Alessandro
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Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 11:49 am

To avoid planes taking off with too little fuel (like the ATR-72 Turinter and Gimli-glider)
and also have the right balance (central gravity) on the plane and notice overloading of aircrafts, would it make sense to have
wieghing scales on all landing gear. I read in a book about UN operations in Congo-Kinshasa where pilots on a aircraft would put their fist inbetween a place in the landinggear if you could
do that the plane wasn´t overload if not it was.
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metroliner
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 11:56 am

Normally, if there's any doubt about the readings from the tanks, you can work out the fuel remaining in them by using the logbook, or by checking with fuellers.

Aircraft do have pressure sensors in the gear to ascertain whether the plane is on the ground or not - for things like thrust reverser and ground spoiler operation.

I also heard something about piezoelectric weight sensors on the gear - but very vague, and I'm not sure if it would be accurate enough to measure fuel load by.

Quoting Alessandro (Thread starter):
UN operations in Congo-Kinshasa

That doesn't sound like standard operating procedure for many airlines!  Smile
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Alessandro
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 12:18 pm

Metroliner, the 1960ies UN operations in Congo-Kinshasa was cowboy flying.
So how did Gimbli glider and Turinter crash happen? Firstly if the pilot had an indication that
the weight was too low, he/she could notice the take off, was it normal or shorter than a
take-off should be for this amount of weight?
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metroliner
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 12:39 pm



Quoting Alessandro (Reply 2):
Firstly if the pilot had an indication that
the weight was too low, he/she could notice the take off, was it normal or shorter than a
take-off should be for this amount of weight

My knowledge is still that of the enthusiastic amateur, so I can only give you partial answers.  Smile

For what it's worth, noticing the load on take-off by feel alone is a pretty difficult thing to do - since all take-offs are different. Also, in the case of the Gimli Glider, the pilots made an error in computing the onboard fuel and then drew the rest of their calculations from it. The Tuninter case is less familiar to me, but I'd suspect the same thing happened.

What is unclear to me is why nobody seemed to check the fuel gauges on either plane before take-off - or perhaps they did, and assumed they were reading kilograms instead of pounds or some such error.

Quoting Alessandro (Reply 2):
cowboy flying

Well, the 'gouge' they used to check the loading is probably based on years of experience, so it has its use too  Smile
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474218
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 12:42 pm



Quoting Metroliner (Reply 1):
Aircraft do have pressure sensors in the gear to ascertain whether the plane is on the ground or not - for things like thrust reverser and ground spoiler operation.

Most aircraft use magnetic (squat) switches or speed sensors on the wheels, for ground sense (thrust reverse/auto ground spoilers).
 
metroliner
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 12:46 pm



Quoting 474218 (Reply 4):
Most aircraft use magnetic (squat) switches or speed sensors on the wheels, for ground sense (thrust reverse/auto ground spoilers).

Ahh, cheers  Smile I wasn't sure exactly how they worked.

But there is no system in place to measure their exact weight?
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zanl188
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 2:01 pm



Quoting Metroliner (Reply 3):
What is unclear to me is why nobody seemed to check the fuel gauges on either plane before take-off - or perhaps they did, and assumed they were reading kilograms instead of pounds or some such error.

Fuel quantity indicating system on the AC 767 was inop. This was the first item in the accident chain.
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oly720man
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 2:37 pm



Quoting Metroliner (Reply 5):
But there is no system in place to measure their exact weight?

The problem with any sort of mechanism for weighing an aircraft, mounted on the aircraft, is that the mechanism itself becomes part of the load bearing structure of the aircraft, with all the design, safety and reliability issues that then become involved. It would be very difficult to integrate such a device into the undercarriage and I expect operators would not be favourable.
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jetstar
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 3:05 pm

I think it could be done very easily

I would do it with the main landing gear strut compression. As long as the strut pressure is set right, a remote pressure gauge could be used to verify strut pressure then a potentiometer measuring the angle between the upper and lower drag links or one drag link and the strut could be calibrated to convert angle to weight.

On the JetStar, I could rough guess about how much fuel the pilots landed with by just looking at the extension of the struts.

JetStar
 
EMBQA
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 3:42 pm



Quoting Alessandro (Reply 2):
So how did Gimbli glider and Turinter crash happen?

People not paying attention....
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tdscanuck
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 5:33 pm



Quoting Alessandro (Thread starter):
To avoid planes taking off with too little fuel (like the ATR-72 Turinter and Gimli-glider)
and also have the right balance (central gravity) on the plane and notice overloading of aircrafts, would it make sense to have
wieghing scales on all landing gear.

It's present on some aircraft now, but it's very hard to do it accurately. You can do it well enough to make sure you've got the stabilizer set right for the CG, but I think that's about it.

Quoting Metroliner (Reply 1):
I also heard something about piezoelectric weight sensors on the gear - but very vague, and I'm not sure if it would be accurate enough to measure fuel load by.

I don't think it's accurate enough for measuring fuel load, but the 777 uses a piezo sensor on the main landing gear beam as an air-ground sensor, so the concept can certainly work.

Quoting Metroliner (Reply 5):
But there is no system in place to measure their exact weight?

The FQIS measures exact weight. It really all starts from there...if the gross weight you put in the computer doesn't match OEW+payload+fuel, you know you have a problem.

Tom.
 
Tristarsteve
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 5:39 pm

The B747 and B777 have a pressure sensor on the nose gear strut that measures oleo pressure. This used to drive the green band on the stab trim indicator. But only in sectors. It gives the crew a quick balance check on dep.

Many years ago STAN was fitted to the BEA Merchantmen and worked in much the same way, but was on all three gears and showed weight of aircraft. But it was not very accurate.

There is no reason not to fit these to aircraft, but the problem is getting them accurate. If you consider it a gross error check OK, but you can't weigh aircraft on strut pressure accurately on the ramp. Biggest problem is that the struts have a lot of stiction and don't slide smoothly during loading, they move in jerks.
 
Part147
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 6:25 pm

Hmmm, nice idea, but maybe a better solution would be to park the aircraft on a number of scales set into the concrete at the gate (like a weight bridge for trucks) - and that will solve the 'extra weight in the aircraft' problem!

This would only be useful to verify the fuel uploaded at the gate though - IIRC the gimli glider lost it's fuel during the flight itself so that kind of problem wouldn't have 'shown up' on the ground anyway!
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Alessandro
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 6:47 pm

Part147, I think the Azores flight did, Gimli was too light when it took off.
Got another answer that the flying tigers tried two different systems out, first on the DC8 and then the B742, none of them worked so they where scrapped.
See, still things to invent out there, I wonder about helicopters, would that be easier to measure?
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Alessandro
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 6:50 pm



Quoting Metroliner (Reply 3):
My knowledge is still that of the enthusiastic amateur, so I can only give you partial answers.

For what it's worth, noticing the load on take-off by feel alone is a pretty difficult thing to do - since all take-offs are different. Also, in the case of the Gimli Glider, the pilots made an error in computing the onboard fuel and then drew the rest of their calculations from it. The Tuninter case is less familiar to me, but I'd suspect the same thing happened.

What is unclear to me is why nobody seemed to check the fuel gauges on either plane before take-off - or perhaps they did, and assumed they were reading kilograms instead of pounds or some such error.

Tuninter was a faulty fuelgauge, an ATR-42 gauge instead of an ATR-72 installed on the plane.
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oldtimer
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RE: Weighing Scale In The Landing Gear?

Sun Apr 12, 2009 10:06 pm

The Gimi glider had both fuel gauges u/s. Fuel loading was carried by using the dripsticks in the tanks, unfortuneately, the crew thought the readings were in kg but they were in lbs so basically they had less than half the fuel they thought they had.

Can't remember now but either the L1011 or DC10 had a gross weight indicator, haven't worked on either since early eighties, but the F/E's said thay never used them as they were not certified for use so used load sheets for gross weight.
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Okie
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 1:35 am



Quoting Oldtimer (Reply 15):
Can't remember now but either the L1011 or DC10 had a gross weight indicator, haven't worked on either since early eighties, but the F/E's said thay never used them as they were not certified for use so used load sheets for gross weight.

Gross weight indication was an "advertised option" on the 741 during its public introduction.
Most operators did not buy the option and from what I can remember that there were either reliability issues involved with the system or inacuracies and was abondoned quickly.
Whether the reasons were engineering in nature or too few units to sustain factory/manufacturing support or both I do not know.

There were a lot of items on aircraft from that era that do not exist anymore. 1,000 round dial gauges, and a F/E for starters.

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DocLightning
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 4:49 am



Quoting Oly720man (Reply 7):

The problem with any sort of mechanism for weighing an aircraft, mounted on the aircraft, is that the mechanism itself becomes part of the load bearing structure of the aircraft, with all the design, safety and reliability issues that then become involved. It would be very difficult to integrate such a device into the undercarriage and I expect operators would not be favourable.

Not really. It merely needs to measure how compressed a spring of known compressibility is. The landing gear have shock absorbers, which are springs of known compressibility. All you need is the measurement device, which is neither integral nor load-bearing.
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411A
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 5:47 am



Quoting TristarSteve (Reply 11):
Many years ago STAN was fitted to the BEA Merchantmen and worked in much the same way, but was on all three gears and showed weight of aircraft. But it was not very accurate.

Many Boeing 707 freighters had the STAN (Sum Total and Nose) system fitted, and it proved to be very accurate, IF properly maintained.
 
Tristarsteve
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 5:52 am



Quoting Metroliner (Reply 1):
Aircraft do have pressure sensors in the gear to ascertain whether the plane is on the ground or not

The B777 uses strain gauges on the landing gear support beam to provide a Weight on Wheels signal.
I wonder if a more sophisticated version could provide weight output?
 
zappbrannigan
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 10:09 am

Without being an engineer, I think strut-based weighing devices wouldn't be able to calculate ramp weight accurately enough to make them a replacement for the current methods of determining ramp/TOW. I can imagine them requiring excessive levels of frequent calibration to produce a legally usable TOW with aircraft that weigh in excess of 400 tonne in many cases - which would immediately fail the all-important $$$ test.

However I can definitely see a system that can detect obvious errors in weight calculation - e.g. the EK A340 that departed here a few weeks ago with an apparently massive error in the FMS weight input - closest Australia's ever been to a catastrophic air disaster - a good 300m of tail-drag off the end of a 3600m/11,000' runway - very large discrepancy in calculated vs. required V-speeds. When the input vs. calculated ramp/TOW/ZFW is out by tens of thousands of kilos, then perhaps this is a case where such a system would save the day.
 
zappbrannigan
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 10:27 am



Quoting Alessandro (Reply 2):
So how did Gimbli glider and Turinter crash happen? Firstly if the pilot had an indication that
the weight was too low, he/she could notice the take off, was it normal or shorter than a
take-off should be for this amount of weight?

I read in the official Flight Safety magazine in this country on the 20th anniversary a couple of years ago that the Gimli takeoff was conducted at higher thrust than normal for the calculated weight (let alone the actual weight), and due to this the crew may not have noticed any unusual performance increase.

Quoting Metroliner (Reply 3):
What is unclear to me is why nobody seemed to check the fuel gauges on either plane before take-off - or perhaps they did, and assumed they were reading kilograms instead of pounds or some such error.

I'm sure it's been mentioned heaps in this thread - the Gimli Glider's problems began with inop fuel gauges (which were properly MEL'ed), coupled with errors in litres to pounds vs. kilos calculations by ground crew.
 
David L
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 12:26 pm



Quoting Alessandro (Reply 14):
Tuninter was a faulty fuelgauge, an ATR-42 gauge instead of an ATR-72 installed on the plane.

Yes, so they did check a gauge that appeared to indicate that they had enough fuel. It was the fact that it showed the wrong quantity that was the problem. This was compounded by the assumption that the overnight change in the indication was due to refuelling, with no check of the corresponding paperwork.

If you add another system to indicate the aircraft's weight (using the same safeguards) you could still fit the wrong part and get an incorrect weight indication. It might just be simpler to prevent the wrong fuel gauge from being fitted, as was the case with the ATR fuel gauges after the Tuninter accident... and to check the paperwork before departing.  Smile
 
EMBQA
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 4:54 pm



Quoting Alessandro (Reply 14):
Tuninter was a faulty fuelgauge

The fuel gauge was not faulty... it worked just fine. The issues was it was not for the type of aircraft it was installed in.
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redflyer
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 5:08 pm



Quoting TristarSteve (Reply 11):
If you consider it a gross error check OK, but you can't weigh aircraft on strut pressure accurately on the ramp.

IMO, wouldn't a gross-error check make it worthwhile? The thread on the A345 tailstrike in MEL last month is reporting that the pilots entered the planes T/O weight incorrectly and it was off by around 100t. If the system could measure the weight to, say, 10% of actual it could act as a general "catch-all" safety check. Not something to be used for T/O performance calculations, but simply as a "Are you sure your numbers are correct?" warning.
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Ceph
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RE: Weighing Scale In The Landing Gear?

Mon Apr 13, 2009 5:19 pm

Oh man... I was beaten to this... I was thinking of making something like this for my Aeronautical Engineering diploma Final Year Project. Had that idea a few days ago then I saw this thread.  Smile
 
tdscanuck
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RE: Weighing Scale In The Landing Gear?

Tue Apr 14, 2009 2:17 am



Quoting DocLightning (Reply 17):
It merely needs to measure how compressed a spring of known compressibility is. The landing gear have shock absorbers, which are springs of known compressibility. All you need is the measurement device, which is neither integral nor load-bearing.

The problem is that there isn't an obvious candidate for the "spring". Oleo displacement is a horrible choice because, as noted, their spring constant varies all the time with temperature and nitrogen service level, plus they have ferocious stiction (there's no design requirement for an oleo to respond smoothly to small load changes, which is what you need for a good weight sensor). The MLG beam is a nice choice from a purely theoretical point of view, but now you're trying to weight the whole airplane with a strain-gauge tied to something that's not supposed to deflect that much, and is reacting multi-axis loads so you need some way to back out all the assorted torques and loads to find the one that's actually due to gravity. It's certainly doable, but it's not trivial.

Quoting RedFlyer (Reply 24):

IMO, wouldn't a gross-error check make it worthwhile? The thread on the A345 tailstrike in MEL last month is reporting that the pilots entered the planes T/O weight incorrectly and it was off by around 100t. If the system could measure the weight to, say, 10% of actual it could act as a general "catch-all" safety check. Not something to be used for T/O performance calculations, but simply as a "Are you sure your numbers are correct?" warning.

The issue is that you need to certify the thing, or you can't use it for any flight crew annunciations. So, to certify it, you need to be able to guarantee performance. And, if you can't certify it, you can't take any credit for it being there. Still a potential safety improvement, but not nearly as cost effective as beating on the flight crew and dispatches to make sure they actually put in the real weight.

Tom.
 
wingscrubber
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RE: Weighing Scale In The Landing Gear?

Fri Apr 17, 2009 1:49 am

The accuracy of a weight measuring 'strain gauge' would depend on consistent gas strut pressure, shocks leak air just like tyres, so deflection can be different for a given weight, although if you paired your strain gauge with a shock pressure transducer it would be pretty simple to compute the weight that way. Pardon me for not listing formulae but it could be done... Maybe I'll suggest doing that on my airplane  Smile
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tdscanuck
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RE: Weighing Scale In The Landing Gear?

Fri Apr 17, 2009 4:42 am



Quoting Wingscrubber (Reply 27):
The accuracy of a weight measuring 'strain gauge' would depend on consistent gas strut pressure, shocks leak air just like tyres, so deflection can be different for a given weight, although if you paired your strain gauge with a shock pressure transducer it would be pretty simple to compute the weight that way.

How do you back out friction in the strut seals?

Tom.
 
A342
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RE: Weighing Scale In The Landing Gear?

Sat Apr 18, 2009 5:09 pm

Well, many excavators and forklifts have integrated weighing equipment, and they also work with hydraulics. Somehow this should be applicable to aircraft landing gears.
Exceptions confirm the rule.
 
tdscanuck
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RE: Weighing Scale In The Landing Gear?

Sat Apr 18, 2009 11:27 pm



Quoting A342 (Reply 29):
Well, many excavators and forklifts have integrated weighing equipment, and they also work with hydraulics.

Interesting...what's the accuracy requirement? Also, construction equipment has effectively no weight constraint, so that might be why it's practical for them, but I'm not sure how much such a system would weigh.

Tom.
 
A342
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RE: Weighing Scale In The Landing Gear?

Sun Apr 19, 2009 12:24 am



Quoting Tdscanuck (Reply 30):
Interesting...what's the accuracy requirement?

I have no clue.

Quoting Tdscanuck (Reply 30):
Also, construction equipment has effectively no weight constraint

Trucks do. Earth, gravel etc is weighed by the excavator before being put in a dumptruck. IIRC these systems add the weights of multiple "bucketloads". If you enter a predetermined value for maximum truck payload, they will tell you when to stop, and you can empty your last "bucket" until you've reached the permissible weight limit and then dump it into the truck.

Or do you mean that there is no weight limit for the excavators themselves? You should consider that these are also transported by truck, and as such, they are as light as possible (or at least they should be).
Exceptions confirm the rule.
 
Okie
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RE: Weighing Scale In The Landing Gear?

Sun Apr 19, 2009 2:36 am



Quoting Tdscanuck (Reply 30):
Interesting...what's the accuracy requirement?

I think the big issue is not what the accuracy of the total weight of the airframe but where the weight is in relation to the MAC of the aircraft.

Extreme example 10,000 lb object loaded directly above MLG would have a totally different effect on the CoG of the airframe than 10,000 lb object loaded at the tail. The MLG would see the 10,000 lbs but not where it is in relation to the MAC. I am sure computations can be made to accommodate a relationship between the NLG and the MLG but I am guessing this is where the issue lies not necessarily the accuracy of the weight.

Okie
 
rwessel
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RE: Weighing Scale In The Landing Gear?

Mon Apr 20, 2009 11:24 pm



Quoting Okie (Reply 32):
I think the big issue is not what the accuracy of the total weight of the airframe but where the weight is in relation to the MAC of the aircraft.

Extreme example 10,000 lb object loaded directly above MLG would have a totally different effect on the CoG of the airframe than 10,000 lb object loaded at the tail. The MLG would see the 10,000 lbs but not where it is in relation to the MAC. I am sure computations can be made to accommodate a relationship between the NLG and the MLG but I am guessing this is where the issue lies not necessarily the accuracy of the weight.

If the scale on each gear leg is reasonably accurate, then determining the CG is trivial. It's exactly the same calculation as the normal weight-and-balance/CG determination. For simplicity, treat the mains as a unit. Pick a reference point, multiply the weight on the nose gear and the mains by their respective distance from the reference point, divide by the total weight on both points, that's where the CG is relative to your reference point.

For example, let's say the reference point is the nose of the aircraft (completely arbitrary - you just need to pick a point, it doesn't need to even physically be on the actual aircraft), the nose gear is 100 inches back from the nose, and the main gear is 500 inches back. Let's say you measure 1000lbs on the nose gear and 20,000lbs on the mains. The CG is at (100*1000+500*20000)/21000, or 481 inches back from the nose.

Extensions to aircraft with more mains at difference positions (like the 747) is trivial as well - just add additional terms.

Given that you'd probably want to measure right/left balance as well, this gets slightly more complex, and you end up computing the weighted center point of the triangle (pentagon for a 747, rectangle for a B-52) instead, but it's still trivial.

Of course given the difficulties many pilots seem to have with weight and balance calculations...  duck 
 
Okie
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RE: Weighing Scale In The Landing Gear?

Tue Apr 21, 2009 1:47 pm



Quoting Rwessel (Reply 33):
Of course given the difficulties many pilots seem to have with weight and balance calculations...

Thanks for the explanation Rwessel.

That works great as a redundant check. You still need to know the correct weights before loading to have your CG in limits rather than shuffling freight/cargo/bags around in the hold while on the ramp waiting to push back.

I have been on a quite a few ATR's where they shuffle the SLF around for CG issues. Whether it is for being out of limits or to just get more towards a the center of the CG envelope is unknown. However, the ATR has got the cargo-hold forward of the mains.

Okie
 
scrubbsywg
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RE: Weighing Scale In The Landing Gear?

Tue May 05, 2009 5:10 am



Quoting TristarSteve (Reply 19):
The B777 uses strain gauges on the landing gear support beam to provide a Weight on Wheels signal.
I wonder if a more sophisticated version could provide weight output?

what kind of strain gauges are used in service like this? My only experience with strain gauges was during my thesis and they were fairly delicate and i would not want to use them in service on an aircraft.
 
tdscanuck
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RE: Weighing Scale In The Landing Gear?

Tue May 05, 2009 5:41 am



Quoting ScrubbsYWG (Reply 35):

what kind of strain gauges are used in service like this?

Piezoelectric. Very durable.

Tom.

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