Vr

[Steamhead]

Target Theshhold Speed (TTS) is calculated as 1.3 Times the stalling speed in the configeration at the time.
How is Vr calculated? Obviously the lee way between stall and Vr will be less than that used for TTS, because the main wheels are still on the ground and the aircraft is still accelerating.
The Question is raised because of the appalling accident recently at Brum, some witnesses have said the aircraft dropped a wing at liftoff so could the EFIS have shown the wrong Vr because of a fault or wrong programming.

[HotDog]

B.C.A.R. determine VR by one of four criteria:
1./ 1.05xVMCA
2./ 1.10x the minimum power off stalling speed VMS
3./ 1.1 or 1.05x the minimum unstick speed VMU
4./ A speed which allows the greater of 1.1 VMCA or 1.2 VMS1 to be achieved at the screen height of 35ft with one eng. inop.

The above from Perf.A notes.

[john_tullamarine]

Apart from the odd rule requirements which have to be addressed, from a practical point of view VR is determined by the manufacturer so that, having had a failure at V1, the aircraft hits V2 going through 35 feet.

[747FOCAL]

That is not entirely correct. That was the case in the old days. Now as we get more and more history and gain confidence in our safety margins Vr is calculated at much higher rates than is necessary to meet FAR 25. Much like and for the same reason that CDA approaches are in effect over in the EU. If he had said Vrmin that would be correct, but Boeing and Airbus are fast moving toward Vrmax at this time. Airbus was probably the front runner in that.

[mutt]

747FOCAL

VR is calculated exactly as John_Tullamarine explained.(FAR25.107 e)

Your statement Vr is calculated at much higher rates than is necessary to meet FAR 25, relates to the use of improved climb/overspeed but even then the relationship between V1 /VR /V2 remains the same.


Mutt <img src="smile.gif" border="0">

(J_T, did I get it right this time??? <img src="smile.gif" border="0">

[ 12 January 2002: Message edited by: mutt ]</p>

[john_tullamarine]

Now you both have me scratching my head .. just a little. How ought I interpret your joint use of the term "rate" ? ... and, surely, for an overspeed V2 schedule (improved performance or whatever term might be in vogue here and there), then the associated VR is still basically scheduled so that the aircraft, having had a failure at the scheduled V1, still hits the scheduled higher V2 around 35 feet ? To do otherwise would infer that the pitch rotation rate also has to be rescheduled ?

If we are going to use an overspeed schedule then the V2min schedule, naturally enough, has no particular significance for the particular takeoff.

If things are not as I imagine, please do improve my knowledge on these matters and lift the dreadful veil of confusion from my eyes ..

G'day, Mutt, good to see that you are still in the land of the living ..

[ 12 January 2002: Message edited by: john_tullamarine ]</p>

[john_tullamarine]

.. don't be like that, mate .... :) :)

[2high]

I have only ever heard of "overspead takeoff" once about 15 years ago from the now defunct Affrettair (Zimbabwe based) who were flying DC8's - does any other airline use this expression??!!!

[john_tullamarine]

2high,

I think that the Boeing fraternity tend to use the "overspeed" term .. descriptive as the actual V2 scheduled exceeds the minimun permissible V2. "Improved Performance" tends to find favour with the Airbus mob. Again, quite descriptive as the main reason for doing it is to gain an improvement in initial climb gradient.

A long time since I have looked at a Fokker AFM but my hazy recollection is that they also inclined to the overspeed term.

Doesn't matter one iota which term you prefer ..

[ 12 January 2002: Message edited by: john_tullamarine ]</p>