Flex/Derate take-off

[Mohit_C]

I have a question regarding this topic. When you use a flexible or derated take-off, you have an assumed temperature greater than the OAT for a given weight of the aircraft. Using the assumed higher temperature, you apply a TO/GO throttle setting lower than the maximum TO/GO throttle setting for the actual OAT and weight of the aircraft.

The concept I'm stuck on is the thrust setting. If you assume that the OAT is higher than the actual, the density is lower and therefore the aircraft needs an advanced throttle setting for the same TO/GO thrust at the actual OAT. That would mean the complete opposite to my first paragraph.

Or does the higher assumed OAT got to do with the Turbine Inlet Temperature?

Thanks.

[Stanley Eevil]

An analogy: At an OAT of +50, a jet engine at 100% HPRPM delivers, say, 70 `bananas` of effective thrust. If we now transport that engine (same RW,AUW, surface wind and pressure etc) to an airfield with an OAT of +15, the colder, denser air will make the engine more efficient, and less HPRPM (eg approx 95%) is required to achieve the same net THRUST.

[Mohit_C]

That is exactly what I meant in my second paragraph, but what I don't understand is that if you use an assumed temperature higher than the OAT how can the throttle setting be reduced for TO/GO Power? Or am I missing some point here?

[FE Hoppy]

Hi,
What you are missing is that the engine is flat rated.

Up to a certain OAT when you set toga you always get the same thrust. Above that temp the engine would normally be temperature limited so you would get a lower thrust to keep the ITT/TGT within limits.

So we are finding the OAT at which the ITT limited thrust would be sufficient for the actual weight and setting that thrust.

Now how we set that thrust is a different matter.

With a FADEC controlled engine we just lie to the FADEC about the OAT. It sets the thrust for that false(assumed) Temp.

However, as the real temp is lower and therefore the density higher the fadec has to make a correction to the thrust to take account of the temperature difference. So it needs to know the actual temperature as well as the assumed temperature to correct the thrust setting.

[kijangnim]

Greetings
Why the assumed Temp is more restricted than derated Thrust, in terms of runway condition, antiskid/one brake inop? :}

[barit1]

One of the best things about Pprune is its search function - when it works :}

You often find that a subject or question has been discussed exhaustively in months or years past - in this case here (http://www.pprune.org/forums/tech-log/313674-de-rated-power-settings-question.html)(and many other times as well).

Don't get me wrong - both newbies and oldtimers gain a lot from fresh discussions. But do us all a favor and do some homework!

[Mohit_C]

Ok, I think I understand this now. Please just confirm this:

If we have an OAT of 20ºC and we set an assumed temperature of 50ºC, the FADEC would calculate a throttle position which would be at a lower throttle position than if we set the OAT of 20ºC, in order to keep the Turbine Inlet Temperature within its limits. Now as we need to input both the assumed temperature and actual OAT, taking into account the density, the position of the throttle that the FADEC calculates would be, as a matter of fact, even lower than the assumed temperature as the density is the actual density at 20ºC and not at 50ºC.


Thanks a lot.

[FE Hoppy]

Your almost there.

The throttle is not conected to the engine. It's connected to the FADEC.

For all take offs on modern jets you put the Throttles in the TOGA position. This tells the fadec to set the appropriate thrust.

So if you change throttle to N1 in your answer then it's correct.


And yes please try the search function. This is a regular topic.

[Wizofoz]

For all take offs on modern jets you put the Throttles in the TOGA position.

FE, If the 777 counts as a "Modern Jet" that isn't universally true. On this (and therefore I would surmise the 787) throttle position is still related to thrust, and "Firewall" gives max rated thrust.

On a reduced thrust takeoff (E.G most takeoffs), the Autothrottle moves the T/Ls to an intermediate position to give the required T/O thrust. At 80 knots it reverts to Throttle Hold, and you need to simply push the levers forward if Max thrust is required.

[TO MEMO]

My Gosh!!!

I flew the 737 old gen 12 years ago and the throttles worked that way... they would also go to Throttle hold at 80 kts and then you would have to manually adjust the thrust to -1%, +0,5% N1, if I`m not mistaken, because ATHR never set the precise thrust... are you telling that such and advanced airliner such as the 777 still works that way...????:E:E:E:E:E:E

ihihihihihhi:ok:

[Mohit_C]

Ok I understand this. Thanks a lot guys!

[The Real Slim Shady]

Why the assumed Temp is more restricted than derated Thrust, in terms of runway condition, antiskid/one brake inop?

Derate and assumed are different concepts: derate is the same as unbolting a 26K engine and bolting on a 22k engine for the duration of the flight.

Assumed is just what the learned colleagues have described.

Assumed is restricted by system failures.

[kijangnim]

R.S.S thanks for your reply, however the question was not regarding DERATE Versus FLEX (Assumed) as a power reduction technic, but why if you use assume or Flex you are more restricted in terms of runways conditions brake anti-skip... than when you use Derate. :)

[john_tullamarine]

the aircraft needs an advanced throttle setting for the same TO/GO thrust at the actual OAT

I think that the point of the exercise is being overlooked

(a) depending on circumstances, a full thrust takeoff may not be "necessary" .. ie at full (not necessarily rated) thrust the aircraft uses less than the available distances, climbs at a gradient greater than the minimum required, etc.

(b) in such circumstances, if one were to conduct the takeoff at a somewhat lesser thrust, these conservative margins might be reduced toward (or become coincident with) the minimum required performance.

(c) the assumed temperature approach is a simple way to adjust the thrust downwards. If today the OAT is 20 deg but the RTOW chart indicates that, at the actual weight, the takeoff can be conducted at, say 40 deg, then to set the thrust to a level appropriate to 40 deg will be adequate. It will also be conservative as the actual OAT is less than that assumed for the calculation.

ie there is no intent to operate at full thrust .. the intent is to operate as some lesser thrust .. the benefits being related to engine costs.

you are more restricted in terms of runways conditions brake anti-skip... than when you use Derate

May we have some specific examples ? the question confuses me a little.

[kijangnim]

Greetings John T and thanks for taking the time.
For examples, One brake unit inop no assume authorized but Derate permitted :confused:

[john_tullamarine]

Without being able to review the specific TC data, my presumption is that the abnormal FMEA considered only full thrust takeoff for the particular aircraft you are considering. The inference is that there is some consideration with flex which creates a problem with the numbers .. for the life of me I can't imagine why ?

Derate IS the certification full thrust basis so that thrust would have been considered in the derate FMEA.

One needs to keep in mind that flex and derate are totally different animals from the certification viewpoint.

There are a couple of other considerations in the back of my mind .. where does the restriction to which you refer actually appear ?

[downwindabeam]

When you drive your car, if you press the gas pedal all the way in you accelerate faster right? Let's say from 0MPH to 30MPH (or 0KMH to 50KMH if you're form a Metric part of the world). Don't complicate it with different gears.... let's assume there's only one gear and our whole speed spectrum is 0 to the end of 1st gear... for simplicity let's call it 30MPH.

Lets say on a nice day, with what ever temperature it takes you 20 seconds.

Now try going back into your car and pressing the gas pedal half way instead of full way. Let's assume that took you 43 seconds.

We have a table, actually alot of tables that tell us how long it'll take us to reach the "limit of 1st gear", or in other words, how long it'll take us to reach from 0 to Vlof (Velocity Lift Off). Ofcourse when you talk about airplanes that time/distance could be expressed in many different forms, one of which could be MAX TO GW for a given runway for a given set of conditions.

Since tempreture is the one thing that always controls thrust output, they decided that telling the engine which tempreture to use is the way to go to determine if you're going to press the gas pedal full, half full or anywhere in between on the takeoff run.

In the older 747 classic, they just bug an EPR/N1 setting derived from the surplus tables. Different names for the same thing.

You call it assumed tempreture, I call it a bug setting on the N1 gauge, the next airplane guy will call it levels 1 through 10. Doesn't matter. We use the tempreture because the system is designed that way to account for density and other factors. Just happens to be the way it is.

As for antiskid inop, packs on (engines)/off (apu or off in general) etc - we go back to our car example. Let's say that one of the things that controls the amount of gas at the full position would give it less gas.

Let's assume that if the air conditioning system is on full blast because it's miserably hot, it'll take us 27.5 seconds to get form 0MPH to 30MPH at full pedal position.... well then, now you have your answer. That needs to be taken under account considering we only have a certain amount of runway available to us and within the runway we have to provide all margins of safety provided by law. (field limit, climb limit, obstacle limit, brake energy limit, tire limit - lower of all five. Please note different airplane types will be limited differently and call these items by different names to account for limitations of different types but the concept is the same).

If I had to draw a diagram here's an assumed takeoff profile on a runway with full power:
(R - Takeoff run, V1 - Abort point, A- Abort pavement)
[Rwy36[RRRRRRRRRRRRRR-V1-AAAAAA]-----Rwy18]

Now for every degress of flex, assumed whatever you want to call it we trade an A, for an R

[Rwy36[RRRRRRRRRRRRRRRRR-V1-AAAAA]---Rwy18]

You utilize the pavement available to trade weight that isn't there and favorable atmospheric conditions and favorable systems operational status to preserve engine life. Each time you add a degree or reduce a .1%N1 you trade an A for an R.

Simple?

[kijangnim]

Greetings, the restriction was on B727, B737-200 (both of these did not have derated takeoff) on the B767 non fadec engines, we had assumed temp, and derated takeoff, and we could do derated + assumed as long as total reduction was not more than 25%.
the restriction was on the MEL for the B727 B737.

[john_tullamarine]

When you drive your car

That's fine .. but it misses the point that you are invoking a non-limiting situation .. ie there ought to be no problem.

the restriction was on the MEL for the B727 B737.

.. and that's precisely the main consideration I had in back of my tiny mind ...

While the MMEL is regulatory based, the MEL is operator derived from the MMEL so one can see all sorts of conservatisms incorporated .. ie, the likelihood is that the restriction may be the choice of the relevant management type who was controlling the MEL development ...

Going back many years ago, the Chief Pilot and I (for a particular operator) imposed a number of conservatisms which seemed to be appropriate for us .. but which were over and above the regulatory requirements .. was that a good thing ? can't rightly say but it was our considered opinion ... funny thing that operationally experienced ops engineers (such as me, Mutt, OS, etc) have a different paradigm approach to these matters when compared to the "straight" engineering types ....

[plain-plane]

derate vrs assume..

Edit, opps I was wrong...

Assumed thrust method: ATM IS NOT considered a limitation.
Derate IS considered a limitation.

?????
but that is what the NG FCTM has to say about it... FCTM 3.15-16

And during T/O thrust levers should only be advanced in a condition requiring thrust increase on both engines: ie windshear.

[FE Hoppy]

Well there are lots of variations on this.

On the e-jets we have ATTCS, TO1 TO2 and sometimes TO3 depending on the engine.

The ATTCS system will in the event of an engine failure automatically cancel the flex and boost thrust to reserve for the rating selected.

VMC are based on TOx-RSV

Advancing the Thrust levers to MAX will never give more than TOx-RSV when you are engine out. But when both engines are running If you have chosen TO-3 the ATTCS will give you TO3-rsv if you have windshear warning but the pilot can push to max and get the equivalent of TO1-rsv.

It's all getting a bit out of hand don't you think?

[NinerVictor]

Assumed thrust method: ATM IS NOT considered a limitation.
Derate IS considered a limitation.

I believe this has to do with Vmcg. ATM is not considered a limitation because Vmcg is derived using actual OAT. So even if you increased the thrust during take-off and lose an engine, you should still be able to maintain directional control (so long as you do not increase above the rated thrust for your take-off, ie TO or TO-1).

In the case of de-rate. Let's say Full rated thrust (TO) of the engine is 50,000 lb and de-rated thrust (TO-1) is 45,000 lb. The Vmcg for using TO-1 is derived using 45,000 lb thrust. Now if you have a take-off situation where V1 = Vmcg using TO-1, and for some reason you decided to firewall the engines at V1 and happen to lose an engine, directional control will not be assured with full rudder. This is because Vmcg for TO is higher than Vmcg for TO-1.

So this brings me to another question. If you take off using TO-1, and you encountered "Windshear! Windshear!" at V1. What will you do? I was thinking that on contaminated runways or other situations when V1 was reduced, the V1/Vr spread is larger and the exposure to this scenario can be significant. Are my worries without a cause?

[mutt]

I believe this has to do with Vmcg
Correct, if you were to use the VMCG for the full rated thrust you could remove the limitation, however you would also lose the benefits.

FE Hoppy, we chose to operate with only TO-1, do you know of many operators that are using all 3 ratings?

Mutt

[john_tullamarine]

So this brings me to another question. If you take off using TO-1, and you encountered "Windshear! Windshear!" at V1. What will you do? I was thinking that on contaminated runways or other situations when V1 was reduced, the V1/Vr spread is larger and the exposure to this scenario can be significant. Are my worries without a cause?

Simple solution .. if you continue, you consider/do firewall(ing) the levers ...

(a) the risk of dying due serious windshear is real and staring you in the face

(b) the risk of then having an engine failure and departing .. is far smaller

(c) if you've already had the failure and then get the windshear ... it just wasn't your sort of day and you are probably best positioned to take your chances in the overrun.


... first, attend to the thing which is going to kill you first ... noting that, if it succeeds, then the next threat doesn't even get the chance to try ...

[Airbusalltheway]

Just for clarification:
most engines(i know anyway) are flat rated.
A flat rate is quite simply a way of protecting the engine from surpassing it's limits when temperatures drop.
f.e.
when we do power setting calculations we refer to a form of maximum thrust, call f.e. N1ref
engine flat rated at 23°C will produce it's maximum thrust at it's maximum RPM(say N1ref=95%N1) at 23°C.
Going into a lower temperature, say 20°C, you enter the purely structural/thrust-limited section and you would see the resulting required RPM is lower, say N1ref=92%N1.
If you go into a higher temperature, say 30°C you enter the engine's temperature limited zone. this is where you meet your EGT/TGT/ITT whatever you call it - limits. again, result is a lower N1ref=92%

Flexing is making the engine believe it's hotter outside than it actually is.
Depending on you runway and conditions you can calculate a maximum ambient temperature for which running your engines at N1ref would give you still sufficient margin on your takeoff/climb performance required. (This is usually done by the company's engineering dept and presented to you in a performance chart specific for 'a runway/intersection/flap setting/runway condition' and contains values to correct for anti icing, bleed selections and wind.
So if it were really 'that' flex temperature outside, we'd be very sweatty and selecting our max thrust N1ref, would be able to takeoff from that runway.
So now we do this with a temperature which is actually lower than that flex temperature, giving us an additional margin on performance. And most importantly, we will have a lower N1ref setting sparing our engine and increasing its life thus reducing company costs.
It's important to know aswell that when your flex temp for a given rwy/condition/flapsetting is lower than the flat rate of your engine, you should not perform flex takeoff.