Operating a O-360 A3A
 

Hi,

I just got familiarized with a Robin DR300 that has a Lycoming O-360 A3A (180hp). I have flown O-360s in Cessnas before, but the following two operating techniques as described in the DR 300's POH were new to me:

1) It is advised to use the RPM range between 2,150 and 2,350 only for a short period of time.

2) Starting the engine shall be done on the left magnetos only. Once the engine is running switch to both.

Does anybody have an explanation for those techniques? As for starting the engine, perhaps the 'crank' position of the key (like in off-left-right-both-crank) in those Cessnas I use to fly does use left magnetos only?

Thank you.

The A3A variant of the engine has a hollow crankshaft, A4A has a solid crankshaft. There are various props that set up a resonance with the hollow crankshaft and cause vibrations at certain RPM, it is quite noticeable as you move the throttle through that range. Check the prop that you are using, not all are affected, there should be a placard in the aircraft.

Most key ignitions cut the right mag. out in the start position but some don't, certainly the older Robins I have flown (like the DR300) didn't. The left mag. is the impulse mag (retarded ignition for start up) so the right one isn't used until the engine has started.

Enjoy the aircraft, MUCH nicer than any of the American spam cans.

Thanks a lot, Max Angle.

O360 A3A
 

Max Angle,
I have a PA28 180 with the O360 A3A. is there another propeller manufacture that makes a prop that will not create the harmonic vibration in this engine? If I have to go with an A4 A, it will cost me about $28K and I can get a new prop for $5K.....

I would contact a club of PA-28 owners and ask if they have a list of prop change STCs for your model. I see only two on the FAA's list on rgl.faa.gov, but that list may be incomplete.

I wouldn't worry too much about the rpm avoid range, it is not really a problem and not worth buying a new prop for. As far as I know the only way to avoid it is to use a composite (wood or carbon) prop if that is allowed on your aircraft. We have an A3A and don't find it an issue, that said when we replace the engine we will go for a solid crank. To be clear the restriction is there to avoid damaging the prop rather than the engine, the vibrations that set up when operating in the avoid range (2150-2350) can eventually lead to a catastrophic failure of the prop which would cause a very "bad day".

The hollow section at the front of the crankshaft is to allow pressurised oil into the the hub of a constant speed prop which provides the force to change the pitch. If you are not intending to ever use a VP prop the solid crank is the way to go at engine replacement time. You could also buy a VP prop and solve the problem that way, be bit more than $5000 though!