Incident
On 23 June 2007, at 13:00 UTC, a Piper PA-28-161 Cadet, registration I-APCE, departed from Brescia Montichiari Airport on a local visual flight rules (VFR) training flight as part of a private pilot license (PPL) program. The aircraft was equipped with dual controls; an instructor occupied the right seat and a student the left seat. During maneuvers over Lake Garda at 5,000 feet, the instructor noted difficulty in controlling the ailerons and elevator (horizontal stabilizer). After verifying the aircraft remained controllable, the instructor notified Garda Approach of the issue and decided to return to the departure airport. The aircraft contacted Brescia Montichiari Tower and landed on runway 32, with firefighting vehicles on standby. After landing, a misalignment of the control yokes was observed. A subsequent visual inspection beneath the instrument panel revealed a fracture of the T-bar control rod.
Aircraft and Flight Details
The Piper PA-28-161 I-APCE was a four-seat, single-engine, low-wing aircraft with fixed landing gear, manufactured in the United States. It was equipped with a Lycoming O-320-D3G 160 hp engine and a fixed-pitch propeller. The aircraft had a maximum takeoff mass of 1,055 kg and a cruising speed of 120 knots. It was first registered in 1989 (serial number 41045) and held a valid airworthiness certificate for VFR and IFR operations, with usage classifications for training and touring. Total airframe hours were 8,996 hours 45 minutes. The last 100-hour inspection was performed on 2 June 2007; 29 hours had accumulated since. The flight was a training sortie. The two occupants (instructor and student) were unharmed, and no damage was caused to third parties.
Investigation
The aircraft was immediately placed in a hangar for detailed assessment. Document analysis revealed that the failed T-bar control rod had been installed since manufacture and had accumulated 8,996 flight hours. Visual inspection indicated the fracture occurred at a weld zone of the component. Metallurgical and fractographic examinations were conducted at the laboratories of the Italian Air Force Flight Test Center, in coordination with the ANSV (Italian National Agency for Flight Safety). The tests revealed the fracture was intergranular, associated with the original weld area. The failure was attributed to hydrogen embrittlement, a phenomenon caused by atomic hydrogen entering the crystal lattice and recombining molecularly, reducing ductility and breaking load. This indicated an incorrect execution of the original welding process, typically linked to excessive process temperatures (which increase hydrogen solubility in the ferrous alloy) and too-rapid post-process cooling (which inhibits hydrogen diffusion). Further evidence of poor weld quality included numerous blowholes and subsurface cracks, consistent with rapid cooling. The material used, AISI 4130 high-strength steel, is known to be susceptible to such issues. The fracture initiated at the weld zone where defect concentration was highest, locally reducing mechanical properties. The plastic deformation pattern was consistent with a pitching maneuver, which could be expected during training. The failure occurred only because of the pre-existing defects; the fact that it happened after about 9,000 hours was considered coincidental.
Cause
The cause of the event was identified as the failure of the welded zone of the T-bar control rod, generated by a pre-existing hydrogen embrittlement problem associated with an incorrect execution of the original welding process.