Sign in to follow this  
Guest TooLowFlap

Sudden problem

Recommended Posts

Hey fellow 737NG drivers,I'm really happy with my Bobby and I haven't had any issues to solve so far. Well but now for a week or so all the 737NG models behave weird. For example, the -600 and -700 calculate take off speeds which are just off the line. 152kts V1 with a payload of 103000lbs payload seems way to much, actually it should be something around 115kts. Then the next thing, my -800 doesn't calculate any speeds for me at all anymore. I checked the GW but it's within normal parameters.If I get the bird off the ground all this continues...FMC suddenly says "unabale CRZ Alt" although I haven't change any of my procedures. And the FMC calculated top of descent is suddenly 180nm away from my destination?!? You see, all this doesn't seem normal. I think it all started after I installed either the Lauda 737-600 livery or after the installation of the SAS Star Alliance livery for the 738!Here my system specs:AMD athlon 2200+Geforce Fx 5600512MB DDR-RAMASUS MotherboardAny help would be greatly appreciatedkind RGDS,Flo (TXL, Germany) http://www.precisionmanuals.com/images/forum/800driver.jpg

Share this post


Link to post
Help AVSIM continue to serve you!
Please donate today!

Well I don't know what a "bobby" is but you V speeds appear to be the Kg or Lbs related issues discussed many times ;D. Please double check the menus....Best Wishes,[h4]Randy J. Smith[/h4][h3]P M D G's 747-400[/h3][h4]coming to a runway near you[/h4][/font color]Caution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro | Windows Xp Pro 64 |

Share this post


Link to post

Hey Randy,well "bobby" is sort of a nickname for boeing :)I'll check the menus again but as you probably read, this is not the only problem. Any ideas why the FMC suggests the top of descent 180nm away from the destination?kind RGDS,Flo (TXL, Germany) http://www.precisionmanuals.com/images/forum/800driver.jpg

Share this post


Link to post

"Any ideas why the FMC suggests the top of descent 180nm away from the destination?"Perhaps the FMC is using the aircraft's weight in its decent predictions? If your aircraft weighs 200,000 Kilos, it has a lot of inertia. A heavier aircraft takes longer to slow down (so I've been told by a 747-400 pilot).Please let us know if changing your PMDG and MSFS weight units fixes both of your problems.Cheers.Ian.

Share this post


Link to post

The FMC does indeed use the Gross Weight as a calculation for the VNAV PATH. This is demonstrated in real life when one of the fuel gauges is unservicable. If this occurs the MEL (min equipment List) states that the Fuel summation unit (the gadget that reads the fuel gauges on behalf of the FMC) be disconnected completely to prevent incorrect VNAV calculations. This disconnection renders VNAV unservicable. Basically the FMC is told at the start of the flight how heavy it is without any fuel (ie you put in the ZFW). The FMC then uses the fuel summation unit and adds the total fuel to the ZFW in order to give Gross Weight (GW). If the Fuel gaues are busted then the FMC thinks it has more or less fuel that it actually has and thus its GW is incorrect. I know this cannot happen in the PMDG aircraft but now consider if your aircraft is set up in KG's and you put in a ZFW in LBS. The aircraft now thinks that without fuel it weighs far more than it actually does (1 kg = 2.2lbs so 100,000lbs = 45454 kg) but the aircraft thinks you meant 100,000kg (220,000lbs) it then adds on the fuel and hey presto the old adage becomes true:Garbage IN = Garbage OUT!If it isn't a weights problem then I'd be interested to see what else could cause it!Kris

Share this post


Link to post

Hey everyone,thanks for the help. Indeed you were right, I must have accidently switched the kgs/lbs button in the PMDG styles menu when I tried out some new settings. So now everything works just fine. Again, thanks Kris for the detailed information!kind RGDS,Flo (TXL, Germany) http://www.precisionmanuals.com/images/forum/800driver.jpg

Share this post


Link to post

"the MEL (min equipment List) states that the Fuel summation unit (the gadget that reads the fuel gauges on behalf of the FMC) be disconnected completely to prevent incorrect VNAV calculations. This disconnection renders VNAV unserviceable."Understood, Kris, but weight would have more effect on climb performance and cruise (optimum and max alts) rather than descent. Would doubling your GW really have such a dramatic affect on descent distance once you had achieved a particular cruise altitude? If your aircraft is twice as heavy as it should be, should this almost double your descent distance? Does a 744 take 4 times as long as a 737 to descend from 35000'? ;-)Cheers.Ian.

Share this post


Link to post

Yes VNAV is more affected in the climb and for working out opt cruise levels but a GW which is significantly out would impact the VNAV PTH descent quite markedly at least it does on the 300. The difference is in the angle of descent and also the times/distanced that the FMC computes to be required for decellerating. For example if you had a right wing fule gauge U/S your aircraft will think it is 4600kg lighter than it actually is (potentially I know you will have burned fuel off ). The FMC computes the decell distances for an aircraft weighing less thus the distances are way less than needed. This in turn now puts you high on the profile and leaves you in a pickle for a nice approach, now you need one of two things more miles or more drag! A 747 will not take 4 times longer to descend because everything is relative. The 747 may weigh 4 times as much but it also has 4 times the thrust and 4 times the drag, effectively all you are doing is scaling things up. I know this is inaccurate but lets remember that these are totally different aircraft. My argument is that the weights being incorrect WILL cause errors in the VNAV PATH both in climb/ cruise and descent. Hope that clears matters upKris

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this