Flex T/O vs. V1/Vr

[Leo77]

Hi y´all!

New to this forum i came here thanks to google and i will try to find an answer to following:

Im flying F70 and F100s but i think the question can be valid for all aircraft.

Why is it required to increase V1 and Vr values when using Flex T/Os?
I mean, theres a note on the Take off and landing cards (Speed booklet) saying that you have to increase V1 and Vr by 1 knot every 5° OAT above 30°. The manual says that this also applies for assumed OATs. eg 4 knots more when using flex 50° T/O.

I can imagine some answers but i think they are not 100% correct.

Thanks in advance and

kind regards

Leo

[Old Smokey]

Hi Leo, welcome aboard.

I'm not familiar with the F70 and F100, but a look at basic performance principals should give you the answers you're looking for.

I think the 'key' to the answer lies within your own statement that the V1 / Vr increase is required for Normal, as well as for Flex operations.

It would appear, on the surface' that your engines are Flat Rated up to 30°C, and decline in thrust thereafter.

In using the Flex Temp technique, basically you're reducing the engine thrust to the point where, at that assumed temperature, the aircraft would be performance limited at the given weight. Therefore, any further performance discussion will be identical for both a high temperature day with the aircraft at limit weight, and a Flex situation where you've artificially put the aircraft in a performance limited situation.

From your remarks, I assume that for your aircraft, and for a given weight, the V1 / Vr / V2 relationship is the same up to 30°. Above 30° where thrust is declining (or you're flexing), V2 must stay the same as it is a minimum speed (unless you're using an increased speeds schedule where V2 > 1.2 x Vs).

The problem now lies in the acceleration from V1 to Vr at lower thrust, and the acceleration from Vr to V2 within the Takeoff Distance Available (TODA), also at lower thrust. If these two acceleration distances are excessive, one solution is to increase the V1 and Vr, bringing them closer to V2, thus decreasing the overall acceleration distance from V1 to Vr to V2, and containing it within the TODA. This works well from the performance perspective, provided that the increased V1 does not encounter Accelerate-Stop limits within the ASDA. For 2 engined aircraft (e.g. F70 and F100), the requirement to accelerate from V1 / Vr to V2 is usually a tougher one to meet than the accelerate-stop case.

[Leo77]

Smokey, thanks for your answer.

Finally i read and read it through but there is still that strange thing:

Once you get these new V1 and Vr values, you still need to check them versus your runway performance tables ( for any given or individual airport) right? If you find a lower V1 for the actual temperature and actual weight, then you cant flex at all. But this can only be a problem when field lenght is limiting factor. The lowest V1 found is still ruling isnt it? E.g.: OAT 30°C. Standard speed is 130 for V1 and Vr at a given weight. Now Rule says increase V1 and Vr by 1 knot every 5°OAT above 30°C. Meaning a flex 50 needs a speed increment of 4 knots on Vr and V1.

Now i have a look into the runway table and find lower speeds for V1 and Vr at 50°OAT or assumed OAT. (assuming the weight is just correct with what ive git on my loadsheet, which is seldom, i know) Which V1 shall i take then?

Another thing a bit off topic. Is it right that flexing below 30°C is not allowed because it is the flat rated temperature?

regards

Leo

[Old Smokey]

Hi Leo,

I emphasise that I'm not familiar with the F70 / F100 or their engines, so this comes from general principals. Taking apart your post, quote by quote -

QUOTE - "Once you get these new V1 and Vr values, you still need to check them versus your runway performance tables ( for any given or individual airport) right? If you find a lower V1 for the actual temperature and actual weight, then you cant flex at all".

No, you can't flex here, because individual runway analysis (I assume you're referring to RTOWs) has established that you're Accelerate-Stop limited for that runway, and raising V1 would be out of the question.

QUOTE - "But this can only be a problem when field lenght is limiting factor".

Yes, specifically the ASDA (Accelerate-Stop Distance Available) is limiting. Don't forget that TODA may be much longer than ASDA.

QUOTE - "The lowest V1 found is still ruling isnt it?"

Ruling in the sense that it will govern whether you conduct a normal or a flex thrust takeoff.

QUOTE - "Now i have a look into the runway table and find lower speeds for V1 and Vr at 50°OAT or assumed OAT.........Which V1 shall i take then?

An RTOW arising from a specific runway analysis has already considered all implications for that runway. Unless your instruction to adjust V1/Vr applies in ALL circumstances, my feeling is to go with the RTOW speeds.

QUOTE - "Is it right that flexing below 30°C is not allowed because it is the flat rated temperature?"

I don't know your engine, but if, as I suspect, 30° is the upper limit of the flat rating, it would be pointless to flex to a temperature below 30°, the engine would be delivering full thrust.

Again, I emphasise that I'm not familiar with the F70 / F100 or their engines, take this as general information, there may be flaws in my understanding.

[Leo77]

Hi!

Thanks. I think its not really fokker related and my questions can be answered without knowing aircraft details.

Ok, now i start understanding the problem. Acceleration from V1 or Vr to V2 is limiting when spread between those is too big regarding the EO Scenario.

As far as i know most modern Jet Engines use Tref= ISA + 15°C (SL)
E.g.: actual OAT 15°C. So what happens when i select TOGA Thrust at 15°C actual OAT, Flex 25° (Which isnt allowed on a F70 with RR Tay 620-15 Engines) or when i firewall.

Same question for OAT of 35°. (5 above Flat rated temp.)

What is that flat rating all about? What do i get in the end?

Thank you!

Leo

[Old Smokey]

Jet engines have 3 BASIC limits - Pressure limit (internal pressures), Temperature limits (Turbine temperatures), and Speed limits (N1, N2, N3 etc.).

At low temperatures (and therefore high air density), it's possible to reach the internal pressure limit well before the Temperature or Speed limit. (This will show as a constant EPR on EPR gauged aircraft, or steadily increasing N1 with increasing temperature on N1 gauged aircraft).

As Air Temperature increases, more and more fuel must be injected, with rising Temperature and Speed, until the limiting turbine temperature is reached), so far, thrust has been constant, hence the 'Flat' rating.

As air temperature increases further, fuel must be limited to maintain turbine temperatures at their limits (Both EPR and N1 now begin to decrease). The engine has now reached full rating, and thrust continues to decline as air temperature begins to rise.

At much higher altitudes (usually), the engine speed limit is reached. This is not usually a problem at typical takeoff altitudes.

So....Flexing to a temperature below the flat rating limit temperature would achieve nothing, except full thrust.

[SuperRanger]

leo,

when we 'flex' we end up with a whole new higher set of speeds.

i personally like to think of it as a 'inverse' of improved v speeds (hope you are familar with improved v speeds) i.e. for a fixed ambient condition and runway length, you use 'flex' because you have too much thrust. when you reduced your thrust, you need to increased your v speeds to 'compensate' for the 'lost' of performance.

had you start off with insufficient thrust, then similarly you could achieve a successful takeoff by increasing the v speeds. obviously, the runway need to be sufficiently long.

i know it's a crude way of describing it but i hope it helps...

SR

[Leo77]

Hi!

But there is a small difference in increasing V Speeds based on a Data card which is designed using STD ICAO Atmosphere Values and using speeds found in the runway tables for the specific airport at the current ambient AND assumed temperature.

According to all what i learned here and read its not to compensate a thrust loss, (which is compensated in the relevant Runway table) but to increase your kinetic energy upon lift off. A V1 and Vr nearer to V2 also helps to perform better in Engine Failure cases. But i know what you mean and it comes, in the end, to the same result.
We do increase from flex to TOGA thrust in case of Engine failure after V1 but it is not compulsory or required by performance requirements. Acceleration from V1 to V2 must be possible when you used speeds according the individual runway. BUT it must not be possible when using standard values like the ones found in a speed booklet.

Of course you cant play with it when youre runway field length limited but in all other cases it really makes sense.

Leo

[john_tullamarine]

"We do increase from flex to TOGA thrust in case of Engine failure .."

At low V1, especially, do be very wary of (rapid) increases in thrust, OEI, especially if you overshoot the mark.

I was involved with an accident investigation some years ago where the conclusion was that the pilot created much of his problem by doing just this and ending up with a Vmca sort of problem .... lots of bodies ...

[MAXRATE172]

From the point of view of a pre-BGT piston pilot, this is a very interesting discussion. I don't mean to hijack the thread or anything but what types use flex?

[Leo77]

Of course you can create even more problems by careless increments of thrust. Although it shouldnt be too much of a problem on a F70 or 100 as this increment is max. 0.15 EPR at normal ambient conditions. (e.g. from 1.57 EPR with flex 54 to 1.70 up to 1.72 EPR which is TOGA thrust.) Its up to the PNF to make this and i hope that we wont have a problem using this procedure.
In the end you want to have the best margins and reserves when flying single engine. Isnt it? Normally, even if you firewall the thrust levers (which is done automatically by Autothrust when you get into a windshear), you dont go into the red instantly. It´ll take some time and thats enough time to adjust the thrust as required.

Leo

Maxrate172:

Flex is the Airbus or Fokker terminology for reduced or derated thrust.
I think that almost every airliner even turboprops have the ability to derate the "Power" required for departure. they just call it something else.

Leo

[Old Smokey]

Jumping back to John Tullamarine's post, I heartily agree with his caution regarding 'too rapid' thrust advance following engine failure during a Flex / Assumed Temp. Thrust takeoff. Having coped with the initial yaw following engine failure, it is common to see trainees in the simulator unprepared for the additional rudder input required for the additional thrust.

Flight testing / certification procedures still use the 'Full Thrust Rating' thrust when considering Vmca / Vmcg, and adequate control should exist, within the certification parameters, to advance to the full thrust rating. It is considered normal (although technically not required) by most operators to do so.

Where great caution is required is when the aircraft is certified with several different thrust RATINGS for the same engine, e.g. the RR Trent has a 100%, 92%, and 80% rating, selectable for the particular operational requirement for a given takeoff. Vmca / Vmcg DOES consider the lower rated thrust, and if REDUCED thrust is used against a DERATED thrust, pilots should be VERY cautious in only setting the RATED thrust following engine failure. For the RR Trent example, engine failure during takeoff using the 80% rating could lead to dire consequences if thrust was advanced to the full 100% rating (it's possible) at V1/Vr/V2 speeds.

The same caution applies to engines which operate normally on full Takeoff thrust rating, but employ 'overboost' (e.g. Automatic Performance Reserve (APR)) as an option. In these cases a takeoff with APR off and appropriately set V1/Vr/V2 could lead to control problems if thrust was manually advanced to APR following engine failure.

Read carefully, don't confuse Derated thrust with Reduced thrust in these lines.

(The 100%/92%/80% RR Trent ratings I refer to here are for my current aircraft. It is accepted that other operators may use different derates, as a customer order from the manufacturer).

[mutt]

Leo77,

I believe that JT was referring to control problems rather than overboosting the engine.

Maxrate172,
Just about every airliner uses some form of derating power, this can be either "fixed derates" or "flex (Boeing's Assumed Temperature Method)". I'm presuming that Old Smokey is referring to a B747-400 with Trent engines, combing both the fixed derate of 20% plus the assumed temperature reduction of 25% you end up with that aircraft taking-off with approx 65% of the available power!

The purpose of this is to enhance engine life.

Mutt.

[Leo77]

Wow. Thats an interesting fact or point of view.

Guys, do you really want to say that for a given scenario like a flex T/O where V1 and Vr and V2 are a bit distanced from each other an increment in thrust could lead to a runway excursion cause of inability to hold the runway centerline during acceleration from V1 to V2??? That because force vectors or arm change during thrust increment?

I personally dont think so, but i am willing to believe. What i would ask next is: If youre doing a TOGA T/O and you get an engine failure at V1 you will be able to maintain centerline as long as your V1 was chosen properly. So if youre having a problem at lower thrust, you should still be able to advance thrust without losing centerline. Why should you lose it anyway? Only because of thrust change? Well, if the action is exagerated (is it the right word?) and thrust limits are exceeded, then maybe yes, but otherwise i dont believe that full rudder deflection or Nosewheel steering couldnt do the job.

Leo

[Old Smokey]

Leo77,

Your original post has lead to some interesting twists. If you are doing a REDUCED thrust takeoff (i.e. Flex takeoff), you can have full confidence in advancing the thrust to the engine rating, because VMC has been calculated for this. I think John Tullamarine's warning related to a too rapid advance of thrust which may not be caught by the pilot, I've seen this.

The point I was making (2 posts ago) was that some engines have multiple ratings, with associated multiple VMCs. Reduced (Flex) thrust may be allowed for each of these Ratings, and, if an engine fails, the thrust may be confidently advanced to the rating, NOT to maximum (unless that was the rating used). A similar situation exists for those aircraft with permitted overboost (APR). (We're not talking about a engine limit exceedance here, as might be the situation in a wind shear recovery).

An example of this - The B777 / Trent uses 20% derate (80% of maximum) thrust for operation on icy / slick runways. This significantly reduces VMC (highly desirable on icy / slick runways), allowing much lower V1s than for Full rating. If, following engine failure on an icy / slick runway just above V1, with a long acceleration to Vr to follow, thrust were advanced above the 80% Rating, to the full 100%, a directional control disaster is in the making. Advancing to the 80% Rating from a lower 'flexed' setting should cause no problem, as long as the pilot anticipates the extra swing.

There's a big difference between Derated thrust, and Reduced thrust (Flex).

Smokey

[Leo77]

Thanks again old smokey.

I´m now really confident about the procedures and even more important is, that i know the background. That makes it easier to believe and understand.

Finally it is not so easy at all. You really need time to study through all relevant take off performance things. This topic is huge and there are a lot of books about it but you cant read them all. Finally you still have to work and dont want to spend all your spare time with tons of books giving you information on how things work but in the end there are even more new questions.

regards,

Leo

[john_tullamarine]

It's not surprising that many people have difficulty accepting the Vmcg problems which have been discussed above - very few pilots ever get to play in the Vmcg-OEI sandpit.

The observed fact is that, for operations very near to (the real) Vmcg (for the day) the onset of alarming lateral deviation is rapid and occurs within a few knots of Vmcg.

Like many, I wasn't convinced .. until I was involved in several test programs which looked at the specific problems on the runway ... I still have some VERY interesting videos taken from the runway head which display a "now you see it, now you don't" sort of steering problem. If I were a doubter before .. I became an INSTANT convert ...

The reason that several of us raise this problem in PPRuNe every now and then is for the reason that it is generally not understood at all by the non-test background pilot fraternity.

It's not a day-in, day-out problem, but some background knowledge might just save your neck some day when you are tempted to takeoff in a high risk situation .. when there may be an alternative option available.