V1 or not ???
 

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

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.

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.

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

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

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

Mutt.

This 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
------------

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??

Thanks for your replies everybody ;)

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

Sonia

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

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.

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!!

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.

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.

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

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.

Mad Scientist,

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

:)

Sonia

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:

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.

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

I guess that you get what you pay for.....................................


Mutt. :):)