Mag Troubles

[GAHorn]

Just because an engine won't start, is not a valid un-grounded magneto or "hot-mag" test.

The point I hoped to make is that sometimes WARM engines will not start as easily as cold engines.
The assumption that a hot-mag test on a warm engine will prove out ... is not necessarily valid.
(And I've always assumed a prop is "hot"... but I clearly had a severe brain pharrt that day.)

When I was getting ready for the GLS convention, our fellow member and right-seater course instructor, Bob Lavery, was getting familiar with my airplane, one that ALWAYS starts easily hot or cold, never taking more than a blade or two to roar to life.

Bob had an injured right shoulder and suffered pain whenever he attempted a pull on the starter knob unless he used his left hand to assist his right. He would pull with both but he was being too gentle to overcome the strong spring-action of the clutch sufficiently to engage the starter button electrical contact. He was concerned he'd hurt my airplane if he yanked it too hard.
In the middle of our familiarization flight, we'd made a Coke-stop at Hamilton.

To our disgust the FBO was closed.
(Here's Bob panning for the camera, mimicing extreme disgust.)

So we decided to hop over to Stephenville for a Coke instead. I got back into the right seat, and Bob tried his gentle pull on the starter cable knob and ...again... he didn't pull it sufficiently hard to engage it so nothing happened.

I admonished him to "PULL --- You won't hurt it, Bob!"
He yanked as instructed.
The entire control cable came out of the panel and his double-handed fists struck me in the chest.
We both sat there immobile...... He, embarrassed (needlessly) that he'd broken my airplane. Me, breathless having had the wind knocked out of me.

We then both enjoyed a long chuckle over the event. It was a Laurel-and-Hardy moment.

I got out and I tried hand-propping the engine I knew to be an easy-starter. I absolutely wore myself out, and it wouldn't start. I was now embarrassed that it wouldn't start, and I was especially sensitive because I was afraid it was making Bob feel even worse about being the operator of the broken start-control cable.
I went back to hand-propping with even greather gusto. No deal. Not even a backfire.

Then Bob, very casually remarked to the so-called 170 maintenance expert... "George, isn't this broken cable normally connected to a lever on the engine?" (I didn't realize it at that moment, but He was doing what I've witnessed him do at his regular job at Simuflite teaching Hawker jets.... Bob is absolutely the best instructor I've ever known. He is so very good at what he does. He takes experienced jet pilots that have forgotten some detail they may or may not have ever completely assimilated... and when they've reached the extreme edge of their knowlegeability.... he asks them probing questions that lead to the correct conclusions in such a way... that they believe they have come upon the solution themselves! ... and that makes it such a memorable experience for them. It's embedded forever in their memories and they'll never forget it again! But Bob works his craft so well with the use of subtle, probing questions they are never in a position of embarrassment in front of their peers. What a fine talent he puts to good use!)

"Well, YEAH Bob! It just pulls a lever that operates a clutch-drive and then at the end of it's travel it operates the electric starter-motor to activate the starter-gear to spin the engine!" (What kind of dummy is he", I thought, "who would talk about such details which are so useless at a time when the control cable is broken and of absolutely NO USE! "Jeesh!"--- I thought.)

"Well", he continued, "... do you suppose you could see that lever through a cowl-door if the door were opened?"





I got it! The famous Cessna 170 "maintenance guru" GOT IT! (Thanks, Bob. You didn't have to make my dim-ness so obvious!)

I opened the cowl door, reached past the starter and pulled the lever. It started right up.


Conclusion: Just because an engine WON'T START is not a valid un-grounded magneto test. There are several reasons engines don't start besides magnetos. Warm engines can actually be more difficult to start than cool ones. Attempting to start a warm engine from a dead stop is not as good a "hot-mag" test as a switching mags off on an engine running at idle. Just a personal opinion that it's better to have the engine at low-idle and make the hot-mag test by momentarily switching both mags off, then immediately back on.

If the engine is accidentally allowed to die during a regular high-rpm run-up, then let it come to a complete stop, and make a normal re-start. Do not make the mistake of quickly switching it back on or you risk damaging counterweights and mufflers.


Hand-propping hint for dummies : One needn't hand-prop a 170 simply because a start-cable is broken. One can stay away from that prop over in front of the wing-strut where the cowl door is located.

[dacker]

That's funny. I now have that "starting tidbit" stored way back in the memory neurons in the "break glass for access to useful stuff when stranded" portion of my brain. I probably would have pulled the prop off trying to start my airplane in that situation! Thank your friend Bob for saving me headache in the future. Now, see if he can teach you to hand start one of those "fancy boy" corporate jets you guys fly.
David

[170C]

Good story George and a good point made. Lots of times we get so involved in the situation at hand that we can't see the forest for the trees! At the risk of sounding like a dummy , when reading the story, you mentioned your friend having an injured "right" shoulder and thus having to use his left hand to assist in pulling the starter cable. Now the question, are C-170 starter cables normally on the right side of the left yoke (ie: between the two yokes?) While I have flown several C-170's and have observed many, many instrument panels in 170's, I can't remember where that starter knob is located on a 170 & being out of town, I don't have any of my data here to look at (too tired to do a search) My 172TD has the starter cable knob over on the left side of the left yoke which may or may not be the correct location. Again I will need to look at my operators manual to be sure when I am back home. I can see where the starter knob being closer to the middle of the sub panel would reduce the bends in the cable and thus should make the cable system work a lot smoother than like mine is snaked around to the left side of the sub-panel.

[Showboatsix]

"I got out and I tried hand-propping the engine I knew to be an easy-starter. I absolutely wore myself out, and it wouldn't start. I was now embarrassed that it wouldn't start, and I was especially sensitive because I was afraid it was making Bob feel even worse about being the operator of the broken start-control cable.
I went back to hand-propping with even greather gusto. No deal. Not even a backfire."

"G"

As I was saying, in your case the engine would not start even with BOTH mags hot, so it would even be harder to start with only one mag hot. (if there is a harder to start when it won't start) But it started right away with the starter, ie it is spinning much faster, the impulse is triggered better and it starts, same thing with one mag, hell a Cherokee w/ Lyc.160 starts on one mag only per operators manual. So it can be done.

Dean

[GAHorn]

Dean, at the risk of sounding didactic... you are not getting it. If an engine doesn't start by turning the prop with both mags...or either mag "HOT".... it isn't a valid test of "HOT" mags to attempt starts. The ONLY valid test that mags don't remain HOT when switched OFF.... is to switch them OFF while the engine is running and watch that engine shut down solely due to the mags being switched OFF.

A failure to start ..... by any means, either hand propping or by electric starter.... is NOT a valid HOT mag test. There may be other reasons an engine may fail to start even when mags are HOT!

[Brad Brady]

Soooo....... where did the original problem on this thread go......and is it fixed?

[Showboatsix]

Well, the point I was making is..... some aircraft engines you start on a SINGLE mag only and then switch to BOTH after start. (Piper Cherokee 150/160 Lyc engine)

So my point was, if that single mag on that engine was hot (either by key or broken wire) and the starter was engaged, or perhaps a stray prop rotate, it might/will start! The mag does not care how it became ungrounded!

Here is a question for all of you, when you do the standard "CIGARS" how many of you check to see if the "mixture" cable is really connected to the carb? Do you pull out the mixture during run up to see if you get a drop in RPM?

Dean

PS... AWAYS assume that the MAGS are HOT on all engines!

[GAHorn]

Dean, I feel pretty strongly about this. It's a pretty simple matter.... to check for an inadvertent hot mag...swith both mags off while the engine is running. A starting test is NOT valid.

Don't take my word for it. Look at what Sacramento Sky Ranch, Hartzell, and Teledyne Continental co-authored:

Magneto Check
Periodically ensuring that your magneto’s p-leads have not broken is a good defense against unexpected starts on the ground. These leads, which are connected to the ignition (or magneto) switch in the cockpit, are responsible for grounding the mags to keep the engine from running. Sometimes the wires or connections between the switch and the magnetos break or come adrift. In this case, one or both mags may be "hot," or ready to deliver spark whenever the prop is turned. To test them, instead of shutting down the engine in the usual manner with the mixture control, use the key. Allow the engine to cool normally and idle down. Move the key slowly through both Right, Left, and then to the Off position. The engine rpm should drop slightly at both of the individual-mag positions and shut down completely in the Off detent. Allow the prop to stop and then move the mixture to the idle-cutoff position. Do not try to "catch" the engine before it comes to a stop because a dangerous backfire might occur. If the engine does not stop when the key is in the Off position, shut it down with the mixture, prominently mark the prop as being "hot," and contact maintenance personnel immediately.

[Showboatsix]

If "single" mag on start up IS not a valid test....How the hell do you start a Piper Cherokee with a Lyc. 150/160 engine, which by the manual states, START with left mag ON ONLY, switch to both upon start?

I agree that you can turn off a running engine with the mag switch as a check, however engines DO start on single mag operation, many a Piper pilot does this daily.

If your Cessna will not start on a single mag, (left or right) you better ground that plane, as I would not like to be the one doing a test flight behind those mags!

[GAHorn]

I don't know how to be more explicit, so if you STILL don't get it after THIS explanation... then you should just do whatever you think is best and I'll add you (and all those around your propeller) to the prayer-chain at church.

SOME installations start on only ONE mag because....... only ONE mag is retarded sufficiently to start without back-running the engine.... or only ONE magneto has a "shower of sparks" system which involves a retard set of breaker points.

NONE of which has ANYTHING to do with the safety issue of this discussion which is: Performing a "hot magneto" test as a regular part of operations by switching both magnetos to off with the engine idling.

[Brad Brady]

Jud,
Haven't heard if you found your problem with the switch change. did it work? if so cool....By the way...the only way to do a mag check is with the engine at idle.....switch the mag switch to off....just long enough to see that the engine is going to die then back to both.....nothing else works (CORRECTLY)...just my two cents.....sense this thread seams to be stolen I thought I might through that in.....

[Bruce Fenstermacher]

Stolen is such a harsh word. We're just keeping it alive till Jud gets to back us

[N2255D]

Every time I shut down after a flight I check 121.5 for an ELT and do a MAG grounding check at idle. On my C140 you couldn't forget the grounding check, since there was no idle cutoff, you had to shut the Mags off to stop the engine. I went to go fly one cold morning. My technique for starting, when it was cold, was to use 3 shots of prime, ignition keys in hand, then turn 4 blades while doing the preflight. The second blade and the engine fired. My fingers were sore for weeks where the prop hit. Lucky I still had the tail tied down!
My point here is no matter how many times you check the P leads the MAG/MAGS could be hot next time you move that prop!

[flyguy]

Every time I shut down after a flight I check 121.5 for an ELT and do a MAG grounding check at idle. On my C140 you couldn't forget the grounding check, since there was no idle cutoff, you had to shut the Mags off to stop the engine. I went to go fly one cold morning. My technique for starting, when it was cold, was to use 3 shots of prime, ignition keys in hand, then turn 4 blades while doing the preflight. The second blade and the engine fired. My fingers were sore for weeks where the prop hit. Lucky I still had the tail tied down!
My point here is no matter how many times you check the P leads the MAG/MAGS could be hot next time you move that prop!

GREAT ADVICE ON A VITAL PROCEDURE! AND THE "GOD" OF THE CLOUDS WAS WATCHING OVER YOU. SO QUICKLY THE BLADE CAN CHANGE YOUR LIFE

We have had far too many "fatals" happen in our ranks by unsuspecting folks turning the "blade" and having the engine spring to life. The injuries can be horrific if not fatal. If you intend to move the blade make sure you are clear of that arc and the plane is securely restrained. Also keep an eye on any bystanders - people or pets.

While we were managing the local airport here in Sabine Parish, we were providing instruction to several students. Our instructor asked me to let him demonstrate to his students how lethal it could be to move the prop on Deana's C150. He circled about 5 or 6 students near the front of the plane. He had me sitting in the cockpit to hold the brakes and with the mags hot. While he lectured the students on the danger that lurked at the tips of the blades and how one could lose life or limb by not paying attention to your body parts relationship to the arc of "death", he moved the blade 1/4 turn and you should have seen the amazement on the faces of those students when the O-200 leaped to life. It even startled the instructor and he was expecting it. Bottom line - -
====================== NEVER MOVE IT WITHOUT EXPECTING THE THING TO "START".===================

[rupertjl]

Sorry I haven't gotten back but haven't had a whole lot of time lately...but the new switch didn't fix things either. I'm going to start over and troubleshoot again, hopefully this week. I have the timing box, ignition lead tester, spark plug tester, the works...reattach and see what it gives me...if nothing else, I'll take it off and send the mag back to check it's internal timing...