History of Flight
On May 2, 2011, at about 1440 Pacific daylight time, a Cessna 421B, registration N270CS, experienced an uncommanded retraction of the right flap during the downwind leg for landing at Truckee-Tahoe Airport, Truckee, California. The airplane was operated by California Shock Trauma Air Rescue (CALSTAR) under Title 14 Code of Federal Regulations Part 91 as a repositioning flight. The commercial pilot and two medical flight crewmembers were uninjured, and the airplane was undamaged. The flight departed McClellan Airfield, Sacramento, California, at 1412, with a planned destination of Truckee. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed.
The pilot reported entering the left downwind leg for runway 28 at about 8,500 feet mean sea level (msl). He stated that the pattern altitude was 7,500 feet, so to descend he reduced airspeed to 140 knots, extended the landing gear, and incrementally deployed flaps to 45 degrees. Just as the flaps reached their maximum limit, he heard a popping sound and the airplane simultaneously rolled about 80 degrees to the right. He countered with full left aileron input, retracted the flaps, and applied full engine power. Once he attained a positive rate of climb, he retracted the landing gear.
The pilot noted that wings-level could only be maintained with full left aileron and full left roll trim, so he assumed a right flap failure. He circled Truckee airport to troubleshoot but could not extend the right flap or retract the left. The flap motor circuit breaker was closed; the flap indicator showed zero, and the flap drive motor would not engage regardless of handle position.
Assessing options at Truckee, Reno, or South Lake Tahoe, the pilot elected to return to McClellan due to terrain and wind. He could perform left turns with about 5 degrees of bank; right turns were possible but recovery to wings-level was slower. In this configuration, he maintained a climb rate of about 300 feet per minute, and at 13,000 feet msl he could maintain wings-level with about 90 percent control deflection. For the remaining 35 minutes, a medical crewmember helped maintain left aileron deflection. The pilot landed at McClellan without incident after a left downwind approach.
Tests and Research
Flap Control System
The Cessna 421B flap system uses an electric motor driving a gear reduction unit. Two sprockets on the output shaft drive four chain-connected cables that actuate bellcranks in each wing, with push-pull tubes moving the flaps. A cam and limit switches control the preselect system, which includes a preselect lever on the instrument panel, a control cable routed to the right wing upper flap extend cable, and microswitches.
Initial Examination
A cursory inspection by the operator revealed that the right wing upper flap extend cable (part number 5000008-62) had separated about 11 inches from the inboard turnbuckle and chain fitting, inboard of the preselect cable clamp. A steel braided oxygen line had come loose into the area but had not contacted the cable; the separation was adjacent to the inboard fuselage pulley. The left wing lower flap extend cable (part number 5000008-63) exhibited frays to multiple strands near its corresponding inboard fuselage pulley.
Further examination by the NTSB investigator-in-charge (IIC) showed the flap actuator motor undamaged. The upper and lower limit switches clicked appropriately when triggered. The preselect cable and assembly were intact and functional.
The upper right and lower left flap extension cables and their inboard pulleys were sent to the NTSB Materials Laboratory.
Materials Laboratory Examination
Both cables were 1/8-inch diameter 7X19 wire ropes, about 63 inches long, with a turnbuckle on one end and a swiveling clevis on the other. The right extend cable had separated about 11 inches from the turnbuckle; most wire breaks occurred within a 0.5-inch segment, with others 2 inches away. High magnification revealed fatigue cracking on over half of 24 examined wires; the rest showed overstress necking. No external wear or significant internal wear was found. Fraying (at least 24 broken wires) was detected 20-25 inches from the clevis end, near the outboard wing stub pulley. The left extend cable had about 20 wire fractures concentrated between 51 and 53 inches from the outboard clevis. Both inboard pulleys showed no wear and turned smoothly.
Maintenance
The airplane had 4,832.1 total flight hours at its last annual inspection on March 19, 2011. Records showed the flap extend cables had never been replaced since manufacture in 1975; no life limit exists. The Cessna 421B service manual calls only for visual and physical examination with a cloth at pulleys and fairleads, stating that no more than three broken wires in any 10-inch length are acceptable. The mechanic who performed the last inspection reported examining the flap cables per the manual and detecting no damage; the frays were only noticeable after cable removal and manual flexing.
Service Difficulty Reports
A review of FAA Service Difficulty Reports (SDRs) for Cessna 401-425 series airplanes found 33 reports of flap cable wear or failure on 25 airplanes. Fourteen reports documented cable failures, all during landing approach, resulting in asymmetric flaps. Some reported unusual attitudes after failure. Most described inability to retract the extended flap, with causes including gearbox damage, cable entanglement, or preselect cable interference. None reported left flap cable separation in flight; all separated cables were right extend type or unspecified, and separations occurred near the inboard fuselage pulley and preselect cable clamp. Four reports described frayed cables found during routine inspections; fourteen noted fraying discovered during rigging or after cable removal, and that the damage location prevented detection during routine inspections. Total flight times on damaged cables ranged from 3,049 to 13,636 hours.