NTSB Factual Report

HISTORY OF FLIGHT

On May 21, 2010, about 1600 Pacific daylight time (PDT), a Schweizer G-164B AgCat, N3633C, during landing with a suspected brake problem, the airplane veered off the runway and came to rest in an irrigation ditch near Robbins, California. Sunrise Dusters, Inc., was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 137. The certificated commercial pilot sustained serious injuries; the airplane sustained substantial damage to the left and right wing spars. The local agricultural aerial application flight departed Robbins about 1530. Visual meteorological conditions prevailed, and no flight plan had been filed.

The pilot reported that during the flight he noticed that the right brake pedal was stiff. He saw what appeared to be hydraulic fluid leaking on his foot from what he thought might be from the parking brake valve. The pilot tried without success to set and release the brake to correct the stiff brake pedal. He radioed in to his base of operations; he notified ground personnel of the possible brake problems, and his intentions to return to the airfield.

During the landing, as the right wheel touched down the airplane pulled to the right; the pilot attempted to bring the airplane back towards the left. During the correction, the pilot felt the wind shift. The airplane veered back towards the left, departed the runway, and came to rest in an irrigation ditch.

The airplane sustained substantial damage to both the right and left wing spars. The propeller was substantially damaged, and a large quantity of soil entered the engine intake. The left and right main landing gear struts were damaged when the aircraft went into the ditch. The right brake calipers separated from the airplane, and were found within a few feet of the initial touchdown point of the right main landing gear tire.

The airplane was recovered from the ditch, and moved to the company’s hangar for further examination.

The Federal Aviation Administration (FAA) accident coordinator examined the airplane and the accident site.

The right wheel was found to have the entire inboard flange separated from the inboard wheel half. The tire was a tube type and the tube was still inflated. The tire expanded beyond where the flange once was and pushed the broken wheel flange pieces into the brake disc assembly. The tire was deflated and the parts were retained for examination.

The attach points of the brake disc were found to be bulged and stressed. The attach points of the disc are normally located on the wheel assembly inner diameter. The force of the inflated tire and tube pushed inward radially against the brake disc with great force. The FAA inspector opined that the wheel flange failed in-flight, and gave the pilot a locked brake pedal feel.

The installed tire on the accident airplane was 29x11.0-10. The manufacturer of the wheel assembly for the Ag-Cat G-164B (Part number 40-101) reported that the proper tire size should be 8.50-10.

The FAA accident coordinator contacted the type certificate holder of the Ag-Cat G-164B and was advised that some operators of the accident airplane model have obtained a field approval to use oversize tires from their local FAA Flight Standard District Office.

As a result of this accident and the subsequent investigation, the FAA issued a Special Airworthiness Information Bulletin SAIB CE-11-06 on December 15, 2010, making recommendations to operators who may be operating oversized tires. The SAIB made the following recommendations:

“All operators of 14 CFR part 21, section 21.25 Restricted Category and 14 CFR part 23 Normal Category certificated airplanes that are currently operating with or may modify their aircraft to operate with oversized tires different from the aircraft’s approved type design, should acquire an STC or field approval supported by FAA-approved data with similar original equipment manufacturer (OEM) restrictions for installation, pressure, etc. before doing so. The approved data should include flight test requirements and their results as well as system design and structural analysis. Guidance for replacing OEM tires with oversized tundra tires is provided in FAA Advisory Circular (AC) AC 23-17B. The AC summarizes the results of flight tests recommended by the National Transportation Safety Board, and conducted by the FAA for evaluation of tundra tires installed on a Piper PA-18. In addition, the AC provides information of these tires as well as testing of their installation. Although the guidance in the AC is based specifically on the Piper Aircraft, Inc. PA-18, it identifies issues and guidance that can be extended to other airplane types equipped with oversized tires. The AC also identifies the possible performance effects and flight and ground handling characteristics that may be altered with the installation of the tundra tires. Potential propulsion system effects such as unusable fuel may be affected by changes in normal flying attitude. Review of all these potential effects as identified in AC 23-17B should be evaluated.

Before installation of larger tires operators should do the following as a minimum:

1.

Initially inspect wheels for cracks using an industry standard fluorescent penetrate process with annual inspections thereafter.

Install only approved vendor supplied tires.

Install new tubes.

Determine the appropriate tire pressure for the specific tire, aircraft, and aircraft operations in consultation with the approved tire manufacturer and FAA Advisory Circular, AC 23-17B.

Check for clearance between the tire and brake components.

Check the tire pressure and abnormal wear at 2 week intervals or less.

Additional safety information to the public of potential problems associated with aircraft with oversized tires is also provided in Safety Alert for Operations (SAFO) 10007, dated 5/24/10. The SAFO provides recommended actions for persons interested in operating aircraft with oversized tundra tires as well as with skis, or wheel/ski installations.

Owners and operators of Restricted Category and Normal Category airplanes who intend to modify their aircraft with oversized tires should review the above documents for guidance and the issues to consider and evaluate when replacing the manufacturer’s recommended main landing gear wheels and/or tires with oversized replacements.”

NTSB Probable Cause

The failure of the wheel hub flange, which allowed the over size tire to expand and impinge on the brake disc and caliper, which resulted in a loss of control during the landing.