V1

[twistedenginestarter]

I was about to answer a question on a different forum about aborting a take-off due to a passenger event. I realise I can't remember what the V speeds are for.
Looking on the Internet gives a mixture answers relating to two separate constraints. The first is the speed that allows you to take-off with a single engine failure. Below this you must stop. The second constraint is the speed which is highest you can stop within the length of the runway. When these are the same, you have the maximum weight for the runway in question, but in general they won't be unless you de-rate. So what are the two names for the two different speeds? When you call V1 what are you saying - you can continue or you must?

[TckVs]

A very basic answer.
The two V1's normaly called low/high speed V1. The below the low speed V1 you must stop, so the high speed V1 you can just stop......
Normaly the company decides if its a go/stop airline, either it uses hi or low or even mid way. But I think most airlines use a lower V1 and would be a GO company.

Now ducking from all the flack thats coming my way.

[Tinstaafl]

Like you said, V1 is selected from within range of speeds. Upper end = maximum speed where stopping is still possible & the lower end = minimum speed where a continued take-off is still possible. WAT, TODA & SDA affect those limits, reducing the gap to zero as they converge to a balanced field length.

Continuing the take-off (from a lower V1) is mutually exclusive to stopping (from the higher V1). Once you continue through Vr you're no longer in a situation covered by performance data for stopping - so a higher V1(stop) can't apply. Once you start stopping then you're no longer in a situation covered by performance data for continuing the take-off - so a lower V1(go) can't apply either. End result: a single V1.

No doubt it would be possible to have a range of V1s for each take-off, each for various criteria, but the continue & stopping performance can be so
critical that taking time to decide which applies & then acting would lead to an unacceptable risk. A significant point about using a single V1 is that it simplifies decisions at a critical time ie above you go, below you stop.

[TheWanderer]

V1 is called the decision speed, at which the decision to abort or to continue a takeoff must have been made.
Vmcg is the lower limit for V1 and the lower of
a) VR or
b) VMBE or
c) the speed at which the accelerate stop distance equals runway lengh available (+stopway)
the upper limit for V1.

However, V1, is critical for the length of the takeoff distance and the length of the accelerated stop distance.
A lower V1 results in a longer takeoff distance (longer acceleration with engine out) but shorter accelerate stop distance (braking starts earlier).
A higher V1 has opposite effects, takeoff distance becomes shorter and accelerate stop distance becomes longer.

In case the choosen V1 is such that takeoff distance equals the accelerate stop distance, it is called a balanced V1 or balanced field length.

Balanced field usually results in the highest takeoff weight achievable, unless a clearway or stopway is available to further increase the takeoff weight.

So generally, for an actual flight the V1 is choosen from an available range of V1 between minimum and maximum V1. During takeoff, there is only one V1 used, the choosen or calculated V1 according to the requirements for the given takeoff.

On a wet runway a lower V1 is generally more desirable as poor braking conditions may exist and increasing the available stopping distance will increase the safety margin.

[muduckace]

As stated above simply it is the safe speed calculated to abort a takeoff. This is at the pilot's discretion, engine failure no doubt, each aircraft has different operating limitations.

A passenger event would be an unlikely one allthough possible as it is a relitively shory piriod of time between commencing T/O and V1. Just about all aborted T/O's are due to system failures.

[Flaperon777]

So with the above in mind,what would be the V1 for my single engine Cessna 172.....:confused:

[john_tullamarine]

So with the above in mind,what would be the V1 for my single engine Cessna 172

V1 is not defined for and, from a certification point of view, irrelevant to light aircraft operations.

The generally accepted definition these days is along the lines of "if you have not already commenced stopping by V1, keep going". This assumes, of course, that the aircraft is within the accepted certification parameters and capable of flight.

[LeadSled]

V1 is called the decision speed, at which the decision to abort or to continue a takeoff must have been made. Folks,
This is a very common misconception, see a very useful little book:

http://www.faa.gov/other_visit/aviation_industry/airline_operators/training/media/takeoff_safety.pdf

In particular, page 2.10. V1 is the speed at which the takeoff should be continued if the abort has not already been commenced.(unless is a catastrophic failure and the aeroplane will not become safely airborne -- cf; PANAM on 34 (L) in Sydney, multiple engine failure at gross weight)

The effect of this has been incorporated, with a number of useful amendments, from the original SFAR 422B through to the present amendment level of FAR 25. The words in italics appear elsewhere in FAA Docs. Not withstanding all of the above, in many other FAA Docs., they persist in calling V1 the "takeoff decision speed" ---- confusing, unless you accept it for what it is, the decision to take off ---- what it is NOT, is the stop/go decision speed.

Despite the best efforts of airline training departments, and I can speak of three on this subject, United, Delta and Qantas, V1 being referred to as the stop/go decision speed persists, including being incorrectly defined in several well known training texts, and many training school course notes.

Tootle pip!!

[TyroPicard]

From a recent EASA amendment to something..... they seem to agree with the FAA..
"the speed V1 (at which the pilot is either continuing the take-off or is initiating the first action to abort the take-off)".
That's the important bit - initiate the first stopping action by V1.

Or in simpler terms... Happiness is V1 at Lagos ....

[TheWanderer]

Boeing has a fantastic training video for this, the video is called "The Go - No Go decision" as far as I remember.
Unfortunately I don't know where the video is available except from Boeing.
But maybe you can ask your training department if you like to see this video?

[LeadSled]

Folks,
As a quick PS to this thread, it is instructive to look at the record of high speed aborts, caused by engine failures --- the conclusion I draw is, don't get too worked up about runway remaining at V1, in the event of an engine failure, you are going off the side due to loss of of directional control.

Getting the thrust levers back on the stops is, based on the statistics, even more important than getting on the brakes, to achieve a successful rejected takeoff. An abort brief: "Thrust off, brakes on" --- in that order ---- separated by milliseconds --- any more than that, either psychologically or in practice, and the statistics say you are a candidate for off the side.

As for the arguments about "autobrake" versus good old feet ----- that's a whole area of inadequate consideration, but I know my decision in advance, I am not going to wait to see if the autobrakes work as advertised.

Tootle pip!!

PS: V1 at Lagos ---- couldn't agree more, having several times slept on the floor of a freighter, because it was the best accommodation available at the time. V1 at Luanda is much the same.

[A37575]

but I know my decision in advance, I am not going to wait to see if the autobrakes work as advertised

By that convoluted logic I guess you don't use autothrottle in case it doesn't work - you don't use a flight director in case it gives you wrong directions.
But I bet you use the autopilot even though it may not work.

I don't know about other aircraft but the autobrake system in the 737 using RTO position will give you maximum braking. On countless occasions during simulator training we have seen pilots over-riding the RTO (either deliberately or inadvertently) even though it is operating correctly. In nearly all cases the stopping distance using manual braking was significantly greater.

This was especially on an abort due to engine failure where the combination of yaw prevention as reverse thrust is actuated and the slightest delay in correcting the initial asymmetric yaw on the runway when an engine fails, does not affect the RTO braking which is equal on both sets of brakes. Try that with manual braking and it is obvious from the instructor's panel read-out of individual pedal pressures that pilots often have varying brake pedal deflections when coping with an engine failure and abort - especially if a crosswind is present.

Presumably if you do not trust the RTO system you ensure it is selected off before take off? If so, how does that accord with your company SOP's.

[boeingdream787]

Very good question,Flaperon777.
Although I seriously doubt that it was intended to be a question.Rather some issue you'd like to have discussed!
As you pointed out,V1 is a very unique function of a multi engined aeroplane ONLY.That is, an aeroplane (jet or otherwise),with MORE than one engine.Imagine trying to work out the V1 of a single engine jet.Hmmmm....!! No engine no go...simple as that.
So as far as the V1 on your C-172 is concerned,I got a good feeling that you wont be going far without that prop turning bro......:ok:
Cheers now....
I have a feeling i'm going to start a whole new thread here.Whether or not a single engine jet can/should have a V1...??
Sorry for this deviation,if at all...:ooh:

[LeadSled]

A375757575,

I guess that the B737 must be the odd one out in the Boeing family, in fact Boeing (in the 767 and 744) state that max manual braking will actually produce as good or better (depending on how you read the English expression) as autobrakes.

As the actual braking depends entirely on the brake hydraulic pressure ported to each brake ( as may be modulated by the anti-skid) there is no engineering reason why max. manual braking will not produce exactly the same results as autobrake in a rejected takeoff, once the brakes are applied--- Check with Boeing.

It is more than likely that the sim. result you have seen is a characteristic if the sim, not the aeroplane -- sims are great devices for what they are good for, aircraft certification or operations at the margins of the envelope are not what they are good for, representational, but not precisely accurate. In a real abort, you better be precisely accurate.

Instead of making a string of rather silly and sarcastic remarks, have a close look at the time sequence to activate the autobrake, in a rejected takeoff, compared to applying max. manual braking as the thrust levers are being retarded. Do you have figures for MTBF for the autobrake system?

As to company SOPs, and it is made very clear that it is Captain's choice, they are all quite capable of making that choice. RTO is always armed.

The same goes for the rest of the equipment, unless it's down to low weather minima procedures, and, as most of us understand, that's where it gets very specific. Unlike other companies I could name, keeping thoroughly current on hand flying is also regarded as a good idea. In fact, being competent and current throughout all the combinations and permutations of operating is a requirement --- to be demonstrated during cyclic training.

It sound to me like you work for an "automatic" company, I'm sure you understand what I mean. More than likely, that's why some of your pilots produce the results you have observed on the instructor panel.

Have you ever been involved in a maximum effort abort --- I have, twice, both major engine breakups within a couple of knots of V1, around 150kt, with little in the way of surplus runway in each case ---- Let's just say it is a lot more "realistic" than the sim, or any sim I have been in, and that is quite a few --- the sim. visuals just don't get the big flash out past the front of the aircraft from a massive compressor surge --- both were at night, quite spectacular. All cause by compressor stages turned into something that looked like a well chewed cob of corn.

Tootle pip!!

[stilton]

Think you are a little confused here. RTO Autobrakes will always give you maximum available braking with minimal delay.


Maximum MANUAL braking is more than maximum selectable autobraking for landing.


RTO Autobrakes are absolutely invaluable and will nearly always outperform a human Pilot.

[Fratemate]

I'm with Stilton on this one. From the Boeing manual:

Rejected Takeoff
Selecting RTO prior to takeoff arms the autobrake system. The RTO mode can be
selected only on the ground. The RTO autobrake setting commands maximum
braking pressure if:
• the airplane is on the ground
• groundspeed is above 85 knots, and
• both thrust levers are retarded to idle
Maximum braking is obtained in this mode. If an RTO is initiated below 85 knots,
the RTO autobrake function does not operate.

Landing
Five levels of deceleration can be selected for landing. However, on dry runways,
the maximum autobrake deceleration rate in the landing mode is less than that
produced by full pedal braking

So, from this we can see that max manual braking does not exceed the braking efforts of RTO, only 'Max Auto' when landing. Sorry, LeadSled, but I reckon you should set RTO, let it do its job and concentrate on keeping straight etc, as opposed to seeing if your feet are quicker than Boeing's systems. Of course, max manual braking should be used if RTO is not doing its job (and, from my experience, you know full well when it's working :ok:)

[CFMFan]

Slight deviation guys but if you have a same side (presumably there would be a yaw as well as a bang) double tyre failure in a 737 at just over 80 knots (approx 50 knots below V1) on a 2,500 Metre runway what would be your decision?

(a) Contine trying to drag the aircraft into the air because you do not have your normal wheel braking.

(b) Assume the aircraft is unsafe to fly as you are not confident of accelerating to V1 before running out of runway.

As you would not be more than 500 Metres into the take off roll in still air I would favour (b). There are an increasing number of people in my company insisting we only stop for fire/ eng failure/ or predictive windshear (and don't like thinking about the variable unsafe to fly area).

[BenThere]

I've always been uncomfortable with V1 as the decison speed.

Old engines, old airplanes reach V1 speed as calculated at a point x feet beyond where the pristine engine and airplane might have reached that speed.

Also, when you have 14,000 feet of runway, for example, you might reach V1 at 10,000 feet remaining and plenty of time to stop, yet you're committed to go, even with an engine fire/failure.

USAF used to calculate Refusal Speed, the calculated speed at which you could no longer stop. While having its drawbacks, I think that is a better substitute for V1 under some circumstances.

The best tool to have on board is an old, gray haired pilot when the unexpected happens. He will have thought of all that.

[twistedenginestarter]

Incidentally - I understand the event was a passenger having a serious fit during the take-off run. The take-off was aborted. The passenger may have died - my information is unclear on the final outcome.

[john_tullamarine]

There are an increasing number of people in my company insisting we only stop for fire/ eng failure/ or predictive windshear (and don't like thinking about the variable unsafe to fly area).

The problem I see is the inference that we can set up the RTO planning in anticipation of a guaranteed outcome - ie black and white.

Not the case.

MOST times, if the T/O is rejected up to a margin below V1, the RTO will be successful. MOST times, if the T/O is continued beyond V1, the T/O will be successful.

The bit in the middle has a LOT of caveats attached to its successful outcome. For routine civil airline operations we don't have the time luxury to discuss and try to memorise a tome of information for this takeoff .. we work on the basis of reasonable historical probabilities skewed to the most likely means of achieving a successful outcome.

.. but that doesn't mean that the operation cannot come unstuck if you are near to the tail of the distribution curve on the day ...

[PEI_3721]

CFMFan.
A). Tyre failure – bang, yaw. You don’t know their origin; it’s only your perception – assumptions; as is the lack of braking. At the relatively slow speed it’s safer to stop.
B). How do you know that the aircraft is unflyable – you only attempt a rotate after V1 – at those speeds there isn’t a go/no-go decision, only mitigation of the inevitable accident / speed at which you go off the end.

Never assume.
Regulatory based procedures have evolved from hard earned experience, often from the deaths of other pilots, don’t think that you know better, particularly at that critical moment in time when things may ‘appear’ not to be as expected – check the situation, follow the procedure. There is time for both - practice.

BenThere, few pilots know the distance travelled to V1 – many variables. Similarly the distance (not time) remaining to stop. What are the conditions - dry, wet, tailwind; again many variables, which even a grey-haired oldie cannot compute in a reasonable timescale.
Flying with an engine failure / fire is quite safe enough for the conditions and subsequent procedures – certainly better than attempting to stop with so many unknowns.

[CFMFan]

John - Agreed.It is this black and white view that I object to....the monkey see monkey do brigade even brief predictive windshear as a V1 -1 knot stop on a nil wind/CAVOK/high pressure day!

PEI - We actually agree on the decision "at the relatively slow speed it is safer to stop" but I have not got my point across. In my company if you abandon at 90 knots for a tyre failure you will be asked why you deviated from SOPs.

What I was seeking was clarification on peoples thoughts as to WHY they favour a course of action.

You say at the relatively slow speed it is safer to stop ... if the reason is not "that you are not confident of accelerating to V1 before running out of runway"...then please advise why you would stop.

What I was saying here about the aircraft being unsafe to fly is that if you are trying to accelerate on one set of wheel rims you may well not reach VR speed.

The only other thing I can think of which would cause a ban and yaw at 90 knots would be an engine failure...which would be a stop item anyway. Again here I was just trying to get the experience of someone who has had a double tyre failure as to if these were there symptoms.

Can't agree with your never assume comment though. We get paid mega bucks to make a decision...sometimes when you are above 80 knots (and as John describes it) at a margin below V1 things other than engine fire/failure/PWS will happen. Sticking with the procedure may well result in a take off considerably less safe than the low speed abort.

[PantLoad]

It would be of interest to read Airbus' "Getting to Grips with Aircraft Performance'. In it is explained the evolution of the certification
standards for "V1". Interestingly, depending on when/how/under what FAR/JAR your aircraft was certified, the definition of V1 is different.

Things all started in March of 1978 with "Amendment 42". This FAA
certification amendment allowed the decision to be 2 seconds beyond
V1. The Airbus A-320 was the first and the last aircraft to be
certified under "Amendment 42".

Then, in March of 1992, the issue was revisited. As a result, now, we
have "Post Amendment 42". The two-second rule was re-adopted, but
was changed to the 'equivalent distance of two seconds'. There were
other certification changes, such as: the brakes can be worn to the
certification limits at the start of the takeoff roll (and you still must
be able to stop), dry/wet accelerate/stop certification definition and
requirements, etc. You can read it for yourself. This Airbus
publication is available on-line to anyone.

As per the Airbus FCOM, the criteria for rejecting or continuing the
takeoff are pretty explicit. Boeing uses 80 knots as a dividing line.
Airbus uses 100 knots. As per Airbus, below 100 knots, it is
'reasonable' to reject for just about any Master Caution. But,
above 100 knots, a rejected takeoff should be performed only for
a Master Warning or four Master Cautions (Sidestick, Engine Failure,
Reverser Fault, Reverser Unlocked), any fire warning or severe damage,
sudden loss of engine thrust, or an unambiguous indication that the aircraft cannot fly safely.

It should be noted that Airbus cautions against rejecting in this 'high speed regime' for reasons such as high EGT, wheel/tire vibration, etc.

Interestingly, Airbus is also explicit with regard to the issue of a
tire burst during takeoff. When within 20 knots of your V1, should
a tire burst occur, assuming there is no collateral damage to engines,
the takeoff should be continued. I used to brief, should this occur,
we'll continue the takeoff (sans serious engine damage), but we'll
keep the gear down after takeoff. The purpose of this is to preclude
jamming hot/burning/damaged gear up in the wheel wells. The
problems associated in retracting the gear in this instance are obvious.

Of course, as always, your company's SOP is the bible. However, any
issues/problems you see with the SOP can and should be brought to the
attention of your company's training department.


Fly safe,

PantLoad

[clivewatson]

What use is it to consider a safe abort at the computed speed if one has not achieved the desired acceleration?

Why, with the aid of GPS and other devises that are able to compute acceleration, are we still using a decision based simply on speed?

For as long as I can remember, Falcon have been training their pilots to reley on accelartion BEFORE even looking at a V1 speed.

Is this not the same in airlines?

[Pugilistic Animus]

Unfortunately, I think accleration charts complicate the matter greatly and they don't tend to be available at least on the 'front line'anyways as that matter tends to be absorbed within the engineering assumptions made while deriving those speed. some operators comply with certain regulations in different but approved ways such as the AoA indicating system on the Lear 20 series; Falcon or those who provide the OpSpecs have perhaps gotten alternate approval for your operation



Great thread! I'm learning a good deal:D

PA

[PantLoad]

Clivewatson,

You've made an excellent point. The accel readout on the Falcon is quite useful. Unfortunately, Airbus hasn't got one like that. Maybe, somewhere in the inner workings of the MCDU, you can pull that up. I've never seen it, though.

The USAF method is pretty reliable, and I use a modified version of that.
It's unapproved, not SOP, probably illegal, but it also works well. That is,
as you pass the three-thousand foot marks, your speed should be at least
100 KIAS. Typically, it's 120 KIAS or thereabouts. But, if the 100 KIAS
callout occurs near or after the 3000 foot marks, you have a problem. I've used this in the Boeings and the Airbus. It works.

Now I am ready to have rocks thrown at me for this post....:):):)


Fly safe,

PantLoad

[411A]

Now I am ready to have rocks thrown at me for this post

No rocks from this sector, however, we are often just coming up to 100 knots at 3000 feet in a very heavy weight TriStar...especially at Haj time.:}

[9.G]

great thread, I agree. The topic "aborting for reactive wind shear" was brought up here somewhere earlier, I believe. In my eyes, that's probably one of the kind and can cause great deal of trouble. I'd even regard to that as something like "toss the coin" decision. If one had a bit less tail wind, in case of rejected take off, no overrun would have happened though if there was a bit less tail wind, in case of continued take off, one could have got airborne safely. Not sure if there's a reliable panacea for this nature's trick except to delay the take off. The dilemma is caused by a difference between the actual speed of a/c and the indicated one, making void all the inputs used for take off calculation. The rest is pretty much clear and any kind of assumption isn't really appropriate. My 2 cents.
Cheers :ok:

[PEI_3721]

CFMFan, within our agreement, we appear to have similar lines of thought, but perhaps from differing conceptual bases.

RE: if the reason is not "that you are not confident of accelerating to V1 before running out of runway"
These are not my words or thoughts. Judgement of such an acceleration capability would require great experience and ideal conditions in situations with a vast range of variables.
It is often within an individuals’ quest for confidence that error arise. We have confidence from every day operation that normal take-off performance provides the required level of safety; why then do some people question abnormal performance at a critical stage of operation.

IMHO the addition of phrase “if the aircraft is unable to fly” in the Take Off Training Aid (FAA doc – Boeing addition – Airbus followed for consistency) creates many problems and has decreased safety. If ‘something’ occurs before V1, the ‘decision’ requires you to predict the future – will it fly.
The text introduces a new evaluation and decision loop for a subject (will the aircraft fly with this ‘unknown/assumed’ failure) where few pilots have training or experience (knowledge).
As stated previously, the prime indications of inability to fly occur after V1, at or after rotation, when the stick is pulled back; even then it might fly later at a higher speed. All of this has nothing to do with engine failure recognition, V1, or RTO training – other than the mistaken perceptions. See: Canadian view – After V1. (http://www.tc.gc.ca/civilaviation/systemsafety/newsletters/tp185/1-97/666.htm)

The example of a tyre / wheel failure reducing performance is based on the premise that the pilots knows that the tyre / wheel has failed – and that it will reduce performance by a hazardous amount.
I suggest that both of these are most unlikely; a bang and yaw are not unique features – a galley door + wind gust, an engine failure, etc. At low speed there may be opportunity for discrimination based on salience or pilot experience, but at high speed, past accidents identified human fallibility, particularly when stressed, often by the surprise of an event. For a performance example see this Accident Report. (http://www.bst.gc.ca/eng/rapports-reports/aviation/1994/a94w0026/a94w0026.asp#a4)

Many operators brief only to stop for problems other than engine failure and fire below a certain speed (80kts).
At lower speeds it appears to be assumed (operational regulations/training) that other non-engine failures can be evaluated / established and that the risks in stopping are low – and little is lost in being wrong.
At higher speeds the decision task should be simplified to identifying an engine failure – specifically lack of thrust, as that is a critical parameter for continued safe flight.
IF engine failure (lack of thrust), AND speed less than V1, THEN stop; caveat about having taken the first actions before V1 (certification regulations).
Thus training should concentrate on engine failure recognition (thrust parameter and confirming indications) and the human frailties involving startle, loud noises, vibration etc, false beliefs / perceptions, and assumptions.

Assumptions? – “there are no truths, only perceptions”. I agree that pilots in rare situations are required to make risky judgments – based on assumptions from their perceptions. However, these decisions should not be made without first verifying the basis of the assumption (the risks in assuming …).
Certification regulations balance risks to match the required level of safety, i.e. there is some margin, and thus operating procedures should guide pilots in most circumstances – engine failure +/- V1 is covered.
RTO guidance for non-engine related failures is weak, hence the FAA training aid, but when that also has weaknesses … … Perhaps this is the reason for operators using V1- 20kts or 80kts for non-engine related failures, but not to treat these situations as the same thing as an engine failure.

I agree that crews should follow the procedures, but it does not prevent them enquiring about the basis of procedures. I wonder what management might have assumed about RTOs with tyre / wheel failure; how do they expect crews to identify the fault (an assumption) – how are crew’s to differentiate this from an engine failure. Are you trained for that, is it written down?
Don’t let management pass the buck to the individual – seek clarification.

Also see:
PSM+ICR (http://www.flightsafety.org/fsd/fsd_nov-dec99.pdf). (http://www.flightsafety.org/fsd/fsd_nov-dec99.pdf)

Briefing Notes; (http://www.airbus.com/en/corporate/ethics/safety_lib/index.html) see ‘Revisiting the “Stop or Go” Decision’, under Takeoff and Departure Operations.