What happened
On June 7, 2020, a Tecnam P912 ECHO, registration SP-SAWS, was performing a private flight from Elbląg to Katowice. While cruising at approximately 2,200 feet near Łódź, the pilot experienced a sudden jolt and a loud bang, followed by immediate engine failure.
Seeking an emergency landing site, the pilot identified a nearby meadow. During the approach, the pilot encountered a ditch crossing the landing area and applied back pressure to clear it. While the aircraft successfully cleared the ditch, it lost airspeed, entered a stall, and impacted the ground. The pilot and passenger sustained no injuries. The aircraft sustained damage to the nose gear, engine mount, and propeller hub cover. Following the impact, the pilot discovered that one propeller blade had detached from the hub; the missing blade was later recovered from a nearby property.
The investigation
The PKBWL investigation focused on the structural failure of the propeller. Physical examination of the aircraft revealed that the blade attachment failed at the last thread of the propeller nut. Metallurgical analysis conducted at the Silesian University of Technology confirmed that the failure was a fatigue fracture.
Investigators examined the propeller's history and found that the KASPAR propeller had been operated in an unbalanced state for 22 hours prior to being sent for balancing. Additionally, the aircraft had been upgraded with a more powerful Rotax 912S engine, which the user had been verbally authorized to use with the existing propeller despite warranty notes suggesting a three-blade propeller was required for that engine type.
Findings
- The primary cause of the accident was a fatative fatigue fracture of the propeller blade attachment.
- This fracture was caused by the simultaneous occurrence of two factors: an improperly balanced propeller and the presence of a geometric stress concentrator (the propeller threads) at the point of maximum bending moment.
- The propeller had been operated with significant vibrations due to an imbalance for 22 flight hours.
- The failure occurred at the outer edge of the propeller nut threads, where the bending moment is highest.