Part 25 takeoff/landing performance

[Roger Roger]

I don’t have a particular question per se, just looking for input for a relatively new jet captain at a 135 where (because 135) we do all our own performance calculations using the APG Genesis app. I think I’ve got a decent understanding of landing numbers (115% wet, use of 60% or 80% factoring, how to dig into the manual to determine which level of contamination) and I understand in practice using our APG to prove engine out performance, including use of an alternate ODP. Regarding takeoff performance, kinda curious about why V1 is always the same (105) on a wet runway, I’m guessing Vmcg issues? What does it mean when V1=Vr? Only happens when we’re really pushing max gross.

What are other common misconceptions and gotchas? Or any cool nuggets of theory behind how the numbers come to be that people don’t know?

 

[ppragman]

text sent.

not a jet guy, but realistically I reckon the limitation is due to the accelerate-stop difference combined with the fact that V1 cannot exceed Vr.

 

[Roger Roger]

text sent.

not a jet guy, but realistically I reckon the limitation is due to the accelerate-stop difference combined with the fact that V1 cannot exceed Vr.

Reading a little further it looks like the the wet runway issue has to do with the following:
-trying to get required runway as short as possible
-with a wet/contaminated runway that means using accelerate go vs accelerate stop (because stopping distance is so long) edit: or maybe it just means basing your accelerate stop off the lowest possible airspeed
-V1 can’t be less than Vmcg.
-so by artificially lowering V1 you get “better” takeoff numbers

it would be kinda interesting if APG spit out accel/go and accel/stop numbers just as an FYI thing

 

[Roger Roger]

So thinking through the V1=Vr thing.
Takeoff performance is supposed to be the better of accel/stop and accel/go right?
So what I’m guessing is, at heavy weights the stopping performance is so much better than the first segment climb that all the way up to Vr your accelerate stop is still shorter than accel go.

 

[ppragman]

So thinking through the V1=Vr thing.
Takeoff performance is supposed to be the better of accel/stop and accel/go right?
So what I’m guessing is, at heavy weights the stopping performance is so much better than the first segment climb that all the way up to Vr your accelerate stop is still shorter than accel go.

Been awhile since I thought about this, but I think this is wildly aircraft dependent, and typically stopping distance will be the limiting factor?

 

[Roger Roger]

So thinking through the V1=Vr thing.
Takeoff performance is supposed to be the better of accel/stop and accel/go right?
So what I’m guessing is, at heavy weights the stopping performance is so much better than the first segment climb that all the way up to Vr your accelerate stop is still shorter than accel go.

I had this kinda wrong and I feel dumb for not knowing. Takeoff performance accommodates balanced field length and factored all engine takeoff. The result is still the same, at heavy weights your stopping performance is so much better that you wouldn’t actually get a balanced field until past Vr.

 

[Roger Roger]

Also I find it funny that the people who complain about JC being all about politics are the least likely to participate in the tech talk threads

 

[Roger Roger]

Ok next question. You’re running numbers on a short runway to see if it works. Landing distance is fine, that’s factored so you know you have a buffer. But takeoff distance shows say, 3930’ required of a 4000’ runway. If I’m understanding it right you’re guaranteed a 15% factor all engines operating under that number, but no cushion if one goes kaboom at V1. Do you send it? If not, what kind of buffer do you want from the calculated distance before you’re comfortable?

 

[ppragman]

I had this kinda wrong and I feel dumb for not knowing. Takeoff performance accommodates balanced field length and factored all engine takeoff. The result is still the same, at heavy weights your stopping performance is so much better that you wouldn’t actually get a balanced field until past Vr.

I need to draw a free body diagram to grok this

 

[ppragman]

Ok next question. You’re running numbers on a short runway to see if it works. Landing distance is fine, that’s factored so you know you have a buffer. But takeoff distance shows say, 3930’ required of a 4000’ runway. If I’m understanding it right you’re guaranteed a 15% factor all engines operating under that number, but no cushion if one goes kaboom at V1. Do you send it? If not, what kind of buffer do you want from the calculated distance before you’re comfortable?

I can’t speak to Jet land, but anything within 20% has always seemed sketchy to me.

When I was operating the Pilatus into 1600’ of gravel it wasn’t uncommon to have about 10% margin if memory serves. I know it’s a different kind of flying, but those flights were meticulously planned and we always had reasonable assurances that the weights were accurate to within 10 lbs or less, but still, you’d stress about the flight all the way down.

Regardless, anything less than 20% felt extremely uncomfortable to me, anything with less than 10% margin meant I was bumping weight. We eventually (after some people scared themselves) put together a company limit of 30% - bump freight until you had 30% margin on whatever numbers you come up with.

 

[Roger Roger]

I can’t speak to Jet land, but anything within 20% has always seemed sketchy to me.

When I was operating the Pilatus into 1600’ of gravel it wasn’t uncommon to have about 10% margin if memory serves. I know it’s a different kind of flying, but those flights were meticulously planned and we always had reasonable assurances that the weights were accurate to within 10 lbs or less, but still, you’d stress about the flight all the way down.

Regardless, anything less than 20% felt extremely uncomfortable to me, anything with less than 10% margin meant I was bumping weight. We eventually (after some people scared themselves) put together a company limit of 30% - bump freight until you had 30% margin on whatever numbers you come up with.

So would you run the all engine numbers from the spaghetti chart and then reassess? Or base your decision off the worst case scenario of “V1” *loud metallic thump* “expletives”

 

[Roger Roger]

Reading a little further it looks like the the wet runway issue has to do with the following:
-trying to get required runway as short as possible
-with a wet/contaminated runway that means using accelerate go vs accelerate stop (because stopping distance is so long) edit: or maybe it just means basing your accelerate stop off the lowest possible airspeed
-V1 can’t be less than Vmcg.
-so by artificially lowering V1 you get “better” takeoff numbers

it would be kinda interesting if APG spit out accel/go and accel/stop numbers just as an FYI thing

Ok sorta had this wrong as well. Just like the V1=Vr thing it comes down to balanced field length. On a wet/contaminated runway your stopping distance is so long that your V1 for a balanced field length is less than Vmcg, which of course you can’t allow, so we’re actually artificially bumping the V1 UP to Vmcg.

 

[Roger Roger]

Next question.
Under 135, are we really free to fly whatever alternate engine out procedure APG spits out? Don’t need any ops spec approval?

 

[ppragman]

Next question.
Under 135, are we really free to fly whatever alternate engine out procedure APG spits out? Don’t need any ops spec approval?

presumably 135.19 has you covered there

 

[CFI A&P]

Next question.
Under 135, are we really free to fly whatever alternate engine out procedure APG spits out? Don’t need any ops spec approval?

Some will encourage you to choose the one that most aligns with your assigned SID, however you may find that going a different direction may have lower obstacle thus higher takeoff weight, or higher temp.

For example: hypothetically you're assigned SID that has a right turn in it, and the APG to the right gives you a lower MTOW penalty because of an obstacle in that flight path. You may find that another APG procedure, even from the same runway with the only difference being a left turn will allow for a higher weight because of a lower or non-existent obstacle in that direction.

Also, worth noting that not all APG OEI procedures are for obstacles. A good example is departing the 30s in KSJC - the OEI procedure is for airspace avoidance with a disabled aircraft. Do you really want to fly "straight out" and into Moffett or SFO's bravo?

 

[Roger Roger]

Some will encourage you to choose the one that most aligns with your assigned SID, however you may find that going a different direction may have lower obstacle thus higher takeoff weight, or higher temp.

For example: hypothetically you're assigned SID that has a right turn in it, and the APG to the right gives you a lower MTOW penalty because of an obstacle in that flight path. You may find that another APG procedure, even from the same runway with the only difference being a left turn will allow for a higher weight because of a lower or non-existent obstacle in that direction.

Also, worth noting that not all APG OEI procedures are for obstacles. A good example is departing the 30s in KSJC - the OEI procedure is for airspace avoidance with a disabled aircraft. Do you really want to fly "straight out" and into Moffett or SFO's bravo?

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.

 

[Fly_Unity]

... On a wet/contaminated runway your stopping distance is so long that your V1 for a balanced field length is less than Vmcg, which of course you can’t allow, so we’re actually artificially bumping the V1 UP to Vmcg.

I am not following this logic. Our V1 numbers always go down when we input a wet runway. I am assuming the reason is we need to abort sooner is because of a. We need more stopping distance on a wet runway, and b. we need to have 115% factor in there.

I do not follow the logic of V1 artificially bumping "up". V1 can only artificially bump "down" so that it does not exceed Vr (we dont want to abort the takeoff after you rotate).

 

[Roger Roger]

I am not following this logic. Our V1 numbers always go down when we input a wet runway. I am assuming the reason is we need to abort sooner is because of a. We need more stopping distance on a wet runway, and b. we need to have 115% factor in there.

I do not follow the logic of V1 artificially bumping "up". V1 can only artificially bump "down" so that it does not exceed Vr (we dont want to abort the takeoff after you rotate).

V1 is the speed at which accel stop and accel go are equal, am I understanding that right? That’s the concept of balanced field length? But on a wet or contaminated runway, your actual V1 speed would be LOWER than Vmcg, because it takes you so long to stop with a contaminated runway. So instead of using the actual V1 speed for balanced field length, you have to artificially set V1 higher, at Vmcg. It makes sense in my head. But perhaps I’m not communicating it well. It’s lower than V1 on a dry runway, but higher than the “actual” V1 for a balanced field under the conditions.

 

[Fly_Unity]

V1 is the speed at which accel stop and accel go are equal, am I understanding that right? That’s the concept of balanced field length? But on a wet or contaminated runway, your actual V1 speed would be LOWER than Vmcg, because it takes you so long to stop with a contaminated runway. So instead of using the actual V1 speed for balanced field length, you have to artificially set V1 higher, at Vmcg. It makes sense in my head. But perhaps I’m not communicating it well. It’s lower than V1 on a dry runway, but higher than the “actual” V1 for a balanced field under the conditions.

I think I am following you now. I just ran a bunch of scenarios with APG in our CJ3, and a wet runway ALWAYS lowered V1. However I can see that certain airplanes (probably aircraft with engines that are not so centerline thrust) Vmcg would go up therefore raising V1.

The part that really confuses me is if V1 is the maximum speed at which you can abort and still have enough runway, then how can you increase that number without running off the end of the runway in the event of an abort. I guess its a range that meets both criteria?

Edit:

A Quick Google search says this:
When operating at the minimum allowable field length for a given weight, there is only one speed that will satisfy both of the criteria of the V1 definition. However, if the runway available is greater than the minimum allowable length for the aircraft weight, there may be a speed range available for V1 from which the operator can nominate a specific value according to the following criteria:.....

• Minimum V1 - the minimum permissible V1 speed for the referenced conditions (temperature, wind, pressure altitude, weight, thrust, runway etc) from which the takeoff can be safely completed after the critical engine has failed at the designated speed (VEF)
• Maximum V1 - the maximum possible V1 for the referenced conditions at which a rejected takeoff can be initiated and the aircraft stopped within the remaining runway (or runway plus stopway where available)"

So yea, It all makes sense, I think...

 

[Roger Roger]

I think I am following you now. I just ran a bunch of scenarios with APG in our CJ3, and a wet runway ALWAYS lowered V1. However I can see that certain airplanes (probably aircraft with engines that are not so centerline thrust) Vmcg would go up therefore raising V1.

The part that really confuses me is if V1 is the maximum speed at which you can abort and still have enough runway, then how can you increase that number without running off the end of the runway in the event of an abort. I guess its a range that meets both criteria?

Edit:

A Quick Google search says this:
When operating at the minimum allowable field length for a given weight, there is only one speed that will satisfy both of the criteria of the V1 definition. However, if the runway available is greater than the minimum allowable length for the aircraft weight, there may be a speed range available for V1 from which the operator can nominate a specific value according to the following criteria:.....

• Minimum V1 - the minimum permissible V1 speed for the referenced conditions (temperature, wind, pressure altitude, weight, thrust, runway etc) from which the takeoff can be safely completed after the critical engine has failed at the designated speed (VEF)
• Maximum V1 - the maximum possible V1 for the referenced conditions at which a rejected takeoff can be initiated and the aircraft stopped within the remaining runway (or runway plus stopway where available)"

So yea, It all makes sense, I think...

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!

in the Lear 45, every weight/weather combo I’ve run with a wet runway gives the same V1, 105. Which I think is Vmcg plus maybe a knot? Correction, Vmcg+5