Range for V1?

[Will]

When I run takeoff performance in TOPCAT, it returns a range of values for V1, like in the picture below. (Please ignore the maximum performance line at the top, and concentrate instead on the assumed temperature lines below.) You'll see that with 49C, V1 ranges between 122 kts and 125 kts.

   


I don't understand why that would be a range. Is the pilot supposed to know which value within the range is best for a given takeoff?

[Hardy Heinlin]

Maybe headwind and slope determine the final V1 within this range?

[Will]

I only showed a clipping from the main screen... Headwind is factored in, as is slope. In this case, headwind was 3 kts and slope was -0.1%. As are runway condition, altimeter setting, obstacles, and several other variables.

For what it's worth, I checked the TOPCAT manual and it only says that this column refers to the "V1 speed". The manual doesn't even acknowledge that the range is present. I guess it assumes we know what to do with it. (I don't.)

[FlyItLikeYouStoleIt]

V1 is really two definitions, a minimum can-continue speed after an engine failure, and a maximum begin-to-stop speed.

Code of Federal Regulations

Perhaps that is why?

[Jeroen Hoppenbrouwers]

If the range widens when you add more thrust on the same runway, that may be an indication?

[Mariano]

When performance permits or when "unbalancing" the field is allowed (not only by current conditions but by company policy), you might get a range of V1 (some company software applications are set to always output only one single V1 value).

Selecting a lower V1 from the allowable range, increases the stop margin, but decreases obstacle clearance and vice-versa.

This is because if you reject at a lower V1, you have used less runway up to that point, thus you will have more runway to stop. If, at that same lower V1 you continue the takeoff with one less engine, it will take a longer distance to accelerate to VR and V2, lowering the vertical clearance from obstacles on the departure path (to not lower than 35 ft. on a dry runway or 15 ft. on wet/contaminated runway, and 50 ft. in a turn (FAA)). I believe that EASA (JAA?) increases the obstacle clearance by distance traveled from the departure end of runway on a percentage basis.

So, if your company lets you see the range of V1 in their approved calculation method, and you are using a long runway with plenty of stop margin but with considerable obstacles on the departure path, you might benefit from using a higher V1. The opposite goes for a short runway with a low stop margin and no considerable obstacles.

I know that some companies always output the lower V1 value favoring stopping performance, and some applications actually select a V1 value that considers both stopping performance and obstacle clearance.

United (on the 744 at least) used to never unbalance the field, which added a nice, conservative safety margin to many runways (at the expense of payload sometimes). Kalitta used to publish only the lowest V1 allowed.

Regards,

Mariano

[Will]

Very helpful explanation Mariano, thanks.

[MFarhadi]

When performance permits or when "unbalancing" the field is allowed (not only by current conditions but by company policy)

On A300-600, A310, and A330, my previous company used to always lean on "the configuration and speeds that maximized FLEX TEMP while keeping V1 and VR as close as possible." When we were light, relative to the runway length which was 99% of the time there with so many ~4KM runways, you could see a single number for all the three speeds e.g. 151/51/51 for V1/R/2. I'd personally like to see some flexibility in regards to V1 in order to capitalize on 2nd segment climb performance when limited to, and field length (accelerated stop distance) when that's the limiting factor. I never really delved that deep into Airbus' SmartCockpit software, but we were told that the software tries to optimize for any scenario when possible.

[Mariano]

I assume that most operators would favor using the lowest V1 allowed in order to maximize stopping performance, thereby decreasing the inherent risks involved with RTOs, seeing as we perform them only twice per year in a controlled environment, in which we also are expecting them.

As a rough approximation, there are a hidden/unpublished 100 meters of additional stop margin (or also less distance to VR and V2) per 10C difference between assumed temperature and OAT, due to density effects.

So, when using the lower V1 in the range together with ATM, we still have a small additional obstacle clearance that can offset some of the lost obstacle clearance from using this lower V1 value. That being said, some extra obstacle clearance is still available with a higher V1, albeit not as much.

I have never seen VR and V2 at the same value. Certification-wise, V1 cannot be lower than VMCG, and cannot be higher than VR, VR cannot be lower than 1.05 VMCA, and must result in a VLOFF of 1.10 VMU (all engines), or 1.05 VMU (engine out), and V2 cannot be lower than 1.13 VS1G, or lower than 1.10 VMCA.

In your case, it seems that the software was increasing VR to 1.10 VMCA to match V2, and then increasing V1 to match VR; very neat!

Regards,

Mariano

[DougSnow]

I assume that most operators would favor using the lowest V1 allowed in order to maximize stopping performance,

We'll do that on a wet or contaminated runway, but on a Dry runway, you get an Optimum V1, which is roughly 1/2 way between the Lowest V1 and the highest V1, or whatever comes out of the Boeing software.