View Full Version : 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.
Thanks
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.
rudderrudderrat
11th Nov 2009, 09:26
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.
Mach E Avelli
11th Nov 2009, 09:55
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.
SILENT_BADGER
11th Nov 2009, 10:16
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
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.
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.
Dan Winterland
11th Nov 2009, 12:51
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.
main_dog
11th Nov 2009, 13:17
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)?
MD
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.
Boss Raptor
11th Nov 2009, 13:59
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
....still intact and registering remarkably accurately
Likewise.
STAN (Sum Total and Nose) was/is remarkably accurate...IF maintained properly.
Meikleour
12th Nov 2009, 10:26
Provided that you also factor in the AOA accuracy level of +- 3kts!!
main_dog
12th Nov 2009, 22:45
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?)
MD
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.
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.
Ocampo
13th Nov 2009, 03:00
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...)
After much googling, can't find a pic for you main dog, sorry. I'll see if any cohorts on Freight Dogs have one...
barit1
14th Nov 2009, 12:59
I believe some DC-10's (the KSSU fleet?) once had a wt & balance system - but seldom maintained.
Love_joy
15th Nov 2009, 10:36
A Captain I flew with recently was talking about this aircraft; Vickers Vanguard (http://www.brooklandsmuseum.com/index.php?/explore/vickers-merchantman-vanguard-1969/)
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.
Meikleour
15th Nov 2009, 16:24
Twas fact - I was there!!! ( 1972-1977)
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.
Improved STAN....worked good, lasted a long time.
Others...only hoping ( the load and trim sheet was accuratre)...'hey Sidney, what did the load sheet actually say?
Hey, Bubba?
STAN worked good!
ChristiaanJ
15th Nov 2009, 20:10
The Vanguard/Merchantman at Brooklands is still pretty much alive.... they still do regular engine runs!
So go and ask, maybe even the STAN still works?
CJ
john_tullamarine
15th Nov 2009, 21:21
The original question included "accurately" and that is the killer for on-board systems in respect of for what the output might be useful.
(a) Typical commercial weighbridges probably will come in at something in the order of 0.5-1.0% and the output might be skewed up to as much as 4-5%.
(b) Aircraft (in hangar) weighing systems are considerably more accurate but, at the end of the day, the end result is constrained by the technology in the particular load transducers and the care with which the weighing is performed. The sort of technology currently available is quite amazing and, for instance, some of the platform kits are "accurate" with no significant calibration error through the usable loading range.
Having weighed something upwards of 2000 aircraft (gliders through to airliners) over the years (with a nominal 0.2% requirement) it became very obvious, very quickly, that, even in tightly controlled circumstances, accuracy (or errors) remains an ever present concern. Indeed, for many aircraft weighing systems, the idea of achieving the normal accuracy and repeatability requirements is, at best, fanciful.
The folk doing the job can only endeavour to constrain the errors to a reasonable minimum. That is to say, when you read your empty weight and CG figures in the cockpit, don't fall into the trap of thinking that they are "accurate" .. they are approximate but should be reasonably accurate.
(c) On board systems should be somewhere between the two, with any sort of luck.
Overall, the on-board systems can be comforting to the crew (I never went to work on the Electra without my on-board weighing kit in my back pocket ..) and, at times, a life saver (we had a couple of major ULD weighbridge weighing errors which were picked up by the system .. and, at least, one which wasn't .. that caused some consternation in the ensuing circuit and landing) but they are unlikely ever to replace in hangar weighing for starting or dispatch figures.
The question of standard weights and related accuracies introduces a whole bunch of other problems but they are not related, per se, to the weighing equipment itself. Certainly the potential is there for the on-board system to pick up significant load build errors, whether they be inadvertant or deliberate.
Love_joy
16th Nov 2009, 09:30
The original question included "accurately" and that is the killer for on-board systems in respect of for what the output might be useful.
Accurate weights are the true 'ideal', but consider the real world.
Commercial operations, such as mine (passenger) use standard weights for passengers and their baggage. Even the DOW of the aircraft includes an allowance for me, a figure I am still trying to aspire to! And I'm slim!
There have been a few occasions where I have been watching rotund figures bouncing towards the aircraft, with impossibly heavy carry on bags. I often wonder how the standard figures would stack up if I weighed my sponsors and their cargo.
Of course, the vast majority of passengers in a large group will average out - thus the reason we can use standard masses. But the point to be made is that we don't actually need to know to a +/-1kg degree of accuracy. On the two commercial types I have flow, both very different, we fly using figures calculated from the nearest half ton when rounded up.
Of course, the smaller the aircraft - the bigger the error even a few kg's could create.
The point of a Weight & Balance system is to have a "gross error" check versus the loadsheet. At my previous outfit, a difference of 5T was to be "investigated" (B747-400F). At the end of the day, the loadsheet figures are used; any big differences noted for the engineers to deal with, after all, W&B systems need calibration from time to time.
Love Joy,
If memory servers me correctly you should find the STAN on the Mechantman behind the Captains seat on the side panel.