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Originally Posted by B2N2
(Post 10595174)
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 ?” |
Originally Posted by oggers
(Post 10594474)
:rolleyes: Does he have no understanding of wake turbulence separation.
Unless, of course, a minute had already elapsed. I think this is more likely to be some sort of separation requirement, either imposed by a next sector to reduce overload, or by a procedure. |
Some interesting comments in this thread. (I note that the following summarises a number of comments made by others) -
would these be invalidated if you started your take off run say 309m earlier I can't bring any situation of concern to mind. Of course, one might choose to redo the sums but using the original calculations will produce acceptable data for the new starting position for the takeoff. Caveat - if there is a turn required, then the commencement of the turn/configuration and speed changes have to remain at the same point/sequence so that lateral terrain clearance is not a problem. When this cone is moved back, it is possible to "capture" an obstacle that was not considered from an intersection departure. Not really a concern. The splay starts at the end of the TODA. The only problem, then, is how do you achieve that starting point when you can't track visually, realistically, for a jet ? Always a vexed question. A tad easier since the days of GPS and very accurate FMS tracking capabilities. Your V1 is essentially out the door Only to the extent that it now becomes non-optimised for the takeoff. One can either redo the sums or accept a conservative set of takeoff data. Now you’re forced to take a sick puppy airborne. If this is seen to be a concern, one redoes the sums. I think his point here was that the performance was calculated and the aircraft perfectly capable of an intersection departure, so he didn't NEED a backtrack. Indeed ... but, were I to end up in an accel-stop, might I appreciate the additional (new) fat in the actual ASDA ? 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. I might be more interested in the comment had it come from the ops engineering SMEs ? Often the training folk, although well-intentioned and well-motivated, are not always across all the nitty gritty data. there was a change to EOAA that needed to be accounted for. What might be the objective basis for this statement ? "If something were to go amiss during takeoff, would my choice of using the original intersection data be considered proper?" I think so .. unless someone can come up with a technically sustainable concern. |
The presentation below may be of interest for those discussing these issues - future developments.
https://skybrary.aero/bookshelf/books/4658.pdf |
Originally Posted by VTH
(Post 10595466)
If they were actually given two minutes, there wouldn’t be any difference to the wake turbulence separation since the previous departure would have started rolling from the same holding point. Three minutes would be correct for an intersection departure. Unless, of course, a minute had already elapsed. I think this is more likely to be some sort of separation requirement, either imposed by a next sector to reduce overload, or by a procedure. Two minutes separation required due to same track (the OP said as much; it is not for wake turbulence it is just the ICAO departure separation if No1 is 40kts faster) But for the intersection departure 3 minutes wake turbulence separation might apply subject to aircraft cat (not specified in this case) By chosing full length instead, the No2 could save up to 1 minute by reducing required separation to 2 minutes again |
Correct, but that would only be applicable if they had “two minutes to go”(out of a total of three) and not two minutes in total. The way that original post was worded, makes it seem like there’s a two minute separation required in total, in that case a backtrack wouldn’t make any difference if the reason for the delay was wake. |
Originally Posted by B2N2
(Post 10594507)
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 I’m not a performance engineer, but I so believe we all do takeoffs such as this every single day. Let’s assume you are doing a ferry flight will full thrust off a very long runway. Your calculated V1(min) and Vr are 120kts, but you could safely stop with V1(max) 180kts. Are you going to use V1max, just in case something catastrophic happens? Unless there is a 10000ft mountain dead ahead, I’m assuming the answer will be no. |
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 ? |
Originally Posted by VTH
(Post 10595681)
a backtrack wouldn’t make any difference if the reason for the delay was wake.
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