My C172 engine "hiccupped" - why?

[tgrayson]

he gave me some explanation about enriching the mixture too quickly causing it.

I suspect he was making that up.

 

[TXaviator]

lol. for about 6 out of 8 months I was flying the mapping gig, my plane would miss on a cylinder pretty regularly.

mag check = only runs on 3 cylinders.

had the mags torn apart, had them rebuilt, some gear/distributor thing rebuilt, new plugs, plugs gapped, compression checked, etc etc etc

nothing ever fixed it.

after they tore the mags down though it went from running 3 cylinders on the R mag to running 3 cylinders on the L mag.

*shrug*

definitely "exciting" at night over long stretches of louisiana with lots of hungry gators below and not a road or streetlight in sight....

still never heard if they found the problem. not flying that plane again next season.

 

[jrh]

after they tore the mags down though it went from running 3 cylinders on the R mag to running 3 cylinders on the L mag.

*shrug*

definitely "exciting" at night over long stretches of louisiana with lots of hungry gators below and not a road or streetlight in sight....

still never heard if they found the problem. not flying that plane again next season.

But you kept flying the plane for the rest of that season on 3 cylinders? That might be a hard one to explain to the feds if something were to happen.

 

[TXaviator]

But you kept flying the plane for the rest of that season on 3 cylinders? That might be a hard one to explain to the feds if something were to happen.

It didn't continually run on 3 cylinders. It was a documented intermittant problem with LOTS of MX to back it up. I'd squawk it. Plane gets looked at. I'm told "its fixed". Problem comes back intermittantly later down the road.

That's just how it goes. Can't really say "hey I promise its doing this weird thing every now and then but its not repeatable so I'm not gonna fly the plane"

I'm sure they would have found another pilot who would fly it. :-D

 

[matt152]

I was doing a Wright Flight in a 172 when there was a big hiccup on climbout. Immediate return to the airport. Turned out the timing on one mag was 10 degrees off. Did not notice anything unusual during the runup.

 

[HeyEng]

I also vote probable carb ice. If this was recent in the AUS area, the atmosphere has been ripe for it. Nice and humid out there...ESPECIALLY at night. I just LOVE the Gulf of Mexico...

 

[jrh]

It didn't continually run on 3 cylinders. It was a documented intermittant problem with LOTS of MX to back it up. I'd squawk it. Plane gets looked at. I'm told "its fixed". Problem comes back intermittantly later down the road.

That's just how it goes. Can't really say "hey I promise its doing this weird thing every now and then but its not repeatable so I'm not gonna fly the plane"

Ahh, that makes sense. Intermittent problems are the worst. I thought it continually only ran on 3 cylinders on one mag.

I'm sure they would have found another pilot who would fly it. :-D

I'm sure they would have. There's been more than once when I've been happy to walk away and let another pilot fly something.

 

[trafficinsight]

I was doing a Wright Flight in a 172 when there was a big hiccup on climbout. Immediate return to the airport. Turned out the timing on one mag was 10 degrees off. Did not notice anything unusual during the runup.

I, personally, think that 1700rpm, which is where a lot of people do their runups in Cessnas is not sufficient to show a bad spark plug, it'll show fouled plugs just fine, but a bad plug wont fail until it's under pressure. I've had a couple times where the plug "checked fine during the runup, but acted fouled in flight." Sure enough, do a runup at 2000 or 2200rpm it shows up every time.

In your case, assuming that someone didn't make a huge mistake, the timing being that far off is usually the sign of a bad capacitor in the magneto, once the capacitor goes bad the points burn and the timing wanders, by a LOT. Again, at lower RPM it may not show up.

 

[LatitudeDancer]

I've had enough hiccups to count on one hand and each one was unpleasnt. Prior to the hiccup and after the hiccup the engine showed no indications of roughness, loss of RPM, sputtering or anything... was completely normal before and after each one which revealed no definite indication of what may of caused it.

:yeahthat:

Happened to me the 1st time in a fuel injected Diamond 20. It was just as described. I thought it was the mixture, but was leaned fine. The only thing I could come up with other than that was a drop of water maybe. Who knows. I was in cruise flight for a while after doing some maneuvers earlier.

 

[matt152]

I, personally, think that 1700rpm, which is where a lot of people do their runups in Cessnas is not sufficient to show a bad spark plug, it'll show fouled plugs just fine, but a bad plug wont fail until it's under pressure. I've had a couple times where the plug "checked fine during the runup, but acted fouled in flight." Sure enough, do a runup at 2000 or 2200rpm it shows up every time.

In your case, assuming that someone didn't make a huge mistake, the timing being that far off is usually the sign of a bad capacitor in the magneto, once the capacitor goes bad the points burn and the timing wanders, by a LOT. Again, at lower RPM it may not show up.

Good stuff. But I just fly 'em. I leave the maintenance issues to the pros.

 

[TXaviator]

Ahh, that makes sense. Intermittent problems are the worst. I thought it continually only ran on 3 cylinders on one mag.



I'm sure they would have. There's been more than once when I've been happy to walk away and let another pilot fly something.

well even frustrating me more was that after the plane would get looked at ...the problem WOULD go away *for a while*

so we're dumping money at the plane, it seems to work, and then i have to gripe to the boss that my plane is broke yet again....!!

 

[trafficinsight]

yeah, that is one of the most irritating things I can think of.

 

[Texguy]

Thank you, everyone! Good info. Y'all are great.

 

[Texguy]

I'm gonna save this graph. Yes that night in early June was a little humid. Is there a website where I can check the dewpoint from then?

I'll have to read more about carb ice. I went back to my old "Cleared for Takeoff" book which says between 20-70 degrees plus humidity are favorable conditions for it. Honestly I don't understand how ice can form if it's 80, 90, 100 degrees but apparently it can.

 

[Texguy]

I, personally, think that 1700rpm, which is where a lot of people do their runups in Cessnas is not sufficient to show a bad spark plug, it'll show fouled plugs just fine, but a bad plug wont fail until it's under pressure. I've had a couple times where the plug "checked fine during the runup, but acted fouled in flight." Sure enough, do a runup at 2000 or 2200rpm it shows up every time.

In your case, assuming that someone didn't make a huge mistake, the timing being that far off is usually the sign of a bad capacitor in the magneto, once the capacitor goes bad the points burn and the timing wanders, by a LOT. Again, at lower RPM it may not show up.

Interesting, thanks.

 

[Texguy]

I'm gonna do a little more study on Mixture, too. I'd be grateful if anyone can point to a simple guide on 172s and mixture, besides the POH.

 

[HeyEng]

I'm gonna save this graph. Yes that night in early June was a little humid. Is there a website where I can check the dewpoint from then?

I'll have to read more about carb ice. I went back to my old "Cleared for Takeoff" book which says between 20-70 degrees plus humidity are favorable conditions for it. Honestly I don't understand how ice can form if it's 80, 90, 100 degrees but apparently it can.

Dewpoints can be found on most weather sites and ASOS/METARs reports. they are right after the temp...eg: right now, it's 31/19 here in SAT. 31 degrees and 19 dewpoint...about 50% relative humidity. By morning the spread won't be quite as much and the RH will be around 70%. A good rule of thumb here in S/SW Texas is that if it's dark, it's humid. Maybe not a lot, but when the southern winds blow off the gulf, there is GOING to be high humidity. By day break here in SAT, it's down right miserable.

The formation of ice involves the carb. I won't get into too deep, but the temperature drops by quite a bit, and during humid conditions can be as much as 10-15 degrees celsius. Sure it's hot on the ground, but several thousand feet up it gets quite a bit cooler. On a recent long XC, it was consistantly 100+ degrees on the ground, but I had to run the heat at 9500 feet to keep the wife happy.

Check out this site. Probably a lot of good info if you put the time into it to find what you are looking for!

http://forums.cessnaowner.org/

 

[killbilly]

I also vote probable carb ice. If this was recent in the AUS area, the atmosphere has been ripe for it. Nice and humid out there...ESPECIALLY at night. I just LOVE the Gulf of Mexico...

The owner of the 172 I've been flying wants me to lean to about 25 degrees rich of peak on his plane, so I've been doing that. At 5500 feet around AUS lately, even at night, that translates to a lot more than an inch of the mixture knob being out.

 

[CoffeeIcePapers]

HAHA

Ripping the wing apart?

Back tot he topic, isn't is SOP to go full rich when descending?

The only reason you enrichen the mixture in the descent is because it will be too lean when you lose a few thousand feet if you keep it at the same setting. You should enrichen it some, but not full rich.

 

[Bumblebee]

Honestly, from your discription, sounded exactly like a little bit of carb ice to me. Not a mechanic.

:yeahthat:

Temp was 80-85 degrees and you were at 3500? Hmm doesn't sound like a carb ice issue to me.

The 3500' alt is where he experienced the hiccup, the point at which the water melted and went through the carb. (my opinion only)

It sounds like the perfect recipe for cab ice to me. 80-85 on the ground makes it 60-70 at the altitudes you were flying. What was the dew point? Look on this chart to see where they intersect. I'd be willing to bet you were in the "Serious Icing (glide power)" zone.

great post, some opinion, some fact and even a chart to help us all apply some knowledge!:clap: