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On May 11, 2010, about 1335 eastern daylight time, a Bell 407 helicopter, N31VA, operated by the Virginia State Police, was substantially damaged during an emergency landing following an engine failure and autorotation near Virginia Highlands Airport (VJI), Abingdon, Virginia. The certificated flight instructor (CFI) and the commercial pilot were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the public use instructional flight.
According to the crew, who were both Virginia State Police (VSP) officers, they and the helicopter were based at VJI. The purpose of the flight was to provide aircraft orientation training for the pilot. The pilot conducted one takeoff and landing at VJI, and then proceeded to the northwest to practice confined area operations at a field about 3 miles from VJI. When the flight was enroute to the practice field, the crew heard a noise that they described as a "very low growl" coming from the engine compartment. As they approached the field at an altitude of about 250 feet above ground level (agl) and a speed of 80 knots, they heard a "very loud growl," and the engine "surged" twice. The "FADEC DEGRADE" caution light illuminated, and an aural "ENGINE OUT" alert sounded. The CFI informed the pilot that he was taking control of the helicopter, and then initiated a 180 degree right turn, and an autorotation to the field. At about 50 feet agl, the CFI flared the helicopter, and then landed it. The helicopter bounced one time and came to rest on a "slight slope" in the field, with the right skid on the uphill side. The crew shut down the helicopter and exited normally. The pilot reported that the flight duration was seven minutes.
Two days after the accident, the helicopter was recovered to a Bell Helicopter completion and maintenance facility. The engine, including the electronic control unit (ECU) and fuel control, was removed and shipped to the Rolls-Royce facility in Indianapolis, Indiana for detailed examination and testing. Since the helicopter was a public use aircraft, a week after the accident, the VSP formally requested that the Federal Aviation Administration (FAA) "conduct a formal investigation" into the accident.
Pilot and FAA records indicated that the CFI held an airline transport pilot certificate, with several ratings, including rotorcraft-helicopter, and a flight instructor certificate with rotorcraft-helicopter and instrument helicopter ratings. The CFI's most recent FAA second-class medical certificate was issued in July 2009, and his most recent flight review was completed in August 2008. He reported that he had 3,278 total hours of flight experience, which included 2,316 hours in helicopters, of which 1,100 hours were in the accident helicopter make and model.
Pilot and FAA records indicated that the pilot held a commercial certificate, with several ratings, including rotorcraft-helicopter, and instrument helicopter. The pilot's most recent FAA second-class medical certificate was issued in April 2010. He reported that he had accumulated approximately 4,328 total hours of flight experience, which included 4,007 hours in helicopters, of which 147 hours were in the accident helicopter make and model.
According to FAA records, the helicopter was manufactured in 2000, and was first registered to the Commonwealth of Virginia in January 2001. Examination of the maintenance records revealed that the records system utilized three separate hour-tracking categories, as well as an engine "cycles" value. The three hour-tracking categories were "Hobbs," "Aircraft TT (total time)," and "Engine TT." Examination of the records from October 2009 to the date of the accident indicated a constant difference between the aircraft and engine TT values; the aircraft TT value was 198.0 hours more than the engine TT value. In contrast, the Hobbs value did not maintain a constant difference from those values, but was about 43 hours more than the aircraft value.
The helicopter was equipped with a Rolls-Royce 250-C47B engine. According to the maintenance records, the most recent annual inspection was completed in October 2009. The most recent 50 hour/3 month airframe inspection was completed on April 20, 2010, and the most recent 150 hour engine inspection was completed the following day. As of those two latter inspections, the aircraft TT was about 3,887 hours, the engine TT was about 3,689 hours, and the engine had accumulated 6,631 cycles. The helicopter and engine accumulated about 14.5 hours between those inspections and the accident.
The VJI 1343 recorded weather observation included wind from 200 degrees at 10 knots, with gusts to 18 knots, visibility 10 miles, scattered clouds at 3,700 feet agl, broken cloud layers at 4,200 and 5,000 feet agl, temperature 21 degrees C, dew point 12 degrees C, and an altimeter setting of 30.20 inches of mercury.
WRECKAGE AND IMPACT INFORMATION
Representatives from the FAA and Rolls-Royce arrived at the accident scene the day after the accident. They reported that both landing skids were splayed in the outboard direction, and that the right skid exhibited more deformation than the left skid. The upper 3 inches of the left vertical stabilizer was missing, and one main rotor blade had paint transfer marks consistent with stabilizer contact. The tail skid and tail rotor blades were intact. The forward-looking infrared (FLIR) turret that was mounted on the underside of the fuselage below the left rear seat was pushed up, and penetrated the cabin floor. The "Night Sun" lamp that was mounted on the underside of the fuselage, below the left front seat, was damaged, but did not penetrate the cabin. The remainder of the airframe, main rotor and tail rotor were otherwise intact. Movement of cockpit controls confirmed continuity to all control surfaces.
The helicopter had approximately 790 pounds of fuel on board at the time of the event. All fuel, lubrication, and pneumatic lines were checked for damage, continuity and security; all were intact. The engine was found securely in position, with all attaching hardware in place and secure. A visual inspection of the engine exterior did not reveal any damage.
Checks were then made of the N1 and N2 drive trains. Motoring of the engine to approximately 10 percent rpm resulted in smooth and continuous rotation from the starter generator to the compressor. Rotation of the main rotor head showed resultant smooth and continuous rotation to the No.4 power turbine wheel. No attempt to start the engine was made.
Engine Data Recorder
According to the Rolls-Royce representative, the ECU was equipped two separate non-volatile memory (NVM) units, known as the "maintenance terminal" (MT), and the "incident recorder" (IR). The MT recorded discrete events relevant for maintenance purposes, and the IR recorded time history data of engine parameters. The IR recording was designed to start whenever a "trigger" (parameter exceedance) was detected; the recording would capture data from 12 seconds prior to the trigger, and continue after the trigger.
Visual examination of the ECU found it to be securely in position, with its data connectors in place. The ECU NVM data were downloaded. Examination of the data revealed that two "engine surge" events were captured in the "Last Engine Run Fault" section of the recording. Also, the "Accumulated Faults" data revealed only one temperature exceedance. That value was a gas temperature exceedance of 1.25 seconds duration, with a maximum temperature of 1,712.9 degrees F. The exceedance did not have a time-of-occurrence associated with it, but it was the opinion of the Rolls-Royce representative that it most likely occurred during the engine anomaly/fault event.
The Rolls-Royce Maintenance Manual (MM, Sec 72-00-00 p 20) required the following:
• During engine starts, gas temperatures between 1,700 and 1,830 degrees require an inspection of the turbine, and entries in the engine maintenance records (including temperature and duration)
• During power transients, any gas temperatures above 1,661 degrees requires that the turbine be removed for "heavy [maintenance] or overhaul"
No previous engine overspeed or overtemp exceedance events were noted in the engine maintenance records.
Detailed Engine Examination
On June 10, 2010 an engine investigation was conducted at the Rolls-Royce facility. In attendance was an FAA inspector, and representatives of Rolls Royce, VSP, and Bell Helicopter.
Visual examination of the compressor module exterior revealed no damage. When rotated manually, the compressor exhibited smooth operation, both before and after separation from the accessory gear box. Disassembly and inspection of the compressor front support, compressor rear support, impeller, and compressor shroud revealed no damage.
Prior to separation of the engine modules, manual rotation of both the N1 and N2 drive trains at the tachometer generator pads revealed smooth and continuous rotation of the N1 and N2 gear trains through the accessory gear box. Visual examination of the gearbox interior revealed that it contained clean oil, and no damage was noted.
The outer combustion case and both air discharge tubes were properly positioned, and no external damage was noted. Removal of the outer combustion case revealed a metal strip, approximately 2 inches long, lodged between the basket and the inner wall of the outer combustion case. A second, similar metal strip was observed bent around, and lodged in, one of the dilution holes of the combustion liner. Visual examination of the interior of the combustion liner revealed no unusual streaking, or other evidence of thermal damage. The support plate (the "-6" component in the manufacturer's Illustrated Parts Catalog (IPC), commonly referred to as the "deflector plate"), which was normally located at the aft end of the combustion liner, was absent.
Examination of the turbine module revealed heavy metallic spatter across the aft face of the No.1 nozzle shield, and a smearing of a yellow substance around the outer rim of that nozzle shield. A foreign strip of metal was found bent around a first stage nozzle vane saddle. The No.1 nozzle exhibited discoloration of the vane surfaces consistent with excessive thermal exposure. Several turbine vanes and the No.1 turbine wheel of the gas producer section exhibited foreign object impact damage. All the turbine blades were damaged, and four blades were missing the majority of their airfoil length.
The No.2 turbine wheel exhibited metal spatter across the blade surfaces with foreign object impact damage to many blade leading edges. The No.2 turbine nozzle exhibited nicks across the leading edges of several vanes. The trailing edges of approximately 20 percent of the vanes exhibited thermal damage consistent with over-temperature. Metal spatter was noted across the vane surfaces. The Nos.3 and 4 turbine wheels all exhibited nicking across the blade leading edges, and light spatter across the blade surfaces.
The lower chip detector was clear of any metallic particles; the upper chip detector exhibited light metallic particles or slivers. All other engine components appeared normal and undamaged.
Combustion Liner History
The combustion section consisted of an outer combustion case and an inner combustion liner. The liner was supported at the forward end by the gas producer nozzle vane assembly, and at the aft end by the fuel nozzle, which was mounted in the aft end of the outer combustion case.
Review of the helicopter and engine maintenance records indicated that the accident combustion liner (part number 23064570, serial number PHI-0020) was originally manufactured by Rolls-Royce. In 2008, the liner was sent to Cadorath Aerospace Lafayette LLC (CAL LLC) for inspection, with the possibility for overhaul if required. At that time, CAL LLC was a Rolls-Royce designated "authorized repair facility."
According to the repair facility's work order "traveler" document, the liner was received and visually inspected. Subsequent detailed inspection revealed that the liner did not conform to the inspection criteria, and was therefore rejected, which denoted that it was no longer an airworthy component. The document indicated that cracks were present in the liner and the deflector plate, and that attempts to weld-repair those cracks were unsuccessful. The document also indicated that the liner was partially disassembled, a new deflector plate and associated spacers were installed, and the liner was reassembled. The liner disassembly and re-assembly process included cutting, machining, brazing and welding. In October 2009, when the engine had a TT of 3,544.9 hours, its combustion liner was removed, and the overhauled combustion liner was installed.
Combustion Liner Repair Details
As noted above, the combustion liner deflector plate, which normally surrounded the fuel nozzle boss, was found absent from its normal position. Failure analysis of the remaining liner revealed that the required circumferential fillet weld between the liner and the deflector plate was not performed during the repair at overhaul; only the plug (positioning) welds were present to affix the deflector plate to the liner. The combustion liner was approved for return to service on October 24, 2008. The TT on the combustion liner could not be determined, but at the time of its failure, it had accumulated 158.4 hours since overhaul and installation in the accident engine. The manufacturer's MM-specified "recommended time between overhaul" (TBO) for the combustion liner was "On Condition," which the MM explained as the component "May remain in service provided operation and condition are satisfactory."
The engine manufacturer's maintenance documentation, included the Overhaul Manual (OHM), the Overhaul Procedures (OHP) manual, the Parts Repair Procedures Letters (PRPL) and the Illustrated Parts Catalog (IPC). The OHM provided top-level repair information, and specific guidance was contained in the OHP and PRPL. Examination of those documents revealed that while the engine manufacturer permitted the deflector plate to be replaced, there was no specific guidance for that procedure in the OHP or PRPL, and therefore the repair facility was not authorized to conduct that procedure. In addition, the repair facility did not possess the applicable guidance for replacement of the deflector plate. The investigation was unable to determine the specifics of how or why the repair facility replaced the deflector plate, and then inspected and approved that replacement.
Although the combustion liner was approved for return to service by the repair facility about 19 months prior to its failure, the investigation did not locate any information that indicated that either the repair facility or the FAA principal maintenance inspector (PMI) for the repair facility was aware that the repair facility had accomplished a procedure that it was not authorized to conduct. In addition, there was no evidence to indicate that the FAA or the repair facility attempted to remove the subject combustion liner from service prior to its failure.
On December 21, 2010, after the repair facility was advised of the deflector plate failure mode due to the improper repair, the repair facility identified other assemblies which had a known or suspected improper repair, and recalled those assemblies from their customers. That action was accomplished by means of a repair-facility-issued "Urgent Stop Use and Product Recall Notice," which listed a total of 19 units.
On March 24, 2011, the engine manufacture sent a "letter of finding" to the repair facility. The letter formally advised the facility that the deflector plate replacement was not an authorized procedure per OHP 72-40-14-01, and provided details regarding the deficiencies of the repair facility's procedures as executed. The letter instructed the facility not to conduct any such repairs in the future, to identify and recall any previously-affected combustion liners, and to notify the manufacturer once all suspected liners were successfully recalled. The FAA PMI was notified of those findings and actions. On April 18, 2011, the repair facility informed the engine manufacturer in writing that all 19 suspected combustion liners had been successfully recalled, and that no additional in-service failures had occurred.
In September 2011, the engine manufacturer modified section 72-40-00 of its OHM to more clearly state that replacement of the deflector plate by a repair station could only be accomplished by replacement of the next-higher assembly, the pre-ignition sub-assembly.