FTW99FA132

While in level cruise flight, the propeller separated from the airplane. The pilot made a forced landing at the Claremore Municipal Airport. During the approach, the airplane porpoised, so the pilot elected to touch down in a grassy area between the runway and ramp area. During landing, the nose gear collapsed and the nose wheel separated from the airplane. Examination of the propeller attaching bolts revealed that they failed in fatigue. Examination of the aft spinner bulkhead revealed that crescent shaped arcs had been punched out, elongating the bulkhead's bolt holes. According to FAA service difficulty reports and reports from mechanics, the potential exists for the aft spinner bulkhead to slide off of the crankshaft flange bushings and rest on the propeller bolts during installation. The mechanic will then torque the propeller bolts with the aft spinner bulkhead pinched between the propeller spacer and the bushings. Over time the pinched sections of aft spinner bulkhead fail, leaving the propeller loose.

History of flight

On May 8, 1999, at 0830 central daylight time, a Grumman American AA-5B airplane, N4546J, was substantially damaged during a forced landing following the separation of the propeller while in cruise flight near Claremore, Oklahoma. The commercial rated pilot, sole occupant and owner/operator of the airplane, was not injured. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 personal flight. The flight originated from the pilot's private grass strip near Ketchum, Oklahoma, at 0815, and was destined for the Oklahoma City Downtown Airport.

During an interview conducted by the NTSB investigator-in-charge (IIC), the pilot stated that the airplane was in cruise flight at 2,500 feet msl, when he noticed a vibration. According to the pilot, the vibration became more violent, and subsequently, the propeller departed the aircraft's engine. The pilot initiated an emergency landing to a field; however, noticing that his glide ratio had improved without the propeller, he decided to land at the Claremore Municipal Airport. The pilot stated that during the approach, the airplane started to porpoise, so he elected to touch down in the grassy area west of runway 35 to absorb some of the landing energy.

During the landing, the nose landing gear collapsed and the nose wheel separated from the aircraft. The airplane slid across the runway and came to rest between two runway lights.

Personnel information

The instrument rated commercial pilot had accumulated approximately 8,000 total flight hours. The pilot was rated in single-engine and multiengine land airplanes, single-engine sea airplanes, and was also a certificated flight instructor. The pilot was issued a second class medical certificate with no limitations on December 23, 1998.

Aircraft information

The single-engine, low-wing Grumman American AA-5B airplane (serial number AA5B-1283) was manufactured in 1979, by Gulfstream American Corporation. A Lycoming O-360-A4K engine (serial number L-27139-36A) was installed in the airplane during manufacture. According to the aircraft maintenance records, the last annual inspection was completed on May 8, 1998, at an aircraft total time of 1,734.1 hours. The airplane was originally equipped with a McCauley propeller, model 1A170FFA7563. With the McCauley propeller installed, a caution range was placed on engine RPM during descents to prevent hazardous vibrations induced by the harmonic frequencies from the engine and propeller combination. On July 15, 1983, the registered owner purchased a Supplemental Type Certificate (STC-SA1195NW) from Fletcher Aviation, Inc., of Houston, Texas, to replace the McCauley propeller with a Sensenich propeller and an SK 143-2 Spinner Kit. According to the FAA and the STC owner, with the Sensenich propeller installed, the engine RPM limitation does not apply.

On August 5, 1995, N4546J was involved in an accident, where it received damage to the propeller. On October 27, 1995, a new Sensenich propeller, model 76EM8S10-0-63 (serial number 30328K), with the original SK 143-2 Spinner Kit, was installed by Fletcher Aviation, Inc., at an aircraft total time of 1,583 hours. The STC section titled "Detailed Installation Instructions" states, in part: "2. Install Sensenich, 76EM8S10-0-60 through -65, propeller and SK143-2 spinner assembly IAW installation drawing No. AM130-ID-1 and Gulfstream American service manual."

The Gulfstream American AA-5 Series Maintenance Manual, under the section titled "Propeller Assembly-Maintenance Practices," subsection D, step 3, reads: "Install aft bulkhead assembly (10) on starter ring gear (11) and use a temporary method of securing the bulkhead in place. (One temporary method is to tape the bulkhead to the nose cowl.)." There is also a caution following this step, which states: "THE AFT BULKHEAD CAN BE DAMAGED IF NOT HELD SECURELY IN PLACE DURING PROPELLER INSTALLATION. REMOVE TEMPORARY SECURING MATERIAL AFTER PROPELLER MOUNT BOLTS ARE TORQUED."

There are two STCs available for the installation of the Sensenich propeller on the AA-5B airplanes. STC-SA3326NM, which was not purchased for the accident airplane, under "Detailed Installation Instructions" states, in part:

2. Install Sensenich propeller/spacer assy. using bolts supplied with new propeller (Sensenich bolt kit P/N 76EM8S10) and spinner assembly, in accordance with installation Drawing No. KD110-ID-1. USE EXTREME CAUTION IN PLACING THE BULKHEAD OVER THE PROPELLER DRIVE BUSHINGS (in crankshaft flange) AND WHILE SEATING THE PROPELLER!!

SPECIAL NOTE: It is very easy for the spinner aft bulkhead to slip off the drive bushing shoulders and rest on the propeller bolts when the propeller spacer is seated before tightening the bolts. Should this happen, the bulkhead holes will be deformed and prevent proper center alignment. IF ALL HOLES ARE DAMAGED IT MAY BE UNAIRWORTHY AND SHOULD BE REPLACED! (If re-using an SK143-2 spinner assy. the bulkhead should be inspected for this and replaced if determined to not be airworthy.)

To prevent damage during installation, one person should hold the back plate in position while another seats the propeller spacer over the drive bushing shoulders and installs two of the propeller bolts. When only two bolts have been 'snug-tightened' the propeller should be then pulled through by hand, or spun with the starter WITH MAGS OFF, to see if the spinner is rotating evenly in relation to the front cowling. If there is any doubt the bulkhead is properly centered the propeller should be removed and the aft bulkhead holes inspected.

One option may be elected, which involves an added expense but may aid ease of installation and prevent damage to the bulkhead during later re-installation of the propeller, is detailed in the 'Options' section of these installation instructions. If it is to be done, parts should be obtained prior to beginning installation since they are not usually found in stock at most Lycoming Distributors and may be back-ordered.

The "Options" section of STC-SA3326NM states, in part:

A. Replacement of Propeller Drive Bushings: When the propeller is removed, it will be noticed that two of the six drive bushings are shorter than the other four. These may be replaced with Lycoming Part Number 74248 (1ea. Indexing), & 74249 (1ea.) which are longer than the other four originals. This will help hold the aft bulkhead in proper position while the prop is being installed and lessen the chance of it being damaged. If this option is chosen the replacement should be done per Lycoming instructions by qualified personnel only. This change must be recorded in the aircraft engine log.

According to Lycoming Service Instruction No. 1098F dated May 22, 1995, the crankshaft flange is allowed a thickness of 0.38 inches +/-.010 inches. The shank length for the two "shorter bushings" is 0.59 inches (as measured from the forward face to the point corresponding to the aft face of the crankshaft flange). The shank length of the other four bushings is 0.78 inches. With the bushings installed in the crankshaft flange, the two "short bushings" protrude 0.21 inches +/-.010 inches from the front of the flange. The other four bushings protrude from the flange 0.40 inches +/-.010 inches. With the starter ring gear (which measures 0.25 inches thick) installed, approximately 0.150 inches of bushing protrudes from the starter ring gear on four of the six bushings. The two "short bushings" do not protrude from the starter ring gear. Additionally, the forward face of all the bushings are manufactured with break on the outer edge, which decreases the level area by an additional 0.03 inches, leaving 0.120 inches to support the aft spinner bulkhead