V2 and Runway Change

[Mariano]

Hi,

Perhaps someone can shed some light on this.

As per regulations (CFR Chapter 14, FAR Part 25) for airplanes certified after 1986, V2 may not be less than 1.13 Vs1-g or less than 1.10 Vmca.

The other day, we had a runway change during taxi out. After receiving performance numbers for the new runway, we noticed that the new V2 was quite different than the V2 we had set for the original runway.

Environmentally, nothing had change but wind direction and speed (hence the runway change).

We were a bit confused as to the reason for this. We checked the ACARS numbers again and also cross-checked them with the TLR, which agreed.

It was an assumed temperature takeoff, with the same assumed temperature as the one originally entered (so our V speeds were calculated for full takeoff thrust (thus Vmcg and Vmca-protected), negating any V2 corrections for the >=1.10 Vmca requirement (since Vmcg was already protected)).

Flaps setting also remained unchanged.

Considering all of the above, why would V2 change for a runway change and nothing else? New runway was only 1,000 ft. shorter than the one for which we had originally planned.

Not that it makes any difference, but terrain or obstacles weren't an issue.

I'm sure that I'm missing something very, very basic here.

Thanks and best regards,

Mariano

[Hardy Heinlin]

Hi,

was the taxi distance to the new runway longer than that to the previous runway? Was your expected gross weight different at the new runway?

Did the pressure altitude change? Was it different at the elevation of the new runway? Or was it different because your taxi time was longer and the weather changed during this time?

However, I don't think that such a nuance would make any difference greater than 1 knot ...


Cheers,

|-|ardy

[Mariano]

Taxi distance was just a bit longer, but negligible on fuel burn. QNH and OAT did not change.

As you said, none of these should change V2 by no more than one knot.

I just briefly looked at some of our TLRs and at some other carries' runway tables, and our TLRs show consistent V2 differences whenever assumed temperature changes, even by one degree, but nowhere near as much as we saw that day, which stopped us on our tracks. Other carriers' takeoff performance runway tables do not have V2 changing with assumed temperature changes, as expected.

I am beginning to believe that this might be due to a particular method our vendor must use to calculate takeoff performance into which I still need to do further research, apparently.

The V2=>1.10 Vmca requirement seems to be the most likely culprit here, but shouldn't matter on an assumed temperature method takeoff (from D-TO only, not D-TO1 or D-TO2, of course), especially not for the same assumed temperature value.

Best regards,

Mariano

[dhob]

Was the V2 from AeroData different than the FMC calculated V2 speed?

Takeoff thrust has no bearing on V2. For a given weight, flap setting, and pressure altitude, V2 remains constant across TO, TO1, and TO2, including any ATM used. This is evinced by the 747-400 Flight Planning and Performance manual takeoff speed tables, and can be verified by the FMC as well. As you noted, V2 must be the highest of the following:
- 1.13*VS1g,
- 1.10*VMCA

We use AeroData as well, and I'm curious as to how you know terrain and obstacles were not a factor? Does your AeroData presentation (FDP or TLR) list the performance limiting weight for the takeoff (Field, Climb, Brake Energy, Obstacle or Tire Speed)?

It could be that the new runway had an obstacle or climb limit that required the use of improved climb speeds. In general, the benefits of improved climb speeds are increased second segment gradient limit weight, clearing a distance obstacle, or reduced thrust if performance is climb or obstacle limited. This is done by using excess runway to increase V2 to optimize climb performance. This is an option for AeroData to use, however it is not identified in anyway on the FDP or TLR at our company. We would have no knowledge improved climb V-speeds were used except by the difference between AeroData speeds and FMC speeds.

For any significant change to V2 without a change to gross weight and flap setting, improved climb would be my first guess.

[IefCooreman]

Anything that allows for an increase in V1, VR will result in a increase in V2, if the performance tool allows unbalanced TO performance. Improved climb comes to mind, stopways will allow V1 increase, etc... V2 is not really a calculated speed, it is more a selected speed based on VR.

[Mariano]

I too immediately thought of improved climb, but vaguely recalled a ground school instructor stating that our operation does not utilize improved climb. I was on the jumpseat during this IOE flight, so I asked the check airman about improved climb. He too stated that we do not use it, although both of us certainly could be wrong.

V1 and Vr were identical to the original ones, which tells me improved climb was not used, although at that moment, I could not come up with any other explanation.

There is an improved climb section in our FPPM (which is quite Boeing-generic), but it is not mentioned anywhere else.

Engine failure procedure was to simply track runway heading over quite featureless terrain, just as it was for the original runway. Perhaps there was a significant temporary man-made obstacle, although it would/should have been included in the "RUNWAY INFO" section.

Our TLRs/FDP printouts do not display limiting factor on the takeoff section, only on the landing section, which would certainly clear all this up.

Thanks for the input, gentlemen. I'm hoping to visit the Dispatch offices soon, during which time I will inquire about this. Should I find the answer, I will post it here.

Best regards,

Mariano

[Will]

Did the two runways have the same length, stopway, and obstacle clearance rules?

It's true that minimum V2 is Vsr x 1.13 (changed in 2002, from Vs x 1.2). V2 is limited by the engine-out. accelerate-go distance, which is the distance needed to accelerate to V1, have one engine fail at least one second before V1, continue the takeoff, liftoff, and reach a point 35 feet above the runway surface at V2.

So if the engine-out, accelerate-go distance is limiting for a particular configuration, then decreasing V2 decreases the amount of runway required.

[DougSnow]

Since V2 changed - does your operation use clearway?  If you do use it, V2 can actually occur over the clearway (no more than halfway down it if I am not mistaken), and that could be the reason for the adjustment of V2.  With the newer runway with a shorter length (there is not enough detail to see if the runway lengths permit an unbalanced takeoff, in other words are TORA, TODA, and ASDA all equal, or are they unequal), it is possible that Aerodata was using clearway to further unbalance the field to lift the same weight off the shorter runway.

I'd have your engineering group ask AeroData - but report back, I'm intrigued.  My airline (a purple box hauler) doesnt use clearway for anything.

[United744]

What was the runway slope of the two runways? My guess is maybe it was shorter, but flatter/more downhill, permitting a higher V2 in seemingly less distance (higher acceleration).

Try a MTOM takeoff in PSX and look at the time to Vr on an uphill vs. downhill takeoff - the difference is surprising.

[Mariano]

Thanks guys for all the input.

Been very busy in the last two days.

Will read with my full attention tomorrow.

Best regards,

Mariano

[simonijs]

Thanks for the input, gentlemen. I'm hoping to visit the Dispatch offices soon, during which time I will inquire about this. Should I find the answer, I will post it here.

Best regards,
Mariano

Hi Mariano,

Did you ever get a satisfactory answer to this problem (of a higher V2 during a runway change)? With a specific interest in Aircraft Performance, I also would like to learn more about any particular reason for this increase in V2.

This week, I was discussing Radius to Fix maneuvring capabilities of an aircraft at V2 with a Dutch pilot. He considered them to be unsafe. To be on more solid ground in this discussion I opened up EASA CS-25.107 again, for that matter with exactly the same wording as in FAR-25. I then noticed, that the criteria for V2 as used in the forum discussion above were about V2 MIN only (25.107b). V2 is further discussed in 25.107c, where it says: 'V2, in terms of calibrated airspeed, must be selected by the applicant to provide at least the gradient of climb required by CS 25.121(b) but may not be less than (...) V2 MIN".

Could the climb gradient requirements for the new runway be such, that the "old" V2 could not comply with these requirements, and that V2 had to be adjusted accordingly? ATENA 2A/2C SID's (VHHH) for instance require at least 4,1 or 4,9 % up to 1400 ft...

Regards,
Simon

[Mariano]

Simon,

Apologies for the delay.

I had set up a visit to our Operations Center for early March (after my February vacation), during which I was going to tour the Dispatch Center and meet with dispatches, to whom I was going to bring up the issue in question.

With the Coronavirus outbreak, all visits have been cancelled until further notice.

Since Dispatch has a PC version of the FDP software we access via ACARS and with which TLRs are created, I was hoping to run the scenario above in one of their stations. I assume the runway/runway departure track in question must have an issue of which we are not aware in the line, which might help explain the odd V2 behavior (their software may have several detailed notes regarding each runway).

As soon as I am able to come up with an answer, I will definitely post it here.

Best regards,

Mariano