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.