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Class A take off performance invalidated by backtrack
In a nutshell if we calculate take off performance for a multi engine transport jet from intersection B at given airport and we then are offered and accept a backtrack to full length how does this invalid the performance we have calculated from that intersection ... in a nutshell ? |
Um, it doesn't?
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Clearly, you've been challenged on that point - What did they say? (I'm with rudestuff here!)
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Aircraft told two mins delay due previous same route so offered backtrack to which they accepted. I was flying with a very senior and respected training Captain (large European airline ) to which he said “why backtrack do they have no understanding of aircraft performance. .... idiots “ anyway it was a short flight and never got to ask him why. I began thinking of TODA TORA ASDA BALANCED FIELDS CLEARWAYS etc all being calculated from a certain point and would these be invalidated if you started your take off run say 309m earlier , airfield and terrain dependent of course ?? |
My outfit‘s performance tool applies a shortening for the lineup loss according to the runway entry method (90 degree angle for intersection and 180 degree for backtrack). The OPT tool indicates the method used, if we would do a backtrack to a 90 degree calculated entry point crews are expected to do a shortening of 9 metres for the type I fly or apply common sense – thankfully still allowed in my company. Usually this is only of concern on short runways with holding bays at the runway end (90 degree angle used) e.g. LCY or FLR. Edit: I just read the question again, no I see no reason to worry about performance if you start your TO earlier than calculated as the previous posters state. |
I think what your pompous colleague was attempting to convey was that having worked out the performance for the intersection departure, there is little practical benefit in using full length with the same figures. |
Originally Posted by spitfire747
(Post 10594357)
“why backtrack do they have no understanding of aircraft performance. .... idiots “ |
Aircraft told two mins delay due previous same route so offered backtrack to which they accepted. I was flying with a very senior and respected training Captain (large European airline ) to which he said “why backtrack do they have no understanding of aircraft performance. .... idiots “ |
There actually is an odd case in which the performance numbers might change - obstacle clearance height. The area that is surveyed for obstacles extends out in a narrow cone. When this cone is moved back, it is possible to "capture" an obstacle that was not considered from an intersection departure. I've seen the engine-out acceleration altitude increase 20 or so feet when using full length vs an intersection. Not much I know, but it could be one of those "gotcha" things if you have a check pilot or FAA-type on board.
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Your V1 is essentially out the door. Given the additional length you could still safely reject above the calculated V1, you just don’t know by how much. So right after your calculated V1 you have a catastrophic failure for which you could have rejected had you requested new performance calculation. Now you’re forced to take a sick puppy airborne. Which is well and good in the sim but in the real world with a low overcast and a crosswind and 100 people screaming in the back it’s different. Bit academic as there will literally only be a few seconds between the “old” V1 and the “new” V1 so realistically that window is really short but yes if it’s a substantial back taxi I would recalculate. With a V1 of 160kts you’re moving at 253 feet second so somewhat arbitrarily if the backtrack and 180 turn and line up give me a gain of 1000’ I would recalculate. So let’s say a total of 1500’ or more from the intersection I originally planned to use. So here’s your solution: When ATC assigns a runway and an intersection ask for both when you send for your performance on the ACARS. Lets say airport is using 7L (10,000’) and 7R (8000’) for departure and I have not been assigned a runway I’d ask for both and once I’ve been assigned a runway I’ll cross out and circle or fold over. Still leave the numbers legible in case they change their minds. Which ATC never does :suspect: |
B2N2 - When you re calculate for full length, your lovely performance software may well give you a greater flex temp/derate moving your V1 back down the runway making it relevant again. This ,for me, kinda makes your first argument irrelevant. For me, the risks of putting in wrong performance data during that 1min backtrack far outweigh the benefits... There is always a flip side - Our engineers would of course point to the benefits of the increased engine life!
Tomaski - In EU land (if I recall correctly!) we start the 'obstacle accountability area' at the end of the TODA, not at the liftoff point. So for our regs, that wouldn't change this side of the pond... |
Originally Posted by Cough
(Post 10594660)
Tomaski - In EU land (if I recall correctly!) we start the 'obstacle accountability area' at the end of the TODA, not at the liftoff point. So for our regs, that wouldn't change this side of the pond...
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Originally Posted by spitfire747
(Post 10594357)
“why backtrack do they have no understanding of aircraft performance. .... idiots “ By giving yourself more runway you're not invalidating the performance figures, if anything you're giving yourself an extra safety margin. If you've got a wet runway and a V1/Vr split then a recalc might give you a slightly higher V1 or a slightly higher de-rate. But on a dry runway with a full de-rate and no split I wouldn't bother. V1 can't get any higher and engine power can't get any lower so the numbers will come out the same. |
More runway is never a bad thing.
So maybe your V1 is not quite right now but it is more conservative. FFS, runway behind you is useless and now you want to belly ache about being given more ahead of you??? |
Originally Posted by oggers
(Post 10594733)
Nor the other side. FAA = "End of runway". Either way, changing from intersection to full length departure doesn't change the OAA.
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Originally Posted by Tomaski
(Post 10594844)
And yet I have actually seen this difference in the Engine Out Acceleration Altitude (EOAA) happen when comparing the takeoff data for full length vs intersection during preflight, so something changed. The explanation I gave is the one given to me by our training department.
For the intersection departure, the thrust requirement is a large one to clear the close in obstacle. As you have a large amount of thrust on, the OE climb angle is steeper and acceleration segment is shorter and you climb away without the second obstacle having any effect. But for the full length departure, the thrust requirement to clear the first obstacle isn't too great so the performance app reduces the thrust to just clear the first obstacle be the required margin. But because the subsequent climb out and acceleration are that much longer the second obstacle comes into play, requiring the performance app to raise the EOAA to avoid this... But say you take off from the full length with intersection data - Because you'll have the higher thrust selected, you'll not only avoid the second obstacle by miles, you'll also avoid the first by a greater margin... |
Originally Posted by Cough
(Post 10594909)
So, theorising what might have occurred (please understand - This is just one of many possibilities!). Say you have two obstacles to be cleared, one close in, one further away.
For the intersection departure, the thrust requirement is a large one to clear the close in obstacle. As you have a large amount of thrust on, the OE climb angle is steeper and acceleration segment is shorter and you climb away without the second obstacle having any effect. But for the full length departure, the thrust requirement to clear the first obstacle isn't too great so the performance app reduces the thrust to just clear the first obstacle be the required margin. But because the subsequent climb out and acceleration are that much longer the second obstacle comes into play, requiring the performance app to raise the EOAA to avoid this... But say you take off from the full length with intersection data - Because you'll have the higher thrust selected, you'll not only avoid the second obstacle by miles, you'll also avoid the first by a greater margin... That may very well be a more correct description of what caused the change. Nevertheless, from strictly an operator perspective the important point is that there was a change to EOAA that needed to be accounted for. For this reason and a few others stated above, while the intersection data will in the vast majority of cases be reasonably close for a full length departure, it may (depending on a particular airline's policies) not be considered technically legal. The question the PIC really needs to ask is, "If something were to go amiss during takeoff, would my choice of using the original intersection data be considered proper?" |
Originally Posted by Tomaski
(Post 10594940)
Nevertheless, from strictly an operator perspective the important point is that there was a change to EOAA that needed to be accounted for
Originally Posted by Tomaski
(Post 10594940)
"If something were to go amiss during takeoff, would my choice of using the original intersection data be considered proper?"
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Originally Posted by spitfire747
(Post 10594357)
Aircraft told two mins delay due previous same route so offered backtrack to which they accepted. I was flying with a very senior and respected training Captain (large European airline ) to which he said “why backtrack do they have no understanding of aircraft performance. .... idiots “ anyway it was a short flight and never got to ask him why. I began thinking of TODA TORA ASDA BALANCED FIELDS CLEARWAYS etc all being calculated from a certain point and would these be invalidated if you started your take off run say 309m earlier , airfield and terrain dependent of course ?? |
Morale of the story, next time you’re faced with a potential choice requests both and see what the actual differences are. In any case I know plenty that would redo the performance numbers. NTSB judge : “ Did you or didn’t you use the correct performance numbers ?” |
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