T/O thrust reduction and speeds
 

There is a topic about R-TO and FLEX.
but my questions are:

Do you modify the speeds V1 and Vr when doing a FLEX take-off?

Why some captains do not do a FLEX T/O on the first flight of the day?

and why they do not reduce when there is noise sensitive areas?

(edited to add a Q)



[This message has been edited by fernando (edited 15 June 2001).]

Quick answer (without too much thought)

1. V1 is based on runway performance, and therefore will change with reduced or flex TO thrust. (slower accel) Perf figures for the thrust setting should take account of this.

VR however is based on a/c weight and therefore will not change with thrust setting.

2. First flight of day - could be max thrust for thrust assurance check, or just too damn early to have brain engaged to think about the option!

3. Max thrust in noise sensitive areas will increase the climb angle and therefore the altitude, to reduce the noise footprint by the time you get to the measuring point.

Any other thoughts anyone?

When flexing V1 will decrese.

Thanks.

I always though so that V1 should be lower when derrating.
But captains add a few knots when using flex cause they say that if the real temperature was 50ºC and elevation 1000ft (or what ever the airfield elevation is) the speed booklet adds some 2-5 knots to V1 and 2-3 to Vr.

I never say anything cause I am a copilot with limitted experience and in any case those few knots dont make a big difference. As derrating only takes place on long runways.

In the case of the footprint.
How do you know, when you are high enough so that that higher power setting causes less noise that if you were lower with a flex power?

Temp is just something to derate the engines.
V1 decreses because you will achive VR further down the RWY than you would have if you had been using TKOF thrust, hence giving less distance to stop on.

Concerning the NAP, PANS-OPS standard proc A stipulates that you should climb with TKOF power to 1500FT and than use climb power. I suppose that this is something ICAO has thought to be the least noisy proc for the largest group of aircrafts.


------------------
-----------
Está esto el cambio de marchas?

The aircraft performance data for each runway should have all environmental data, Temp, head/tail wind, QNH etc as entries or adjustments; to enable the correct V1 to be calculated.

Therefore I would not be comfortable with someone adding speed to the V1 on a limiting runway without referring to the perf data.

VR and V2 tables are generally more broad based and for a given weight at a specific airport, one speed will cover a range of temperatures and pressures . Adjustment would be made if (say) it was somewhat hotter than ISA.

Most noise abatement climbs require the a/c to climb at a steep angle, (T/O thrust and V2+10) until passing a given height above airfield elevation, i.e 1500ft, then for thrust to be reduced to climb and for speed to be maintained at V2+10 until passing 3000ft, in order to keep the noise footprint as small as possible away from the airfield.
It is my understanding that flex or reduced T/O thrust is done to preserve engine life; the amount of reduction does not significantly affect the EPNdB (noise)produced. (The quote here is that is you can reduce ITT by 14 degrees C, you will double the life of the engine, for that phase of flight. A simplistic statement, because below a certain figure (dependant on engine type) further reduction will have no effect, but at T/O and climb settings, where the engine is hot, it will have a marked effect. That is why de-rated take-offs are used wherever possible).

I remember when noise monitoring points were marked on the SID charts, where the technique was to use all available thrust to climb until that point, where the a/c was levelled off and thrust markedly reduced until the monitor was well behind. Cheating really, but it kept the recorded figures correct!!

Then, should I comment about adding those knots to V1 when derrating or should I just leave it as it is?

We do use t/o power until 1500AGL and then reduce to climb power. And, I know that we will be higher up if we use full power (thrust). But how do they know that using full power is less nosy?

are they just rambling?

----------------
***********
Lo dif'cil es pasar de la cuarta a la marcha atrás.

Baggy,

But I guess that you do not derrate in a limited runway, hence, there should be no problem to stop the aircraft in case of a RTO when derrating and adding knots to V1.

This is also why I have kept my mouth shut, but again I am letting you know because it is not 100% clear to me.

Fernando

As baggywrinkle wrote there is little difference in dB level between TKOF power an and derate. However altitude seems to have a greater inpact on noise.
Concerning the V1/VR issue. If you use the (pre)calculeted values with the described corrections it should be accurat.

------------------
-----------
Está esto el cambio de marchas?

[This message has been edited by Knold (edited 15 June 2001).]

Fernando

Firstly - seperate the discussions about reduced take-off thrust and noise abatement climb! There is no real difference in noise levels. Reduced thrust take-offs are done to extend engine life; noise abate climbs are done to avoid annoying the neighbours. Two different issues.

As to not using derate on a limited runway - a broader discussion is required, because you may be artificially limiting the runway by using reduced thrust!

Let us say that for a given (short) runway at airport AAZZ, a dive into the perf data for a flap15 take off produces an RTOW of 22,100Kgs using a reduced thrust setting.
For a max thrust take off, the RTOW increases to 26,500Kgs. This is the same as the MTOW (Airframe limit) of 26,500Kgs.

The dispatcher now arrives with a loadsheet showing an actual take off weight of 24,200Kgs.

Quite clearly, reduced thrust can not be used and max T/O thrust is the order of the day. If, using Max take off thrust, the V1 at RTOW/MTOW is 134Kts, but the VR at 24,200Kgs is 129Kts, the V1 and VR are both 129 as the decision speed can not be greater than VR (OK?). Another five knots on the V1/Vr, although not good practice, would still allow us to accel/stop on the grey stuff. (And some to spare, because the actual TOW is less than RTOW and the a/c would therefore accel to V1 faster. Also stop faster. (Yes?)

If, however, the loadsheet shows a TOW of 22,050Kgs; this figure is less than the RTOW for a reduced thrust Take off which would then be used in order to extend engine life.
The RTOW is less because the acceleration with reduced thrust is less and it takes longer to accelerate. The V1 is less for the same reason, but in this case, the actual take off weight is quite close to the RTOW. We have in effect artificially made the runway limiting, because our T/O weight is close to the RTOW for the thrust setting.

In this case, you would want to derate the take off to preserve engine life but arbitrarily increasing the V1 figure above that quoted in the perf data would not be a good idea as we are close to RTOW and there would be the risk of overrun in the event of an aborted take off.

On very long runways, where the RTOW would, if calculated, be well above the the MTOW, it would not be a problem stopping in the event of a malfunction. In this case though, I can't see why anyone would want to arbitrarily increase the V1.

As to commenting to your Captain when he adds a few knots to the V1, I would perhaps run it past your performance specialist or Chief Pilot as a hypothetical question first.

Make sure that you fully understand the calculations.

Hope this helps.

Hi Knold and Baggy,

Ok for the time being I wont mention noise on this debate again but I want to concentrate on the FLEX T/O.


If taking off from a not limited rwy with an ATOW of 60.000 kg at sea level and 15ºC using standard flap setting (11º)
we have:
V1 - 130
Vr - 136.5
V2 - 145.5

but if the temperature is 50ºC we must add 4 and 2 knots to the V1 and Vr, hence
V1 - 134
Vr - 138.5
V2 - 145.5

These figures are from the AOM, and I think the reason why captains add those knots when flexing is behind this temperature correcction.

But again, if taking off from a hotter airport the engines are developing less power and we will need more rwy to reach the same speed as if the temp was lower. Also, if we take into account that the speeds used are IAS we will find that the TAS (which is what it matter to stop the aircraft) is higher with higher temp. ¿correct?

Same thing if we take off from a higher field, knots are also added to V1 and Vr. but lets just concentrate in the temperature increase.

Could you please check your AOM for a situation like I have mentioned. This will help me to undestand a bit more why captains vary the speeds when derrating.

I might going a bit too far, but I believe that I can only solve my doubts within the pprune community.


gracias chicos.

Regarding the Vr and V2 speeds on a reduced power take off, and what follows refers to the B737-34 and probably the rest of them too!

We have two types of B737, ones with a dumb FMC where you type in the speeds you are going to climb out with, and another which produces the speeds given a selected weight, flap, and reduction (assumed temp/flex - whatever your language - same thing!)

The first one, we enter the speeds from a card. Done.

The second one - we take the weight, and get an idea of the speeds and bug these from the card. You then go to the FMC, and enter the ZFW, it will come up with the Vr and V2 speeds (V1 down to us). You enter the reducation and the Vr and V2 speeds change (partcularly at light weights). Because....(IMHO)

The idea following standard rate rotation is to achieve V2, following an engine failure at V1. If you use speeds for a full power take off this will be true. However, at a reduced power, because the aircraft is not accelerating as much the V2 will not be achieved at the end of the rotation. You will have to accelerate a little in the air to achieve V2. Hence the FMC will increase the Vr so that at the end of the rotation, you end up at V2. Cunning!

CCcccccccc....ough

Cough

Does the 737 not uptrim the remaining engine in the event of a failure then - either automatically or by pilot input?

V1 changes are complex and best left to someone who knows what they are talking about - ie not me!

Vr increases because part of the upward force at rotate ('lift') is engine thrust. Reduce that and...........

V2 decreases because you can fly slower and still control engine out yaw with a lower 'grunt'

Knold.your right inthat the V1 is reached earlier,but since your derating the thrust to use more runway,you have to compensate for 'true'airspeed and bug up the speeds to equate to a 'hot'V1

Knolt,if you didn't bug up to compensate for the 'Warm' day your engines are putting out,you'd always reach V1 early(your right there),but it wouldn't be balanced(equal stop/equal go.One uses 'flex' to ease the engines and use the runway,the question is should one use up all the runway credit in goin 'full'reduced all the time-no'buffer'left!!

Hi, baggy, knold, cough, boac, m+v and anybody else interested in the V1/Vr,

I have done some deep book diving and came out with a conclusion. I quote from the 737 AOM:

*********
For reduced thrust takeoffs based on assumed temperature, determine V1, Vr and V2 using airport pressure altitude and assumed temperature.
Adjust V1 for slope, wind clearway and stopway.
Compare adjusted V1 with minimum Vmcg from table using airport altitude and actual temperature. This is to cover the case where the pilots elects to advace the thrust levers to max take off thrust.

***********


Vmcg (min ground control speed) is related with the thrust the engine is developing. If it is at high thrust, as in low temperature and low elevation (low DA Density Altitude) we need a higher speed to counteract the assymetry.

But this is all I know.

¿why increase V1 and Vr when derrating?
¿is it to operate in a balanced field?
anybody is welcome to check his/her Operating Manual and compare the results


--------------
vuela bajo y despacio me dice siempre mi abuela.

fernando-see my previous about Vr. I refuse point blank to dig a hole on V1 however!

Anybody got any comment on the possible problems with control if you opt for max power after engine failure when you are at the (lower) V2?

Baggy : The T/O reduction takes into account the engine failure on take off account, and at that thrust setting, you will climb away from the ground, meeting min gradient requirements, obstacle clearance etc.

We do always keep a trick up the sleeve whenever we reduce - We always have the option to increase power to full G/A thrust on the live engine should you want to.

Ccccccoooou...gh

BOAC - Talking 737-400 here!

Provided V2 is above Vmca (bear in mind, as Vmcg is done on flying surfaces only, Vmca is probably going to be somewhere around Vmcg = 115/117 as applicable) and if you look at the speed card we have, the lowest rotate speed at 115 kt (no lower!) then the increase in engine power is not going to be an issue here! Given that the lowest V2 is 131, you should get an adequate margin of control!

Now why the V2 decreases I dunno - Bear in mind that a higher V2 gives a better climb (hence increased V2 climb out terrain limited places) I am struggling on that one!

Fernando - Was only talking about V1 in general terms, not increasing it! But having said that. Given a long enough runway (737 doesn't need much!) the ammount that you can reduce ceases to become balanced field based. Instead climb out performance (single engine/WAT limit) is the limiting factor. We don't aim to just crawl off every runway out there! So because you typically end up with a runway that is longer than you require, V1 will normally be the same as Vr. (Not always though!)

CCCccccccoug......h

Some thoughts ..

(a) it makes more sense (to me) to adjust the VR/V2 speeds to suit actual RTOW data with several considerations

(i) if you keep the V2 around V2min plus 20 or so, then the improved gradient is good value
(ii) if the procedure involves a turn for terrain, then make sure that your V2 rescheduling is compatible with whatever the ops engineers designed into the procedure from the point of turn radius.
(iii) I would be cautious about playing with V1 unless you have some quantitative data to hand upon which to base any changes. It would be normal for the performance manual to have some sort of guidance in this regard. In addition, be wary of reducing V1 to V1min values in conditions of strong crosswind.
(iv) the question of derating to a limiting condition has been well canvassed in other threads

(b) alternatively, keeping with the speeds for the higher RTOW data ought not to present a problem unless

(i) the bird has a desire to lift up its skirts and fly below those higher speeds.
(ii) the operator prefers to keep the shake rattle and roll wear and tear down during the ground roll operations - and we ought to keep in mind that, as pilots, we owe a duty to our employers in respect of desired standards and methods of operation.

(c) noise (sound pressure level) is a distance squared animal, so height is most important. With a significant derate operation, the SPL meter ought to record a difference in sound energy.

(d) BFL is not an a priori consideration. BFL often is a convenient calculation basis, but generally doesn't offer the best RTOW figures.

(e) if your preference is to increase thrust with a failure, then do so steadily rather than abruptly and after the rotation flare is completed, gear selected up, and the aircraft in stabilised climb conditions - why make an awkward situation even more critical ?

I know of at least one fatal where throttle advance was implicated. The normal limit speed boundaries ought to protect against directional control problems presuming that the pilot is appropriately competent.

From my observations in simulator exercises, I would earnestly suggest that you practise such exercises in sim free time so that you are comfortable with the sometimes surprising handling effects.