Today's Engine Puzzler

[MCA]

I was an early adopter of PMags and had my share of issues. Fortunately, they have addressed the issues since then.

It hasn't been mentioned explicitly so I'll say it here. PMags only generate their own power above a certain RPM, I believe it is around 1200-1500 RPM.

Therefore, PMags require ship's power to start an engine or operate at low engine RPM.

It is also important that pilots realize in the event of an electrical failure (as Rob mentioned above) you must keep the engine RPMs up at least 2000 RPM or higher, even on approach.

As mentioned in the Puzzler, pulling back power to a lower engine RPM during stall practice would cause both PMAGS to fail. Makes sense.

[flyvans.com]

Originally Posted by MCA

...
It hasn't been mentioned explicitly so I'll say it here. PMags only generate their own power above a certain RPM, I believe it is around 1200-1500 RPM.
...

Marc, that must have been a really early model then...

I can assure from tests that the latest 114 models quit somewhere around the 800 rpm range. At the same time, flying/gliding at reasonable speeds you will never see below 1000 rpm (provided there is no mechanical or other similar malfunction/blockage).
So that limit is really mostly relevant during rollout, taxi and of course startup, which is impossible without external power.

[MCA]

Hi Bernie, it has been a while. Your numbers are certainly better than mine.

[DavidHarris]

Regarding the question about roll-out, I paced off the distance from when the tracks first appeared on the sand until I came to a stop, and it was 200-250 feet. I had the good fortunate of a strong headwind (more than 20 kts at altitude, uncertain of what it was on the ground but it was definitely blowing as the desert often does in the spring) and sand that was firmer after a recent rain. I don't recall the back wheel coming off the ground during rollout. Off roaders had left tracks in the river bed as well, which might be misleading. The aircraft had come to very nearly a complete stop by the time the nose wheel touched the ground and promptly dug in. I was in a five-point harness, but recall very little force coming to a stop, and no tail rise. There is a small black mark between the bottom of the anti-Splat and the nose leg, indicating possible contact of the two, but no scratches, chipped powder coat, or visible bends.

I discussed the possibility of a teardown with two mechanics and concluded it was not necessary. I do not want to debate this decision in a public forum and would kindly ask for others to PM me if you wish to critique my choice.

[D Weisgerber]

My P Mag will self power to just below 800 RPMs.

[DavidHarris]

I agree with almost everything Bill said but disagree with one point of his analysis.

Originally Posted by N941WR

Walt,
The problem here is the OP turned off the P-mags and didn't realize it until he was on the ground. Had he not touched the P-lead, or turned it back on, the engine would have started up after he lowered the nose and his RPM's came back up.

I had turned off one of the two PMAG switches, most likely in turbulence before the event, though possibly while troubleshooting or as pilot error at the end of testing it during run-up. The other PMAG switch was in the ON position, and the key was in the BOTH position.

I am convinced the root cause was the bad crimp in the power wire between the main bus and the PMAG switches causing intermittent loss of main power to both PMAGs.

I believe that if I had lowered the nose sufficiently, the engine would have started up again based on the PMAG internal alternators. During a subsequent ground test, I found my left PMAG alternator runs down to 850 RPM and the left down to 780 RPM, which is consistent with expectations. By the time I attempted lowering the nose, I was too close to the ground to do it very aggressively or for long enough to manipulate the power and mixture settings while doing so.

It is also possible that the engine would have restarted better with the mixture briefly at cutoff, as in the normal starting procedure.

[swaircraft]

David: Thank you for the kind word's!

Glade to hear ever thing is going forward, FLY SAFE

Jack Vandeman

[Mike S]

Jack, welcome aboard the good ship VAF