I find it rather great that PMDG's FMS is nice enough to calculate Vspeeds for me, because it means I don't have to do math. However, those Vspeeds seem a bit...Optimistic. On almost every takeoff in the 73-8, if I were to reject the takeoff just before V1, there's absolutely no way I'm getting that thing stopped before the end of the runway. There have been times when I've hit V1 and then Vr  less than one second later, and the threshold is disappearing under my nose just as I lift off. Is there some step I'm missing, or are the FMS calculations off?

 

The speeds that the box has are balanced field numbers. All that means is the distance it takes to accelerate to V1 and reject is the same distance as it would take to accelerate to V1 and reach 35 feet above the runway. Both are based on single engine performance...i.e. engine failure at V1 - decision to go - 35' above the runway: Same distance as if you were to decide to reject the takeoff. The point you stop would be the same as when you reached 35' above the runway.

Using a performance tool, your speeds may vary to a pretty high degree. If you're taking off on an 11,000' runway, you may only actually need 6,000' of actual pavement. Using a performance calculator, your V1 may increase. Our V speeds are generally quite a bit higher than the balanced field numbers...just the other day, we had a V1 that was up at 158, while the balanced field numbers in the box were way down at 140-something. Conversely, if the runway is contaminated, your V1 may be less than the basic dry numbers that the box spits out. You can go into page 2 and select WET and you'll see your V1 drop. That's just because your stopping power may be compromised, so you need to decide whether to GO or STOP earlier.

20 minutes ago, eslader said:

There have been times when I've hit V1 and then Vr  less than one second later

When in doubt about runway distance, you can always use flaps 1, 5, 10, 15, or even 25. You'll see those speeds change quite dramatically based on flap settings. You can also go to page 2 of the Takeoff performance, and in R1 select WET. That'll give you a lower V1, VR and V2.

I recently purchased the Virtual Performance Tool (and of course PMDG finally announced EFB is ready right after I bought it😆)  I have actually noticed that once you get your assumed temp from VPT, the default PMDG V-speeds match up perfectly every time (VR and V2).  V1 will vary of course.  Anyway, it was interesting to see that after wondering how accurate the PMDG numbers were....

Basically it's because the PMDG FMC has no idea how long your runway is or its condition.  It's just spitting out appropriate numbers based on the flap and thrust selection you made.

In reality off a long dry runway, V1 and VR will be almost equal.  Short, wet or especially contaminated runway, you'll see a potentially large split.  You would also likely be using a higher thrust setting, and/or a greater flap setting, than in your example (more flaps gets you off the ground sooner but your second segment climb gradient suffers so it depends if the limiting factor is runway length or obstacles in the 2nd segment.)

Basically, the FMC isn't calculating anywhere near full takeoff perf data for you.  The PMDG box just spits out some V speeds but first you have to determine the thrust and flaps you'll need.  There are 3rd party tools that'll do it, the PMDG tablet will do it when released, or you can just take a guess.

10 hours ago, eslader said:

Is there some step I'm missing

The EFB 🙂. Luckily PMDG knows the release date already 

https://forums.x-plane.org/index.php?/files/file/38471-vas-airplane-toolbox/ Honestly I use this and works quite well.

13 hours ago, eslader said:

There have been times when I've hit V1 and then Vr 

Even using an external performance calculator (Topcat) this is usually the case for me in narrow bodied aircraft unless the runway is wet or very short.  Widebodies (especially when heavily loaded) have a better chance of v1 and vr being further apart.  

It’s just the reality that in a narrow body with a long dry runway you’ll normally take off with lots of runway to spare.

3 hours ago, carlanthony24 said:

https://forums.x-plane.org/index.php?/files/file/38471-vas-airplane-toolbox/ Honestly I use this and works quite well.

I use this too, but I've found lately it's a little buggy.  For example, I cannot set flaps or engine bleed settings anymore.

You might to look at this website: Virtual Performance Tool

They are about to introduce Ver 1.0.0, which is take it is their full release version.  They recently released a YouTube video on how the takeoff caluclations work: 

 

In real life, you would very surprised by how little runway a reject takeoff at V1 actually requires.  On field length takeoff, one where balanced field length required is equal to the runway available, it would not be unusual to have 2000' or less of runway remaining.  Modern brakes are highly effective in stopping the airplane. 

Rich Boll

39 minutes ago, richjb2 said:

In real life, you would very surprised by how little runway a reject takeoff at V1 actually requires

Truth.  Until you've seen a high speed abort, you don't really have an understanding of what the airplane is capable of.  It's not comfortable at all... but impressive!

14 hours ago, Stearmandriver said:

Basically it's because the PMDG FMC has no idea how long your runway is or its condition.  It's just spitting out appropriate numbers based on the flap and thrust selection you made.

In reality off a long dry runway, V1 and VR will be almost equal.  Short, wet or especially contaminated runway, you'll see a potentially large split.  You would also likely be using a higher thrust setting, and/or a greater flap setting, than in your example (more flaps gets you off the ground sooner but your second segment climb gradient suffers so it depends if the limiting factor is runway length or obstacles in the 2nd segment.)

Basically, the FMC isn't calculating anywhere near full takeoff perf data for you.  The PMDG box just spits out some V speeds but first you have to determine the thrust and flaps you'll need.  There are 3rd party tools that'll do it, the PMDG tablet will do it when released, or you can just take a guess.

On the PMDG 737 FMC, I set the takeoff runway, so the FMC knows the length before it gives me the V speeds, plus flap settings and on page 2 of the Takeoff Ref I can set wet or dry runway conditions. 

1 hour ago, Bobsk8 said:

On the PMDG 737 FMC, I set the takeoff runway, so the FMC knows the length before it gives me the V speeds, plus flap settings and on page 2 of the Takeoff Ref I can set wet or dry runway conditions. 

Wet or dry will make a small difference (not really a realistic one) in the V1 - VR split, but setting the takeoff runway makes no difference at all.  Runway length isn't taken into account.

In reality, hardly any 737 FMCs compute takeoff speeds at all; this dialog in the PMDG can be treated as a simmism.  Much older versions of FMC software used to calculate balanced field speeds but basically no one used them anyway as specific runway analysis for conditions was always a safer option.

2 hours ago, Stearmandriver said:

Truth.  Until you've seen a high speed abort, you don't really have an understanding of what the airplane is capable of.  It's not comfortable at all... but impressive!

This is why we set autobrake to max in the takeoff checklist I suppose...

I have never practiced an aborted takeoff in the 737 at V1.  I'll have to try that sometime.  Maybe see if the brakes smoke with the PMDG.

9 minutes ago, Mace said:

This is why we set autobrake to max in the takeoff checklist I suppose...

I have never practiced an aborted takeoff in the 737 at V1.  I'll have to try that sometime.  Maybe see if the brakes smoke with the PMDG.

Not Max for takeoff, but RTO.  Max is for landings only of course, but it's also not even close to the same braking force.  RTO (as antiskid will allow) is the same as max manual - 3,000psi to each brake.  It's like an arrested landing from 130kts... and yep, often results in melted/fused brakes, melted fuse plugs etc.  But you stop.  Dear god do you stop lol.

Edited October 10, 20232 yr by Stearmandriver

1 minute ago, Stearmandriver said:

Not Max for takeoff, but RTO.  Max is for landings only of course, but it's also not even close to the same braking force.  RTO (as antiskid will allow) is the same as max manual - 3,000psi to each brake.  It's like an arrested landing from 130kts... and yep, often results in melted/fused brakes, melted fuse plugs etc.  But you stop.  Dear god do you stop lol.

Oops, I've been flying A310 lately, they have "MAX" and not RTO, forgot about 737 jargon.  Look how rusty I get not flying 737 for two months.