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Can aircraft weigh themselves accurately

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Can aircraft weigh themselves accurately

Old 15th Nov 2009, 16:24
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Twas fact - I was there!!! ( 1972-1977)
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Old 15th Nov 2009, 19:46
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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!
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Old 15th Nov 2009, 20:10
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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
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Old 15th Nov 2009, 21:21
  #24 (permalink)  
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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.
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Old 16th Nov 2009, 09:30
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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.
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Old 16th Nov 2009, 14:42
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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.
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Old 16th Nov 2009, 14:43
  #27 (permalink)  
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Love Joy,

If memory servers me correctly you should find the STAN on the Mechantman behind the Captains seat on the side panel.
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