Dumb Question

[uwochris]

Hey guys,

Perhaps this is a stupid question, but I'll ask anyway...

If the ignition system is independant of the electric system, why must the master switch be on prior to starting the plane? ie) You can't start the plane without the master on, but in flight, the plane can still run if the master is Off.

I've been told by several instructors and manuals that it is used for the electric starter to turn over the engine, although I cannot seem to visualize this.

Anybody wanna clear this up for me?

Thanks!

 

[Dep676]

I may not have the best answer either. I will give a shot though. The spark comes from the mag's. They get there energy from the engine therefore they don't need the battery. The master supplies power to the starter to turn the engine over. Go to the MX dept and have someone show you it will make sense to see it. Hope this helps. Anyone else care to chime in?

Remember the only dumb question is the one that isn't asked.

 

[s.o.sJTB]

As already mentioned the master switch is needed to energize the starter to crank the engine. In flight the engine is already cranking due to the propeller windmilling in the airstream. You could hand crank the engine without the master switch being on on and get it to start completely independent of the electrical system. The Magnetos have an Impulse coupling, kind of like a mini generator, that winds up a coil as the engine cranks, at a certain point the coil uncoils rapidly spinning a magnet and providing an electrical current to provide a spark for cumbustion.

 

[tarp]

Correct.

Magnetos (and hence the ignition system) on light airplanes are independant of the battery/electrical system. The magneto makes it's own spark(power) and "distributes" it to the spark plugs via the ignition wires.

The battery, alternator and rest of the electrical system are used for the starter motor, the lights, the radios, etc.

As for the process of starting the engine. On the ground you either need the starter motor and the power supplied by the battery or you need a good strong armed person to "hand prop" the plane. One way or another, somebody or something has to start the turning process so that the magneto will turn and start creating it's own spark.

In the air, the air itself is the turning power, since it moves over the prop and keeps it "windmilling". No need for the electric starter unless the prop comes to a complete stop. Also, in the air or on the ground, once the engine is running there is really no need for the master switch or all that other battery powered stuff. Just like a lawn mower, the prop will keep turning as long as there is fuel and some oil.

The engine will come to a stop when you either starve the engine of fuel (pull the mixture back) or "kill" the magnetos by grounding them out (turning the ignition switch off). The second method is not recommended if you want to keep your magnetos in good working order.

 

[Slick]

I'll assume you're speaking of a piston powered aircraft, (not a turbine).

The master must be on in order to get electrical power from the battery to the starter, which obvioulsy turns the crankshaft until the engine starts.

After the engine starts, the crankshaft turns a shaft inside the magnetos which produce the electricity to fire the spark plugs. Because the electrical power to the spark plugs is being provided by the mags, (not the battery or alternator), the engine becomes kind of a perpetual motion machine. It produces it's own electrical energy, for it's own spark. The only outside ingredients needed to cause continued combustion are air and fuel.

So because the engine is producing electrical energy via the mags, we can turn the master switch off, (battery and alternator), and the engine will continue to run. Of course, we won't have any lights, radios, navs, intercom, etc. But at least the engine will continue to run.

At least, that's my understanding of how it works. I'm sure others will feel free to chime in and correct any mistakes I've made here.

As an additional related matter of interest, you may want to make a mental note that when the alternator decides to poop out, you may not notice that anything is wrong, (unless you see the red dash light come on or the volt meter misbehaving). The engine will contine to run and the radios, navs, transponder, and lights will continue to work, (off of the battery). Eventually, the battery will drain and you'll be left flying around with only an engine. Your navs won't work, (although the needles will look like you're perfectly on your desired path). Your radios won't work. Your intercom won't work. Transponder go bye-bye. You won't have any lights, (including landing light if flying at night). And you're electic fuel guages will (incorrectly) indicate that you're quickly loosing fuel. It's a bad set up, especially at night. When you loose your alternator, plan to land within 1/2 hour, assuming you became aware of the exact time that the alternator quit. And it may not be a bad idea to give ATC a heads up, so they can get the light gun ready.

 

[katanabob]

I may not have the best answer either. I will give a shot though. The spark comes from the mag's. They get there energy from the engine therefore they don't need the battery. The master supplies power to the starter to turn the engine over. Go to the MX dept and have someone show you it will make sense to see it. Hope this helps. Anyone else care to chime in?

Remember the only dumb question is the one that isn't asked.

You don't need a starter, it's just a convenience. THere are still a great many Cubs, Champs, Luscombes, Stearmans, etc. out there that don't have an electrical system at all, let alone a starter.

The Mags are geared to the engine, and their internal structure makes it possible for them to take rotational energy and turn it into electrical spark. (Think of them like little alternators, though they discharge in jolts of electricity) They require only this rotation to operate; hence the importance of turning the mags off when the engine is supposed to be shut down AND the importance of performing a p-lead check prior to shutdown. The engine can literally light off with the mere bump of a prop. In contrast, your car requires electrical power to generate a spark (via the coil or spark generator) which it then sends out via the distributor/rotor to the plugs. Believe it or not, many older radial engines have one distributor and one magneto. THe best (and the worst) of both worlds.

Turning the prop with the starter is just like putting out the flaps; you're using electricity from the battery to turn a motor, which is geared to the engine and in turn cranks the engine.

Hope this helps.

 

[Rank&File]

While we're on this subject, can anyone think of any aircraft engine that is not self-sustaining? Even the Allison A1/P engines (which I am most familiar with) that is totally dependent on several computers, known as FADECs (Full Authority Digital Engine Control) can run without the aircraft's electrical system being energized. A Permanent Magnet Alternator supplies electricity to the ignition and FADECs just like mags make the spark plugs spark.

 

[tarp]

Porsche Mooney

 

[fly4ever]

magnetos

First - remember the only dumb question you'll have is one you don't ask. Always seek out answers to questions just like you're doing now.

The magnetos are an independent ignition system that if properly installed have P-leads(grounding wires) which ground the mags to the airframe. The P-leads effectively cause the mags to quit sparking when the ignition is turned off. If the P-leads are not properly grounded the mag remains "hot" (electrified by the battery) therefore capable of running even without the ignition on. This is a requirement for starting the engine.

Once the engine is running the the magnetos produce their own source of energy because of their design and no longer need the battery to continue running. In effect, they become their own little generator (parts - a "field" which is a magnet and an "armature" which is the structure that supports the conductors). There's more to it but these are only the basics. It is correct that you can shut down the entire electrical system in flight of an airplane equipped with magnetos and the airplane will continue to run. The mags are self-sustaining, that is the back-up in the event of electrical failure. But, if the ignition is turned off and the P-leads are properly grounded the engine will quit. The ignition is wired to bypass the P-leads in the left, right or both position for starting. That's why it is always a good idea to check the P-leads for proper grounding while shutting down after flight by turning the ignition to the off position momentarily and listening for the engine to stop running then turning the ignition back to "both" before pulling the mixture to idle-cutoff.

I hope this helps a little - I know there are A & Ps out here that can probably explain it a lot better than I but your post suggests that the CFI's you have been talking to don't know much about the equipment they are flying. There is an article in the May 2001 Private Pilot that may also add to your understanding.

Good Luck with the training!

 

[Singlecoil]

Re: magnetos

If the P-leads are not properly grounded the mag remains "hot" (electrified by the battery) therefore capable of running even without the ignition on. This is a requirement for starting the engine.

Right idea, but perhaps a little misleading. The magnetos are never "electrified by the battery". What they do is generate electricity and send it to the spark plugs whenever the prop is turning and they are not grounded. If you have a broken ground wire, or P-lead, then the spark plugs will receive a spark if the prop is turned even if the mags are switched off. You are right, this could be dangerous if you are turning the prop for some reason such as priming with fuel and oil on a very cold day. If the ground wire is broken, then the magnetos are delivering spark to the spark plugs, which may cause the engine to fire and cost you a finger. Hopefully you had the plane tied down!

 

[fly4ever]

Singlecoil,

Thanks, I got myself a little turned around in my post. I hope the general idea came through.

 

[uwochris]

Hey guys,

Sorry to bring this thread back from the dead, but I just wanted to make sure my understanding is clear. I have a "general flight test" coming up, and in case this gets asked, I want to make sure I can impress the CFI.

Anyway, my understanding is this- The spark required for combustion is generated by the magnetos. The mags get the energy required to make this spark from rotational energy from the crankshaft. ie) a spinning magnet generates an alternating current... this current is transformed into a direct current and is distributed to the spark plugs by the distributor.

While it may seem paradoxial that the mags get their energy from the engine, yet they are required to start the engine, it is because an electric starter, which uses electrical power, cranks the crankshaft and turns over the engine afterwards. As the crankshaft turns, it enables the magneto to produce its own spark, which is used for combustion. After this is complete, the starter is no longer required unless the props stop windmilling.

Well, in general, is this a good understanding? I want to make sure I understand it.... and not just be able to regurgitate it.

Thanks in advance.

 

[avbug]

The magnetos aren't outputting alternating current; it's DC all the way. It does change polarity, but this is still DC current. It's never AC changed to DC. The first plug fired gets DC, then the next reverse DC, and so on. That's one of the reasons that spark plugs should be rotated, top to bottom, and one forward in the firing order.

 

[SelectZoneFive]

Let us ask a practical question. What does it all mean in the end?

It means stay the hell away from the prop arc when you are turning the prop or just generally anywhere near it.

Those whirling blades of death can be quite short tempered.

 

[310]

also if the engine(s) are fuel injected hand proping will not work if your battery is dead because the boost pumps are battery powered. If your flaps are electric and you have a charging failure in cruise and the battery runs down- you will loose all radios, lights etc but also you will fly a no flap approach- if the gear is electric and battery goes dead due to a charging problem the gear will have to be manually extended- so think ahead and know your gear extentsion procedures including remembering to select gear down so any latent energy in the battery does not work against you during the gear extension and selecting gear down allows cranking the gear into the over-center locked position.

 

[avbug]

310,

If that's true, I've made the accidental mistake of handstarting ("propping") more than a few fuel injected engines.

It's still possible. Then again, if your battery is dead, you need to reevaluate what you're doing starting the engine in the first place...where are you planning on going with an electrical system failure?

There exist a few occasions when this is necessary; stuck where there is no help, and you need to get to help. Certainly in airplanes that lack electrical systems...a no-brainer.

However, if you have an inoperative electrical system and a dead battery, and prop the airplane you are flying an unairworthy aircraft. That should be a red flag right there. It's one thing to develop a problem enroute and get to where you need to be for repairs. It's another matter entirely to take aloft an airplane with a known problem.

 

[310]

I am not advocating doing this and have never- just stateing what i thought was the obvious to the new guy that posted the thread- Question avbug; how do you get a injected engine going without the boost pump- I'm not disputing it, but want to know.Was it a high wing so you get a little gravity provided fuel pressure?

 

[avbug]

Advocate away...there's nothing wrong with handpropping an airplane. Advocate teaching it...nobody should complete even a private pilot certificate without receiving hands-on training in hand-propping operations.

Many of the fuel injected engines I've hand started have been high wing airplanes, yes. However, many fuel injected airplanes also utilize a wobble pump or manual primer, and all one needs is enough prime to get the engine to fire. After that, fuel flow is per the engine driven fuel pump. No problem.

 

[GopherEvertt?]

Just remembered a lecture from the 4th grade. The teacher was talking about electricity. Upon describing a magneto, he said, "Does anyone know why it is called a magneto?" Answer: "Because it has a MAGnet, and the way it works is NEATO!" Well, it seemed funny when you're 10 !

 

[RockyMnt1]

Dumb question

I was involved in hand propping a C-421 engine (bad starter). Like Avbug said, we primed it a whole bunch and pulled it through a couple of times and it was just shear luck that it caught. Just about lifted the mechanic off the ground and if it didn't catch on the first try we were going to forget about it. The prop didn't seem to move very far so watch those prop blades!!

I tried to hand prop a C-310 engine (another bad starter) and it didn't budge. Had to get a new starter for that one.