Recip Technology: Rotax Engines

Understanding and troubleshooting


Rotax engines are a popular choice for certified light and homebuilt aircraft, auto gyros, and military UAVs. Though you may not have encountered one yet, chances are you will in the future.

So what sets the Rotax apart, and what do you need to know to troubleshoot it? To answer these questions, AMT spoke with Rotax expert Dean Vogel of the Aero Technical Institute (ATI; www.aerotechnicalinstitute.com). Affiliated with Lockwood Aviation in Sebring, FL, ATI offers Rotax maintenance training courses.

AMT: First, tell us about the Rotax engine. They are quite different from a Lycoming or P&W, aren’t they?

Vogel: Yes. Rotax engines have much more in common with motorcycle engines than they do with traditional aircraft engines.

AMT: What makes them different?

Vogel: Well, Rotax engines use liquid cooling to keep their size and weight down. They have five-piece crankshafts, rather than the traditional one-piece forging used in most aviation engines. And their oil tank is located outside the engine proper; allowing for a lubrication system that is effective with a lower oil quantity and significantly less weight. As well, Rotax engines use motorcycle oil — aviation engine oils are totally unsuitable — and automobile fuel.

AMT: Now that we understand how different Rotax engines are, what are the three most common troubleshooting issues associated with them?

Vogel: The three most common troubleshooting items, in probable Pareto order, are carburetion, kickbacks, and ignition. Please understand, if these engines are properly set up and maintained, problems are next to non-existent. But if they do have problems, these are the most commonly seen.

AMT: What symptoms do carburetion problems present, and how do you diagnose them?

Vogel: With carburetors, the symptoms will be engine roughness, and it may or may not be accompanied by a decrease in power. Once in a while the problem will show up during the mag check, and it will be difficult to determine if the problem is ignition or carburetion.

One quick troubleshooting tip is to put the engine where it is not happy, rpm-wise. If that means on one mag, put it on the mag with which it’s the most unhappy. Then pull on the choke. If there is no change, it’s likely that the problem is not carburetion. If the problem disappears, or gets dramatically worse, the problem is the carburetor or something in the air or fuel delivery systems.

AMT: What about kickbacks?

Vogel: They show up as difficulty in starting the engine. There may be a loud banging during start with an associated, apparent pause of the propeller. When the problem is advanced, the propeller may stop altogether and the starter may spin free — and this often can be heard from the cockpit.

The problem with the diagnosis is that several possible starting problems will look the same, and — if left uncorrected — will eventually result in sprag clutch failure. So if you have kickback you may have need to replace the sprag clutch, but if you don’t correct the other issue(s), you’ll be going right back to damaging the clutch again.

One of the easier ways to tell if the sprag clutch has been damaged to the point it needs replacement — regardless of the presence of other issues — is the amperage going to the starter. Place a clamp-on type ammeter around the cable that powers the starter. You will need one that has data hold. Press the data hold button and hit the starter. If you see a startup amperage of less than 75 amps, the sprag is too far gone and needs replacement. You will obviously want to make sure your power supply is adequate before doing this test. Given the expense of changing the sprag, you wouldn’t want a false reading to come from a low battery.

The usual other culprits are a weak battery, too long a cable from the battery to the starter with insufficient gauge, carburetor problems, gearbox maintenance, a propeller with too high a moment of inertia, a resonance in the engine mount or airframe, or the need for a Soft Start module on the ignition.

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