greater v1 or lower v1

[barry3327]

Hi everbody. I have problems with this question and I would like to be clear with this V1 question. Thank you very much

The aircraft is not field length limited, with multiple V1:confused:
available. Some operators prefer to use a lower V1.State the benefits of using a high V1
?
(i) Requires less take-off distance for a confirmed take-off.
(ii) Better vertical clearance above an obstacle.
(iii) Greater stop margin in abandoned take-off.
a. 1 and 2.
b. 1 and 3.
c. 2 and 3.
d. All of the above.

[Tinstaafl]

Unless I've misunderstood something it wouldn't be a greater stop margin in a rejected take-off. The faster you go the more distance needed to stop which reduces stopping margin. That eliminates all options except one.

That being the case this is an example of a poorly written question. Or rather, the options are poorly written. You only need to know a single factor (go faster => more room needed to stop. Hardly rocket science) to eliminate all except one option. You don't even need to know the (correct) information that the question is supposedly testing you on.

[Admiral346]

With same weight and same thrust, you obviously have a greater stopping margine using the smaller v1.

On the other hand, it will make you take an engine failure (or fire or any other fault) into the air. And in case of thrust loss, you will have to accelerate the portion from v1 small to v1 large with less thrust available and then to vr, therefore it will take longer and give you WORSE clearance height of any obstacle in your departure path.

So for long runways, I tend to opt for v1 large, giving me more opportunitiy to stop and stay on the ground, but on short runways or in unreliable conditions (contaminated) I'll take the v1 small to give me more margin for error of the calculation (I do take any calculation with contaminated RWY as a good, educated guess by the manufacturer, but for not more than that).

X-wind might make me opt for a higher v1 again, as the faster you go, the more stable you'll fly on one engine out...

Nic

PS: To answer your question after all my babbeling, answer (a) seems to be the correct one.
With larger v1 you will reach your screen height quicker and you will have therefore more vertical clearance towards any obstacle.

[MarkerInbound]

Look up "improved climb."

[FE Hoppy]

Low V1 gives safer stops and more likelihood of a OEI go. Downside is your OEI go could be right on the limits.

High V1 gives less safe stops and more likelihood of a stop. On the plus side better OEI climb.


So Low gives safer stop case and high gives safer go case.

and as above A is correct.

[john_tullamarine]

As always the devil is in the detail ... and examination questions invariably have to dumb down the detail for the purpose at hand -

(a) is the intended answer. Although (a) is not quite correct, (b) and (c) are wrong so (a), by default, is the way to go in the exam.

There is a caveat which should be understood: if the question relates to

(a) a situation where there exists a range of V1 which may be utilised for the SAME V2, then (a) is correct

(b) an overspeed schedule takeoff (which we suspect is the purpose), the actual screen position may be a long way further down the runway to achieve the higher V2. It follows that the INITIAL takeoff flight path will be LOWER for the higher V1 schedule.

Thus, you might well see considerably reduced clearance margins above first and early second segment obstacles. Progressively, however, the better climb gradient at the higher V2 will close the gap between the two calculated flight paths and, eventually, the higher V2 path will be higher and the clearances better.

In the real world it's a case of horses for courses - where is the critical obstacle(s) ? It is this which determines the principal desirability for an increased speed schedule or not.

[bubbers44]

Well said John. Spot on. It is the higher V2 associated with higher V1 and Vr that improves the 2nd segment climb for obstacle concerns below your flight path on an engine failing at V1. Of course this will take a bit more runway.

[AerocatS2A]

It seems to me that ii is correct for a given weight because more of the acceleration on the runway has been on all engines therefore the take-off is earlier and the obstacle clearance greater. For the extreme case of V1 being equal to Vr the failure must occur at Vr at the earliest, so the ground roll in the engine failure case will be the same as the ground roll all engines operating. On the other hand if V1 was 20 knots slower than Vr you have a significant portion of the ground roll conducted with a failed engine which gives a longer ground roll and reduced obstacle clearance. Once again, assuming the same weight.

[General Disaster]

Lets consider a r/w like the old Kai Tak at Hong Kong. One way you climbed out over the sea, the other way you climbed out over the blocks of flats.
This is my thinking.
You definetely do not want to overrun on an abort and go into the sea. Therefore you would choose a low V1.
you definetely would not want to be climbing out with the loss of an engine over the blocks of flats. Therefore you would choose a high VI.

I think you can apply that logic worldwide and then consider other aspects as well such as narrow runways and fog. If foggy do you want to be trying to keep an ac straight after an engine failure with a low v1. Answer probably no.
Same with a narrow runway me thinks. With a high V1 your time assymmetric is reduced giving you less time to veer off the edge.
Thaaaaaaaaaaaaaaaaaaaaaaaaaaaaanks:E

[bubbers44]

Everybody refers to V1. V1 isn[t what gives climb performance, with obstacles on departure after initial climb V2 is. With improved climb performance you give up runway length to get a higher 2nd segment climb after initial obstacle clearance. It is easy to look up.

[AerocatS2A]

V1 doesn't give climb performance but it does change the position on the runway that you lift off. A high V1 means more of the takeoff is on all engines and therefore your takeoff is earlier. This means you have better obstacle clearance, not a better gradient, you have the same gradient clearing all obstacles by a bigger margin.

[john_tullamarine]

One can't look at V1 in isolation from V2 and get a comprehensive result .. Your position is correct ONLY for constant V2 which may not be available for the particular Type.

[bubbers44]

Here is Boeing's explanation of improved climb performance.

http://www.captainpilot.com/files/BOEING%20PERFORMANCE/Improved%20Climb.pdf

[AerocatS2A]

Bubbers, I'm not talking about improved climb performance, I'm talking about the same climb performance commencing closer to the start of the takeoff run due to more of the takeoff being conducted with all engines running. It does not give you better climb performance but it will give you more obstacle clearance than a lower V1, all else being equal, as John notes. I think that's what the exam writers are going for, if you increase V1 only and leave the weight and other V speeds the same you will have better obstacle clearance for an engine failure at V1.

[bubbers44]

If true, you have different rules in Australia than the FAA rules.

[john_tullamarine]

if you increase V1 only and leave the weight and other V speeds the same you will have better obstacle clearance for an engine failure at V1.

And I can bring to mind several aircraft Types for which this is perfectly correct - albeit that the terrain clearance delta is minimal and the main reason for changing V1 in such a scenario is to gain a TODR/TORR advantage.

The consideration here is whether the AFM gives the data necessary to play with V1 in isolation.

The present problem is that we don't really know what the exam question is driving at. However, this sort of discussion is good to have from time to time to bring it up for the new chums around the sandpit.

[JammedStab]

Bubbers, I'm not talking about improved climb performance, I'm talking about the same climb performance commencing closer to the start of the takeoff run due to more of the takeoff being conducted with all engines running. It does not give you better climb performance but it will give you more obstacle clearance than a lower V1, all else being equal, as John notes. I think that's what the exam writers are going for, if you increase V1 only and leave the weight and other V speeds the same you will have better obstacle clearance for an engine failure at V1.

If true, you have different rules in Australia than the FAA rules.

Why would the rules be different. It seems to me that some transport category aircraft have the ability to have their V1 speed adjusted at least some of the time. If all other minimum criteria are met, one can have V1 as low as VMCG or as high as VR.

If two exactly same aircraft in terms of type, weight and loading, takeoff at exactly the same time from two parallel runways of the same length which have thresholds beside each other with the same obstacles, but one of them has a V1 20 knots lower than the other but both have the same V2 speed and both of them lose an engine at V1 , I should think that the one with the lower V1 will take a much longer length of runway prior to reaching VR, and will be much closer to any obstacle.

[bubbers44]

Yes, it is good to discuss because we learn from each other. Using a higher V1 but the same V2 does give better, marginally, initial climb performance but doesn't help any on the rest of the second segment climb. Now it decreases because of the lower speed to the same climb as a normal V1. I guess in the US we went with improved climb higher V2 speeds if runway length allowed it with chart to prove we are safe. Are there any charts for your Australian procedures you use to increase V1 but use original V2? You would clear the 50 ft trees at the end of the runway but what about the hill you need to clear 2 miles past at your reduced climb rate after V2 is reached because you didn't use the higher V2 to get that performance? Yes, I know you may have lifted off a few hundred feet shorter if V1 was with both engines but do you have charts to calculate take off performance using that procedure? If you do fine, we don't have any of those charts.

[john_tullamarine]

but do you have charts to calculate take off performance using that procedure

It will depend on the particular AFM - some give all the limits explicitly and provide significant flexibility. Others combine limits and make the processing a tad easier for a loss in user flexibility and choice.

Not something to fuss over too much - so long as the procedure adopted is consistent with the AFM, things should be reasonably OK.

[bubbers44]

I guess so. 23,000 hrs and never heard of that procedure. Maybe it depends on what aircraft you fly. Does anybody do this on a Boeing?

[AerocatS2A]

Bubbers, I'm just addressing the specific question in this thread. Whether it is allowed by the rules or the flight manual is immaterial as it seemed to be a purely theoretical question.

[President Bush]

I enjoy aborting after V1 from time to time. Now that we know it's variable we know V1 isn't actually V1 after all. Thought as much.

[galaxy flyer]

Nice to see your newest handle, SSG.

GF

[bubbers44]

All I know is I would be in deep trouble if I aborted after V1 at my company, a major US airline. Maybe Australia has different rules but we can't do that.

[donstim]

I'm amazed at how difficult many of you are making this. The question is purely about using a high V1 speed when a range of V1 speeds are available and can be used (i.e., the takeoff is not field length limited and multiple V1 speed data are available in the AFM). It says nothing about changing V2, using improved climb, or overspeeds.

Aerocat is spot on. Using a higher V1 results in a shorter accelerate-go (i.e., takeoff) distance, which will also increase your vertical clearance over obstacles. You get that increased vertical clearance because you got into the air in a shorter distance; there isn't any effect on your climb gradient.

[AerocatS2A]

All I know is I would be in deep trouble if I aborted after V1 at my company, a major US airline. Maybe Australia has different rules but we can't do that.
It's good that nobody is suggesting aborting after V1 then.

[Admiral346]

I don't think that the original question is about variable v2 also. At least that is not indicated in the question.

I remember that on the A320 there was something called a "variable kvs factor" and had to do with shifting v2 by pretendig to need a higher stall margine to increase v2 and therefore being able to pull out more weight in case of engine failure. That was the reason why even on very long RWYs we got rather small stopping margines. One would expect around 1500m+ on a 4000m RWY on a twin like the 320 but it was way less.

I doubt that this is the examn's question. On our EFB, the takeoff data calculation does show a v1 range, but always, alyways only one vr and one v2. Wether that v2 is "kvs factored", is not important, as there is no range to choose from.

So i beleive, aerocat and I agree, and do give the correct answer.

But who knows, thinking back on my examn, and how many wrong answers were the ones you had to check to get the points...

Nic

[bubbers44]

Take-Off Speeds for the 737-300/400/500
TOW 737-300 20K 737-400 23.5K 737-500 18.5K
1000kg V1 VR V2 V1 VR V2 V1 VR V2
70 158 162 168
65 154 155 160 152 154 162
60 147 148 154 144 147 155 147 147 152
55 140 141 148 137 139 149 140 140 146
50 133 133 141 129 131 143 132 132 139
45 123 123 133 121 123 136 123 123 132
40 114 114 126 112 115 130 113 114 124
35 104 104 117 104 104 117

ISA Vmcg=111 ISA Vmcg=115 ISA Vmcg=106
Typical wet V1= dry V1-10kts.
All speeds assume balanced field, flap 5, pa<5000ft, OAT<35C, nil slope, nil wind, runway dry.



Looks like increasing V1 and leaving Vr and V2 alone doesn't change anything enough to be readable or none on a 737.

[donstim]

Sorry, but that ain't it. That info only provides a single, balanced field V1 value for each weight. You cannot use that info to select an unbalanced field V1 value, which is what you are doing when selecting from a range of possible V1 values.

You need to get that info from the AFM, not from a basic ops speeds card.

[bubbers44]

Since in my long career through several airlines I have never heard of variable V1 speeds except for improved climb which changes all the speeds where do you find this information. Also by adding an average of 1 knot to V1 and leaving the other V speeds the same what does that accomplish? It seems we are wasting a lot of bandwith here.

[JammedStab]

Since in my long career through several airlines I have never heard of variable V1 speeds except for improved climb which changes all the speeds where do you find this information.

Most likely, the performance engineers at your companies has heard of this for your aircraft. But, in the name of simplicity, for all the pilots on the line at your company, for a given set of conditions, the same V1 speed is always used.

[FE Hoppy]

Since in my long career through several airlines I have never heard of variable V1 speeds except for improved climb which changes all the speeds where do you find this information. Also by adding an average of 1 knot to V1 and leaving the other V speeds the same what does that accomplish? It seems we are wasting a lot of bandwith here.

You may find the V1 used quoted on your Runway analysis tables (IRT). The choice is generally made by policy for each runway or for an entire fleet or operation.

The software used to produce the IRTs will have a setting along the lines of:
Where a range of V1s is available choose: Highest, Lowest, Balanced or fixed V1/V2 ratio.

The end user on the line doesn't normally see the choices but the type of V1 chosen should in my view be printed on the IRT for information.

[BraceBrace]

The examiner talks about the engine failure case which is the reason why V1 was designed anyway. Fairly obvious A is correct I would say.

The question is clearly with a fixed V2. V2 and V1 have regulatory no direct link as far as I know (and I presume this is a question for an official exam). V2 is limited by rotation speed, minimum control speed (air) and stall speed. If a company uses fixed V1/V2 relationships than that is their choice as long as it makes sure the minimum climb requirements, etc, are fullfilled.

Improved climb uses available excess field to increase Vr, hence V2, and hence creating better climb gradients. Different story that has initially nothing to do with V1 changes (although V1 is likely to change because Vr and V2 change). The note about the field limit might be confusing, but it's only there to show V1 can be increased, field length is not limiting your stopping distance because of a higher V1.