Overview
On May 28, 2021, about 1040 mountain daylight time, a Pipistrel Alpha Trainer, registration N119JP, was substantially damaged when it crashed near Fort Collins, Colorado. The pilot/airplane owner sustained minor injuries, and the flight instructor sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.
Flight Sequence
The flight instructor reported that the purpose of the flight was to give the pilot a flight review. After takeoff, they proceeded to a practice area where they conducted flight review maneuvers, emergency procedures, and the flight instructor demonstrated a power-on stall. The pilot then took over the controls and added power, but the engine did not respond. He manipulated the throttle several times, but the engine power remained at idle and would not change. The pilot informed the flight instructor that the throttle cable was loose. The flight instructor took control and initiated an emergency landing. The airplane descended to a nearby field and touched down. The nosewheel sank into a small ridge, causing the airplane to flip upside down and slide to a stop. The airplane sustained substantial damage to the fuselage structure and empennage.
Examination Findings
The engine had two carburetors, each with a throttle and choke cable. The cables ran to a throttle/choke assembly that contained a splitter. During a postaccident examination by the Federal Aviation Administration, the throttle cable from the cockpit was found fractured near the throttle/choke assembly. The cable did not exhibit abnormal bends or rubbing on other components; damage was confined to the fracture area. The throttle/choke assembly and cables were sent to the National Transportation Safety Board materials laboratory.
Examination of the splitter revealed that the single throttle input cable had separated within the terminal fitting end of the splitter assembly, disabling throttle control. The cable was coated with flux paste, inserted into a terminal fitting, clamped with a set screw, and soldered through a hole. Metallurgical examination showed a void where the solder had not filled the joint. The throttle cable strands indicated progressive cracking over time, and some thread edges were flattened, consistent with rubbing within the terminal due to the lack of solder. Additionally, the solder did not fill the output side of the splitter cavity. The choke cable attachments on the splitter were normal.
Maintenance Information
Revision A00 of the Pipistrel maintenance manual, dated March 22, 2019, requires replacement of throttle and control cables every 5 years. It also notes that these time limits must be considered during 100-hour/annual inspections, with removals or overhauls entered in the aircraft technical logbook. An engine logbook entry dated March 1, 2019, stated: "5 Year Maintenance LOA (#027-17) per Pipistrel’s and Rotax 912 UL instructions." However, it could not be determined if the throttle cable was replaced during this maintenance action.
As a result of this accident, Service Bulletin SB-100-00-80-012 was issued on July 26, 2021, calling for replacement of throttle and choke cables before or at the next scheduled maintenance if either cable is 5 years or older or has been replaced at any point in the aircraft's lifetime.