747-400 Vref30 V2 relationship?

[StopStart]

Whilst I appreciate one is comparing apples and aardvarks here, does anyone know the relationship/common ground between VRef30 and V2 on the 744?

Manoeuvre speeds are referenced to "VRef30 +" however it seems V2 is used in place of VRef30 in practice. I'm just interested to know how and why it is that these two speeds, referencing two different phases of flight, should be the same (or as close as makes no difference).

Coincidence or are they linked through the Vmca relationship?

[HazelNuts39]

If the speeds happen to be the same , on what basis do you state "it seems V2 is used in place of VRef30 in practice"?

[StopStart]

Because we do.

[Wizofoz]

You're really not explaining yourself- WHEN are you using V2 in place of VREF 30?

VREF 30 is only relevant when AT flap 30. It is 1.3VS at flap 30.

V2 will change with a number of Take Off parameters. It will be different for a flaps 5 TO then for a flaps 15.

They are used for different things at different times- HOW do you use one as the other?

[parabellum]

What Wizofoz says. If you are using Vref30 you are in a landing config and if you are using V2 then you are most likely in a TO config.

[noip]

Couple of points ...

WizofOz is referring to a 767 as far as takeoff flap is concerned.
V2 varies with factors such as need for tailscrape avoidance, angle of bank requirement after takeoff etc - for example the departure from Cairns rwy 15 required an addition to V2 to allow extra bank angle.

The only time it might come into some sort of relevance is in a go-around situation, where at Vref+5 you can retract to F20 (with restricted bank angle) ...

[StopStart]

Apologies, this question is directed at other 747-400 users, hence the title.

The minimum manoeuvre speeds associated with various flap settings are referenced to margins above VREF30 e.g. the Flap UP bug speed is VREF30+80.
On departure, VREF30 is the same as (or as near as makes no difference) to V2 and as such V2 is used as the reference to which additions are made (if operating non VNAV or Speed Intervening). This is because V2 is readily available through the FMC and is already dialled up on the MCP.

I am merely interested to know why it is that V2 and VREF30 are so very similar. Is it just a massive cosmic coincidence especially given that V2 applies at take off with Flap 20 and VREF30 is for landing with Flap 30?

I suspect it's down to the margins above Vmca that must be attained for a minimum V2 and a minimum Vref....

[SMOC]

I think what needs to be added here is for the most part on the 744 F20 - V2 is the same as Vref 30 for all weights except below something like 230T, and most times F10 - V2 is 5 knots greater.

So it appears as if there is a relationship such as.

F20 V2 = Vref 30 = F10 V2+5

However an accurate computer generated RTOW V2 shows that these figures are not the same and the generic weight vs V2 vs Vref chart is conservatively rounded making it appear that the above statement is true when it's not.

The -8 V2 and Vref are so different that the generic chart is not possible and in fact we have begun removing this same chart from our 744s, as it's somewhat of a carryover from the classic where a simple chart was referred to for V2 & Vref.

[InSoMnIaC]

Stopstart i think you need to fly Airbus. They like to over complicate things. Boeing keeps it simple vref is vref v2 is v2

The only thing in common the 2 have is that they are both based on Vs.

In the absence of improved climb V2 = 1.2vs whereas vref is 1.3vs (for their reepective flap settings.)

1.2 x a higher speed (flap20 Vs ) is close to 1.3 x - a lower speed (f30 vs)

[JammedStab]

Originally Posted by StopStart

On departure, VREF30 is the same as (or as near as makes no difference) to V2 and as such V2 is used as the reference to which additions are made (if operating non VNAV or Speed Intervening). This is because V2 is readily available through the FMC and is already dialled up on the MCP.

I am merely interested to know why it is that V2 and VREF30 are so very similar. Is it just a massive cosmic coincidence especially given that V2 applies at take off with Flap 20 and VREF30 is for landing with Flap 30?

I suspect it's down to the margins above Vmca that must be attained for a minimum V2 and a minimum Vref....

I think V2 does have to be a certain amount above VMCA by 10% I believe. Also, I think it is at lower weights in general that approach speed can be limited by VMCL which is a different flap setting. So if this is correct(and it is subject to confirmation), it might explain why these two numbers can be close together. Our performance planning computer does show Vref30 on the same page as the takeoff V-speeds and I have noticed a difference of several knots at times.

So, after an engine failure during takeoff without VNAV(or in the case which has happened to me in the sim where we forgot to enter a cruise altitude for our planned failure and the command bars after a while started directing a descent), you can go to FLCH and then set the speed to Vref+100(pretty much V2+100) which was what VNAV was supposed to target anyways.

But how do you know what V2/Vref is. It might be best to memorize it as the speed window closes on the MCP soon after departure at about the same time that all the V-speeds in the FMC disappear. Or maybe try to be somewhat familiar with the speeds. If we are heavy, maybe V2 is 185, medium 165, light 145. Add 100 knots to that and you have your target climb speed.

[HazelNuts39]

Insomniac is correct in that V2, Vref and the flaps up bug speed are all three based on the Vs in the respective configuration.

At low weight, altitude and temperature V2 and/or Vref can be limited by minimum control speeds, i.e. V2 is not less than 1.1 VMCA and Vref is not less than VMCL. Adding 80 kts in that case results in a conservative stall margin flaps up. Since 80 kts is based on the ratio of stall speeds flaps30/flapsup at high weight it is conservative at low weight.

[Wizofoz]

Hazelnuts et al-

MINIMUM V2 is based on Vs, but that isn't always the one that is used.

It seems that the point is that Stopstarts operation uses a fairly simple definition of V2.

As Jammedstab pointed out, it CAN be different, particularly if using a performance optimising tool.

In that case, V2 can't be less than Vr, and both can be considerably more than Vref 30 in order to utilise excess runway to improve RTOW.

[HazelNuts39]

Originally Posted by Wizofoz

It seems that the point is that Stopstarts operation uses a fairly simple definition of V2.

Yes, it would seem so. In his second post he clarifies that he is using V2 for flaps 20, he should have said that in his first post. I would think that optimisation of takeoff weight limitations (i.e. improved climb performance for WAT-limit or obstacle clearance) would take you first to a reduced takeoff flap setting, and then to V2 speeds greater than 1.2 Vs.

From another post I gather that flaps 20 is the discontinued approach flap setting for landing with flaps 30. According to the regulation the stall speed in the discontinued approach configuration must not exceed 110% of the stall speed for the related landing configuration. That means that minimum V2(flaps20) will not exceed 101.5% of Vref30 (1.1*1.2/1.3*100%), except where limited by Vmca.

To cut a long story short, it is not entirely coincidental that in many conditions V2(flap20) is close to Vref30.

[Intruder]

Vref is a good crosscheck for V2 calculation, since they are very close -- Vref30 for F20 T/O, or Vref25 for F10 T/O. For an emergency return, the V2 can be used in a pinch if the PM can't get the FMS set up in time for the landing.

The Vref can be gotten from either the QRH or Preformance Manual, or the FMS Approach Ref page.