Incident Description
On March 10, 2011, about 0745 Pacific standard time, a Cessna 208B airplane, registration N930FE, experienced a loss of directional control while taxiing to parking at Meadows Field Airport in Bakersfield, California. The airplane impacted unoccupied parked vehicles. The flight originated from Ontario, California, about 0642 as an instrument flight rules (IFR) cargo flight under 14 CFR Part 135. The sole pilot, an airline transport pilot, was not injured. The airplane sustained substantial damage to the left wing spar and a skin panel outboard of the fuel tank. Visual meteorological conditions prevailed, and an IFR flight plan was filed.
The pilot reported that the brakes and steering system operated normally during taxi for departure at Ontario. After landing on runway 30R, he taxied about 1.9 miles using the customary route toward the FedEx cargo unloading area, making one right and one left turn without anomalies. Prior to reaching the destination, he applied right rudder and brake pressure to negotiate the last 90-degree right turn. He applied "full right braking action" to avoid a collision with FedEx vehicles on the left side. The airplane did not turn as sharply as expected and impacted two FedEx trucks and a FedEx van.
A witness standing on the tarmac observed the airplane taxi toward the unloading area and saw the collision. The witness reported that the airplane's taxi speed appeared normal throughout its approach and final turn.
Aircraft and Systems
The Cessna 208 uses independent left and right brake/rudder pedals for ground steering and braking. Each pedal has an individual master brake cylinder (MBC). A rocker foot pedal with an actuator rod attached to the MBC allows the pilot to apply independent braking pressure to the left and right main landing wheel brakes.
Maintenance and Examination
The airplane was maintained by West Air, Inc. A review of logbook records revealed no outstanding maintenance squawks regarding the brakes or steering system. Due to additional brake issues on other airplanes reported to the operator's maintenance department, the accident airplane's two master brake cylinders were removed and sent to Engineering Systems, Inc. (ESI) for examination, along with MBCs from other airplanes. Cessna was advised and participated in the evaluation with the operator, the NTSB investigator-in-charge, an NTSB Materials Laboratory technician, and an FAA air safety inspector.
Findings from MBC Examination
The ESI examination results, reviewed by the NTSB Materials Laboratory, revealed several issues that could potentially cause or contribute to brake anomalies:
- The right pedal assembly may interfere with the adjacent floorboard when the MBC is compressed to or near the full internal travel stop, limiting overall travel.
- Additional clearance could be achieved by extending the MBC length via clevis adjustment.
- No single component was isolated as the sole cause, but numerous components could keep the MBC from fully extending after brake release, preventing adequate travel (reduced pressure) on subsequent applications. There was also a possibility of brake fluid bypassing the piston O-ring or Stat-O-Seal if parts were damaged or excessively worn.
Further findings from inspection of multiple MBCs: 1. The MBC inspection criteria is "on condition" (external visual only), with no recommended internal inspection/overhaul/replacement period from Cessna. This means the MBC may be operated until failure. 2. Compared to new Cessna MBCs, there were differences in machining depths/shape of the brake cylinder. These manufacturing differences appeared to cause problems with the MBC actuating rod fully extending, potentially reducing spring extension force and causing severe spring bending. Spring bending could cause side loading on the piston, preventing full extension and leading to inadequate MBC stroke/compression of brake fluid to stop the airplane. 3. As the MBC wears, it is possible for pilots to "walk" the MBC down to a mid-travel position, where travel may not be adequate to compress brake fluid and stop the airplane.
Service Bulletin
As a result of the investigation, on March 14, 2014, Cessna, in conjunction with the FAA, published Service Bulletin (SB) CAB-32-01, with one revision. The SB requires disassembly and inspection of all MBC parts within a prescribed time period, with replacement/service as necessary. Additionally, the SB requires installation of an added externally mounted brake pedal return spring assembly.
Taxiway Clearance Review
During the investigation, NTSB and FAA personnel, along with airport, FedEx, and West Air management, reviewed the accident site area for conformity with FAA advisories and airport taxi-lane usage policies. The review of the taxi path found that the path was sufficient for Airplane Design Group I aircraft (wingspans less than 49 feet). However, the Cessna 208 is classified in Airplane Design Group II, with a wingspan of 52.1 feet. The required Taxi-lane Object Free Area (TOFA) for Group II is 115 feet (57.5 feet either side of centerline) per FAA AC 150-5300. The narrowest point along the taxi-lane was 102 feet in width (from bollards to the FedEx hangar). Following the review, the airport rerouted Airplane Design Group II aircraft along an appropriate taxi route.