Use of TOGA with Flex speeds
 

Interested to know your thoughts on the following:

Regarding take-off performance.

The scenario is, you are on stand and plan for a Flex take-off. Whilst taxying out, due to bad weather in the vicinity of the airfield, Captain suggests the use of TOGA instead. You do not however re-calculate the performance based on TOGA, instead you keep the speeds from the Flex calculation.

My question is, is this safe?

A couple of things which I would like to clarify:

a) with the use of TOGA, normally the performance calculation would give a reduced V1, so your decision point on the runway would be earlier. The Flex V1 would be higher/later.

b) in relation to point a) - say our Flex V1 was 140 kts. With TOGA, you would reach this speed at an earlier point on the runway, allowing a slightly longer remaining stopping distance, therefore is the use of TOGA with Flex speeds considered more conservative?

Thank you for any replies in advance.

Of course it's safe!

You said it yourself. You'll reach the originally calculated V1-speed earlier down the rwy, so if you abort at V1 you'll be able to stop earlier down the rwy as well.

Also, your climb performance will be better.

I don't understand what makes you having doubts about using the speeds calculated for the flex TO with TOGA thrust.

Regards,
Erasmus

Not always I'm afraid...

If your performance calculations for your flex take-off have a very low V1, then sometimes, that V1 is lower than Vmcg for Full thrust T/O. This depends on how your performance is calculated, and can be an issue if your airline uses min V1 as standard for flex. take-offs or when RWY WET.
Therefore, you should never use a V1 that is lower than Vmcg full thrust just in case of your said scenario or a panick reaction by PF at V1...
Remember that V1 is calculated for your reduced temperature thrust input, Vr and V2 are calculated for actual aircraft mass.

The same in case of windshear during the t.o. run the concept of V1 is lost.
Or when you forget to set Flex Temp. and TOGA is applied but with Flex calculated speed.

Maybe safe but WRONG,why the rush?:confused:

One thing I have to consider on my aircraft (777), if changing thrust setting on the roll is to recheck the stab trim setting, as the nose-up pitching moment with the thrust increase wiill require a different setting....apart from VMCG considerations at lower weights..the increased thrust will give you guaranteed performance..... Zlin.

Our company's laptops always calculate the speeds with Vmcg for max thrust as a minimum V1 speed.

Starting up those laptops, inserting the new data, do the calculation and insert them into the FMGS will take 3 or 4 minutes at least. Sometimes you have the time and sometimes you don't. I will not delay my TO (and delay traffic taxiing behind me) just to do unnecessary TO speed recalculation. Now if you're talking about delaying the TO due to the weather, of course that can be considered if it looks unsafe to take off.

Laptop guys....you dont have qrh speeds in your fmc?Or a handy fcom?
Walking on thin ice i say...

DE-FACTO,

The speeds that may be generated by some FMS systems or from the QRH are not optimized and are not taking into account things like runway slope, obstacles etc.

To get the RTOW charts out (if you have them, if you have LPC laptop you won't) would take just as long as starting up the laptop and cross checking the numbers.

Airbus specifically warns that the speeds must not be mixed and matched, so best to take a moment and update the FMGS with the updated information.

I can't speak for other types but from the A320 series FCOM

"Using the same takeoff chart, for a given weight it is possible to :
‐ Select a temperature lower than the maximum determined one and keep the speeds defined at maximum temperature"

If you decide to reduce below TREF use TOGA.

Wake up at the back!

Surely VMCG only refers to Full T/O thrust? On a flex takeoff it is always safe to apply TOGA if necessary. e.g. windshear, engine failure.....

But if you DERATE the T/O thrust VMCG will decrease.....

TP

Agree on having to look at the rtow charts as well however If you aint climb or runway limited in the first place (as they used ass temp),they wouldnt be limited using full thrust...obstacles and slope are the same..

Not flying Airbus but that IS exactly my point.

It the same as ohhh the runway is now wet..let s just cut 10kts off V1..:rolleyes:

As has been pointed out, V1 for flex or Assumed Temp is always available, and calculations always take into account VMCA/G for TOGA thrust.

HOWEVER- A Fixed-Derate (TO1 or 2 on some Boeings) only takes into account Max thrust at the derated setting, so using fiew-wall TOGA may exceed the allowable thrust re VMCA/G

Not needed Zlin- the 777 has a speed trim, so the setting is the same regardless of thrust.

de facto, that's absolutely not what we're talking about here.

The question is: "You have calculated a reduced thrust setting with the corresponding V-speeds. Is it now "safe" to use the same speeds with a higher thrust setting (TOGA), all other factors (rwy conditions, weight...) remaining the same?"

The simple answer is yes! If Airbus talks about not mixing and matching speeds, they're not talking about setting TOGA. Setting TOGA at any point during the TO roll is always an option. (On the A320 anyway. I would be amazed if it were different on any other jet)

Don't worry, if it suddenly starts raining and the rwy gets wet, the laptop will come out and new TO settings will be checked. (If it was likely to start raining, we would probably have had those numbers available already :ok: )

Sounds like music to my ears:ok:

I suggest you guys read

"Getting to grips with Aircraft Performance" before you start fooling around with TOGA Derates and Flex speeds. :ok:

Specifically look at the nice color graph on page 49.

http://www.smartcockpit.com/data/pdf...erformance.pdf

@nitpicker330

I fail to see what your graph offers in this discussion.

The question here is about increasing thrust from Flex to TOGA. So lets look at the effects:

V1's relationship with VMCG hasn't changed as VMCG was based on TOGA.
VR and V2 were based on weight which hasn't changed and VMCA which also hasn't changed.

The only thing that has changed is the acceleration rate of the aircraft. We now reach all of the speeds earlier.

In the case of engine failure before V1 we stop the same mass from the same speed (energy) but starting at an earlier point on the runway and therefore stopping earlier.

In the case of and engine failure above V1, again this would be at an earlier point than in the FLEX case (failure at the same speed) and therefore we reach screen height earlier and subsequently our gross and net climb are above those we would have had with flex.


As many have already pointed out. The gotcha is not from flex to TOGA (RATED) but from De-rated to TOGA. In this case VMC may change and therefore the V speeds need to be checked that they are still above the minimums for the new thrust.

The bigger problem I see is the mixing of balanced and un-balanced calculations by grabbing speeds from other sources.

I looked at the graph and I fail to see how it would be relevant to this discussion.
(I didn't have to download the book. I've had it since I started flying the A320. The book was mainly written by an ex-colleague from SABENA:D who joined Airbus after the bankruptcy. :{)

Want to do some other reading? Then read this: Understanding Takeoff Speeds by Airbus.

The note on page 6 says:
"Flexible Thrust is a thrust reduction, designed to save engine life. This thrust is reduced
to take advantage of the available runway length, when full thrust is not necessary
(from a performance perspective), but takeoff speeds with full thrust still apply."

Now, are you guys still not convinced that there's nothing wrong with using TOGA with the "FLEX-speeds"?

Well ok but.....

1/ we are not Test Pilots
2/ keep it simple
3/ follow your company SOP's with regards to selecting TOGA during De-Rated thrust T/O's etc
4/ if you make up your own rules and get away with it then lucky you, if you don't then expect a lot of sh** to come raining down on you from high up :ok:

5/ if that means delaying T/O for 4 mins while you re do the numbers then so be it........

Remember that if you start operating outside the normal green band SOP's then mistakes can and do happen. You do so at you own peril no matter how simple it may seem at the time.

Basically if you stuff up your Balls are theirs.....:sad:

Thanks for the link to that Airbus doc. I think I've read it before but now I've downloaded it. :ok: