NTSB Factual Report

On February 11, 2005, about 1535 eastern standard time, N403BK, a Cessna 402C, operated by Island Airlines, was in cruise flight when the airline transport pilot noted that the airplane required aileron trim to maintain level flight. The airplane had departed Barnstable Municipal Airport (HYA), Hyannis, Massachusetts, and landed at Nantucket Memorial Airport (ACK), Nantucket, Massachusetts, without incident or injury. Visual meteorological conditions prevailed, and no flight plan had been filed for the solo positioning flight conducted under 14 CFR Part 91. A subsequent inspection of the airplane by maintenance personnel revealed damage to the left wing spar.

During an interview, the pilot stated he departed runway 33, at HYA, climbed to 1,000 feet, and preceded direct to ACK. The winds were out of the northwest about 28 knots, and the ride was "bumpy" throughout the flight. Shortly after leveling in cruise flight, the pilot noticed that the airplane tended to fly left wing low, and required a 10-degree right aileron control input. The pilot stated that he thought that the ailerons were "a little out of rig" or perhaps a flap was not completely retracted. The pilot utilized aileron trim to maintain level flight, and proceeded to ACK without further incident. After landing, he contacted the director of maintenance (DOM) at Island Airlines to inquire if any maintenance had been performed on the airplane's flight controls. The DOM stated that no maintenance had been performed on the airplane's flight controls, and he would look at the airplane when the pilot returned to HYA. The pilot then departed ACK with 9 passengers for a flight conducted under 14 CFR Part 135. The airplane landed at HYA without incident. The pilot stated that the out of trim condition did not change at all during the flight, and he had no problems controlling the airplane. After landing, the pilot wrote up three discrepancies; "Lower center static strip on windshield loose," "Nose strut seems low," and "RT aileron trim required in cruise." There were no further flights in the airplane.

According to the DOM, he began inspecting the airplane on February 12, 2005. He checked the aileron control cables and began to drain the fuel tanks to compare the actual fuel, against the cockpit gauges. He noted a fuel imbalance of 8 additional gallons in the left tank; however, he also noticed two rivets missing on the underside of the left wing, between the heat shield and a factory-installed doubler. The DOM removed the fuel pump/selector access panel and observed a vertical crack on the backside of the forward wing spar. He also removed the heat shield and found that it concealed a crack on the skin. On February 14, 2005, he notified the Federal Aviation Administration (FAA), and Cessna Aircraft Company. He also notified the DOM for Cape Air at HYA, who maintained a fleet of 49 Cessna 402Cs.

The airplane was examined on February 16 and 17, 2005, by representatives from the FAA aircraft certification office, and Cessna. It was noted that the airplane did not contain, nor was it required to contain a lower front wing spar cap strap modification. The lower forward spar cap was cracked at wing station (WS) 114, at a rivet hole located just outboard of the outboard engine beam. The web doublers, both forward and aft, were split near the end of the inboard wing spar web at WS 112.5. The skin was cracked from the front spar lower cap up to the first stringer location behind the front spar. The cracked portion of the forward spar, skin and the aft web doubler were removed and examined at Cessna, with the participation and supervision of a Safety Board Metallurgist. Examination of the spar cap crack revealed fatigue damage, which originated at the forward portion of the rivet hole on the spar cap at WS 114. Blackening and corrosion were observed on the fracture surface; however, the examination did not reveal any evidence of pre-existing heat or corrosion damage (For further information, please see the Cessna Material and Process Report located in the public docket).

The DOM for Cape Air observed the damage on the Island Airlines airplane, and ordered a fleet-wide inspection on all of their Cessna 402Cs that were not equipped with a spar strap modification. Emphasis was placed in the same location where the cracks were located on N403BK. On February 17, a crack was identified through the left wing lower forward spar cap of N991AA, operated by Cape Air. According to the DOM, a mechanic reported that he observed a faint black line, aft of the rivet hole located at WS 114. The DOM looked at the area and "thought" he saw a faint black line forward of the WS 114 rivet. Only after the skin underneath the spar cap was "pulled-back," was the crack confirmed. The crack extended from the forward spar cap, to the WS 114 rivet hole, and then continued from the rivet hole aft.

The Safety Board was notified of the spar cracks on February 17, 2005, and a Safety Board investigator inspected both airplanes on February 19 and 20, 2005.

On February 20, 2005, the skin underneath the right wing, at WS 114, on N403BK, was pulled-back to inspect the spar cap. The examination revealed that the spar cap was cracked at the WS 114 rivet hole forward. There was no visible crack noted aft of the rivet hole.

Examination of the right lower wing spar cap crack from N403BK, and the left lower wing spar cap crack from N991AA was conducted by a Safety Board Metallurgist. The examination revealed that both cracks were fatigue cracks that originated in rivet holes at WS 114. Neither spar cap contained evidence of metallurgical abnormalities or significant mechanical deficiencies (For further information, please see the NTSB Materials Laboratory Factual Report located in the public docket).

At the time the cracks were discovered, N403BK, and N991AA, had accumulated 20,510 and 20,349 total airframe hours; respectively. Both N403BK, and N991AA, were required to have their respective forward, aft, and auxiliary wing spars inspected for cracks per FAA AD 2000-23-01, at intervals not to exceed 110 hours time-in-service (TIS). According to maintenance records, N403BK was inspected on February 5, 2005, about 12.3 hours prior. N991AA, was inspected on February 6, 2005, 31.2 hours prior.

On February 20, 2005, the FAA issued emergency AD 2005-05-51, which increased the repetitive visual inspections required in AD 2000-23-01, and emphasized the following areas:

"Just Outboard of the Engine Beam

The lower spar cap at WS 114.

The three rivets on both the inboard and outboard sides of WS 114 (total of six rivets) in the lower spar cap as viewed from the access hole.

The spar web at WS 112.5.

Just inboard of the Inboard Engine Beam

The lower spar cap between WS 80 and WS 89.

The two attach bolts just inboard of the WS 89.18 rib."

On March 2, 2005, the FAA issued emergency AD 2005-05-52, which introduced an eddy current inspection procedure, in addition to the visual inspection procedure. The combined inspection techniques allowed for longer intervals between the repetitive inspections contained in AD 2005-05-51.

On June 22, 2005, the FAA issued AD 2005-12-13, which included a requirement for a spar strap modification dependent on the airplane serial number and time in service. The spar strap modification would terminate the repetitive inspection requirements contained in AD 2005-05-52.

NTSB Probable Cause

The mechanic's inadequate inspection, which failed to discover cracks in the left wing spar around wing station 114.