The International Cessna^® 170 Association

1952 170B
McCauley B3D36C424-E Prop
Lycoming O-360-A

When the aircraft is at cruise for 20 minutes or so, 2400 RPM, 24” Man Press, the aircraft will start to shake violently. So bad it actually opened the door. Pilot reported the engine seemed to be moving up and own 2 inches. Power was reduced and the nose of the aircraft was raised and the shaking stops. Ran power back up to 2200 RPM and 22" and after 40 minutes it started shaking again. No vibration felt in the cockpit flight controls. The last maintenance on the engine before this started was replacing all cylinders, all pistons and two rods. We have replaced all engine mounts with no improvement. We have inspected the entire aircraft and can find no reason for this. We balanced the prop to .06 IPS with no improvement. Put on a different prop with no improvement. We pulled the engine down and weighed the pistons and rods. Nothing out of the ordinary found. Any ideas out there?

Sticking valves? Check out Lycoming SB388C for valve guide condition and carbon buildup. How many hours on the engine? Cylinders new or overhauled?

I'd start by checking the magnetos for carbon tracking or a bad condenser. When was the last mag inspection? Do you have an engine monitor? Did you try running on one mag when the roughness started?

Lycomings just like to run rough. I'm a Continental man myself.

The cylinders were replaced with new Superior parts when the shaking first started. Then they were replaced 25 hours later with Lycoming parts. The engine runs smoothly all the time. All gauges are stable throughout the shaking episodes. Engine has 1150 hrs TSN.

We had a similar issue with the New Lycoming O360A1A we installed in my friends 172 tail dragger. It was a brand new engine not an overhaul. It has roller tappets and wide cylinder flanges as all of the new Lycomings do. The old O360 A1A narrow deck engine did not demonstrate this issue. The airplane has a 6 probe EGT CHT JPI system. The number one CHT and EGT was going to the moon. I would post the numbers here but you would not believe them. My friend was afraid to fly the airplane it shook so bad. The Lycoming tech rep came down and observed the engine run and down loaded the JPI data. He had the engine sent back to Lycoming. The tech rep said he had never seen any thing like it. Lycoming said that they could not find anything wrong with the engine and suspicion d the Carb air box which was bolted directly to the bottom of the AVSTAR New style machined body Carb. The engine was sent back and the OIl sump was 5/8ths of an inch taller. It was obviously changed because the brackets that previously fit on the engine now did not. Lycoming claimed it never happened. In any case the problem was better once the engine was re-installed however it was not completely gone. Steve ended up taking the Avstar carburetor off and replaced it with a overhauled Marvel Shebler carburetor. It was better but not resolved. Steve bought a different Carb air box off of some thing or another and that finally solved to issue. Lycoming had said they thought the issue may have been being caused by the STC'd carb heat box not having a neck between the base of the carburetor and the carb heat box causing flow separation through the induction system. In any case the #1 cylinder evidenced by the JPI Data EGT was going extremely hot and the associated CHT was going over the read line limit. On a few of the flights prior to resolution Steve was afraid he was not going to make it back the airfield. One weird experience.
Jim

This might be a good argument to have an EI engine monitor or the like installed. Although they're expensive in themselves, they are cheap in comparison to the total investment.

The aircraft has a JPI system. There were no changes to any temps when it shook (unfortunately)......

If you're certain that the engine is smooth and temperatures steady, why did you replace all the cylinders as a fix? If it isn't an engine problem then it's a scary airframe vibration. To me it still sounds like an ignition problem, but I can't explain why that (or any engine problem) wouldn't show up in EGTs. You might also check for an internally blocked muffler, but that normally causes a smooth power loss.

Does the crankshaft in your engine have counter weights?

Edwards1 wrote:The aircraft has a JPI system. There were no changes to any temps when it shook (unfortunately)......

Is this based on gauge observation, or actually downloading and reviewing the data? The gauge more or less shows trends. The data may show actual events (if they are there). You may have to up the capture rate too. I'm not sure what the stock JPI setting is.

DEM

If the engine runs smooth for 20 mins after starting so how can it be airflow or engine balance issues. What can change after heat soak? Take off is the most stressful period for a engine . However the magnetos can be effected by heat. It's either the coils or condensers. Maybe carbon tracking but I doubt it. The quickest and cheapest check is to replace the condensers and check the point gap and check the resistance of the coils. Another quick check is run the engine at night without the cowls but get that engine really hot. If the leads are shorting out you will soon see them.

Edwards1 wrote:1952 170B
McCauley B3D36C424-E Prop
Lycoming O-360-A

When the aircraft is at cruise for 20 minutes or so, 2400 RPM, 24” Man Press, the aircraft will start to shake violently. So bad it actually opened the door. Pilot reported the engine seemed to be moving up and own 2 inches. Power was reduced and the nose of the aircraft was raised and the shaking stops. Ran power back up to 2200 RPM and 22" and after 40 minutes it started shaking again. No vibration felt in the cockpit flight controls. The last maintenance on the engine before this started was replacing all cylinders, all pistons and two rods. We have replaced all engine mounts with no improvement. We have inspected the entire aircraft and can find no reason for this. We balanced the prop to .06 IPS with no improvement. Put on a different prop with no improvement. We pulled the engine down and weighed the pistons and rods. Nothing out of the ordinary found. Any ideas out there?

When was the 3 blade prop installed? (engines hours at install, and engine hours since installation)

Here are a couple of links that may help.

http://www.eaavideo.org/video.aspx?v=4773862922001 Vibration diagnosis

http://www.eaavideo.org/video.aspx?v=1367499350001 Pre-flight and in-flight mag checks start around 32:43

http://www.eaavideo.org/channel.aspx?ch=ch_webinars Link to all EAA webinars