Take Off Perfomance

[The Real Slim Shady]

I had a discussion with a colleague about take off performance; essentially, the topic was the use of assumed temperature and the choice of speeds for V1,Vr and V2.

His assertion was / is that we should use the speeds associated with the weight which corresponds to the assumed temperature. I disgree: I believe that we should the speeds corresponding to our actual TOW. The FMC, interestingly computes Vfr and Vfto based on the actual weight.

His belief is that by using the higher speeds we are also employing the improved V2 climb technique, however, I am of the opinion that we can only use that technique when we have unused distance available on the runway and we are WAT limited (climb limited).

Taking the issue to extremes to highlight the point, if our airplane MTOW is 300 tonnes ( ISA, still air, 3000m TORA/ASDA/TODA, Dry,no obstacles - basically everything going for us) but today, same conditions, we weigh 250 tonnes, we should use the 250 tonne speeds even if we tell the engine computer that the assumed temperature is +55 and the RTOW at that temp is 300 tonnes !

If we had a limiting obstacle, using the higher speeds (OK I concede that at 250 tonnes we would go up like a rocket with all 4 turning / 2 turning), would keep the airplane on the ground longer; TODR increases and the distance to the obstacle reduces. Additionally, the obstacle domain changes as the end of the domain is based on the end of TODR, thus another obstacle, not accounted for in the analysis, could be introduced.

I think he is confusing saving engine life and the improved V2 climb technique but would be grateful of a more scholarly input.

[Flying Dispatcher]

In my opinion in an assumed temperature take-off, you would use your V1 speed for your current weight as you require the highest possible V1. The Vr, V2 speeds are then calculated based on your assumed temperature as you have reduced thrust from your powerplants.

[mutt]

Both systems work, if stands to reason that if you can accelerate to V1 @ 55° with 300.0 you will be able to do it even quicker if you weigh 250.0

You are basically using improved climb.

Around here people dont like fast V-speeds, so we use the speeds associated with the actual weight, the FMS produces the Vspeeds so this method eliminates the need for the crew to adjust the FMS for the limiting runway weight or the requirement to manually adjust the Vspeeds.

Mutt.

[The Real Slim Shady]

Thanks guys.

Mutt, as I understand the improved V2 technique, it is available only where the aircraft is WAT or climb limited rather than field length limited.

I believe, though don't have any definitive document to substantiate it, that the UK CAA do no permit the assumed temperature take off to used with the improved V2 climb. Don't know about JAA or FAA. Although, I can't think of situation where you would want to do that anyway - but it's early and my first coffee hasn't kicked in ;)

[Alex Whittingham]

I think you've hit the nail on the head. The assumed temperature take-off starts with the aeroplane in an unlimited condition then effectively takes you back to a limited condition by using less than full thrust. If you were not limited in the first case why would you want to use the increased V2 procedure?

[john_tullamarine]

Some thoughts ..

(a) "higher speeds ... improved V2 climb technique.. unused distance available on the runway and ... WAT limited "

V2 higher than minimum V2 for the weight will give you a better climb gradient than at minimum V2 .. has nothing to do with whether you are/are not WAT limited. However, with spare runway, and providing that obstacles permit, overspeed can be used to trade that better gradient capability for extra weight to bring you back to WAT-limited. (The mighty dollar rules).

(b) " limiting obstacle, using the higher speeds ... would keep the airplane on the ground longer; TODR increases and the distance to the obstacle reduces".

True .. but it is a case of balancing the thing ... extra distance on the debit, but better climb on the credit. Not a cases of new obstacles popping up .. the analysis looks at all obstacles and figures the RTOW on the critical set of obstacles for the weight and speed schedule combination.

(c) there are other pros and cons with speed choices .. eg, lower V2 make the flying more critical and the obstacle clearance further out worse ... higher speeds add to aircraft wear and tear and eat into the ASD pads.

(d) speeds have to be consistent with an analysis. .. so, for instance, you can't pick this V1 and mix it with that VR and V2. Otherwise, you pick one according to you preferences.

[SR71]

(d) speeds have to be consistent with an analysis. .. so, for instance, you can't pick this V1 and mix it with that VR and V2. Otherwise, you pick one according to you preferences.


Although to me sometimes it would seem to be a sensible thing to do.

I've mentioned this before with respect to PRG.

Your Improved Climb speeds have a V1 about 20kts higher than the V1 for normal performance at the same weight.

For the purposes of an RTO, a TO may be rejected safely at any time prior to the highest V1 of the two cases above.

Yet, in the case of a normal performance TO (as opposed to improved climb TO), the brief I receive often constrains me to continue with a TO after the lowest V1.

This is somewhat contrived, AFAIK, because, we, as pilots are not trusted to mix the two together.

Neverthless, accelerating at 3kt/sec, it would allow me another 5/6/7 secs of time to decide whether to RTO.

Of course, in a normal performance TO, we would probably be airborne by then but in principle...

[mutt]

Slim,

The aircraft was RWY LWY at the higher weight, as your actual weight is lower, you will actually be WAT or climb limited! So the improved climb concept works!

Alex,
90% of our B777 operations are conducted using assumed temperatures, as there is a big performance difference between an actual OAT of 50° and an assumed temperature of 50°/OAT 30°, the aircraft isn’t as limited as you might think.

SR71.
Have you ever considered optimized V-speeds?

Mutt

[Alex Whittingham]

That's interesting. I'm assuming the thrust will be the same at an assumed temp as it will at the real temp, presumably the difference is the CAS/TAS separation that doesn't occur on the reduced thrust take-off?

[oldebloke]

AS i've mentioned before with the advent of 'assumed' thrust takeoff techniques one 'bugged 'up the V speeds to the assumed T/Off performance-to equalize the actual to assumed...
In fact one would reach V1 earlier with the actual(cold)temp than the 'assumed'.ergo the accel/stop wouldn't be equal..the padded V2 would also perform in the air as per the reduced thrust (hot)V2
Cheers..:ok:

[SR71]

Mutt,

SR71.
Have you ever considered optimized V-speeds?

Nope. I would but I don't write the manuals or incept the policies...

How do they work?

[mutt]

Alex,

The EPR is the same but the amount of thrust available at V1/Vr/V2 is greater, your accelerate go/ accelerate stop distances are therefore less.

SR71,
Remind me of your email and i will send you a presenation on the subject.

Slim, Mr Boeing states...
Standard Takeoff Speeds.
V1/VR/V2 should be obtained from the FMS or QRH based on the Actual Aircraft weight and the assumed temperature used for thrust setting. VMCG must be based on OAT.

Optimized V1 or Improved Climb
Speeds must be obtained either from a takeoff analysis for a specific runway or a real time calculation, ie, BLT.

So they are happy for you to use both methods, the decision then rests with how you actually want to do it.


Cheers

Mutt.

[The Real Slim Shady]

Mutt,

Many thanks. I would be grateful for sight of your presentation.

Slim