B737 Take off V speeds versus weight

[Hudson]

Various Boeing documents state that take off V speeds are a function of actual BRW. This means if the actual BRW is significantly less than the performance limiting weight and for operational reasons the pilot decided to use full rated thrust, then the V speeds chosen reflect the actual BRW.

Others have a different view in that regardless of the actual BRW, if the pilot decides to use full power for take off, then the V speeds used must be those of the performance limiting weight - not the actual BRW. Example; If the performance limiting BRW is 65 tonnes (requires full thrust) but the actual BRW is 55 tonnes (but the pilots wants to use full thrust anyway) then the V speeds for 65 tonnes are to be used.

Which is correct?

[mutt]

Speeds go with thrust setting used. thrust setting used goes with temperature.

Therefore if you want to use Max Thrust at the actual temperature you would use the associated speeds.

Mutt.

[john_tullamarine]

Morning, Mutt (evening for you, I guess) .. not often we coincide on the forum .....

I suggest either schedule may be used although I would be interested in Mutt's views should he take a hardline position ... I suspect that his position has a standardisation basis. I can't see that either approach necessarily comprises any airworthiness or flight standards concern .....

If one uses the actual weight schedule, all is well and the ASDR is minimised .. along with gear and flap wear. The only time I would be wary is if the weight is very low when there will be a significant overspeed due to acceleration and, in the case of a significant crosswind, handling problems in the event of a failure are needlessly increased.

If one uses the limiting weight schedule, one gets an improved climb performance at the expense of increased ASDR. At the reduced weight, the actual performance is superior to that inherent in the limiting weight calculations .... given that I suspect I know the operator to which you refer and the background of its management, I am surprised to see them adopt this protocol. ... but it is really a case of which horse management wishes to back ....

Perhaps the operator or its performance personnel made a flight standards decision to keep things simple for the troops in a period of unseemly expansion wherein a great number of comparatively inexperienced pilots has been employed ..... ?

Then again ... a nice cold morning ... sea level airport .... empty aircraft ....full thrust .... makes a change from seeing the far end of the runway at close quarters .....

[mutt]

Morning J_T, sorry that I missed this last night, I got dragged away for some mischief :):)

My stance is based on our desire to use the FMS as much as possible. On most of our aircraft, crews don’t have the ability to “readily” find the speed charts. So while both methods work, in our case it would involve the crew inputting an assumed temperature into the FMS, finding out the V-speeds, removing the temperature and then over writing the FMS V-speeds for the actual temperature! Quite feasible to do, but cumbersome.

It’s also worth remembering that the shortest runway that we operate to is 8000 ft. So the extra speed isn’t a factor for stopping.

Mutt.

[Hudson]

Mutt. If I understand you correctly, you are implying that the Boeing Performance Manual in relation to V speeds versus actual BRW is wrong? Try as I can, I cannot find any reference in any Boeing manufacturer's publication to where it states that if you use full thrust you must therefore use false BRW figures unless the take off is performance limited. Exactly what is incorrect about using V speeds that are relevant to the actual BRW?

[mutt]

I used the statement....

So while both methods work

That doesnt imply that the Boeing method is wrong but in our case with FMS equipped aircraft, the easiest method is to use the actual temperature and the associated V-speeds.

What are your reservations about using the higher V-speeds?


Mutt.

[Hudson]

Mutt. With all respect to your high qualifications. My concern about using higher V speeds than is necessary for the actual BRW is that it is not a Boeing approved procedure - If it was, and there were significant advantages accrued, then I am sure that it would have been recommended by the manufacturer.

Deliberately using V speeds that are in excess of that needed for the actual weight, simply because it is perceived as too tricky to fiddle the FMS, would cause a good QC to rub his hands with glee in court - in event of an emergency on take off that turned into an over-run due excess unnecessary speed additives. Slippery runways come to mind here.

History has shown that there are significant hazards involved with high energy aborts - so what is the point of holding down on the runway to get extra speed when it is not needed?

[alatriste]

First of all I´m not a B 737 pilot but MD 80. Our company SOP call for V speeds according ATOW.
But I do change that criterium ocasionally.
Let´s figure out we are taking off in a 12.000 feet runway, for enviromental conditions our RTOW is MTOW 160.000 lbs and corresponding V1 152 Kts, VR 157 Kts.
If the ATOW is just 110.000 lbs and V1 112 KTS, VR 115KTS, I always increased V1 to equalls VR. ASD will increase but I´m quiet sure will be lower than ASDA cause we are allowed to perform a RTO with 152 Kts and 160.000 lbs.
I think is easier and safer to remain in ground until VR when taking off with such a low weight.
I think it would be unsafe, not only because performance and controlability but also regarding legal matters, to increase all speeds as we were taking off with the MTOW or RTOW.

[Flying_Tuur]

TO speeds are calculated with the Actual TO weight. The TO speeds have nothing to do with the thrust setting for TO.
The speeds are designed for aircraft weights, Thrust settings have a minimum for a certain weight (your assumed) but can be increased for different reasons (fog, CB's in vicinity, slippery runway, ...)
For example, light aircraft on a foggy runway.. Full Take Off thrust is mandatory to get you in the air as soon as possible. Which idiot would use higher speeds, and make life difficult with staying on that foggy runway, or in the worst case having to abort a take off at a critical point near V1 with all the hassle of keeping the aircraft at high speeds on the runway, while lower speeds are available??
BRGDS,
Tuur

[alatriste]

HI TUUR, TO thrust setting affects V speeds when assumed temperature is used. This topic was discussed previously under " V1, Vr corrections".

[Hudson]

Most light aircraft - Chieftains, Cessnas, Barons and so on, use full power for take off. In the POH for these aircraft there may or may not be a table of take of speeds for actual weight. I have never seen a manufacturer's recommendation to use the speeds for the performance limiting weight instead of the actual weight.

If therefore for some reason an airline decides to use full power for every take off in a 737 regardless whether or not the aircraft is light or heavy, then it would seem logical to use V speeds for the actual BRW just like a light piston twin.

[SuperRanger]

Hi Hudson,

We tend to view takeoff performance i.e. Assumed Temp and V Speeds, as one fixed set of numbers for a particular TOW. This is only true if you are taking off at the max. limited TOW on a balanced field.

At any weight that is lower than the max. limited TOW, we are opening up a range of Ass. Temp as well as V speeds. This fact will be clearer if we were to calculate our takeoff performance using the Generalised Method which is similar to Performance A exam.

But in the real world (as in my company), we have Specific Takeoff tables to help simplify our task. Instead of having a range of speeds, we get a fixed set of speeds together with a fixed Ass. temp. Unfortunately, in simplifying things, the workings behind it is discarded and over time it can become confusing (i know it did for me).

If you read carefully, Mutt, John and Alatriste have pretty much answer your question.

PS: Tuur, Alatriste is right, the V speeds does change with thrust. In fact, V speeds INCREASE with INCREASE Ass. Temp (REDUCING thrust). This is to use the increased V speed (utilising the extra runway length) to counter the decreased in second segment climb performance.

[Hudson]

Super Ranger. Sorry I am a bit slow in following your argument for V speeds that do not bear resemblance to the actual BRW. As I said earlier, there is no reference to this policy in the Boeing Flight Planning and Performance Manual, and for the life of me I cannot see what is wrong with using the tabled V speeds in the manual versus the actual weight. It does not matter whether the take off is climb, obstacle or runway limited. From Boeing Document D632A003-KG310: Quote:

Take off speeds. The speeds presented in the take off speeds table as well as FMC computed take off speeds can be used for all performance conditions except where adjustments must be made to V1 for clearway, stopway, brake de-activation, improved climb,contaminated runway situations, unbalanced for brake energy or obstacle clearance with unbalanced V1.
These speeds may be used for weights less than or equal to the performance limiting weight.
Normal take off speeds V1,VR, V2 with anti-skid on are read from the table by entering with take off flap setting, brake release weight and appropriate column. If take offs are scheduled using these simplified speeds in conjunction with airport analyses that include clearway and/or stopway, adjustments to V1 speed are required.

I fail to comprehend the rationale for using V speeds that are manifestly in excess of that required for the actual BRW just because the pilot decides to use full thrust.

[john_tullamarine]

A point or two ..

(a) QRH data is simplified and inflexible

(b) Vol 3 or similar is far more flexible but needs a modicum of interpretative skills

(c) the AFM and performance program outputs provide the most useful data

(d) there is no cogent reason why the speed schedule ought always to be minimum or always be maximum (as limited by the RTOW) for the runway in question. There are advantages and disadvantages to both. It ought to come down to either a standardisation issue (which is an operator/regulator negotiation matter) or a rational risk assessment and risk mitigation issue .....

[Slick]

Hudson a couple of points, I havnt read all the posts fully, so dont shoot me if Im repeating someone.

Your TOW is 45T and your limiting TOW (RTOW) is 70T, you go rated thrust and use the speeds for 70T.

There are if I have grasped you QN correctly a few flaws with this line of thinking. No1, the V1 at 45T compared to 70T, and No2 the V2.

Im looking at this as simply as possible. Your no1 engine ****s itself at 115kts, your V1 for 45T is 110kts, However your using a 120kts for an artificial weight of 70T. So Mr Hudson can you please explain why you decided to abort your takeoff 5kts past V1?

V2, if you have bugged a V2 for 70t instead of 45T, has anyone considered your 2nd segment climb graident, what will you achieve if your flying a 45T aircraft at a 70T V2 +20?, I sure dont know.

I suppose your perf dept may have a reason for wanting to use the higher V speeds at an artifically higher weight. But generally speaking you should be using the Vspeeds for the actual aircraft weight. And anyway if there is such a policy it really should have been explained better during your perf day when you joined. Problem with pilots (and I am one) is that years of BS and C wispers associated with perf have severly muddied the waters.

Your Vspeeds do change when using an assumed temperature, as they will ever so slightly when considering slope, wind ect, but I dont think your question relates to that, does it?

Ok thats me!

Rgds

[john_tullamarine]

Slick,

I suspect that you may have missed the point of the discussion...

Hudson, in fact, is a very experienced big bird instructor .. who personally prefers the minimum speed philosophy and is endeavouring to stir up some interesting discussion on the subject. While he is not a performance engineer .. he knows a lot more about performance than the great majority of pilots do ... trust me on that one ..

Looking at your comments ..

(a) if the V1 adopted for the takeoff is 120 kt (providing that that speed is not less than the minimum permitted nor greater than that scheduled for the RTOW) and a decision is taken to abort at 115 ... where is the problem ? ... if the 70T case can schedule a 120 kt V1, then a lightweight reject from 115 kt is hardly going to present a greater problem than the heavyweight case for the pilot on the day ....

(b) second segment climb gradient will be rather wonderful at 45T using 70T weights ... if it is OK at 70T it is going to be much more OK at 45T .... think about it a little

(c) we all concur that performance scheduling is full of old wives' tales .....

[FlareArmed]

For Info:

I ran the Boeing Laptop Tool with a takeoff weight of 55T for both assumed temperature and max rated thrust of 24K.

The speeds for the Maximum Rated Thrust takeoff were 117/120/129 and for a 60 degree assumed temperature were 124/125/129. Improved climb was not used.

Interesting to note that the full thrust V speeds were lower than the reduced thrust figures.

[Slick]

John I suspect your right and I never questioned Hudsons or anyone elses qualifications, however I think the question was Vspeeds to use for takeoff BRW of 55T if your perf limired TOW was 65T, and I still stick by Vspeeds for 55T not 65T.

As you said you have a range of Vspeeds for a weight, If you want a higher V1 why not just get jep or your own perf dept to run max V1 as an option for 55T, if thats you aim, or min V1 if thats your other aim?

Either way I wasnt suggesting u wouldnt get away with using those higher speeds associated with an artificialy higher weight, just that technicly I believe it incorrect unless the analyses has been done.

If ive still missed the point then bollocks to it.

Best rgds

[alatriste]

Here goes my point of view regarding Slick questions to Hudson.

When rejecting a TO at 115 Kt and ATOW 40T using 70T speeds, I´m absolutely sure that the airplane will came to a complete stop within the ASDA,cause the runway is enough for a RTO at 120 Kt and 70T.
When climbing at V2 of 70T instead of the real 40T, my climb gradient will be higher than minimun 2.4%, cause my weight is 30T less.

All the above will be true as far as 70T is not greater than RTOW for the airfield conditions.

Sorry my last post was useless. All questions were already answered. I didnot realiced there was a second page in the thread. Regards

[Hudson]

Thanks for the replies everyone. A little background to my query is called for, as you have all gone to a lot of trouble to reply.

My old airline of countless years ago operated out of several Pacific atolls. The airstrips were built from compacted coral and were often rough as guts. The aircraft really took a beating on take off and landing. We used reduced thrust because invariably we were very light, and the speeds were low anyway.

The boss chief pilot decided that due to the rough runways it would be better if we got airborne with minimum take off run - understandably. So he put out a directive that we were not to use reduced (assumed temperature) thrust but instead go for full power. 2.18 EPR in those days. But then he got the idea from somewhere that if using full thrust you must therefore also use the V speeds for the performance limiting weight which required full thrust.

Yes - you guessed it. We were now holding the aircraft on the ground (and getting the bejeezus belted out of the poor 737) for anything up to 15 knots higher than previously.

I queried this and was told that too bad, that was the legal way to do it - ie use V speeds for theoretical max performance limiting weight. It didn't make sense then and it doesn't make sense now. But there are operators who still do it that way when lighter than the performance limiting weight.

[mutt]

Apologies for the delay in answering....... Needed to pop up to the local pub for a Guinness.....http://www.stopstart.fsnet.co.uk/smilie/guin.gif

Hudson,

With the introduction of our new fleet aircraft a couple of years ago, we had two schools of thought in relation to the V-speeds at actual temperature. Some wanted to use the convoluted method that I described above. They justified that with the argument that if you satisfy all of the takeoff requirements at 70T, the surely if you use the same takeoff thrust at the lower weight of 45T with the higher V-speeds, you will reach the V1 point sooner with a greater distance to stop and with less mass to stop.

Is this safe?? We couldn’t find any regulation that said that it wasn’t.

Is it prudent?? We operate in an environment of excessively long runways with severe Temperature inversions (+15°C) during the summers. Some felt happier taking the extra speed into the air. Using the higher V-speeds introduced the Boeing philosophy of Improved Climb (Airbus Over-speed.). The extra runway length was traded for greater climb performance.

Your situation is totally different; I seem to remember that you or someone else mentioned using a takeoff flap setting of 25 for takeoff from those atolls. If this was the case then keeping the aircraft on the ground for that extra 15 kts totally defeated the purpose of selecting the higher flap setting!

Some may argue that you should never have a higher V1 than absolutely necessary, but considering that we are using Balanced field V1s, Optimized V-speeds, Improved Climb or even just rounding V1 up to Vmcg. You may find that its an everyday part of our lives.


Mutt. :ok:

[Hudson]

Thanks Mutt - all understood now. Interesting debate though, isn't it?;)

[Captain Phaedrus]

Sorry to blow the dust off this after so long, but it's an interesting thread.

Some speeds (V1, Vref etc) are related to runway length in its various guises.

Others (V2, VMU, etc) are related to aircraft weight.

To mix and match assumed temperature V1s and V2s is mathematically invalid, which would be fine if it had been proved by flight test, but it hasn't.

So, we should all be using actual speeds for actual weights.

Following an engine failure, do you want to use a 'wrong' V2, and thus invalidate your performance calculations? (V2 has ONLY to do with AUW).

[mutt]

Some speeds (V1, Vref etc) are related to runway length in its various guises.


Not sure if i agree with this......... look at the simple Boeing V-speeds, I think that you will find that V1,Vref are weight dependent REGARDLESS of the runway length.

Mutt.

[BOAC]

Yep - interesting thread!

Mutt - my understanding is that the Boeing V1s in the (simplified) QRH are for the old 'balanced field', and that there should ALWAYS be a correction available if the field/obstacles are worse than this?

As you say, Vref is ALWAYS weight/flap dependent.

...and Hudson - I think your CP was nuts!

[mutt]

BOAC,

You are right that the V1 in the QRH is based on balanced field, however what that means is that for a certain weight you need a specific amount of runway to satisfy the balanced field requirement.

For discussion sake lets say that a B737 needs a balanced field of 5000 ft at maximum takeoff weight. If you are operating from a 10,000 ft runway, do you adjust the V1 for the additional runway? (Forget about improved climb and derate). I would guess that the answer is no, you use the same V1.

The same goes for obstacles; neither the QRH v-speed chart nor the FMS provide you with the means to adjust speeds for obstacle clearance. You are required to reduce the takeoff weight and then get the associated V1.

So i believe that V1 is based on weight and not runway length.

Mutt.

[FlareArmed]

My company operates B737-700 with a Boeing Laptop Tool to derive performance calculations. This is a self dispatch tool that covers takeoff and landing performance as well as weight and balance.

With regards to V1, there are five selectable options:

-Optimum

-Balanced

-Min

-Max

-Balanced Plus

Our company uses Optimum because it allows the highest takeoff weight and also matches the FMC derived V1 (assuming no other factors such as MEL performance degradations).

The BLT also allows the option of Improved Climb as previously discussed in this thread. As a point of comparison a takeoff on a longish runway, using Improved Climb, gave an increase in weight of 9T and a V1 25 kts higher.

Effect of Obstacles

I plugged in a limiting obstacle that reduced the MTOW quite markedly (by 15T) and used a wet runway to force a larger V1/Vr split.

When I cleared the obstacle, the V1 at minimum thrust remained constant. Minimum thrust dropped by about 4% N1.

The obstacle reduced the MTOW but had no effect on V1 for the equivalent weight at minimum thrust. Interestingly enough, for a fixed thrust and weight, clearing the obstacle forced the V1 DOWN by 4 kts.

Effect of Thrust

A decrease in takeoff N1 using either derate method produces an increase in V1 and Vr. V2 remains the same regardless of takeoff thrust.