The NTSB coupled old-fashioned “kicking tin” with highly technical investigative assistance from safety agencies in Germany and Switzerland to solve a perplexing mystery–what caused a Pilatus PC-12 to crash while attempting to land at the airport in Butte, Mont., killing everyone on board the big turboprop single.
The flight had departed Oroville (Calif.) Municipal Airport on March 22, 2009, en route to Gallatin Field in Bozeman, Mont., but diverted to Butte for unknown reasons. The pilot and all 13 passengers (six adults and seven children) were killed and the aircraft was destroyed by impact forces and a post-crash fire.
“This accident investigation was truly a case of tracking down clues to solve a mystery,” said NTSB chairman Deborah Hersman at the outset of a board meeting last month to establish a probable cause for the crash (see Accidents on page 64). Because there were no onboard data recorders, investigators initially lacked critical information about what happened on the leg from California to Montana.
The PC-12 originally departed Redlands, Calif., as a Part 91 flight and flew to Nut Tree Airport in Vacaville, Calif., where it picked up passengers. It then flew to Oroville, where more passengers were picked up. The single pilot then filed an IFR flight plan from Oroville to Bozeman, with Butte as the alternate.
The airplane departed at 1210 local time. At 1403, with the PC-12 cruising at FL 250, the pilot asked Salt Lake City Center to change his destination to Butte, but he gave no reason for the diversion.
At 1428, he reported Bert Mooney Airport (BTM) in sight, and ATC terminated radar service. At 1429, ATC called the aircraft in the blind with no response. The accident was reported to local authorities at 1433.
Initial reports from ground witnesses indicate that the airplane was flying approximately 300 feet above ground level in a north-northwesterly direction. Shortly thereafter, the nose pitched to a nose-low attitude and struck the ground.
One witness with aviation experience reported the PC-12 was west of the Runway 33 centerline and appeared too high to land on the runway. The witness then saw the airplane bank to the left and fly farther west when it rolled, pitched down and descended out of his view.
Although there is no tower at BTM, the local FBO lineman monitoring the radio as the airplane approached the airport heard the pilot transmit that he would be landing on Runway 33.
The Search for Data
For months after the crash, the NTSB was uncertain it would be able to learn enough information to connect all the dots. But then, after sifting through the wreckage using the painstaking, hands-on process that investigators call “kicking tin,” NTSB airworthiness group chairman Dennis Diaz eventually uncovered a non-volatile fuel memory chip from the PC-12’s annunciator panel.
At first, not even Pilatus Aircraft could mine useful data from the find. So the NTSB turned to the German Federal Bureau of Aircraft Accidents Investigation and the Aircraft Accident Investigation Bureau of Switzerland for “pivotal guidance” and support. “By facilitating data recovery from the aircraft monitoring system,” said Hersman, “they helped us connect many of the dots in this investigation. I think future investigators will be reading case studies on the chip-level recovery in this investigation.”
NTSB staffers determined that at the time of the crash, the left wing tank of the turboprop was almost completely full, while the right tank was almost completely empty, creating a significant fuel imbalance. Once investigators stumbled upon the chip from the caution warning system computer, the Board was able to ascertain that the pilot would have been getting a “low fuel pressure” indication for much of the flight from Oroville. If the pilot had done a complete preflight before departing from Oroville, he might have detected ice at the airframe fuel filter, said Diaz.
“We also learned that the pilot neglected to follow the airplane flight manual’s requirement to add a fuel system icing inhibitor–commonly known by its brand name Prist–to the fuel,” Hersman said. “The pilot didn’t put it in himself, nor did he direct anyone who fueled the airplane to do so. That decision set in motion a series of consequences that culminated in the deadly crash.”
At the time of the accident, the wind was from 320 degrees at 10 knots, visibility 10 statute miles, clouds were 4,400 few, 8,000 feet overcast, and the temperature was 7 degrees C.
The NTSB ruled that the probable cause of the crash was the pilot’s failure to ensure that a fuel system icing inhibitor (FSII) was added to the fuel before the flights on the day of the accident, his failure to take appropriate remedial actions after a low fuel-pressure state (resulting from icing within the fuel system) and a lateral fuel imbalance developed, including diverting to a suitable airport before the fuel imbalance became extreme, and a loss of control while the pilot was maneuvering the left-wing-heavy airplane near the approach end of the runway.
Although the Pilatus was 400 pounds over gross when it departed Vacaville and 600 pounds overweight when it took off from Oroville, the NTSB said that was not a factor in the accident. “Although the number of passengers on board the airplane during the final flight leg did not comply with the PC-12 airplane flight manual limitation requiring no more than nine passengers, the four additional passengers on board the airplane did not directly affect the outcome of the accident,” the NTSB found.
The Board faulted the pilot for not diverting to a nearby airport once the maximum allowable fuel imbalance had been exceeded. The pilot eventually diverted to Butte, likely because he recognized the magnitude of the situation and his attempts to resolve the increasing left-wing-heavy fuel imbalance had been unsuccessful. Meanwhile, he overflew three suitable airports on his way to attempt to land at Butte.
As a result of the crash, the NTSB made several recommendations to the FAA and the European Aviation Safety Agency. These include amending the certification requirements for aircraft requiring fuel additives, including FSII, so that those limitations are highlighted in the airplane flight manual and placarded at the fuel filler.