C&D heater low-time combustion tube failures

[roger]

We just experienced our 2nd failure of a C&D heater combustion tube at low time (first at 892 hours, this at 865). Symptoms both times exhaust smell and CO detector changing color in cockpit, then fail PDT when checked. Will be working through warranty process with C&D (now Hartzell Engine Tech). Worrying because C&D airworthiness limitations don't require a PDT until 1000 hours heater time. C&D acknowledges that they had issues with the Chieftain setup, but our airplane has all the latest greatest stuff from them installed and same issues. Some ideas we're going to institute to try to keep them running longer:
-Check fuel pressure and cycling switch temperature at filter change intervals (250 hours heater time). We've had issues with the C&D cycling switches shifting-to the point where on the ground the heater will hit the overheat switch. Suspect that isn't good for the heater tube.
-More pilot training on heater cool-down procedures. I rode with an ACP a couple weeks ago and found out he wasn't using (or teaching) proper cool-down after landing (if heater was left on until landing). Suspect the issue is wide-spread.
Other than that, don't know what to do other than proactive PDTs before they are due, and submit SDRs.
Edit: we do a lot of short trips, so lots of heat-cool cycles on the heater. Wonder if that may be part of the issue as well.

[klausnw]

You know how I feel Roger. The pilots need to pay more attention and stop abusing everything they touch. The heaters are fragile at best and protocols need to be followed.

There was a heater installation that I worked on that had the squat switch on the fuel pump. The flame would only work in the air or when the spring loaded override switch was engaged.

Good luck keeping the heaters working.

[roger]

Update: Set up new heater yesterday. Interesting thing, when we had the previous issue with the cycling switch, we found that it had a huge deadband, like it would turn fuel off at 220° and not back on again until ~70°. Didn't seem quite right, but we went with it and got it turned down so we stopped tripping the overheat breaker. A new cycling switch only has about a 20° deadband, so fuel off at 220°, on again at 200°. So when we had the issue before the cycling switch was malfunctioning and should have been replaced. Whether that could have contributed to the problem or not I don't know, but I would definitely keep an eye on it (like I said check cycling switch setting and function at 250 hour intervals, or more often). Also learned that the low boost pump doesn't like to run too long with no fuel flow through it. Didn't kill it but did have it vapor lock.

[Tim Glubaskas]

We just started installing C & D and all ours have a pretty tight cycling band also - maybe 20-30 or so.

We have one now with a finicky cycle switch so I hope these don't become bigger issues going forward