How can I tell whether my gear cushions or donuts need replacement? What might it cost? Why should I worry about them, if everything seems OK?
(Search strings: cushion replacement, donut replacement, landing gear repair)
Bob Steward, A&P-IA:
Last time I priced donuts about $800/ stack (times 3 gear legs) was the Rapid price. A couple hundred per gear to jack, remove, clean, prep, and install would be in line with what I’d expect.
I certainly hope more people will look closely at the lower attach point where the trailing arm meets the bottom of the donut stack. I see this all the time, the pivot is at or above the lip of the landing gear casting. A dead giveaway that their donuts are out of spec and UNAIRWORTHY per the manufacturer’s published specifications. That means that in flight the gear drops down with no weight on it, and then at touchdown it SLAMS back up against the hardened stack of collapsed donuts and transmits the jolt to the wing spar rather than initially slowing the travel of the gear and spreading the G load of the impact over more time. Same energy, just longer to absorb it. Those with wrinkles in the top wing skins near the gear attach point can probably thank 30 or even 40 year old donuts for the permanent damage to the airframe.
The sad part is that these pieces would not be ignored if they were more visible. However they are out of sight and out of mind. Bill can attest to how much nicer the plane rides over the uneven pavement, and what he didn’t mention is that you can SEE the difference in the way the plane sits on the ramp. You REALLY need that step to get up on the wing with the increased height of the new donuts! Its hard to justify the money when you have other items like a paint job or avionics upgrade, which both have eye appeal.
TECHNICAL EDITOR’S COMMENTS:
– When the donuts get beyond 15 years or so in age, they have become too hard to provide adequate dynamic cushioning. So (for example) when you taxi against a 1″ offset piece of asphalt or concrete (like a raised lip or a pothole), the airframe feels 6 to 10 Gs (or more) instead of 2 or 3 Gs. The fact that the duration is short does not mean that there is no adverse effect from shock loads like this. This is very hard on the gear brackets inside the wings, and on the fore and aft wing attach brackets. I don’t think that the main spar splice plates are as susceptible as the much smaller auxiliary brackets.
– The effect of a hard landing is similarly magnified. With the donuts already compressed so far, and age-hardened, there is much less dynamic range to absorb a hard landing. Sort of like landing on a quarter-inch thick layer of Styrofoam over concrete, instead of a mattress over concrete.
The Styrofoam will help, but….
– When the donuts are still pliable, they recover to their installed height pretty quickly, once the load is removed. This is particularly true in warm weather. This means that if you take off and fly for an hour or two, that inch of ‘set’ that you noticed in the gear may be gone by the time you next touch down. This helps maintain full cushion travel during actual service, even though you may find gear travel from ‘set’ when you jack the plane (or look at your Sierra’s main gear). As the donuts harden with age, the set becomes more and more permanent. So much so that a removed stack of very aged donuts will never regain their original height, despite weeks or months sitting unloaded on a workbench. Contrast that with the fact that you have only a few minutes of working time to get a new donut stack installed into the gear leg, despite the fact that they were first pre-compressed and frozen at their minimum installation length. By the time you put the tools away, they will have warmed enough to completely expand to their service length. The same thing happens with still-serviceable donuts, each time you take off.
– The cushioning comes from dynamic compression of the rubber, NOT from friction on the stack tube. When it is heavily compressed under load, and is not excessively age-hardened, the rubber acts like a very thick gel, and absorbs the load as the stack compresses.
– Much like the Cessna spring-steel gear, good donuts WILL store some energy, and have the potential for rebound. Age-hardened donuts that have taken a deep ‘set’ don’t do much cushioning, and they store very little energy. The severe rebound that can occur from a hard landing on old donuts is the result of the spring effect in the spars, which has virtually zero damping. Now isn’t that good for them.
– Cessna dampens out rebound by using ‘tire scrub’. The tires are forced outward (and sometimes back) as the gear legs flex upward; then inward when the gear legs rebound like a giant leaf spring. The scrubbing dissipates some energy, at the expense of tire life. Tires don’t last long on Cessnas used for landing practice in primary training.
– The Beech gear get some damping from the ID of the donuts rubbing on the stack tube, but not much. It is more on the Sierras, where the donuts are full-width in the center; less on the fixed gear, where the rubber tapers to half-thickness at the center. In both fixed and retract gear, the rubber has to be flexible in order to create damping. If it is age-hardened and set, the ID does not change, as there is very little ‘squish effect’ to push the rubber in toward the stack tube. The rubber disk is more like a hockey puck with a hole in the middle.
The first thing an owner notices with new donuts is a higher airplane. The second thing is that it seems like you can identify more of the small imperfections on the taxiway (like filled asphalt cracks). But then you realize you are not being jarred any more by the big imperfections, and firm landings no longer have any ‘edge’ to them; you just hear the tires chirp.
And if you have the plane re-weighed, the empty CG can become an inch or more further aft (because the main gear tire contact patch is now further forward). This has nothing to do with flight performance, but it can often solve W&B issues from a ‘legalities’ standpoint.
If you send me photos of the sides of your main and nose gear, I can express a valid opinion on whether they are still serviceable. But even if they look OK in terms of degree of set, their hardness is still suspect if they are original. If they aren’t more than 15 years or so old, they can probably keep going for a while, if they are not too flattened. For perspective, when I repair a landing gear on the workbench, I won’t put the old donuts back in if they are ten or more years old.