Pawnee air-oil bear strut pressure

On Monday, April 5, 2021 at 2:23:07 PM UTC-4, Larry Ruggiero wrote:
On Monday, April 5, 2021 at 2:11:19 PM UTC-4, Larry Ruggiero wrote:
On Tuesday, March 9, 2021 at 8:24:22 AM UTC-5, wrote:
On Monday, March 5, 2018 at 9:37:44 PM UTC-5, Charles Longley wrote:
OK I give up. What part are you talking about?

Charlie
Pilot/ A&P
Larry, all you need to do is call Dan at Aerial Banners in Ft. Lauderdale, Florida. Dan is the most knowledgeable person around concerning Pawnee's, he operates about 60 of them and his rebuild shop is impressive. Tell him that I sent you, he will be the best contact available for Pawnee information. Bob
Thanks all. To close this out, I just finished overhauling its left strut with new O-rings and backup rings, and it’s fairly straightforward, although the fact that it’s now weeping 5606 out the bottom weep hole tells me that its days are numbered and we’ll probably convert to the hydrosorb/bungee struts after this season. Also the carb airbox wasn’t as worn as I thought and just needed new rubber bushings, which took awhile to get. Btw, the person referenced at Aerial Banners never returned my call.

Larry Ruggiero, A&P/IA
Carolina Soaring Association

You should have called back! FWIW Dan at Aerial Banners is a great guy and has many parts for Pawnee's and is always willing to assist.

On Monday, April 5, 2021 at 2:23:07 PM UTC-4, Larry Ruggiero wrote:
On Monday, April 5, 2021 at 2:11:19 PM UTC-4, Larry Ruggiero wrote:
On Tuesday, March 9, 2021 at 8:24:22 AM UTC-5, wrote:
On Monday, March 5, 2018 at 9:37:44 PM UTC-5, Charles Longley wrote:
OK I give up. What part are you talking about?

Charlie
Pilot/ A&P
Larry, all you need to do is call Dan at Aerial Banners in Ft. Lauderdale, Florida. Dan is the most knowledgeable person around concerning Pawnee's, he operates about 60 of them and his rebuild shop is impressive. Tell him that I sent you, he will be the best contact available for Pawnee information. Bob
Thanks all. To close this out, I just finished overhauling its left strut with new O-rings and backup rings, and it’s fairly straightforward, although the fact that it’s now weeping 5606 out the bottom weep hole tells me that its days are numbered and we’ll probably convert to the hydrosorb/bungee struts after this season. Also the carb airbox wasn’t as worn as I thought and just needed new rubber bushings, which took awhile to get. Btw, the person referenced at Aerial Banners never returned my call.

Larry Ruggiero, A&P/IA
Carolina Soaring Association
I would convert that back to the hydrosorb/bungee configuration whenever you can. If you convert back you can alter the bungee cords used and go to two cords vs three for a much more docile landing bounce. I did this on all of my Pawnee's and it works well. not having 135 gallons of chemical makes a big difference. I might just have a \n old set of hydrosorbs in my stockpile. Bob

On the subject of Pawnee landing gear - be very careful of the Hutch conversion 2 piece slab type gear that is fixed with 3 vertical bolts. If the centermost bolt fails (as it can from vibration over the years) the gear collapses, the prop strikes the ground, and you have a totaled plane from a situation that really can't be detected/inspected before the failure. We lost a towplane that way in a very light landing on grass when the bolt snapped.
The bungee system has some redundancy and the Cessna one piece gear slab is good - but the 2 piece Hutch gear is a problem.
ROY

On Tuesday, March 6, 2018 at 10:55:02 AM UTC-5, Roy B. wrote:
On the subject of Pawnee landing gear - be very careful of the Hutch conversion 2 piece slab type gear that is fixed with 3 vertical bolts. If the centermost bolt fails (as it can from vibration over the years) the gear collapses, the prop strikes the ground, and you have a totaled plane from a situation that really can't be detected/inspected before the failure. We lost a towplane that way in a very light landing on grass when the bolt snapped.
The bungee system has some redundancy and the Cessna one piece gear slab is good - but the 2 piece Hutch gear is a problem.
ROY

The PA-25 SB 25/32/04 dated 01/03/2008 describes a Main Landing Gear Shock Absorption System modification adding a cable to restrict lateral movement of the landing gear leg should a piece of the Hydrasorb shock absorber crack and fail (which they do). We had a Pawnee mod'd (bungee gear system) and had the failure of one support on landing. The down wingtip didn't touch, and neither did the prop. Laviasa had a kit (KIT LA-K-005) with support fittings, a security cable, and longer bolts. Money well spent. I have a soft copy of the SB (2 pages text, 3 pages drawings) if anyone wants one. Also pics of the cable having done its job, a closeup of the installation, and a pic of an unluckier Pawnee without the mod kit.

For light reading, I attach a link to a Canadian Civil Aviation Safety Alert showing the fatigue cracking of the part. https://www.tc.gc.ca/eng/civilaviati...11-02-1344.htm

Dan

Thanks Dan, I will look into this mod. Our gear’s pivot bolts have been replaced in the past few years, but I totally agree redundancy is good.. At least one operator has shared with me a story of her experience with that bolt failing— scary. Right now I need to get its gear unsplayed and softened back up. The POH was written to accommodate a full-up gross weight ag plane, which ours will never be again. Unless greater wisdom appears here, I will start by reducing air pressure in modest increments and see what happens.

Larry

If the gear is splayed, it means you need MORE pressure, not less.

Boggs

More than the recommended 575psi? Knowing that these struts are back-asswards in function, maybe I shouldn't be surprised. The hydraulic side of it isn't compressible but the air is. Might it also mean some of the hydro has to come out as well? Since the IA and I had a big disagreement about its towhook, and since it was suggested I take our business elsewhere, I ain't going back. Second annual in a row where he left me with more crap to deal with than it had when dropped off, and all I get is shoulder shrugs and BS about it being an old airplane when I complain. Yet his business flourishes. ****es me off...

Anyhow, wish I had looked more closely at it when he had it torn apart on the bench so I could wrap my head around its function. The service manual's really no help.

Larry

Well having read through the above data, it's not applicable. Ours doesn't have the hydrasorb gear that also incorporates bungees. Ours is the 2 massive air-oil strut units Piper had on at least one series of Pawnees. I'll try posting a photo.

Larry

That may be worthwhile on this and other models (no, I'm not a A&P....).
I believe, due to other gear failures on a Cub and a Pawnee, the routine is to just replace some major gear bolts every few years.
Can't really check them every day, history has shown they wear out over time. When they wear out, broken bolt, then gear collapse follows right after.
On both of our normal towplanes, we have droop tips. This means we get a "wee bit more clearance" before a prop strike.
No, not perfect.
Yes, may be quite a bit cheaper.

On Tuesday, March 6, 2018 at 11:46:11 AM UTC-7, Dan Daly wrote:
On Tuesday, March 6, 2018 at 10:55:02 AM UTC-5, Roy B. wrote:
On the subject of Pawnee landing gear - be very careful of the Hutch conversion 2 piece slab type gear that is fixed with 3 vertical bolts. If the centermost bolt fails (as it can from vibration over the years) the gear collapses, the prop strikes the ground, and you have a totaled plane from a situation that really can't be detected/inspected before the failure. We lost a towplane that way in a very light landing on grass when the bolt snapped.
The bungee system has some redundancy and the Cessna one piece gear slab is good - but the 2 piece Hutch gear is a problem.
ROY

The PA-25 SB 25/32/04 dated 01/03/2008 describes a Main Landing Gear Shock Absorption System modification adding a cable to restrict lateral movement of the landing gear leg should a piece of the Hydrasorb shock absorber crack and fail (which they do). We had a Pawnee mod'd (bungee gear system) and had the failure of one support on landing. The down wingtip didn't touch, and neither did the prop. Laviasa had a kit (KIT LA-K-005) with support fittings, a security cable, and longer bolts. Money well spent. I have a soft copy of the SB (2 pages text, 3 pages drawings) if anyone wants one. Also pics of the cable having done its job, a closeup of the installation, and a pic of an unluckier Pawnee without the mod kit.

For light reading, I attach a link to a Canadian Civil Aviation Safety Alert showing the fatigue cracking of the part. https://www.tc.gc.ca/eng/civilaviati...11-02-1344.htm

Dan

Dan, thanks for sharing this information. I'd be very interested in the LAVIASA Service Bulletin kit information. LAVIASA has been very difficult to get ahold of. How can I get in touch?

And for those operating Piper PA-18 Supercubs, F. Atlee Dodge has an STC'd mail landing gear safety cable kit, which is very widely used by "off field" operators, e.g., in Alaska, viz: http://www.fadodge.com/safety-cables-for-mlg-std/

I've been of the opinion that the MLG bolts should be changed out, and attach points closely inspected based on number of landing cycles, rather than just hours, especially for tow operations.

Thanks. Curt