Folks:Yesterday, I was departing KJAC (Jackson Hole WY). Runway length is ~6300 ft and the airport altitude is ~6500 ft. The was an 8 kt headwind.737-800, weight 151K lbs. Max T/O Power, Max Climb power.I've been trying to follow procedures, so I've started using the PMDG manuals to confirm that I have enough runway. (Reference the PMDG Takeoff Manual for the -800.)So:Required (adjusted) T/O Field Length = 6000 ft (headwind compensation)Rwy Length Limit Wgt = 6000ft, <13C = 181.9K lbsSo, my weight of 151K is well below the limit weight of 181.9K; I should have plenty of runway, right???V-speeds:Here's what the manual says (Dry rwy, flaps 5, wgt 151K) (use interpolation):V1 147, Vr 151, V2 156But, since KJAC is at 6500 ft and the temp was -11C, you have to correct the V-speeds:V1 +4, Vr +3, V2 -2So:V1 151, Vr 154, V2 154But the FMC said:V1 132, Vr 142, V2 152Now, here's my two issues:1. Why are the FMC V-speeds so far off?2. On my T/O (after confirming enough rwy available) I went right off the end of the rwy before reaching the FMC calculated Vr of 142. Ideas?Thanks all,James

James,I'm afraid the PMDG manuals can't really be used to do your takeoff analysis. The PMDG manuals don't identify what altitude and flap setting the runway length-limited performance information is based on, but I suspect it is for sea level. If you go to the Boeing website (http://www.boeing.com/assocproducts/aircompat/acaps/737wsec3.pdf), there is some takeoff performance information there that can be used to get a general idea about what the maximum takeoff weight would be for your situation.Depending on what engines PMDG has modelled, you would need a runway length of anywhere from about 8100 to 8600 feet to take off from a runway at a pressure altitude of 6500 feet at a standard day temperature. This is with your air conditioning bleeds turned off for takeoff and no wind.By the way, the wind speed correction is backwards in the manual. A headwind is considered positive, so your corrected runway length would be greater than the actual runway length.Can't help you on the speeds. I don't know why the speeds from the FMC differ so much from the manual. Maybe someone from PMDG could comment on that.Don S.

Hi James,according to the 737 Flight Manual the correct speeds should have been in your reported conditions:V1=141/Vr=143/V2=148With Flaps 15 you should have gotten: 137/138/143Are you sure to have put all the correct numbers into the FMC? That is ZFW, Fuel, Temperature, Flaps ?I checked randomly the FMC numbers with the Flight Manual's and they were correct (+/- 1 kt).

Is the airport altitude equal to your density altitude? (Does MSFS "do" density altitude?)And don't just limit your analysis to the runway but also factor in the first climb segment and obstacle clearance too.Regretfully, I am not familiar with the parameters of the 737 or I'd jump in with a better answer. But based on what I am reading those are the questions I'd be asking first.Tim__757