V1 or not ???

[Sonia767]

Hiiiiiiiiii,

I have a bit of a confusing question on V1 speed.

Lets say you are really heavy and your balanced field V1 is 160 knots.

That means you could abort the takeoff at V1 and SHOULD be able to stop on the runway.

Now what if you were light and V1 was 130 knots.Then comparing it with the case above wouldnt your Vr be before V1 since you know you could stop even if V1 was 160 knots.

Yes, i know V1 has to be before Vr but ........................Oh im :confused:

Could someone help out

Sonia

[Miserlou]

The max value for V1 is Vr.

See also V1/Vr thread below. Discussion on whether to call V1 in said case, if I remember rightly.

[mutt]

Sonia767,

Remember that a Balanced Field Length may be less than the actual runway length. In your case above assuming that V1=160 Kts uses all of the available runway, your V1=130 Kts wont use the same amount of runway but it will still be a Balanced Field!

VR is a function of V1 and would therefore be associated with the V1=130 Kts, you wouldnt use the VR associated with the higher V1.

Mutt.

[*Lancer*]

mutt, I thought V1 was independant from Vr/V2. Ambient temp, press alt, aircraft wieght affect them all, but Vr/V2 is uninfluenced by the field length available, whereas V1 is.

Even so though, in Sonia's case the weight would bring Vr/V2 down (possibly below the V1 at the higher weight) but still greater than 130... or am I totally on the wrong track :O

Lancer

[mutt]

Oops, i should check read my messages when sober........ sorry..

*Lancer* you are on the right track.....

Mutt.

[FlyingForFun]

I thought V1 was independant from Vr/V2This is not the way I was taught it.

V1 could, theoretically, be any speed between Vgo (the slowest speed at which you can go on a single engine) and Vstop (the fastest speed at which you can stop). Normally, you'd pick a number in the middle.

However, there are certain other speeds which V1 is not allowed to be either more than or less than. For example, it must be less than Vmbe, but it must be more than Vmca. If your initial attempt at finding V1 falls outside any of these restrictions, then you must move it - although it must still be between Vstop and Vgo (if you can't satisfy this, then you have to lose some weight).

In the case where your initial attempt at finding a V1 gives you a number which is greater than Vr, then you reduce V1, such that V1=Vr.

At least, that's the way I remember it being taught...

FFF
------------

[NW1]

Sonia - (my 2/100 euro's worth) looking at it practically: V1 is limited by Vr (ie. made equal to Vr) at the lighter weights (if theoretical V1 > Vr) because you can stop at Vr and after the rotation manoeuvre is started the performance calculation ensures that an engine failure can be safely carried through into the air. And since closing the good engine(s) and re-landing after Vr is not taken into account there is no point having V1 higher than Vr.

Let's say in your example of V1 being 160kts but the a/c weight and conditions on the day give Vr=130 (say). You might be able to stop from 160kts on the runway, but since 160kts > Vr you would be well established in the climb and you would probably not be able to re-land and then brake inside of the TORA (theoretical, of course, because such a manoeuvre would be, er, dramatic to say the leaast - but it demonstrates why V1 is limited by Vr?).

Of course if something more significant than a single engine failure occours after V1 or Vr then you need to think outside of the perf A box.....

Is this where you were coming from in your question??

[Sonia767]

Thanks for your replies everybody ;)

NW1 could you elaborate what you mean by "think outside the Perf A box ? " :)

Sonia

[Dogma]

NW1, Mutt, etc,

This post illustrates perfectly why not to ask technical questions on this type of forum.

Granted you have made some accurate observations but some of this is utter drivel:

Let's say in your example of V1 being 160kts but the a/c weight and conditions on the day give Vr=130 (say). You might be able to stop from 160kts on the runway, but since 160kts > Vr you would be well established in the climb and you would probably not be able to re-land and then brake inside of the TORA (theoretical, of course, because such a manoeuvre would be, er, dramatic to say the leaast - but it demonstrates why V1 is limited by Vr?).

This is rubbish and only some of it.

Vr is 1.3 VS, Simple as that.

V1 is a function of the accelerate stop capability of the aircraft on the day.

Vmcg is the ability to control the aircraft on the ground using solely aerodynamic means. Very important on the boeing 75/76 type. Vmca is not a factor on most Boeings.

Please for accurate gen' please read the books not Pprune:8

[Mad (Flt) Scientist]

Posted by Dogma
This post illustrates perfectly why not to ask technical questions on this type of forum.
Granted you have made some accurate observations but some of this is utter drivel:
<snip>
Vr is 1.3 VS, Simple as that.
V1 is a function of the accelerate stop capability of the aircraft on the day.
<snip>

I'm sorry, but your info above is far more misleading.

Vr has No rpt NO relationship of that nature with stall speed. I don't know where you can have got such a wierd idea. One of the regulatory minima for V2 (which is always higher than Vr) is 1.2Vs. There would be no point in defining this if Vr were always 1.3Vs.

V1 may or may not be a function of the accel-stop performance; it depends on the aircraft and the conditions, as do issues of whether Vmc or Vmu are limiting for determining takeoff speeds.

edit - OK, I think I can guess where you are getting that idea from...
Vr is rotation speed (and is the subject of all the above discussion)
Vref is defined as not less than1.3Vs (note it need not be equal to 1.3Vs) and I suspect that's what you are bringing into the discussion.

[springbok449]

In the climb not being able to reland and stop on the RWY??
Jeepers some guys are on drugs, reland in a C-152 maybe but in Performance A a/c??
If I ever see such a manoeuvre I hope it will only be on Flightsim 2000!!

[Dogma]

Mad Scientist: I do accept that I have confused Vr and Vref, but this further illustrates why not to ask tech questions on an open forum.

"V1 may or may not be a function of the accel-stop performance; it depends on the aircraft and the conditions, as do issues of whether Vmc or Vmu are limiting for determining takeoff speeds."

Irrelevant, you cannot sum up V1 in these terms, "Vmc" ? Is it Vmca or Vmcg that you are alluding to?

Vmu is not I repeat not considered when determining take off speeds on a balanced field.

[Chimbu chuckles]

Sonia767,

This is one of those classic cases where thinking about someting too much...or questioning something too much is counterproductive.

V1 is a 'derived' speed, based on many variables one of which is weight.

Some days on long runways at light weights it is clearly possible to stop successfully beyond V1...BUT WE DONT unless something dramatically more disastrous than a fire or failure occurrs....both of which are, performance wise, 'non events'. To do otherwise is to invite a multitude of personal variations on when it's Ok to stop and when it's not...far, FAR too subjective and the very reason why smart men came up Perf A/FAR 25/CAO 20.7.1b back in the 50s...to stop needless crashes caused by "But I thought I could stop"..or "I thought it would clear the trees"

Please accept also that V1 is NOT 'Decision speed' but 'decision made speed'. Or a 'first braking aid applied' speed.

In the 'runway limited' case your 'decision speed' is more like V1-5-10kts because inertia will take you beyond V1 even after a sudden and complete power loss...particularly the fire case which may have no significant associated power/thrust loss.

When you have a balanced field the actual stopping bit is balanced on a knife edge...I'm sure during CRM Initial years ago we were told rejected takeoffs were the least well understood and most likely accident scenario of all.

Chuck.

[Sonia767]

Hello,

Seems Dogma is really after me for asking this question in this forum.....:(

Hmmmmmm so then what questions do you ask in this forum.

How do you define what to ask and not to ask here............:confused:


Sonia

[Mad (Flt) Scientist]

Dogma

How would you sum up V1, then? t is determined by consideration of other than the accelerate-stop criteria, otherwise you could have V1>vr.

I said Vmc because I couldn't be bothered typing "Vmcg or Vmca" as both matter. And, incidentally, 25.149(a) actually uses the terminology "Vmc" rather than "Vmca"; it is conventional to add the "a" for the in-air T/O config Vmc, but not necessarily 'official'.

And Vmu sets criteria which must be met for all takeoff speeds defined for an aircraft. Whether the field is balanced does not affect the need to respect the conditions set by Vmu (or any other conditions). As the takeoff speeds (V1,Vr,V2) are interrelated any single condition (Vmbe, Vmcg, Vmca, Vs, Vmu,..) may drive all the speeds.

Sonja

You might get a less adverserial response (and no less help, I think) if you asked this in Tech Log. But I don't see why you shouldn't ask here.

[Sonia767]

Mad Scientist,

Thats what a i thought.A questions a question...................and this is a QUESTION FORUM.Cheers

:)

Sonia

[Dogma]

Sonia767,

The question is a good one, my sole concern relates to the accuracy of the data in the responce! Most information on Pprune is wildly inaccurate.

P.S Surely you do not fly a 767? :confused:

[qwertyuiop]

Bloody Hell!!!

Best advice I can give is KISS (Keep It Simple Stupid).

Use the figures provided and dont think toooo deeply. As you can see, there is soooo much uninformed and confused advice.

Use the figures provided and don't be clever.

[Sonia767]

Dogma thats why this is a forum.

Everybody gives out their views ( not necessarily all correct) and then you judge for yourself what you want to retain or to discard.

Dont wanna argue with you on this one because i figure youre much to knowledgeable for me

:uhoh:

Oh by the way why shouldnt i be flying a B767........

Sonia

[mutt]

my sole concern relates to the accuracy of the data in the responce! Most information on Pprune is wildly inaccurate.
I guess that you get what you pay for.....................................


Mutt. :):)

[Dogma]

"Oh by the way why shouldnt i be flying a B767........

Sonia"

Because the you should know that light weights give rise to a range of V1 and thus enables us to use an assumed temp...etc,!

Basic stuff:8

Your not Doctor Sonia are you?

[NW1]

Brilliant use of irony - had me laughing out loud! To accuse me & others of talking "utter drivel" and "rubbish" and then come out with stuff like "Vr is 1.3 VS, Simple as that" and then more comments which would indicate a lack of understanding of large aircraft performance. Funny stuff.

If you were able to carefully read my original post and surpress the risable overreaction, you might understand that the point is that V1 cannot always be based solely on the accelerate/stop consideration.

The original question Sonia had was very valid - and it is a fact that V1 may (and often is) limited by a low Vr (due, in the questioner's valid case, to light weight). I emphasised the impossibility of V1 being greater than Vr by pointing out that that situation would imply the possibility of a reject after liftoff - clearly absurd (so no, springbok449 - not on drugs, just pointing out that absurdity, you may relax) - and so in the performance calculation when (and it can happen) V1 comes out greater than Vr, it must be set equal to Vr.

Furthermore, before sounding off to others, you need to understand that Vmcg will directly affect V1 - there will often be a minimum V1 (due to Vmcg and so the two are actually related), on my aircraft it is 132kts. So if V1 comes out of the calculation at less than 132 (it may at light weights), you must set it equal to 132kts.

So, once again, here's an example of V1 not being purely about accelerate/stop considerations - it must be set not less than Vmcg and not greater than Vr.

At the end of the day, this is the equivalent of a bar room chat - nobody here is trying to re-write manuals or alter company SOPs. So it would be very helpful if you refrained from emotive language and also perhaps bear in mind that some correspondants here might actually be very experienced in the field they are discussing.

Your sarcastic question to Sonia : "Surely you do not fly a 767? " was totally unwarranted: her willingness to ask open basic questions shows an attitude very well suited to the flight deck - your critical and scathing responses indicate the opposite.

[Dogma]

NW1, was quite good was it not!

Beautifully illustrates my main point, if you want accurate gen' get your head in the books!

But have to admit your contribution, thou well meaning, was utter tosh:ok:

All power to you Sonia!

[NW1]

"your contribution, thou well meaning, was utter tosh"

..oh puhlease. In that case, do tell us all when V1 may be less than Vmcg, or greater than Vr. This should be even more fun than your previous.

[john_tullamarine]

This thread is a little disturbing as it emphasises the oft-touted lemma that a little knowledge is a danger.

One of the problems behind the scenes is that there is a lot of work which goes into certificating a particular aircraft type ... most of which never sees the light of operational day (ie to those of us who hang onto the stick) other than as information embedded within the various certification data (AFM etc).

Another major problem is that the typical airline tends to teach the pilot on a "monkey see, monkey do" philosophy .. which often leads to the pilot only getting a small part of the overall story.... and there are those who believe that the regulatory examination processes in various parts of the world don't do much better ...

When it comes to performance scheduling (one aspect of which is the thrust of this thread) there are quite a few things which the performance engineer takes into account in arriving at a solution to a particular runway problem. Often there are several available and reasonable options and flight standards, commercial, or other influences may dictate the option finally chosen.

It is always a bit of a risk to put a very hardline and didactic viewpoint in an arena in which one is not an expert .. the expert, in fact, usually tends to avoid this and provides often numerous qualifications to his/her observation so that anciliary considerations can be addressed or, at least, identified.

Be aware that Mutt is a very experienced airline performance engineer (i.e. he makes his crust doing this stuff day in and day out and has done so for many years ... in any case he drinks Guinness ... so he can't be all that bad ...) and that Mad (Flight) Scientist is a (large) aircraft manufacturer engineer and flight test man as I recall ... one would be well advised to listen to their counsel with some degree of regard.

If the suggestion is that PPRuNe is peopled by individuals of scant knowledge and skill, then consider that more than a few of the people herein have PhDs in their particular disciplines. At least one, to my knowledge, holds a technical discipline Chair at a highly respected University .... just some food for thought, people ...

One of the most useful aspects of PPRuNe, regardless of which forum a thread choses to be born, is that people are able to express their views ... (it would be nice if this were to happen without those views being lambasted .. .regardless of their weight) ... and those who have relevant professional knowledge are able to hop in from time to time with a hope that errors of great magnitude can be corrected in a friendly environment.

Maybe, just maybe, a few people can learn a few things along the way ..... the aim is not to present an academically rigorous approach as this would put most people off their beer ... rather just to ease the knowledge base a little away from ignorance towards the actual story ...

A certification-based V1 is not a simple "pick a unique number" thing ... however, airlines often adopt a simplified approach to V1 selection for various reasons... and there is nothing inherently "right" or "wrong" with such an approach ... just a matter of narrowing down the available options for whatever reason.

The available range of options increases greatly when the aircraft's performance capability in the circumstances results in a moderate section of runway being surplus to minimum length requirements. So, for instance, V1 may well be dictated by obstacle considerations in a particular set of circumstances (presuming, of course, that a V1 driven to high numbers is not going to become an ASDR problem).

It is facile to exhort people to "read the books" when the very books with the information sought may well not be available to the man in the street ... Mutt often refers to Boeing Performance Engineers' (specific Type) manuals ... easy to come by if you are an ops engineer and your airline operates the particular aircraft ... otherwise as scarce as RHS.

Might I assure the readership that Vmcg and/or Vmca can be VERY relevant to takeoff at light weights for MANY types of aircraft ... as many of my endorsement students have found out to their wide-eyed surprise ... Boeings or otherwise ..

I am particularly disturbed when I read things such as ... "I do accept that I have confused Vr and Vref, but this further illustrates why not to ask tech questions on an open forum" .... if one has the story not quite right ... then PPRuNe is a very useful vehicle to put an erroneous point of view and have it gently corrected in a friendly environment (for techo things, especially in Tech Log).


Sonia .... you just keep on asking questions .... it gives us techies something to scratch our grey heads about ... (OK .. follicly-challenged heads .. whatever .... )


... and Dogma might question his/her motives ? ..... then, again, perhaps he/she is just a wind-up merchant as one might be forgiven for inferring from the username ?

[Dogma]

Good Post John!:O

Apologies to those whom my antics peeved.

Catch you all later;)

Dogma

[Sonia767]

Dogma,

I dont know why im still responding to you :D but i think this is due to the fact that you have p***** me off.

Okay i think i remember someone mentioning assumed temp to me on route training (duuuhhhhhhhhh)

Yes you can use an assumed temp and thus correspondingly the takeoff speeds increase.........

But what if the Runway was WET with standing water........then i guess you dont do an assumed temp takeoff or do you :rolleyes:

Hope im not showing my ignorance here.

So the way i see it its not as basic as you seem to think it is and its back to square one again

Someone shoot me down please ;)

Sonia

P.s Thanks for sticking up for me NW1 and as for you john_tullamarine dont you worry i still have loads more questions to come ;)

P.s. Dogma you lost me there with the Doctor Sonia bit.....

[knackeredII]

Sonia, I have included some extracts from the definitions section of an old DC-9 Flight Study Guide which I think explains the answer to your qestions pretty well. I think most of the information is still pretty accurate today. I've only included the points relevant to your query.


Accelerate-Stop Distance:
(a) The accelerate-stop distance is the sum of the following:_
(i) All engine acceleration distance from brake release to speed V1.
(ii) Distance from V1 to application of full braking.
(iii) Distance with full braking applied to stop. (One braking aid not considered.)

(b) A recognition time, i.e. between actual engine failure and recognition by the pilot (V1) is built into the accelerate-stop distance


Balanced Field Length:
The balanced field length occurs when the accelerate-stop distance required equals the engine-out distance to 35 ft.
Decision Speed (V1):
(a) The V1 speed is the speed which the pilot uses as a reference in deciding whether to continue the take-off or to abort.
(i) V1 cannot be less than VMCG or greater than VR.
(ii) The speeds given for V1 have been selected so that:
1. if an engine failure is recognised at or above the V1 speed, the take-off may be continued to a height of 35 ft above the available take-off distance, or:

2. a stop may be made on the runway from V1, or at speeds less than V 1 and without, in either case, exceeding the scheduled accelerate-stop distance.
(b) The scheduled take-off field lengths are based on stopping if engine failure is recognised before reaching V1 and on continuing the take-off if engine failure is recognised after V1.

(c) Appropriate reaction time is allowed between the failure and recognition of the failure.

(d) When stopping, appropriate time is allowed between recognition of the failure and completion of all actions required for rejection. It is also required that one of the available braking systems (e.g. reverse thrust) must not be used during certification or if all braking devices used that an additional distance factor (e.g. F.28: + 10%) be applied.


Lift-Off Speed (VLOF):
The lift-off speed is the speed the aircraft first becomes airborne.


Minimum Control Speed - Air Vmca
VMCA is the minimum speed at which the aircraft can be kept within certain specified limits of attitude and heading, when, in the second segment configuration and with take-off power on all engines, the critical engine experiences sudden, complete failure.


Minimum Control Speed - Ground Vmcg
VMCG is the minimum speed on the ground, at which the aircraft can be kept within certain specified limits of heading using primary aerodynamic controls alone, when, in the take-off configuration and at maximum power or thrust on the operating engines, the critical engine experiences sudden, complete failure.


Minimum Unstick Speed (VMU):
The minimum unstick speed is the minimum speed at which an aircraft can be made to lift off the ground and, maintaining a positive climb, continue the take-off without displaying any hazardous characteristics. Certification requires demonstration with all engines operating and with the critical engine inoperative.


Rotation Speed (VR):

(a) Rotation speed is that speed at which the pilot begins to rotate the aircraft to the lift-off attitude.

(b) The rate of rotation can vary but it should normally be at the rate of 2° - 30 per second depending on the aircraft certification.

(c) Rotation at the maximum practical rate will result in a minimum lift-off speed. Rotation at the normal rate will result in attaining the V2 speed at or below 35 ft, with one engine inoperative, or slightly exceeding the V2 speed at 35 ft with all engines operating. VR is such that it is not less than:_

(i) V1
(ii) 1.05 VMCA
(iii) a speed which permits the attainment of V2 prior to reaching 35 ft.
(iv) a speed which will result in at least the minimum required lift-off speed. This speed must have a margin over VMU.
(v) a speed which will not result in increasing the take-off distance if rotation is commenced 5 kt lower than the scheduled VR during one-engine-inoperative acceleration, or 10 kt lower than the scheduled VR during all-engine-acceleration.


Take-Off Distance Available:
The available take-off distance available for an aircraft to accelerate from brake release to the 35 ft point either with or without engine failure at V1.


Take-off Safety Speed and Initial Climb Speed (V2):
V2 is the take-off safety speed and is achieved prior to 35 ft. This speed must not be less than:
(a) 1.1 VMCA

(b) 1.2 VS (or 1.15 VS - 4 engine propeller aircraft)

The correct V2 is a result of proper rotation and lift-off procedures and it allows the aircraft to maintain a specified gradient in the climb-out flight path.



Hope this helps.

[Sonia767]

Thanks for the informative post KnackeredII.......:ok:

Sonia

[df1]

V1 is a composite speed. Computed using gross weight, flap setting, pressure altitude and temperature. If you increase GW,PA and/or temp then you increase V1!! Reducing the factors will reduce V1. Increase weight for example and you impair the post V1 performance, hence the need for a higher decision speed. Set V1 too low and you may not have capability to attain safe lift-off point and speed.

Vr (not to be confused with Vlof (lift off)) is the speed reference used to safely accelerate to V2/35'.

Vr is never lower than V1 [operationally]. Although for a given set of conditions you may find that V1 could occur at a higher speed (runway length), but then we would be in a nonsense situation, whereby we have accelerated beyond a sensible take-off speed, and we should ask why we are still on the ground at that point (no emergency).

Contaminated surface will reduce V1 since braking capability is reduced (discounting reverse). Uphill sloping runway will increase V1, downslopes the opposite.

Some pilots only call Vr during the take-off roll. This is an example of computed V1 being higher than Vr. Should a reason to RTO occur then Vr becomes decision speed (some argue a dangerous situation).

Check smartcockpit.com for their pdf downloads on RTO's etc.

[northwing]

Read this post as from a stupid engineer, late at night after a (strong) vodka and tonic.

"V1 is less than or equal to VR" - means "You don't change your mind after deciding to take off because you could float for miles while trying to land again after abandoning a takeoff with the wings producing lift."

"V1 must be greater than Vmcg" - means that you don't commit yourself to flight before you can control the aircraft using the aerodynamic controls you will have available to you in the air."

[Mad (Flt) Scientist]

Northwing:
"V1 must be greater than Vmcg" - means that you don't commit yourself to flight before you can control the aircraft using the aerodynamic controls you will have available to you in the air."

Actually, two different things:

"V1 must be greater than Vmcg" - don't get into a position where you have an engine failure and decide to "go" i.e. continue to accelerate along the runway, but find you have insufficient rudder power to keep the aircraft on the runway (which is what Vmcg is about) (The idea being that if Vmcg is high, you make sure that following an engine failure at or below Vmcg you are stopping, and so reducing thrust on the good engine(s), which reduces the directional control problems.

"V2 must be greater than 1.1 Vmca" - means that you don't commit yourself to flight before you can control the aircraft using the aerodynamic controls you will have available to you in the air.