V1 Speed and Intersection Takeoffs

[jtrain609]

You guys have books with all the data permanently there?

We get all that same info on every single release.

Yup.

Saves paper eh?

It's pretty simple. Find the page with the runway/intersection you want, and the flap setting you want (standard is flaps 9, only used flaps 22 once). On the page it's broken up by temp and power setting, and it tells you the max weight you can have.

Under the weight? Life's good. Over it? Go to the next power setting. Still over it? Go to the next power setting. Still over it? Check out flaps 22.

 

[Polar742]

Come again?

If the performance on the back of the checklist says I need 3,230' you're telling me I have to use all 9,000' of runway?

...normally I just tell ground or tower "I can take 4,700' if an intersection helps" (we add 1500' to "book numbers" for rolling takeoffs) and they point me in the right direction since they have those numbers and I don't have them readily available.

Are you saying that's "illegal"? I don't see how, but I'm willing to learn.

-mini

Sorry, I left out the "in 121 operations" thing.

I have no idea on 135, and part 91 is, well, Part 91

...and, if you take the 4700' and the performance says you need 3200, and you, for some reason, have a boo-boo, just one more thing to go into the "Pilot Error" file, rightfully or not.

The present operation, we have a tablet PC into which we input our data and it spits out a handy-dandy electronic data card and even has an image of the FMC screen so you can plug and play. The backup system is they send generated numbers with the release that already takes into account a 10,000 kilo adjustment above Takeoff weight (which deals with last minute additions, or extra fuel. It's about 2 knots for the speeds).

If neither of those is copacetic, then we can ACARS dispatch for the right info, and we have a limited Airport Analysis Manual onboard.

There is a company called APG that did the data at the old shop. Recently I saw one of their advertisements for a performance logorythm for a PDA for your specific airplane.

 

[minitour]

Sorry, I left out the "in 121 operations" thing.

I have no idea on 135, and part 91 is, well, Part 91

Raja. It seems that procedure would be operator specific and consistent with their approved manual/procedures/training?

-mini

 

[Polar742]

Raja. It seems that procedure would be operator specific and consistent with their approved manual/procedures/training?

-mini

There are rather common elements. It's not like I had to learn a foreign language leaving the last shop and coming here.

However, there are operator specific elements, but there is a standard framework dictated by FAR 121 of what inputs you need for takeoff.

Performance and High Altitude/High Speed Aero were two of the most misunderstood subjects during recurrent time. And to me, the fundamental knowledge of which can keep your ass out of the sling more times than not (from first hand experience).

 

[Orange Anchor]

Of course, we had a mission computer that would compute it all, or it would take all day to do it by hand. The guys who flew tankers before the computer would do it, though.

I was challenged by my ops officer to become proficient in perf data because I came into the tanker knowing that any aircraft with more than 1 engine or more than 1 seat had an inherent design flaw.

We had an ORI where they cleared a swath in the center of the runway. Inside the swath, braking action was good. Outside the swath it was close to nil. After the big briefing I asked the commander about the problem posed with an engine loss. The Dash 1 said with OPTIMUM technique you could expect to deviate from center line by at least 50ft (old rudder). With the trucks now in the braking action nil zone, VMCg was higher than rotation speed. PROBLEM.

Solution via the commander, 'Don't lose an engine. Any more questions?"

 

[germb747]

V1 is the same as Refusal Speed...which DOES depend on actual runway length.

It's the maximum speed that you can accelerate to, lose an engine, and stop in the remaining runway distance.

Hacker, in big jets V1 can be based on a lot more than just the Refusal speed, and it usually varies from jet to jet.

I'll give an example. In the KC-135 S1 (decision speed, or go-no go speed... same as V1) was the largest of Vmcg, Vcefs, Vnefs unless Vrotate was less than the largest of those three, in which case it was Vrotate. In no case could the S1 be larger than Vrefusal or Vmax brake, or you would be in a take-off No go scenario. (Basically you aren't allowed to take off).

So, some definitions:

Vmcg or V "min control on the ground"- the minimum speed at which you could lose the downwind, outside engine (worst case) and still maintain directional control within particular limits (which I can no longer remember) and without pulling power on any other engines or using differential braking. So basically, the min speed at which you can continue the take-off and maintain directional control of the airplane if you lose the worst case engine. Vmcg was a controllability number, rather than a performance number. There was also a Vmca- min control airborne, but it was guaranteed to be less than Vrotate, so we didn't worry about it.

Vcef or "critical engine failure speed"- The speed at which if you accelerated to it on all engines operating, and then lost an engine it would take the exact same distance to continue the take-off as it would to stop the airplane. That distance was defined as the critical field length. Vcefs is a performance number, and it basically says that before you reach that speed, stopping takes less distance than going. After that speed going takes less distance than stopping, and AT Vcefs, they're the same.

Vnef or V "non-critical engine failure speed"- Some civillian airplanes define a non-critical engine failure speed that is based off of the engine that is NOT the worst case failing... but that is not what a KC-135 Vnef meant. In the 135, Vnef was basically an adjusted refusal speed. You took the actual runway available and subtracted 2000' and with this adjusted runway you computed a refusal speed. This adjusted refusal speed was our Vnef, and basically if Vnef was greater than Vcef you had a "non-critical takeoff." If Vcef was bigger, you had a critical takeoff. In a critical takeoff, we were not allowed to reduce the thrust, unless we needed to for controllability. It is a stop speed, but it allowed us to keep a bad airplane on the ground longer than using Vcef or Vmcg for decision speed, and still retain a 2000' margin of safety on stopping. We would only use these numbers if the take-off were a non-critical scenario, or in other words, if the take-off were easy to make on the available runway.

Vrot - Rotate speed. self explanatory

Vrefusal- Refusal speed. The fastest speed you could accelerate to and still stop in the remaining runway available. It's a stop speed.

Vmax brake- The fastest speed you could accelerate to and still stop the airplane before the brakes exploded. Interestingly enough, sometimes this number was smaller than Vrefusal, and if it was it meant that your Vrefusal number was bogus. It's also a stop speed.

So the philosophy was basically that you had to be faster than your fastest minimum "Go" speed, and still slower than your slowest maximum "Stop" speed to have a safe take-off.

Of course, we had a mission computer that would compute it all, or it would take all day to do it by hand. The guys who flew tankers before the computer would do it, though.

For those of you who are confused, the civilians/FAA uses somewhat different terminology.

Anyway, V1 may or may not be impacted by having a shorter runway. Usually, our V1 speed is based on rotation speed. So in that case, there would be no impact. However, if the remaining runway becomes short enough, your V1 speed may be lower than rotate speed by virtue of the fact that there's less room to put on the brakes and stop. AKA "refusal" speed.

 

[BobDDuck]

It depends too one whether you use a balanced field length requirement also. If that's the case, V1 *should* equal VR and your accelerate stop distance (the distance it takes to go from standing still, V1 or V1-5 and then come to a complete stop) should be the same as your accelerate go distance (complete stop to rotation and I think clearing a 50 foot tree but I'm not sure about that part) all while corking an engine at VR.

We actually don't use balanced field length which means that we just have to trust the data we get from Aerodata will work and follow any Alternate Single Engine TO procedures they have listed.

 

[fish314]

I was challenged by my ops officer to become proficient in perf data because I came into the tanker knowing that any aircraft with more than 1 engine or more than 1 seat had an inherent design flaw.

We had an ORI where they cleared a swath in the center of the runway. Inside the swath, braking action was good. Outside the swath it was close to nil. After the big briefing I asked the commander about the problem posed with an engine loss. The Dash 1 said with OPTIMUM technique you could expect to deviate from center line by at least 50ft (old rudder). With the trucks now in the braking action nil zone, VMCg was higher than rotation speed. PROBLEM.

Solution via the commander, 'Don't lose an engine. Any more questions?"

Hmmm.... I think I may have HAD that commander at one point in my career...

Or perhaps at several points. Maybe they all come from the same family or something?

 

[Orange Anchor]

Hmmm.... I think I may have HAD that commander at one point in my career...

Or perhaps at several points. Maybe they all come from the same family or something?

That ORI got him his star. On an earlier sortie we were holding for wx. We didn't have take off mins but the mission had come down from Offutt and it was a must go. "Alpha" asked us to open the door and drop the ladder. We did. He came into the cockpit and asked what was the problem. I explained we didn't have mins for takeoff. He asked what we needed and what the rule of thumb was. I said, "1/4 mile.. that's about eight runway lights." He smiled. He the pointed and counted 4 lights on the left and 4 lights on the right. "... that's 8 lights. Good night Captain. Have a safe flight."

 

[minitour]

It depends too one whether you use a balanced field length requirement also. If that's the case, V1 *should* equal VR...

Not quite. They're only based on the accel/stop and accel/go distances, not the speeds. Your V1 may not = your Vr. Ours frequently does not, but our numbers are for BFL.

-mini

 

[TUCKnTRUCK]

Yeah, the only requirement is that Vr is not less than V1.

For us in the Q, we either do it via acars, or dispatch. We don't have a paper based program for weight/balance/performance.

Our release has a TLR section that has allowable weights for departures, including most normal intersections... like 22R at W in ewr. Otherwise when we send our request, we can manually shorten a runway, say for a notam.

A normal request for runway performance data via ACARS is simply the runway ident. the next is an intersection departure, the 3rd is a manually shortened runway

RUNWAY1
22R
RUNWAY2
22R/W
RUNWAY3
22RSW2000F

in the accars request, 22R/W means 22r, intersection W. 22RSW2000F breaks down to 22R, with the SW(south west) end shortened by 2000 (f for feet, m for meters) It's important to know that shortening a runway a runway end that has emass,clearway or another stopping aid will remove that safety device from planning, and may significantly change performance.

We also have provisions for runway conditions, arresting gear, wind, temp, flaps, special procedures etc, ground icing, deice fluids, en route climb ice speeds etc.

There are so many variables that it is much easier to let the computer do it. We also have 2 fms units with acars.. but only 1 antenna.

verbage varies by FMS/ACARS manufacturer.

 

[BobDDuck]

Not quite. They're only based on the accel/stop and accel/go distances, not the speeds. Your V1 may not = your Vr. Ours frequently does not, but our numbers are for BFL.

-mini

Not doubting you, but what accounts for the V1/VR split then?

 

[gatorbuc99]

Isn't a 25R at F takeoff a frequent thing at LAX?

 

[Polar742]

Not doubting you, but what accounts for the V1/VR split then?

It's been a while, but VR can be no less than V1. V1 is a pure hypothetical number, where VR is based on the airplane actually flying the airplane.

Off hand, I could think a braking limitation could be an issue. Say it would take more distance to stop an airplane than it would to continue the takeoff. The Vr would be further down the runway since the airplane would make the screen height at the same distance the plane comes to a stop.

 

[BobDDuck]

It's been a while, but VR can be no less than V1. V1 is a pure hypothetical number, where VR is based on the airplane actually flying the airplane.

Off hand, I could think a braking limitation could be an issue. Say it would take more distance to stop an airplane than it would to continue the takeoff. The Vr would be further down the runway since the airplane would make the screen height at the same distance the plane comes to a stop.

I guess I just assumed the VR having to be >= V1 was a given.

As far as the second point, if that was the case, then it wouldn't be a balanced field length would it? I mean, I guess in theory you could have a balanced field length that was longer than the actual runway, but that wouldn't be very practical either as in the case of a reject you would run off the end every time.

Performance data is one of my weakest areas (my company does a TERRIBLE job teaching it) so any discussion I can get about it is a good thing.

 

[minitour]

I guess I just assumed the VR having to be >= V1 was a given.

It is. But that's not the definition of, nor does it affect BFL. BFL is only based on the distances, not the speeds.

Performance data is one of my weakest areas (my company does a TERRIBLE job teaching it) so any discussion I can get about it is a good thing.

It's a tough concept for me to grasp sometimes too. Thankfully they spend 7 days on that and electrical systems at Flight Safety, so my weak areas are hammered into my head before I leave.

-mini

 

[Polar742]

As far as the second point, if that was the case, then it wouldn't be a balanced field length would it? I mean, I guess in theory you could have a balanced field length that was longer than the actual runway, but that wouldn't be very practical either as in the case of a reject you would run off the end every time.

Performance was a real weak area at my former company. I tried to change what I could when I was in a position to bolster that.

The BFL is purely based on a set number of feet. Let's say that number for this discussion is 7000'. So now all the numbers need to line up to get the airplane either: a) Accelerated to V1 and stopped in 7000' or b) Accelerated and continue the take off to the screen height by 7000'

For our hypothetical airplane, we'll say the brakes take 3500' to stop the airplane, so obviously the V1 must reflect that distance. We'll say our performance numbers come up with 105 kts for this number.

Our hypothetical airplane has a second segment issue, so we'll used a reduce takeoff flap setting, say flaps 5 instead of 10. Through our performance machinations, we figure out that to get the airplane to screen height in 7000' we can used a reduced flap setting, but we are also using a reduced thrust setting for T/O. Due to the lower amount of flaps set, we don't rotate the airplane until 115 kts.

So, V1 is 105, Vr is 115. We have two different speeds, yet both allow the airplane to use balanced field because both requirements are met in the same distance.

Now, where does unbalanced field length benefit?

We know that we need 7000' to go or stop the airplane at the given weight from the previous example - and to prevent any other machinations, the environment is the same (ISA and same Field Elevation and 0 slope on any runway).

Well, now we find ourselves at some outstation with one short runway - 6500 feet for this example, however there is a clear way (the departure end of the runway ends and there is a body of infinite water in that direction).

The FARs allow us to take a "Clear way" credit as there are no obstructions on a specified clearance plane (My brain says at a 1.25% gradient for some reason - but that's a recall). We know it takes 3500 feet to stop the airplane, and 7000 feet to get to 35'.

So what we do in this instance is reverse engineer to the 3500'-to-go distance on the runway. Using our performance computations, we figure out what speed we'll be at that point in space, say 95 kts. Obviously we know we need 115 kts to fly the airplane.

So in this instance 95 kts is our reject speed, based on available runway, and our Vr remains at 115. We are still able to carry the same weight as we were off of the 7000' runway.

I hope this makes sense.

 

[BobDDuck]

Yep, it does.

Thanks.

 

[minitour]

Well esplained.

-mini

 

[Bigey]

Thank you everyone for responding and chiming in such great detail! I really appreciate it! It's about as crystal clear it can be for me now. I gotta try and pan it out with the toy runway and plane now!