# Question regarding Required Takeoff Field Lengths

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I definitely see how this is confusing. Going by the paragraph describing how to use the table, it makes perfect sense that a headwind would reduce the takeoff roll. However, taken in the overall context, it seems that these tables are intended to be used to convert the true length of the runway to the actual length that should be counted on given the winds. In that case, a headwind should convert "available length" to a larger number.If the paragraph description is right, and these tables are to be used to calculate actual takeoff roll, then the table headers are certainly erroneous since we would not be working with "available runway length" or "wind corrected field length". Instead they should say "estimate zero wind takeoff roll" and "wind corrected takeoff roll".I know my reply hasn't answered your question, but I thought I would show that I share your confusion. :)

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Thanks for reading my story and replying, Ross. I am glad I am not the only one that got confused. Lets see what the experts reply.Cheers,Henri

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Yeah, I'm not sure I follow these tables, either. If the negative values are tailwinds it makes more sense to me. Say you have a 10,200' runway, but a 15 knot tailwind so the corrected runway length is 8,140. Now suppose it's middle of August in Texas with temps over 100 degrees F. The table on the next page would suggest that if your gross takeoff weight is over 148,000 lbs, you ain't gonna make it off the ground before running out of runway. The lower your weight, the less runway you'll need to get up to rotation speed, and as weight decreases for a given temp, so does corrected field length. With a headwind you can carry more weight given the longer corrected runway length. That's how I read it, anyway. Seems a bit harder to use than a table that gives takeoff distance required for a given weight. Also, I would think you'd need an adjustment for high pressure altitudes somewhere in there.

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Yes, I read the tables exacltly as you describe: 148,000 lbs indeed is too much (be it in the case of single-engine failure, for normal operation you will rotate in time). I agree that there should be a correction for density altitudes.Best regards,Henri

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Yes Henri, you're correct. It should read (negative numbers indicate TAIL wind).

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Thanks, Iz. I am wondering what PMDG says about it.Cheers,Henri

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Just a quick bump to see whether somebody from PMDG can throw any light on this matter.

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Bump again. PMDG, it would be nice if you could confirm that the manual is indeed incorrect in stating that negative values in the required takeoff field length table are for head winds. ie. that the positive values are really headwinds, thus increasing the 'equivalent' runway lengths for use in the runway length limit weight table.Gary

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Don,Thanks for this long explanation. I am really convinced now that a few things in the manual have been mixed up. Best regards,Henri

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Don, Thanks for that. What you say makes sense.And now, a word from our sponsors .... (PMDG, this is your cue ;))Gary

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Hi All, i bring this topic up again just to check i got all which has been said.. :) Let's talk about the 700.. Required Takeoff Field Length (PAGE 1-8)If the runway is 5800 feet long and i have 20 knots headwind i can suppose the runway to be 6380 feet long for further calculations. (headwind as positive number not negative as stated) On wet runway I should consider the corrected figure - 5%? Runway Length Limit Weight (page 1-9)Using the figure from the previous step (6380)OAT 26

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G'day HenriYou are 100% correct in that the manual is incorrect and should be the other way around,well spotted.EnjoyDarren Howie

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Hi again,Maybe instead of a long message I should have posted a of screenshot from a commercial product called ultra-nav to better illustrate the complexity of the issue raised in first place.DISCLAIMER:The above reflects only a personal opinion and was written without any intend to offend, harm or criticize in any way or manner any individual, institution, association or company including PMDG.Michaelhttp://forums.avsim.net/user_files/70817.jpg

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Thanks, Darren.It turned out to be a rather informative thread, didn't it.Thanks everybody for the useful information.Cheers,Henri

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With the info from Don for the PMDG charts reading and Micheal for the "real part" I think I'll get to something useful.. :) Thank you, Luca

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