Is V1 called 'Decision Speed' or 'Action Initiation Speed'?

[hawtin]

I always thought V1 was called the 'Decision Speed' but according to an airline (Boeing) pilot, the correct name for V1 is 'Action Initiation Speed'.

Every book/online resource I have refered to says that V1= Decision Speed. I have never come across this 'Action Initiation Speed' term. Now I know that the action should have been initiated before V1 is reached but you can also say that the decision needs to be taken before V1 is reached.

Can somebody please tell me what the correct term for V1 is?

I'm asking because there was a question in a recent airline exam I had taken asking for the correct term for V1. I marked 'Decision Speed'.

[vilas]

hawtin
I have not herd the term action initiated speed but it is correct to say V1 is the maximum speed at which first action to reject take off is taken. You do not decide at V1. Decision speed is the older definition of V1.

[hawtin]

Thanks for replying..but then what is the correct term for V1 as in if you were asked, what would be your answer?

[vilas]

It is still termed as decision speed. Below is reproduced from Airbus document.


V1: Decision Speed
V1 is the maximum speed at which a rejected takeoff can be initiated, in the event of an

emergency. Additional information on this “Go/No-Go” decision can be found in the

Flight Operations Briefing Note entitled

[hawtin]

Thanks Vilas :ok:

[HazelNuts39]

From FAR Part 1, Sec.1.2, Abbreviations and symbols:

V 1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V 1 also means the minimum speed in the takeoff, following a failure of the critical engine at V EF , at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.

[hawtin]

:confused: So what IS the correct term for V1? Airbus says 'Decision Speed', Boeing Pilot says 'Action Initiation Speed'. FAA says something else. Transport Canada has another definition, Indian DGCA says..er..well..nevermind..:ugh:

[ExSp33db1rd]

I reckoned V1 was the "Go" or "No Go" speed, ie. at V1 one has enough runway left to safely take off with a power failure, or can safely brake in the distance remaining and not run off the end.

I'd made my "decision" before I reached V1 !!

But then I'm an Old Fart.

[HazelNuts39]

To my knowledge there is no universally accepted "correct term for V1". The Boeing term describes its role in the accelerate-stop, but in the continued takeoff there is no action at V1 (except for the PF taking his hand off the TL's).

Historically, V1 started out as the "critical engine failure speed". When the recognition time delay was accounted for it became the "engine failure recognition speed". Still later the regulation was amended to take account of takeoff rejection for reasons other than engine failure and V1 was termed "takeoff decision speed". That term was abandoned because the abort decision should not be taken at V1, and the definition in FAR Part 1 was adopted instead.

[ASRAAM]

It's also important to remember that these days for a modern airliner V1 is an almost arbitrary speed based around minimising engine wear.

Obviously it is a speed that can by stopped from and also a speed from which the take off may be continued, however it would frequently be possible to stop from a faster speed, or indeed continue from a slower speed.

Unfortunately the pilot will not usually have this information and will be unable to tell on a daily basis where in the stop/go bracket V1 has been located.

[PEI_3721]

‘V1’ depends on the context being considered; design, certification, operation.

Definition: V1 is defined as the takeoff decision speed (via ICAO)

Certification: V1, in terms of calibrated airspeed, is selected by the applicant (manufacturer); however, V1 may not be less than VEF plus the speed gained with the critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognises and reacts to the engine failure, as indicated by the pilot’s initiation of the first action (e.g. applying brakes, reducing thrust, deploying speed brakes) to stop the aeroplane during accelerate-stop tests. (EASA CS 25 certification – FAR similar)

Operations: V1 means the maximum speed in the take-off at which the pilot must take the first action (e.g. apply brakes, reduce thrust, deploy speed brakes) to stop the aeroplane within the accelerate-stop distance. V1 also means the minimum speed in the take-off, following a failure of the critical engine at VEF, at which the pilot can continue the take-off and achieve the required height above the take-off surface within the takeoff distance. (EASA operations)

Thus V1 is variable, it not uniquely VEF, it relates to decisions and action, and as a pilot it is very important to understand the operational implications of V1.

[Jazz]

I have a recollection that falls in line with the Boeing guy. V1 is the latest speed by which stopping actions must commence. I believe that in certification, a time of 1 second is allowed for the decision to stop. The actions must commence by V1 itself at the latest, and 2 further seconds from V1 speed are allowed for the stopping configuration (ie. applying brakes, selection reverse and spoilers as installed).

So, decision must be made 1 second before V1, which is where many operators state in OM-A that the "V1" call must be completed by V1. One second to decide, two seconds commencing at V1 to configure the stopping actions.

[bleeke]

Although in some cases it might be better and safer to stop the TO roll even after V1. The aircraft could become unflyable due to a trim runaway, blocked flight controls etc.

[PEI_3721]

Jazz, the quotes @#11 are from the relevant certification and operations requirements.
You might clarify your recollection and ‘false’ beliefs about certification by referring to CS 25, the info is around para 25.107 and associated AMC.

bleeke, you might benefit from reading the Takeoff Safety Training Aid and CS 25 as above.
As far as you should be concerned (certification assumptions about probability, exposure, effect, etc) the trim will not run away in the time scale being considered, nor would there be a control block providing you complete before takeoff control check correctly. Certification assumes that checks are done, but accidents indicate that the human is still the weakest link in safety.

[john_tullamarine]

As HN39 and PEI 3721 have observed, the meaning for V1, and its sensible operational implementation, has varied over the years. One needs to look at individual TCDS and then the cited frozen design standards to match one with the other.

General approach for quite some time now -

(a) by V1 the first stopping action has already been initiated

(b) if not, the the aircraft is launching with the problem.

based on the observation that the risk involved in stopping is much greater than that associated with continuing.

(.. acknowledging that such is an idealised construct and sensible based on statistical history .. there will exist circumstances, on occasion, where the commander's outcome will be better doing things differently)

As to what one might call it, doesn't really matter .. decision speed, Friday night at the movies, etc., so long as the embodied strategies are understood both for the particular certification and sensible operational risk management.

As always the underlying rule is "don't crash".

[aviatorhi]

PEI,

Still though,

Dual Engine failure (on some airplanes you'll have no choice anyway).
Reverser operating.
Control malfunction (ie rudder hard over).
Multiple main gear tire failures.
Collision with foreign object (ie truck on the runway).

A few more that I can't think of at 2 am...

While extremely remote I know of at least two of these occurring recently. Timely action by the flight crews (in one case after Vr) saved their lives.

[tommoutrie]

Have I misread that or are you stopping after Vi with tyre failure?

Cos I wouldn't..

[hawtin]

As to what one might call it, doesn't really matter .. decision speed, Friday night at the movies, etc., so long as the embodied strategies are understood both for the particular certification and sensible operational risk management.

Thanks John but unfortunately it does matter, I was asked this question recently in an airline exam. Not 'what is V1' but 'What is the correct term for V1'. There were four choices, two of which were 'decision speed' and 'action initiation speed'. Which one would you pick?

Going by the above posts, would it ok to say that there isn't any?

ps: thanks everyone for replying.

[john_tullamarine]

unfortunately it does matter

Like so many questions in so many exams .. a bit silly.

Presuming that one of the other two alternatives was "none of the above" and that the fourth was irrelevant, I would go for "none of the above".

However, presumably the relevant course provided the "approved" answer somewheres along the way, as is the normal way of doing things ?

As we all understand, in the exam it is a matter of providing whatever canned answer is appropriate to the particular examiner's foibles regardless of its degree of sillyness. No benefit in falling on one's sword to make an unwinnable point.

Unless, of course, we are talking about a specific Type. Then, one would refer to the relevant TCDS, the relevant frozen Design Standards and ACs .. the answers you have cited may, then, well be the correct (if outdated) items. They would, however, remain pertinent to the particular certification.

[aditya104]

http://i.imgur.com/YsIIqAk.png

Thanks vilas, HazelNuts39, PEI_3721 for the definitions. This question was recently asked in Jet Airways' written exam and has already invited a lot of healthy discussion.

One thing everybody seems to agree on is the significance and applicability of this speed i.e. V1 is the maximum speed at which a rejected takeoff can be initiated, in the event of an emergency.

There are a lot of conflicting definitions.
Airbus, in its Flight Operations Briefing Notes-Understanding Takeoff Speeds (http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-TOFF_DEP_SEQ07.pdf) and Revisiting the “Stop or Go” Decision (http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-TOFF_DEP-SEQ04.pdf) clearly states that V1 is the Decision Speed.
Then there is FAA's Pilot Guide to TakeOff Safety (http://www.faa.gov/other_visit/aviation_industry/airline_operators/training/media/takeoff_safety.pdf). This is a training document published on FAA's website and not an actual regulation. Quoting from it:

One common and misleading way to think of V1 is to say “V1 is the decision speed.” This is misleading because V1 is not the point to begin making the operational Go/No Go decision. The decision must have been made by the time the airplane reaches V1 or the pilot will not have initiated the RTO procedure at V1. Therefore, by definition, the airplane will be traveling at a speed higher than V1 when stopping action is initiated, and if the airplane is at a Field Length Limit Weight, an overrun is virtually assured.

But I haven't yet come across any source that defines V1 as "Action Initiation Speed" where as I have come across many that define it as "Decision Speed".

[PEI_3721]

John, your general view (#15) summarises the operational practicalities, but more often the industry depends on a precise view of the subject which is assumed to apply to operations. Differences in viewpoint can represent a safety-gap between what is expected and what happens; the breeding ground for accidents.

Whilst the comments in parenthesis have good intent and are technically correct, they play on human weaknesses of false belief and inaccurate risk assessment which often dominate in sudden and surprising events.
I assume that you would agree that the decision re RTO is one of the few really critical time-dependant assessment in operations, and thus where possible should be couched as a simple ‘if then’ activity. This places emphasis on situation recognition which should be based on training and tech knowledge. Extremely remote (improbable) events can be discussed during training so that some pre-reasoning might dispel bias (strategic thinking), thus clearing the mind for a tactical decisions with a failure.
Unfortunately the FAA training guide introduced doubt by discussing ‘what if’ above V1, apparently without reference to certification and safety assumptions etc; from this we encounter these types of discussion.

aviatorhi, you can always find exceptions, both good and bad, but aviation safety is based on pre-assessed risk. The current probability of the exceptional events is far less than those of erroneous human judgement in situation assessment and choice of action. The required level of safety is maintained with aid of design and certification requirements, industry wide safety initiatives, and the assumption that pilots will act as accordingly.
Thus if you think (how do you know) that you have encountered one of your example situations you are almost certainly wrong, and thus expose yourself to a higher probability of an overrun. Also when considering the combined probabilities of failure, exceptional event, and speed, the exceptions do not warrant further concern.
Thus because of the human biases in assessing risk and situation recognition, extensive training and strategic reasoning is required to curb these deficiencies for this critical operation, and most others.

hawtin, I agree. The problem is not that there isn’t a suitable definition, but in the poor wording of the question. The examiner appears to have ignored context, and also pre-packaged some answers, expecting only one; whereas in reality V1 depends on how a pilot asks the question and knowledge of the subject before the event. These cannot be answered with a single exam question.

[capt. solipsist]

For AIRBUS, V1 is Decision Speed, and you have 2 seconds more to initiate the reject. This is the "startle factor" included in Airbus performance computation. (Reference: Airbus Perormance Manual 9.4.3)

[FE Hoppy]

From Airbus A318/A319/A320/A321 PERFORMANCE TRAINING MANUAL


Operating speed: V1
Definition: TakeOff decision speed chosen by the applicant.
V1 is the speed limit at which the pilot can interrupt TakeOff in case of failure.

The 2 seconds doesn't belong to the crew.

[aviatorhi]

Tom,

If you have a dual failure on the left side of a narrow body you may very well have no choice except to stop due to loss of directional cpntrol.

[ExSp33db1rd]

and .......... the V1 speed calculation is variable depending upon whether full thrust is used, or reduced thrust, which is often the case. With reduced take off thrust the V1 speed will be lower, to take into account the slower deceleration to the "V1 speed" at a point further down the real estate.

but ......... in the case of an engine failure around V1 then using reduced thrust for take off, then the applicaton of full power on the remaining donkeys will alter the maths !

Nothing's ever easy ! "When in doubt, lash out " i.e. it is often better to get airborne and have time to think than end up a steaming wreck at the end of the runway ( or beyond it ! )

Not my problem anymore, best of luck chaps !

( just revert to Stick and Rudder - i.e. fly the beast, don't rely on Bill Gates' magic box. )

[john_tullamarine]

more often the industry depends on a precise view of the subject which is assumed to apply to operations

There are two views under consideration -

(a) certification, which varies according to the Standard relevant to a particular Type's certification. Refer TCDS info. As close as we can get, reasonably, to something relating to precision, whatever that might be.

(b) SOP driven practice. Main aims are repeatability and rule based simplicity. Scant consideration of precision in any aspect.

depends on a precise view of the subject which is assumed to apply to operations.

Absolutely the case - which is why (b) must be strongly rule based other than in the extreme situation where the commander has really good reason to believe that it isn't going to work on this occasion.

you have 2 seconds more to initiate the reject

Whoa, there. Absolutely NOT the case. A/L 42 acknowledged the reality that the previous accel-stop standards were a recipe for disaster on limiting runways in the real world. Rejection philosophy doesn't change ... the numbers just have a bit of a pad (and not all that much) built in to give the crew a chance of stopping on a REALWORLD limiting runway ... with all its contamination, pilot startle, etc., etc... The rejection still MUST have been started with respect to the speed schedules.

The 2 seconds doesn't belong to the crew.

Concur .. but it might just save their necks.

If you have a dual failure on the left side of a narrow body

.. and we can come up with a number of scenarios .. cross wind min V1 situation, for instance. If circumstances conspire to put you right outside the presumed certification envelope .. well, that was the day you shouldn't have dragged yourself out of bed ..

the V1 speed calculation is variable depending upon whether full thrust is used, or reduced thrust

Careful, I suggest .. For flex takeoff V1 min is based on rated thrust. For a derated takeoff, speed will be based on the lower derate thrust - one of the reasons for using derate on some critical runways. If, then, you add flex to derate, V1 min still reflects the derate thrust level.

in the case of an engine failure around V1 then using reduced thrust for take off, then the applicaton of full power on the remaining donkeys will alter the maths !

No, that's why V1 min is based on rated rather than flex thrust. Different ball game with a derated takeoff if you are able (and choose) to push the levers up rapidly and too far.

[hyzhao11]

it is Decision Speed, while the term used by THAT pilot is not formal.

[compressor stall]

I always thought the 2 seconds was the time you have to initiate the abort - having already made the decision to abort at V1 - and still stop before the end of the runway (or calculated distance).

If V1 is 150 kts you travel 77m a second.

If you aborted at exactly V1, the test pilots determined that the aircraft would stop in X metres. To account for our mortal reflexes and the "sh1t F#ck factor" as I call it, the performance calculations give you X plus 144m to stop.

[vilas]

solipsist and compresser stall
V1 call is a GO call period. You are misinterpreting the accelerate stop distance definition after Amendment FAR 25-42
To summarize,
Amendment 25-42 required the accelerate-stop distance to include
two seconds of continued acceleration beyond V1 speed, before the pilot takes any action to stop the airplane.
That is when first action to reject is taken at V1 not two seconds afterwards.

[compressor stall]

Yes - a rereading of 25.109 states that the first action to reject is at the V1 - which is practically what you do anyway.

[john_tullamarine]

Stallie .. the two seconds is intended to be hidden from the pilot's actions and represents money in the bank for the accel stop. You probably recall the equivalent Oz change to 101.6 at A/L 62.

[bubbers44]

At ballanced field length for US was V1 meant you could stop at the end of the runway or be at 35 ft at the end of the runway. No 2 second buffer in the US. That is the way I was taught. The two second buffer to react seems pretty sloppy for an airline crew performance. I taught this in corporate jets and V1 was a go or abort speed, no buffer. My major airline was the same. No pilot I know would take 2 seconds to decide what to do. That is why V1 is there. It is not advisory, it is a decision speed. Some countries might do it differently but that is how we have always done it.

[vilas]

bubbers 44
Your balanced field lengh understanding is correct the only change is at V1 you don't decide but first action to reject is taken. For that to happen it has to be before V1 call. V1 call is GO, actually according to Airbus GO call is only before V1 not afterwards. Many are mistaking the two seconds. There is no question of taking 2 seconds to decide after V1. V1 call is go call. Two seconds acceleration is added for ASDR calculations since it will take some time for decelaration to begin. It is not for pilot. This ammended definition of V1 is from FAA.

[vilas]

hawtin
Out of the options you had the correct would have been "Action initiation speed". That was the closest.

[vilas]

Since this topic is not a matter of opinion but a regulatory and safety issue,to put an end to all arguments and opinions below is from Boeing document
"V1 and the GO No Go Decision"
V1- is the speed at which the takeoff should be continued unless the stopping maneuvre has already been initiated.
You can review this document on smartcockpit.com under FLT OPS.

[bubbers44]

Since we ar talking about less than a knot of airspeed at V1 it really doesn't matter much, does it? Before 1 knot below V1 abort, at V1 continue. How many pilots have had a problem in that one knot range? Maybe none?

[HazelNuts39]

Since we ar talking about less than a knot of airspeed at V1 it really doesn't matter much, does it?Since we must also consider an abort with all engines operating, we are actually talking about much more than 'a knot', more like 7 - 8 kts IIRC.

[Owain Glyndwr]

The two second buffer to react seems pretty sloppy for an airline crew performance.In the long ago days when I earned my living by calculating takeoff performance for inclusion in the FM, we included a failure recognition time of 1 second and a decision/reaction time of another second. Given that the recognition time includes the aircraft developing enough motion at the cockpit to be noticeable that doesn't seem really sloppy. About 1 second recognition time was what the test pilots achieved even when they knew they were doing a RTO test. The other second IIRC was a statutory value dictated by the rules.

Anyway, the 2 seconds is built into the calculation of V1, so it is not an additional buffer. HN39 may confirm my memories?

[HazelNuts39]

The 2 seconds of continued acceleration after V1 was introduced in the accelerate-stop distance specification of FAR 25.109 with FAR Amdt. 25-42, effective date March 1, 1978 (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFinalRule.nsf/0/A95AF4C6BE88807E862568240073CDCD?OpenDocument&Highlight=25-42).
The current regulation is FAR Amdt. 25-92, effective date March 20, 1998 (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFinalRule.nsf/0/B1759D97FF77F95086256936004D6E44?OpenDocument&Highlight=25-92).

P.S.

Advisory Circular AC 25-7C "Flight Test Guide For Certification Of Transport Category Airplanes" (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/BD2675E7774B4C4786257AC200546ACE?OpenDocument&Highlight=25-7c) provides additional background on how the flight tests are conducted, and how the flight test data are 'expanded' to AFM distances.

Appendix 3 describes the Historical Development of Accelerate-Stop Time Delays

[john_tullamarine]

No 2 second buffer in the US

Only for those aircraft for which the frozen design standard included A/L 42. TCDS gives that story but, given the use of grandfathering, one is looking at somewhat further down the date track.

The two second buffer to react seems pretty sloppy for an airline crew performance

A/L 42 was not intended to address crew incompetence. Rather it was to put in some conservatism to allow for real world things such as startle factor, surface contaminants in the stopping region of the runway (lots of rubber deposits from oppposite direction landings), etc

As Vilas and others note, there is a moderate to significant speed overshoot prior to the deceleration's actually resulting in the speed's (rather than the acceleration's) decreasing. For the AEO case, that overshoot can be quite significant.

the 2 seconds is built into the calculation of V1, so it is not an additional buffer

Actually, the recognition and reaction times must still be there (unless the current crop of TPs have, indeed, evolved to be superhuman) ... the A/L 42 two seconds is an additional pad added to the calculated (and is included in the demonstrated) accel-stop distance.

[john_tullamarine]

HN39, an extremely pertinent observation, good sir. I would entreat you to reinstate it for the benefit of the readership.

[HazelNuts39]

Thanks John,

I withdrew my remark because on reflection is wasn't entirely pertinent and didn't add anything to what had already been said better by other posters.

[LeadSled]

Multiple main gear tire failures.Aviatorhi,

No way, a lot of people died in Spain when a pilot did that (nose gear tyres, in this case)

This is too critical a matter for any pilot who is actually flying heavy transport aircraft (FAR 25 or equivalent) to be any doubt, please do not read the manufacturer's description like a bush lawyer, the definition ----- and what you have to do if you are confronted with a potential go/no go decision must be clear and simple.

The reason V1 is not a "decision speed" is that the decision must be made before V1, think of V1 as the speed at which the takeoff must be continued if the abort has not already been commenced.(That's how Boeing describe it, in an extensive discussion paper on the subject)

Don't get all tied up in in certification standards, if you are the pilot.

In the case of an abort, or continuing the takeoff you have to keep things very simple.

Some time before V1, something has happened and you decide it is important enough to abort.

or

Something happens so close to V1, that it is going to be passed before any reject action, so you must continue, unless the failure is so severe that the aeroplane will not get airborne and fly.

Boeing (and, no doubt Airbus) have excellent training material on the subject, the words used may seem different, but the "definition" of V1 should be clear, you can't start making the decision at V1, you have either commenced the abort by that speed, or you go.

The 2 seconds doesn't belong to the crew. FE Hoppy,
That is correct, very correct, the 2 sec. is a matter for performance engineers preparing takeoff data.

PLEASE, PLEASE don't any of you who are actually flying these aircraft think you can commence an abort 2 seconds after V1.

If you think otherwise, you are misinterpreting something that that must be very clear in your mind.

[john_tullamarine]

hear ! hear !

[good egg]

Having been preparing a paper on windshear I noticed this link referring to V1...

http://flightsafety.org/asw/jul-aug11/asw_jul-aug11_p23-24.pdf?dl=1

Any comments?

[HazelNuts39]

The title had me a little bit worried. But then the author doesn't really want to change the definition of V1. All he wants is a descriptive name for it, to replace the misleading name 'decision speed'.

In that respect, I agree with Vilas in post #34. However, it is not a very good name because action to stop can be initiated at any speed less than V1.

If one wants a really short name, I would suggest 'go speed'.

[tommoutrie]

Its called V1. It means we are continuing the take off. Maybe it used to be called decision speed because you've already made the decision. But once V1 is called, you have made the decision to continue. If you have made the decision to stop but happen to be so close to V1 that your reaction time means the aeroplane accelerates through V1 before the application of brakes and closing the levers etc starts to have an effect, you are stopping. Thats because you made the decision before V1. Lets call it Decision Made Speed! You'll never get a bunch of manufacturers to agree what to call it but as long as we know what it means, who cares? Short is good, lets go with HN39's idea of go speed.