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VREF40 on takeoff?

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Hi,

The other day when I was watching a video on Youtube of someone showing a bit of the takeoff calculations for a 737 using an app on their iPad, i saw the speeds for V1, Vr, V2 and also VREF40 which was a value between Vr and V2 (I can't remember the exact figure). So I was wondering why you'd want to know the VREF40 for takeoff since i thought VREF only had an affect on landing.

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Vref can stand for two different things. To most pilots, Vref means Reference Speed (for landing), but the military also use Vref to mean Refusal Speed (the maximum speed during your take off roll from which the aircraft can still stop within the available remaining runway length at the density altitude for its weight and configuration, including external stores attached to the thing). Military pilots use that in combination with Vrot (the rotation speed) and Vs1 (the speed at which the aircraft will be controllable in a particular configuration).  I'm guessing the differentiation will be particularly related to having munitions slung under the wings, which would obviously be a consideration with things which might explode or cause control problems when hanging off your aeroplane as it brakes or staggers into the air on one engine.

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I'm not familiar with the 737 in particular but certainly on the 757/767 (and the 747 though in a different way) all the flap speeds are based off Vref30.

Thus as part of the takeoff calculation in the 757/767 you would note Vref30 and bug Vref30 + 40 (Flap 15 manoeuvring speed and Vref30 + 80 (Flap 1 manoeuvring speed).

Thus you would retract from 15 to 5 once above the Ref 30+40 bug, from 5 to 1 at ref30 +60 (not bugged) and 1 to up at the ref30+80 bug (all if my memory serves me correctly).

It would seem logical that the 737 would be similar. 

On the 747 the other useful trick is that Vref30 is roughly equivalent to V2 so quite a useful crosscheck.

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28 minutes ago, skelsey said:

I'm not familiar with the 737 in particular but certainly on the 757/767 (and the 747 though in a different way) all the flap speeds are based off Vref30.

I'm guessing it's because on the 737, you can actually set the flaps to 40, whereas with the rest, the most you can do is flaps 30. That said, I don't know how common flaps 40 gets used on the 737. My understanding is it's not used all that often, but I don't fly it, so I wouldn't know.

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27 minutes ago, Captain Kevin said:

I'm guessing it's because on the 737, you can actually set the flaps to 40, whereas with the rest, the most you can do is flaps 30. That said, I don't know how common flaps 40 gets used on the 737. My understanding is it's not used all that often, but I don't fly it, so I wouldn't know.

Absolutely -- my assumption (in the very roundabout way that I wrote that post!) is that the 737 flap speeds are probably based off Vref40.

The 767 is a "Vref30" aeroplane -- there is only one Vref, Vref30, and all the speeds are always based off Vref30 whether you are landing Flap 30 or Flap 25.

The 747 is a bit different in that Vref is landing configuration dependent -- so your flap speeds are either Vref25 + 20/40/60/80/100 or Vref30 + 20/40/60/80/100. However, on takeoff as I say, Vref30 in the 744 is roughly equivalent to V2, and as far as I know on takeoff the flap retraction speeds displayed on the PFD are based on increments to Vref30.

I would imagine as I say that the 737 flap retraction schedule is built upon increments to Vref40, hence why it is displayed as part of the takeoff performance calculation.

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