purpose of "de-rated" (TO1/TO2) takeoff thrust?

Hi,

It's nothing to do with efficiency or anything like that - it's to do with reducing thrust, and reducing engine wear as a result.

I hate de-rate and flex. I'm all for using as much power as possible, because you need it if the brown stuff hits the fan. You can't be there waiting for the turbine to spool up (in the case of FLEX) right when you need it, because all the time you're losing airspeed.

This is obvious, but you move the throttle levers forward, you get a certain amount of thrust.

Assume the throttles are at idle. You get 20% N1 rotation.

At full TO thrust, you get 103% N1.

With de-rate, what it does is reduce the actual thrust rating of the engine.

Assume you took TO-1 de-rate, which hypothetically reduced the thrust rating of the engine from 22k to 20k.

You open the throttle levers fully again, only this time, because of the de-rate, you only see 95% N1.

Note that you can NOT push the throttles forward any further - you've got all the thrust you're going to get.

Now in an engine-out scenario, this is bad, because you've limited the thrust rating of the engine. It is capable of producing 22k lbs of thrust, but you've restricted it to 20k lbs.

Assume now you took TO-2 thrust de-rate. You've limited the engine now to 18k lbs of thrust.

Again, you open the throttles fully, but this time only see 88% N1.

Again, in the event of an engine out, you can't open the throttles any further, so you're now stuck with 18k lbs of thrust until you do something with the de-rate.

De-rated thrust is typically used at light loads, but IMHO you're better knowing the performance of the aircraft in this situation.

Once you've lifted off, there is nothing at all to say that you can't throttle back, and effectively fly the aircraft seat-of-the-pants.

Too many simmers think LNAV and VNAV rule, and FLEX is the only way to takeoff, but because they're not contemplating failures, and are unaware of what reduced thrust really means, they think it is the only way to fly, and anything else is wrong.

Some RW airlines state to use FLEX at every opportunity, but all the pilots I know throw that rule out the window. They want everything the thing has got to get airborne.

Interestingly, the pilots who favor FLEX are all young (less than 30 years old) and generally haven't flown anything other than Boeing or Airbus.

The older guys who flew things like Viscounts, Tridents etc.. don't like to use de-rated or FLEX thrust because of the performance penalty involved in doing so.

In any case, if you de-rate, you use more runway than you would otherwise, which reduces the available braking distance. Yes, you figure the numbers that say you take x to takeoff and y to stop at V1, but that assumes certain things. Nothing wrong with having more stopping distance than required.

The other point is that by using the full thrust available, you get to V1 quicker, there is less time for a fault before V1 and a potential stop, before getting airborne.

Statistics have shown that generally, a GO is safer than a STOP, so by extending the time on the ground, you're actually increasing the time for a failure before V1, which isn't necessarily the best option (see the statistics).

Hopefully that sheds some more light on it for you.

My opinion: use all thrust available. You can always throttle back when airborne, but you can't increase thrust when de-rating.

* De-rating from 22k to 18k means a 19% reduction in available thrust (near enough 1/5th). That's a lot of thrust!

Best regards,
Vulcan.

Yes.

There are tables for de-rate as for FLEX. Note that you can NOT de-rate and FLEX at the same time!!!!! This is a critical point.

Not all aircraft can FLEX and/or de-rate. As you point out, FLEX is more "accurate", but the idea is the same really; just a different way of achieving it.

In the days of aircraft such as the 707, instead of FLEXing or de-rating, the FE (Flight Engineer) would calculate target values for N1/N2, for TO and climb power.

That kind of aircraft had three thrust settings: TO, Climb Phase 1 (to acceleration altitude) then Climb Phase 2 (after the aircraft was cleaned up). He would then monitor engine parameters, and tweak the power as necessary.

The main difference between the two is that FLEX reduces TO thrust without limiting performance - you have the option to slam the thrust lever to the firewall and get maximum power. You can't do that with de-rate.

Best regards,
Vulcan.

From what I understand... flex is an assumed ambient temperature derate, fooling the system into believing there's less thrust available, so 20 degrees flex provides a degree of thrust reduction that’s always in relation to the outside air temperature. Outside air temperature influences thrust available, so that way you get a thrust reduction in step with true engine power.

 

TO-1 and TO-2 derates are a way of reducing engine thrust to that of a less powerful engine variant, ‘independent of the environment’, a fixed thrust reduction is regardless of the ambient temperature. So not really just a different way of achieving it [as Pointy said] and would be a relatively greater risk than flex as the percentage of thrust reduction would be a greater chunk of total engine power, for example in a very warm environment.

 

 

With de-rate, what it does is reduce the actual thrust rating of the engine. Assume you took TO-1 de-rate, which hypothetically reduced the thrust rating of the engine from 22k to 20k. You open the throttle levers fully again, only this time, because of the de-rate, you only see 95% N1. Note that you can NOT push the throttles forward any further - you've got all the thrust you're going to get.

I'm not a Boeing man but the press of a button on the FMC gives you full thrust. The pilot would have that page open and ready. Wouldn’t surprise me if there were a button on the yoke also, or perhaps the glare shield? Not to have full thrust available easily would be a mistake.

 

The other point is that derate is only used if the conditions, aircraft weight, environmental conditions etc, permit it, in which case the chosen derate would be perfectly adequate and provide enough thrust for a single engine takeoff. Otherwise that derate wouldn't be permitted.

 

Some RW airlines state to use FLEX at every opportunity, but all the pilots I know throw that rule out the window. They want everything the thing has got to get airborne.

 

If that’s true I'll eat your shorts. Any thrust reduction must enable a takeoff in the event of an engine failure. Unless an expert tells me different. And pilots generally comply with the instructions of the airline.,they don't take off on full power at super light weight with their nose in the air like a fighter, and then throttle back frantically when the passengers scream.

 

The other point is that by using the full thrust available, you get to V1 quicker, there is less time for a fault before V1 and a potential stop, before getting airborne.

 

That’s ridiculous Pointy, so you would pilot a 757, under weight and take off on full thrust? Pitched up at 30 plus degrees no doubt and accelerating beyond the appropriate climb out speed? Wouldn’t want to be your passenger.

 

There are tables for de-rate as for FLEX. Note that you can NOT de-rate and FLEX at the same time!!!!! This is a critical point.

 

Yes you can! In a Boeing, not permitted in Airbus aircraft. And before you say the bus doesn't have derate, it does, introduced as an extra option a while back.

 

Doesn't make sense to me, you advocate taking off on full thrust regardless of weight, obviously resulting in excessive nose up, uncomfortable for the passengers, and then throttling back manually. Then complain about waiting for the thrust to spool up with a derate. Lose your engine after you have calmed your horrified passengers by throttling back and the engine remaining would obviously still require time to spool up.

 

As you point out, FLEX is more "accurate

 

Is it? Don't think so, because engine power is related to the temperature of the air that enters. Therefore flex is directly proportional to the temperature of that air. If you derate by 20 degrees with flex, then its 20 degrees in relation to the true outside air temperature. If you derate the thrust by a fixed amount regardless of ambient temperature, then your thrust will vary considerably depending on how hot it happens to be that day. And you’ll still be stuck with the same percentage derate.

 

Think we need Chris to make an appearance and clarify before we embarrass ourselves courtesy of our ignorance.

''Note that you can NOT push the throttles forward any further - you've got all the thrust you're going to get.''

''Now in an engine-out scenario, this is bad, because you've limited the thrust rating of the engine. It is capable of producing 22k lbs of thrust, but you've restricted it to 20k lbs.''

You dont read everything do you?

You might prefer it, but you don't need it. All you need is sufficient thrust to make a safe take off, if it can be done safely in the event of an engine failure, [with flex and derate as well if you are a Boeing pilot and require it] then you don't need more. That’s what the regulations are for, to ensure a safe take off. They wouldn't certify derated takeoffs as safe if they weren't

 

 

thats what i was trying to get at, every aircraft is designed to fly safely on one engine at takeoff, if the aircraft is light then using de-rate will be fine

Because the alternative is to take off in your 757 at very light weight like a space rocket, in the most likely scenario that your engine didn't fail.

Tell that to the guys who love the performance then!   The yaw would be greater initially, but nothing you couldn't handle. Below V1? Easy - pull the power back - asymmetry is no longer a problem.   Above V1 and you should have control authority anyway to deal with it.

 Which makes a complete mockery of your suggestion. Of course they don't derate if they have no need to.

Don't forget the 'I' Hotty!