737 Assumed Temperature Takeoff Explained — Derate, SEL Temp, and the FMC N1 Page

Every few weeks someone posts a version of the same question on the PMDG forums. How do I do a FLEX takeoff in the 737? Why is my FMC N1 wrong with a double derate? If I have a derate and a SEL temp set, can I still firewall it if things go bad?

Those three threads — #340612, #335614 and #237005 — are the most-asked 737 performance questions in the community, and they all come from the same root confusion: the 737 has two completely different ways to reduce takeoff thrust, they can be used together, and they behave differently the moment something goes wrong on the runway.

This guide untangles them. What each one actually does, what to type where, how they stack, why your EFB and your FMC disagree, and — the one that genuinely matters — whether you are allowed to push the levers up.

Everything operational here is verified against a Boeing 737-700/800 Flight Crew Operations Manual and PMDG's own tutorial documentation. Where an operator's SOP is stricter than Boeing's baseline, that is flagged.

Two methods, one goal

Reducing takeoff thrust lowers exhaust gas temperature and extends engine life. Boeing puts it plainly: reduced thrust takeoffs "lower EGT and extend engine life" and are used whenever performance limits and noise abatement procedures permit. On a 3,500 m runway at half load you simply do not need every pound of thrust the engines can make, and using it costs real money in hot-section wear.

The 737 gives you two levers to pull.

Fixed derate (TO-1, TO-2). You pretend the aircraft has smaller engines. A 26K 737-800 selecting TO-1 becomes, for certification purposes, a 24K aeroplane. This is a discrete, pre-certified rating with its own performance data in the Airplane Flight Manual.

Assumed temperature method (ATM). You tell the FMC it is hotter outside than it really is. Thrust falls off with temperature, so the FMC computes the N1 it would set on that hotter day and sets that instead. Airbus calls the same trick FLEX; Boeing calls it assumed temperature, and the CDU field is labelled SEL.

They are not two names for the same thing, and the difference is not academic.

Fixed derate: a smaller engine, and a real limit

Selecting TO-1 or TO-2 on the N1 LIMIT page selects a different certified thrust rating. Boeing's line titles show it explicitly — on a 737-700 you will see 22K DERATE and 20K DERATE; on a 26K 737-800, 24K DERATE and 22K DERATE. The CFM56-7B family spans roughly 18,500 to 27,300 lb across the NG line, and the -7B24, -7B26 and -7B27 share the same core hardware certified at different ratings, which is exactly why a derate is a paperwork exercise rather than a hardware one.

Because the aeroplane is certified at that lower rating, everything downstream is recomputed at that rating — including the minimum control speeds. That is the derate's superpower. Boeing's guidance for a light aircraft that cannot produce valid V-speeds is exactly this:

Selecting derate thrust is the preferred method as this will reduce the minimum control speeds. Note that the assumed temperature method will not help this condition as the minimum control speeds are determined at the actual temperature and therefore are not reduced.

That comes from the FCOM's discussion of the V SPEEDS UNAVAILABLE scratchpad message, which appears at full rated thrust and light weights when the required speed increase exceeds certified limits. Takeoff is not permitted in that condition. Your three options are a smaller flap setting, a derate, or more weight in the form of fuel. A derate is the preferred fix. An assumed temperature does nothing for it at all.

The flip side is the constraint. With a fixed derate selected:

the thrust setting parameter is considered a limitation for takeoff; therefore, thrust levers should not be advanced further except in an emergency. A further thrust increase following an engine failure could result in a loss of directional control while on the ground.

That is the whole reason the distinction exists. Vmcg was certified at the derated thrust. Shove the good engine to full rated thrust after losing the other one, and the rudder may not have the authority to keep you on the centreline.

Assumed temperature: a softer reduction, and no limit

Assumed temperature works on the physics of a flat-rated engine. PMDG's tutorial explains the mechanism cleanly: "The engines are designed to produce their rated thrust at an actual outside air temperature of 30C (ISA + 15C). If the temperature is higher than this, the air becomes less dense and the engine produces less thrust at the same N1 setting." Enter a higher temperature and the FMC computes a lower N1 to match.

Two hard numbers govern it.

• Maximum SEL entry is 70 °C (158 °F). Above that the CDU rejects it.
• Maximum thrust reduction is 25 percent below any certified rating. Even if you enter a temperature implying more, "the FMC, however, will limit the N1 to 25% takeoff reduction."

Crucially, the minimum control speeds are not recomputed. It really is 12 °C out there, and full rated thrust really is available at the push of a lever, so Vmcg and Vmca stay at their full-thrust values. Which is precisely why Boeing does not treat ATM as a limit:

When the assumed temperature method is used with full rate, the reduced thrust setting is not considered a limitation. If conditions are encountered where additional thrust is desired, the crew can manually apply full thrust.

There is your answer to thread #237005 — at least, half of it. The other half is coming.

Stacking them: the double derate

You can use both at once, and airlines routinely do. Boeing is explicit: "Derated takeoff rating can be further reduced by assumed temperature."

What the CDU shows you when you do is the tell. The N1 line title normally reads 24K N1 or 26K N1. Add a SEL TEMP and it becomes RED XXK N1 — and Boeing spells out the arithmetic:

If a SEL TEMP and a DERATE are both selected the data line title will change to "RED XXK N1," and the effect on thrust will be additive.

Additive, but bounded. The 25 percent cap is measured from the certified rating you selected, not from full rated thrust. A 26K 737-800 on TO-2 is a certified 22K aeroplane, and the assumed temperature may cut at most 25 percent off that.

This is the configuration behind thread #335614, where pilots compare the FMC's double-derate N1 against their own table lookups and find a gap that widens as OAT drops. Whether that discrepancy lives in the add-on's engine model or in the comparison method is not something this guide can settle, and a forum thread is not a source I can verify a number from. What I can tell you is what the number should represent, and how to read the CDU so you know which reduction you actually have.

What to type, page by page

N1 LIMIT page

Press the N1 LIMIT key. The preflight version of the page lays out like this:

LSK	Field	What it does
1L	SEL / OAT	One line, two values. SEL is the assumed temperature (max 70 °C / 158 °F); OAT is the actual outside air temperature
2L	TO XXK	Full rated takeoff thrust
3L	TO-1	First fixed derate
4L	TO-2	Second fixed derate
1R	XXK N1	The computed takeoff N1
2R	CLB	Full rated climb
3R	CLB-1	Climb derate 1
4R	CLB-2	Climb derate 2
6L	PERF INIT	Back to PERF INIT
6R	TAKEOFF	Jumps to the TAKEOFF REF page

PMDG's own tutorial flight walks that sequence exactly: LSK 4L for TO-2, then type the assumed temperature and line-select it into LSK 1L.

SEL and OAT share one field, and the slash decides which one you set. The line displays as ---°/ °C — SEL on the left, OAT on the right — and it obeys Boeing's Data Pairs rule. The outboard value (SEL) is typed bare: 50 sets an assumed temperature of 50 °C. The inboard value (OAT) needs the slash: /15 sets the actual temperature. Type 50 when you meant to correct the OAT and you have just set an assumed temperature instead. The same rule governs every slashed CDU field on the aircraft; the 737 FMC constraints guide works through it on the LEGS page, where it bites hardest.

Order matters. Select the rating first, then enter V-speeds. Boeing warns that "selection of a new rating after V speeds are selected on the TAKEOFF REF page causes the V speeds to display in small font, and the NO VSPD flag to show on the airspeed indication." If you have ever seen NO VSPD appear on the PFD after you were certain everything was set, this is why: you changed the thrust rating afterwards and invalidated the speeds.

The climb derate arms itself. Selecting TO-1 normally arms CLB-1; TO-2 arms CLB-2. That is not decoration. It exists so the transition from takeoff to climb thrust does not require the levers to move forward, which is what would happen if climb N1 exceeded your reduced takeoff N1. Boeing describes the logic as selecting "the highest climb thrust available (CLB, CLB-1, CLB-2) which would not result in a thrust lever push."

CLB-1 cuts the climb limit by 3% N1 (roughly 10 percent thrust), CLB-2 by 6% N1 (roughly 20 percent thrust), and the reduction "gradually increases to full rated climb thrust by 15,000 feet." That is why you watch N1 creep up through the climb and see the CLB 1 annunciation drop to plain CLB somewhere around FL150.

Check the OAT sign. This sounds trivial. It is not. Boeing issued a flight crew bulletin after three incidents of erroneous OAT entry, one where the actual temperature was +28 °C and −28 °C was datalinked into the FMC, which "resulted in a significant thrust shortfall on takeoff." On airframes without aspirated TAT, OAT is a manual entry on this page. Check the value, and check the sign.

TAKEOFF REF page

Press 6R, or INIT REF then TAKEOFF. Here you enter takeoff flaps at 1L, line-select CG to get your trim setting, and line-select V1, VR and V2 onto the speed tape. If you want the full CDU preflight in order, the PMDG 737 FMC tutorial covers every page from IDENT onward, and the page-by-page CDU explainer covers what each key does.

Two things on page 2/2 matter here. The SEL/OAT line is repeated at the top of it, so the assumed temperature can be entered there instead of on N1 LIMIT — same field, same value, same effect. Boeing's own description of it is literally "repeats data shown on the preflight version of the N1 LIMIT page."

The other is a pair of heights that people routinely conflate:

• THR REDUCTION — where takeoff thrust transitions to climb thrust. Boeing's allowable entries are 800 to 9,999 feet, default set by the operator.
• ACCEL HT — where you lower the nose and accelerate for flap retraction. Entry is optional, 400 to 9,999 feet, default from the operator, with a separate EO ACCEL HT for the engine-out case.

They are different fields doing different jobs, and only the first one is about thrust. PMDG's tutorial reduces thrust at 1,500 ft AGL and calls that height the acceleration height — fine as shorthand for a tutorial flight, but on the page they are two entries.

Reading it back on the glareshield

The thrust mode display on the upper engine display is your confirmation that the box did what you asked:

• TO — full rated takeoff
• TO 1 / TO 2 — fixed derate, no assumed temperature
• D-TO — assumed temperature only, off full rate
• D-TO 1 / D-TO 2 — derate plus assumed temperature, the double derate
• CLB, CLB 1, CLB 2 — the climb equivalents

Some airframes use a simplified annunciation set where anything reduced shows as R-TO, and Boeing notes explicitly that in that case "R-TO does not indicate the type of reduced takeoff." If your aircraft shows R-TO, the display alone will not tell you whether you have a derate, an assumed temperature, or both. Go back to N1 LIMIT and read the line titles.

The question everyone actually asks

With a derate and a SEL temp both set, can you push to full thrust during the roll?

No. And this is the subtlety the forum threads trip on, because the answer flips depending on what you stacked the assumed temperature onto.

• Assumed temperature on full rate (D-TO) — the reduced setting is not a limitation. Full thrust is yours if you want it. Vmcg was computed at full rated thrust, so the aeroplane is controllable there.
• Fixed derate, with or without assumed temperature (TO 1, TO 2, D-TO 1, D-TO 2) — the derate is a limitation. Boeing: "If the assumed temperature method is applied to a fixed derate, application of additional power should not exceed the fixed derate N1 limit as loss of directional control could occur while on the ground."

So on a D-TO 2 takeoff you do have thrust in reserve — but only back up to the TO-2 N1 figure, not to full rated thrust. The assumed-temperature portion of the reduction is recoverable. The derate portion is not, because Vmcg was never certified there.

And "except in an emergency" is doing real work in Boeing's phrasing. Losing directional control on the runway is also an emergency. The judgement is whether the obstacle ahead is worse than the swing to one side — which is exactly why dispatch does the sums beforehand, so the situation never arises.

When reduced thrust is off the table

This is where a lot of sim pilots quietly break the rules, because the FMC will happily accept a SEL TEMP that the real operation would forbid.

Boeing's baseline prohibitions on assumed temperature:

• Anti-skid inoperative. This is an AFM limitation, not advice: "Operation with assumed temperature reduced takeoff thrust is not permitted with anti-skid inoperative."
• Contaminated runway — standing water, ice, slush or snow. Not allowed, "whether alone or in combination with a fixed derate."
• Potential windshear conditions. Boeing's wording is "not recommended" rather than prohibited, but treat it as a no.

The wet-versus-contaminated distinction is the one people get wrong. They are not the same category:

• Wet runway — reduced thrust (fixed derate, assumed temperature, or both) is allowed, provided the performance calculation accounts for the longer stopping distance on a wet surface.
• Contaminated by slush, snow, standing water or ice — fixed derate is still allowed if the calculation accounts for the surface condition, but assumed temperature is not allowed at all.

That asymmetry follows directly from what each method does. A derate is a genuinely certified lower-thrust aeroplane with its own accelerate-stop numbers. An assumed temperature is a fiction layered on top of real conditions, and on a contaminated runway the margins it eats are the ones you are about to need.

Operators go further. The FCOM this guide is verified against carries an operator SOP list that also excludes assumed temperature with a thrust reverser inoperative, with the EEC in Alternate Mode, after de/anti-icing, with engine bleeds off, on landing-gear-extended ferry flights, where the airfield brief prohibits it, and — notably — when the crosswind component exceeds 10 knots, as a tailstrike prevention policy. That last one is that operator's rule, not a Boeing limitation. If you fly for a virtual airline with its own SOPs, theirs wins.

Why SimBrief and your EFB disagree with the FMC

Three separate boxes will offer you a takeoff N1, and there is no particular reason for them to agree.

SimBrief added takeoff and landing performance to its Dispatch tools, covering reduced-thrust takeoffs and optimised flap settings. It is a generic performance model built to span a wide fleet, not a reproduction of one airline's certified AFM data for your exact airframe, engine rating and winglet fit. Its job is to hand you a plausible, self-consistent set of numbers for a flight plan. If you are wiring SimBrief into the sim more generally, the SimBrief to FMC workflow guide covers the import path.

Your add-on's EFB computes against that add-on's own performance tables.

The FMC computes N1 from the model/engine database in the box, against the current aircraft state — and that state includes the bleed configuration. Boeing's own takeoff N1 tables are published "based on normal engine bleed for air conditioning packs on," with separate adjustments applied for other configurations. Run the numbers with packs on in the EFB, then take off with packs or bleeds off, and the FMC will show you a different N1. That is not a bug. It is two calculations being asked slightly different questions.

The practical rule: the performance tool decides what reduction is legal — the rating and the assumed temperature. The FMC decides what N1 achieves it in the aircraft's current configuration. Type the tool's rating and SEL TEMP into the box, then fly the N1 the box gives you. Do not hand-tune the SEL TEMP until the FMC's N1 matches a figure you copied off a screenshot; you would be changing the actual thrust reduction in order to chase a display.

Quick reference

	Fixed derate (TO-1 / TO-2)	Assumed temperature (SEL)
What it changes	The certified thrust rating	The temperature the FMC computes against
Airbus equivalent	Derate	FLEX
Vmcg / Vmca	Recomputed lower	Unchanged — full-rated values
Fixes V SPEEDS UNAVAILABLE	Yes	No
Is it a limit?	Yes — do not exceed except in an emergency	No, on full rate — full thrust available
Contaminated runway	Allowed, with the right calculation	Not allowed
Wet runway	Allowed	Allowed
Maximum reduction	Fixed, per certified rating	25% below the selected rating
CDU entry	LSK 3L / 4L on N1 LIMIT	LSK 1L on N1 LIMIT, or TAKEOFF REF 2/2
Annunciation	TO 1 / TO 2	D-TO
Both together	D-TO 1 / D-TO 2 — effects additive, ceiling is the derate N1

Practise the entry, not the theory

Reading this once will not stop you fumbling the sequence at the gate with a pushback clearance pending. The entry order — rating first, then SEL TEMP, then V-speeds — only becomes automatic by doing it.

Assumed Temperature — Reduced Thrust Takeoff in our browser-based 737 CDU trainer walks you through three assumed-temperature values on the N1 LIMIT page so you can feel the trade-off between engine wear and performance margin. No sim, no add-on, no install — it runs in the browser.

Then pair it with TAKEOFF REF — V-speeds, which covers the other half of the preflight performance entry: flaps, V1, VR and V2 onto the speed tape, in the order that does not trigger NO VSPD.

Where to go next

• Boeing 737 CDU/FMC explained — every page and key on the CDU, if N1 LIMIT is not the only page giving you trouble.
• PMDG 737 FMC tutorial — the full preflight in order, from IDENT through TAKEOFF REF.
• 737 FMC constraints — the LEGS-page formats, for when the departure has crossing restrictions on it.
• FMC programming guide — the general Boeing workflow, including MOD and EXEC.
• PMDG 737 cold and dark startup — everything that has to happen before the N1 LIMIT page is even worth opening.
• A320 vs 737 in MSFS and X-Plane — if you also fly the bus and want the FLEX-to-assumed-temperature mapping in context.

The short version, if you remember nothing else: assumed temperature is a suggestion, a fixed derate is a rule, and stacking them makes the rule the ceiling.