Why do pilots have to calculate their center of gravity? Couldn't this simply be measured?

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?

• Just out of curiosity, which episode/incident was that? Commented Feb 26 at 17:32
• @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
Commented Feb 26 at 18:16
• 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. Commented Feb 26 at 20:11
• A landing gear sensor system to calculate CG wouldn't work well on windy days. Commented Feb 27 at 12:06
• Commented Feb 27 at 12:09

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.

• "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." Commented Feb 26 at 20:07
• Yeah imagine the chaos if the industry was forced off the "standard passenger weight" value used for loading the cattle car. Commented Feb 26 at 20:21
• Also add the fact that the equipment will add to the empty weight of the aircraft, thus reducing its useful load. Commented Feb 27 at 9:14
• 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. Commented Feb 27 at 9:50
• On very small planes (<20 pax like Twin Otter or Cessna Caravan), it's pretty common to weigh every passenger anyway. Commented Feb 28 at 4:51

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.

• 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? Commented Feb 27 at 15:03
• @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. Commented Feb 27 at 15:19
• 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. Commented Feb 27 at 18:55
• But wouldn't weighing the pallets give you the weight, not the mass? Commented yesterday

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.

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.

• 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? Commented Feb 28 at 15:15
• @StephanSamuel They did, it's just that all the parts want to abandon formation when they hit the ground. Commented Feb 28 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
Commented Feb 29 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? Commented Mar 1 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
Commented Mar 1 at 16:25

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...

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.

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.