Planes that weigh themselves

[south coast]

I was having a few drinks with some pilot friends the other night when one of them said...

'in this day and age, why aren't planes built with a self-weighing system?'

since we can go to the moon, auto-land planes in bad weather, why aren't planes built with such a capability and do away with averages and estimates regarding actual payload carried?

can anyone shed some light on this perhaps?

[Dualbleed]

Good question, can't wait to see a good answer here :ok:

[john_tullamarine]

.. such systems have been around for decades ... on freighter operations, they can be a very useful backup to all the other systems when it comes to catching out significant errors.

However, as with any weighing system, the underlying technical considerations are accuracy and repeatability .. which go hand in hand with cost.

Overall, there is no real advantage in going to all the hassle and cost of setting up a reweigh standard on board system.

So far as using average weights is concerned, provided that the sample numbers are adequate, the residual errors are quite small generally.

[chiglet]

Well,
IF "Aquatic Mammals" want to weigh themselves tney go to the
"Whale Weigh Station!" so a/c would go to the "Play[n] Station"
watp,iktch

[CR2]

As JT says, they've been around for ages on freighters.

The sensors are located in the gear (I'm talking 744F here). The "problem" with this is that a perfectly flat base (ie ramp/taxiway/runway) is required to get a accurate reading. This of course is rare... The tolerance on the 744F is roughly 2%.

Considering the aircraft has a MTOW of close to 400T (more on the -ERF), you could have a difference of +-8T with this tolerance.

In our discussions with Airbus for the A380F, we brought up a request for a 1% tolerance. We were told this would be unaffordable.

Our guidline is that any difference of +-5T (again 744F) should be "investigated" :confused:

[Tarnished]

Another technical issue in addition to the flat surface (which only really makes CG computation more difficult) is the fact that such systems tend to work base upon undercarriage oleo compression to work out how much mass is pressing down at each location. However the compressibility of an oleo is affected by ambient pressure and temperature, whilst these figures are readily available from other systems on the aircraft, the resulting calculation could only ever be a comparison with the load sheet. If there was a significant disparity between the two the resulting extra effort would outweigh (no pun intended) the cost of gaining certification to use the self-weighing device.

T

[sgt_pepper]

MD-11 has an optional Weight and Balance System (WBS) and requires the following conditions:
- Wind velocity less than 25 kts
- Fuel and engine ignition switches off
- Cargo and passenger loading complete
- Tug not attached
- Fuel scheduling and system testing complete
Two weight and balance computers control, operate and test the WBS. GW, CG and ZFWCG of both W/B computers are compared and averaged to obtain output data that is transmitted to the FMCs.
.... however, I never seen it working.

[john_tullamarine]

The prime consideration remains accuracy and repeatability - which are usually expressed in terms of full scale reading.

Typical commercial weighing systems might be in the order of 5%.

A serious aircraft weighing application looks to much tighter limits .. Australia, for instance, requires 0.2% ... never mind that the majority of weighing kits can't really make that limit anyway.

Aircraft weighing kits are dramatically more expensive than their commercial cousins because of this sort of requirement ... and the errors in weighing aircraft and traps for young players .. let's not go there ... another hobby horse of mine.

On board systems are affected by a variety of error-inducing boundary conditions and, at best, are a useful guide only. I have never seen an on board system agree with the load sheet data but one should see a sensibly, if approximately, constant error. The secret is to watch for a divergence in error which is cause for investigation.

Keep in mind that we are not all that worried about very minor errors.

The value of such systems lies in picking up those errors which come out of left field and bedevil the safeguards built into traditional weight and load control systems used by airline and other operators.

Things like

(a) a defective weighbridge causing all the can weights to be straight out of fairyland .. I recall one such event where the bridge picked up some FOD which affected the system articulation .. picked up by a freighter crew which queried a sudden change in the on board system error history.

(b) cans loaded in the incorrect order .. shouldn't happen but does so far too easily at oh-dark-thirty in the rain. Good reason for the flight crew to do a followup check in those aircraft which have access post loading .. does your loading protocol have an independent person ticking off the cans as they go into the hole ?

(c) cans intended for one aircraft ending up in another .. likewise

(d) significant fuel misloads ... the Gimli glider was only one of many which has been caught by this gremlin.

I'm sure that others can add to this list ....

[Seat1APlease]

Back in the early 70's BA converted their remaining Vanguards into Merchantman freighters. In the process they fitted a system which I think they called "STAN" which gave a readout of the weight and trim using sensors on the wheel oleos.

It wasn't used as the basic information but as a gross error check and as such proved very useful on occasions.

It is in someways surprising that thirty years later, aircraft can still take off with gross errors in weight and trim, especially since the Captain has little real chance of picking up errors in the actual loading process as opposed to errors in the paperwork.

[411A]

Yep, STANS was on the B707 frieghter too.
No, i'm not saying it was all that accurate, but it did pick up gross errors vs. the loadsheet.
PanAmerican kept theirs in reasonable condition...but it was expensive.

[Lu Zuckerman]

Maybe it has been already stated but what about monitoring the gas pressure inside the oleo struts. This pressure reflects the weight of the aircraft on that particular strut. The internal pressure is very stable and not greatly influenced by compression during landing, as the gas is Nitrogen, which is inert. Entered in the computation is the weight of the landing gear below the shock strut and the pressure readouts can be totaled with a not very complex black box.

:E :E

[john_tullamarine]

Lu,

I don't have any design details on any specific systems but I believe that most work on strut pressures for those reasons.

[Stearperson]

According to folks I used to fly DC-8s with the 8s also used to have this type of system installed.
It did work off nitrogen pressure inside the oleo strut.
It was also difficult to maintain and prone to failure. By the time I was flying the planes of these systems had been removed. Of course so had many other useful items like apu's, air conditioning and turbocompressors in some of the planes.
Perhaps the real reason these self weighing systems are not in widespread use today is that operators do not want the crews or the FAA knowing how much weight we are really carrying.

[Boss Raptor]

As already stated Pan Am B707's with freight doors were fitted with a system called 'STANS' from as early as 1965 (believe it was a customer option on all production B707's with freight door fitted)which used the moment/transducer type weighing system to give a weight on an indicator/readout in the cockpit - operating a B707-321C in late 90's it was still fitted and appeared remarkably accurate

I have never seen this system or anything like it in any other Western aircraft

[eastern wiseguy]

Regarding the system om Merchantmen freighters


which gave a readout of the weight and trim using sensors on the wheel oleos.


was this why they stopped EVERY time after a few metres of taxiing ?I recall as a controller these aircraft could always be relied upon to call for taxi ....taxi ,probably no more than 100 yards ....stop ......sit for a few moments ...then proceed as cleared .Were they doing a final "gross error" check?

[Seat1APlease]

Yes,
When loading, the oleos tended to "stick" a little, and give a false reading.

To to get an accurate reading the engines would be started, then taxy and use a few gentle presses on the brakes before an accurate reading would be given, sometimes it had to be repeated a couple of times to get a reading within limits.

[Jhieminga]

.... or to get the door closed! :}