Today's Engine Puzzler

[wsquare]

The doomaflachi disconnected from the wachama thingy.


Wayne
RV-9a purchased flying and still grinning

[flyinhood]

Originally Posted by PhatRV

So what is the answer. I don't have time to peruse threads that arnt active from the day before. I realized the OP wants to do this like IMC club but I don't have the wealth of knowledge spanning decades like you guys. So saying what was wrong up front is helpful for me. Like the AD about the double adel clamp or the open plug on the spider valve a while ago.

Exactly

...where is the "Like" button.

[Bavafa]

Originally Posted by flyinhood

Exactly

...where is the "Like" button.

I doubt this is going to get deleted, so if one doesn't have time or patience, we can check back in a week or so to check on the fix.

Just saying...

[Adam]

Im not sure I like this puzzle and I'm not sure this is the way to handle this issue! I use and support this site because of information that i can use so I can prevent future issues, I have that similar engine, should I not fly it till I get the answer?, I'm not that paranoid but others maybe, not a big fan of your puzzle.

[Captain Avgas]

Originally Posted by DavidHarris

About 8 hours into Phase 1 testing, my engine quit during stall testing (at the top of a stall at 48 KIAS, with partial flaps). Much to my surprise, it would not restart in the air despite playing with the boost pump, mixture settings, throttle, and starter, and pitching the plane down to increase the windmilling. I shut everything off and made a forced landing on the sandy bed of the dry Mojave River...... On the ground, I observed the right PMAG switch was off. I likely bumped it while entering a transponder setting during turbulence.
David

OK, I have a few observations here, neither of which addresses your teaser question but which may be valuable to other pilots.

Firstly, if you were able to do something as dangerous as knocking off a PMag switch while "entering a transponder setting" then I would suggest your panel ergonomics leave a lot to be desired. Honestly, as a TC I see builders constantly putting very important switches unprotected in vulnerable locations.

Secondly, your emergency check list for engine failure did not include switching the left and right PMags off and on to check them independently. This is a "must do" for Pmags. These devices can totally lose their timing (many recorded cases) and if the timing becomes significantly advanced on one PMag it will cause the engine to quit...and in such a case there is no redundancy in the second PMag. The only way to get the benefit out of the second PMag in such circumstances is to determine which one has failed and switch it off...thus the required PMag check.

Finally, I suspect a number of people are getting a bit annoyed by the teaser question. And fair enough. Having people guess what went wrong with your engine is not achieving anything in these particular circumstances.

[krw5927]

Add me to the list of people who don’t understand why someone would prefer to play games instead of helping people. Gosh if someone got injured or worse as a result of a similar issue that may have been alerted by reading this thread with an actual solution to an engine-out problem...

[N941WR]

Originally Posted by Captain Avgas

...

Secondly, your emergency check list for engine failure did not include switching the left and right PMags off and on to check them independently. This is a "must do" for Pmags. These devices can totally lose their timing (many recorded cases) and if the timing becomes significantly advanced on one PMag it will cause the engine to quit...and in such a case there is no redundancy in the second PMag. The only way to get the benefit out of the second PMag in such circumstances is to determine which one has failed and switch it off...thus the required PMag check.
...

This issue was resolved a number of years ago and no longer applies.

However, your comment regarding ergonomics is spot on.

[mbauer]

Bird nest or a critter home of some kind in the air intake.
Mike

[DTARM1]

You've not provided enough information. I assume you've done this to make yourself feel better when some 75 year mechanic with 4 or 5 Charles Taylor awards under his belt working for NASA can't simply state the problem. You've created one of those "infomercials" that says ever 45 seconds "you'll be amazed at what I'm about to tell you". As previously stated you're technique is annoying at best and likely matches your personality of "oh look a shinny ball"

Good luck with your suckering people to make yourself feel smart.

[DavidHarris]

First, my sincere apologies to folks annoyed with the puzzler. I used to enjoy listening to the puzzlers on CarTalk and thought this could be fun (glad you liked it Ray), but I'm sorry for those who didn't like the format and will avoid puzzlers in the future.

The issue proved to be an intermittent open connection in the common power wire to the PMAGS.

My son (age 14) and I had looked at the LEDs on the PMAGS while we were retiming them, and he knew they should be yellow or green when power they were properly powered and the key is in the OFF position. When I was unable to start the engine, he noticed that both PMAG LEDs were off.

We followed up with a voltmeter and found the PMAG power inputs were both 1.2 instead of 13 V. Then they went back up to 13 and then back down to 1.2 as we poked around. The only wires in series with both PMAG power inputs are the main power system between the battery and bus, and the short shared wire between the bus and the split to the two PMAG test switches. The rest of the aircraft was operating normally, so it wasn't the main power system, leaving only the short shared wire. We also looked at the ground connection, but it is short, sturdy, and showed continuity on the meter.

This shared wire had spade connectors on it. I spent a good deal of time shaking it the next day and briefly made the PMAG power drop out, but it was surprisingly hard to reproduce. We cut the wire out and replaced it with a wire with heat shrink solder sleeve instead of spade connectors. We then dissected the spade connectors and found a poor crimp that could be pulled apart with a good tug after all the heat shrink had been removed.

The aircraft has been starting reliably since that change, and now idles fine down to 750 RPM and the engine hasn't quit at any point.

In hindsight, this makes perfect sense. The crimp was good enough that it rarely acted up, and I'd been unable to detect the bad connection by shaking the wires back in March or earlier. When the PMAGS had a good connection, they would operate reliably down to lower RPM. When the connection was poorer, they lacked main power and would only operate off their internal generators, which I have now measured need 850 RPM on the left and 780 on the right PMAG, so they would drop out at low RPM as I was trying to set the idle or as the prop stopped windmilling after landing. The weak connection also explains my intermittent starting difficulties. I'm guessing that vibrations during starting and operation may have caused the connection to get better or worse at different moments.

The engine failed when I was at the top of a stall with partial flaps. The PMAG power wire must have not been good at the time, so the PMAGs were operating off internal power. Once the RPM became very slow at the top fo the stall, the PMAG generators stopped producing power, so the PMAGs both stopped making spark and their tach readings both dropped to 0. Although I pitched the nose down to increase windmilling, I was too close to the ground to do this for long, and must never have achieved high enough RPM and appropriate mixture settings to restart before having to commit to the landing.

I agree with Bob's comment about the PMAG switch ergonomics. My switches came with my Advanced QuickPanel. Brad at PMAG says he now recommends a "non-latching momentary pushbutton" but doesn't yet have a specific switch in mind. I'm not sure I love that because I'd also like the PMAG switch to have a locking OFF position so I can completely shut off power while working on the engine. Stein sells Milspec PMAG switches that can be used ON, locked OFF, or momentary OFF (for test). Unfortunately, the holes in my panel are about 1/16" too close together to accommodate these switches. Still thinking about what to do here...

I also like the notion of one standard magneto so that the ignition has independent failure modes.

David