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phllax
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Joined: Wed Jan 11, 2006 6:53 am

V Ref Speed Question

Sun Jun 21, 2020 10:39 pm

Hello,

Is it possible in a STOL or any commercial airplane for V1 to be after Vr and V2 if the runway is long enough and the headwind component is strong enough? One would think it could be possible on DEN's 34L, which is 16,000 feet long?

Also, for the pro's out there, if coming in to land do you ever bump the landing Vref speed by 5-10 knots to give you a cushion knowing there may be gusty or windshear conditions on final, so that it's one less thing to deal with at such a critical phase of flight?
 
JustAnFO
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Joined: Tue Jul 10, 2012 5:47 pm

Re: V Ref Speed Question

Sun Jun 21, 2020 11:52 pm

#1 No. FAR certification regs say that V1 must always be less than or equal to Vr.

#2 Yes, though generally at an airline, it's not a "technique", but a defined procedure for calculating Vtarget. It varies with type and operator policies.


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GalaxyFlyer
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Joined: Fri Jan 01, 2016 4:44 am

Re: V Ref Speed Question

Sun Jun 21, 2020 11:55 pm

By definition, V1 can not exceed Vr; however, refusal speed (accelerate-stop) may exceed Vr but it’s not used as a performance number as you’d be airborne. There are times for unbalanced field length calculation that you need to know the refusal speed.
 
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zeke
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Joined: Thu Dec 14, 2006 1:42 pm

Re: V Ref Speed Question

Mon Jun 22, 2020 12:14 am

phllax wrote:
Hello,

Is it possible in a STOL or any commercial airplane for V1 to be after Vr and V2 if the runway is long enough and the headwind component is strong enough? One would think it could be possible on DEN's 34L, which is 16,000 feet long?

Also, for the pro's out there, if coming in to land do you ever bump the landing Vref speed by 5-10 knots to give you a cushion knowing there may be gusty or windshear conditions on final, so that it's one less thing to deal with at such a critical phase of flight?



It is possible for V1 to be technically after Vr, however to meet regulatory requirements for Vr to be >= V1 they will increase those speeds. The performance charts or software will make reference to that relationship when generating the speeds.
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Starlionblue
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Re: V Ref Speed Question

Mon Jun 22, 2020 12:16 am

To add to the increase in Vref (Vapp on Airbus) in gusty conditions, this is also tied in to autothrust. It takes a moment for autothrust to react to a speed excursion. Having a 3-5 knot buffer means if you have a downward speed excursion by the time you are down to "actual" Vapp, autothrust has reacted already.
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mmo
Posts: 2054
Joined: Thu Apr 18, 2013 3:04 pm

Re: V Ref Speed Question

Mon Jun 22, 2020 9:45 am

Starlionblue wrote:
To add to the increase in Vref (Vapp on Airbus) in gusty conditions, this is also tied in to autothrust. It takes a moment for autothrust to react to a speed excursion. Having a 3-5 knot buffer means if you have a downward speed excursion by the time you are down to "actual" Vapp, autothrust has reacted already.


Just an addition on Boeings if you have the autothrottles engaged there is no need to add the gust component to the Vref speeds. However, the FCTM also advises, as a technique, you also use a reduced landing flap configuration. I flew for SQ on the 744 and their preferred landing flap configuration was Flaps 30, so in this case, landing flaps would be Flaps 25. The reason is with 30, the additional drag makes the aircraft more susceptible to the gusts and the autothrottles will have a tendency to lag. With the reduced drag with 25, the autothrottles can handle the gusts with no problem.
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