an "impossible" snag on a BE200

[Tim]

Wondering if anyone can shed light on the following snag.

During a batt start on the #2 engine I had the generator come online with the starter engaged. At some point prior to me starting the engine the #2 gen switch was placed into the on position (I'm assuming I hit it getting into the cockpit or by some other means). During start with the gen in the on position the gen came online as N1 spooled up with the starter still engaged and without moving the switch to reset. Through subsequent troubleshooting I also discovered that I could use the starter to activate the gen by leaving the gen switch in the on position. With the gen offline but the switch in the on position, engaging the starter would immediately turn the gen on. All indications were the gen was running normal - light out & 28V. I aware that this is supposed to be impossible, I'm also aware that I should not have found the snag, since it was due to an improper start. None the less, the snag was discovered. Maint keeps telling me what I'm saying is impossible. The fault could not be replicated on the #1 engine and the starter/gen switches and indications were normal for that side.

I've heard of this "impossible" situation happening before, but only anecdotally and don't know what the fix was.

Can anyone shed any light on this or share a similar story? Just trying to make sense of it...

[CID]

Isn't a fault simply abnormal behavior of a system? In other words, a system that behaves in a manner that is "impossible" if the system is in good working order?

Anyway..... if I understand the snag correctly, I would tell them to check the GEN switching units to see if one is welded and if that doesn't fix it have them replace the GCU on one side at at time.

[Tim]

Sorry, forgot to add GCUs were swapped out with no effect.

[GA MX Trainer Dude]

Tim


There is a "Field Disconnect Relay" in the system that is normally closed and powered to the open position when the start switch is selected.

If that fails to open then you can have the conditions you have described.

It will also often cause - especially if doing 2nd engine battery starts - a slower than normal start and possible higher temp as well.

Hope this helps - and I did cover that issue when I did the B200 training course.

Mx

[frozen solid]

I'm sorry if this is a silly question, but I'm not familiar with BE200s particularly. Does the GCU incorporate a reverse current relay or is that a separate box? It really sounds as if the RCR is fused. I'm assuming the starter-generator incorporates a circuit that opens the start relay when the engine accelerates through a certain N1 speed. If the RCR is fused, the generator would kick on as soon as the start relay opens. I had a problem like this once when I shut down an engine without switching the generator "off", as soon as the generator voltage dropped below 24 volts it would begin functioning as a starter again until I switched off the generator. If I had shut down the engine correctly I would never have discovered this. Your problem sounds similar, but with a different relay fused in the RCR unit.

Electrical stuff has always been my weakest subject. If what I just said was really stupid, don't be too hard on me. I also don't work on Be200s.

EDIT:
Oh- I see in the post above this one, an actual engineer said more or less the same thing, more intelligently. Never mind.

[Tim]

Super helpful posts guys, thanks!

2nd eng batt starts are rare for me, and I have not done one on the a/c in question. Gen assist starts lead to a normal start on #1 (less than 700 deg ITT). I have noticed that the #2 ITT on start creeps up higher than your average batt start on other BE20's, but not to the point of concern (~800 deg).

Next question, what are the potential associated failures if the field disconnect relay is indeed the culprit? Are we talking gen or engine malfunctions in flight? or just land somewhere to find your starter doesn't work any more?

[GA MX Trainer Dude]

Tim

For aircraft with S/N above BB 88

Think of it this way:

When the start switch is selected "on" the field disconnect relay is supposed to "open" and remove the ground to the generator field circuit. This in effect will not allow the generator to begin producing electrical power during the start. If the disconnect relay doesn't work it won't hurt anything but because the starter/generator is now trying to produce electrical power as well as start the engine less motive force is available for spooling up the engine resulting in slower spool up times and the possibility of warm starts or possible hung starts if the battery is really bad. It will also cause your starter generator temperature to increase with the additional time required for the start process.

A quick check is to make sure the field disconnect relay is working - first thing I would do is replace / swap it with the other side. I would think that it would be the relay because if it was anything other than a short to ground before the field wire got to the relay the system would be open and the generator would not operate. Also most - but not all shorts to ground often result in an intermittent operation as a result. If it is the control wire from the switch to the relay then you will have the same symptoms after you change out the relay - of course you could check that with a meter pretty easy as well. My money is on the relay but I have seen broken wires from switches to relays as well. Of course the last possibility is the grounding of the relay - it is possible that the relay is good - the circuit from the switch is providing power - but the relay is not grounded and thus is not working.

In the same thought process - have you ever forgot to move the starter switch to the off position after a start?? With a correctly operating system the clue you have done that is the generator won't come on line. Again because a correctly operating system with the start switch on will power the field disconnect relay open.

Once the engine is started and the start switch is off the non working relay is in the correct position for continued operation of the generator.

BTW the ~ 800 Deg start temp is what I would expect if you are doing gen assisted or cross gen starts on the later models or the modified ones with this issue.

It's been a while since I taught the course and when I get home I will check my notes as to where the field disconnect relays are - I think they are on the generator control panel IIRC.

I asked when on course so many years ago why Beech has the locking type start switches rather than the common spring loaded momentary contact type and the answer I got was - "so that the generator does not come on line with an inadvertent start switch to off - which at a critical point in a battery start sequence could cause the engine to overtemp".


Hope this explains the system a bit better for you.

Mx

[Tim]

Wow, thanks MX. Very informative and much appreciated!

[GA MX Trainer Dude]

Tim

You are welcome!!! Nice to see that some on here are grateful for a bit of info.


I checked my notes and the field disconnect relay is on the generator control panel as I thought.


Whatever it is that fixes the problem - let us all know.


Regards,

Mx