V1 question.
 

I always thought V1 was calculated as the last point in the take off roll that the take off could be safely abandoned and the aeroplane brought to a stop on the runway. Therefore, a problem occuring after V1, unless catastrophic, would be taken into the air and dealt with once airborn. It follows (I'd have thought) that the V1 calculation must take into account available runway length (is there enough length left to stop in?).

Talking to a retired 747 / 777 BA pilot the other day it transpired this is not the case. He though this was daft, and it does seem so.

If you are taking off from a dry salt lake bed with 20 miles of 'runway' available, surely you've never reach V1 as you could abandon the take off and safely stop right up to the point of rotation and lift off.

So, what's V1 really about, then?

So, what's V1 really about, then? In regard to the question, it’s about safety.
Runway length is a practicable limit and must be considered; we don’t have the luxury of infinite runway length.
Safety is about risk and managing risk. Stopping at speeds above V1 involves proportionately greater risk, and limits such as tyre speed and brake energy have to be considered irrespective of runway length.
Talk to your ‘Captain’ about brake temperatures, likelyhood of fire, tyre burst; then what risk is there in infinite operations – perhaps infinite risk unless it is bounded; V1 is just one boundary.

It is. Or at least, it should be:rolleyes:

Sometimes it's more length to stop, V1 is not always limited to runway length, e.g. it can't be greater than VR. No margin = balanced field.

Some performance charts or performance calc computers tell you the margin which is left stopping at V1.

It is what you thought it was, have you not ever had V1 and VR just about the same speed? Glad this captain is retired.

That is only half the definition.



If those two calculations end up with the same number, you have a balanced field.

Indeed, but this wasn't the question.

Similar subject....

http://www.pprune.org/rumours-news/4...eport-out.html

Never. I don't use V speeds in the lovely Chipmunk, hence my question. :O

Other issues such as maximum tyre speed, rotate and V2 also impose their own limitations on V1 and don't forget that this is just a decision speed, not a maximum speed. So on most take-offs an abandonment would result in you stopping well before the runway end. And if you want to continue the 'infinite runway' argument, you'll find that an aircraft will also have to meet a minimum climb gradient when airborne so before dispatch, the thing has to be able to fly at some time... then the limits above start becoming relevant.

PM

Shaggy, i like your way of thinking. I like to use extreme cases to validate hipotheses.

In the infinite runway case, you still have limits: tyre speed, for instance. Not maximum brake energy speed, because all you would have to do is retard the throttles and smoke a cigar while you slows down.

In not so big airplanes such as B737, A320, etc... optimum V1 is just the same as VR, meaning that you have more runway than you need to stop the airplane (basically you have "infinite runway length" in case of stop). In these airplanes you will hear something like "V1, Rotate" in most of the take offs.

In the big birds, however, you will hear something like "V1", ".......", then "Rotate".

It is in the big birds where V1 is truly meaningful. At the V1 call they remove the hands from the levers. From then on they are going to fly whatever happens.

V1 also means that the airplane will be able to lift off, maintain v2 and then reach 35 ft by the end of the take off distance available. At V1 you can either stop or go safely. Depending on the circumstances, most of the times you can choose a V1 within a speed band. The trick is choosing the optimum one (the one giving max take off weight or max assumed temperature).

Then you have the climb and obstacle clearance consideration, which can have effects in the determination of the optimum V1, too.

OK, so V1 can never be greater than rotate speed (after that, it's irrelelvant as you'll be flying). But at rotate speed, given sufficient runway length remaining, surely the power could be pulled off, reverse selected, and the brakes used normally (not agressively) to bring the aircraft to a gentle stop.

Most runways in the real world are not long enough to allow that, hence my assumption that V1 takes into account available runway length. Does it?

In the 'infinate runway' scenario, wouldn't V1 always be rotate speed, and therefore not relevant as a descision speed?

Yep, that's right. Not relevant, we can say.

we posted simultaneously

When V1 and VR are the same, runway lenght is not limiting ("infinite") for the stop case. And we are using the maximum possible V1, which is the best for the Go case (shorter take off distance). So performance is very good, runwaywise. But you can still have marginal performace for climb and obstacle clearance, though.

refer to my other post, if you like
regarding the rejecting take off after VR... Never an option. In the infinite runway it would be a quick take off and landing. Once you rotate, you never ever reject take off.

Yes :ok:

Thanks guys, that makes sense.

As a point of interest, why is a T/O never rejected after rotate but before lift-off?

And remember that Vr is not a fixed speed either, it can be moved to enable a better climb performance after take off (obstacle limit). With all the performance tools available you can use a 4000m runway completely even with a lightweight 737 (our ATC brethren really hate it when we do that).

When you calculate V1 you are using an unreliable fudge to find the point on a balanced field runway from which you may stop or accelerate to Vr before the end.
I say 'fudge' because, ideally, you'd have a mark on the runway as I seem to recollect was used by the V Force.
The use of IAS doesn't take into account an unnoticed power shortfall or a dragging brake which will place your V1 point too far down the runway.

Like Vmcg and Vmca there are woolly areas but they are the best we have at present.

A slight drift but have been in an aircraft (light and poorly loaded) that "auto rotated" before VR. The thrills of flying ACMI freight!

When V1 and VR are the same, runway lenght is not limiting ("infinite") for the stop case

Nit picking, I guess, but it might be better to say that the runway length may not be limiting. V1=VR certainly doesn't preclude the takeoff's being ASD limited.

why is a T/O never rejected after rotate but before lift-off?

Procedural protocol. In addition, operational technique may become critical from the viewpoint of repeatability for distances.

And remember that Vr is not a fixed speed either, it can be moved to enable a better climb performance after take off

Not really the case. VR generally is predicated on V2 and provides a suitable delta V such that, for the AFM rotation technique, VR will produce V2 - OEI - as the aircraft passes screen height. V2 may well be increased in some situations to take advantage of the improved climb gradient capability associated with an overspeed V2 takeoff.

When you calculate V1 you are using an unreliable fudge to find the point on a balanced field runway from which you may stop or accelerate to Vr before the end.

Not at all unreliable. Considerable effort goes into the flight test program and, for the conditions inherent in V1's establishment, the distance figures are sensibly repeatable. V1 does not necessarily presume BFL and can, for those aircraft whose AFM permits, apply for an unbalanced field length scenario.

I trust, for BFL, that you mean achieve OEI/AEO screen before the end rather than VR ? The latter might be quite disconcerting in many situations.

Well, at least the 737 performance software from boeing tries to optimize every take off, to do that it usually does not use the lowest V2/Vr combination but rather slightly higher to much higher speeds. Over 95% of all take offs have an increased V2/Vr combo. Difference between Vr and V2 is usually only 2 to 3 kts anyway which is not enough even for the OEI case to just reach V2 at screen height, therefore the recommendet technique is to fly the actual speed of V2 to V2 + 20 and usually it is at the higher end of the airspeed band.

So in all practical application V2 and Vr are not fixed values, they can be changed considerably to provide best possible performance or highest possible thrust reduction.

Several decades ago, a BAC-111 (Dan?) captain rejected T/O at LGW above Vr - the a/c had been misloaded and the nosewheel stayed firmly on the runway when he pulled back - a/c stopped inside the available length no problem.

A few years later an Iberia DC-10 captain rejected T/O at Malaga above V1 with a burst nosewheel tyre and mucho vibration - went off the end resulting in a burnt-out hull and significant loss of life.

haha..I agree wz you. Glad he's retired!
how many commercial airliners takes-off from SLC?.ha!

too fast n too much inertia n too little tarmac left at V1.
You are in the high speed regime, above 100knots .if u read annex 6 and FAR25, you will get all the answers.read up on " Balanced Field Length".
I ever had a RTO at 87knots and it was anything but gentle.
In performance, we are not actually learning what the aircraft can do but rather what it CANNOT do. that's called Limitations and that's why we have to calculate Field Length limit, climb limit, obstacle clb limit..fwd CG, tailwind limits, bla..bla..bla..
Nothing is perfect, machines or humans. safety is numero uno!
What they guys say here are correct but shouldn't u have learnt and passed this subject in Perf A before flt? Don't get me wrong , im just curious.
Cheers n happy flight!

"Thanks guys, that makes sense.

As a point of interest, why is a T/O never rejected after rotate but before lift-off?"

to do that it usually does not use the lowest V2/Vr combination but rather slightly higher to much higher speeds

V2 overspeed (improved performance in AB lingo) takeoffs will improve weight compared to min V2 schedules until whatever limitation occurs to preclude further V2 increase.

Difference between Vr and V2 is usually only 2 to 3 kts

Which is why we don't normally concern ourselves with the V2/VR ratio - rather V1/VR

not enough even for the OEI case to just reach V2 at screen height

Can't speak to the NG but, for the Classic, rotating at VR, OEI, will approximate V2 at screen. AEO, of course, screen speed will be substantially greater than V2.

So in all practical application V2 and Vr are not fixed values

Absolutely correct save for the following caveat. VR is constrained by V2. At very low weights, V2 WILL be constant with changing weight (Vmc limited) until the weight increases to the point where V2 becomes stall limited. Overspeed schedules are used to exploit the improved climb performance at speeds modestly in excess of V2 min.

I recall one of Emerald's 748s chopped the power and landed
on the remaining runway very safely after he'd copped a fire
warning just after lift off.

There was a lot of truths, half-truths and total bull**** on the
relevant threads at that time. I for one learned a lot from the
timely actions of that bloke.

landed on the remaining runway very safely after he'd copped a fire
warning just after lift off.

Given enough runway, of course that is an option.

However,

(a) the captain presumably had some explaining to do - I vaguely recall the incident but not the investigation.

(b) had the outcome been a mishap, the consequence might have been far more difficult for the captain.

While acknowledging that SOPs don't cover all situations and captain's prerogative, routine risk management generally dictates SOP compliance as being the reasonable way to go.

IIRC that 748 had a fire in the engine or wing at or very soon after lift off. The captain judged that the fire was so severe that the structure would not stay intact for even a tight circuit, so he chopped the power and landed ahead. I think they all walked away.

Report No: 3/2001. Report on the accident to HS748 Series 2B, G-OJEM, at Stansted Airport on 30 March 1998 - HS748 Series 2B, G-OJEM is presumably the incident being referred to. (?)

JT,
I think I gave good reasons for the term 'fudge' but, as I said, it's the best we have.
Recollect once querying why one twin jet we flew had a lower Vmcg (or was it Vmca? - forgotten now) on the version with the more powerful engines. Came the answer: "It's what's demonstrated by the test pilot on the day."

I am very happy that everybody escaped with their lives after this crash (this wasn't an accident). But...

And let me be very clear - that is called luck! To assess that the wing would have burnt off (because of a bang, a few screams in the cabin and possibility that the No. 1 told you that there was an engine fire) and that from 60-100 ft above the runway at 11:30pm that there would be enough room to land is something only worthy of a someone who believes in Intelligent Design and Creationism.

I have also learnt a great deal from that bloke as well.

PM

From the report, it was hardly "very safely" - they overran the remaining runway at 62kts according to AAIB, which is non-trivial overrun speed. Depending on who you ask, 70-80 knots represents the speed at which an overrun should be considered potentially Catastrophic, which is the most severe category for a safety assessment.

Though the AAIB don't, in fact, challenge the pilot's decision.

Wow..surprised the PPRUNE server isn't melting down at the acknowledgement that all planes don't magically fly after V1.

I ponder how many people JT has banned who simply made the comment that a captain with 5000 feet of runway post V1, can stop his aircraft safely...

Hell's freezing over....

"It's what's demonstrated by the test pilot on the day."

That MAY be true but are you sure that the engine upgrade didn't involve anciliary mods which might have had some relevance to Vmc considerations ?

The captain judged that the fire was so severe that the structure would not stay intact for even a tight circuit

Command prerogative is there for such decisions. However, no-one in the circumstances could make a rational call on that so the decision can only be seen to be a knee-jerk reaction to the situation. All of us who have had time on Darts would have a heightened concern about uncontained engine fires so the captain's decision is not entirely strange.

Overall, continuing for a circuit has the better outcome numbers on risk history. Having said that, if I were faced with similar circumstances and, say, 10-14,000 ft of runway, I'd be doing the same thing. However, the report suggest he landed with not very much seal in front of him and proceeded to depart the runway head at VERY significant speed for a Draggie. Probably not a good call to try and justify to the Chief Pilot over tea and bikkies - actually, probably without the tea and bikkies ?

it was hardly "very safely"

That's an understatement. Thanks for the link - I had read this report long ago and had the same thoughts then.

all planes don't magically fly after V1

Of course they don't.

Certification Standards and SOPs address reasonably expected circumstances. If the situation is dreadfully out of left field, then all bets are off and the captain earns his year's salary in the next few seconds or minutes ....

The very great majority of situations, however, will have a better likelihood of a satisfactory outcome by following sensible SOPs. No guarantees at all .. but the historical numbers are on the side of SOPs.

I ponder how many people JT has banned who simply made the comment that a captain with 5000 feet of runway post V1, can stop his aircraft safely

Actually, I have no idea how many folk I have chastised - probably not more than I could count on the fingers of one hand and certainly none for the sort of reason that you cite.

Caveat - a couple of troublesome folk who have gone through an interminable succession of re-incarnations have had recurrent slaps on the wrist applied. On that point, should I hold some interest as to why your comment is vaguely familiar in its style ?

Those who wish to form their own conclusions and/or learn from the HS 748 accident should readthe full report.
The aircraft suffered an uncontained engine failure and subsequent fire; it was at night and there was a bright glow visible in the cabin.
The AAIB report handles the HF aspects very sensibly, with consideration of the likely surprise (shock) of the situation (my supposition), which probably contributed to the decision to land, as it may have done in the failure to follow all of the shutdown procedures – failure to close the LP fuel cock.

Errors stem from the influence of factors in the situation. At times, these factors exceed the human capabilities of analysis and rational decision making, which with the pressure of time and personal threat can result in non standard actions.
These issues are often found in accidents when attempting to stop after V1.

Gee whiz....you haven't rotated yet, you see a mile of runway left...is it really that hard to consider keeping the burning wreck on the ground rather then fly it up in the air?

At issue here isn't logic, but the fact that the airlines, and military have culled so hard for followers rather then thinkers that any concept no matter how logical, that hasn't been handed down from the chief pilot, must be dangerous.

Like lemmings off a cliff, I ponder how many pilots have pulled a broken aircraft into the air, only to fly it to the scene of the accident.

Whenrealityhurts

If "broken" airplanes were taken airborne and crashed, when they could have stopped, I don't think the SOPs surrounding V1 would be THE industry standard. The record is quite clear--the statistics say taking it airborne is safer. Yes, statistics and safety is about probabilities, not absolutes. On almost 15,000 feet of runway at JFK, a TWA L-1011 aborted AIRBORNE after a false stall warning, everyone got out of the fire following a very hard landing and severe damage. Was that the right decision?

GF

PS: At least in the USAF I graduated from, pilots were anything but followers. My acquaintances in the RAF, RAAF and RCAF would be shocked at such an accusation. An AF doesn't have any combat capability with a flight crew force of followers. I include the enlisted members in that assertion.

For those who haven't looked at the report, the very first page of the "Appendicies" pdf contains a diagram of the runway, with points of interest marked. Also shown is the distance from the start of RWY 23, which was approx 3000m long. The following are estimated, but good to at least 100m, maybe 50m.

• rotation : 1150m
• Bang : 1250m
• Engine debris : 1300m to 1800m
• HP Disc fragments : 1550m
• Touchdown : 2600m
• Aircraft Stop Point : 3200m

So the actual event happened with 1750m of "runway left". Deciding to get back down, and then doing so, took more than 3/4 of that distance. Leaving just over 1000ft in which to stop. Which should give pause for thought for anyone thinking of putting a damaged plane back down again. Especially if they start any further down the runway than this event...

At issue here isn't logic ..

On the contrary I suggest that that is the pivotal issue.

Like lemmings off a cliff, I ponder how many pilots have pulled a broken aircraft into the air, only to fly it to the scene of the accident

Somewhat fewer, I suggest, than those who have rejected post-V1 and come to grief. There are no guarantees, only probabilities. Sensible folk stake their lives and superannuation on the numbers game. Only in the rarest of situations is the spur of the moment call going to win the day. Of course it happens - sometimes - but the odd successful exception doesn't invalidate the general rule.

I think that GF and MFS have summarised the situation succinctly ?

GF, you may not have picked up on the new persona we are blessed with - our old friend. On the other hand I may be telling you how to suck eggs. :E JT, to his credit, has him sussed.

Love it.

Thanks MFS for that report link. I knew eventually someone
would have it.

Particularly interesting on the report is..... 3. Conclusions -
(a) Findings - paragraphs 4 and 5 (p83/87).

I don't think anyone is familiar with just how bloody dangerous a
wing mounted Dart fire can be. That bloke certainly did, and faced
the enquiry very much alive, as did his crew and entire pax.

Anyway it was all said here before, 13 odd years ago.

JT,
Don't know. Did cross my mind that the rudder was larger but never checked.