As I am sure you all know; paragraph (b) of AD 77-13-21 (Prevent Landing Gear Collapse) deals with replacing the landing gear bungee cords. AD 77-13-21 (b) requires this be done every 3 years or 500 hours which ever comes first. However, I bought the Bogert bungie tool and ever since I have owned the plane, my A&P/IA and I have chosen to replace the bungies at every single annual as a standard practice.

Least year when I was ordering new bungies from Webco, Phillip said "oh you're one of those...." [i.e. people who replace them every year]. He questioned why I thought it was necessary to do that but he happily sold me new bungies. When we took the last set of bungies off they seemed like new. That said, I did not do any kind of strength / stretch test or analysis on them and obviously just from visual inspection one can't really tell. Since they sit on the ground partially stretched it seems logical to assume they loose some strength even in just 1 year.

Even though I have the Bogert tool, replacing the bungies requires great care and focus. When the bungie is on the tool it's under tremendous load and if anything is done incorrectly damage or injury can result. Last year as we were going through the process we (my IA and I) debated if we should start doing them say every other year. After all there is wear and tear every time a bolt is un-screwed and re-tightened. Maybe replacing the bungies every year is excessive. We resolved to revisit this at the next annual.... that next annual is now upon us.

I am debating whether to replace the bungies again this year (as I have at every annual) or just leave them for next year's annual. Any thoughts or opinions on this appreciated. Do the rest of you change them every year as well or do you go 2-3 years? I don't think I'll ever go more than 2 years but I can see going 2. Thoughts?

Thanks,

- Charles

So I replaced my bungees according to the schedule. Also, if during a regular annual inspection, they looked odd, frayed, "strechy" or what have you, then I would replace them then as well.

It is important that they not be forgotten for several years unto uselessness. And different bungees will degrade at different rates. But no repetitive maintenance is without costs, and those costs are not just monetary, they also include other risks to the equipment. Much wear and tear on equipment is actually due to repetitive (and perhaps un-necessary) maintenance rather than useful service.

At any rate, today Phillip at Webco shipped me 2 new bungies and they arrive Monday or Tuesday. I am sticking with annual.

- Charles

last weekend we replaced the bungees on our Comanche. We bought her three years ago so I thought it is now time to change them. Actually I don't know for how long they were installed since there was no record in the books.

I don't share the worries of collapsing gear. The bungees support the motor and the mechanic during retraction of the gear. The gear is solely locked by the elbow levers. The bungee is not involved in this at all in normal operation. The AD speaks about "To prevent collapse of the landing gear after manual extension".
Our old bungees were about 1" longer than new. But I can't tell if the extension is already there after one day, one week, one month, or three years.
Bottom line: Since it is not a safety issue we stick to the official cycle of the AD.

To share some of our experience since we just changed the bungees: When you know how to do it, it only takes half an hour to finish one side.
Personal opinion: David's bungee tool (see attachment figure 1A from Comanche Flyer June/2010) does the job and is easy to build.
To reduce the load of the bungee cord it is good to retract the gear. Measure the distance between the pulley and the other end of the bungee and adjust the tool accordingly. Let the gear come down a bit by hitting the master so that you can insert the tool between the bungee cords. Retract the gear again and the bungee sits on the tool. Now you can turn the tool and unscrew the bolt with the pulley. Easy!
Best regards
Christian

Zach

Some of the above posts demonstrate a lack of knowledge of the Comanche gear system. Our gear is held down by one thing only: OverCenter action. The springs and bungees do help hold the gear over center when down, especially when manually lowering the gear. Due to the cam action of the bungee arms, the bungees do help raise and lower the gear, but what holds it down is the OVERCENTER action.

I had the pleasure of speaking many times with Maurice Taylor and have some of his videos. He explained in exquisite detail how the gear was designed and functioned.

As far as changing intervals, anything sooner than every 3 years is up to the owner. I personally find the risk of wearing out the airplane fasteners, etc more than the benefit of changing sooner than required. I am a huge believer is the Mike Busch theory of maintenance, that is, there is such a thing as maintenance induced failures.

LeWayne Garrison, A&P, IA

Zach