Why don't aircraft weigh themselves?
 

Following on from the thread in R&N, where an a/c had a tailstrike following a ten tonne loadsheet error - why aren't there sensors connected with the landing gear that provide an actual weight after loading? Also, assuming that the CoG is somewhere within the geometry of the wheels (surely a safe assumption!), that could be detected and any out-of-safety discrepancy reported.

There are plenty of factors that armchair designers miss, cost/benefit tradeoffs and the usual unforeseen consequences of adding yet more complexity to the already fearsomely complex mix that is a modern aircraft, but none suggest themselves in this case. There are already squat switches - just need to get the resolution up a bit from the "one aircraft" range.

R

How do you account for the "lift" created by the wind whilst on the ground?

Mutt

Simple algorithm fed by the air data computer...:}


Besides didn't the Vickers Merchantman weigh itself oh what 30 years ago.

Weight sensors on the landing gear was an option on the B-747-200.

The data was fed into a small "computer" and weight as well as CG could be read in the cockpit. Kuwait Airways had that option on some, if not all of their -200s.

It's not hard, or expensive.

In the early 1980s I designed a system, for a major UK articulated lorry fleet operator (some 200 lorries IIRC) who wanted to load them to the limit without getting busted on random weighbridge checks, on total weight or axle weight.

It involved fitting strain gauges to the leaf springs but they could go in other places, near the suspension mounting points.

Production implementation weas handed over to somebody and I don't know how far they got, but I remember that many patents had existed on these systems even back then.

On a large jet, the simplest way would be to measure the compressive strain of the steel of each landing gear centre leg. Very easy with a strain gauge, and accurate to maybe 1%.

Obviously it would only work at standstill.

This was actually done on the Merchantman(Vanguard Freighter) as long ago as 1972 The system was called STAN (sum total and nosewheel) and weighed the aircraft and computed the c of g. Also available on B747-200F in the eighties. Both systems were notoriously unreliable and their use discontinued.

The Air Bridge Merchantmen always used to stop on a level piece of taxiway and weigh themselves on their way out to the runway.

I seem to remember that the Lockheed C-5 also has an inbuilt system.

747-400F also has a weight & balance computer. Tolerance is about 2%; given that mtow is almost 400T, you could be 8T out. Discrepancies of 4-5T are investigated prior dispatch. One other detail is that for the system to work properly, the aircraft has to be on a flat bit of tarmac; slopes affect the readings.

At the end of the day, proper care and attention must be given when issuing a loadsheet. There really is little else to say.

I always have thought there should be sensors at the TO point of the rwy with a digital readout of your actual TO weight in view to the pilot. As mutt said the wind could be a factor. But could it be a solution for sure. I prefer an alternative to the weight measured by the operator and an alarm sign to some pirates.
Tailwinds

I believe that if you actually weighed every a/c and got the actual weight (not some estimated value-using standard pax and baggage figures) 9 out of every 10 a/c wouldn't have the performance for some of the more limiting airports.

Especially if you've got Americans on board.:ok:

randomair

the problem with any kind of weight and balance takeoff warning system (other than the economic issue cynically (but perhaps accurately) referenced above) is that the accuracy of the available systems would be such that either:

1. The system has a false warning rate so high as to render it more of a liability than an asset; or
2. The system would have warning thresholds set so unconservatively as to render the system all-but useless except in the case of errors so gross that massive system failures would be required to get there in the first place.

It's analagous to the "takeoff acceleration" warning systems people keep proposing; the idea is nice, but there's no real "sweet spot" where you could have a warning system that worked when it was supposed, and didn't go off when not required.

There are already conservativisms in takeoff performance and in weight and cg envelopes which account for normally expected variability, and procedures should be designed to account for these two. If those procedures have broken down to the extent of allowing a seriously unsafe loading to develop, the "takeoff weight warning system" might just end up being one more thing the accident report records as being ignored/bypassed or simply inop....

Also on the B707, worked OK in my experience, although expensive to maintain.

The concept is simple (usually leg loads) and, from personal experience with a system on a small fleet of 727 freighters, usually there was a reasonably constant error delta but the variations from the expected error position correlated consistently with the load sheet data... ie the system was a very useful taxy check on the paperwork.

On those occasions folk taxied back for a check, the load was wrong with respect to the paperwork.

My first reply here,

The MD-11 is fitted with just such a system, comprised of strain gauges on each axle which feed data to two weight and balance computers.
The calculated a/c weight is compared with the loadsheet prior to dispatch.
The system is also used as an alerting system in the event of possible a/c tip-over during loading.
The alert is triggered by the load on the nosewheels decreasing below a set value.
Once triggered it also cuts power to the cargo loading systems.
Works fine, as long as the housing for the strain gauges is kept perfectly sealed from any fluid leakage and carbon dust from the brakes.

Hi,

I've indeed flown MD-11's that have this system as some 747 (200's and 400's) as a customer option. I know of some cargo airlines that have them installed in their fleet.

So it's already there, and seems to be working allright.

I was involved with Airbus for the A380F; we asked them for a 1% tolerance and were told that was nigh-on impossible & the costs would be horrendous.

As an aside, if I remember correctly, BOG has a weighbridge which ATC directs you to at random.

STAN system
 

The system on the Merchantman worked by measuring the hydraulic pressure in each undercarriage strut using a transducer mounted at the top of the leg. The main inaccuracy came if the strut wasn't charged correctly or was leaking. The system was tested by the crew and gave a fixed weight and C of G value determined from set resistors in each transducer - this ensured the continuity of all the system wiring. It was still in use at AirBridge in the late 80's.

I recall studies being done (80s? 90s?) on installing weighing machines on taxiways and/or aprons, but not the outcome.

This would be a much more cost effective way than having each aircraft weigh itself..it's not only the cost of installing the sensors etc in the aircraft. but the extra weight they crate, maintenance costs etc.

The difficulty was allowing for the different dimensions of different aircraft, as well as wind etc (previous posts) but they should not have been insuperable, I would have thought.