another one on v1

[av8r76]

I'm finding the concept of v1 a bit confusing. According to my manual v1 increases with t/o weight. My question is that doen't it take longer to accelerate a fully loaded aircraft and to bring it to a full stop due to its momentum. That in turn should reduce v1. This point can be made with the fact that v1 decreases with high ambient temperatures. Again due to reduced engine performance it should take the a/c longer to accelerate to a predetermined speed.
I think I need a bit of training on this topic.
Also why is Vmcg not affected by xwind component http://www.pprune.org/ubb/NonCGI/confused.gif

[quid]

Many pilots find the concept confusing, so don't feel you're alone. <g> Perhaps I can help clear it up.

First, we must understand that V speeds are based on "balanced field length". This has NOTHING to do with the amount of runway available. It simply means that the distance to accelerate-stop equals the distance to accelerate-go.

Second, if you are heavier, it will take longer to stop. But, it will also take longer to "Go". So, your balanced field length will increase.

When the balanced field length equals the runway available (some carriers/countries also figure stopways and clearways) you are at maximum t/o weight. When there is an increase in density altitude, you decrease your max t/o weight. The V (basic) speeds will change very little. They will increase approx. 2 kts. per 5000'.

It's also important to recognize that there are "basic" V speeds and "minimum" V speeds. V1 min is driven by Vmcg. Vr min is driven by Vmu and Vmca, and V2 min is driven by Vmca and Vs. In no case may any of the V speeds be less than the corresponding Vmin speeds. That is why on some very short runways, you can't go even if you're very light.

AT higher density altitudes, many of the Vmin speeds will decrease due to the lesser thrust output of the engines, therefore lowering Vmcg and Vmca.

Actually, V1 basic and V2 basic will INCREASE with higher density altitudes (either temp or pressure altitude). It is the V1 and V2 MINIMUM speeds that decrease. So, if you are limited by V1 min, for example, and you find the temperature has increased, you will see a reduction in V1. Recognize also, that your max t/o weight will be reduced.

To summarize: You determine the t/o weight first. Then get you V speeds corresponding to it. I bet you'll find that the speeds for a 10,000' runway will be the same as for a 20,000' runway.

P.S. Sure, in real life the X-wind will have an effect on Vmcg. The trick is in knowing WHICH engine will fail! ;)

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[This message has been edited by quid (edited 15 September 2000).]

[fart]

Quid , you seem to have these V-numbers under control. What is the storey behind Vr taking you to 10 m or 35 feet above the runway all about? Has it to do with terrain clearance or what?
Many thanks.

[Cough]

I think: Refering to the 1st segment of the Perf A climb out which ends at around 35ft. Start at rotate and at the 35 ft point you will be in the engine out pitch attitude with the gear retracting at V2 assuming a post V1 engine failure...I Think!!! :)

[stardust]

On a Bae 146-200 series ONLY above 29,500Kg Vmcg = 82 and increases 5Kt/10Kt of crosswind

Bae146-100 Vmcg = 88 Kt no X-wind add
Bae 146-300 Vmcg = 86 KT no X-wind add

Can any pilots tell me why X-wind has no effect on the 100 series Vmcg (shorter fuselage) or 300 series Vmcg (longer fuselage)?

I used to operate on a 30 m large runway, has it been taken into account in Vmcg calculation?

thanks in advance.

[martair]

Except in the balanced field length case there is a choice of V1 from V1min to V1max.
V1 max is the accelerate/stop case and V1min is the accelerate/go case.
The reason that in tabulated V speed charts the V speed increases with increasing mass is that you must be able to accelerate from the point at which the engine fails to VR and meet the screen height at the end of the TODA.
Incidentally the first segment is from 35 feet to the end of gear retraction.

[twistedenginestarter]

V1 can seem odd if you think of it as one speed. It is only one speed in the balanced field case. Otherwise there are the two 1)first speed that allows continued takeoff with a single engine failure, and 2)the last speed you can stop from.

[quid]

fart-

The 35 feet is important in so far as the obstacle clearance in the t/o flight path is concerned. You must miss all obstacles by the net climb gadient (this is different depending on low many engines you start with) plus 35 feet. So at 35 feet at the start of the t/o, you are just starting your obstacle clearance path.

stardust-

All other conditions the same, i.e. rudder throw and square footage, thrust, etc., the Vmcg WILL vary with the fuselage length. Your carrier (for whatever reason) may not choose to use any modification of Vmcg for x-wind on those models. A longer fuselage has a greater lever, so the Vmcg will be lower.

The width of the runway has nothing to do with the value. The only concern is how close you can keep it to centerline, not how far from the edge.

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[hopharrigan]

Working out a V1 for every single runway in order to achieve max payload would not be practical or efficient, especially since the airplane is rarely limited by the runway length. So the manufacturer or contractor tasked to do this for the company will produce charts for each runway based on balanced field performance. When this is done the computer (or mark one eyeball) is used to determine the "best" V1/VR. (V1:VR Ratio). VR is based on the value of V2 and so is a weight related number. This makes V1 weight related too, for this type of assumption. This ratio is used for the whole chart, and is not necessarily the best for every case on that chart. Most jet airplanes use about 85% as a ratio for balanced field. This optimises the "STOP" ability. Turbo props or pistons are happier with 100% (or 1, ie V1=VR since their problem is usually a "GO".
So you see the V1 is an arbitrary figure for most cases. There are always two V1 figures for a real calculation, if it was considered necessary to work it out; a Min V1 which would allow for a continued flight, and a maximum V1 which would allow for the stopping case. In many cases, with a long runway, the minimum V1 is below the Vmcg and the max is above the V2, so the pilot would have a choice of any figure in that range. As the runway is shortened, the two V1 figures come together so that at a true balanced field they would be equal.
Way too complicated for the average pilot and usually not necessary. Although it would be nice if we knew the max V1, so that in extreme cases we could opt for an abort rather than be forced to continue the takeoff simply because the speed is above the charted V1 figure But this opens a real can of worms, especially for the training people, and is best left alone.
So the company (or manufacturer) will chart every airfield for you, using an assumption that each takeoff will use only enough runway to satisfy the balanced field performance, and an arbitrary V1/VR ratio. Much easier.
If you have time some day, look at the performance charts in the Flight Manual.(The Ops Manual charts and the FMC calculations use the same assumptions as the RTOW charts). There are explanations on the use of the Flight Manual charts and an example. Pick a runway that is way too long and work out the V1(s). You will have to teach yourself how to do it, since most companies do not cover this manual in training. Look again at a short runway and do the same. Illuminating.

[Checkboard]

Our company's performance department produces V1 charts for every intersection on every runway we use on the network, and tabulates the data in a big book. This allows maximum weight uplift for each take-off.

[quid]

Checkboard-

Does this tab data list a V1 for every weight, every temperature, and every altimeter setting?

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[jtr]

quid, the answer to your Q. is yes, yes, and yes for the mob I`m with. (and I think it`s the same mob as CB`s

Temp in increments of 2 deg C
QNH in 10 hpa inc.
Two best flap configs.
Wet/Dry rwy.
Every applicable intersection.

It`s a lot of pages.

[This message has been edited by jtr (edited 25 September 2000).]

[Streamline]

quid

I think some of your marbles may be on the ground.

1.The V speed concept applies for balanced as well as non-balanced take off performance calculations.

2. V1 goes up withe the weight as it is also related to V2 wich only depends on weight as it is in IAS wich equals EAS at low speeds and density altitudes. As long as there is enough runway ahead to stop it's OK to increase V1 with the weight.

3.As far as Vmcg is concerned:

Regulations do not require that x-wind is taken into account.

If one day this would become a requirement then the case of the most critical engine (similar concept like on the piston thing) would be applicable, hence it would not matter wich engine would fail.


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Smooth Trimmer

[mutt]

Quid,

We produce takeoff charts for every runway that we operate to and also include commonly used intersections. This gives the maximum takeoff weight for a balanced field situation for these runways.

If the takeoff weight is less than the maximum, reduced thrust can be used to utilize the whole length of the runway.

The V-speeds are dealt in different ways depending on the aircraft type. With the Airbus and optimized V-speeds, the V-speed is printed for each takeoff weight on each runway.

For the older Boeings, tabulated V-speeds are presented showing temp, pres alt, weight, thrust rating, flaps and necessary corrections for slope, clearway, packs.

On the newer Boeings the FMC will provide Balance Field v-speeds for whatever weight you input. The FMC doesn’t care what the runway length is, it only uses the weight and actual conditions.

Mutt.

[Streamline]

So Mutt

Would the new Boeing FMC allow a 220 Ton aircraft take off from a 5000 ft runway ???

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Smooth Trimmer

[mutt]

B747-400 or B777-200

The easy answer is YES, the FMS would allow you to takeoff from any runway. It presumes that you have gotten the field length limit weight from another source!

Anyone care to tell me where there is a page for you to enter the runway length in the FMC?

Mutt

[quid]

Streamline-

answer to 1. Don't really know what you're saying here. We use balanced field (theoretical) numbers to operate in unbalanced (real life) situations. Most pilots in their whole career will never be faced with a true balanced field takeoff.

2. V1 only increases with weight if you are heavier than the weight that drives V1min. That in turn is determined by density altitude. Below that, V1 is the same for all weights.

Once a V1 is determined for your weight, (unless your carrier has adopted an increased V1 speed procedure) it is not increased if you have a longer runway.

V2 may or may not depend on weight. Once again, V2min may be driven my Vmca requirements. The IAS-EAS statement is not a factor in this case.

3. My remarks concerning Vmcg were in response to questions by av8r76 and stardust. In most cases, "Vmcg" in jet aircraft is not adjusted for winds. In actual practice, however, your actual ability to control the aircraft IS greatly affected by the x-wind.

jtr-

Sounds like a helluva complete book.


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[Streamline]

av8r76 & quid

May I try to put it in a nutshell

It’s easier to explain when you reason it out backwards

V2 = 1.2 Vs

Vr must be such as to reach V2 at the latest at 35 ft and above the runway end in the balanced case.

So the bigger the difference between V1 and Vr (the one engine acceleration part) the more runway you need, hence the reason to increase V1 closer to Vr (it accelerates better with two then one) as long as there is enough runway left to stop at this increased V1.

V2 depends only on weight when expressed in EAS (and IAS=EAS in the low range of speeds and density altitudes)

So if weight goes up, V2 goes up, so Vr goes up and hence V1 goes up if enough runway is left over otherwise the weight can not go up and you have reached the maximum for that runway.

I know there are some factors as Vmu, Vmca ,Vmcg Vmbe involved as well but these do not affect the aforementioned principle of reasoning to a large extend if we concentrate on the original question "How does weight influanse V1" or am I overlooking something here ?

I know that V1 has some upper and lower limits dictated by brake energy, tyre speed and Vmcg but thats besides the original question I think ?

But do you agree with the following?

In the case of balanced Take Off we set TOD = TOR as a condition for the calculations.

If you want to squeeze out a bit more performance you can go unbalanced and use clear and stop-way to a larger extend.

As far as I know balanced or not has nothing to do with the practical or theoretical concept they are both theoretical but any feedback on this is welcome.


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Smooth Trimmer

[This message has been edited by Streamline (edited 28 September 2000).]

[SID the STAR]

who needs manuals when you've got PPRUNE!

no offence Mr Boeing........

[Streamline]

Hello Mutt

As far as the field lenght limit is concerned referring to your Boeing FMC 747/777 wonder.

Suppose ATC or any other reason asks you if you can take an intercection for T/O, how do you know if you can make it with your 747/777 FMC.

I have flown on the 777 but the company I worked for didn't use the FMC for their V speeds yet ?

Thanks for your feedback

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Smooth Trimmer

[jtr]

mutt, I am confused
If the FM`S calculate the speeds for you, then surely there must be a way to adjust for an intersection departure.

If it`s not too much trouble, would you be able to explain (chronologically) the process you perform for calculating T/O data, from start to finish.

I understand how it could be done, but it seems as if it would be too limiting on TOW to cover all scenarios.

[This message has been edited by jtr (edited 28 September 2000).]

[twistedenginestarter]

You can - I think I'm right in saying - always make a field a balanced field. You derate the takeoff. This makes the engines last longer and saves you having to pick one of the range of V1s we have been talking about. Because now you only have one.

[Feather #3]

Mutt,

Only re r/w length in the FMC; in our B744, the FMC does know the runway length. It's found on the APPROACH REF page for the departure/arrival runway in the active route.

However, as you correctly say, the FLL has to be obtained elsewhere.

G'day

[mutt]

As sad as it may seem, the FMC (744/777) is pretty limited. It doesn’t have an obstacle database built in to it as and Feather #3 has pointed out the Runway Length is part of the Navigation database and not the Performance database.

If ATC offers an intersection takeoff, the FMC has no way of calculating the takeoff weights , so unless the crew have predetermined takeoff weights from that intersection or are ACARS equipped, the intersection can’t be used.

JTR , the chronological is something like this….

Enter a tailored takeoff chart for the airport/runway/surface condition/MEL/ Flap Setting/ Thrust Setting.
Read the limiting takeoff weight at the actual temperature
If the actual weight is equal or lower than the limiting takeoff weight, enter the Actual Weight in the FMC. The FMC will produce takeoff speeds. Over write the V-speeds for any necessary corrections, such as wet / contaminated or MEL reductions.
(CAA certified aircraft has the option of Wet V-speeds on the FMC, FAA certified aircraft do not.):confused

The main place where this is limiting is for intersection takeoffs, we therefore add new runway’s (36* / 36# or similar) based on the takeoff distance from commonly used intersections. The end result is a lot of pages, therefore some are carried on the aircraft and others such as MEL Takeoff charts can be produced on demand in dispatch.

That’s our way of doing it, I’m sure that other airlines have totally different methods that suit them. If someone does know of a way of doing intersection takeoffs in these aircraft, I’d be interested in learning how.

Boeing has brought out a laptop computer program to allow crews to calculate takeoff weights in the cockpit. This was primarily designed to assist BBJ operators, but is now available for other aircraft types. (For a fee, of course..) ;)

Twistedenginestarter, if you are doing a derated takeoff, you still have the same range of V-speeds.

Mutt. http://www.pprune.org/ubb/NonCGI/cool.gif


[This message has been edited by mutt (edited 29 September 2000).]

[jtr]

Thanks mutt, just wanted to make sure, in my worried little head, that the T/O weight calculation was a different one for an intersection departure. Now it all makes sense.

[Streamline]

Thanks for the info, seems that I was not far from the truth.

What's the next question ?

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Smooth Trimmer

[TOGA Thrust]

Vmcg is demonstrated in Zero Wind conditions.
If setting V1 to Vmcg it is worth considering the crosswind. If operating close to the crosswind limits, you are now operating at 2 limiting conditions and probably on a contaminated runway.

One has to decide if you really want to do this.

[Alex Whittingham]

Vmcg is tested according to the certification requirements. In the old days BCAR 25 (British) required 7kt of adverse crosswind and FAR 25 (US) did not. This meant that a jet certificated under British regs could have a much higher Vmcg than the identical jet certificated under US regs and could, on occasions, be limited by Vmcg. When JAR 25 (European) replaced BCARs they standardised to the American Way.

This doesn't change the problem, only the stated speeds. It means that big twins can legally operate from shorter runways but, if they lost an engine above V1 and above stated Vmcg but in an adverse crosswind, they might not be able to keep the aircraft straight. To my knowledge it hasn't happened yet......

[twistedenginestarter]

Mutt

I have to disagree.

If you have a runway where by 140 kts you have reached a high enough speed to lose one engine then if you derate you will take longer to get to 140 kts and take longer afterwards. Most likely V1 (one engine out)will move towards Vr ie you need to get as close as possible to lift off to stand a chance of making the obstacle clearance.

In the meantime V1 (last chance to stop) reduces because, for any given speed, you are now further down the runway.

The very least this will do is to narrow the range of V1s.

My point is you are always able to narrow this band to zero by choosing a suitable de-rate. When you have done this you have - in effect - a balanced field ie you are seeing the same balanced field that would be seen by another aircraft fitted with less powerful engines - less powerful by the factor of the chosen de-rate.

In practice you may not want to de-rate to this figure because you would need to put power on after lift off in order to get a decent climb out, thus more-or-less defeating the point of the exercise (being kind to your RB211s). Maybe this is why you don't recognise the link between de-rates and affects on V1 range.

[quid]

av8r76-

You got quite a discussion started here. Very simply, after the min speeds are met, V1 will increase as weight increases. But as you see, there are many other factors. Is "balanced field" clear now?

V1 MIN will decrease with higher temperatures, V1 BASIC will increase due to the lesser thrust to accelerate to Vr.

streamline-

V2 doesn't necessarily = 1.2 Vs. AT lighter weights, it is often driven by the Vmca requirement. V2 can't be any LESS than 1.2 Vs.

The spread between V1 and Vr will vary signifigantly with density altitude. For example, in my a/c with a t/o weight of 300,000 lbs, at sea level the spread is 14 kts. At 7000 feet the spread is only 8 kts.

All the t/o performance charts I've dealt with use IAS and CAS. What charts are you referring to in mentioning EAS?

twistedenginestarter-

For purposes of this discussion, can we agree that derate and assumed temperature procedures accomplish the same thing? It's the takeoff weight that is reduced by the runway requirement if using assumed/derate. The V1 speed won't change all that much.

If using reduce/derate and I lose one on t/o, I know that all my obstacle and climb requirements have been met. I also have the added performance of the engines really eating colder air than the assumed, so my performance will be better than planned. I'm not an advocate of pushing up the power to get a more "decent" climbout. Why stress the remaining engine(s) at red-line if I've already cleared the obstacles? Once I'm past the obstacles, I'd rather have speed than altitude anyway.

(I know that will take some flak).

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[This message has been edited by quid (edited 30 September 2000).]

[mutt]

twistedenginestarter

I should have clarified my statement, when you are doing a Derate takeoff you still have the choice of Min V1, Bal V1, Max V1. The actual speeds are different to the maximum rated speeds.

It is possible to have all three of these at the same speed as shown below.

Min V1 = 142.3 KIAS
Max V1 = 142.3 KIAS
Bal V1 = 142.3 KIAS V1MCG = 124.6 KIAS at 15.0 DEG C
VR = 149.6 KIAS VMCA = 116.9 KIAS at 15.0 DEG C
V2 = 157.9 KIAS
VS = 133.6 KCAS

These are for a B777-200, 7000 feet runway and 239872 kgs, GE90-90B.

However, if we start dealing with a runway length of 10,000 feet and a weight of 200,000 kgs which allows us to use a 20% Derate, the figures are a lot different

The lower that you derate the engine the lower the VMCG becomes, the V2 will still remain around the same at 1.2 Vs.

Takeoff Speeds B777-200, 200,000 kgs, 10,000 feet runway.

No Derate.

Min V1 = 125.3 KIAS V1MCG = 125.3 KIAS at 15.0 DEG C
Max V1 = 134.3 KIAS
Bal V1 = 126.1 KIAS
VR = 134.3 KIAS
V2 = 145.3 KIAS
VS = 121.1 KCAS

20% Derate.

Min V1 = 113.3 KIAS V1MCG = 113.3 KIAS at 15.0 DEG
Max V1 = 137.5 KIAS
Bal V1 = 134.0 KIAS
VR = 137.5 KIAS
V2 = 143.9 KIAS
VS = 121.1 KCAS

Look at the Bal V1 in relation to V1MCG and V2.

So getting back to your original point, yes it is possible to have the Min V1 / Bal V1 / Max V1 speeds the same, either at a Maximum Rated Thrust or a Derated Thrust.

That also applies if you use a maximum assumed temperature reduction (25%) on top of the 20% Derate.


Mutt


[This message has been edited by mutt (edited 30 September 2000).]

[This message has been edited by mutt (edited 03 October 2000).]

[Streamline]

Quid

I know my statement about v2= 1.2 Vs is not always 100% correct and I have stated in my answer that factors like Vmca,Vmcg, Vmbe etc play a role as well.

I just wanted to keep things simple and concentrated on the original question from av8r76 “How does weight influence V1”.
Once that is sorted out you can go into details.

As far as the spread between Vr and V1 is concerned and the influence of density altitude, I think I addressed that issue in my answer.

If you want to go into details too early while answering the original question the guy is going to get lost.

The way we calculate out RTOW charts is company secret and we exist for over 80 years now so we are one of the oldest Airlines on the globe.


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Smooth Trimmer

[twistedenginestarter]

Mutt

Once again you are wrong.

However your example neatly shows what this thread has been about ie that V1 is a zone of decision rather than a single figure.

Your example shows V1s for 0% and 20% de-rate.

I said V1min will rise with increasing de-rate; and V1max will fall and thus eventually meet at the balanced field V1. Yet yours both do the reverse. Why?

The reason is both V1 figures are nothing to do with V1. The lower figure is following down Vmcg until it meets the rising 'V1min'. Similarly the higher figure is tracking Vr until again it meets the falling V1max.

If you had carried on to 40% 60% 80% derate, this pattern would have reversed to conform to what I proposed.

A good real life example.

I guess also that strictly speaking you were correct since in practice you can't always de-rate enough to get a balanced field because of operational limitations.

[mutt]

twistedenginestarter

I guess that as I haven’t worked on an aircraft that is capable of 60 – 80% Derate, I’ve failed to see your point in real life.

For clarification, V1min tracks VMCG and not the other way around. From the above you can see that the V1 for the accelerate-go is below V1MCG, it is therefore getting forced up to meet the V1MCG.

Mutt http://www.pprune.org/ubb/NonCGI/cool.gif