[QUOTE=iamthewalrus(:3=]
I’m slightly unclear on how this situation is even possible. It seems like someone dropped the engineering ball big time.
Are the runways at that particular airport especially short? Why aren’t planes designed so that the number of seats multiplied by the weight allowance of a passenger (baggage plus person) can be lifted on all the runways it’s expected to fly on in no wind?
Or are there actually flyable wind conditions that are worse than having no wind? What happens if there’s enough wind to take off with a certain load and there’s not enough wind at the destination to land?
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Chicago Midway is a very old airport, built back in the day of prop plans with low take-off speed, and is hemmed in so that it can’t really go further expansion. This, combined with the often deleterious environmental conditions in Chicago, limits the conditions and load weights of aircraft. While I’m not familiar with the specifics of Chicago Midway, I do have some passing familiarity with take-off restrictions for military cargo aircraft from a few remote locations (Kodiak, Kwajalein, Wake) and wind and temperature conditions can be a real concern when taking off with near-rated load. Even with the built-in safety margins, if there is any doubt, they don’t fly.
Wind conditions may dictate approach vectors (i.e. north versus south). A strong tailwind from one direction can make landing and taking off in that direction hazardous, and strong crosswinds across a runway may peclude a safe landing, and the flight will be cancelled or diverted. (This happens a lot at high altitude airports where the reduced air pressure can result in mushy handling to begin with. Don’t even bother trying to fly into Flagstaff, AZ, for instance, unless the conditions are pristine.) Under normal conditions aircraft don’t require a headwind to land; however, on an aircraft carrier, most winged aircraft do require a significant headwind to land; this is achieved not by relying on the wind, but making your own; that is, turning the ship (and indeed, the entire carrier group) into the wind and steaming ahead at nearly full speed. (Launching and landing operations on a Nimitz-class supercarrier, for instance, are conducted at 25+ knots.)
[QUOTE=dropzone]
Maybe nobody thought of it before. At first glance it’s an excellent idea. There are others here more technical than I who can give it a second glance and tell you how it’s the dumbest idea they ever heard and why it won’t work. Then others will have ideas how it could work that get shot down and things go back and forth with namecalling and thrown pencils and eventually a design is laid out.
Ah, the Engineering Process in action! 
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I can think of a couple of reasons; one is that you’d have to retrofit this onto existing aircraft, and it would have to be robust enough to survive environmental and high impact loads, yet precise enough to measure the small fractions total mass distribution difference. (As previously noted, the live cargo weight of the plan is a small fraction of overall mass.) Another is that over time and with temperature change the suspension components will change their mechanical and response properties, which will again change readings; either you’d have to account for this somehow or the change in loading characteristic would have to be larger than these mechanical changes. A third is that if you’re going to use this system to provide critical flight data, it needs to be redundant and essentially infallible. Anyone who has played with LVDTs and strain gauges will tell you that they are in fact far from foolproof. So it would end up being either something that would require regular maintanence and calibration, or at best a backcheck against the loading plan.
Given the slim profit margins in the commerical aircraft industry, the hefty safety margins built into commerial airliners, and the cost of a development program of a landing gear/suspension program to make this sufficiently reliable (versus using essentially the same landing gear design on every airliner, which is essentially what is done now) it’s probably deemed not worth it. I wouldn’t regard it as practically infeasible, but it’s doubtful you could justify the cost when a rule-of-thumb measurement of the cargo and passenger weight should suffice. A design to do this wouldn’t be the $50 strap-on unit that flight estimates (heck, a simple stainless dowel pin per NAS spec will probably run you more than that); it would more likely add $10k+ per plane, not counting development costs, calibration, and maintanence. Mind you, that’s a pittance compared to the overall cost of the plane, but without a pressing need for it, I don’t see it happening.
An easier and cheaper solution, were it so desired, would be to have static load cells placed into the tarmac that the plane would back up onto as part of the gate egress maneuver. This would obtain an accurate estimate of weight distribution without concerns about the suspension system (you’d be measuring the entire weight of the aircraft too, not just the sprung weight), no modification to the aircraft, a robust and easily calibrated measurement device, and reduced cost (emplacement per gate rather than per aircraft). Since this isn’t done, save for vibration and dynamic testing of aircraft during development, I’d say there’s no perceived need to do so.
Stranger
