PDA

View Full Version : A321 actual weight Vs FAC GW

Feather Boa
29th Jan 2008, 10:38
Often in our operation the gross weight as calculated by the FAC's is in excess of 3T greater than that on our loadsheets.

Is this also the norm with other operators?

Fx

tom775257
29th Jan 2008, 10:53
What is the alpha call up for FAC computed gross weight? GW (If memory serves) gives simply the inserted ZFW + FOB - I tested this by adjusting the ZFW entry, and the GW alpha call up changed.

Are you just working backwards from the green-dot speed on the PFD to get the FAC computed GW?

Cheers, Tom.

airseb
29th Jan 2008, 11:12
the difference comes from the fact that your GW is calculated from your loadsheet (from the weights used for the passengers and freight) whereas the fac calculates a weight from how the aircraft is really flying. the use of standard masses for the pax can make a real difference.

Dream Land
29th Jan 2008, 14:53
This used to be the case in the states before the BE1900 crash, standard weights changed, where I'm at now my load sheets reflect reality, sounds like your pilot group need to fix the problem.

LanFranc
29th Jan 2008, 15:25
Alpha call-up in the AIDS for FAC calculated gross weight is GWFL for weight in pounds, and GWFK for weight in Kilo's.

We use standard weight that aren't too realistic and at very light weights, say a ferry flight, the FAC calculated weight is very close to the weight on the load sheet. With more and more passengers or bags the difference increases. In the summer with a full load in the A320, 5,000 pounds difference is common at our operation.

airseb
29th Jan 2008, 16:08
the standard weights are icao defined. the only way you'll get exactly the same weight between fac and gw is if you weigh all your passengers individually before the flight (and keep them at the same weight ...).

the standard weights have been corrected following stated accident but that's still what they are: standard. the only option is to break down these weights into categories: male, female and child.

LanFranc
29th Jan 2008, 19:14
Standard weights are defined by certificating authorities. Standard weights in the US come from this document, page 29

I have flown for a half dozen carriers in different parts of the world and each carriers manuals clearly defined the approved weight and balance program. It was my experience that standard weights always favor the airline companies and a full aircraft at close to gross weight on the load sheet, is almost certainly over gross. The thing to remember is that standard weights are a statistical derivation not guaranteed to be 100% accurate 100% of the time.

I certainly keep it in mind, especially at high gross weights when there is no FLEX.

john_tullamarine
29th Jan 2008, 21:14
If, and that is a big "if", the regulator and operator are honest in their processes, the use of standard weights for larger aircraft usually is adequately accurate ... if the system skews the procedure, it's anybody's guess ...

The cited FAA AC standard weights are, I suggest, significantly out of date and current analyses would suggest that they need to go up a bit ... my comment is based on studies done in Australia some time ago and presume that the US and Australian populations are sufficiently similar in the extent of obesity to draw such a conclusion.

LanFranc
29th Jan 2008, 22:22
"The cited FAA AC standard weights are, I suggest, significantly out of date and current analyses would suggest that they need to go up a bit ... my comment is based on studies done in Australia some time ago and presume that the US and Australian populations are sufficiently similar in the extent of obesity to draw such a conclusion."

These latest FAA weights were promulgated in 2004 after the crash of a commuter aircraft (a B1900 in Charlotte I believe) which was legal on paper but overloaded. In regulatory terms, very up to date. It is, I believe, indicative of the FAA's complicity with the Airlines to keep the figures low and therefore aircraft more profitable. For the FAA and the airlines its a risk they see as worth taking. Afterall, how many engine failures are there in operations with an approved averaged weight weight&balance program? How many of those happen at/near V1? How many of those were at/near gross weight?
Aircraft with technology like the A320 family shows these programs for what they are, smoke and mirrors. I have over 10 years/7,000 hours on the A320 and I check my FAC calculated weight against the load sheet on almost every leg. On empty legs where no weight averages are used (other than the 2 pilots) the weights are within a couple of hundred pounds. In regular operations I'd say 5 to 10% of the time the weights are within 1% of each other. 90% of the time the FAC calculated is 2500 lbs more than the load sheet.
Now, is the FAC calculated weight 100% correct/accurate. I'd say not. But bearing in mind it very close to accurate when the aircraft weight is known (near empty) I'd say it is more accurate than the load sheet. I know that the aircraft performance more closely matches the FAC weight than the calculated weight.
Bottom line I suppose, is be aware.

john_tullamarine
29th Jan 2008, 23:04
In regulatory terms, very up to date

.. but probably not so up to date in respect of population statistics ?

Bottom line I suppose, is be aware.

.. a totally reasonable view, I suggest ...

Feather Boa
29th Jan 2008, 23:18
I'd suggest it is not only standard pax weights that create the problem but the cabin baggae allowance. Thee would be very few of our pax coming on board with less than 3kg!
The (almost!) standard practice in our operation is to add 8kts to the VLS calculated by the FMGC to ensure a 5 kt buffer over the VLS displayed on the PFD...

Ps, KA get upset about overweight landings of a few hundred Kilo's, it seems we are often landing at least 1 or 2T overweight, but on paper it's ok!

Fx

JimGreen
7th Apr 2008, 03:43
Hi

Is it advisable to update the ZFW inflight to bring the AUW value to the GWFK weight ? ( especially if the Commercial Figure is significantly LOWER than the FAC computed weight).

I have found that on days where the FAC read out is higher (approximately 1.5 tons higher than the trim sheet figure), the aircraft squats immediately on Flare. Not sure if this is due to the weight difference. (this observation is made considering a 'stable' :E stabilised approach)

JG

Airbus_a321
7th Apr 2008, 15:13
@ tom775257

GWFK

Airbus_a321
7th Apr 2008, 15:57
@ JimGreen

update of ZFW in flight with the value received by GWFK in my opinion and according my experiences I did on this nice aircraft is not really recommended. Although we all know very well the discrepancies which may occur if StandardPaxWeight and StandardBaggageWeight are used for loadsheet computation and the "real loads" boarding our aircraft.
The GWFK value received via AIDS is obviously a kind of an "aerodynamical weight" value only and basically coming from the AOA and computed by the FACs. (as far as I know.)

Resons for not recommending to adjust the GW on the FUEL PRED acc the GWFK value (found by own observations and by "playing" with the AIDS machine):

This GWFK weight figure may change by tons !! during the flight.

Example:
during climb out I checked GWFK: 2.x tons more than on the loadsheet - I adjusted GW on the FUEL PRED page

after hours in cruise I checked GWFK: again 1.x tons more than the already adjusted GW :confused: - now in total an increase to the loadsheet mass by more than 3 tons.

on final approach last check of the GWFK value (to the wiseacres: Yes I know we should have some other pages open on the FMCs during final approach :8 but: by surprise and not expected now nearly 3 tons less than the cruise value, which means: finally the GWFK value stayed quite close to the expected LW according loadsheet.

Hope this helps a bit.

JimGreen
8th Apr 2008, 03:38
Thank you AB321. I was not aware of the changes that occur over long flights. So far Ive done just 2 or 2.5 hr flights.

Regards,
JG

john_tullamarine
8th Apr 2008, 07:23
Airbus_a321 .. you are not overly fond of loadsheets, one would opine. Do keep in mind that a well designed load sheet is as good (functionally) as a long hand calculation. However, as for any calculation - GIGO is the operative acronym ....

Chris Scott
8th Apr 2008, 17:02
Quote from Feather Boa [Jan29/22:18, currently #11]:
The (almost!) standard practice in our operation is to add 8kts to the VLS calculated by the FMGC to ensure a 5 kt buffer over the VLS displayed on the PFD...
[Unquote]

Was a little concerned to read this, as an ex-A320/319 driver (6 years retired). If there is no significant discrepancy between the gross weights calculated by the FACs and the FMCs, this seems to be an unnecessary and potentially problematical practice.

Just in case the previous posts have not made it entirely clear, the quickest way I know of picking up any discrepancy is to compare the 2 FAC-displayed green-dot speeds (on the 2 PFDs. They should be roughly the same. Now, look at the FMC green-dot speed on the PERF page. If you are above FL200, add 1 knot per 1000ft to the FMC green-dot speed. [Or, do it below FL200, and as you slow down approaching the destination airfield. If you are at high IAS, of course, green-dot may be off the bottom of the PFD scale.]

If the difference between FMC green-dot is, for example, 10kts, there is a 5-tonne discrepancy between the FMC GW and the FAC GW on the A320/319. I guess it is the same on the A321, having the same wing when clean?

In my company, we used to ignore discrepancies of less than 5 tonnes, but I can’t comment on whether that would be appropriate on an A321, if the FMC GW was 5 tonnes too low. Perhaps not.

By the way, there’s the quick way of calculating green-dot speed on the A319/320, provided you know the true gross weight:
[GW (in tonnes) X 2] + 85; then add 1 knot for every 1000ft you are in excess of FL200.
Does that also work on the A321?

Apologies to those − including Feather Boa − for whom this may all be old-hat,

Chris

JSF1
8th Apr 2008, 17:52
321 = GW x 1.5 + 110

Chris Scott
10th Apr 2008, 19:38
Thanks JSF1,

So A321 Green-Dot speed calculation is different from the A320 one that I posted above. Instead, it is:

[GW (in tonnes) X 1.5] + 110

Do you still have to add 1 knot for every 1000ft you are in excess of FL200?

The 1.5 factor means that, referring to my above post, a Green-Dot discrepancy of 10 kts would imply a GW discrepancy of 6.7 tonnes on the A321; and a 2-tonne GW-discrepancy would produce a Green-Dot discrepancy of only 3kts.

fdcg27
11th Apr 2008, 00:48
If I am not seriously mistaken, the CLT accident had everything to do with an elevator trim system rigged backwards, and nothing to do with the aircraft weight or balance.
The crew thought they were triming nose down, but were really triming nose up, resulting in a stall shortly after breaking ground.

LanFranc
11th Apr 2008, 02:57
Well I didn't say that incorrect weights were the sole cause of the CLT accident. What I said was -

"These latest FAA weights were promulgated in 2004 after the crash of a commuter aircraft (a B1900 in Charlotte I believe) which was legal on paper but overloaded"

You may review the NTSB findings in their report DCA03MA022. The relevant portion is this:

The National Transportation Safety Board determines that the probable cause of this accident was the airplane’s loss of pitch control during takeoff. The loss of pitch control resulted from the incorrect rigging of the elevator control system compounded by the airplane’s aft center of gravity, which was substantially aft of the certified aft limit.

Contributing to the cause of the accident were (1) Air Midwest’s lack of oversight of the work being performed at the Huntington, West Virginia, maintenance station; (2) Air Midwest’s maintenance procedures and documentation; (3) Air Midwest’s weight and balance program at the time of the accident; (4) the Raytheon Aerospace quality assurance inspector’s failure to detect the incorrect rigging of the elevator control system; (5) the Federal Aviation Administration’s (FAA) average weight assumptions in its weight and balance program guidance at the time of the accident; and (6) the FAA’s lack of oversight of Air Midwest’s maintenance program and its weight and balance program.

So, no, it didn't have "everything to do with an elevator trim system rigged backwards, and nothing to do with the aircraft weight or balance" and yes, you are seriously mistaken.

OutOfRunWay
11th Apr 2008, 09:52
As I recall, neither the elevator nor trim were rigged the wrong way round, but the cables were tensioned incorrectly, so that the elevators DID move in the correct direction, but did not give maximum deflection. Control was insufficient to recover.

May this never happen to us!

OORW

slip and turn
11th Apr 2008, 10:26
At my local airport I have long wondered why having put us to the trouble of taking our jackets and shoes off, that they don't then weigh us.

This guessing lark based on how thin we all were when we were poor and ate our greens isn't exactly a 'professional' way to weigh and balance an aircraft, is it?

I go over 220lbs, all in reasonable proportions you understand ;), but the other day I saw a guy who must've bust 350lbs shuffling from one plate of meat to the other in the Priority queue ahead of me. I studied his ar*e in the interests of science and wondered what seat would actually accommodate it ...

Turns out he could still squeeze it into a standard aisle seat and then rest his chin and forearms on what didn't fit between the arms of the seat! Actually, I now wonder if he was cheating and sitting on one cheek with the one or both seat arms folded up :p ... he did look perched rather precariously ... Nah, probably there's a nack to it ... sit down, spill out, then put the seat arms down :ok:

I pray to God I don't come back in the next life as a low-cost seat cushion :hmm: