Assumed Temperature — Reduced Thrust Takeoff

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Reduced-thrust takeoff, Boeing's way: the assumed temperature method. Tell the engines it is hotter than it is and they produce less thrust.

What this scenario covers

Engine thrust falls as ambient temperature rises. ATM exploits that: enter an assumed temperature higher than the actual one and the FMC schedules a lower N1 — enough thrust for today's weight and runway, and no more.

Trying several values makes the trade tangible: more assumed temperature, less thrust, longer ground roll, shallower climb — all still inside the certified margins, because the assumed temperature came from a performance calculation that respects them.

How it applies in flight ops

The reason is engine life. Full-thrust takeoffs are the harshest duty cycle an engine sees, and reducing N1 even modestly extends time on wing across a fleet flying several sectors a day.

The limits are firm: no reduced thrust on a contaminated runway, none with windshear reported, and never below the minimum certified thrust for the conditions. Full thrust remains available immediately by advancing the levers, which is what makes the whole practice acceptable.

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Step-by-step: Assumed Temperature — Reduced Thrust Takeoff

  1. Open the N1 LIMIT page. ASSUMED TEMP lives at the bottom of it.

    The 737's Assumed Temperature Method (ATM) is set on N1 LIMIT. The FMC uses the assumed OAT to compute a reduced takeoff N1 — the engines spool to a lower fan speed than full thrust. The actual outside temperature does not change; only what the engines "think" it is.

  2. Try ASSUMED 40°C first — a small thrust reduction. Type 40 and press 5L.

    Assumed 40°C gives only a small reduction over the real +15°C OAT shown above. Real airlines rarely use this little because the engine-wear saving is small — but on a hot day where the real OAT is already 35°C, even +5°C of ATM is meaningful.

  3. Now try ASSUMED 50°C — a typical European short-haul value. Type 50 and press 5L.

    50°C is a common day-to-day value for the 737NG on a mild day. The FMC drops takeoff N1 by 3–5%. Engine T5 (turbine temperature) runs cooler, hot-section parts last longer, and you still have enough performance margin on a long runway.

  4. Now try ASSUMED 70°C — pushing toward the maximum. Type 70 and press 5L.

    70°C is around the upper limit accepted by Boeing's ATM on the CFM56-7B. The real limit is the airline's performance calculation — you cannot assume more than the runway, weight, wind, and weather actually allow. Forbidden on contaminated runways and at limit weights. Above this you must use full TO thrust.

  5. Set ASSUMED back to 50°C — the realistic operational value. Type 50 and press 5L.

    Real flights use whatever ASSUMED TEMP the EFB performance calculation produces — rarely a round number. You enter exactly what dispatch hands you. 50°C here is a stand-in for "the value computed for today's conditions, weight, and runway".

What to take away

  • ATM = assumed temperature: a higher figure schedules a lower N1.
  • The margin is preserved by the performance calculation, not given away.
  • Engine time on wing is the payoff, at fleet scale.
  • Prohibited contaminated or with windshear; TOGA is always one push away.

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