With all due respect, I believe the explanation you linked to is misleading at best, and wrong at worst.

See "Misconception" on this page:
http://www.mountainflying.com/pages/mou ... rb_va.html

I can say the same about your reference, too.

Consider an airplane that has a maximum allowable gross weight of 3,000 pounds. If it encounters a +30 fps gust that results in an additional 2-g load factor, the airplane experiences a total of 3 Gs load factor. Multiply the 3-g load factor by 3,000 pounds and the wings are supporting 9,000 pounds.

Note that 3 G is within the minimum load spec required by the FARs. Nothing is going to break on this plane.

Assume the airplane is loaded to 1,500 pounds and that it is subjected to the same gust. With half the inertia, the gust acceleration is doubled, causing the airplane to experience a 5-g load factor (4-g force plus 1-g level flight). Multiply 1,500 pounds by 5 gs and the wings are supporting 7,500 pounds.

Note that 5 G is NOT within the minimum load spec required by the FARs. Something COULD break on this airplane, like engine mounts for example. The wing spar won't break, but if you break an engine off, it won't matter.

Now here is the most important point: in both situations, the MAXIMUM force the gust can impart on the airplane is the same. It is the same wing, which stalls at the same maximum angle of attack, which produces the same force. The plane being lighter or heavier doesn't change the wing's ability to generate lift.

Thus, if the plane weighs 1 pound or 10,000 pounds, the wing spar carries the maximum load it can before stalling and is under the same ultimate stress.

Consider this thought problem. You fly a parabolic path through a thunderstorm. That is the wing producing zero lift. Can the storm break your wing off?

Of course it can. Lightening the load on the wing does nothing to protect you in severe turbulence from an airframe break up.

Try another thought problem. A really heavy airplane is flying at maximum angle of attack, say around 18 degrees. A gust hits it. Can the wing generate any more lift? No. So there is no added force to the airframe.

Compare with a very lightly loaded airplane with near zero angle of attack. Gust hits it. Can the wing generate any more lift? Absolutely, and parts of that airplane are now subjected to huge accelerations beyond the FARs.

Basically, the higher the angle of attack, the less additional lift the gust can generate. How do you get additional angle of attack? Add weight or reduce speed. That is why you have to go slower when lighter in turbulence.

Being over gross weight does not make you more susceptible to in flight break up from turbulence, as odd as that sounds.

Mike C.

Gust loads are calculated independently from the maneuvering envelope. The g from gust loads can be outside the maneuvering g limits. The plane is designed to whatever of hundreds or thousands of loads cases is the highest stress.

Gusts build up in space and time and heavy airplanes have more inertia, so they can penetrate further into a gust before reacting. The acceleration of a lighter airplane as it flies into the gust is load relieving. Because it starts to move upwards with the gust as it flies into the edge, before it gets to peak speeds at the center.

It's really academic anyway because planes are designed for gusts up to the top of the green arc whether light, heavy, loaded fwd or aft, etc etc.

Byron

I think I'll just leave it at "I'll agree to strongly disagree."

There are a lot of trade offs with a 685….cost of engine o/h and general support being chief among them. With full fuel (esp. w/extended range fuel option) you can’t carry much more than a few people and some bags. Even with 435hp/side it’s an underpowered airplane…..very high empty weight. In general they fit a very narrow mission profile…..compared with some of thier shorter bodied piston relatives. The 680F is a great all around performer in comparison.

A better choice John, you might fantasize about an Aero Commander 680FL or 680FLP w/ 400 hp direct-drive Lycoming IO-720 engines. It's called a "Mr. RPM" Grand Commander. Good birds, if you can get ahold of one anymore. Some flew a lot of mail... and held up pretty darn good. ~ ME

https://flightlevelsonline.com/2016/loo ... commander/

There's one for sale right now on Controller or TaP.

This original post was November, 2014. Lots of water under the bridge since then. To say that I am not interested in a 685 Commander would be quite an understatement. As much as I have always been attracted to Commanders, I am surprised that i never owned one. In 2014, I did look into the details of the 685, more with curiosity than any real interest in purchasing. I'll have to say that it may be the single most impractical piston twin ever produced. You could own and fly a wide assortment of turbo props for much less hassle and money.

Jg

The Cessna 431 (441 predecessor with these engines) was cancelled when it had 3 engine failures in 100 hours. Not sure if performance was up to expectations either.

If anyone is interested in a nice 685 w/low engine times PM me.

All of the 685s had their engines changed by the factory from GTSIO-520-F to GTSIO-520-K because the F was such an unreliable engine. Even so, the K (at 435 horsepower) was asking more from the available cubic inches than any other Lycoming 520 engine. It wasn't known for long life.

The 680FLP used separate high pressure hydraulic pumps to pressurize the airframe instead of turbo bleed air. Hydraulic pumps on each engine drove a hydraulic motor that drove a compressor. It used Skydrol, which is toxic.

If you want a pressurized fast Commander on a budget, save your nickels for one of the early turboprop models.

I spoke to a gentleman in Iowa years ago - he was looking to buy an Aerostar.

He told me that he was pilot for some folks that bought a 685 Commander new. They lost the first engine at 11 hours. They lost the second engine at 18 hours. They lost the third engine at 29 hours TT.

Then it went back. As I recall they traded it on a Turbine Commander.