B747-200: Vref, V2 and Vmcg

[john_tullamarine]

I have to concur with Mutt, the statement to which maxalt refers misses some important points of the runway limited takeoff calculation -

(a) most runways don't have much in the way of clearway declared and, in any case, even if a lot of clearway is declared, a takeoff for a particular Type is limited in the amount of clearway which can be used due to TORR considerations - recall that at least half (US rules) the airborne distance to screen has to be over the TORA, which is normally threshold to threshold... not much point running off the end of the hard bit before getting airborne ...

(b) BFL is a simplified approach to the sums, mainly of use for typically level-ish runways without any clearway declared or where a quick, manually-derived, answer is needed and we are prepared to forgo any benefit to be derived by considering the small margin of TODA over TORA. Consider that many flight manuals provide a BFL subset of charts which is significantly easier and quicker to use than the detailed charts. In this case the declared TODA should include the stopway (typically 60m or so) or remaining strip length and the declared TORA usually is not going to be limiting (ignoring the strange climb antics of helicopters). There will be little advantage in playing with all the charts as the end result will be much the same weight.

As Mutt indicates, there is a very important advantage for an operator with lots of runways to look after in that simplified rules can be generated for flight crew use (correction data and the like) ... in my view, this is a very valid consideration and the main reason that many people emphasise the BFL approach.

The Boeing BFL quote referred to has to be read with these points in mind... if we talk about "runway", specifically, we mean "no clearway" and I suspect that the Boeing statement is looking at the barebones runway .. ie no stopway either.

But it is important that pilots realise BFL is just a constraint on the more general takeoff weight analysis and, if you need the last kilo RTOW on the day, then the generalised analysis is the better procedural way to go. As Mutt suggests, this can become quite an involved, pain in the neck, process when done manually. However, if it be simulated on the trusty PC, then it becomes a doddle ... working up the simulation is quite straightforward but takes a lot of time. When the work has been done, the production of optimised RTOW tables is quick and straightforward ...

Mutt, I presume that the 30 kt delta on the A300 relates to an obstacle limited takeoff from a long runway ? I have no background with the Type but is that a typical sort of variation ?


Now that, as so often happens, this thread is totally off the original question .... I wonder where it will head off next ?