Tampa, FL, USA
N550AJ
CESSNA S550
The airline transport pilot and commercial-rated copilot, who was the pilot flying, were conducting a positioning flight. The copilot reported that, during takeoff when the airplane was at 400 ft, he retracted the flaps and turned right for the published departure procedure. He noted that, during the turn, the controls were stiff, and he was feeling pushback. Both pilots then began pushing on the control yoke, and the copilot tried to use both the electric and manual pitch trim to reduce the control forces without success. He then engaged the trim disconnect and pulled the pitch trim circuit breaker, but the secondary pitch trim was still frozen. He again attempted to use the manual pitch trim without success. The copilot subsequently declared an emergency with air traffic control, returned to the airport, and landed uneventfully. During postincident troubleshooting of the pitch trim system anomaly, the left screw of the left elevator trim tab actuator was removed, cleaned, regreased, and then reinstalled. Subsequently, the primary and secondary sprockets were found separated from the left internal screw, which was fractured due to overstress; no evidence of a preexisting fracture was noted. The two actuator shaft assemblies were examined, and both assemblies were missing flat roller bearings. Maintenance records indicated that the actuator was newly installed over 24 years, or 6,560 flight hours, before the accident and had not been disassembled since that time. Given that fresh grease was found in the actuator in an area that normal maintenance is not performed, it is likely that undocumented maintenance was conducted on the actuator at some point and that the missing bearings were not reinstalled at that time. The maintenance records also revealed that the pitch trim actuator was last lubricated as part of a phase 2 inspection about 1 year 7 months, or 197 flight hours, before the accident, which was within limits; the process did not require the removal of the left internal screw from the actuator housing. Although postincident troubleshooting precluded the determination of the amount and condition of the grease on the left internal screw, it is likely that maintenance personnel did not adequately lubricate the left elevator trim tab actuator during this inspection. Therefore, it is likely that excessive play caused by the missing bearings, which resulted in wear, and the inadequate lubrication of the left elevator trim tab actuator caused the overstress fracture of the left internal screw and the subsequent failure of the pitch trim system.
On October 4, 2014, about 0837 eastern daylight time, a Cessna S550, N550AJ, experienced a pitch trim malfunction shortly after takeoff from Tampa International Airport (TPA), Tampa, Florida. The airline transport pilot and commercial-rated co-pilot were not injured, and the airplane was not damaged. The airplane was registered to North Florida MBA Investments LLC, and operated by Gulf Atlantic Airways, Inc., dba University Air Center, under the provisions of 14 Code of Federal Regulations (CFR) Part 91 as a positioning flight. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The flight originated from TPA about 0836, and was destined for Gainesville Regional Airport (GNV), Gainesville, Florida.The captain stated that they had flown that airplane on 3 previous legs that day and there were no issues pertaining to the pitch trim on any of those flights. Following the landing at TPA of the 3rd flight, they offloaded the passengers, and prepared to depart for GNV. The co-pilot was the pilot flying from the right seat, and before departure they performed the pre-takeoff briefing. The flight departed and at 400 feet at V2 plus 10 knots they retracted the flaps and turned to the right for the published departure procedure. At that time the co-pilot said the controls were stiff and he was feeling push back. He (captain) advised him twice to trim the forces away and noticed his arms were fully extended pushing on the control yoke. He too assisted on pushing the control yoke and tried using the electric pitch trim but that did not work. He also attempted to use the manual pitch trim but that too did not work. Simultaneous to that he hit the trim disconnect in case there was a trim runaway, and pulled the pitch trim circuit breaker, but the secondary pitch trim was still frozen. He again tried the manual pitch trim and again there was no effect. He did not feel it was a runaway trim issue. He declared an emergency with air traffic control, and reduced power aggressively but this caused a slight pitch up (normal). He then added power, then slowly reduced power and when the airspeed was less than 200 knots, lowered the flaps to the approach setting. When the airspeed was less than 172 knots the landing gear was extended, and them the flaps were placed in the landing position. He stated that when they slowed the airplane was more controllable with respect to pitch as the pitch setting was set to the takeoff range. The controller asked if they wanted to perform an ILS approach to runway 19L to which they advised the controller that the flight was VFR and they wanted a visual approach to the runway. They flew downwind, base and final and the flight landed uneventfully at 0844. Postincident examination of the airplane was performed by the Assistant Director of Maintenance (ADOM) of the operator with oversight by a Federal Aviation Administration (FAA) operations inspector. With no power on the airplane and using normal hand pressure of the manual elevator pitch trim, the trim would not move more than an inch and when released, the trim pop right back to the same position. Further, with manual pitch trim, the nose down movement felt like it was binding while the nose up trim appeared normal. With electrical power applied and application of electric pitch trim, the trim moved, however it was very sluggish. As part of the trouble shooting procedures to isolate the issue, the left screw of the left elevator trim tab actuator assembly was removed, cleaned, inspected, re-lubed and then re-installed. After this was completed, further inspection of the actuator revealed the primary and secondary sprockets were separated from the left internal screw, (part number (P/N) 5565450-45). The left elevator trim tab actuator assembly P/N 5565450-79, serial number 0378-80 with fractured left internal screw was retained for examination by the NTSB Materials Laboratory. According to the NTSB Materials Laboratory Factual Report, the 1-sprocket assembly shaft freely rotated within the actuator while the 2-sprocket assembly shaft was jammed inside the actuator and could not be manually rotated. The shaft and internal components of the actuator assembly were disassembled from the actuator housing, and both assemblies were missing the flat roller bearings that should have been adjacent to the flanges on the shaft side opposite the sprockets. Fresh grease with a pale pink tint was overlaid on top of black-colored grease on the portions of the components internal to the actuator housing. Some of the black-colored grease was dry and caked on, particularly around the flanges on the shafts. Following removal of the grease, corrosion was visible on the flanges and ends of both shafts. Corrosion was also noted on the bearings that mated with the ends of the shafts. The 2-sprocket assembly was disassembled and the separated end of the mating shaft was found in 2 pieces between the sprockets. Detailed inspection using a stereo zoom microscope of varying power revealed the fractures through the shaft end of the internal screw had occurred at the holes for the stake pin. Inspection of the fracture surface revealed it exhibited evidence of dimpled features consistent with overstress on areas that did not have mechanical damage. Additionally, circumferential scoring was noted on the outer diameter of the left internal screw between the flanges. Review of the airplane maintenance records revealed the airplane was maintained in accordance with the manufacturer's maintenance program. The left elevator trim tab actuator assembly is considered an on-condition item, and was last installed new on August 8, 1990, at airplane total time 1,545.1 hours. There was no record that it had been removed, overhauled, or replaced since then. A Phase 2 inspection that in part contains the inspection and lubrication of each elevator trim tab actuator assembly required every 300 hours or 24 calendar months, whichever occurs first, was last performed on March 7, 2014. The airplane total time at that time was recorded to be 7,908 hours, while the airplane total time at the time of the incident was 8,105 hours. A review of the FAA Service Difficulty Report (SDR) data pertaining to the part number of the elevator trim tab actuator assembly from 1995 to present revealed a total of 10 reports were submitted. Further review of the submitted reports revealed three associated with pitch trim. One of the three reports indicates the sprocket separated from the internal screw due to separation of the pin that secures the sprocket to the internal screw. The second report indicated that following inoperative manual and electric pitch trim, water was found inside both actuator housings. The third report indicates that following a jammed elevator trim, the actuator was worn and the chain on the actuator was very loose and noted to bind when up trim was selected. There were no entries indicating the internal screw was fractured. Review of reports for the same P/N from prior to 1995 revealed a total of 14 reports, of which two indicate the internal screw was fractured.
The lack of flat roller bearings in the two elevator trim tab actuators and maintenance personnel’s inadequate lubrication of the left elevator trim tab actuator, both of which resulted in the overstress fracture of the left internal screw and subsequent in-flight failure of the pitch trim system.
Source: NTSB Aviation Accident Database
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