You would need a massive carbon dust buildup – of the order of 1-2cm?
Why can’t one have an earthing strap going to the engine body, near where the starter generator is mounted?
Apparently there is enough carbon in the brushes that carbon dust can establish a current carrying path to the drive shaft. I would think you don’t need a solid 1-2 cm bridge, just enough dust to allow arcing to happen.
It’s been established that the path of lowest resistance from the driveshaft to earth is via the #1 bearing (both in a carbon dust leakage event and a hard short where degraded armature contacts the housing).
I would assume the earthing strap wasn’t a good fix (both APC and TRW S/Gs are earthed to the engine case btw) since the solution (at least on Pilatus TRW S/G) was to isolate the shaft from the armature.
Did they tell you anything which could have prevented this with additional maintenance?
BerlinFlyer wrote:
Did they tell you anything which could have prevented this with additional maintenance?
I had a look in the manual:
Event 1/2
Remove bell housing and inspect brushes for wear
500 hours
Remove, clean and inspect starter generator per Starter Generator Cleaning and inspection 24-30-20
1000 hours
Overhaul or replace starter generator
I had a look at 24-30-20 and you are supposed to remove the thing from the aircraft and blow it out
with air. Then measure the DC resistance between output terminal B and the machine frame which
is supposed to be at least 10kOhm.
Did they tell you anything which could have prevented this with additional maintenance?
My Piper Germany Approved Maintenance Schedule calls for Starter Generator Cleaning every 300 hours. I notified them about the problem and but had very little reaction from them.
Sebastian’s email is very interesting. I didn’t know inspection 24-30-20 actually called for the SG to be removed and a leakage check carried out. Maybe I will ask Piper Germany what the leakage reading was last time they did the inspection….
PhilG wrote:
Maybe I will ask Piper Germany what the leakage reading was last time they did the inspection….
And what if they haven’t done it at all? Do you have the access to the paperwork?
This generator “carbon short circuit” is weird. The resistance measurement will show nothing until thereis a pretty good connection – because the carbon particles need to actually join up. I reckon people must be implementing a procedure involving a regular inspection and/or blowing it out with compressed air.
Seeing 10k ohms is too late.
Nowadays you would use a 3 phase alternator, with an inverter driving it to form a starter motor 
In a system without any contaminating dust or other leakage, a simple resistance measurement should yield resistances much greater than 10k.
That is why I think a threshold of 10k sounds about right. In a 24v system, it represents a parallel leakage of around 2-3mA through the AGB assuming the gears/bearings/shafts/oil films to aircraft system ground is low in comparison.
There have been some suggestions that EDD can occur with currents as low as 50-100mA so a x50 safety factor in the shunt leakage seems reasonable.
More of a question for me if the resistance check was actually done or if a quick waft of 100psi air was considered enough.
Emir wrote:
Stuff like this always have scared me of the turbines. However, the rest always attracts me.
Running a PT6 is estimated at USD175 per hour. Fot that, one can buy a JSSI program that covers scheduled and unscheduled maintenance. I understand that this engine did 1400 hours, which is 245k USD when multiplied and 180k converted to GBP. Funny coincidence.
Correction. See here that for the Piper, JSSI is estimated at 136 USD per hour. So a program would save money in this case.
I think programmes are the same as any insurance. For the significant majority of owners, you would be better off financially taking the risk as issues are relatively rare and don’t affect the vast majority of operators.
But for those unfortunates who are at the other end of the statistical distribution the financial consequences of an issue are huge.
If (it’s a big if) I ever buy a turbine aircraft again, I will take a programme any day rather than hope I will be one of the lucky majority.
