Assumed Temperature — Reduced Thrust Takeoff
PremiumbeginnerReduced-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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The CDU checks every entry as you make it, and tells you when an entry was accepted but wrong.
Step-by-step: Assumed Temperature — Reduced Thrust Takeoff
- Open the N1 LIMIT page. The assumed temperature lives on its top line, labelled SEL.
The 737's Assumed Temperature Method (ATM) is set on N1 LIMIT, on the SEL/OAT line at the very top of the page: SEL before the slash is the temperature you are pretending it is, OAT after it is what it really is. The FMC uses the assumed figure 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.
- Try SEL 40°C first — a small thrust reduction. Type 40 and press 1L.
Assumed 40°C gives only a small reduction over the real +15°C OAT shown after the slash. 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.
- Now try SEL 50°C — a typical European short-haul value. Type 50 and press 1L.
50°C is a common day-to-day value for the 737NG on a mild day. Watch the computed takeoff N1 opposite it at 1R fall as you raise the assumed temperature. Engine T5 (turbine temperature) runs cooler, hot-section parts last longer, and you still have enough performance margin on a long runway.
- Now try SEL 70°C — the maximum the FMC will take. Type 70 and press 1L.
70°C is exactly the ceiling: the FCOM puts the maximum allowable SEL entry at 70°C (158°F), and the FMC separately caps the result at a 25% takeoff thrust reduction however high you type. The operational limit is usually lower still — the airline's performance calculation, which cannot assume more than the runway, weight, wind and weather actually allow. Reduced thrust is forbidden on contaminated runways and at limit weights; there you use full TO thrust.
- Set SEL back to 50°C — the realistic operational value. Type 50 and press 1L.
Real flights use whatever assumed temperature 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.
The rest of Performance & Fuel
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📍 EGLL → LSGG — pre-flight planning⏱ ~5 min📊 7 stepsTAKEOFF REF — V-speeds
PremiumintermediateTakeoff flap and V-speeds on the TAKEOFF REF page — the three numbers that turn a runway analysis into a takeoff you can fly.
📍 LEMD → EGLL⏱ ~3 min📊 6 stepsCost Index — What Does It Do?
PremiumbeginnerThree cost index values on PERF INIT — 0, 30 and 80 — and the speed schedule changes under you.
📍 LIRF → LEMD⏱ ~3 min📊 5 stepsMid-Cruise Fuel Check (PROGRESS)
PremiumintermediateThirty minutes into the sector: read the predicted destination fuel on PROGRESS, then reconcile PERF INIT against what the gauges actually say.
📍 EGLL → LSGG — cruise FL360⏱ ~5 min📊 5 steps
Background reading
- Guide737 Assumed Temperature Takeoff Explained — Derate, SEL Temp, and the FMC N1 Page
How the 737's assumed temperature and TO-1/TO-2 fixed derates work, how they stack, what to enter on N1 LIMIT, and when you can push the levers up.
- GuideThe Boeing 737 CDU / FMC Explained: Every Page
What is the 737 CDU and FMC? A plain-English guide to every Boeing 737 CDU page plus the EXEC/MOD workflow. Practice free in our CDU trainer.
- Guide737 FMC Constraints — 250B/, 10000A, and Every LEGS-Page Format That Actually Works
Every 737 CDU constraint format that works: A and B suffixes, speed/altitude pairs, the trailing slash rule, windows, DELETE, and why entries get rejected.