Can aircraft weight themselves?

[Self Loading Freight]

Do aircraft weigh themselves prior to takeoff through the displacement of the gear suspension? If not, why not?

R

[SchmiteGoBust]

Good idea! Could be done on air pressure in the oleo, not displacement though. Displacement is dependant not only on weight,but on ambient temperature and ,of course, correct servicing in the first place.
However a system that did it on pressure over all three oleos would certainly be feasible! Don't know why it hasn't been done-probably because aircraft undercarriages are designed by engineers not pilots!!

[static]

Hi,

Some aircraft do; for instance the 747-400.
But still we use the loadsheet figures for performance calculations. Only if the difference between the systems is more then 7000kgs, we have to look into it more closely, but still then the assumption is, that the loadsheet is correct.
The a/c system even tells you the balance figures!

Static

[m&v]

Like the 47=400,there was a system available at one time to weight the DC8 freighters,gross weight picked off the main oleos,c.o.g picked of the nose oleo. I didn't hear of many installations though

[tired]

The A340 weighs itself too, and is usually within a couple of hundred kgs of the loadsheet. I have no idea how it does it.....maybe I should find out??!! ;)

[Self Loading Freight]

Thanks.

Until now, I had no idea that there was anything called an oleo, let alone what it was. I think that's probably healthy. (I'm still not entirely sure, but I guess it's an hydraulic shock thingy).

It's just that nobody's ever weighed me before boarding, and an A3XX full of porky SLFs would probably notice it...

R

[Pdub]

sress/strain guages on each load bearing member of the gear would probably be a better way of doing it, v. light and cheap and accurate.

Retro fit would be relatively straight forward installation wise, as there would already be a wire going into the fusealage for brake temp etc. Tricky bit would be calibrating it, as you'd have to weigh the whole airplane to get a base figure to work from, for all I know this is done anyway, but I bet working out the how the stresses (ie weight) effect the reading got from the guages would be fun, lots of fun with load bearing items at funny angles.

[F/O Junior]

Also on the A320 family and the A330 it's possbile to get a computed (measured) weight, but only once airborne. As far as I remember, it's based on AOA. I'm aware it's not quite the same, but it's pretty accurate.

[spannersatcx]

are you sure that they weigh themselves, I'm not. Is it not that the FMC knows the weight of the a/c as set in software and just adds the weight of the fuel and then cargo/baggage is added after manually. Having done the 744 and 330/340 courses there has never been any mention of this. There used to be a c of g system on the classic that never worked but that just knew where the weight was shifting. I stand to be corrected......

[This message has been edited by spannersatcx (edited 29 November 2000).]

[Silky]

The A320F weighs it's self when it is airborne only when slats are retracted aircraft speed is 250kts or below and the alt is 14500 or below, it does this via the FAC which takes it's inputs from fuel flow and alfa. This is memorized and updated through out the flight.

This is very relevant on the PFD as it is from this information that "coffin corner" is diplayed, even if the aircraft is a lot lighter it will cause the systems to react as if it is at the calculated weight (proctection activation) :) :) :)

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Gravity always wins....

[Genghis the Engineer]

There's a challenge Schmitegobust; stick a Chartered Engineer in a 747, and an ATPL in an aircraft design office and see who screws up first.

Might just give both breeds a bit more respect for each other!

It's the sort of thing that I'll bet was either thought up by an Engineer who flies, or by a pilot with Engineering qualifications.

G

[VTOL]

Pdub, nice idea, but in my experience strain gauges tend to be better at measuring temperature than stress (OK I know about bridge circuits and the like)! But that still begs the question of how you're going to calculate the weight of the a/c from strain gauges on the undercarriage - they don't measure compression too well. A pressure transducer on the oleo would be more practical.

VTOL

[Pdub]

oops , I'll get me coat ;)

Just shows you not to trust what a 2nd year engineering undergraduate says at midnight.

I imagine the temperature of the gas in the oleo could be monitered straightforwardly allowing compensation. Of course none of this gives you an inflight reading, and a bit of rain would add a fair chunk to the weight too.

Out of interest are aircraft weighed at at any maitenance checks? As I understand that light aircfaft have a habit of putting on weight throughout their lives, does the same aply to the big stuff?

[thermostat]

For the A320 we can use the "green dot" speed (which is the single engine best lift/drag speed) to determine the gross wt. of the aircraft in flight. eg -
Green dot spd ......220 Kts
Altitude............31000 ft
From 220 subtract 11. (this is one knot for every thousand feet above fl 200) = 209.
From 209 subtract 85 (CFM), or 80 (IAE) so 209 minus 85 = 124 (CFM) then divide by 2 = 62. 62,000 kg.
This is in the FCOM.
Hope this helps.

[avoman]

Yes aircraft are reweighed every few years. Also after repaint or relevant ie very substantial mods.
The equipment is like giant bathroom scales under the wheels, using strain gauges.

[Genghis the Engineer]

It's not uncommon to carefully position several strain gauges so that the total result can be adjusted to give a continuously correct indication. However, I think the designer would probably rather have legs with integrity, rather than be asked to strangely shape them to suit strain gauge placement (look up a "Sting" as used in wind tunnels - works well but you get some rather funny internal shapes).

I'm with the oleo pressure method - and I can't see why a temperature transducer can't be sat next to the pressure transducer. Thereafter the maths should be pretty straightforward.

There's no strict rule on weighing intervals, but I'd be surprised if many aircraft get away with less than a 5-yearly re-weigh. Balance sheets are constantly adjusted the rest of the time as equipment is added / removed and then specific loading sheets are prepared for each trip.

G

[Moonbeam Purple]

Spanner - The 744F's I fly, all have the weight&ballance system installed. It gives you the grossweight as well as the CG.

Before leaving the gate, you punch the figures from the loadsheet into the FMC manually, and you then compare the two. Usually they're pretty close.

Moonbeam P.

[gas path]

Spannersatcx
You're showing your age there, it was 'donkeys' years ago that the the classic weight and balance system was ditched.
Agreed it never worked though. :)
Oh! my God......I can remember it too!!!!!!!!

[spannersatcx]

Unfortunately Air Hong Kong's 'new' a/c came with it on but was removed about 18 months ago as it was always u/s anyway. I'm not that old, sob sob

[Cornish Jack]

Spanners
Our -400s are so fitted and the output is displayed as advisory. It is noted and compared to the loadsheet figure but unless there is a large discrepancy, the l/s figure is used.

[veewunrotate]

I think that the trident had a weight and balance system installed, though reputedly it was not too reliable. I think it used pressure sensors in the oleos of all three undercarriage so that you could also get the c of g. WRT the A320, whilst you can work out the weight from the green dot speed, it is not that accurate, and the green dot is frozen above FL300 anyway. Airbus say that an error of only 0.3 degrees in alpha position can give quite a large error.

[Silky]

I refer to my previous answer...... :) :) :)

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Gravity always wins....

[QAVION]

Spanners.... Do you have access to KLM's Boeing MM? Their 400's have the W&B System fitted (some of them at least). The system operates on strain sensors in the axles(nose wheel) and strain sensors in the bogeys(horizontal section) of the main gear. They measure the bending of the axles and bogeys.

A computer in the MEC does all the number crunching (Another input to the system is a dedicated pitch sensor (for sensing aircraft attitude). The system is mentioned in Chapter 31-41, I believe.

Cheers.
Q.

[QAVION]

P.S. Once the W&B has been computed, this info is then sent to the FMCs.

Re: Gross Wt and CG... The 400 FMC only knows what data is sent to it by the W&B Computer (or punched into it manually).

[ballpoint]

As additional info I'd like to inform you
that on our KLM 747 classics and 400's the
W&B system is connected to the maindeck loading system.
This to shut down the system when the center of gravity moves too much backwards ant tail tipping might occur.

Regards.