Bonanza 35
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 15.4; Flight Crew Total 300.1; Flight Crew Type 85.9
Confusion
Aircraft Equipment Problem Critical
Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Aircraft; Human Factors
Late afternoon local time on October 2014; a Bonanza C35 was on a practice ILS approach into ZZZ receiving vectors from ATC to fly heading 030 when; the pilot encountered an uncommanded high RPM. The pilot immediately reduced throttle. RPMs settled at approximately 1500 RPMs.  The pilot found the prop control knob approximately half way out from its normal cruise position. The pilot pushed the prop control knob to full or coarse pitch; but doing so did not change the RPMs or the pitch of the prop.  With no control over the prop; and having limited available power; the pilot decided to land as soon as possible. ATC noticed the Bonanza deviating from the assigned heading of 030. The next communication from the pilot was requesting immediate vectors to the airport; and stating: 'we have an engine problem.' The pilot decided to land at ZZZ1 airport since this airport was in view and closer to the original destination of ZZZ.  The pilot decided landing into the wind on runway 27 at ZZZ1 was not an option due to low altitude; a high descent rate and limited available power.  The pilot landed safely on runway 9 and taxied to an FBO on the field. Engine: E-225-8 Prop: Hartzell propeller with Hartzell A-1C governor with Hartzell T-Drive assembly: C-137-3B. While on the field; an Airframe/Powerplant (A/P) Technician removed the prop governor; fuel pump; and T-Drive assembly and sent these components out for inspection to [a propeller shop]. [The shop] found the T-Drive assembly spiral gears had failed.  The A/P Technician also removed the oil filter and cut and inspected the oil filter pleats and found metallic particles in varying sizes on each pleat. This is believed to be from the failed spiral gears inside the T-drive; which drives the prop governor and Thompson fuel pump. To prevent this type of event recurring; the FAA or Hartzell should immediately demand the inspection of all in service Hartzell T-Drive assemblies for worn spiral gears. Failure to act could result in a recurrence and the outcome could be fatal.
Reporter stated he purchased the Beech-35 a couple of years ago. He and an Airframe /Powerplant (A/P) Technician were in the aircraft when the Technician noted the engine RPMs appeared to be hunting just before the engine suddenly went to high RPMs and a low (fine) prop pitch. He later learned the Prop Control knob would not respond after the prop governor internal spiral gears had failed; because the propeller governor is spring loaded to a high RPM setting with a low prop pitch when governor control is lost. The fuel pump was still functioning but the prop pitch was so low they could not maintain altitude or forward airspeed.Reporter stated that from what he understands; the T-Drive spiral gears had failed due to the bearings had seized and spun the bearing race in the housing causing unrepairable scoring. The propeller Repair Shop noted they had not seen that type of disintegration and scoring before. The T-Drive had a placard dated 1997 but no other information could be found. The Continental engine was overhauled in 1997. A Hartzell Propeller Service Bulletin (S/B) issued approximately July 2002 required inspection of all C-137 Governor-T-Drive units manufactured or overhauled between 1996-2002 and removal of any  B-6508 bearings with 'INDIA' stamped on the bearing race. The bearings that had seized and scored the housing had those markings. The prop Governor was not repairable. The T-Drive bearings and main shaft were replaced.
A Private Pilot reports he encountered an uncommanded high RPM and loss of prop control in their Beech BE-35 aircraft. A propeller shop found the Hartzell Propeller T-Drive Assembly spiral gears that drive the prop governor and Thompson fuel pump had failed.