NTSB Factual Report

On March 10, 2011, about 0745 Pacific standard time, a Cessna 208B airplane, N930FE, experienced a loss of directional control while taxiing to parking and impacted unoccupied parked vehicles at the Meadows Field Airport, Bakersfield, California. The airplane was registered to Federal Express Corporation (FedEx), and operated by West Air, Inc., Bakersfield, as an instrument flight rules (IFR), cargo flight, under the provisions of 14 Code of Federal Regulations Part 135. The solo 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 flight originated from Ontario, California, about 0642.

The pilot reported that the airplane's brakes and steering system had operated normally during taxi for departure at Ontario. Upon landing on runway 30R, he taxied about 1.9 miles using the customary route toward the assigned FedEx cargo unloading area. During this period, he made one right and one left turn without experiencing any anomalies. Prior to reaching his destination, he applied right rudder and brake pressure to negotiate the last 90-degree right turn following the taxiway center line toward the FedEx unloading/parking area. He reported that he applied "full right braking action in order to make the aircraft turn and avoid a collision with the FedEx vehicles on the left side of the aircraft." The airplane did not turn as sharply as expected, and impacted two FedEx trucks and a FedEx van.

The accident occurred in an area of the airport that was not visible to Federal Aviation Administration (FAA) air traffic control tower personnel. A witness, who was standing on the tarmac near the accident site, observed the airplane taxi toward the FedEx unloading area and observed the collision. A witness reported that the airplane's taxi speed had appeared normal throughout its approach and final turn.

The Cessna 208 uses independent, left and right, brake/rudder pedals for steering/braking on the ground. Each pedal has an individual/independent master brake cylinder (MBC). A rocker foot pedal with an actuator rod attached to the MBC allows the pilot to apply independent and varied braking pressure to the left and right main landing wheel brakes.

The airplane was maintained by West Air, Inc. A review of airplane logbook records revealed no outstanding maintenance squawks regarding the brakes or steering system.

Due to additional brake issues on other airplanes that had been reported to the operator's maintenance department, the accident airplane's two master brake cylinders were removed and sent to Engineering Systems, Inc. (ESI), along with MBC's from other airplanes for examination.

Cessna was advised of the accident, and participated with the operator, the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), an NTSB Materials Laboratory technician, and an air safety inspector from the FAA, in the evaluation of the MBC's.

The ESI examination results were reviewed by the NTSB Materials Laboratory, and revealed several issues that could potentially cause/contribute to the brake anomalies.

The right pedal assembly may interfere with the adjacent floorboard when the MBC is compressed to or near the full internal MBC travel stop. The interference may cause an additional stop, limiting the overall travel of the MBC.

Additional clearance between the assembly and the adjacent floorboard could be increased by extending the length of the MBC by adjusting the clevis.

The MBC examination also revealed that there was no single component of the MBC assembly that could be isolated as the sole cause for the brake system failures reported. There were however, numerous components/items that could keep the MBC from fully extending after the brakes are released. Failure of the MBC to fully extend can prevent adequate travel (reduced pressure) when the brakes are actuated on subsequent applications. There is also a possibility the brake fluid may by-pass the piston O-ring or Stat-0-Seal if parts are damaged or excessively worn.

The following findings were the result of the inspection of numerous MBC's.

1. The MBC inspection criteria is "on condition" (meaning external visual inspection) i.e. there is no recommended internal inspection/overhaul/replacement period established by the Cessna Aircraft Co. This means the MBC may be operated until failure.

2. Relative to new Cessna Aircraft Company MBC's, there are differences in machining depths/shape of the Brake Cylinder. These manufacturing differences appeared to be causing problems with the MBC actuating rod fully extending. The differences in machining may cause reduced spring extension force as well as severe spring bending. The spring bending can cause side loading on the piston, which can keep it from fully extending. Failure of the MBC actuator to fully extend can cause inadequate MBC stroke/compression of the brake fluid to stop the airplane.

3. Relative to pilot related problems associated with the MBC, as the MBC wears it is possible for the pilots to "walk" the MBC down to a mid-travel position. Then the MBC travel may not be adequate to compress the brake fluid and stop the airplane.

As a result of this investigation, on March 14, 2014, Cessna in conjunction with the FAA, published Service Bulletin (SB) CAB-32-01, with 1 revision.

Essentially, the service bulletin (copy attached to the public docket for this accident) requires the disassembly and inspection of all MBC parts within a prescribed time period, and replacement/service as necessary. Additionally the SB requires the installation of an added externally mounted brake pedal return spring assembly.

During the course of the investigation, NTSB and FAA personnel, in conjunction with Meadows Field Airport, FedEx, and West Air, Inc., management personnel, reviewed the accident site area to ascertain its conformity with applicable FAA advisories and airport taxi-lane usage policies.

The review of the taxi path of the accident airplane, found that the path was sufficient for Airplane Design Group 1 aircraft having wingspans of less than 49 feet. No objects are allowed to be within the Taxi-lane Object Free Area (TOFA) except for those objects that need to be located in the Object Free Area for air navigation or aircraft ground maneuvering purposes. Adjustment of the taxi-lane centerline markings ensure that adequate clearance is provided for Airplane Design Group I 79' (39.5' either side of taxi lane centerline).

The Cessna 208 is classified in Airplane Design Group II, with a total wingspan of 52.10'. Airplane Design Group II includes wingspans of 49-79 feet. TOFA width equals 1.2 times airplane wingspan plus 20 feet. The TOFA is an area that enhances the safety of aircraft operations by having the area free of objects, except for objects that need to be located in the TOFA for air navigation or aircraft ground maneuvering purposes. Applying the TOFA formula to the Cessna 208 specifications provides a TOFA of 82.52' (41.26' either side of centerline), however, in the interest of safety and when applying and approving standards, the FAA applies the standard for the most demanding aircraft in the design group.

Aircraft classified in the Airplane Design Group II require 115' to be clear of objects (57.5' either side of centerline) as dictated in Table 4-1 in FAA AC 150-5300, Airport Design. The narrowest point along the taxi-lane is 102' in width measured from bollards to the Fed Ex hanger. Airplanes classified as Airplane Design Group II and larger do not have sufficient TOFA clearance.

Following the review, the airport has rerouted Airplane Design Group II aircraft along an appropriate taxi route.

NTSB Probable Cause

The malfunction of a master brake cylinder, which resulted in a loss of braking action and subsequent loss of directional control and collision with three vehicles during taxi. Contributing to the accident was the inappropriate design and manufacture of the master brake cylinders.