On May 20, 2015, about 1440 central daylight time, an Ayres Corporation S2R-G6 agricultural airplane, N3300U, made a forced landing into a field following a loss of engine power near Tivoli, Texas. The commercial pilot, who was the sole occupant, sustained minor injuries and the airplane was substantially damage. The airplane was registered to AT Rental Properties and operated by AT Ag Services, LLC, Tivoli, Texas, under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. The local flight originated from a private strip near Tivoli, Texas.
The pilot reported that the accident flight was the twelfth aerial application flight of the day. He was carrying a load of fertilizer to the destination field when the engine made an uncommanded power acceleration. He noticed the engine temperature increase so he retarded the power lever, but the engine did not respond. The pilot jettisoned the load of fertilizer and shut off the fuel supply to the engine. He then made a forced landing into a field; during the landing the airplane nosed over and came to rest inverted.
A postaccident examination of the airplane was conducted by the responding Federal Aviation Administration (FAA) inspector and a representative from Honeywell Aerospace. The examination revealed no anomalies with the engine rigging, but the engine's fuel control unit stub shaft spline appeared to be worn. No other anomalies were noted.
On June 9, 2015, the fuel control unit was examined by Woodward, Inc., Loves Park, Illinois, with representatives from the NTSB, FAA, and Honeywell Aerospace. The examination revealed that the stub shaft spline was worn beyond the allowable limits. The internal spline was also checked using a specific go/no-go gauge, which should not fit between the teeth of the spline; the gauge rotated freely. This wear can allow the fuel control underspeed governor to be driven at a speed which is lower than the engine speed and subsequently increase fuel flow and engine speed.
On August 24, 2006, the FAA issued Airworthiness Directive (AD) 2006-15-08, for Honeywell TPE331 series turboprop engines. The AD stated in part, "The AD results from reports of loss of the fuel control drive, leading to engine overspeed, overtorque, overtemperature, uncontained rotor failure, and asymmetric thrust in multi-engine airplanes. We are issuing this AD to prevent destructive overspeed that could result in uncontained rotor failure, and damage to the airplane."
The AD specifies that Honeywell Service Bulletin (SB) TPE331-A73-0262 be used to accomplish the maintenance work. An excerpt from the SB:
Initial Inspection of Engines with Affected FCU Assemblies
(f) At the next scheduled inspection of the fuel control drive, but within 1,000 hours-in-service after the effective date of this AD:
(1) Perform an initial dimensional inspection of the fuel control drive for wear or damage.
Information on spline inspection can be found in Section 72-00-00 of the applicable maintenance manuals.
(2) Repair or replace the fuel pump, if the spline fails the dimensional inspection, with any serviceable fuel pump.
(3) Repair or replace the FCU assembly, if the splines fail the dimensional inspection, with a serviceable modified FCU assembly.
Repetitive Inspections of Engines with Affected FCU Assemblies
(g) Thereafter, within 1,000 hours since-last-inspection:
(1) Perform repetitive dimensional inspections of the fuel control drive, for wear or damage. Information on spline inspection can be found in Section 72-00-00 of the applicable maintenance manuals.
(2) Repair or replace the fuel pump, if the spline fails the dimensional inspection, with any serviceable fuel pump.
(3) Repair or replace the FCU assembly if the splines fail the dimensional inspection, with a serviceable modified FCU assembly.
A review of the aircraft maintenance records revealed that AD 2006-15-08 was accomplished on March 16, 2015. The records noted that the AD inspection was to be completed again in 1,000 hours.