V1 - Vr split on long runways

[Robbie-Rocket-Pants]

Hello Chaps,
here is a question which I know may have been asked many times before, but after searching I couldn't find a specific answer.

I fly Boeings and my company uses a Computer Take-Off Performance Program (CTOP) to calculate V-speeds.

Q. When operating from a VERY long wet runway without obstacle hazards (eg MLA), why would CTOP return speeds with a split between V1 and Vr ?

i.e. there is absolutely no way the aircraft would run off the end of an 11000' runway if a reject was made between V1 and Vr.

Thanks.

[rudderrudderrat]

Hi Robbie-Rocket-Pants,

For every take off there will be a spread of V1s.
V1 GO will allow you to suffer an engine failure and still accelerate to VR.
V1 STOP will be the last point you can safely stop.
Practically V1 GO can't be lower than VMCG, and V1 STOP can't be bigger than VR.

Your CTOP has probably selected a V1 midway between V1 Go and V1 Stop.

[Robbie-Rocket-Pants]

Thanks rudderrudderrat.
I still don't understand why CTOP doesn't set V1=Vr in such conditions. For example I would much prefer to reject if I had a fire rather than be "forced" into taking it into the air.

[hvogt]

I'm a newbie, and I might well be wrong, but here's how I would answer your question, Robbie-Rocket-Pants:

V1 for the wet runway will be slower than V1 for a dry runway. This is because
a) the screen height can be reduced to 15 ft which reduces TODR and
b) reverse thrust may be taken into account for the stop case which in many cases will decrease ASDR.
On the other side, VR is still limited by the requirement VR >= VMCA. In other words, V1 decreases while VR cannot decrease.

[rudderrudderrat]

I still don't understand why CTOP doesn't set V1=Vr in such conditions.Our software allows you to look at the V1 min (go) and V1 max (stop). It's acceptable to choose any V1 from within the range, but it is recommended to choose the one in the middle.

[mustafagander]

On heavy 4 engine jets a split of 15 or so knots is the norm for long haul ops. In fact my mob tend to get mixed up on the very rare occasions when V1 = Vr, often we call "V1" and miss "Vr".

At max TOW, say 412 tons, dry, sea level, no obstacles the figures are around V1 154 Kt, Vr 170 Kt, V2 182Kt.

It figures that the V1 is closer to Vr on twins because they lose 50% thrust where quads lose 25% - one hell of a difference when you are trying to accelerate 200+ tons.

As an interesting aside, we were able to lift more out of NZWN, Wellington, by using RTG 1, a fixed derate, which lowered Vmcg which was the driver on this very short runway.

[Microburst2002]

RRR

that seems reasonable. V1 GO could be dangerously close to VMCG and V1 STOP is too much, by the time you realise there is a problem you are too fast. A speed in the middle is a good compromise.

Besides, I prefer to go airborne with an engine in flames than rejecting in marginal conditions.

Maybe the could refine the software and give V1 STOP when you have a large excess of runway where to reject the take off nicely

[fireflybob]

If you have a range of V1s (assuming you are provided with this information by the Company via performance information etc) then it makes sense to pick the V1 which is most appropriate.

For example if you are departing a runway which has a vertical drop at the end of the runway (eg Luton, Funchal) select the minimum V1 since the last thing you want to do is go off the end in the event of a reject close to (maximum) V1. Also in the event of a continued engine failure once you have passed the end of the runway you'll have more than the screen height.

I you are taking off from a relatively long runway and are well below the field length limit but there is an engine out escape procedure (aka emergency turn) it might make sense to select the maximum V1 since stopping isn't a problem but do you want to be clambering round an emergency turn on one engine when obstacle limited?

Just selecting the "middle" V1 is not taking advantage - think about the situation and what makes more sense.


For every take off there will be a spread of V1s.

Not on every take off - on some there will only be one V1.

[fireflybob]

I still don't understand why CTOP doesn't set V1=Vr in such conditions. For example I would much prefer to reject if I had a fire rather than be "forced" into taking it into the air.

Really? Not me! I'd rather not have to reject at high speed and history shows that taking an engine fire warning into the air is far less risky that coping with it on the ground!

[rudderrudderrat]

Not on every take off - on some there will only be one V1.
That's called a balanced field length where V1 go = V1 stop.

By using Flex Thrust method, you can alter the performance, for the given runway length, terrain etc. so you end up with V1 go = V1 stop.

There will be a maximum temperature allowed for flex calculations, so if you reach max flex on a very long runway with no obstacles, you will generate a range of V1 go & V1 stop.

Our performance software allowed us to look at the V1 range with different flex figures.

I agree with fireflybob. Sometimes it's better to take the problem into the air and sort it out, sometimes it's better to stop - but you won't know which one was better until you've had tea and biscuits. V1 in the middle seems a very good compromise to me.

[Robbie-Rocket-Pants]

Thanks for your replies chaps.

FireflyBob, I understand what your saying about limiting runways and that in such cases getting airborne would very often be the safest course of action.

My orginal question was only considering a very long runway with no obstacles. If it's totally possible to safely stop all the way up to Vr, I still don't get why I should be forced to take an aeroplane which may be filling with smoke into the air if I don't really need to.

[aerobat77]

depending on the type of aircraft, the actual take off mass as well the flap setting for t.o you may be brake energy limited regarding the split between v1 and vr .

[rudderrudderrat]

Hi Robbie-Rocket-Pants,
I still don't get why I should be forced to take an aeroplane which may be filling with smoke into the air if I don't really need to.
Having tried to explain the logic behind the maths and the CTOP presentation, if you are still worried about having a smoke filled flight deck which only became apparent after V1 but before VR - then I think you may be in the wrong profession.

[Robbie-Rocket-Pants]

rudderrudderrat, I know I am being pedantic about it but my (very) hypothetical situation may be V1 145kts, Cargo Fire Warning 146 kts, VR 155kts, smoke in flight deck at 1000' and wishing I was on the ground. [Having thoroughly enjoyed being a pilot for 25 years I'm happy to continue in the profession !;)]

[clearandcopy]

Aerobatt 77 has given the most relevant reply to you original question. As he said - Brake Energy Limits have to be considered for V1 max.

[KBPsen]

Q. When operating from a VERY long wet runway without obstacle hazards (eg MLA), why would CTOP return speeds with a split between V1 and Vr ?

A. It is likely that a fixed wet V1 reduction is applied regardless of runway length.

[fire wall]

Robbie, is the data reflecting an increased V2 climb for 2nd segment clb performance and hence increased Vr ? Agreed Vmbe may well be in play and contribute to split.
Used to see this on 767 out of DXB on a hot day /inversion using southrerly runway. This can be the only reason I can think of.

[Robbie-Rocket-Pants]

Vmbe is not a factor since I can increase the weight and get a higher V1. Also if the runway is dry V1=Vr.
I guess it must be the reason that KBPsen gave: "A. It is likely that a fixed wet V1 reduction is applied regardless of runway length."

[rudderrudderrat]

Hi Robbie ,
but my (very) hypothetical situation may be V1 145kts, Cargo Fire Warning 146 kts, VR 155kts, smoke in flight deck at 1000' and wishing I was on the ground.
Ah. That's easy to fix.

Since the chance of a Cargo Fire Warning is time dependent and not engine power dependent, then simply delay any take off by about 5 secs (or the same time it would take you to accelerate from V1 to VR.) Thus ensuring that the hypothetical Cargo Fire Warning would now occur below your mid value V1.

[Robbie-Rocket-Pants]

Good idea rudderrudderrat. I wonder what the delay code for that is.:)

[Phoebus]

If you are really interested in the reasons for V1/VR splits, maybe Boeing produce an equivalent to the Airbus "Getting to grips with
aircraft performance".

Some time back, I can remember using performance charts that displayed both V1min and V1max, but guidance from some quarter (I can't remember whether it was JAR or CAA) was to have SOP geared towards being 'GO Minded' and subsequent revisions to the paper performance only displayed V1min.

In the Airbus document "2.4.3.3. MTOW limited by three limitations" gives the reasons for having a range of V1. While there are some aspects of the required factors that are different for the Airbus fly-by-wire, the overall subject is just as relevant to other manufacturers/types.
Airbus Performance Optimization appendix (http://dl.dropbox.com/u/55933302/Performance_Optimization.pdf) read and enjoy!

[mutt]

Or you could just ask your company about the inputs used in the software, much easier solution than having us guess......

[Centaurus]

depending on the type of aircraft, the actual take off mass as well the flap setting for t.o you may be brake energy limited regarding the split between v1 and vr .

Tontouta airport, New Caledonia has a long runway (10,000 ft plus) with significant obstacles close in for take off runway 11. Many years ago the company I was with often operated to Tontouta. In the 737-200 with JT8D-17 engines we were obstacle limited for departure 11. To counter this our performance engineers designed a runway analysis for runway 29 (no obstacles in that direction) and with a 10 knot tail wind and a 23 knot spread between V1 and VR we could take off at max structural (53 tonnes). The V1 was low enough even with a 10 knot tail wind to make a relatively comfortable abort. On the other hand the runway was long enough to permit the long take off run on one engine to take up the 23 knots of V1/VR spread.

[Robbie-Rocket-Pants]

Interesting reading Phoebus.
Mutt, the algorithms that CTOP uses come from Boeing.

[mutt]

True, but the input parameters come from the airline.

[aerobat77]

hm... i would like to highlight a scenario that this software is in real something like a freeware app or a program for a pc game and the only answer is that is has a program error while several people here perform a brain storming.

in this case you will get no answer- in the case your question is for real i would suggest to contact your technical department instead an anonymous forum which should help you.

when you claim brake energy is not a limiting factor i personally have no idea what is going on.

best regards

[bubbers44]

One of our normal runways at MIA was 13,000 ft long taking off to the west. Our airline required us at V1 to continue the takeoff. A friend of mine took off and had a thrust reverser deploy at V1 taking off to the east in a 727. He managed to get it back and land but aborting was the safest procedure. He followed company policy and returned for landing. All was sucessful but sometimes staying on the ground is the best answer. Remember the DC10 at Chicago crash? Sometimes company procedures will kill you.

[rudderrudderrat]

Hi bubbers44

Remember the DC10 at Chicago crash? Sometimes company procedures will kill you.
How often do you have V1 above VR?

Report of DC-10-10 AA Accident near O'Hare (http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/ComAndRep/OHare/NTSB/COPY/ohare-full.html)
"During the takeoff rotation, the left engine and pylon assembly [see fig. 16.1] and about 3 feet of the leading edge of the left wing separated from the aircraft"

I remember it very well. All was going OK at their initial climb out speed. The crew increased the rate of climb and reduced the speed towards V2. The left wing then stalled.

ASN Aircraft accident McDonnell Douglas DC-10-30CF EC-DEG Mlaga Airport (AGP) (http://aviation-safety.net/database/record.php?id=19820913-0)
"The airplane continued to accelerate through VR. As the captain tried to rotate by applying up elevator, the vibration was of such magnitude that he feared that the plane might become uncontrollable after takeoff. He decided to abort the takeoff. At that point, with a maximum speed attained of 184 kts, there was 1295 m (4,250 feet) of runway left. The captain retarded the throttles and tried to select reverse thrust. The nr. 3 throttle slipped from his hands, causing a power asymmetry. The airplane veered slightly to the left. The Dc-10 overshot the runway at a speed of 110 kts, colliding with an ILS building, causing engine number 3 to separate. The airplane went through a fence and crossed a highway were it damaged three vehicles. It then collided with a farming construction, causing three quarters of the right wing to break off, as well as the right horizontal stabilizer. The aircraft stopped 450 m (1,475 feet) past the end of runway 14. A fire erupted in the rear of the fuselage."

All because of nose wheel shimmy!

[Pub User]

If you are really interested in the reasons for V1/VR splits, maybe Boeing produce an equivalent to the Airbus "Getting to grips with
aircraft performance".

Here it is:

"www.flightwork.com/library.html?eID=dam_frontend_push&docID=722"]

[ImbracableCrunk]

Here it is:

http://"www.flightwork.com/library.h...sh&docID=722"] (http://%22www.flightwork.com/library.html?eID=dam_frontend_push&docID=722%22])

Is that a 3- or 4- credit course? ;)

Thanks for the link.
"] ("http://%22www.flightwork.com/library.html?eID=dam_frontend_push&docID=722%22)

[Crossunder]

Like mutt said; why not just ask your chief pilot?

This is most likely due to a company policy to have the computer give you a V1 optimised for, say, slippery runways? They want their pilots to be "go minded".
There must be a reason why you don't get the whole V1 range presented; they do not want you to choose your own V1 based on runway length vs. runway condition. Maybe they don't trust their pilots to make a sound judgement?
My company wanted V1GO to be used for every take-off, because we operate on many short and sometimes slippery runways. They were afraid that some schmuck would some day select V1STOP and be "stop minded" instead of "go minded" when taking off on a short, slippery runway. This caused many pilots (idiots) to start increasing their V1 to a ramdom "suitable" value on 3.000m+ runways when the computer came up with a V1 of e.g. 93 kt and a Vr of 130. FOs started filing reports, and consequently, the company changed their policy: We now get a more optimised (higher) V1 when taking off from long (2.500m+) runways. However, we are still not given a V1 range, like some other companies I know of. As a captain, I'd like to be able to choose from a V1 range (baed on actual conditions and my own experience) rather than have some desk jockey decide what's the "on average best" V1 for me for any given take-off...