Difference GW calculation between FAC/MCDU
Thread Starter
Join Date: Nov 2000
Location: Worldwide
Posts: 102
Likes: 0
Received 0 Likes
on
0 Posts
Difference GW calculation between FAC/MCDU
Called up the alpha label parameter for GWFK (FAC GW) and noticed a 2t difference with our MCDU calculated GW. Is this normal and what is the limit in the difference all could find was that a 7t difference causes a GW message.
Join Date: Jan 2008
Location: ...
Posts: 3,753
Likes: 0
Received 0 Likes
on
0 Posts
There is no limit, you are most likely not supposed to play with the alpha call-up. Therefore the aircraft will warn you if the difference is greater than 7 tonnes.
But your Vapp, magenta bug, is calculated based on the FMGS VLS weight (VLS(fmgs) +5kts). The actual VLS is derived from the FAC.
This is the reason that sometimes the Vapp is less or more than 5 kts above VLS on the PFD speed scale.
My company instructs us to adjust the Vapp if Vapp is smaller than VLS +3kts on the PFD speedscale in order to achieve a +5kts margin.
But your Vapp, magenta bug, is calculated based on the FMGS VLS weight (VLS(fmgs) +5kts). The actual VLS is derived from the FAC.
This is the reason that sometimes the Vapp is less or more than 5 kts above VLS on the PFD speed scale.
My company instructs us to adjust the Vapp if Vapp is smaller than VLS +3kts on the PFD speedscale in order to achieve a +5kts margin.
Last edited by 737Jock; 3rd Sep 2012 at 15:51.
Join Date: Jun 2002
Location: Vega Constellation
Posts: 286
Likes: 0
Received 0 Likes
on
0 Posts
These GW differences are not unusual. The W&B sheet is most of the time based on standard weights, e.g 75 KG per pax, or some fixed values for men/women/children. There is no surprise the actual values rarely match the canned numbers.
This should also make you think of what could be your real TOW when you take-off close to the limit...
Alpha call-up is a secondary tool not meant to correct the FMS values, but if by curiosity, I read 6.5 tons difference with the FMS weight, I would prefer to look again to the W&B sheet to search for gross errors or FMS input mistakes.
Another useful clue is to call up AOA. It has happened in the past that AOA sensors could be slightly miscalibrated, and the AOA1 versu AOA2 could show slightly different values (we're talking symmetric flight here, not engine failure!). If the two AOA sense precisely the same values, the W&B is mostly the reason for inaccuracy, or finger trouble while initializing the FMS. If there is a consistant slight discrepancy or more (like 0.3 to 0.5 degrees), I'd be more inclined to trust the W&B.
Funny to note that QAR (or FDM, whatever) seems to record the weights from FMS, so if you land at 64.5T from FMS, you could actually be higher. Unfortunately, this has given bright sparks the idea that they could modify the GW in flight when they see the predictions showing a LW slightly higher than max. Plain stupid.
Bear in mind that 1 tons difference gives about 2kt error at GD, and about 1kt error at Vref.
Flex
This should also make you think of what could be your real TOW when you take-off close to the limit...
Alpha call-up is a secondary tool not meant to correct the FMS values, but if by curiosity, I read 6.5 tons difference with the FMS weight, I would prefer to look again to the W&B sheet to search for gross errors or FMS input mistakes.
Another useful clue is to call up AOA. It has happened in the past that AOA sensors could be slightly miscalibrated, and the AOA1 versu AOA2 could show slightly different values (we're talking symmetric flight here, not engine failure!). If the two AOA sense precisely the same values, the W&B is mostly the reason for inaccuracy, or finger trouble while initializing the FMS. If there is a consistant slight discrepancy or more (like 0.3 to 0.5 degrees), I'd be more inclined to trust the W&B.
Funny to note that QAR (or FDM, whatever) seems to record the weights from FMS, so if you land at 64.5T from FMS, you could actually be higher. Unfortunately, this has given bright sparks the idea that they could modify the GW in flight when they see the predictions showing a LW slightly higher than max. Plain stupid.
Bear in mind that 1 tons difference gives about 2kt error at GD, and about 1kt error at Vref.
Flex
Join Date: Oct 2009
Location: UK
Posts: 1,270
Likes: 0
Received 0 Likes
on
0 Posts
Hi FLEXPWR,
FCOM DSC 22-40-30 PFD Speed Scale management
"Normal accuracy for VLS in CONFIG FULL is about ± 3 kt."
As Flexpwr mentions, that's about 3 tons error (1kt per ton at Vref)
This old OEB 819-1 explains it better.
FCOM DSC 22-40-30 PFD Speed Scale management
"Normal accuracy for VLS in CONFIG FULL is about ± 3 kt."
As Flexpwr mentions, that's about 3 tons error (1kt per ton at Vref)
This old OEB 819-1 explains it better.
Join Date: Sep 2004
Location: France
Age: 47
Posts: 161
Likes: 0
Received 0 Likes
on
0 Posts
Well all this lecture is wonderful and logical But flying wise.. When the Vapp is close to PFD VLS , the aircraft is very unpleasant to fly with high pitch attitude and the autothrust struggling to keep your bug just on top of VLS. That means something is wrong and not just AoA error. Otherwise the aircraft would behave a little bit better.Remember that all protections are triggered by PFD speeds and not MCDU speeds. If you enter AoA protection it s because you were in the PFD AoA threshold and not MCDU VLS.
Consequently fly your PFD and airmanship dictates to land at Vref+5 in any aircraft.
Any aircraft should be flown in a conventional way and as I know your primary instrument is the PFD and not the MCDU.
Many airlines suffered hard landings because of this matter and requested their pilots to have a decent speed on final and not fly in the backside of the curve just because the books say so. Fly the aircraft and not the computer and remember how you flew your cessna 172 .
Consequently fly your PFD and airmanship dictates to land at Vref+5 in any aircraft.
Any aircraft should be flown in a conventional way and as I know your primary instrument is the PFD and not the MCDU.
Many airlines suffered hard landings because of this matter and requested their pilots to have a decent speed on final and not fly in the backside of the curve just because the books say so. Fly the aircraft and not the computer and remember how you flew your cessna 172 .
Join Date: Jun 2008
Location: Spain
Posts: 2
Likes: 0
Received 0 Likes
on
0 Posts
My two cents:
My operator flyes A318, A319 and A320, and the discrepancy between GW (FMGS) and GWFK (AIDS) has some slight variations (normaly AIDS weight is slightly larger) but in the A318, the difference is usually more than 4 tons! A bit worrying. Also the A318 descend profile always is too low, the airlane going too slow/too low.
Regards
My operator flyes A318, A319 and A320, and the discrepancy between GW (FMGS) and GWFK (AIDS) has some slight variations (normaly AIDS weight is slightly larger) but in the A318, the difference is usually more than 4 tons! A bit worrying. Also the A318 descend profile always is too low, the airlane going too slow/too low.
Regards
