Accident Summary
On August 28, 2019, at approximately 2010 mountain daylight time, a Cessna 172 airplane, registration N53432, was substantially damaged during a landing accident near Denver, Colorado. The student pilot, who was the sole occupant, received minor injuries. The airplane was being operated under Title 14 Code of Federal Regulations Part 91 as an instructional flight.
Flight Sequence
The student pilot was on a solo cross-country flight. During the first leg, he noticed the low voltage warning light intermittently illuminating with a faint red glow. Although the voltmeter dropped from 28.5 volts to 28.4 or 28.3 volts, it remained around 28 volts for the remainder of the leg. The pilot did not perceive a problem requiring immediate action. After completing a touch-and-go landing at the destination airport, he began the return flight to the departure airport.
Electrical Issue
Shortly after departing, the voltmeter began to decline more rapidly. The pilot requested visual flight rules flight following from air traffic control and climbed to approximately 10,000 feet. He later reported that he was not confident he fully understood how an electrical system failure might affect the aircraft. As he approached the departure airport, the voltage continued to drop. He informed the tower controller of the electrical issue, requested a straight-in approach, and was cleared to land on runway 03. At that time, the voltmeter indicated about 12 volts.
Landing and Runway Excursion
When approximately 10 miles from the airport, the pilot selected 10° of flaps, but the airplane lost all electrical power and the flaps did not extend. The student pilot had not previously completed a landing without flaps. The airplane crossed the runway threshold at 100 knots and touched down near the 1,000-foot markings. It bounced multiple times. The pilot applied brakes once the airplane remained on the ground, but could not stop before it departed the end of the runway, went over a retaining wall, and struck a guardrail. The airplane sustained substantial damage to the propeller, fuselage, and landing gear.
Post-Accident Examination
The alternator was examined at the manufacturer’s facility under Federal Aviation Administration oversight. It had impact damage to the pulley, cooling fins, and front housing. After replacing the damaged front housing with an exemplar unit, the alternator passed the acceptance test procedure for a new or rebuilt alternator. Wiring shipped with the alternator, including the aircraft-side connector and harness for the alternator control unit, showed insulation cracking and areas of exposed wires. It could not be determined whether this damage resulted from chafing or impact. The aircraft-side alternator control unit connector pin sockets were loose, with no apparent sealing or strain relief. Two wires had crimp splices that remained intact when manipulated.