Weight and balance

A thought brought on, yet again, by this incident.

How is it that, when determining the weight and weight distribution of a big complex expensive airliner, the calculations are based on assumed average weights of passengers and their luggage?

I've thought for years that big commercial aircraft ought to have force sensors in the landing-gear struts, thereby providing three actual weight measurements.  Add them up, and you have total weight; do some further analysis taking the design of the plane into account, and you have balance.

Doesn't even have to impair mechanical integrity; glue-on strain gauges on existing structural members would get the job done, using the strut itself as the load-bearing component of the force sensor.

Bonus: if the computer's opinion of measured weight doesn't match the pilot's guesstimate, it's time to ask some questions.  Like, if the plane comes up light, are we short of passengers, of baggage, or of fuel?

Comments

[Marc]

You don't even need strain gauges. Since most landing gear struts use gas pressurized oil-containing struts (to provide both a spring function and a dampening function), all you need to do is measure the internal pressure (and multiply by piston area). Since the pressure measurement could be done on the existing gas lines (I think it actually is already), the precision would be far greater than a strain gauge system.

That said, I believe this is already done...

[Marc]

Yup.
https://commerce.honeywell.com/webapp/wcs/stores/servlet/eSystemDisplay?catalogId=10251&storeId=10651&categoryId=43752&langId=-3

[Eric Wilner]

Ah! Sure seemed like someone ought to be doing it. Big question is: why isn't everyone doing it?
I had pondered measuring strut pressure, but I've never been quite clear on the construction of landing gear, and the last time I tried to measure weight indirectly it got unreasonably complicated (though that was in a system with a lot more variables). Strain gauges are more of a known quantity in my world.

[Marc]

Pressure is no more indirect than strain gauges (as both technically use an existing structure to translate the force into a measurable quantity). Given the size of the struts, you could have geometric variations in strain (if the attachments weren't simply supported). Pressure gives a nice average.
Though, we are debating around a small variation. Both work around the inability to put a load cell (yes, strain gauge application, but in a structure specifically designed to eliminate error and using well known design and material)

That said, a little googling also found the use of strain gauges on the hub bolts, which would also provide a nice averaging / summing function.

Easiest would be design a part of the strut to have extremely well known and linear strain / gear force relationship, and go with that. But any fear alterations are difficult....did you know they are THE longest lead items in the design/build of an A/C? Its why even near-one-offs (B-2) use gear that is very closely related to others (take a look at the B-2 gear, and at 757 gear, IIRC, its spooky)