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The Puzzler
20th Sep 2005, 09:33
Puzzle me this....

On the Fleet Admin side of the BLT there is a variety of V1 calculation options available. These options control how the program goes about calculating V1 and the effect this has on the allowable take off weight. Straightforward enough. However, what is the advantage/disadvantage of any given setting? We, as an operator, do not carry large weights, as opposed say to Ryanair. Therefore our policy need not be one that generates a high gross weight for t/off. My BLT user manual points the way to an IATA document - 'Standard Computerized Airplane Performance Interface Specs-Take Off' for more info on V1 options allowed. So are there any performance gurus out there who can help out? :ok:

enicalyth
20th Sep 2005, 19:38
Where is Captain Chambo when we need him. I think he is a bit of a guru on the BLT.

Old Smokey
21st Sep 2005, 08:15
Puzzler,

I hate to see a good question go unanswered (and it is a good question), so if you can tolerate a generic answer, and sort the wheat from the chaff, I'll give it my best generic shot in a day or so if no BLT qualified person comes on board.

I'm absolutely staggered that enicalyth didn't have a major contribution. Come on big E, give it a shot.

Personally, my only knowledge of BLT is Bacon, Lettuce, and Tomato sandwich:E

Whilst we wait for Mr. / Mrs. / Ms BLT to respond, back to the next riveting instalment of "Farting in the Cockpit" (i.e. Augmented thrust):ok:

Regards,

Old Smokey

The Puzzler
22nd Sep 2005, 09:50
Puzzle me this....

Thanks Old Smokey. In the interim I have had some good fortune in answering my own question. ;)

As I previously mentioned our operation has the luxury of not having to operate at high gross weights, so the solution we were looking for was based around balanced field and minimum V1's, as opposed to trying to extract high take off weights and the resultant higher V1, and in some instances unbalanced fields. The drop down options in the BLT allow the user/airline to determine what V1 policy they wish to adopt, taking into account the above. Operating costs, vis-a-vis the use of reduced thrust or derates, will also be affected by the take off policy. The IATA manual (which I was fortunate enough to get a copy of in my hour of need) sets out the standard options that aircraft manufacturers will make available in their standard computerized take off performance. It turns out our best option is a balanced V1 which will also equate to the closest speeds to the FMC (which only looks at the runway itself). This is useful to us as it is a good cross check that we've done it right! :ok: For what its worth we operate 737-700's. :D

For those who still use paper performance, the advantages of a system such as the BLT - if used correctly, which is what I'm trying to achieve - is enormous as it gives a great deal of flexibility and is alot less restrictive particularly when operating with DDG items or on short/contaminated runways.

Hope this helps the performance minded out there!! :hmm: Happy take offs! :D

Old Smokey
24th Sep 2005, 03:27
Happy that you found the answers Puzzler, you saved me a lot of typing, but thinking of a response gave me some alleviation from boredom on a boring 2 day stopover.

You're quite right in saying that the balanced field technique generally provides the best optimisation of Accelerate-Stop and continued OEI Takeoff performance to achieve the highest possible Takeoff Weight. Mind you, Balanced Field is not quite the sacred cow that many would have it to be, if Clearway and/or Stopway are available, and you are chasing maximum possible takeoff weight, then go ahead and use them. The problem is that any number of AFM documents, and the FMC, only look at the Balanced Field, or (even worse) advise that the AFM data is for an unbalanced field with the limiting weight being the lesser of the Accelerate-Stop or continued Takeoff cases.

If, indeed, as you say, your 'typical' Takeoff weight is well below the maximum, then you should have an excess of performance available for both the Stop and Go cases. Even though you are below the limits, it serves well to maximise the safety margins, and in this respect taking precaution against the statistically most likely mishap would be well advised. Accelerate-Stop accident / incident numbere far out-weigh accidents / incidents for the continued Takeoff case and utilisation of the speed schedules offering the lowest possible V1 will statistically favour the worst case. Of course, such a speed schedule reduces margins for the continued Takeoff case, but statistically you are far less likely to have a problem here.

It should be no mystery why the Accelerate-Stop manoeuvre is more fraught with danger than the continued OEI Takeoff if you analyse the in-built margins during certification. For the continued Takeoff, the margins are very generous (up to 50% excess) versus very slim margins for the Accelerate-Stop.

In my Performance Engineering tasks, my objectives are first, to achieve the highest possible weights possible, and, having done so, then set about finding the safest means to achieve those weights. (All of the techniques are safe, some are safer than others).

Regards,

Old Smokey

Capt Chambo
24th Sep 2005, 09:43
Sorry I couldn't reply. I am on leave at the moment and the BLT is on the aeroplane, but it seems the question has been answered.

We rarely go into the admin. side of the BLT, we don't want people fiddling around with it and messing it up.

Incidentally does your BLT have the "Ignore obstacles" option when you calculate your V speeds?

mutt
24th Sep 2005, 12:20
FMC (which only looks at the runway itself).

The FMC doesnt care what runway you are on, the vspeeds are based upon weight, temp, thrust, slope, wind and flap. The FMC uses the runway information in the navigation module not the performance module.

The options available in the BLT are identical to those available to the guys producing paper charts, its up to the airline to decide if they want to use balanced field, optimized, min v1 max v1 or improved climb.

In our case, we try to use the FMC for V-speeds, therefore we opted to use Balanced field methodology.

Mutt.

The Puzzler
24th Sep 2005, 16:05
Puzzle me this....

All of the techniques are safe, some are safer than others

Absolutely correct, its just a matter of ascertaining what suits your operations needs most appropriately. We do have the option to ignore obstacles in VMC Capt Chambo, but dont use it. Thanks for the correction Mutt :O quite right. :ok:

Old Smokey
25th Sep 2005, 00:25
Puzzler,

As Mutt has pointed out, don't forget that the FMC also ignores obstacles. I'm looking forward to the day when the FMC database includes obstacle data for the selected runway, and the associated special procedure. With software upgrades, it's possible right now. I've proposed it, my company has proposed it, but the reply is that there's not enough customers interested to make it worthwhile for them.
We do have the option to ignore obstacles in VMC Capt Chambo, but dont use it.
Thank God for that. May you never change.

Fly safe,

Old Smokey

The Puzzler
25th Sep 2005, 10:02
Puzzle me this....

I'm looking forward to the day when the FMC database includes obstacle data for the selected runway

Doesnt the GIV?V do this already? Anyone out there able to enlighten us on this one? :confused: There is alot of technology available Old Smokey that we dont have available to us yet. I suffer this every day on the -700!! I have had a sneak preview of the next version of the BLT, which is going to be 'interfaced' and presented in the same format as the electronic flight bag, so lets hope that its not too long before the FMC is interfaced with the whole sh'bang as well. All the best. :ok:

mutt
26th Sep 2005, 08:38
Doesnt the GIV

I dont think so, we operate the GIV and dont supply them with an obstacle database.

Douglas offered us a system called TOPAZ for the MD11, it was basically a stage 3 electronic flight bag, however they recinded the offer once they tangled with the FAA.

Maybe Boeing will have more patience or a more understanding FAA office :)

Cheers

Mutt.