What happened
On 15 September 2021, a Grob G 120TP-A, registration G-ETPC, was conducting a training flight at RNAS Yeovilton when the crew experienced a sudden loss of forward visibility. While the flight was initially uneventful, the pilot in command noticed the windscreen clarity deteriorating, initially mistaking the phenomenon for ice formation. The situation rapidly progressed as engine oil began leaking from the propeller onto the windscreen.
Despite the loss of vision, all engine parameters remained within normal limits. The crew climbed to a safe altitude to maintain a gliding range in the event of engine failure and declared a MAYDAY. To facilitate a safe landing, the pilot executed a steep final approach using a full right-side slip. This maneuver allowed the pilot to maintain sight through the only remaining clear portion of the windscreen, located in the lower right corner. The pilot relied on the other crew member to monitor airspeed and altitude to maintain focus on the narrow field of vision. The aircraft touched down on Runway 206, where the pilot applied heavy braking to prevent a runway excursion.
The investigation
An examination of the MTV-5 propeller revealed that the piston seal, specifically a nitrile rubber O-ring, had suffered multiple cuts and widespread surface damage. This failure allowed engine oil to penetrate the hub assembly, subsequently degrading the hub bearing grease. The investigation established that the O-ring in use was part number C-047-135.
The propeller manufacturer determined that the damage was caused by the O-ring swelling. This swelling was triggered by high engine oil temperatures combined with contact with a non-compatible engine oil. This expansion caused the seal to stick and roll against the piston during operation rather than sliding smoothly, leading to the physical tears and cuts observed.
Findings
- The primary cause of the visibility loss was engine oil leaking from the propeller hub due to a damaged piston seal.
- The O-ring experienced swelling caused by the interaction between the engine oil and the specific rubber compound.
- The swelling resulted in the seal sticking and rolling during pitch changes, which caused the physical surface damage.
- Although the aircraft was using AeroShell 560, which was listed as a compatible oil in the manufacturer's service bulletin, the seal failure occurred due to the identified compatibility and temperature issues.