Powerflow exhaust cracks
- Thread starter Ak Kurt
- Start date
I got a call from my IA, who was doing an annual inspection on my Husky, telling me that my exhaust was cracked in the same area as pictured above. I called the folks at PowerFlow to see what material was used for their exhaust. They told me that the exhaust is 321 SS and they use 316L SS for filler material. Also, they stated that they back gas their welds when manufacturing. The guy at PowerFlow also stated that the cause of the cracks was either a dynamic balance issue with the prop or that the exhaust was not properly maintained by keeping the slip fits lubed where the header pipes enter the muffler.
After I made the call to PowerFlow, I went to my hangar to see the cracks and to ensure that I had some 1/16" 316L on hand to repair the cracks. First thing out of the IA's mouth was, "it looks like they didn't back gas these welds." I never mentioned that PowerFlow said that they did, he was the one that brought up the poor weld procedure that was used. I looked for my self and saw the black sugary color/texture on the backside of where PowerFlow made the welds.
Yes, the dynamic balancing of the prop is important as well as the annual maintenance on the slip fits on header tubes; but it is also important to correctly weld the parts. I am not saying that the cracks in this area are solely PowerFlow's fault, but I wouldn't be too quick to dismiss the possibility!
After I made the call to PowerFlow, I went to my hangar to see the cracks and to ensure that I had some 1/16" 316L on hand to repair the cracks. First thing out of the IA's mouth was, "it looks like they didn't back gas these welds." I never mentioned that PowerFlow said that they did, he was the one that brought up the poor weld procedure that was used. I looked for my self and saw the black sugary color/texture on the backside of where PowerFlow made the welds.
Yes, the dynamic balancing of the prop is important as well as the annual maintenance on the slip fits on header tubes; but it is also important to correctly weld the parts. I am not saying that the cracks in this area are solely PowerFlow's fault, but I wouldn't be too quick to dismiss the possibility!
Flying Dave
Active Member
This sure does seem to be a common occurrence with this setup. For the money spent I’d be slightly ticked off…
Kent Wien
Well-Known Member
It seems to happen only to the heater box ‘ribs’ that the shroud mounts to. So fortunately it hasn’t been as much of a safety issue, but something that needs to be fixed when it arises. I actually have my prop balanced rather frequently in hopes of never having this occur again.
For clarification, on my muffler it was only the heater flange/end-plates that were not back purged when TIG welded. It looked like they back purged the inside of the exhaust muffler tubes when they welded on the heater's end plates; however, didn't bother with the back side of the heater end-plates. Whether the cracking is from lack of back purging or vibration, it is still a design flaw. To a degree, PowerFlow is using vibration that can be well within the industry's normal acceptable range as a scapegoat for their design flaw. It looks like the cracking started at the heat affected zone of the end-plates where the metal was sugared during welding.
Kent Wien
Well-Known Member
Well, the stock exhaust hasn’t been immune either in our circle of Huskys. We have 5 examples, and on the new ones Aviat really came through for us.
harry harper
Active Member
Had to lookup "back purging". Bought the A-1 just over a year ago and it has a PF. Did what I thought was a thorough annual last winter including a bunch of updates. But we missed the specific maintenance to the PF slip joints. Could find nothing in the logs re lubing the slip joints previously. Annual is coming up soon, so, what to do with cracks, if any? Recommendations please. Boise area location.
I thought about the crack and how to weld it and decided that if I just weld the crack, it would most likely fail again within a year. The problem is that this is a design flaw and a second round of welds on SS will end up with more damage in the metal's heat affected zone (HAZ). This means that the new weld would make the part fail even quicker than before. So I ran the muffler by F. Atlee Dodge in Anchorage to see what they would recommend for a repair. F. Atlee Dodge has been making aircraft muffler repairs for decades and has done reasonably good work for me in the past. F. Atlee Dodge would not attempt the weld repair because they consider it a design flaw that a simple weld would not solve. Their comment was to send it back to PowerFlow or throw the exhaust system away. They said that this is a common problem across many of PowerFlow's exhaust system applications. Turn out that the same design is used for PowerFlow's O-360 systems used on re-powered 170s & 172s. The muffler end plates crack in the same place on PowerFlow systems sold to that market as well. A quick weld over the crack will not solve the design problem.
So I will be putting my old OEM exhaust system back on for now and will address this problem later. My thoughts are to call PowerFlow and see if they will share their repair procedure/method with me. I suspect that they will say that I have to send the muffler in and they will most likely cut off the damaged end plate area and graft a new replacement piece of end plate back on without addressing the weak design. If PowerFlow does not have a redesign for the end plate problem, I will take a detailed look at it myself using FEA to visualize the cause for the stress riser in the problematic crack prone area. Hopefully I can work out a repair procedure that will solve/reduce the design problem and one that will satisfy my IA before a repair is started. The problem is that things like this just can't be blindly beefed up because it could transfer the problem to the other side of the weld which could create an exhaust leak into the cabin heating system.
CheemsAF, did the folks at Aerospace Welding make a doubler plate or a new end plate when they repaired your muffler; or did they just weld over the crack? Is there a chance that you have a picture of the repaired area? If their repair procedure looks good, I will most likley copy it instead of trying to reinvent the wheel.
Regards,
Craig
So I will be putting my old OEM exhaust system back on for now and will address this problem later. My thoughts are to call PowerFlow and see if they will share their repair procedure/method with me. I suspect that they will say that I have to send the muffler in and they will most likely cut off the damaged end plate area and graft a new replacement piece of end plate back on without addressing the weak design. If PowerFlow does not have a redesign for the end plate problem, I will take a detailed look at it myself using FEA to visualize the cause for the stress riser in the problematic crack prone area. Hopefully I can work out a repair procedure that will solve/reduce the design problem and one that will satisfy my IA before a repair is started. The problem is that things like this just can't be blindly beefed up because it could transfer the problem to the other side of the weld which could create an exhaust leak into the cabin heating system.
CheemsAF, did the folks at Aerospace Welding make a doubler plate or a new end plate when they repaired your muffler; or did they just weld over the crack? Is there a chance that you have a picture of the repaired area? If their repair procedure looks good, I will most likley copy it instead of trying to reinvent the wheel.
Regards,
Craig
Proteus
Active Member
Very interesting, and given the price disappointing that they're not made as well as one would expect.
Just to note regarding moving cracking problems to other areas, If you operate with the aircraft leaned out pretty well you don't generate much CO at all. At least measuring with a CO meter in a friends maule we saw pretty big differences by adjusting the mixture in CO levels. With cruise lean it was basically non existant.
Just to note regarding moving cracking problems to other areas, If you operate with the aircraft leaned out pretty well you don't generate much CO at all. At least measuring with a CO meter in a friends maule we saw pretty big differences by adjusting the mixture in CO levels. With cruise lean it was basically non existant.
Tango Foxtrot
Member
When I had the prebuy done on my 07 A1B 180 the IA said I had a burned out flame tube on the stock exhaust. I called Aviat they told me they had issues with the stock exhaust system and Aviat had redesigned a new exhaust for the Husky. I asked about having it repaired and I was told Aviat would not repair the exhaust due to the repaired exhaust systems were failing.
VLB
VLB
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Flying Dave
Active Member
So what was your end result? Did you get a quote in the factory exhaust by chance?
Ak Kurt
Well-Known Member
My original factory system cracked. Replaced it with a PF. My PF developed cracks after about 200 hours or so. I called PF and they said they have never seen this before and its a complete mystery to them, ya right! Anyway, I sent it to them and they repaired it. Then a few months later my brothers PF had the same cracks so I called PF and said another one is coming their way with the same issue. PF did repair mine under warranty but they sure did question me extensively on maintenance issues and if I followed the instructions of continued airworthiness. I followed those instructions precisely and I had my prop balanced. There is no other item under the cowl such as baffling, brackets or fittings that has any cracking except the PF. The instructions state that the system has to be removed at each annual and the joints need to be lubricated, make sure you do this! Actually, this should be done with any exhaust system, PF or Aviat. Also, if I remember correctly there is an hour interval too, I think its every 100 hours. I had to send copies of my log book entries to them to get them to cover the repairs. I now have about 40 hours on the repair and no cracks so far, I’m hoping it holds.
Make DARN SURE you follow the Instructions of Continued Airworthiness if you want PF to stand behind the cracking issue. I am disappointed in both Aviat and PF concerning their exhaust systems. I am hopeful that Aviat has fixed this issue, I have no idea if PF even knows how to redesign or prevent future cracking from the lengthily conversation I had with them. Maybe they are just playing dumb and know they have a problem and are not letting on to it or maybe they really dont know how to fix the problem, either way, after spending big bucks for a supposedly superior system these issues are very frustrating. The PF is a good system except for the cracking issue some of have experienced, it produces an amazing amount of cabin heat compared to the stock Aviat system. As far as the big performance claims, I noticed a small bump in performance but not nearly as much as they claim.
Kurt
Make DARN SURE you follow the Instructions of Continued Airworthiness if you want PF to stand behind the cracking issue. I am disappointed in both Aviat and PF concerning their exhaust systems. I am hopeful that Aviat has fixed this issue, I have no idea if PF even knows how to redesign or prevent future cracking from the lengthily conversation I had with them. Maybe they are just playing dumb and know they have a problem and are not letting on to it or maybe they really dont know how to fix the problem, either way, after spending big bucks for a supposedly superior system these issues are very frustrating. The PF is a good system except for the cracking issue some of have experienced, it produces an amazing amount of cabin heat compared to the stock Aviat system. As far as the big performance claims, I noticed a small bump in performance but not nearly as much as they claim.
Kurt
Ak Kurt
Well-Known Member
I find this blame it on the prop reason questionable, it seems like a “pass the buck”, “not our fault” explanation. My already incredibly smooth out of the box prop was balanced to the point of almost perfection and my PF system still cracked. I think on the PF its a design issue but that is just my opinion and those of the local expert welders who have a lot of experience with airplane exhaust systems I showed the cracks to.
Kurt
Tango Foxtrot
Member
When I called Aviat they told me the redesigned system would be around $2500.00 plus labor. The Shop where I get the annual told me he can get the exhaust repaired.
VLB
Kurt, as stated in a previous post above, a well known shop in Anchorage that has decades of experience with both building exhaust systems along with repairs stated that it is a design flaw and that they have seen cracking in this exact area across many aircraft model lines where a PowerFlow has been installed. Hence they would not make a repair that will not last! It is disappointing that the folks at PowerFlow are playing dumb with regards to their design flaws.
Without going into too much technical details, the U bends on PowerFlow's problem header tubes look like they could be used for a Coriolis flow meter. Here is a link showing/discussing a Coriolis flow meter's vibration: https://www.comsol.com/model/coriolis-flowmeter-fsi-simulation-in-the-frequency-domain-51831 The U tube configuration in a Coriolis flow meter is intended to enhance flow vibrations.
One solution to fix the muffler's flange from cracking might be to stiffen the U bends such that their natural frequency becomes significantly higher than that of the forcing function that makes them vibrate: the normal vibration of an O-360. When I get a chance to make a model for finite element analysis (FEA) of the system, I will post the resulting natural frequency. It might be the exhaust gas pulse frequency causing/driving the problem instead of engine vibration.
I had the same Power Flow exhaust cracking issue on an 0-360 powered C-172 twice in less than 200 hrs. Engine was brand new from lycoming and the prop was brand new as well. We had the prop balanced before the first flight with this set up. I followed the Power Flow ICA applying anti-seize to all slip joints at the 100hrs inspection which was kind of a pain and took some time. After the second crack we pulled the power flow exhaust and put the factory exhaust system back on. Haven't had a problem since in over 300hrs. I didn't notice any performance loss or gain. It sounds like this issue isn't limited to just the Husky's. I'm not trying to bad mouth the Power flow exhaust system but I hope people do a little research before they spend the bucks on a new exhaust system.