Surprise Propeller Motion

[rwtucker]

Today's Pilot Workshop Tip of the Week discusses the following scenario:

“One day a while ago, I jumped into my airplane for a quick local flight and was shocked when I turned on the master—and the prop started turning. I shut off the master right away. Fortunately, nothing—and more importantly no one—was inside the prop arc.

This happened because the starter contactor (which can also be called the starter relay) had failed in the on position. Despite the fact that the airplane keys were still in my pocket, turning on the master powered the starter and the prop immediately started turning."

It is always good advice to treat the prop as if could fire but is anyone familiar enough with Diamond wiring diagrams to know if this kind of rare event could occur in various models? If it is electrically possible, would the failure have to occur before shutdown from the previous flight and wouldn't it therefore be noticable? Interesting issue but I am 1,000 miles away from my wiring diagrams.

[Steve]

It can definitely happen with a DA40 - it happened to me about 22 years ago. Starter contractor contacts welded together due to my running the battery down.

[rwtucker]

Did you experience the prop turn when you turned the Master on Steve?

[Steve]

It actually happened while I was cranking. Released the key and the prop kept turning. Turned off the Master and it stopped. Turned key to off and turned Master on and the prop started turning. The start contactor had welded closed. Had to replace it (which isn't an easy job due to its location).

[Kesk]

I have heard of this being a possibility for years. Never had it happen, where I work we call out and clear the propeller any time we turn power on. Well last year we had this happen to 3 or 4 starter solenoids.

[ecogal]

Hello,

Please check the Push Button / Starter Button. This did happen in the past sometime, contact burned.

[rwtucker]

The distinctive in the Pilot's Workshop example is that the prop turned when the master power was turned on. The ignition was off and the keys were out. I can see how that might occur in a very unusual situation but it seems that the malfunction would have had to occur before the engine was was shut down from the previous flight and that should have been noticeable or even caused damage.

[jwx96]

How fast does the starter turn the prop? I think it would be slower than the engine, but still probably enough to cause harm. I can imagine a scenario where 2 pilots are doing the preflight with one out in front and the master getting turned on and someone getting hurt. For the engine to take over the ignition would have to be on or a p-lead disconnected. I’ve had a disconnected p-lead. Thanks for sharing. This isn’t a scenario I’ve considered.