Hello,please shed a light on this: when I have a calculated V2 speed, which is 150kts let's say, why I need to make a correction adding 20kts more and express it with '+20', and not add it in the V2 speed and make it from 150kts to 170kts. Perhaps my question might sound stupid to the specialists here but I would like to clarify it once and for all.Thank you

It depends a bit on the actual plane in use, but your V2 sets a number for a safe speed (means a reference value), e. g. single engine climb. So that +10 or +20 is a margin you choose or some manufacturer's docs dictate. You don't want to get lower than V2, so you aim slightly higher.You see a similar thing when landing, as you calculate or read your Vref from some charts (again, that's a reference speed value) and then add +5. If your headwind component is rather high, you add more to avoid getting lower than Vref in any case. As said, it's plane dependent and therefore very much bound to the standard procedures that plane is flown at.

Edited February 9, 201214 yr by CoolP

Because the V2 compoment is calculated by the FMC and posted on the PFD, while the +20 margin is fixed regardless of gross weight.Same on the way down WRT to Vref.Cheers,- jahman.

It is about the PMDG MD11. After preparing the FMC I get the calculated V speeds and I see them also on the PFD. Now during the take off roll there are the speed callings, V1, Vr, V2. If I decide at V2 to add +20 kts, where should I insert these 20 kts? Is it via the FMC or adding mannualy with the throttles?

I think the main help when discussing those things is the difference of a reference value and the one you set up with adding a margin to it. The short answer on the reference speeds the FMC gives you is 'leave them as they are'. The rw folks may add some more or less significant reasons to change them, but we sim guys can live with a proper FMC calculated value in almost any case.So, there's V2 on the MD11. What's that thing good for? Well, one purpose is to reference the speed you need to fly at when one of your engines has failed, is on fire and so one right after V1 (another reference) or takeoff.In short, it's thrust value isn't were it should be. That's a calculated value taking into account the weight of the plane and the available thrust of the remaining engines. Some more factors come in, but I think you get the picture.Being lower than that speed could mean that your engines aren't able to produce enough thrust against the increased drag in your config and with a rather high pitch angle. You won't be able to climb for example and if you just took off and lost an engine, you may indeed want to climb right now.So that reference pointer reminds you about not to raise the nose even more (you will loose speed then), but to fly at that speed and keep the two engines at max. cont. thrust until reaching a safe altitude. Therefore, altering that pointer on the PFD and overriding the FMC calculated speed value is not recommended. You would loose some awareness on the touchy speed case in that situation.Another example on the Vref. That's the calculated speed you want to land at. It's not the speed your aircraft would stall and it's not the speed you may fly at the approach, but it's your reference value again. So in a strong headwind, you would want to stay above that reference value in case the wind 'goes away' and leaves you without the headwind component (which will decrease your IAS).Aiming at that speed and calculating it again is plane dependant. I think it usually adds 30% to the real stall speed of the plane, so there's a margin. However, you don't want to use that margin on landing as the slower speed will increase your pitch angle, therefore, and especially on the MD-11, reducing the clearance your tail section has when touching down and compressing the main gear for example.Also, that reference speed is one with a safe angle of attack and also an appropriate flight deck angle, so you can still see where you are landing, which is nice of course.So whatever you add to those reference speeds, which serve certain purposes, it depends on the conditions (headwind, tailwind and so on, even icing), but the reference doesn't change and the pointers therefore help you to determine your current margin when flying.If you want to fly V2 + 10, leave the FMC value on V2 and enter the V2 + 10 value in the speed window of the autopilot panel.

What a great help CoolP! Thank you so much.Please allow me another question:

If you want to fly V2 + 10, leave the FMC value on V2 and enter the V2 + 10 value in the speed window of the autopilot panel

This is crystal clear. In case that AP is engaged and it reads the speeds only from the FMC then what?Thank you once again

You mean when the FCP has a blank speed window and FMS Speed is active? Depends on the situation.V2 + 10 is the max speed it will aim for until reaching the eng out altitude. V2 itself is the lowest speed it will aim for, that's why this entry is important. It will pick the right speeds in that range of V2 up to V2 + 10 depending on the actual speed of the plane, no entry needed.So the FMS logics always aim for a fast gain of altitude in the first section of your takeoff path. The strategic target therefore is to clear obstacles fast and allow an altitude safety margin to establish.Reaching eng out altitude, it will try to gain speed until V3 is established, then you may climb some more which should be easier as you've retracted the flaps and slats, therefore drag is reduced significantly.I just saw something wrong up there. Correcting it just in case.

So that reference pointer reminds you about not to raise the nose even more (you will loose speed then), but to fly at that speed and keep the two engines at max. cont. thrust takeoff thrust until reaching a safe altitude.

The thrust later gets reduced to MCT, but the first thrust target should be that takeoff value until you are at a safe height above the ground.

Edited February 10, 201214 yr by CoolP

I printed already your helpful reply :-)Thank you. I really appreciated

Panos, the docs of the MD-11 are great. I recommend printing them to a certain extend before you are using my conclusions. So maybe navigate to PMDGMD11operating manual and look at the FCOM or the systems docs on the aspects of the automated flight. A way better reference than forum posts in my eyes.

Yes you are right. I did already printed the 500+ .pdf file and I started deepen into the systems of this great airplane