Gear Bungie Opinion

[Charles Schefer]

I'l appreciate other owner opinions on this. I have a 1968 PA-30 "B" but I am sure this question applies to all Comanches - singe and twin...

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

[William Hughes]

My personal opinion about much repetitive (prophylactic) maintenance is that the risk of harm from maintenance is greater than the risk of harm from slightly less effective bungees. This is my opinion regarding a great deal of this type of repetitive (prophylactic) maintenance.

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.

[Charles Schefer]

[Kristin Winter]

Hans did some testing. I forget what the numbers were, but there is a significant lose of strength, especially after two years. My unscientific pull tests agree with Hans' conclusion. The bungees unload the transmission on retraction. Bungees are check. Transmissions are expensive. Enough said. I have been replacing mine every year and will continue to do so.

[Charles Schefer]

[N3322G]

After reading Hans' work, I went to every year.

[Kristin Winter]

[Charles Schefer]

Thanks Kristin - no none of those problems. My DOM is an experienced A&P/IA and quite frankly he's by far the best mechanic and troubleshooter I've worked with EVER. He is extremely meticulous and careful. We have the bogert tool and nothing here is done in a ham-fisted manner. I just meant that every time a bolt is removed and reinstalled there is wear - that's just reality.

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

[Charles Schefer]

Well we changed the bungies last night. It's not terribly hard using the Bogert tool. Because of this thread we paid extra attention to the old bungies coming off vs the new ones going on. I have to say quite honestly the difference was significant. The (only) 1 year old bungies exerted a good force to be sure, but the new bungies were significantly stronger. I can' tell you how much stronger but it was a significant delta. Using the Bogert tool it was very easy to stretch / un-stretch the old bungies. The new ones required significantly more effort.

- Charles

[Kristin Winter]

One note for folks doing their bungees, make sure that the roller on the arm turns. I find that most do not as some mechanic has cranked down the bolt enough the mushroom the bushing inside.

[Charles Schefer]

[Christian Bayerlein]

Hello,

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 Grant L1011jock]

Christian,
Just a quick note on your post here. I may be reading too much into it, but it sounds as though your bungee arms may be swapped side for side. Maximum stretch of the bungees is supposed to happen with the gear retracted about 14 degrees. If as you say retracting the gear does nothing but unload the bungees, then they are backwards. Properly installed they pull down during the final bit of extension, and they pull up as the gear reaches a point when counterbalancing the cantelievered weight is beneficial to the mechanicals. The reason for the down pull is to help get the nose gear out into the slip stream on a manual extension, so yes the bungees do help in extending the gear so that you can get the gear properly over center. Additionally, in the event of having to actually do the manual gear extension, make sure you slow to less than 100 mph IAS as stated on the checklist before attempting. This is also a critical step in getting the gear to extend properly. You are correct in stating that they have nothing to do with keeping the gear down, that is done primarily by the nose "down lock" springs and a proper preload on the push pull cables to the main gear, and secondarily by the relatively weak springs on the main gear. By the way, the Maint. Manual specifically says when you change the bungees, change the down lock springs at the same time.

Zach

[LeWayne Garrison]

First let me say that I have changed the bungees (shock cord rings) many times over the last 26 years of ownership and maintenance. At first I used the Bogert tool, but saw injuries to mechanics and damage (major) to airframes over that time. I now use a tool sold by a mechanic at the Orlando convention several years ago. There is not danger in using this tool because the bungee is on the airframe BEFORE it is stretched.

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 Grant L1011jock]

LeWayne,
First you are correct in that the strength of the gear is in the over center position. How do you think the gear stays "over center? If you said anything but spring tension, you don't See the big picture. In short, the springs on the nose gear are so important to keeping the gear over center, that Piper incorporated a kit to double the spring strength, and we know this because they went from one to two springs, and that was mandated by Paragraph A of AD 77-13-21. It is circular reasoning, but if you rig the gear as per the maintenance manual, you actually time the nose gear to go over center slightly after the main gear does, causing a preload situation on the main gear sidebraces. Without these items installed, properly maintained, and properly rigged, including removing excessive play in the bolts and bushings that would allow the properly tensioned over center action to not be maintained (SL 782), the gear will not remain in the down and "locked" (over center) position regardless of whether the transmission is engaged or disengaged, as the transmission mount is no match for the weight of the complete aircraft.

Zach