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Guest jaredeaston

V Speeds not properly calculated by FMC

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Guest jaredeaston

Hi all. I need help re my V speeds for takeoff on the pmdg 737 700, the FMC calculates the takeoff speeds but when I reach the v1 speed i cant get airbourne, the same with vr and v2, in fact, most times I run out of runway!! can anyone help? Many Thanks, Jared

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Guest jgreth123

While I've had it happen to me a few times as well, there are a few things to keep in mind:1) Altitude - The higher you are, the thinner the air gets. This has a huge effect on the airflow over the wings. Since the air is thinner, you need more of it to pass over the wings, hence higher speed.2) Barometric Pressure - The higher the pressure the easier it is to get off the ground. I'm not 100% sure on the physics of it, but I know that you get 'credited' weight ( subtracted ) from the gross weight on takeoff if the pressure is above normal. ( 220lbs per 1 millibar, not inch )3) Trim - Obvious reasons here, make sure your trim is set appropriately before your takeoff roll.

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This is a trim problem---you need to set the trim to the correct nose up position prior to the TO Run---this is usually about 5.8 to 6.8%----a crude but effective technique is to add in some up trim at VR id the elevators are not providing lift---I hope that this helps

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by the way at V1 you really shouldn't be trying to get airborne... Luca


Luca Benelli

PMDG & WX Radar? read here

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>Hi all. I need help re my V speeds for takeoff on the pmdg>737 700, the FMC calculates the takeoff speeds but when I>reach the v1 speed i cant get airbourne, the same with vr and>v2, in fact, most times I run out of runway!! can anyone help?>Many Thanks, JaredAlso Jared, bear in mind that you don't take off at V1, that's the speed beyond which you are committed to takeoff even if a failure occurs. The procedure is to start pulling back on the controls at Vr (r for rotate) and the aircraft will pitch up and start flying at about V2. So once you've set your trim and picked a long enough runway (a fully loaded 737 needs a fair bit of runway) try that out and see what happens.Cheers!


Mark Adeane - NZWN
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Guest jaredeaston

Ok, ok, a tying error, I know that I can't (or shouldn't) be taking off at V1, mant thanks to everyone for their assistance... i believe it was a trim problem!Thanks all!Jared

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Guest imoz

I have a question that ties into this. I just flew from Atlanta-Hartsfield to Philadelphia International in the 737-800 with the Thunderstorms FS9 weather theme. Weather on arrival was overcast with winds 18 kts at 336 degrees. I selected the 40 degree flap approach and the FMC calculated the approach speed at 126 knots. I plugged this in to the MCP, but the aircraft stalled as it approached this speed. Although ATIS didn't report it, it felt like FS9 was generating much stronger gusts intermittently as well.What did I do wrong or what should I look at to figure out where I went wrong? I also couldn't get the APP button to light up, although I had set NAV1 and NAV2 to the same localizer freq (108.95). I'm thinking I didn't correct for the wind properly but not sure (I was landing on runway 27R). Should I have put 126 + 18 (144) into the MCP?I've also had the same problem at takeoff as above. Tonight I tried setting the trim properly, I was able to rotate properly at Vr but the plane still seemed to use a lot of runway. But at least I get positive climb before the end of the runway now ;-)

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Guest jgreth123

imoz, Please sign your real name to your posts, per the PMDG forum rules. I'm not exactly sure what went wrong with your weather on approach, but here's some guidelines on approach speed. The FMC generates a 0-wind approach speed. Add on half of the constant headwind component. Then add on the full amount of any gust factor. So if you're flying in on 09R and the winds are reported 09016G25 ( 090 at 16kt gusting to 25kt ), your approach speed on the FMC is giving 130 knots. Add half of the constant ( 16 / 2 = 8 ) and the full gust factor ( 25 - 16 = 9 ), so add 17 knots. Your new approach speed should be 147. As for your takeoff problem, using a lot of runway. The FMC only calculated sea-level, 2992 pressure, dry surface, 0-wind speeds. It doesn't account for runway length, altitude, barometric pressure, wind, etc... So some days it'll appear as though it takes off early, and some days late. If someone would be gracious enough to post some real-world charts I'm sure you can calculate to correct these flaws.

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Take-Off Reference P2/2:SEL/OATThe ground OAT is already in there but the first part of that field SEL means?That same page does not allow entry into the psuedo field for runway wind, etc.The take-off and landing charts for the 737s are in the PMDG docs folder for all environmental conditions (at least the Aerosoft CD distributions for the 800/900). How close they are to TRW I do not know.Other than manually adjusting for gusts or runway surface conditions, the V speeds reads out IAS which already considers density altitude, not TAS which does not. The aircraft lift performance depends on IAS. The FMC does not, as you say, account for runway lengths.


Ron Ginsberg
KMSP Minnesota, Land of 10,000 Puddles
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