Shorts SD330 sherpa very PT6A-45R illusive problem, fix
The Shorts SD 330 / C-23A, PT6-45R, has a issue with clogged or obstructed P3 Air Filter inside the Inlet tube of the Fuel Control Unit.
During High Humidity Environments the P3 Filter can become totally obstructed dropping engine to Base Ng of 76% or around 900 rpms.
This can also happen in Icing conditions, Dusty environments and after compressor wash.
This condition is corrected with a new Proper size Environment specific P3 filter fix.
At Zweibrucken Germany, a C-23A Sherpa was taking off when both engines dropped to 900 rpms. There was just enough altitude for the Sherpa to turn back for a short landing.
Subsequent Short Brothers investigation found both engines equipped with 5 micron P3 filters in the FCU, both clogged by fine atmospheric dust.
A fix was the change to a 15 micron P3 filter.
No permanent fix was passed to the FAA as initial certification was done with the 5 micron filter.
Failure to properly replace the P3 filter can and has resulted in both engines dropping to Idle.
Needless to say; This condition will be fatal if both engines experience this problem.
If both engines are shut down and restarted, the engines will appear to run normally, then at the most inconvenient time, the engine will drop again to base Ng around 900 rpms.
Most mechanics have no knowledge that there is a filter in the FCU.
Most Mechanics will write off the condition, when it fails to replicate.
It also must be noted that the fine particulate dust was not visible to the naked eye, upon examination.
It is important to change both P3 filters with new filters and not just one side.
If a Prop is mis-rigged it will reproduce immediately.
If a Prop Governor has internal leaking it will seek and wonder continually.
Prop issues will reproduce immediately.