NYC98IA106

After takeoff, the pilot heard a 'loud bang' followed by severe vibration. The pilot returned and landed at the departure airport without further incident. Examination revealed the left engine's number two propeller counterweight had separated, and exited through the propeller spinner. The counterweight then impacted the number one propeller blade, of the left engine, which separated about 12 inches from the hub. Examination of the counterweight clamp leading edge bolt hole revealed that the internal threads in the hole were stripped by motion of the leading edge bolt out of the hole. Additionally, the hole contained a double set of thread roots, indicative of cross threading of a bolt or mistapping of the threads. The propeller counterweight bolts had been replaced during an overhaul performed by the propeller manufacturer on April 12, 1995. The propeller counterweight bolts were also replaced by a certified propeller repair station, during maintenance on April 8, 1996, and at the last overhaul, which was performed on January 28, 1997. According to the propeller overhaul manual, after removal of a counterweight clamp bolt, an undersized tap should be used to remove the loctite retaining compound which was to be applied to the counterweight clamp threads during installation. The manual also stated '...Visually inspect threads for damage. Cross threading, damaged or missing threads is cause for retirement of the clamp.' A representative from the propeller repair station stated that their mechanics use a compressed air plastic bead spray to clean the clamp threads and they do not use an undersized tap. According to the propeller manufacturer, the use of a compressed air plastic bead spray to clean the propeller counterweight clamp threads was not an approved procedure.

On May 4, 1998, about 0925 eastern daylight time, a Beech 1900D, N832CA, operated by Commutair, and dba US Airways Express, flight 4561, experienced a separation of a propeller counterweight and adjacent propeller blade during takeoff from the Syracuse Hancock International Airport, Syracuse, New York. The 2 crew members and 12 passengers were not injured. Visual meteorological conditions prevailed and an instrument flight rules flight plan had been filed for the flight destined for Boston, Massachusetts. The flight was conducted under 14 CFR Part 121.

In a written statement, the pilot said the airplane departed runway 10. After takeoff, at approximately 200 feet above ground level, he heard a "loud bang" followed by the airplane "vibrating violently." At 400 feet, he noted that part of the propeller blade had separated from the left engine. The pilot then secured the left engine, declared an emergency, and returned to the airport, where the airplane landed without further incident.

Examination of the airplane by a Federal Aviation Administration (FAA) Inspector, revealed the left engine's number two propeller counterweight had separated, and exited through the propeller spinner. The counterweight then impacted the number one propeller blade, of the left engine, which separated about 12 inches from the hub. Additionally, the propeller governor control arm with attached pilot valve was found separated from the propeller governor housing, and the engine cowling in the area of the propeller governor was damaged.

One-half of the propeller counterweight clamp with the top portion of one counterweight bolt still attached, was found about 3,000 feet from the approach end of the departure runway. Also found in the same area was the other counter weight clamp bolt, the separated propeller blade, and some pieces of fiberglass and metal. The second-half of the counterweight clamp with the counterweight arm, was found in a grass area about 120 feet southeast of where the other parts were found.

The propeller counterweight clamp assembly was forwarded to the National Transportation Safety Board Materials Laboratory, Washington, DC, for examination.. According to the Metallurgist's Factual Report, examination of the leading edge bolt hole in the clamp half with the counterweight arm revealed that the internal threads in this hole were stripped by motion of the leading edge bolt out of the hole. The report further stated:

"...the stripped region had a double set of thread roots, indicative of cross threading of a bolt or mistapping of the threads...The bottom of the leading edge bolt hole contained what appeared to be compacted chips of metal...X-ray energy dispersive spectroscopy of the chip area generated spectra nearly identical to spectra from the nearby cut surface...."

The propeller was manufactured by Hartzell Propeller Inc, Piqua, Ohio. It had accumulated about 2,778 flight hours since it was last overhauled, and about 8,703 total flight hours.

Review of the propeller maintenance records revealed that the propeller counterweight bolts had been replaced during an overhaul performed by the propeller manufacturer on April 12, 1995. The propeller counterweight bolts were also replaced by New England Propeller Service Inc., during maintenance on April 8, 1996, and at the last overhaul, which was performed on January 28, 1997.

According to the propeller overhaul manual, loctite retaining compound was to be applied to the bolt threads and internal clamp threads during the installation of the counterweight bolts. The overhaul manual also stated, in part:

"After removing the counterweight clamp from the blade, use an undersized tap to remove loctite residue from the threads of the D-5117-1 counterweight clamp. Visually inspect threads for damage. Cross threading, damaged or missing threads is cause for retirement of the clamp."

In addition, the overhaul manual required the counterweight clamp bolt heads to be drilled and a cotter pin installed. The counterweight bolts on the accident propeller did not contain cotter pin holes.

In a telephone interview, a representative from New England Propeller Service Inc., stated that their mechanics use a compressed air plastic bead spray to clean the clamp threads and they do not use an undersized tap.

According to the propeller manufacturer, the use of a compressed air plastic bead spray to clean the propeller counterweight clamp threads was not an approved procedure

On July 17, 1998, Hartzell Propeller Inc, issued Alert Service Bulletin (ASB) HC-ASB-61-237, which outlined procedures for a one time "on wing" inspection of the propeller to ensure proper torque and safetying of the counter weight clamp bolts, and a one time air flow inspection of the propeller counterweight clamp holes to verify thread quality. Additionally, the use of loctite retaining compound on the counterweight clamp bolts and threads was eliminated. The ASB further stated:

"Caution: Use of a tap to clean counterweight clamp threaded holes can lead to thread damage and is not permitted on assembled clamps."

On August 31, 1998, the FAA issued Airworthiness Directive 98-ANE-53-AD, which in part, required compliance with ASB HC-ASB-61-237.