Engine scare - help?

A

AA5Bman

Line Up and Wait
Joined
Jun 14, 2009
Messages
792
Display Name
Display name:
He who ironically no longer flies an AA5B
Hi guys,

I fly an AA5B Tiger which is powered in the typical fashion by a Lycoming 0-360 A4k. I was flying home tonight in clear air at cruise (leaned) at 4,500 just plodding along, when out of the blue something goes wrong. The engine stutters and starts to shake, power and RPMs drop, moderate - but not severe - vibration in excess of what is normal sets in, and most oddly, the temperatures on my EI 4 probe engine monitor start to bounce up and down, almost looking like the EQ on a (music) radio.

I immediately note the closest airport and do all the things I can to remedy the problem - full rich, fuel pump, switch tanks, carb heat, etc etc. I pretty much try everything, testing both mags and fiddling with anything I can think of. Nothing makes any immediate effect.

Within only about 30 seconds from the start of this, the engine smooths out, RPMs come back up, power returns to normal, temperatures settle right back to where they should be - like nothing ever happened in the first place. I fly home.

After landing, I do a long run-up and try to re-create the problem or figure out what went wrong. Can't get it to happen again, and everything checks out as per normal. I call my mechanic, who after describing the problem and asking some questions, suggests that it's probably a fuel contaminant - either water or dirt - that got sucked through the engine causing the roughness and drop of engine power. As it got digested, the engine returned to normal. The facts that the engine returned to normal operation, checks out in run-up, and that the problem can't be re-created suggest, according to him, that the engine didn't experience a system failure per se. He suggests that I sump the tanks carefully, run up the engine, and if all looks good, fly a pattern and see what happens. I sump the tanks and lo and behold, there is a little - but not a lot - of dirt-looking contamination coming out of one of the sumps (and I think - but now can't remember - that it was on the tank I had selected when the engine burped in the first place). I sump until clean (four or five small strainers), and check the other ones, which are all fine. I jump back in and run up the engine. It sounds and feels great, and responds appropriately to all the run-up checks. I decide to take it around the pattern, and the plane is strong and eager to fly, like nothing ever happened.

It seems like it's a pretty decent diagnosis that something probably went through the fuel lines. Does anyone have any experience with this kind of thing, or think this is a reasonable read on the situation? I actually feel pretty decent about it right now, but wanted to see if anyone out there in POA-land felt differently.

Your thoughts appreciated.
 
Contamination in the fuel is my first guess...water?

An issue with the mags and/or primary wiring between the mags and the plugs.
 
Normally I would blame the ignition first. It's so easy to blame the fuel, but a general power loss would occur with fuel delivery troubles, not the shaking so much. A mag that's going bad could start acting erratic and cause a moderate RPM drop and some shaking, but fooling with the mag switch should have isolated that. Carb ice can cause such behavior; how long was the carb heat on and what were the temp and dewpoint?

But a sticking valve that isn't opening or closing all the way could also do this. If it doesn't open, the pushrod gets bent and will sometimes show up as a bent pushrod tube atop the cylinder. A valve sticking open wouldn't bend the tube but would cause the brief power loss and shaking. If an intake valve sticks open the engine will make popping and banging noises as the mixture in the intake ignites once in a while. One more possibility is that some bit of carbon broke off from inside the head and got stuck under the valve for a little while, keeping it from closing properly.

If I were you I would get the mags looked at and do a wobble check on the valves. Is the engine older than 1999?


Dan
 
Last edited:
AA5Bman said:
Hi guys,

It seems like it's a pretty decent diagnosis that something probably went through the fuel lines. Does anyone have any experience with this kind of thing, or think this is a reasonable read on the situation? I actually feel pretty decent about it right now, but wanted to see if anyone out there in POA-land felt differently.

Your thoughts appreciated.
Click to expand...

I tend to think of my plane and my engine as separate entities, so whenever I fly there are three of us. I also tend to anthropomorphize a bit and think that after a bouncy landing my plane is pretty ****ed and so she won't let the step down for me and who knows what she'd do if I stepped on the flaps getting down. Likewise, the engine is just as fussy and if I haven't leaned properly during my previous flight she'll refuse to bring the mags up to tolerance during run-up. I can't imagine what she'd do if I didn't religiously check my fuel sumps and drain the selector--I think your engine was giving you a little pucker-factor to make sure you check for water and contaminants more carefully. Threesomes are great, but you've got to keep the ladies happy or they turn on you.
 
Haha, nice post. I probably should be more careful sumping, it's true. Once in a while I've noticed just alittle "stuff" in the sump, but we've checked the fuel screen and found little or no contaminants trapped there.

In response to some of your questions, I doubt it was carb ice, and my mechanic and I did discuss that. There was probably at least a 5 degree C spread between temp and dewpoint, I was in clear air (not in clouds), and was at cruise power. We also have a typically dry airmass here in southern California. I didn't leave the carb heat on long, or really long enough to fix an ice buildup (remember the plane fixed itself within probably at most 30 seconds)

Sticky valve? Couldn't say, although the engine was new in 2006 and has been well-loved. It is a possibility, though. Don't the valves start to go bad when the engine is run too hot for prolonged periods of time? It could be this, although I am conservative with engine procedures - leaning properly, and typically running CHT around 380.

This airplane has Slick Mags, and they are coming close to due for their AD required (suggested?) 500 hr overhaul/inspection. I mentioned this to my mechanic, and he said that it would be unusual for a mag to go bad and then "fix" itself. He suggested that mag problems would show up again during mag checks and run-up, and that I'd likely be able to re-create the scenario.

Should I be concerned about flying this plane, do you think? Is it a "get it to the mechanic now" situation? I'm not sure that it is, but don't want to do anything stupid, either.
 
AA5Bman said:
In response to some of your questions, I doubt it was carb ice, and my mechanic and I did discuss that. There was probably at least a 5 degree C spread between temp and dewpoint, I was in clear air (not in clouds), and was at cruise power. We also have a typically dry airmass here in southern California. I didn't leave the carb heat on long, or really long enough to fix an ice buildup (remember the plane fixed itself within probably at most 30 seconds)
Click to expand...

A five-degree spread isn't much, but as it's a Lycoming it probably wasn't carb ice. Ice conditions don't need cloud, either; just a higher relative humidity. We've seen carb ice in our engines on sunny, warm summer mornings here in Alberta, where it's also typically fairly dry.

Dan
 
Have you seen it on a warm sunny day at cruise power?
 
AA5Bman said:
Have you seen it on a warm sunny day at cruise power?
Click to expand...
Given high humidity, it can form from a combination of expansion cooling and evaporative cooling on a warm sunny day.

I have never personally seen carburator icing...but I believe it exists. You don't need to be in the clouds or at 100% humidity

AOPA has a nice chart showing when icing is likely to occur:

nyc02fa025_1.jpg


More information here:
http://www.aopa.org/asf/epilot_acc/nyc02fa025_1.html

I do suggest that you read it.
 
Very compelling. I'm guessing you hit the nail on the head. Today is pretty much like yesterday, so check it out:

Approximately 65 degrees F today with a dewpoint of approximately 55 degrees as reported on the ground (sea level). Take that up to 4,500 for a short-range cruise flight, and assume that both those numbers fall. Look where that puts you: serious carb icing potential on the ground at low power, and serious carb icing potential for cruise power at altitude.

The kicker? Relative humidity today is reported at 79%.

Very surprising to me, but what a fortunate way to learn... So happy not to be the pilot in your attached story.
 
If you found black crap in the tank, and believe that this may have been what caused your malfunction (which I don't disagree with), I would pull the bowl off the carb and see if there is any more in there. While I had the carb off, I would also flush through the rest of the fuel system with an electric fuel pump and service the gascolator.
 
A sticky valve was my first thought, too, although if your CHT's are staying under 400 and you have the high chrome valve guides (which you would if you had new Lyc cylinders in 2006) it's less likely, and if the EGT's on all four were bouncing, almost impossible. Fuel contamination, however, would affect all four at once.
 
The O-360A4K installation in the AA5B seems pretty resistant to carb ice. That doesn't mean it can't or doesn't happen.

I'd go with Hennings suggested diagnostic and cleaning of the fuel system first.
 
Henning, If it was debris in the fuel system how could it clear itself up in flight?

Sounds like water in the fuel to me, no?
 
Geico266 said:
Henning, If it was debris in the fuel system how could it clear itself up in flight?

Sounds like water in the fuel to me, no?
Click to expand...

A speck of something can get caught in the carb jet, disturb fuel flow, and then be pushed through the jet resolving the problem and leaving no evidence. This creates one of those spooky engine problems where you don't really know what happened, whether it was a transient event or if it was a problem that could recurr.
 
I am assuming you EI doesn't record the data? It could be helpful if you could download the data from the event.

Brian
 
AA5Bman said:
Have you seen it on a warm sunny day at cruise power?
Click to expand...

Yes, but not in Lycomings that have their oil at operating temp. The Lyc's carb is attached to the bottom of the sump and the hot oil heats the carb body and reduces the likelihood of icing. Continentals, now, or the old Gipsy Major engine I used to fly, have other ideas about when they'll ice up. Warm sunny days with high dewpoints and they'll ice up in cruise.

Dan
 
Maybe not ice, I would still point out that the conditions, per the AOPA chart, were conducive to possible in-cruise icing which is surprising and might be a good lesson for all of us.

Some one out there (not on this board) suggested a really simple explanation: a fouled plug. Lose a cylinder and you get the shakes and the RPM drop, and the unburned fuel could reek havoc with your temperatures. The fouling goes away, everything goes back to normal, which would explain why I couldn't re-create the situation. This seems like the most intuitive solution.

Also, I've never had this problem before, but aren't O-360 A4ks (or maybe their installation in the Tiger) known for plug fouling?
 
Geico266 said:
Henning, If it was debris in the fuel system how could it clear itself up in flight?

Sounds like water in the fuel to me, no?
Click to expand...

A little bit of rubber gets wedged into the jet which has a hole about 1/8" deep and it takes a moment or two before the fuel flowing past it drags it through. It could be water, but since he sumped out a good few particles, prudence dictates that the bowl be inspected and the fuel system cleared before further flight.
 
AA5Bman said:
Maybe not ice, I would still point out that the conditions, per the AOPA chart, were conducive to possible in-cruise icing which is surprising and might be a good lesson for all of us.

Some one out there (not on this board) suggested a really simple explanation: a fouled plug. Lose a cylinder and you get the shakes and the RPM drop, and the unburned fuel could reek havoc with your temperatures. The fouling goes away, everything goes back to normal, which would explain why I couldn't re-create the situation. This seems like the most intuitive solution.

Also, I've never had this problem before, but aren't O-360 A4ks (or maybe their installation in the Tiger) known for plug fouling?
Click to expand...

The plug issue would only show on one cylinder. You said all the cylinders were getting funny on the engine monitor. The contaminated fuel/fuel system or carb ice are more likely.
 
Ron Levy said:
Fuel contamination, however, would affect all four at once.
Click to expand...


I do not know if the AA series has the propensity to hold water in the tanks like the late model Cessna 172, but wouldn't the evidence of a fuel contamination such as water still be in the gascolator?

If you got enough water coming out of the tanks wouldn't it have to fill the gascolator before it got sent to the carb?

IMHO,,,That's a lot of water and there would still be some to see, some where.
 
flyersfan31 said:
carb ice are more likely.
Click to expand...

Carb Ice at cruise power setting usually shuts the engine down until you add heat and cure the problem, it doesn't cure its self.
 
Henning said:
If you found black crap in the tank, and believe that this may have been what caused your malfunction (which I don't disagree with), I would pull the bowl off the carb and see if there is any more in there. While I had the carb off, I would also flush through the rest of the fuel system with an electric fuel pump and service the gascolator.
Click to expand...

Any debris large enough to plug the cruise jet, coming thru the fuel system should be caught by the screens, if it does get thru the system and into the carb, it will get lodged in the cruise jet and stay there, making the VSI point to where you are going.

There is nothing but divine intervention that would dislodge it, and make every thing run right again.
 
My WAG is a loose exhaust baffle.
 
Tom-D said:
I do not know if the AA series has the propensity to hold water in the tanks like the late model Cessna 172, but wouldn't the evidence of a fuel contamination such as water still be in the gascolator?
Click to expand...
Not if it went through the carburetor and out the exhaust, and if it didn't, you wouldn't experience the symptoms.
 
Ron Levy said:
Not if it went through the carburetor and out the exhaust, and if it didn't, you wouldn't experience the symptoms.
Click to expand...

I don't know about your gascolator, but mine holds about a pint, water being heavier than fuel, most of the water will still be in the bottom of the gascolator bowl. that's kinda why they designed the gascolator to be drained from the bottom.

To my way of thinking you would need enough water to fill the gascolator before the engine would realize it wasn't getting fuel.

Remember the gascolator feeds the carb from the top of the gascolator (water goes to the bottom) the carb feeds the engine from the bottom of its bowl, thru the mixture valve, any water getting to the carb will go directly to the mixture valve and to the cruise jet, the venturi and then the engine.

If he had enough water to make the engine quit, he will have a gascolator full of water when he lands.
 
Ron Levy said:
Not if it went through the carburetor and out the exhaust,

I'm thinking about that,,,,, any water in excess of what the gascolator will hold, that is exactly what will happen.

and if it didn't, you wouldn't experience the symptoms.
Click to expand...

Yer right about that too, any water coming thru the gascolator that is less than the capacity of the gascolator bowl will be held in the gascolator and he would have never had a engine sputter. If ya think hard about it, that's kinda why they put it in the system.

So I don't believe this was a water in the fuel problem.
Ignition problems usually don't fix them selves.
Lead fouled spark plugs don't effect all 4 cylinders or both mags. or fix themselves.

So what's left? Piper's age old muffler failure. that's why they put the bail in the muffler isn't it? Has he got it? did he look?
 
Last edited:
Dan Thomas said:
Now, that's a really good possibility. Old Pipers used to do that. Dan
Click to expand...

there's an AD for it. was this one complied with ? the fix for the AD is not fool proof either, the bail fails.
 
Tom-D said:
Carb Ice at cruise power setting usually shuts the engine down until you add heat and cure the problem, it doesn't cure its self.
Click to expand...

Good point, I forgot what he said about the carb heat.
 
Hmmm... thanks for all the thoughts. Exhaust baffles are another common Tiger issue (er, Lycoming, I should say), if I remember correctly. Fortunately I have some time to think about this and the plane is going to go in for a look-see after I get back in town.
 
Tom-D said:
Any debris large enough to plug the cruise jet, coming thru the fuel system should be caught by the screens, if it does get thru the system and into the carb, it will get lodged in the cruise jet and stay there, making the VSI point to where you are going.

There is nothing but divine intervention that would dislodge it, and make every thing run right again.
Click to expand...

You may be right, but you are also assuming that the screens are actually there and intact, that would not be an assumption I would be willing to make. I've pulled apart too many fuel systems where those screens were deficient or missing.
 
You must log in or register to reply here.
Back
Top