FTW00LA008

The pilot completed the 8th seeding pass over a field and was turning to make the 9th pass when the agricultural airplane shuddered and made three uncommanded banks prior to impacting the terrain. The pilot initially stated that he flew through his own wake turbulence and stalled the airplane while in a 60-degree bank to the right. However, the pilot later stated that he thought the propeller may have malfunctioned resulting in the loss of airplane control. Post-accident examination of the airplane flight controls and the propeller revealed no anomalies that would have prevented normal operation of the aircraft prior to the accident.

On October 7, 1999, at 1620 central daylight time, an Air Tractor AT-502B agricultural airplane, N61768, owned and operated by Aviation Services Unlimited of Bay City, Texas, was substantially damaged when it impacted terrain while maneuvering near Bay City, Texas. The commercial pilot, sole occupant, received minor injuries. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 137 aerial application flight. The local flight originated from the Bay City Municipal Airport at 1606.

During a telephone interview, conducted by the NTSB investigator-in-charge (IIC), the pilot stated that he had completed his 8th seeding pass over the field and was turning to make the 9th pass. He was in a 60-degree bank at approximately 300 feet agl, when he "flew through the wake turbulence from his previous pass and stalled the airplane." The airplane "violently rolled to the right, then to the left." Before he could recover from the "stall," the pilot leveled the airplane and it impacted the ground.

In the enclosed Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the pilot stated that the aircraft was turning to the right at 93 mph and 342 feet msl (290 feet agl). The airplane was in a 60-degree bank and was entering the final portion of the turn prior to entering the north end of the field being seeded. The pilot reported that the "aircraft shuttered twice, made an uncommanded right roll to the inverted position." The pilot applied left aileron and rudder and relaxed the back-pressure on the elevator in an attempt to level the airplane. The airplane was moving toward a level position when the "aircraft made an uncommanded left roll" to an approximate 90-degree bank. The pilot then reversed control inputs to right the airplane, and subsequently the airplane made another "uncommanded roll" to a 90-degree right bank. The pilot made a control input to level the wings and "flared the aircraft" prior to impacting the ground with the right wing and right main landing gear first. The pilot reported that he then retarded the power lever to the idle stop. The airplane bounced, impacted the ground again with the tail wheel, then the left main landing gear, and subsequently rocked forward impacting the propeller. The pilot added in his written statement that the aircraft "was not stalled, because I still had control responses."

According to the FAA inspector, who responded to the accident site, the tail section was structurally damaged and remained partially attached to the fuselage, the left main landing gear collapsed, and both wings sustained structural damage.

The NTSB IIC requested that a mechanic examine the airplane to check for flight control malfunctions. A letter provided by the mechanic reported that the flight controls and engine controls showed "no evidence of pre-impact failure." The pilot attached a letter to the mechanic's statement requesting that the NTSB examine the propeller because he suspected an in-flight malfunction.

On October 29, 1999, an FAA airworthiness inspector, a representative from the propeller manufacturer, and the pilot examined the Hartzell HC-B3TN-3D propeller. The propeller was a 3-bladed, hydraulically operated, constant speed model, with full feathering and reversing capabilities. According to the propeller manufacturer's representative, all three propeller blades were bent aft near their shanks and each blade was twisted toward low pitch. All three propeller blade tips were curled and the tip of each blade had been torn off. Two of the blades had a 4-inch section sheared off from their tips, and one blade had been sheared twice, once at 4 inches from the tip and again at 8 inches from the tip. Examination of impact marks on the propeller's three link arms equated to an approximate blade angle of 23 degrees. The propeller manufacturer's representative reported that, "the propeller was operating at a positive blade angle at impact, in the normal operating range." All of the damage sustained by the propeller blades and the hub was a result of impact damage. "No discrepancies were noted that could have precluded normal operation" of the propeller.

Review of the propeller maintenance records revealed that the propeller (serial number BUA19368) was manufactured on May 16, 1996, and had accumulated 1,424.6 hours since new. The propeller had never been overhauled, but it had been serviced (inspection, profile, and paint) once, 398 hours prior to the accident.