FLEX and assumed temperature, explained

Every airliner takeoff in the sim starts with the same question on the PERF page: what goes in the FLEX box. On a Boeing it is the assumed temperature in the N1 limit page. Most pilots I talk to on VATSIM know it makes the takeoff quieter. Fewer know where the number comes from, why it changes so much between runways, and why some days the answer is “none”. This post explains it from the ground up, and shows how to get it for the aircraft you fly.

What an assumed temperature is

Jet engines make less thrust in hot air. The engine computer knows this and sets the takeoff thrust for the outside air temperature. Reduced thrust takeoffs use that behaviour on purpose: you tell the engine that the air is hotter than it really is, and it sets the thrust it would use on that hotter day.

That made-up temperature is the assumed temperature. Airbus calls it FLEX, Boeing calls it the assumed temperature method, and the idea is the same. If the aircraft can meet every takeoff requirement on a 50 °C day, it can certainly meet them on today’s 15 °C day with the thrust of a 50 °C day. The real day is colder, so the air is denser, which leaves an extra margin on top.

Airlines do this because lower thrust means lower engine temperatures, and that saves engine life and maintenance money. In the sim nothing wears out, but flying the takeoff the way the real crew would is the point, and the numbers you get are the ones a real FMC would expect.

How the number is found

The assumed temperature is the highest temperature at which the aircraft, at its actual weight, still meets every takeoff limit on that runway. To find it, a performance tool keeps raising the temperature and checks each limit again:

• The runway. The all-engine takeoff distance, the distance to continue after an engine failure at V1 and the distance to stop after a rejected takeoff all have to fit inside the declared distances (TORA, TODA and ASDA).
• Climb. With one engine out the aircraft must still climb at the minimum gradients the certification rules set, most importantly 2.4 percent in the second segment for a twin.
• Obstacles. The engine-out flight path has to clear the terrain and obstacles in the departure sector with a margin.
• Brakes and tyres. A rejected takeoff at V1 must stay inside the brake energy limit, and the liftoff speed inside the tyre speed limit.

Whichever limit runs out first sets the answer. That is why a long runway at sea level gives a high FLEX and a short runway at a hot, high airport may give none at all. It also explains why two runways at the same airport can differ by twenty degrees: a different length, a different slope or an obstacle off one end changes which limit bites.

The ceiling: why FLEX stops where it stops

The assumed temperature cannot go up forever. Airbus limits a FLEX takeoff to 25 percent less thrust than full takeoff thrust, and the engine has a published maximum flexible temperature. Below are the ceilings the SkyNexus models use, each read from the documentation for that engine:

• A320ceo with CFM56-5B4 engines: ISA + 55, which is 70 °C at sea level.
• A320neo with LEAP-1A26 engines: ISA + 55 as well.
• A321 with IAE V2533-A5 engines: ISA + 42.
• A321neo with LEAP-1A32 engines: ISA + 52.
• A380-800 with GP7270 engines: ISA + 60.

Because the ceiling is tied to the standard temperature, it drops by about 2 °C for every 1 000 ft of airport elevation. On the 737-800, Boeing publishes the maximum assumed temperature as a table that depends on the outside air temperature and the pressure altitude, and SkyNexus reads that table. The assumed temperature also cannot be below the actual temperature, or below the temperature up to which the engine is flat rated, because below that the reduction is zero.

When you cannot use it

Some conditions rule a reduced thrust takeoff out, whatever the runway length. When one of them applies, select full thrust yourself (or, on the 737, a fixed derate without an assumed temperature):

• A contaminated runway, such as standing water, slush or snow.
• Windshear reported or expected on departure.
• Inoperative items whose MEL entry forbids it. With the anti-skid inoperative SkyNexus refuses the takeoff, because the MEL does not allow dispatch.

A wet runway does not rule it out, but it costs you. Stopping takes longer, so the accelerate-stop limit comes in sooner and the assumed temperature comes down.

Derates are a different thing

On the 737 you will also see TO-1 and TO-2, or in SkyNexus the 26K, 24K and 22K ratings of the CFM56-7B26. Those are derates: a lower takeoff rating with its own V-speeds and its own, lower minimum control speed on the ground.

The difference shows when you need more thrust. With only an assumed temperature, the full rating is still there: push the thrust levers forward and you get it. With a derate, the V-speeds were planned for the lower thrust, so on the ground you stay at the derated thrust.

The two can be combined: a 24K derate with an assumed temperature on top is a normal 737 takeoff on a long runway. Then the derate rule applies: on the ground you may take away the assumed temperature reduction, but not go above the 24K rating.

How to get the number for your flight

Some add-ons have a performance calculator in their EFB, and there are web tools as well. SkyNexus does it from your SimBrief flight plan: weight, runway and the latest METAR are filled in, every takeoff flap is checked, and the PERF page shows the FLEX or assumed temperature with the speeds that belong to it. Change the runway or the intersection and the number follows. Pick the page for your aircraft:

• A320 and A320neo
• A321 and A321neo
• 737-800, with the derates
• Every supported aircraft, including the 777-300ER and A380-800 with indicative numbers

If your number differs from the one in your add-on, that is not always a mistake on either side. I wrote about the reasons in why takeoff calculators disagree.

Questions pilots ask

Is FLEX temperature the same as assumed temperature?

Yes, in practice. FLEX is the Airbus name, assumed temperature (ATM) the Boeing name. Both tell the engine the air is hotter than it is so it sets less thrust.

Why do I get no FLEX on some runways?

Because at your weight one of the takeoff limits is already used up at the real temperature: runway length, engine-out climb, obstacles or brake energy. Then full thrust is the only option. A contaminated runway, windshear or some inoperative items rule FLEX out as well.

What is the maximum FLEX temperature on the A320?

On the A320ceo with CFM56-5B4 engines and on the A320neo it is ISA + 55, which is 70 °C at sea level and about 2 °C less per 1 000 ft of elevation.

Can I combine a derate with an assumed temperature on the 737?

Yes. Select the derate first, then enter the assumed temperature on top of it. SkyNexus works out both for the 26K, 24K and 22K ratings.