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Can aircraft weigh themselves accurately

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Can aircraft weigh themselves accurately

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Old 11th Nov 2009, 09:38
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Can aircraft weigh themselves accurately

Hi, after much discussion about this subject we decided to ask those who fly aircraft that can do this. Can they weigh themsleves and are these figures used for actual take off calculations or are they used as gross error checks.
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Old 11th Nov 2009, 09:47
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The 744F has a weight and balance system that we use against the loadsheet for an error check. We use the loadsheet figures for actual RTOW performance.
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Old 11th Nov 2009, 10:26
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Hi. The Vickers Vanguard Freighter (VC9) in 1970s used something called "STAN". The main and nose oleo pressures were measured, and knowing the area of the piston in each leg, a pretty accurate cross check was made of the weight. Another calculation revealed the C of G.

The load sheet paperwork was used as the official figures. STAN was just a gross error check.
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Old 11th Nov 2009, 10:55
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Fokker also trialled STAN in the later model F27-500 but had some certification problems so I think they gave up on the idea. Possibly the aircraft was a bit too light (20 tonnes approx) and therefore strong winds may have affected the accuracy? Dunno.
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Old 11th Nov 2009, 11:16
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The A320 series don't weigh themselves on the ground. But once you are airborne it can use information from the Angle of Attack probes to work out its actual weight. It uses this to calculate such characteristic speeds for each flap setting. It displays the relevant speeds on the Primary Flight Display and so if you wish you can make the reverse calculation to work out the actual weight the aircraft thinks it is.

There is a formula in FCOM 3.04.10 and also the QRH has a table in 4.01
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Old 11th Nov 2009, 12:22
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Depends on your definition of "accurate". The 744F's W&B system has a 2% tolerance, so MTOW could show 8T plus or minus (in practise I've found it more accurate than that). Was involved in discussions with Airbus for the A380F; we asked for a 1% tolerance, they didn't think that was possible at a reasonable cost.

The system uses sensors in the gear to work calculate the weight and balance. Obviously a flat ramp is required to get an accurate reading.

As an aside, if you ever see a large unexplained error, trying pushing the a/c back and pulling it forward again. Worked for me a couple of times.
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Old 11th Nov 2009, 12:50
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we were discussing how it would be possible to try and trap the errors of incorrect TOM into efb's and one point that came into consideration was the FMS displaying its calculated weight and not allowing or seriously questioning inputs that do not coincide with what it thinks is correct,however I am still are in the analogue age, just got a 742 with old FMS- test bench for the -400FMS ,so, not up to speed with the glass world yet.
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Old 11th Nov 2009, 13:51
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The 747-400 WABC (Weight And Balance Computer) was allegedly not accurate enough and in the company I flew them for, only used for a gross error check as well, like SMOC's compant (may be the same one). The company preferred us to use the loadsheet figures - the cynic in me suspects this directive came from the accountants!

When you saw an aircraft loaded full with passengers who were last at the notional weight when they were considerably younger and their '7kg' of (unweighed) cabin baggage, I was more inclined to belive the WABCs were more accurate than the loadsheet, as I never saw a WABC MTOW that was below the loadsheet figure. Some differences were truely staggering and lead to the addtion of a few knots for good measure.



The A320 series actual mass can be found in the AIDS in the Alpha codes if your company's aircraft has the option. The codes are GWFK for kg, or GWFP for lbs. I always check it if flying an A321, because the aircraft always believe they are a bit heavier than the loadsheet. The AoA probes feed their information into the FACs (Flight Augmentation Computers) which in turn compute the safety speeds including VLS (Velocity Lowest Selectable), However, the VApp is computed from the FMGC's (Flight Guidance Management Computer) weight that is input during initialisation from the loadsheet weights. This often gives less than the required 5 knots difference required, so I input an increased VApp based on 1 knot per tonne increase over the loadsheet weight. The aircraft is more flyable and manageable with the increased VApp, so I suspect the FACs are probably correct.
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Old 11th Nov 2009, 14:17
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Further 744 WABC question

As an aside, if you ever see a large unexplained error, trying pushing the a/c back and pulling it forward again. Worked for me a couple of times.
I'm just a newbie on the 400 (I'm off the 200) so don't flame me, but I was recently told that the WABC-calculated GW and CG values "freeze" at doors closed.

If by any chance the tailstand/nose strap was still attached when the last door was closed then the values would be inaccurate and possibly out of tolerance, but simply opening L2 and closing it again permits the WABC to have another go at estimating the weight. Anyone here know if this is true (there is shockingly little information about the WABC on our manuals)?

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Old 11th Nov 2009, 14:46
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I've never heard of that one main dog; not saying it ain't so. Are you refering to the FMS gross weight value? The read-out on the -F maindeck does not freeze.
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Old 11th Nov 2009, 14:59
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as above regarding STAN/STANS system - I can cnfm that this was also fitted to (eventually ex) Pan Am B707's and maybe others

sadly our ex Pan Am -321 now long gone and I have lost the set of Pan Am manuals we had detailing the STANS although we never used it operational it was still intact and registering remarkably accurately
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Old 11th Nov 2009, 20:36
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....still intact and registering remarkably accurately
Likewise.
STAN (Sum Total and Nose) was/is remarkably accurate...IF maintained properly.
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Old 12th Nov 2009, 11:26
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Provided that you also factor in the AOA accuracy level of +- 3kts!!
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Old 12th Nov 2009, 23:45
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744 Wabc

Are you refering to the FMS gross weight value? The read-out on the -F maindeck does not freeze.
Yep CR2, referring to the CDU read-out (I don't think we have a read-out on our main decks, is that an option?)

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Old 13th Nov 2009, 00:03
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Main dog.. yes. I haven't seen a pure -F 400 (ie non-converted BCF etc etc) without an RDDU. Have seen one with a read-out in the nose, most with read-outs at both nose and side doors, next to L/M panels. They show gross weight and % MAC; digital displays that change as the aircraft gets lighter/heavier and actual cg value during loading/offloading/fueling.

If you're talking pax a/c, all bets are off; have the grand total of zero experience with those. 20 years -F.
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Old 13th Nov 2009, 00:46
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CR2,

Can you post a pic of these things never seen them or heard of them until now our 400Fs certainly don't have them but that's typical of of CXs minimal approach to A/C purchases.
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Old 13th Nov 2009, 04:00
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The Vickers Vanguard Freighter (VC9) in 1970s used something called "STAN".
I'm guessing that system would be the one (or a more updated version maybe?) fitted later on the MD-80 series? They have a gross weight read-out placed near the fuel indicators, and IIRC, it can show ZFW too (easy calculation anyways...)
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Old 13th Nov 2009, 11:59
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After much googling, can't find a pic for you main dog, sorry. I'll see if any cohorts on Freight Dogs have one...
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Old 14th Nov 2009, 13:59
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I believe some DC-10's (the KSSU fleet?) once had a wt & balance system - but seldom maintained.
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Old 15th Nov 2009, 11:36
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A Captain I flew with recently was talking about this aircraft; Vickers Vanguard

He explained that it could weigh itself, accurately - using a rather interesting solution. Apparently, after you pushed, you had to taxi gently and apply the brakes. The aircraft would rock about on its oleo's, and after a few seconds a little guage on the flight deck would display the aircraft mass AND the %MAC.

I used to be a regular at Brooklands, next time I go I'll suss this out as fact or fiction.
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