Part 25 takeoff/landing performance

Roger Roger said:
Right, that’s kinda why I made the thread? I know more than when I started the thread, but I feel like I know less!
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I don’t know if this will help or hurt, but I’m a fan of going straight to the horse’s mouth in FAR part 25 itself:
www.law.cornell.edu

14 CFR § 25.107 - Takeoff speeds.

www.law.cornell.edu www.law.cornell.edu

25.107(a) is pretty basic and leaves a lot up to the manufacturer but it would be interesting to find an example problem.
 
inigo88 said:
I don’t know if this will help or hurt, but I’m a fan of going straight to the horse’s mouth in FAR part 25 itself:
www.law.cornell.edu

14 CFR § 25.107 - Takeoff speeds.

www.law.cornell.edu www.law.cornell.edu

25.107(a) is pretty basic and leaves a lot up to the manufacturer but it would be interesting to find an example problem.
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When I go back on shift I may try to dig into the spaghetti charts and run a bunch of examples in APG to see the effects of weird loading and/or weather scenarios. Or maybe not, I’ve got an annual inspection to finish and another side project to work on.

A lot of this is mostly theoretical, but in the case of 2 airports we might get asked to go to it’s pretty applicable and I want to make darn sure that I understand what that number that APG spits out is giving me before I push the thrust levers up yknow?
 
Roger Roger said:
Right, in our case, we could never depart our home airport with any usable amount of gas and an east wind if we stuck with the OEIs that are similar to the published ODPs. The RNAV OEI dep basically mimics an alaska airlines RNP departure.
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Where is the home airport? I’m curious to run APG out of there and see what I get.

Next up, don’t wet runways need more braking distance, which is why V1 goes down? You’re basically doing a Car and Driver test of 0-100-0, except it isn’t the distance in 0-100 that is limiting (although accelerate distance is longer due to hydrodynamic forces), but the distance required from 100-0 is now longer and that is limiting.
 
CFI A&P said:
Where is the home airport? I’m curious to run APG out of there and see what I get.

Next up, don’t wet runways need more braking distance, which is why V1 goes down? You’re basically doing a Car and Driver test of 0-100-0, except it isn’t the distance in 0-100 that is limiting (although accelerate distance is longer due to hydrodynamic forces), but the distance required from 100-0 is now longer and that is limiting.
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PAJN, checkout the runway 8 numbers. We usually would need to bump about 2k lbs of fuel to make either of the >180° turn options work.
 
CFI A&P said:
Next up, don’t wet runways need more braking distance, which is why V1 goes down? You’re basically doing a Car and Driver test of 0-100-0, except it isn’t the distance in 0-100 that is limiting (although accelerate distance is longer due to hydrodynamic forces), but the distance required from 100-0 is now longer and that is limiting.
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right, but I was curious about why specifically we always ended up with 105. Turns out that must just be as low as they can get it, I’m guessing, there’s probably some reason they don’t set it right at Vmcg.
 
Roger Roger said:
right, but I was curious about why specifically we always ended up with 105. Turns out that must just be as low as they can get it, I’m guessing, there’s probably some reason they don’t set it right at Vmcg.
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Aha! I think part 25.107(a) (1) & (2) linked before explains that part. :)

So if I’m a performance engineer at Learjet doing takeoff calcs and figuring out takeoff performance charts I’d be governed by the following:

(a) V1 must be established in relation to VEF as follows:

(1) VEF is the calibrated airspeed at which the critical engine is assumed to fail. VEF must be selected by the applicant, but may not be less than VMCG determined under § 25.149(e).

(2) V1, in terms of calibrated airspeed, is selected by the applicant; however, V1 may not be less than VEF plus the speed gained with critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognizes and reacts to the engine failure, as indicated by the pilot's initiation of the first action (e.g., applying brakes, reducing thrust, deploying speed brakes) to stop the airplane during accelerate-stop tests.
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So we start out with the assumption that VEF is greater than or equal to Vmcg from 25.107(a)(1), and then add the requirements from 25.107(a)(2).

Learjet and the other manufacturers must have some empirical reaction time number (based on pilots doing V1 cuts in sims maybe?) of the delay time between the engine failure and how long it takes the pilot to notice and initiate the first action…

They should also know the takeoff acceleration provided by the remaining good engine…

This is one of the derivations of the classical mechanics kinematics equations from physics:

1644900149513.png


If you’re not into calculus, it’s basically say that because acceleration is change in velocity per a given change in time, you can multiply that change in time on both sides and take the integral (calculus for area under the acceleration curve) and you get the equation [1] at the bottom.

Equation [1] is saying your final velocity (v) = your initial velocity (v0) + that known acceleration (a) times the change in time (delta t).

For our purposes v0 is VEF which is Vmcg or slightly greater, a is the acceleration provided by your remaining good engine and delta t is the pilot’s reaction time to initiate action after the engine failure, and the resulting v is going to be our V1.

In conclusion if we interpret the FAR part 25 definition of what V1 is and do some physics, it makes sense why V1 > Vmcg. Hope this helps!:)
 
inigo88 said:
Aha! I think part 25.107(a) (1) & (2) linked before explains that part. :)

So if I’m a performance engineer at Learjet doing takeoff calcs and figuring out takeoff performance charts I’d be governed by the following:



So we start out with the assumption that VEF is greater than or equal to Vmcg from 25.107(a)(1), and then add the requirements from 25.107(a)(2).

Learjet and the other manufacturers must have some empirical reaction time number (based on pilots doing V1 cuts in sims maybe?) of the delay time between the engine failure and how long it takes the pilot to notice and initiate the first action…

They should also know the takeoff acceleration provided by the remaining good engine…

This is one of the derivations of the classical mechanics kinematics equations from physics:

View attachment 63376

If you’re not into calculus, it’s basically say that because acceleration is change in velocity per a given change in time, you can multiply that change in time on both sides and take the integral (calculus for area under the acceleration curve) and you get the equation [1] at the bottom.

Equation [1] is saying your final velocity (v) = your initial velocity (v0) + that known acceleration (a) times the change in time (delta t).

For our purposes v0 is VEF which is Vmcg or slightly greater, a is the acceleration provided by your remaining good engine and delta t is the pilot’s reaction time to initiate action after the engine failure, and the resulting v is going to be our V1.

In conclusion if we interpret the FAR part 25 definition of what V1 is and do some physics, it makes sense why V1 > Vmcg. Hope this helps!:)
Click to expand...
Ah great! Yes it’s right in there. V1 has to be Vef plus a margin for reaction, and Vef has to be at least Vmcg. Hence an airplane with Vmcg of 100 gives you a minimum V1 of 105, and because that gives you the best possible number (least runway required) on a wet/contaminated runway that’s what they use.
This is exactly the type of understanding I was hoping to gain here. It would be nice if this was covered thoroughly in either company or type school along the way, but it wasn’t so here we are.
 
Roger Roger said:
Ah great! Yes it’s right in there. V1 has to be Vef plus a margin for reaction, and Vef has to be at least Vmcg. Hence an airplane with Vmcg of 100 gives you a minimum V1 of 105, and because that gives you the best possible number (least runway required) on a wet/contaminated runway that’s what they use.
This is exactly the type of understanding I was hoping to gain here. It would be nice if this was covered thoroughly in either company or type school along the way, but it wasn’t so here we are.
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This kind of reminds me of CFIIs having to dig into the TERPS to explain a crappily explained instrument topic.

“Sorry your training dumbed this down so much that it isn’t actually useful, let’s go read the document governing the people who write the charts.”

Replace TERPS with Part 25 and FAA cartographers/Flight Check people with Bombardier Aerospace Engineers haha!

While I’m glad we found the answer, we probably shouldn’t have had to look that hard.
 
inigo88 said:
This kind of reminds me of CFIIs having to dig into the TERPS to explain a crappily explained instrument topic.

“Sorry your training dumbed this down so much that it isn’t actually useful, let’s go read the document governing the people who write the charts.”

Replace TERPS with Part 25 and FAA cartographers/Flight Check people with Bombardier Aerospace Engineers haha!

While I’m glad we found the answer, we probably shouldn’t have had to look that hard.
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Maybe we shouldn’t, although on a day to day basis most people are just going to punch the information in the app or the FMS, post it to the screens, and forget about it. I’ve just never really been comfortable with my understanding on this topic and I thought there might be a few nerds left on here who are as interested in digging beyond the basics to understand more of what’s really going on there.
 
As a tangent, I've had classroom situations where people were adamant that V1 was always a balanced field length decision speed. That never sat well with me, but I was/am not nearly as knowledgeable on this stuff as you guys are, and so I never felt on solid-enough ground to refute that statement. For future schoolhouse discussions, is this wet runway/ 105 knot V1 a good example of not having a balanced field? Any other examples come to mind?
 
SteveC said:
As a tangent, I've had classroom situations where people were adamant that V1 was always a balanced field length decision speed. That never sat well with me, but I was/am not nearly as knowledgeable on this stuff as you guys are, and so I never felt on solid-enough ground to refute that statement. For future schoolhouse discussions, is this wet runway/ 105 knot V1 a good example of not having a balanced field? Any other examples come to mind?
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I think another corner case at least in the LR45, would be when you are at max gross weight and V1=Vr. I surmise that in that case your accelerate stop is less than your accelerate go, that’s the only reason I can think of that they would recommend that you abort all the way up to Vr. Does it seem like I’m thinking straight there?
 
Autothrust Blue said:
Related generally but not specifically to the above, this is the best "stupid line pilot (no calculus here, @inigo88 )" explanation of transport category performance I've ever read.

https://www.smartcockpit.com/docs/Getting_to_Grips_With_Aircraft_Performance.pdf (PDF target, from Airbus)
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Damn... I was scrolling to the end of this thread so I could post this link!


SteveC said:
As a tangent, I've had classroom situations where people were adamant that V1 was always a balanced field length decision speed. That never sat well with me, but I was/am not nearly as knowledgeable on this stuff as you guys are, and so I never felt on solid-enough ground to refute that statement. For future schoolhouse discussions, is this wet runway/ 105 knot V1 a good example of not having a balanced field? Any other examples come to mind?
Click to expand...

Any time you flex the engine temp (or derate the takeoff thrust), you are moving away from a balanced field length V1.
 
BobDDuck said:
Damn... I was scrolling to the end of this thread so I could post this link!




Any time you flex the engine temp (or derate the takeoff thrust), you are moving away from a balanced field length V1.
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Does the Aîrbùs document explain that? Haven’t gotten that far in it.
 
New question regarding wet runway landing numbers. There are two ways of getting wet runway numbers for the Lear 45.

First, you have the standard part 135.385 (d) 115% of the dry runway number.
Second, you have the actual landing distances for wet runways chart, from the contaminated runway supplement.
Now, those two come out within 100 or 200 feet of each other for the most part.

BUT.

When you get into the fine print, the wet runway actual data considers MAX TR credit. The 115% rule, being based on the dry runway numbers, does not. If you use actual wet distance numbers, and then adjust for deferred TRs (as we have right now), and the basic part 135.385 (b) 60% rule, you need about a 7000’ runway for a max landing weight, calm wind landing. Using the 115% rule, you only need like 5500’ with the 60% factor.

Do you just go with the conservative number and say hey, we basically need a 7000’ runway right now? Do you go with the less conservative number because hey, it’s legal, and with that 60% rule you’ve still got a ton of wiggle room?
 
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