) at maximum takeoff weight and actual ambient conditions. A reduced-thrust departure using ATM must still satisfy every segment.

Segment Definitions and Gradient Requirements

First segment — Begins at liftoff (VLOF) and ends when the landing gear is fully retracted. The airplane must maintain a positive climb gradient OEI with gear in transit. This is the most demanding drag condition of the entire takeoff profile.
Second segment — Gear fully retracted, flaps at the takeoff setting. The OEI climb gradient must meet the minimums specified in Part 25: at least 2.4 percent for two-engine airplanes, 2.7 percent for three-engine airplanes, and 3.0 percent for four-engine airplanes. This segment continues to at least 400 feet AGL and is often the limiting segment for ATM calculations because gradient requirements are most stringent relative to available climb performance.
Third segment (acceleration segment) — The airplane accelerates at approximately constant altitude (or in a shallow climb) from the second-segment speed to the final segment speed, allowing flap retraction. Thrust is maintained; the primary regulatory concern is ensuring obstacle clearance is not lost during the acceleration.
Fourth segment (final segment) — Flaps fully retracted, the airplane climbs at the final segment speed to at least 1,500 feet AGL (or the en-route obstacle clearance altitude). The required OEI gradient is 1.2 percent for two-engine airplanes, 1.5 percent for three-engine airplanes, and 1.7 percent for four-engine airplanes.

All gradient checks use actual OAT and pressure altitude, never the assumed temperature. Because second-segment gradient is so frequently the binding constraint, the maximum allowable assumed temperature is often set by second-segment gradient margin rather than by accelerate-stop distance or obstacle clearance.

Conditions That Prohibit or Restrict ATM

ATM is not universally applicable. Certain operational conditions require full rated thrust or limit the degree of reduction permitted:

Contaminated runways — Wet, slush, standing water, ice, or snow significantly degrades accelerate-stop performance. Unless the operator possesses specific approved contaminated-runway performance data authorizing reduced thrust, full rated thrust is required.
Inoperative items on the Minimum Equipment List (MEL) — Some MEL items explicitly prohibit reduced-thrust takeoffs or restrict the maximum permissible reduction. The MEL and associated CDL (Configuration Deviation List) must be checked before applying ATM.
Obstacle-critical departure procedures — Standard Instrument Departures (SIDs) or obstacle departure procedures (ODPs) that require climb gradients exceeding the standard 200 ft/NM may limit or eliminate the available ATM margin entirely.
Airport, runway, or weight combinations where no margin exists — If a performance-limited takeoff already uses the full available runway and barely satisfies second-segment gradient at full rated thrust, no assumed temperature reduction is possible.

ATM Versus Fixed Derate: An Important Distinction

ATM is sometimes confused with a fixed derate (for example, TO-1 or TO-2 on certain Boeing aircraft). These are fundamentally different in their regulatory basis. A fixed derate establishes a lower thrust rating that is independently certificated with its own AFM performance data. The aircraft is, in effect, temporarily re-certified to that lower thrust level, and all performance numbers in the relevant data tables are derived at the derate thrust level. ATM, by contrast, reduces thrust within the full-rated performance data set using an assumed OAT—no separate certification is required because the calculation is derived from the existing certified data. The two methods may sometimes be legally combined (applying an assumed temperature on top of a fixed derate), but the combined reduction must remain within limits approved in the AFM and the operator's Operations Specifications.

Key Numbers and Rules

Assumed temperature must be at or below T-flat (the engine's flat-rated temperature limit).
Second-segment OEI gradients: 2.4% (2-engine), 2.7% (3-engine), 3.0% (4-engine).
Final-segment OEI gradients: 1.2% (2-engine), 1.5% (3-engine), 1.7% (4-engine).
All performance segments evaluated at actual ambient conditions, not the assumed temperature.
The crew retains authority to advance to full rated thrust at any time during the takeoff roll if conditions require it.
ATM authorization must appear in the operator's Operations Specifications (OpSpecs) issued by the FAA under Part 121.

Common Test Traps

"ATM reduces the required climb gradient margins." False. All gradient requirements are identical to a full-thrust departure; only the thrust level used to meet them changes.
"The assumed temperature may exceed T-flat if the reduction is small." False. T-flat is an absolute ceiling for the assumed temperature, regardless of the size of the reduction sought.
"You may always use ATM." False. Contaminated runways, certain MEL items, obstacle-critical procedures, and weight/field-length-limited conditions may prohibit or restrict reduced-thrust operations.
"ATM and a fixed derate are the same thing." Not correct. A fixed derate is independently certificated to a lower thrust level with its own performance tables; ATM reduces thrust within the existing full-rated data set via an assumed OAT. Their regulatory bases differ, and they may sometimes be combined within AFM limits.
"If the assumed temperature takeoff succeeds, the airplane could not have done better at full thrust." False. Because actual conditions are cooler than the assumed temperature, the engine retains reserve thrust capacity. The crew can always advance to full thrust if an emergency or unexpected obstacle requires it.

Memory Aid

"Flex the thrust, not the rules." ATM allows the crew to flex (reduce) engine thrust for economic and mechanical longevity benefits, but every regulatory performance standard—gradients, distances, obstacle clearances—is evaluated at actual conditions. The rules themselves never flex.