C172 Cowling

[mkosmo]

Story first:

Well, today was interesting. Went to go flying on the first pretty flying weekend day we've had in a while and I taxied across the field for gas. Life was good, took on 22 gallons of gas I've used over the past few weeks and went to go depart to parts unknown. Went to start the airplane and she wouldn't start for anything. I probably tried cranking 6 or 8 times (various intervals in between)... The starter would try and then eventually free-spin at various delays in my attempts to start. No indication of any ignition, no partial kicking, nothing. Eventually, after spending a half hour screwing with that, I decided I had looked like an idiot enough on the ramp and went in and asked the front desk to get me a tug for me an the airplane back to my parking. I hate paying for the tug... but at least my fuel purchase would have given me credit for all but a few dollars of the tow fee.

By now I just figured my battery wasn't spinning the prop fast enough and I'd need to charge or change the battery. The plan was to call the mechanic and let him take a look. The guy who came over to tug the airplane made a comment that he thought it was odd that the starter was still engaged (I thought it was fairly common knowledge that it wouldn't kick out if it didn't start) and was trying to convince me it was just a hung starter. He also made a comment about not feeling any compression... and that last bit got me worried. I didn't want to mess with it too much with the starter still mated, though.

Once he got me back to the other side of the field I figured I'd pull the upper cowling off to take a look before I called the shop tomorrow just to see if anything obvious was going on. It's probably worth noting that there was no leaking fluid under the aircraft on the ramp or in trail during the tow. While I had the upper cowl off, I decided to try starting one more time since I had nothing to lose and it had been a half hour or so since my last try. The blade didn't move two inches before she purred right to life. After a few seconds I shut back down (in hind sight I should have just let the battery charge then...) and put the upper cowl back on. She started up again after that and I let her run at 1k or so while the battery charged.

I'm now figuring the failure to start was something along the lines of vapor lock, but I'm having trouble with that diagnosis considering how cool it was out today and I've never had this issue with hot starting before on hotter days with hotter engines.

My question is really this: Are there any tightening specs for the cowl screws? It's a 76 172M and there are three fastener types: Quick turns on the firewall, coarse threads along the side, and some find threads up front. They're snug for now, but if there's a spec, I'd like to make sure I have them secure correctly before flying again. Otherwise I'll have to call the mechanic and pay him shop rate to tighten some screws lol

 

[UngaWunga]

Screws... tight enough. Use your internal torque wrench. Hard to explain the touch unless you're used to turning a screwdriver. Ain't magic. You probably have them tight enough. It would be silly to ask the mechanic to tighten them for you. If one falls out, it isn't the end of the world. Just replace.

Starter: was it spinning the prop? If so, you might've flooded it. Which would be why it started and ran a while later, after the extra fuel evaporated.

 

[mkosmo]

Screws... tight enough. Use your internal torque wrench. Hard to explain the touch unless you're used to turning a screwdriver. Ain't magic. You probably have them tight enough. It would be silly to ask the mechanic to tighten them for you. If one falls out, it isn't the end of the world. Just replace.

Starter: was it spinning the prop? If so, you might've flooded it. Which would be why it started and ran a while later, after the extra fuel evaporated.

I figured hand tight may be the answer but wanted to ask in case there was a spec.

I doubt it was flooded, I didn't prime it since it was a warm start, but the thought crossed my mind. Same starting technique that's always worked before! It was indeed spinning it for a while each try before it would free wheel.

I'm not very familiar with these starter designs, though. What causes them to let go and free wheel? Only other time it's done that was a very cold day I hadn't primed enough and it wouldn't start. Do they have some kind of safety clutch?

 

[UngaWunga]

What do you mean, "free wheel"? Was the starter not engaging the prop?

Engines are very simple. Suck, squeeze, bang, blow. If there's compression, spark at the right time, and the right mixture of air & fuel, the engine will run. If the engine ran later, you have compression and spark. What changed was the mixture. You were either too lean or too rich.

 

[mkosmo]

What do you mean, "free wheel"? Was the starter not engaging the prop?

Engines are very simple. Suck, squeeze, bang, blow. If there's compression, spark at the right time, and the right mixture of air & fuel, the engine will run. If the engine ran later, you have compression and spark. What changed was the mixture. You were either too lean or too rich.

The prop would stop spinning (it had been) with the starter still engaged and you could hear the motor spin up faster once the prop load was gone. The starter gear was still engaged on the fly wheel, though.

And I know and agree. That's why I was thinking vapor lock. It seems more likely than flooded given today's operation.

 

[GrampaMark]

The prop would stop spinning (it had been) with the starter still engaged and you could hear the motor spin up faster once the prop load was gone. The starter gear was still engaged on the fly wheel, though.

And I know and agree. That's why I was thinking vapor lock. It seems more likely than flooded given today's operation.

Your starter drive (AKA "the Bendix) is slipping. This is what is known as an "overrunning clutch". When the starter is engaged, the clutch (which is ahead of the pinion gear) takes up the load of cranking the engine. When the engine fires and starts to build RPM, the flywheel ring gear starts to drive the starter. This causes the rollers in the clutch to lose contact with the notches in the clutch's outer drum, preventing the starter from being overdriven by the flywheel.

When the clutch gets worn, the rollers will disengage while the starter motor is still turning but before the engine starts. This is why you hear the starter speed up and observe the prop stop turning. A worn starter drive will often engage when cold but fail to catch on a hot start (it only takes a little bit of heat expansion for a worn clutch to slip).

The starter will have to be removed and the drive replaced. Pretty simple fix and not terribly expensive.

Mark

 

[mkosmo]

Your starter drive (AKA "the Bendix) is slipping. This is what is known as an "overrunning clutch". When the starter is engaged, the clutch (which is ahead of the pinion gear) takes up the load of cranking the engine. When the engine fires and starts to build RPM, the flywheel ring gear starts to drive the starter. This causes the rollers in the clutch to lose contact with the notches in the clutch's outer drum, preventing the starter from being overdriven by the flywheel.

When the clutch gets worn, the rollers will disengage while the starter motor is still turning but before the engine starts. This is why you hear the starter speed up and observe the prop stop turning. A worn starter drive will often engage when cold but fail to catch on a hot start (it only takes a little bit of heat expansion for a worn clutch to slip).

The starter will have to be removed and the drive replaced. Pretty simple fix and not terribly expensive.

Mark

Thank you for the explanation! Makes sense and I'm glad to hear it's not horribly expensive!

 

[Silvaire]

The prop would stop spinning (it had been) with the starter still engaged and you could hear the motor spin up faster once the prop load was gone. The starter gear was still engaged on the fly wheel, though...

That would be physically impossible, if the starter motor was "freewheeling" it had kicked back in and was no longer engaged with the ring gear.

First of all check that the starter mounting is secure and make sure the bushing has not come out of the end of the bullnose.

 

[mkosmo]

That would be physically impossible, if the starter motor was "freewheeling" it had kicked back in and was no longer engaged with the ring gear.

First of all check that the starter mounting is secure and make sure the bushing has not come out of the end of the bullnose.

I assure you it's possible. The description above explained how pretty well and confirmed what I was assuming, but with the details I lacked.

 

[GrampaMark]

Thank you for the explanation! Makes sense and I'm glad to hear it's not horribly expensive!

Keep in mind that we're talking about aircraft parts. In this case, a common starter drive which has been rubbed with the magic chicken bone to make it an aircraft part. It won't be cheap as an automotive starter drive would be, but, as aircraft repairs go, it shouldn't be too painful.

Mark

 

[mkosmo]

Keep in mind that we're talking about aircraft parts. In this case, a common starter drive which has been rubbed with the magic chicken bone to make it an aircraft part. It won't be cheap as an automotive starter drive would be, but, as aircraft repairs go, it shouldn't be too painful.

Mark

Of course, add a zero or two.

 

[GlennAB1]

http://www.faa.gov/documentLibrary/media/Advisory_Circular/Chapter_07.pdf