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adamv66

747 Takeoff

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Hi guys,Just been reading the PMDG Takeoff Manual and had a couple of questions hopefully someone might be able to shed some light on!Takeoff flap setting - are there any particular deciding factors for conducting either a flaps 10 or 20 takeoff? I originally presumed it was a factor of aircraft weight but the full range of operating aircraft weights are in both the flaps 10 and 20 tables!!! Derated thrust - "Whenever possible crews should conduct takeoffs using a derated takeoff N1 thrust setting...." Same thing again really - how to decide between 5% and 15% derate? My guess is this decided by the runway length available? Or is it just a case of different policies from airline to airline? Any further insight much appreciated!Many ThanksAdam

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>Derated thrust - "Whenever possible crews should conduct>takeoffs using a derated takeoff N1 thrust setting...." Same>thing again really - how to decide between 5% and 15% derate?>My guess is this decided by the runway length available? Or is>it just a case of different policies from airline to airline?>Any further insight much appreciated!Factors:1. runway length2. weather (temperature, wind, wet/dry runway)3. weight of the aircraftYou derate as much as you can if the factors above allow for it. You can't however, by FAA regulations, derate more than 25%. 25% is the highest thrust reduction allowed.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg


Michael J.

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Adam,There's NO scentific or real-world basis for my personal rule of thumb but it is this:FLAPSBelow 200,000 kg I use Flap 10, above that, Flap 20. Less than 8,000 feet of runway? Then use flap 20 regardless!DERATEAt sea level and ISA (15 degrees c) on a 10,000 ft runway - Rotate speed (Vr) < 140 knots then I'll derate 10% (TO-2) Rotate speed (Vr) > 140 knots then I'll derate 5% (TO-1)Rotate speed (Vr) > 160 knots then I'll use full TO power.If the temperature > 20 degrees c or Airport altitude > 2,500 feet MSL orRunway is < 9,000 ft:Then the rule shifts up one step (ie a 10% derate becomes a 5% derate, a 5% derate becomes full TO power).Not exactly accurate I'm sure but it seems to work. I've only had the far end of the runway sneak up on me once and that was still OK, I cleared the far end of the runway at about 50 feet at 185 knots!


Mark Adeane - NZWN
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Guest ghostrider117

As far as flaps setting is concerned, I can't help, but regarding thrust derate, here's what I suggest :Read the performance section of the manual (look for "Takeoff perf", I suppose... I can't tell as I don't currently own the QOS). There, you should find some tables useful to calculate the max derate you can use, which won't be 15% or 5% derate but you'll simply change the "SEL" temperature to use a higher number than real OAT.First, you compensate rwy length for wind. That gives you wind corrected field length. There should be tables for that.Then, you take a look at the Runway Limit Weight table, which gives you the MTOW when taking OAT and runway length in account. On the line corresponding to your corrected field length, you seek the weight that's the closest to your actual TOW. On the top, you will then read the max OAT for T/O. This means that if this OAT is the real temp, then you'll leave the ground at the end of the rwy because of mandatory derate due to heat.So, this SEL setting will, I think, give you the thrust derate (considering you have selected TO and not TO-1 or -2) that will lead you to the max use of rwy, that means the "min power" t/o, and so the most efficient (the cheapest, if you prefer). Remember that if you want a bit greater margins, you can lower the calculated SEL by some deg...Well, that's what I would do. But maybe I would end in the trees, as I've eaten a bit too much today :-)Please note that it will probably have also an influence on climb thrust... Nothing's perfect, but I don't think it makes sense to derate for T/O but climb full thrust. So, no real disadvantage.Seb

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>First, you compensate rwy length for wind. That gives you wind>corrected field length. There should be tables for that.>You are making things up a bit. There are no such tables. Derate tables come in two types: either there are individual tables for some typical winds (0,+10,+20,etc) or there is only a single table that tells you how to compensate for wind. There was extensive discussion here on this forum just days ago about this very topic.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg


Michael J.

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