[Sir Morley Steven]

Why isn't it in any POH or checklist? Because it isn't a terribly good idea.

[Jonzarno]

Here is an article by Mike Busch about in flight mag checks:

http://www.sportaviationonline.org/spor ... tion?pg=94

Well worth a read.

[Crash one]

I wonder if it's possible to run 8 little LEDs off the ign HT or would they just go pop?

LEDs need 10-20mA, the spark is much less current than this.

Also LEDs conduct at ~2Volts so would short out the spark stopping the plugs from firing.

OK all agreed but, please excuse the daft idea department, some rev counters use a sensor on a, usually No1 plug, lead which reads the HT pulse, instead of converting this into a needle movement couldn't it power a light. I'm no expert on electronics but as magnetos seem to be so unreliable, and having two making it difficult to detect a partial failure it was just a thought for a cheapskate warning thing.

[Rob L]

Get yourself a J.P. Instruments Aircraft management system. The best $'s I've ever spent. It does your checks for you.

From where does said device derive its power? Aircraft battery/alternator? If so, I shall not be heeding your well-meant advice, 'cause I have neither.

Like the Kid, I always do a mag check before coasting out. A friend had an unknown mag failure (at least, he was unaware of it) in the cruise over mountains; only discovered upon shut-down checks after landing. The worst time to have a mag failure is when the other has already failed.

But don't switch to "Off" at high power settings, this could cause the exhaust system to depart the scene.

[Rob L]

As an aside, I have had two mag failures where the impulse spring has failed. By the nature of design, this is only an issue upon starting/shutdown, when the impulse spring is used to retard the spark (typically below ~600rpm). The springs do not fail in situations where the rpm is above this (as in cruise flight).

The first occasion cost me a lot of money in incidental expense (stuck on Lundy island); the second was a non-event (financially) because I now always carry a spare impulse spring in the aircraft. Just a two-hour delay to fit the replacement.

I also have them replaced by rote at the necessary inspections.

[Waveflyer]

Here is an article by Mike Busch about in flight mag checks:

http://www.sportaviationonline.org/spor ... tion?pg=94

Well worth a read.

a

That's the article that convinced me of the merits of in flight mag checks.

If it's good enough for Mike Busch........

[Delta Juliet]

some rev counters use a sensor on a, usually No1 plug

It would require some electronics/power to convert it to be able to run an LED.

You can buy an electronic rev counter with L-R mag monitoring for about £100, an LED wouldn't be as good.

[Gertie]

I would suggest you'd need a very big (value) resistor!

The largest resistor I ever used was a few inches of string.

We were supposed to be doing a static electricity experiment, but nothing would stay charged up, the weather was too damp that day. So we got out the 5kv bench supply and put together a circuit, with a length of string in the circuit as a resistor. Enough electrons got through that whatever-it-was stayed charged up for long enough for us to do the experiment.

(Said it was damp weather, didn't I. Obviously damp enough for the string to conduct however many picoamps it was that we needed.)

I'll bet A level physics students aren't allowed to play like that these days (I think we told the teacher what we were doing).

[Sooty25]

I wonder if it's possible to run 8 little LEDs off the ign HT or would they just go pop?

this might be worth a try.



L1 - LED
D1 - Zener diode to protect the LED about 2v
R1 - say 1kohm

L2 - is just a coil of single core wire wrapped around the HT plug lead. the induced current should make the LED light. You may need to play about with the number of turns.
You may need to play about with the resistor value as well.

repeat for each plug lead.

No guarantees, not CAA or LAA aproved, blah, blah, blah

[Keef]

I think the problem with that design is that the length of the ignition pulse is extremely short - a tiny percentage of the total time. The LED would light for about 50 nanoseconds, and then go off until the next spark came round. At 2,400 RPM the "no light" time between sparks would be in the region of 50 milliseconds. That's a duty cycle of something like 1:1,000,000.

You'd be hard put to see anything unless you drew enough power from the spark to charge up a capacitor to even out the glow - in which case you'd probably use up a fair proportion of the spark energy. It still wouldn't tell you if the plug is sparking, only if the magneto is producing volts.

A better solution would be a pair of sensors, one for each magneto, on the mag switch (P?) connections via a very large resistor, feeding a suitable circuit and LED driver. You could then have two LEDs that glow green if the mags are accling, and red if they aren't.

[Sooty25]

you could try a capacitor across the diodes and wrap a few more turns around the plug lead. It wouldn't affect the plug potential.

No it wouldn't confirm if a plug was or wasn't firing, but it would confirm whether high voltage pulse was at the plug cap. After that you'de have to monitor CHT's

P connection is on the Low Tension side of the magneto so could pulse, even if the High Tension coil had failed.

[Paul_Sengupta]

The largest resistor I ever used was a few inches of string.

Better be sure no one spills their tea on it!

I'll bet A level physics students aren't allowed to play like that these days (I think we told the teacher what we were doing).

I guess I shouldn't mention someone wiring the 6kV PSU to the door knob of the physics lab then...

Regarding LEDs, can I just point out that neon indicators are a lot better for this sort of thing.

[Keef]

If you touched my 3kV power supply, I suspect you'd know nothing about it.

When I first built it, I found one of those large green wirewound resistors - 100k, about a foot long and an inch in diameter. I wired it directly across the reservoir capacitor. It got pretty hot but I was happy that the thing would discharge the PSU once I turned it off.

I was doing some work on the linear and decided on the spur of the moment that, to be doubly safe, I'd short out the capacitor first. I used my long screwdriver with the insulated handle. I had to use a hacksaw to get it off the terminal posts. The big R had gone open circuit. That's about the nearest I've come to harp lessons so far.

I don't mess with high voltage!

[Paul_Sengupta]

We did something similar in the physics lab in school Keef, with a whole desk full of huge capacitors and various large (similar sounding) resistors. However our results were a bit more disappointing. We guessed at the time that it was the 6kV PSU which wasn't quite up to the job.

Our low voltage PSUs could supply quite a current though. They were nominally rated at 8A but they could make a penny glow if connected across one.

The penny made a burn mark on the desk. Not as much as the gunpowder on the tin tray though, but that's another story.

You could also set light to wire wool with the PSU. We used an old washing machine motor with wire wool on the end to do a reasonable impression of a catherine wheel.

[Keef]

The little 10kV PSU I built to feed my first basic oscilloscope (modulation monitor, really) couldn't produce enough output to charge up any sort of capacitor. The lack of decent smoothing made the scope trace a bit wobbly, but it sufficed. Your little one would probably have done the job of charging those capacitors, eventually, as long as it could produce output with a load that looked like a short circuit.

I don't know what "power" an ignition magneto can produce - not a lot, I would think.

I wouldn't turn my mags off in flight, on the basis a fairly short loss of ignition could build up enough fuel vapour to blow the exhaust manifold off. I remember the bang when I turned the key too far on power checks...