Problem with C152

[arthur106]

Don't know if this is right forum, first post here...

I was flying a C152 on a night cross country a few weeks ago and noticed a momentary significant loss of power (maybe 100-200RPM). The engine resumed running normally very quickly before I could determine the issue, but it freaked me out. I was able to convince myself that everything was fine and that I was just overreacting and kept on flying. About 15 minutes later, the same thing happened; even though it only lasted for about 5 seconds, the loss of power was significant enough that I became concerned that it might even go out completely, so I decided to land at the nearest airport. I ruled out carb icing because I was running at 2400RPM and it was not humid that night, so I figured it must have been water in the fuel tanks. I sumped the tanks right after the flight and found nothing, (neither did I find any water before the flight) so I reported it to the mechanic on the field. The mechanic at the airport found nothing wrong with the plane and it was flown back to its home base (an hour away) a few days after the incident.
Just today I went to fly the plane and it started up just fine. It ran for approximately a minute and then quite suddenly lost RPM and quit running. (I had it running at 1000-1100 RPM with a full rich mixture, mags on both, primer in and locked, and the fuel shutoff valve open). It was quite warm outside, so the temperature should not be a concern.

Does anyone have an idea as to what the heck is wrong with the plane? The maintenance manager for the flying club seems convinced its just water and I'm not doing a good enough job sumping it before I fly, but I do a very thorough job (I even shake the wings and lower the tail before I sump so any water trapped behind a spar won't be an issue).

 

[docmirror]

Welcome, this is a good forum for the type of issue you have.

I"m not an A&P but I'll chime in anyway so others can attack, ridicule, and scoff.

Three things needed for an Otto cycle engine. Compression, mixture, and a properly timed spark. Since you have a dual ignition system, that's the first thing I would discount. A single ignition fault would possibly drop the RPM 20-60RPM. Loss of compression is pretty drastic, but you might have a intake boot on the engine(don't know if the O-235 has them) leaking intermittantly and that could drop the RPM a lot if you lose RPM. As you already found, the mixture is the most probable failing setup.

Stoich mixture is anywhere from about 11:1 to a very lean 15:1 air/fuel ratio. If I were you, I would continue to focus on the fuel delivery, and mixture at the carb. If it's been looked at briefly by a mech, and nothing is obvious, it's time to take the carb apart, and have it checked carefully. Before you do that though, try draining the fuel tanks completely, putting in 2 gallons per side, then slosh it around, drain it again, and then refill the tanks and test run it on the ground. If it makes power without the stumble, try around the pattern. If it makes that, do some circuits over the airport, and maybe try a few minutes with the carb heat on full, note and changes.

<edit: I just noticed that you had your first drop of 100-200RPM, which I misread. I have to revise what I wrote, in that it could in fact be a mag issue. Another thing that can be checked pretty well by an A&P. Or, you can try a flight with one mag off, and see if there is a change.>

 

[Tom-D]

Don't know if this is right forum, first post here...

I was flying a C152 on a night cross country a few weeks ago and noticed a momentary significant loss of power (maybe 100-200RPM). The engine resumed running normally very quickly before I could determine the issue, but it freaked me out. I was able to convince myself that everything was fine and that I was just overreacting and kept on flying. About 15 minutes later, the same thing happened; even though it only lasted for about 5 seconds, the loss of power was significant enough that I became concerned that it might even go out completely, so I decided to land at the nearest airport. I ruled out carb icing because I was running at 2400RPM and it was not humid that night, so I figured it must have been water in the fuel tanks. I sumped the tanks right after the flight and found nothing, (neither did I find any water before the flight) so I reported it to the mechanic on the field. The mechanic at the airport found nothing wrong with the plane and it was flown back to its home base (an hour away) a few days after the incident.
Just today I went to fly the plane and it started up just fine. It ran for approximately a minute and then quite suddenly lost RPM and quit running. (I had it running at 1000-1100 RPM with a full rich mixture, mags on both, primer in and locked, and the fuel shutoff valve open). It was quite warm outside, so the temperature should not be a concern.

Does anyone have an idea as to what the heck is wrong with the plane? The maintenance manager for the flying club seems convinced its just water and I'm not doing a good enough job sumping it before I fly, but I do a very thorough job (I even shake the wings and lower the tail before I sump so any water trapped behind a spar won't be an issue).

the first problem was a little lead fouled a plug and then cleaned its self up.

the second, you simply allowed it to ice up and quit. the 0-235 will get carb ice as quickly as the 0-200 when cold.

 

[RichNY]

Read up on leaning the engine on the ground and in flight. Also look at carb ice probability charts.

 

[bnt83]

I read your OP and can't help but wonder what does "quite warm" and "dry outside" mean? It would have helped to give us:

#1 Location your operating out of, which gives us :
#2 better idea of outside air temps
#3 Altitudes you are flying at
#4 Humidity

 

[dell30rb]

I ruled out carb icing because I was running at 2400RPM and it was not humid that night,


Just today I went to fly the plane and it started up just fine. It ran for approximately a minute and then quite suddenly lost RPM and quit running. It was quite warm outside, so the temperature should not be a concern.

1) NEVER rule out carb ice.

2) How warm? If you are in NY, it is not warm.

 

[DaleB]

Concur with the advice to lean. Especially lean aggressively on the ground to keep from lead fouling the plugs. I got a lot of bad advice from flight instructors, like "It's OK to run full rich all the time". It's not.

 

[frfly172]

Agree with both,lean on taxi, worry about carb ice. Always try carb heat.

 

[Tom-D]

Concur with the advice to lean. Especially lean aggressively on the ground to keep from lead fouling the plugs. I got a lot of bad advice from flight instructors, like "It's OK to run full rich all the time". It's not.

Cold engines will not run lean.

Do not worry about carb ice when it is really cold, the ice particles in the air will not melt until they are past the Venturi.

When it is very cold the carb idle mixture may not support the fuel requirements of the engine, too much dense air not enough fuel to run. that's why it quit when the extra fuel from priming was used up.

 

[Tom-D]

1) NEVER rule out carb ice.

2) How warm? If you are in NY, it is not warm.

Yes you can rule out carb ice in the extreme cold of the past few days. This is the same principal as flying thru a very cold airmass then entering a snow storm, the snow will not stick to your airframe.

I've noticed that update NY has been colder than Fairbanks the past few days.

 

[iflyvfr]

Personally, I would not fly it again until someone else (preferably your pooh-poohing maintenance manager) thoroughly checks it out.

I have experienced a smooth rollback of power in a C150 that later was destroyed in a runway excursion that I suspect was caused by something similar.

 

[dell30rb]

I have experienced a smooth rollback of power in a C150

Probably carb ice

 

[arthur106]

Never considered or had a problem with carb I've on the ground, it was 65 degrees and somewhat humid when the engine died on the ground. If it was indeed carb icing, I suppose the rich mixture didn't help and contributed to the abrupt stop of the engine.

On my cross country, it was between 30 and 40 degrees and I was at 4500 feet, not sure about the humidity, but I couldn't tell there was much because my visibilty was practically unlimited. I can't imagine it was water in the fuel, because I very carefully checked for it before and after the flight. The loss of power was intermittent though and then it resumed running normally without any action on my part, this isn't normal for carb ice, is it?

 

[docmirror]

Carb Ice is one of those gremlins that are pretty hard to predict. If you were just on the edge of getting accumulation, and then it broke off the venturi area, it could possibly be intermittent.

It's also one of those things that is so hard to reproduce because you need the same environmental conditions to test in. The only way you can test it is to run the engine with the carb heat on for a long time, and see if your symptoms happen again.

Some pilots have been fooled by carb ice before, because they had it, and when turning on the carb heat the engine ran worse, so they turned off the carb heat. When actually, the engine ran worse because it was ingesting the water vapor from the melting ice in the intake. If you turn on the carb heat, leave it on for a while, and let the ice(if any) work through the engine, then turn it back off completely. If the ice comes back, you can turn on a little bit of heat to just keep the intake air warm enough so that the ice won't form.

Carb ice is a tricky little devil, and without a MP gauge in your plane, hard to diagnose.

 

[Dan Thomas]

I'd also bet that carb ice, especially in the next day's engine failure while warming up. Typical of a Lycoming that's still cold; everyone thinks that the hot oil keeps the carb warm, but that only works when the oil is hot.

The OP's comment "It was quite warm outside, so the temperature should not be a concern" is itself a concern. Carb ice can occur right up to 100°F.

Dan

 

[weirdjim]

Never considered or had a problem with carb I've on the ground, it was 65 degrees and somewhat humid when the engine died on the ground. If it was indeed carb icing, I suppose the rich mixture didn't help and contributed to the abrupt stop of the engine.

That'll happen when the venturi freezes over.

On my cross country, it was between 30 and 40 degrees and I was at 4500 feet, not sure about the humidity, but I couldn't tell there was much because my visibilty was practically unlimited. I can't imagine it was water in the fuel, because I very carefully checked for it before and after the flight. The loss of power was intermittent though and then it resumed running normally without any action on my part, this isn't normal for carb ice, is it?

Between 30 and 40 is the PRIME range for icing. If, for whatever reason, the ice buildup was shedding small chunks on a regular basis then intermittent engine loss sounds about right. Then when a BIG chunk broke off the engine ran normally.

Here is a good test. See what happens on the ground when you turn on FULL carb heat. It should drop around 100-150 RPM or thereabouts. Then when you suspect carb ice in the air, pull on full carb heat. If the engine starts bucking and snorting a whole bunch, that is a very good indication that you are sucking melted ice into the engine. THEN after about 30 seconds of pure terror, the sucker will start purring like a kitten after "burning" all that water. Then you can start playing with the carb heat until it stays running normally.

CAVEAT ... the WORST thing you can do is turn on too little carb heat. Then you get some real nasty carb icing that is a real bitchkitty to get rid of. Too much is better than too little. And when the engine quits and there is no exhaust heat to suck into the engine, you are in a "character building" situation.

 

[dell30rb]

Never considered or had a problem with carb I've on the ground, it was 65 degrees and somewhat humid when the engine died on the ground.

That's right on for carb ice.

When you run up, do the mags first, then lock the throttle, note the RPM, pull the carb heat and leave it out for a few seconds. Once you push the carb heat back in, note the RPM. If you gained some, you had a little ice.

 

[mtuomi]

Stoich mixture is anywhere from about 11:1 to a very lean 15:1 air/fuel ratio. If I were you, I would continue to focus on the fuel delivery, and mixture at the carb.

Just a small correction to terminology if I may. Stoichiometric mixture with 100LL is 14.7:1. This is the air/fuel ratio, where all available oxygen can react with the hydrocarbons in the fuel, which is the definition of stoichiometric mixture.
Too many hydrocarbons for the amount of air would mean a rich mixture and vice versa. So anything below 14.7:1 is "rich" and anything above is "lean".

 

[arthur106]

My comment on temperature not being an issue was in reference to having trouble with startup because of the engine simply being cold, not to carb ice.

One more question: I was able to start up the engine after two or three tries after it quit, is that consistent with carb ice?

Thanks, y'all have all been very helpful!

 

[Witmo]

Another possibility is a restricted fuel vent. If the primary vent under the left wing becomes blocked, negative pressure will build in the tank while fuel flow drops off and rpm decreases. When the negative pressure gets high enough, the vented fuel cap pops open, fuel flow returns to normal and rpm recovers. Without a negative pressure to hold it open, the fuel cap vent closes and if the primary vent continues to be blocked, the cycle repeats.

 

[Dan Thomas]

My comment on temperature not being an issue was in reference to having trouble with startup because of the engine simply being cold, not to carb ice.

One more question: I was able to start up the engine after two or three tries after it quit, is that consistent with carb ice?

Thanks, y'all have all been very helpful!

Yes, because the ambient temperature has melted it. The carb only gets cold while there's a pressure drop in it and fuel vaporizing as it's sprayed into the airflow. When all that stops, the carb body temp will rise to ambient or to whatever a hot oil sump will give it.

Next time you're idling and the rpm starts falling, don't just feed in more throttle. The engine is talking to you and telling you something. Pull the carb heat.

The other thing one should do before every flight: Take a look at the temperature and dewpoint spread on the METAR. If there's only a few degrees difference, expect carb ice at some point. Sunny, warm days can convince the unwary that everything is just fine for flying. But if the temp is 20°C and the dewpoint is 16°C, watch out.

Dan

 

[bnt83]

Did they take the cowling off when they "troubleshot" the engine or just go run it on the ground?

 

[Tom-D]

Another possibility is a restricted fuel vent. If the primary vent under the left wing becomes blocked, negative pressure will build in the tank while fuel flow drops off and rpm decreases. When the negative pressure gets high enough, the vented fuel cap pops open, fuel flow returns to normal and rpm recovers. Without a negative pressure to hold it open, the fuel cap vent closes and if the primary vent continues to be blocked, the cycle repeats.

Vented fuel caps on the 152 are open, or bad. there is no "POP OPEN " feature.

see the ADs on the subject.

 

[Tom-D]

One more question: I was able to start up the engine after two or three tries after it quit, is that consistent with carb ice?

When you have carb ice it requires heat from one source or another to clear the ice before you can start again.

Preheat, ambient temps, or carb heat.

 

[alexgeo]

Hi there,

I had the exact same thing happen to me on my first night XC with my instructor on board the 152. We never did find out the cause of the problem, but it was more than likely a fouled plug causing the issues. Even though we're pretty good about leaning the plugs (instructor and students) the other renters probably do not bother. My flight schools mechanic sent out an email to every instructor with a picture showing how nasty the plugs looked on that poor old 152 after the annual.

It's a pretty gut wrenching experience hearing your engine running rough, even if it's only for a few seconds.

 

[Tom-D]

It's a pretty gut wrenching experience hearing your engine running rough, even if it's only for a few seconds.

Things that go bump in the night aren't always the fault of the aircraft.

 

[Capt. Geoffrey Thorpe]

One more question: I was able to start up the engine after two or three tries after it quit, is that consistent with carb ice?

Yup.
I had a car with a bad control valve for the intake air heat. Bad weather, mostly stop and a little go (snow storm) - it starts running pretty bad. I suspected carb ice, but I kept going. Made it home, whipped off the air cleaner and there it was, the booster venturi was iced up solid. I dashed in the house to grab a camera, but by the time I got back out the ice was gone.

 

[Jim Logajan]

Never considered or had a problem with carb I've on the ground, it was 65 degrees and somewhat humid when the engine died on the ground. If it was indeed carb icing, I suppose the rich mixture didn't help and contributed to the abrupt stop of the engine.

On my cross country, it was between 30 and 40 degrees and I was at 4500 feet, not sure about the humidity, but I couldn't tell there was much because my visibilty was practically unlimited.

This diagram showing carb ice probability may help:

 

[docmirror]

Hey, thanks Jim. Nice chart to have.

 

[Silvaire]

...I was flying a C152 on a night cross country a few weeks ago and noticed a momentary significant loss of power (maybe 100-200RPM). The engine resumed running normally very quickly before I could determine the issue, but it freaked me out...

They do that every once in awhile, mostly over mountains or at night. Perfectly normal.

 

[Witmo]

Vented fuel caps on the 152 are open, or bad. there is no "POP OPEN " feature.

see the ADs on the subject.

The cap vent has a thin rubber membrane that normally seals a vent hole in the cap. Negative pressure within the tank causes the rubber membrane to pull away from the hole and allows air to enter the tank equalizing pressure. If the cap vent was open all the time, rain water would easily enter the tank. Take a look at a vented cap and its operation is easily understood--it is not open all the time unless the rubber membrane is bad. The underlying vent normally allows air to enter the tanks as fuel is used. The vented caps only are a backup and do not allow air into the system unless the primary vent is blocked.

 

[Tom-D]

The cap vent has a thin rubber membrane that normally seals a vent hole in the cap. Negative pressure within the tank causes the rubber membrane to pull away from the hole and allows air to enter the tank equalizing pressure. If the cap vent was open all the time, rain water would easily enter the tank. Take a look at a vented cap and its operation is easily understood--it is not open all the time unless the rubber membrane is bad. The underlying vent normally allows air to enter the tanks as fuel is used. The vented caps only are a backup and do not allow air into the system unless the primary vent is blocked.

The shape of the cap and a gasket keeps the water out, the rubber membrane stops fuel from escaping from the tank thru the vent. that is all it does. when you blow thru the vent from the top down there must not be a restriction.

Were water to get into the vent, it will get into the tank.

see the SIL SE77-6
Quote:

Dual venting can be accomplished on most in service aircraft by installation of vented caps, which are now available. Because of the advantages of dual venting it is recommended that the vented caps be installed on in service aircraft in accordance with the information attached.

Careful pre-flight inspection of the primary vent(s) continues to be necessary to assure a properly functioning fuel vent system, with or without vented caps. Any indication of main vent blockage - inability of the vent line to pass air through it, fuel tank oil-canning heard in flight or post-flight, sudden in-rush of air when the fuel cap is removed, or fuel tank filling with less than expected fuel useage - should be investigated and the cause corrected before further flight.

For those aircraft on which dual venting is not available or if the owner chooses not to incorporate it, a placard, like the one shown below, should be made up and installed on the instrument panel to encourage adequate inspection.


ASSURE FUEL VENT LINE IS OPEN PRIOR TO FLIGHT

 

[nrpetersen]

Doesn't anyone else think it might be a sticking valve?

 

[Witmo]

The shape of the cap and a gasket keeps the water out, the rubber membrane stops fuel from escaping from the tank thru the vent. that is all it does. when you blow thru the vent from the top down there must not be a restriction.

Were water to get into the vent, it will get into the tank.

see the SIL SE77-6
Quote:

Dual venting can be accomplished on most in service aircraft by installation of vented caps, which are now available. Because of the advantages of dual venting it is recommended that the vented caps be installed on in service aircraft in accordance with the information attached.

Careful pre-flight inspection of the primary vent(s) continues to be necessary to assure a properly functioning fuel vent system, with or without vented caps. Any indication of main vent blockage - inability of the vent line to pass air through it, fuel tank oil-canning heard in flight or post-flight, sudden in-rush of air when the fuel cap is removed, or fuel tank filling with less than expected fuel useage - should be investigated and the cause corrected before further flight.

For those aircraft on which dual venting is not available or if the owner chooses not to incorporate it, a placard, like the one shown below, should be made up and installed on the instrument panel to encourage adequate inspection.


ASSURE FUEL VENT LINE IS OPEN PRIOR TO FLIGHT

Your quoted reference does not support your assertion that the vented cap is continuously open to outside air pressure. When it says to assure the vent line is open prior to flight, it's talking about ensuring there are no obstructions in the under wing vent line such as mud daunders or insects.
If you were to slip a rubber tube over the under wing fuel vent tube and blow into it, you would pressurize the tank as the vented fuel cap only opens to let air in when there is a negative pressure in the tank. The primary fuel vent normal operation allows air into the tank to equalize the pressure and fuel out of the tank if it is full and has to expand due to warmer air temperatures during the day. The vented fuel caps are only a backup path for air to enter when the primary vent is blocked. In normal operation neither fuel, air, or water can pass through the cap, in or out. This has been discussed and explained in the Cessna 150-152 Club which I highly recommend anyone who owns or flies a 150 or 152 to join.

 

[Tom-D]

Your quoted reference does not support your assertion that the vented cap is continuously open to outside air pressure. When it says to assure the vent line is open prior to flight, it's talking about ensuring there are no obstructions in the under wing vent line such as mud daunders or insects.
If you were to slip a rubber tube over the under wing fuel vent tube and blow into it, you would pressurize the tank as the vented fuel cap only opens to let air in when there is a negative pressure in the tank. The primary fuel vent normal operation allows air into the tank to equalize the pressure and fuel out of the tank if it is full and has to expand due to warmer air temperatures during the day. The vented fuel caps are only a backup path for air to enter when the primary vent is blocked. In normal operation neither fuel, air, or water can pass through the cap, in or out. This has been discussed and explained in the Cessna 150-152 Club which I highly recommend anyone who owns or flies a 150 or 152 to join.

If you don't believe Cessna in their service letter, I don't know how to help you understand.

 

[Dan Thomas]

In normal operation neither fuel, air, or water can pass through the cap, in or out. This has been discussed and explained in the Cessna 150-152 Club which I highly recommend anyone who owns or flies a 150 or 152 to join.

You really need to go take one of those caps off and actually examine it. The valve is a very thin silicone rubber affair that will let air in under the least negative pressure pressure differential in the tank. It will not let air or fuel out if it's working fine but sure will let air in. It has to open extremely easily to allow the small gravity head pressure of the fuel to overcome the low pressure on top of the wing in flight. Any substantial restriction of that vent would make it useless.

Dan

 

[Witmo]

You really need to go take one of those caps off and actually examine it. The valve is a very thin silicone rubber affair that will let air in under the least negative pressure pressure differential in the tank. It will not let air or fuel out if it's working fine but sure will let air in. It has to open extremely easily to allow the small gravity head pressure of the fuel to overcome the low pressure on top of the wing in flight. Any substantial restriction of that vent would make it useless.

Dan

That's what I'm saying, the fuel cap valve (silicone rubber affair) is normally closed but will open to allow air into (not out of) the tank. Blowing into the cap on the outside is the same as having a negative pressure on the inside--it unseats the silicone rubber to let air (through or) into the tank. There have been instances of the under wing fuel vent being blocked and the head pressure decreasing due to the negative pressure build up in the tank until the fuel cap vent opens to equalize the pressure. RPM drops off somewhat until the pressure equalizes and rpm then is restored. At lower power settings there may be very little rpm drop but at higher power settings it would be quite noticeable. I'm not making this up, it happens. Tom is mistaken saying the fuel cap vents air into the tank all the time regardless of there needing to be a pressure differential to unseat the silicone rubber seal. If that were the case, there would be no need for the underwing vent and water would easily enter the tank from a heavy rain following the same path as air flowing into the tank. As long as the underwing vent is clear, the fuel cap vent is closed and the head space pressure is equal to ambient pressure. Tom always has had a problem thinking that he is unable of being mistaken and his reading comprehension is superior to anyone else's.

 

[SoloEqs]

It has to open extremely easily to allow the small gravity head pressure of the fuel to overcome the low pressure on top of the wing in flight. Any substantial restriction of that vent would make it useless.

Dan

Not to mention that in the 150 and 152 fuel is gravity fed. The slightest resistance to that flow can be a serious problem, which is why the vented caps were mandated to begin with.

 

[Dan Thomas]

Not to mention that in the 150 and 152 fuel is gravity fed. The slightest resistance to that flow can be a serious problem, which is why the vented caps were mandated to begin with.

The vented caps were mandated to offer venting if the primary vent system got plugged by ice or some big bug. Or by a mud dauber making his nest in there. The AD only required a vented cap on the right tank, which would supply venting to the left tank via the vent interconnect. Since the unvented caps haven't been available for a long time, most Cessnas will have them on both tanks.

http://rgl.faa.gov/Regulatory_and_G...225F45DF0B489ACE86256A34006AB0A2?OpenDocument

The vent valve in the LH tank has a spring-loaded poppet in it, and that poppet has a tiny hole in it. The poppet will open if there's a sudden need for lots of air in the tank, but closes to prevent the rapid loss of fuel out the vent when the tanks are full and someone parks the airplane with the left wing low. The tiny hole in the poppet usually does the venting, but also allows fuel to drip slowly out the vent. McFarlane Aviation has an STC'd vent tube that has a high spot in it to discourage the fuel-dripping problem. If an owner has a real dripping/fuel loss problem in his airplane, that vent valve has probably rotated in the tank and is no longer up at the top where it should be but is dipping into the fuel. It's on an offset tube just inside the tank.

Dan

 

[Tom-D]

Not to mention that in the 150 and 152 fuel is gravity fed. The slightest resistance to that flow can be a serious problem, which is why the vented caps were mandated to begin with.

Vented caps came in with the AD in 1974, to the 150 and other 100 series aircraft. They never were offered for the 152, and the AD does not apply to the 152, simply because the 152 came out after that period and had the vented caps installed at the factory.

The under the wing vent valve is nothing more than a swinging door hinged at the top so as to allow it to hang to the closed position. It looks like a cupped washer with a 1/8" hole in the center, and a hinge at the top.

That type of vent system replaced the earlier style ramp air vent system,like the 120/140/150/170/ and early 172. These had a single vent at the top center of the fuselage that acted as the vent for both tanks.
This type of vent system did not have vented caps, thus the retro fit to vented caps on all aircraft mentioned in the AD.

And the latest style that has the rubber membrane in the cap assembly that was mentioned earlier in this thread which creates a oneway check valve effect to stop fuel from being vented over the wing. These caps did not come into being until the 1974 AD required the retro fit.