Starter kept running
 

I was flying the MD500 yesterday, with my friend, the owner's pilot (a bit of recurrent training for me). Everything normal, great 45 minute flight. I landed and shut down, we debriefed the flight. After the engine had stopped, and the rotor was still winding down, I noticed that it had reached a stable RPM, and was not slowing any more. I mentioned this to my friend, who agreed - we could just hear the starter running. At that point, we could begin to smell the burning of the starter generator, as it was turning the rotor. Battery switch off, and everything came to a stop as it should - other than the burning smell. Fortunately, we had landed at a maintenance base, so we could begin to troubleshoot right away.

We let it sit for an hour to cool. I then found that with the battery on, the the generator switch off, everything was normal. However, battery on, crank with the starter, generator on, release the starter and it kept cranking. Gen off, still cranking, battery off, it stopped cranking. Observing starter cool down times (for whatever good it did at that point) we repeated, and the fault repeated. They're working on it now, I hope to learn once they found the problem. In the mean time, I'm suggesting a starter ammeter shunt in the starter line, so the pilot can tell if the starter is being energized or not. For the amount of smoke which poured out of the starter generator, I can imagine a fire potential!

Not familiar with the MD500 (so probably a dumb question)-but am always trying to learn something new:

Does the MD500 have a separate "starter" generator?

Most helicopters i have flown use the starter/generator, so you could either "crank the engine" or use the generator, but never both...

Robinsons have a light indicating when the starter is on and it is indeed possible for the starter to remain on after the engine starts; you're supposed to verify the light is off after starting and shut down immediately if not. Do MD500s not have such a light?

Failed reverse current relay..,

Or the start relay is welded closed.

Yes, it's a starter generator. After the start is complete, generator turned on by switch, and charging current checked. I have checked that prior to takeoff, and it was fine.

The first thing I asked the maintenance guy to do was remove the start relay to starter cable, and my friend tried the starter. The voltmeter on the relay output suggested it was working correctly (and I could hear it clicking with the starter button). When I left, they were beginning to look at the reverse current relay, I have not heard the outcome....

Five decades ago had the same in a Huey on start, don't recall what symptoms alerted us to the fact, probably the voltage, shut down immediately.

I have mistakenly left that stupid B212 starter switch on, picked it up when the Gen would not come on line.

Pilot DAR, I can't understand how the motor can be stuck as a starter, i.e. power coming in and driving it around, can still work as a generator? Are you sure the loadmeter showed a positive reading, not negative?

Let it be, let it be.
 

You seem not to have much faith in your maintainers?
That you post your symptoms here and then you are ask them to disconnect leads and suggest modifications would suggest a lack of confidence in your engineers.

I don't entirely understand either, the loadmeter showed it charging in the green during the pre takeoff check, neither of us gave it more than normal thought, as we did not realize yet that there had bee a malfunction.

"Starter engaged lights" are actually a requirement under some authorities for some aircraft, though I have not heard of them for turbine helicopters before. But now I see the reason. I worked in piston engine maintenance for many years and saw the damage done by a starter which kept turning after the start, pilot unaware. I know of several piston engines which required complete replacement because of the damage done to the whole engine (bits of starter drive scattered throughout the engine). Happily, the Allison design is not as vulnerable this way. My suggestion of a shunt an ammeter was at the request of my friend and the maintainers asking how this could be prevented in the future, fearing that this could have resulted in an engine compartment fire (and there's no fire detection back there). Approving modifications to aircraft is how I make my living, good things can still be made better sometimes...

Sounds very similar to a problem a friend of mine had with a Westland Wessex that wouldn't shut down. He was a liney, new to the squadron, and the crew more or less left him to sort it out. Fortunately he did.

Had that happen on a 206L-1 once …it was the reverse current relay. I flew that 206L-1 over 780 hours that year and that was the only snag I had.

With a reverse current relay failure, as long as the starter/generator is up to speed, the current flowing from the generator will energize the main power buss above battery voltage. When the engine is shutdown, and the generator goes below the rpm needed to generate current, current will flow from the battery thru the generator windings, which spins the starter/generator. In the 369, the generator is controlled by the switch function of the reverse current relay, not by controlling the field circuit.

Turning off the master switch will disconnect the battery from the reverse current relay.

Putting a shunt in the starter wire will not identify this type of failure, because current flow thru the generator circuit is causing the problem...

Thanks rotormatic!