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v1 vs vr

Old 29th Aug 2000, 12:31
  #1 (permalink)  
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Question v1 vs vr

i am studying for my cdn iatra and am wondering why can v1 not be greater than vr.
Old 29th Aug 2000, 12:56
  #2 (permalink)  
Danish Pilot
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There are many reasons, but to make it simpel: V1 is the speed, when an aircraft (mostly airliners) accelerate down the rwy for take off, when the aircraft now have so much speed, that it would not be able to stop on the remaining rwy, if a serious mailfuntion occour. So before V1 you stop, after V1 you continnue the takeoff roll. Vr is the speed that the pilot rotate the aircraft, to get it airborne. If Vr was to be lower than V1, it would be possible to get the aircraft airborne, have a mailfontion, then land and stop on the remaining runway. I can not think of any rwy in the world there are that long.

The limiting factor are that the aircrafts needs a certan speed to get airborne, and the runways can only be a certan length. There are many other factors such as rwy surface, density, system u/s on the aircraft etc.etc.
Old 29th Aug 2000, 21:37
  #3 (permalink)  
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actually in a roundabout way it can be..depending on definitions.....with excess amounts of runway available, it is quite often possible for a jet aircraft to have the ability to stop after Vr is reached, and a rotation is accomplished..especially at light weights...however, a rejected takeoff is statisticaly more likely to result in an accident, than a take-off continued...but a good example is the Detroit runway collision between a NWA B-727 and a NWA DC-9 who entered the runway mistakenly the 727 had actually begun his rotation when his wing collided with the forward fuselage of the dc-9...the 727 capt discontinued the take-off and stopped in the remaining runway...his left wing was so severely damaged, that a successful continued take-off, and flight would have not been answer the original question...when the available runway length exceeds the balanced field length requirement for an aircraft, V1 may well be a higher speed than Vr...but go to your local expert before taking your exam..FAA balanced field length=

1) distance to accelerate to V1 then stop
2) distance to accelerate to V1 lose an engine and attain a height of 35 feet
3) 1.15 times the two engine take-off distance to 35 feet..cheers
Old 30th Aug 2000, 00:53
  #4 (permalink)  
Dan Winterland
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A V1 can be higher than VR. I just can't see any reason to be on the ground at a speed higher than VR if that answers the question.
Old 1st Sep 2000, 10:11
  #5 (permalink)  
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Single engine aircraft have enough performance to take-off, if everything is working.

FAR 35 light (usually piston) twins accelerate to the point where they can safely stop, then go through a "grey area" where they can neither stop nor continue to fly with an engine failure.

Transport category aircraft (usually jets) accelerate to the point where they can safely stop on the runway, and after that point they have a guaranteed performance to fly with and engine failure - i.e. there is no grey area.

Now once the aircraft has reached Vr, it has by definition, enough performance to fly on one engine, so there is no need to abort from this point (we, and the regulators, are talking about engine failure here, not significant damage!). That is why V1 is never greater than Vr.

Note that once the aircraft has rotated, then the distance to stop is very technique dependant as it depends opon how long the pilot takes to put the aircraft back on the runway. Calculated V1s assume that the aircraft is in contact with the runway, and can begine braking immediately. If you were to go into the books, and work out a Vr of (say) 130 knots, and a V1 at the same weight of 135 knots, well once the wheels are off the deck that calculated V1 now has no meaning, as the basis for it (wheels on the deck) no longer applies.
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Old 1st Sep 2000, 20:39
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Errrr - guys

I think some of you are is missing the big picture here.
Practically, V1 CANNOT be higher than Vr because there is not a performance manual in the world that contains an accelerate - rotate - land again - stop distance for an aircraft. By definition it is agreed that at Vr you are committing the aircraft to flight, and that there is adequate performance to meet the obstacle clearance criteria following an engine failure at V1. If you were to reject the takeoff after Vr, how are you going to KNOW that you have runway enough to stop. Come to think of it, you have also got a problem then with gear configuration (i.e. when do you pull it up if the runway is long...)
Old 2nd Sep 2000, 11:32
  #7 (permalink)  
Danish Pilot
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Yep, my point excatly. By the way, it makes me think of an incident SAS had not long ago in CPH. A B767 bound for NRT, the crew had entered wrong data to the FMS (due to human factors and stress) so they ended up with a wrong and lover V1 Vr than req. At rotation the aircraft wouldnt fly, and in theory they had pased V1 ages ago, but decided to reject the t/o. They were ofcourse able to stop, and the aircraft needed some new brakes, and tyres.

My point is, that in this case Vr (the wrong one) was indeed lower than V1 (the real one)
but the aircraft never got into the air, simply because it will not fly; it can not rotate enough to get airborne, because of the tail.

So it have been tested kids, it wont work....
Old 2nd Sep 2000, 20:06
  #8 (permalink)  
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Checkboard mentions a 'grey area' in the ops of light (part 23?) twins. In these types if you have an engine failure after Vr but prior to attaing sufficient speed you will have to land back (or do a forced landing).

In this case V1 is technically after Vr.
Old 3rd Sep 2000, 00:35
  #9 (permalink)  
Danish Pilot
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In a far23 certified aeroplane, you donīt use V1 Vr. According to certifications requirement the far23 plane doesn't have to be able to climb with a engine out. Most far23 manuals advise that IF landing straight ahead is possible, do so. And if not... well to bad. As my old instructor said: "why does an aeroplane have two engines...? Because it wont fly on one!!" Some truth in that.
Old 4th Sep 2000, 12:24
  #10 (permalink)  
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The decision to stop after V1 is not one to be taken lightly but if you know that the A/C will not fly (Wing fallen off) surely its better to go off the end of the runway at 40 knots rather than hit the block of flats 1 mile on at 140 knots.
As Checkboard points out that the regulators assume engine failure.
So V1 guarantees that if you have had an engine failure and decided to stop by V1 you will have sufficient stopway ahead. Beyond V1 the correct descision may be to stop but there are no guarantees as to what happens next.
Just thought- Emerald 748 at STN couple of years ago, Catastrophic failure of No2 Engine and Massive Fire. Already Airborne landed straight ahead everyone walked away. Job Done (pitty about the drainage ditch in the overrun)
Old 6th Sep 2000, 05:16
  #11 (permalink)  
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av8R76, get hold of a copy of "Handling The Big Jets" by B.P.Davies. You will find all the answers to your questions. Best of luck.
Old 12th Sep 2000, 02:16
  #12 (permalink)  
Trash Hauler
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V1 (decision speed) is related to runway available and aircraft performance. It is the speed below which the aircraft can be stopped on the remaining runway if the take off is rejected.

Vr is related to aircraft performance and gross weight (the speed at which the a/c is rotated into the takeoff attitude)

Technically V1 could be higher than Vr however as previously stated once in the flying config rejecting the takeoff in most cases would be far more hazadous than continuing.

I hope this helps.

Trash is better than pax................

[This message has been edited by Trash Hauler (edited 11 September 2000).]
Old 13th Sep 2000, 18:37
  #13 (permalink)  
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V1 is your decision speed as to whether you will continue or abort the takeoff. No point in deciding whether to continue or abort after you're airborne.

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