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I was watching Air Disasters, and one of the root causes of that episode's particular crash was due to the pilots underestimating their payload weight and being improperly trimmed for take-off.

This seems like an obvious and easily avoided danger. Why don't they just include a sensor in the landing gear suspension system to measure the weight? Adding up the weight each gear measures should precisely give you the total, triangulating them should tell you your center of gravity, no estimating necessary.

What's going on here?

Ryan_L
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    Just out of curiosity, which episode/incident was that? – sophit Feb 26 '24 at 17:32
  • @sophit I didn't catch the beginning of the episode, I'm not sure. It was a twin engine propeller plane though. Sorry I can't be more specific. – Ryan_L Feb 26 '24 at 17:36
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    @sophit I suspect it was Midwest 5481. Mayday / AIr Crash Investigation episode "Deadly Weight". (And before you ask, yes I do have a sadly photographic memory of most episodes of this series) – Jamiec Feb 26 '24 at 18:16
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    This series makes me wonder if the Smithsonian doesn't have enough imagination among its staff to come up with a better variety of subjects than constant repeats of Air Disasters. – Juan Jimenez Feb 26 '24 at 20:11
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    A landing gear sensor system to calculate CG wouldn't work well on windy days. – quiet flyer Feb 27 '24 at 12:06
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    Related - https://aviation.stackexchange.com/questions/44731/do-modern-aircraft-have-sensors-to-determine-weight-of-the-aircraft , https://aviation.stackexchange.com/questions/1850/do-some-airplanes-have-weighing-scales-on-the-landing-gear – quiet flyer Feb 27 '24 at 12:09
  • Broadly, how might CoG be measured? How many of which typeof sensor might that need? – Robbie Goodwin Feb 27 '24 at 21:13

7 Answers7

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It would be possible to design a system that computes an airplane's weight and C of G from the landing gear load in real time, either by incorporating load cells into the gear legs, or measuring nitrogen pressure in each oleo (accuracy might be a problem there), and processing the data with an on-board computer. Or you could just include weight sensor pads on the tarmac somewhere, and weigh and compute the loaded airplanes as they taxi over them like trucks at a weigh station.

Problem is, you need to be able to forecast the all up weight and C of G beforehand anyway, to plan the load amounts and load distribution, fuel load etc. An on-board load sensing system only helps you after everything is complete.

Since you know the weight on the gears of the empty airplane before you started, it's just a simple math calculation to work out the numbers, and being able to do it beforehand allows you to adjust and tune your loading. It has a long history of being an accurate and reliable methodology, and incidents are too infrequent to justify major reform of it.

At the end, because a system to measure and compute the load on the gears in real time is kind of redundant, nobody is going to want to pay for such a system with the cost of certifying it.

So basically, there is no market or regulator demand for such a system. If a rash of incidents occurred that made it clear that the existing methods were unsafe, then you would probably see a move in that direction and start to see development of that sort of system.

John K
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    "Problem is, you need to be able to forecast the all up weight and C of G beforehand anyway..." LOL! So true! "Folks, this is your captain, we're sorry, but one of you wasn't honest about your beer belly and now we have to return to the gate and rearrange everything." – Juan Jimenez Feb 26 '24 at 20:07
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    Yeah imagine the chaos if the industry was forced off the "standard passenger weight" value used for loading the cattle car. – John K Feb 26 '24 at 20:21
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    Also add the fact that the equipment will add to the empty weight of the aircraft, thus reducing its useful load. – Mick Waites Feb 27 '24 at 09:14
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    This answer assumes that an adequately accurate load measuring system is merely a matter of cost, however that is not correct. As a result, a system like this could only ever by used as a fail-safe. – MikeB Feb 27 '24 at 09:50
  • @JohnK Actually, Finland has started weighing pax at the gate on a "voluntary" basis. "Hello Mr... umm... Bubba? Would you mind stepping on the scale?" – Juan Jimenez Feb 27 '24 at 15:45
  • It would be interesting to collect data on that and see if the pareto distribution of weights across the population upon which the standard weight is based matches reality. Of course, I'm guessing the average Finn is somewhat lighter than the average North American. – John K Feb 27 '24 at 17:33
  • It would be interesting to collect data on that and see if the pareto distribution of weights across the population upon which the standard weight is based matches reality. Of course, I'm guessing the average Finn is somewhat lighter than the average North American. – John K Feb 27 '24 at 17:33
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    On very small planes (<20 pax like Twin Otter or Cessna Caravan), it's pretty common to weigh every passenger anyway. – SomeoneSomewhereSupportsMonica Feb 28 '24 at 04:51
  • How does that work, please? Why is your load-sensing system not capable of providing readings every so often, if not in real time? – Robbie Goodwin Feb 28 '24 at 21:42
  • @MikeB That's not correct. I believe the Crane Airweighs system on the 777F was certified for operational use, the numbers that came from it can be used directly. It was shown to be within 1% of external aircraft weighing, which is a lot better than using standard passenger and baggage weights. – user71659 Feb 28 '24 at 23:22
  • @MikeB Is that an STC? How does it work? – John K Feb 29 '24 at 01:29
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    @JohnK If that was for me, it was Boeing factory option that apparently very few people ordered. Strain gauges on the landing gears. What I recall the marketing was in situations where you were close to being weight limited, finding you were a half ton lighter could prevent expensive delays and left cargo. Also, containerized cargo is easy to weigh and balance, but you also have charter carriers who are hired to ship a large industrial thing where the mass and CG aren't precisely known. You may have a reasonable upper bound on mass before you get there, but you still need to balance the plane. – user71659 Feb 29 '24 at 08:28
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    Wouldn't it still be potentially useful to have the measurement of the CoG available after loading is complete? This would allow verification of the calculation. if the calculation was too wrong, then there is still an option to adjust. Of course, this has to be weighed against the additional effort and the general severity of these errors. – Chieron Feb 29 '24 at 09:25
  • @Chieron Personally, I think we should have this system (it's standard equipment for military transports) on new aircraft. As a cheap bit of extra safety. But it's hard to justify commercially if it can't replace the manual calculation. – Therac Feb 29 '24 at 11:12
  • Airbus Airplanes have load detection sensors in undercarriage legs, this allows autopilot to calculate CoG while still standing on Ground. Blessings + – Urquiola Feb 29 '24 at 12:26
  • Just for the record, in actual physics you can NOT calculate the true 3D cog of an object using such weight measurements against gravity on one surface. (A simple way to see this, the scales the wheels of the plane are sitting on, simply have utterly no idea of the height of the object (the plane) and hence no possible way of knowing at what point on Y the cog is. – Fattie Feb 29 '24 at 15:39
  • @Fattie but the person who designed the plane knows the height of the object... So they could do a good job at approximating the value... – Questor Feb 29 '24 at 22:10
  • Hi @Questor. you misunderstand. I added the *"a simple way to see this"* purely as *a didactic aid* to show how it is impossible to learn the THREE DIMENSIONAL cog (ie including the Y value) from the "scales under tires" method. I can absolutely assure you that you can't get the exact Y value of the cog from the "scales under tires" method. (Sure you can guesstimate it.) – Fattie Feb 29 '24 at 22:18
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It's not only possible, it's been done on military transports and some cargo aircraft.

However, one still needs to know the aircraft's mass (not weight) prior to takeoff. Gear-mounted strain sensors get their data skewed by uneven surfaces, wind, and other factors.

Yes, installing such a system on all transport aircraft would have prevented a few accidents. It would not, however, free the crews of the need to calculate the aircraft's mass manually or via manual input. Strong wind near the ground can create lift and make an airplane appear lighter than it really is.

It's very useful for military planes, which might need to urgently transport cargo of unknown weight. Even with known cargo, wartime haste can greatly increase the risk of error. But for commercial cargo, weighing each container or pallet and calculating CG based on its location is much more accurate. Strain sensors could still be useful for double-checking, but they're not required and not often requested.

Therac
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    IIUC, if there's wind blowing on the nose of the aircraft, the wings will generate lift which will reduce the measured weight. Any wind blowing from the front 180° of the aircraft will reduce weight by some amount over each wing, possibly throwing the CoG calc off because one gear will report a lighter than actual weight while the other will report greater than actual. Do I understand correctly? – FreeMan Feb 27 '24 at 15:03
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    @FreeMan Yes. Wind anywhere over the wings will also shift the measured CoG. Not so much as to make it useless, but enough to make it a safety check only. – Therac Feb 27 '24 at 15:19
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    For airliners, the A300 had such a landing gear strain gauge system as an option, but nobody bought it. They tried again with the 777F. – user71659 Feb 27 '24 at 18:55
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If I'm right about this being Midwest 5481 the incorrect calculation of Centre of Gravity was not the full cause of the accident.

Most, of not all, accidents are caused by a series of events - what is often refereed to as the swiss cheese model. In the case of this accident, it was a further aft CoG combined with improper maintenance. Had the pilots have had full elevator trim authority, they probably would have corrected the higher than expected pitch up caused by the aft CoG.

So, in answer to your question, the system doesn't exist because it's not needed.

Jamiec
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Because it would be expensive and just add something else to the list of things to maintain. It's easier to calculate the empty weight CG and then add to that anything else you load on the a/c. Such gross mistakes in CG calculation are extremely rare, and the CG envelopes on airliners are relatively large, e.g. on the 777-200 the range is from 14% to 44% MAC (MAC=Mean Aerodynamic Chord, or the average distance from the leading edge to the trailing edge of the wing.)

A system that weighs everyone as they walk into the jetway would make more sense, but only for airlines. They are experimenting with this in Finland but they only ask for volunteers for now.

Juan Jimenez
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  • While this answer is a little less technical, it leads directly to the real answer: cost vs. benefit. The real cost of this system, including maintenance, training, etc., would greatly outweigh what already works. It points back to the (joking) question: if the black box on an airplane is indestructible in an accident, why don't they make the whole plane out of the same thing? – Stephan Samuel Feb 28 '24 at 15:15
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    @StephanSamuel They did, it's just that all the parts want to abandon formation when they hit the ground. – Juan Jimenez Feb 28 '24 at 21:04
  • From 2010 Boeing patent: "the present invention provides onboard aircraft weight and balance systems and methods for using them that are accurate, reliable, inexpensive, light in weight, and easier to calibrate, maintain and use in the field than prior art onboard weight and balance systems.". Note that sensing the forces on the wheels is not sufficient, the attitude of the aircraft must be known too (using the INS in this case). – mins Feb 29 '24 at 18:22
  • @mins This assumes the aircraft is in still air, no wind, etc. That's probably the reason why it's not deployed on Boeing aircraft, to my knowledge. Is it? – Juan Jimenez Mar 01 '24 at 16:11
  • Afaik, it's not deployed. It could be used as a safety net (perhaps precision is not required), but W&B calculation would still be required to not discover at the last minute there is a problem and the aircraft can't take off. – mins Mar 01 '24 at 16:25
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The short answer for cargo aircraft is because you always calculate the CG before you ever load the airplane. You then load to achieve the calculated CG. It's a proactive approach that's simple, accurate, fast, and cost effective.

If you load the airplane out of limits and catch it by measuring after the fact you would have wasted a bunch of time.

Not to mention all the drawbacks of such a system mentioned in other answers...

Michael Hall
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Just to complement the other answers a bit:

Sensors are normally already installed on the landing gears in order to indicate if weight is loaded on them and determine if the aircraft is on ground. These are termed Weight-on-Wheels (WoW) systems and are used to dis-/enable other systems: for example the thrust reversers are enabled on ground while weapons get disabled. Avionics also extensively uses these sensors.

Why don't they just include a sensor in the landing gear suspension system to measure the weight?

WoW systems are therefore already used and considered safety-critical.

There are several types of WoW sensors, ranging from the simplest mechanical switch mounted on the struct to magnetic proximity sensors based on the change of magnetic field associated with the damper's compression. Other systems (only proposed as far as I know) use strain gauges, cameras or even lasers detecting the landing gears extension. Those latter should be easy to be adapted and used to estimate weight and CG location.

sophit
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When Concorde was being developed a knowledge and control of CofG prior to take off was important to maximizing fuel efficiency and range, as poor loading would require excessive trim and increase in drag.

Barney
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