FTW96FA009

During flight the propeller failed shedding one blade approximately 15 inches from the hub. The aircraft was observed to pitch up and enter a flat spin to impact. The propeller installed on the aircraft was a cut-down metal propeller, and the aircraft specifications required that only wood propellers be used. The logs indicated a wood propeller was installed on the aircraft and no history of the metal propeller was found.

History of flight

On October 7, 1995, at 1250 mountain daylight time, a Rupe RV-4 homebuilt, N61TR, impacted near mid field at Vance Brand Airport, Longmont, Colorado. The pilot received fatal injuries and the aircraft was destroyed by impact and postimpact fire. Visual meteorological conditions prevailed for this personal flight operating under Title 14 CFR Part 91 and no flight plan was filed. The aircraft departed Vance Brand Airport at 1215.

According to witnesses, the aircraft was passing over the airport from north to south at mid field about 800 feet above the ground and was seen to be trailing white smoke. Several witnesses described hearing a "popping" noise and saw the aircraft pitch up and then enter a left turn flat spin to the ground, impacting beside runway 11/29 about mid field facing opposite its original direction of travel. Witnesses stated the aircraft began to burn shortly after impact.

Personnel information

Pilot logbooks were retrieved from the aircraft wreckage but were damaged and no useful information was derived from them. According to available information, the pilot was, at one time, a pilot for the U. S. Air Force and stopped flying in about 1957. The pilot started flying the accident aircraft in 1993 and the aircraft logs provided information that he flew approximately 227 hours in the aircraft after he purchased it from the builder. Other flight time accumulated by the pilot is unknown.

Aircraft information

The RV-4 is an all metal low wing monoplane powered by a reciprocating engine in the 150 to 180 shaft horsepower range. Specifications are attached and the aircraft construction manual provided information that a wood propeller was standard.

The RV construction manual states the following concerning the use of metal propellers:

"Ideally, a fixed pitch metal prop would have some advantages over a fixed pitch wood prop. But all of the fixed pitch metal props manufactured were designed for much slower airplanes so their pitch angles are much too low to be effective at the RV-4's cruise speed. While it is true that metal props can be re-pitched for more or less speed, they cannot be re-pitched to the extent needed for the RV-4.

In addition to a loss of efficiency incurred through excessive re-pitching, the stress on the metal renders them unsafe for use. The stresses on metal props are very involved, and the most insidious of these are harmonic vibration stresses which are not detectable through vibrations which can be felt by the pilot. Extensively reworked metal props have a history of losing sections of their blades which can (and have, with fatal results) easily result in catastrophic imbalances capable of tearing the engine completely free of the mount and airframe. This is but a superficial explanation, it gives some idea why fixed pitch metal props were not considered suitable for use on the RV-4."

Attached are additional excerpts from the RV-4 construction manual and portions of RV newsletters that address metal vs wood propellers.

Wreckage and impact information

The aircraft was examined where it came to rest on the north side of runway 11/29 at Vance Brand Airport, Longmont, Colorado. The wreckage was aligned on a magnetic heading of 330 degrees and the remains of the left wing extended onto the runway approximately five feet. No evidence of a horizontal ground scar was found. Fire scorch marks extended from the wreckage on a heading of 300 degrees magnetic for approximately 50 feet.

The left wing was intact with melting of the wing from the root extending outward for three and one half feet. The aileron was attached, the flap was down and unrestrained. Control continuity through the wing was confirmed.

The empennage, horizontal stabilizer and vertical stabilizer were intact. The outer surface was scorched but the inner surface was not. All control surfaces were attached and control continuity was confirmed.

The right wing was attached at the root with fire damage and melting at the root leading edge. The aileron and flap were attached and the flap was down resting on the ground and unrestrained.

The engine remained attached to the engine mounts. Fire and impact damage were present. The mounts exhibited torsional deformation with cracks in all mounts near the fire wall mounting points.

Detailed examination of the engine was conducted on September 10, 1995. Continuity through the engine was present.

The accessories were all accounted for but could not be tested due to impact and fire damage.

Internal examination provided no evidence of preimpact failure or malfunction.

The propeller was attached to the engine. One blade was fractured approximately 15 inches from the hub. The fracture surface was at a 90 degree angle across the blade span. The outer portion of the blade was not recovered. The propeller was an all metal McCauley design number 1C160 DTM, S/N 82576. The intact blade measured 34 inches from the center of the hub to the tip.

An interview was conducted with a mechanic who had balanced a metal propeller blade for this aircraft on March 31, 1994. No evidence was found as to whether this propeller was the one the mechanic had balanced. The mechanic did state during the interview that it was his understanding that there had been a propeller strike on this aircraft but he could not identify the date or what type of propeller was on the aircraft at the time. The mechanic's balancing work sheet is attached.

Medical and pathological information

An autopsy on the pilot was conducted by the Boulder County Colorado Medical Examiner's Office. According to the medical examiner, no evidence was found during their examination that would indicate the possibility of a fire prior to impact.

Tests and research

The propeller and propeller hub spacer were sent to the Safety Board Materials Laboratory for further examination. The results of the examination are attached and provide evidence that the propeller had been cut down and re-pitched. In addition, analysis of the blade fracture area provided evidence that one propeller blade had failed in fatigue.

A copy of the propeller section of the aircraft log is attached. The log provided no record of the metal propeller installation and investigation provided no information as to the history of the propeller or when it was installed on the aircraft.

Additional information

The wreckage was released at the accident scene to the owner's representative, Beegles Aircraft Services, Greeley, Colorado, on September 11, 1995. The propeller and propeller hub spacer were retained for further evaluation. These parts were returned to Beegles following examination.