A new National Traffic Safety Board report points to unsafe electrical wiring near hydraulic lines as the cause of a Cal-Ore Life Flight crash that killed four people after it left Crescent City July 29, 2016.

While it doesn’t specifically state the cause of the crash, the NTSB Aviation Accident Factual report did prompt the Federal Aviation Administration to issue a warning to operators of the same model aircraft that electrical wires need to be regularly inspected and addressed.

The plane was piloted by Larry Mills, 54, and carried flight nurse Deborah Kroon, 49, flight paramedic Michelle Tarwater, 30, and patient April Rodriguez, 35. All died in the crash in a remote section of Humboldt County.

The NTSB report, released March 7, notes that on July 29, 2016, the Piper PA-31T Cheyenne II airplane crashed after the pilot reported the cockpit was filling with smoke.

“At (11:58 p.m.) the pilot contacted the Seattle Air Route Traffic Control Center controller working the flight and stated that he was going to turn back to Crescent City and that he was smelling smoke in the cockpit,” the report states. “Radar data indicated that the airplane’s altitude about that time was 14,900 ft, and the airplane was on a southeast heading.”

The report states the pilot later told the controller it seemed he was about to lose power, was going to turn back to Crescent City and asked that the fire department be notified of the arrival.

“At (11:59 p.m.) the pilot stated that he had three souls on board,” the report stated. “There were no further communications from the pilot despite multiple attempts by ATC controller.”

The investigation

The site of the crash was a remote area of brush and heavily wooded terrain containing a 2,400 foot trail of debris.

“No sign of postcrash fire was evident. There was an even, lightly adhered coating of light soot over the entire fuselage’s exterior skin with several small areas of heavier soot deposits,” the report states. “In addition, evidence of thermal damage was present within the forward section of the fuselage between the pilot and copilot seats.”

Parts of the plane were sent to a NTSB lab in Washington, D.C. for closer inspection.

“A localized area of thermal damage to a section of wiring was located in a large wire bundle that ran through the center tunnel in the floor of the cockpit,” the report states. “All the insulation was either melted, thermally discolored, or missing. One large gauge wire end was found to have beading, welding and melting on one end. All other wire bundles located in this area were intact and sustained minimal thermal damage.”

From there, the report focused extensively on the wiring, even showing detailed photos of melted and bare wires. Hydraulic lines in the area of the burned wires were examined and all were said to have evidence of heat exposure.

“The four hydraulic lines servicing the landing gear system were located in this area,” the report states. “All the lines exhibited signs of thermal exposure with melting and missing sections of material.”

According to the report, the heat from wiring likely caused the hydraulic lines to fail.

“The fractured ends on all the lines were examined and found to exhibit patterns consistent with overstress fracture; some fractures exhibited features consistent with elevated temperature exposure as a contributing factor,” the report stated.

Investigators also looked at six other examples of the Piper PA-31T Cheyenne II that were in near-perfect condition and found wiring and hydraulic lines had come in contact with each other.

“On all six exemplar airplanes, electrical lines and hydraulic lines were found in direct contact,” the report states. “Some of the wires in the exemplar airplanes showed chafing between hydraulic lines and the electrical wires.”

In December 2016, the FAA issued a bulletin concerning the wiring on that and similar aircraft.

“The (bulletin) provided information on wiring conditions in the area below the floor mounted circuit breaker panels that could lead to chafing, thermal stress, or arcing,” the report states. “The (bulletin) recommended best practices for securing high electrical current wires in the aircraft.”

Based on the bulletin, the NTSB issued an urgent safety recommendation “concerning unsafe wiring conditions” and requested the FAA issue directives to owners and operators of similar models

Plane and pilot

The plane, made in 1981, had been inspected just a couple weeks before the fatal crash.

“The operator maintained the airplane in accordance with Piper Progressive Inspections on a 100-hour cycle per Piper document 761-664, dated Oct. 27, 1994,” the report reads. “The most recent inspection was completed on July 14, 2016, when the airplane had accumulated 7,286.6 hours total time in service.”

Along with noting the pilot’s extensive training, experience and certification, the report ruled out other impairments.

“The FAA’s Bioaeronautical Sciences Research Laboratory, Oklahoma City, Oklahoma, performed toxicology tests on specimens from the pilot, which were negative for carbon monoxide, alcohol and drugs,” the report states.