Friday, October 17, 2008

Its Not the Years, It's the Hours

First thing in the morning for the FBO isn't first thing for me, but as soon as he gets in, the engineer starts work on my airplane, even though there is work to be done for his company as well. He has a reconditioned magneto available, and negotiates a trade-in value for the core our magneto, which he will recondition in turn. Paperwork will record all the serial numbers of parts swapped through the aviation world in this manner. It's a pretty normal occurrence. It's actually pretty funny to see one of our airplanes taken apart and to see all the points of origin recorded on stickers or plates on various components. I once had a maintenance shop refuse to do work because an instrument had a sticker on it claiming it was on loan. Fortunately the company whose name was on it was still in business and I was able to get them to relinquish any claim on the equipment. At some point there must have been a trade-in involving that loaner equipment, and no one took the sticker off.

After the installation, which didn't take long, he sent me out to do a runup with one of his apprentices. That's a nice way to do it. I've had shops go and do the runup themselves, come back and sign it off, and I've had shops send me out to test the equipment, and sign it out on my say so. I'm okay with both ways, but this was a best of both worlds. The shop avoids the problem of worrying about messing up my radios or any of my equipment taxiing my plane without me. I get to verify for myself that the magneto is now working correctly. And the apprentice has a strobe-light rpm detector to measure the rpm drop per mag exactly without having to rely on the accuracy of my gauges. I've seen the rpm detector before, when someone was testing the calibration of an RPM gauge, but I'd never seen someone use it to check the rpm drop on a magneto. We taxi back and park. Now it's all over but the paperwork.

It's a bit of a relief to be working in Canada now and know that I don't have to explain journey logs and TTAF to a US AME. This shop has a good system with checklists and workcards that are not loaded with unnecessary overkill items, but have a thorough list of what does have to be done, and I can see they've actually been used on the shop floor as opposed to being all filled in at the end after the work is done anyway. As he signs it off and verifies the billing information, the AME mentions that this is the highest time airplane he has ever certified. My airplane isn't frighteningly old. It just works hard for a living. I think this guy has worked mainly on medevac aircraft, which don't go off on all-day missions like ours.

13 comments:

Paul said...

I gotta ask my mechanic how they get both mags to fire *exactly* at the same time.

I know there are lights for timing the points, but it would seem milliseconds would count. I also wonder how over the life of the mag, the timing doesn't change?

--paul

Samwise said...

Paul,

The good thing when adjusting the mags is that you get to do it with the engine off. This basically means that you can turn the engine by the prop and you can do it as slow as you like. Basically you can see (with a suitable "buzz box") that both mags fire at the same time and that they fire at the proper phase of engine rotation.

Adjusting them with the engine running would be a millisecond-counting operation. :)

Aviatrix said...

I flew for a while in an airplane that had electronic magneto timing, on top of the regular kind. If you turned off the electrical master there was a slight (~25 rpm) drop in power.

jinksto said...

Samwise,
that was a complete "duh" moment for me. I was thinking through what Paul said trying to figure out how the heck they could get it to work. Then I saw your comment and it all got very easy.

majroj said...

Speaking of "old planes", in 1984 a "new" jet mech/hydrauics guy joined our Guard unit. I say "new" because he had no hours with us, but he did have many years with our aircraft type, the McDonnel-Douglas RF4C. The commander gave him a detective job: find out why this one aircraft had flap actuator problems despite replacing everything in sight which impinged on it.
"Well, I hope I can, We had one like this back in Nam at Udapao, same pesky...oh for the lovea Xrist, look at these papers!".

It was his old nemesis, come back to haunt him, twenty years later.

steve said...

Although it may seem odd, the timing of the two mags does have a tolerance!

the clue is in the fact that you get a rev-drop by switching one off.- The reason being, each spark-plug initiates a flame-front in the combustible-mixture,thus the mixture burns faster and more completely in a given time-span....logically, your tolerance would be the time taken for the flame-front of either plug ,to overlap the "field" of the other one,at normal operating RPM.

Of course, there are arrangements to retard for starting also "impulse start" and "shower of sparks" start.

So, the initial spark timing is fairly critical,the second can lag without detriment...also bear in mind the relatively slow,low-compression ,constant-speed nature of most piston aero engines.variations of several degrees, due to tolerances,wear and backlash are inevitable.

Paul said...

Thanks, Steve.

So, it seems we can say running on a single mag retards the timing of the engine. That is, the flame front takes longer from the spark of the single plug to completely burn the mixture in the cylinder. When ignited by two plugs the mixture is ignited more quickly--a few more degrees BTDC.

Interesting.

--paul

Anonymous said...

Alfa Romeo (italian car maker, part of the FIAT group) used this principle in one of their automotive engine series (not to make it more fail-safe, but to enhance performance and to comply with stricter emission controls):

http://en.wikipedia.org/wiki/Alfa_Romeo_Twin_Spark_engine

nec Timide said...

I had a Nissan pickup that had dual ignition as well, but it was for performance like the Alpha. The second set of plugs was only fired under high load conditions.

Soaring Student said...

nec timide.... I wonder what the fouling was like on the second set of plugs, since they were in the cylinder full-time, but not not sparking full-time. And why Nissan would ever do that... it's not like the reduced wear&tear on the parts was worth it.

paul.... internal combustion engines are timed so that the spark ignites the air/fuel mixture a few degrees before top dead centre, since the initial burn doesn't produce much power. However, when the exhaust valve opens the mixture still has not fully completed the burn... the engine is timed this way to keep a bit of compression in the engine and get the exhaust cycle underway as soon as possible... since the last bits of the burn don't produce much power anyway it is better for performance to be more effective at getting the spent charge fully out of the engine. In fact, most things in an engine happen before TDC or BDC. Timing (retarded or advanced) isn't changed whether you're on one mag or two... the spark happens at the same time either way.

In an aircraft, dual plugs are there primarily for redundancy and safety. A secondary benefit is that you get a more complete burn, therefore mileage and better environmentals.

A Squared said...

Adjusting them with the engine running would be a millisecond-counting operation. :)

True, but not difficult at all to measure those milliseconds. An oscilloscope will do that. The plane I used to fly had one built in, and the engineer could check the timing in flight. The same could be done on the ground with any dual trace oscilloscope. Adjusting them, though with that method, would be a lot of trial and error.

So, it seems we can say running on a single mag retards the timing of the engine?

No, that would not be correct. The timing (when the spark occurs) has nothing to do with how many magnetos are operating. The RPM drop on one magneto is because less power is produced with slower combustion, not because the timing has changed.

I had a Nissan pickup that had dual ignition as well, but it was for performance like the Alpha. The second set of plugs was only fired under high load conditions.

Are you sure? My dad had one of those also, and I was under the impression that both plugs fired all the time. Like Soaring Student, I can’t imaging a reason why they *wouldn’t*, and I would think you might have some pretty severe fouling problems, if one set weren’t firing at low speed, low power.

internal combustion engines are timed so that the spark ignites the air/fuel mixture a few degrees before top dead centre

"a few" is typically 20 25 degrees BTDC

nec Timide said...

Re my Nissan:

Just reporting what the owners manual and mechanic told me, I suppose both could be wrong and both plugs could fire all the time. A little Googling found this Nissan used the dual spark plug ignition system as a way to lean out the fuel mixture as much as possible and the two spark plugs guaranteed ignition of the leaner fuel mixture. The exhaust side spark plugs cut out under certain conditions, such as hard acceleration or heavy load, to prevent detonation. In essence it is two separate ignition systems, each with it's own coil. Which is the inverse to what I remember.

So let's blame it on my memory (it was a 1984 truck). After all the internet can't be wrong, can it?

A Squared said...

The exhaust side spark plugs cut out under certain conditions, such as hard acceleration or heavy load, to prevent detonation.

Memory is such a fickle thing. That seems to me to make more sense.

That truck would fire up more readily on a sub zereo (farenheit) morning that any engine I've operated before or since. I always wondered if hte dual ignition had anything to do with that.