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HISTORY OF FLIGHT
On January 16, 2007, approximately 1155 mountain standard time, a Piper PA-46-500TP, single-engine turbo propeller airplane, N747BL, experienced a nose landing gear collapse and loss of directional control during landing roll at the Parowan Airport (1L9), Parowan, Utah. The airline transport pilot, who was the sole occupant, was not injured. The airplane was registered to a private individual and operated by the pilot. Visual meteorological conditions prevailed for the personal cross-country flight, which was conducted in accordance with 14 CFR Part 91, and a flight plan was not filed. The flight departed the McCarran International Airport (LAS), Las Vegas, Nevada, about 1055.
In a telephone interview, and according to the Pilot/Operator Aircraft Accident/Incident Report (NTSB Form 6120.1), the pilot reported that during the approach to landing the red gear warning light remained on for about 4 to 6 minutes, but then went off. The pilot stated that at the end of the landing rollout and at a minimal taxi speed the aircraft [veered left], and impacted a snow bank off the side of the runway before coming to rest in an upright position. The pilot also reported that when the nose gear was lowered onto the runway, "...it started to shimmy a little bit before pulling to the left."
The pilot reported that 2 to 3 landings prior to the incident landing he noticed the airplane constantly pulling to the left on taxi and takeoff, and also during landing. The pilot stated that while taxiing he had to exert unusually heavy pressure on the right rudder to keep the nose straight, and at times he had to use braking to taxi straight. The pilot further reported that on a number of other occasions the landing gear warning light came on in flight and remained on for brief periods, and on one flight after lowering the landing gear the nose wheel indicator light was red; a normal landing followed without incident.
The pilot held an airline transport certificate with single and multiengine land ratings, and a commercial pilot certificate for single-engine seaplanes. The pilot reported a total time in all aircraft of 6,171 hours, with 181 hours in make and model. The pilot further reported having flown 60 hours in the last 90 days and 20 hours in the preceding 30 days. The pilot also reported having completed his most recent biennial flight review on November 3, 2006, which was conducted in a P-46-500TP Meridian turbo propeller airplane.
The pilot held a third class medical certificate dated April 24, 2006. Under limitations, the pilot's medical certificate stated the pilot must wear corrective lenses.
N747BL, a Piper PA-46-500 turbo propeller single-engine airplane, serial number 4697226, was manufactured in 2006, and issued a Federal Aviation Administration (FAA) normal airworthiness certificate. At the time of the accident the airplane had accumulated a total of 181 hours. The airplane was equipped with the original engine mount, part number 102460-002, serial number 815909.
According to maintenance records the airplane underwent its most recent annual inspection on December 1, 2006, at a total airframe time of 162.2 hours. On November 17, 2006, Piper Service Bulletin (SB) 1154A, Engine Mount Inspection, was complied with: "...no cracks or defects noted." SB 1154A, dated July 26, 2006, stated, "It has been determined that cracks may develop on the engine mount in the area of the nose gear actuator feet. This condition typically occurs when the nose landing gear is subjected to excessive loads, possibly through hard landings, rough field operations, excessive speed turns and/or improper towing of the aircraft." PART I of SB 1154A requires the repetitive inspection of the engine mount. PART II requires replacement of the engine mount if cracks are found during the inspection of PART I. Compliance time was to coincide with the next regularly scheduled maintenance event but not to exceed 100 hours.
The nose landing gear assembly (trunnion) attaches to the two forward lower attachment points on the engine mount. The nose gear downlock actuator attaches to the rear of the engine mount. The upper actuator fitting is secured in the rear engine mount attach feet with an AN 7-17 bolt, two washers and a locking nut.
WRECKAGE AND IMPACT INFORMATION
The initial onsite examination of the aircraft, conducted by an FAA airworthiness inspector and a representative from Piper Aircraft, revealed that the airplane had sustained minor damage. Examination of the nose gear found that the rear actuator bolt was missing and the actuator was pushed against the aluminum rear wall. A subsequent search of the excursion area along the runway with a metal detector located the head end and the nut end of the actuator bolt. A subsequent examination by the NTSB Materials Laboratory in Washington, D.C., revealed that the bolt fracture signature was consistent with shear overstress. An examination of the engine mount with a flashlight from under the airplane revealed no damage or cracks.
TEST AND RESEARCH
On March 21, 2007, a Senior NTSB investigator, a NTSB Structural Engineer, a FAA Structural Engineer, and Piper engineers examined the engine mount at the Piper Aircraft, Inc., facilities in Vero Beach, Florida. In addition to the incident mount, 4 mounts from other accidents and incidents were examined (Ref: NTSB Accident/Incident Reports - DEN07LA030, MIA07LA038, CHI07IA067, and LAX05LA116). An examination of this airplane's engine mount revealed that the mount was bent near the nose gear actuator attach feet, and no cracks were noted.
According to NTSB Accident Report, LAX05LA116, a review of the FAA database revealed there have been 32 PA-46 aircraft that have veered off the side of the runway. The majority of the runway excursions occurred in the 310 and 350 series of the PA-46, and not the 500 series. Since the LAX05LA116 report, 4 Piper PA-46-500TP aircraft (including this aircraft) have experienced a nose landing gear failure and/or loss of directional control.
According to Piper, due to the recurring cracked engine mounts, the original engine mount was redesigned. Piper stated, "The redesign for the engine mount started with the determination, from customers in the field, that cracks were developing on the engine mount in the area of the nose gear attachment feet and in the areas immediately outboard of the nose gear pivot points. Piper reviewed several broken engine mounts and determined that a stronger mount was necessary. Working with the FAA and with their approval, Piper redesigned the mount and have not experienced any failure with the new design." The redesign process included finite element model analysis and various testing programs. Piper stated, "The steps taken by Piper included additional testing and analysis beyond the minimum levels required by the FAA in order to insure that the new design would have a higher level of durability and safety." The original mount design had not incorporated these types of design tools. Post-event engineering examination of the original engine mount and STCd engine mount revealed that there was compliance (flexibility) in the engine mount actuator attach feet structure, sometimes resulting in the cracking of the engine mount at the actuator feet. This compliance issue was sometimes exasperated at various speeds, resulting in a high speed shimmy effect and subsequent uncommanded left or right turn of the nose wheel during takeoff and/or landing.
The fractured AN-7 nose landing gear actuator bolt was sent to the NTSB Materials Laboratory in Washington, D.C., for examination. A Senior NTSB metallurgist reported that the bolt was fractured through the grip area about 0.89 inches from the underside of the hex head, with an equal length of the bolt contained in the nut. Examination of the fracture disclosed smooth matte gray fracture faces with adjacent deformation of the bolt typical of a sheering overstress separation. No indications of preexisting cracking or corrosion were uncovered. (Refer to the attached Materials Laboratory Factual Report, #07-025)
On March 28, 2007, Piper issued SB 1154B, Engine Mount Inspection. According to the SB details, SB 1154B supersedes SB 1154A and SB 1154. SB 1154B shortens the repetitive inspection requirement for the engine mount from 100 hours to 50 hours. The SB states that Piper has developed corrective action to eliminate the on-going inspection of the mount. A new mount with improved service life has been developed; P/N 102460-036. The original mount will no longer be available as a service part replacement. If the original mount 102460-002 is replaced with the new mount, the repetitive inspection will no longer be applicable.
The new engine mount, P/N 102460-036, incorporates a one piece design at the actuator attach point, eliminating the two piece feet design. The one piece design provides a less compliant structure, which prevents relative motion between the two feet in the original engine mount design.
According to Piper personnel, all original mounts will be removed and replaced with engine mount P/N 102460-036 by November 2007; as of March 2007, 107 PA-46-500TP aircraft needed the engine mount replaced. Piper stated that all PA-46-500TP customers have been contacted regarding the replacement of the mount and a replacement schedule has been developed. The NTSB has requested that Piper update the NTSB regarding the status of the mount replacement.
The airplane was released to the owner's representative.