Incident
On June 9, 2010, at 1820 central daylight time, a Piper PA-34-220T, registration N104NW, experienced a hard landing on runway 36 at Ankeny Regional Airport (IKV), Ankeny, Iowa. Runway 36 is 5,500 feet long and 100 feet wide, with a concrete surface. The airplane was operating under 14 Code of Federal Regulations Part 91 as a personal flight. It had departed Detroit Lakes (DTL), Minnesota, at about 1622, with IKV as the destination. Visual meteorological conditions prevailed, and an instrument flight rules (IFR) flight plan was filed.
The pilot reported that the flight was normal until the final approach for landing. During the flare, pulling back on the control yoke did not cause the nose of the airplane to pitch up. The airplane landed hard on the nose wheel and bounced several times. The pilot maintained directional control and stopped the airplane on the runway before taxiing to the ramp. The firewall and fuselage sustained substantial damage.
Examination
A Federal Aviation Administration (FAA) airworthiness inspector examined the airplane. Maintenance personnel removed the floorboards and tunnel panels to inspect the control cables. The inspection found that the stabilator control cable had fractured into two pieces. The fracture was located in the unobstructed space in the tail cone, not associated with a pulley, fairlead, or bulkhead. No rubbing or chafing was found in the area of the fracture or tail cone. The entire length of the cable was covered with dried black grease. The cable was removed and sent to the National Transportation Safety Board’s (NTSB) Materials Laboratory for examination.
The NTSB Materials Laboratory determined that the cable’s composition was consistent with stainless steel. The cable fractured approximately 18 inches from the turnbuckle end and approximately 65 inches from the ball portion of the single-shank ball-end terminal. The entire length of the cable was covered with grease. Individual cable wires in the fracture area were partially unraveled, and exposed surfaces were covered with grease. Most fractured wire ends exhibited wear damage at a slant plane relative to the cable length, reducing the ends to a knife edge and reducing the majority of a wire’s cross section. The ends of the remaining fractured wires exhibited elongation deformation and cup and cone fracture features typical of overstress separation. Fractured wire ends showed micro-void coalescence dimple features typical of overstress separation, with no evidence of fatigue cracking.
Background
On January 3, 2001, the New Piper Aircraft Company issued Service Bulletin (SB) 1048, “Stainless Steel Control Cables.” This bulletin identified Piper-manufactured airplanes with stainless steel cables installed. According to SB-1048, the accident airplane (N104NW, serial number 34-8133059) had stainless steel control cables installed during manufacture. The bulletin stated that service reports indicated stainless steel cables in aircraft control systems had a considerably shorter service life compared to galvanized cables for the same application. It mandated inspection and maintenance of control system cables in accordance with FAA Advisory Circular (AC) 41.13.
On July 11, 2001, the FAA issued Special Airworthiness Information Bulletin (SAIB) No. CE-01-30. The SAIB recommended that owners or operators of affected aircraft perform an inspection and possible replacement of stainless steel cables as outlined in Piper SB-1048. The FAA recommended inspections at each 100-hour or annual inspection.
FAA AC-41.13, “Acceptable Methods, Techniques and Practices – Aircraft Inspection and Repair,” stated that inspection and repair of control cables should be accomplished at annual inspections and during 100-hour inspections. It stated that a cloth should be passed over the cable to snag on broken wires.
The aircraft maintenance logbooks indicated that the last annual maintenance inspection was conducted on March 18, 2010. The total time on the airframe at that inspection was 7,240.2 hours. The airplane had flown about 17 hours since the last annual inspection. The logbook entry indicated that control cables were checked for correct rigging and cable tensions, and stated “Stabilator OK.” There was no indication of any repair or replacement of the stabilator control cable.