Narrative:

I have had three events now where crew's were interpreting the headwind/tailwind corrections on the takeoff performance wrong. I had a crew today departing and due to runway construction and the shortened runway; they have some significant performance hits to take for each departure depending on weather conditions. Today's flight I planned using runway 33 which had a tailwind because even with the tailwind you get better performance numbers than if you were to use runway 15. The part where there seems to be repeated confusion with the crews is below their maximum weight section of the takeoff performance; they have their corrections for both headwinds and tailwinds. It reads for a tailwind tw/10 KT then has -465 for their correction. This means you have to take 465 pounds off your max takeoff weight per knot of tailwind. In my instance I had figured up a 5 KT tailwind. That equates to 2325 pound penalty for the tailwind correction. Crew calls up and wants me to run new numbers because the max performance weight they came up with was much higher than the number I came up with. I had a hunch they were doing the same thing as the previous two crews and only used the 465 pound penalty once believing it is a 465 pound penalty for any amount of tailwind up to 10 KTS. I feel the crews are confused about this and are not aware that it is per knot up to 10 KTS. Crews are thinking they can takeoff a few thousand pounds heavier than they can and that is a problem. I can see where the crews get confused as it doesn't make it clear on the max weight section of the takeoff performance. There is nothing there to state that tailwind adjustments are per knot. Our head of training has been instructed that this is the way it's supposed to be.

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Original NASA ASRS Text

Title: Air Carrier Dispatcher reports that flight crews do not understand the tailwind correction for takeoff performance calculations.

Narrative: I have had three events now where crew's were interpreting the headwind/tailwind corrections on the takeoff performance wrong. I had a crew today departing and due to runway construction and the shortened runway; they have some significant performance hits to take for each departure depending on weather conditions. Today's flight I planned using Runway 33 which had a tailwind because even with the tailwind you get better performance numbers than if you were to use Runway 15. The part where there seems to be repeated confusion with the crews is below their MAX WT section of the takeoff performance; they have their corrections for both headwinds and tailwinds. It reads for a tailwind TW/10 KT then has -465 for their correction. This means you have to take 465 LBS off your max takeoff weight PER KNOT of tailwind. In my instance I had figured up a 5 KT tailwind. That equates to 2325 LB penalty for the tailwind correction. Crew calls up and wants me to run new numbers because the max performance weight they came up with was much higher than the number I came up with. I had a hunch they were doing the same thing as the previous two crews and only used the 465 LB penalty once believing it is a 465 LB penalty for any amount of tailwind UP TO 10 KTS. I feel the crews are confused about this and are not aware that it is PER KNOT up to 10 KTS. Crews are thinking they can takeoff a few thousand pounds heavier than they can and that is a problem. I can see where the crews get confused as it doesn't make it clear on the Max WT section of the takeoff performance. There is nothing there to state that tailwind adjustments are per knot. Our head of training has been instructed that this is the way it's supposed to be.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.