NTSB Factual Report

HISTORY OF FLIGHT

On May 1, 1994, at 1220 mountain daylight time, N1905Y, a Mooney M20E, was destroyed during a forced landing in Broomfield, Colorado. The private pilot and passenger were fatally injured. Visual meteorological conditions prevailed.

Shortly after taking off from runway 11L at the Jefferson County Airport, the pilot requested clearance to make an immediate landing; he did not indicate the nature of his emergency. A witness said he heard a loud metallic noise coming from the engine as the airplane passed overhead. He said the pilot made a steep right bank in an attempt to land on runway 29L and impacted terrain.

WRECKAGE AND IMPACT INFORMATION

A ground scar was located abeam the 2,000 foot distance-to-go marker of runway 11L, between runways 11L and 11R. Just beyond this scar was a 60 foot gouge in the earth, aligned on a magnetic heading 250 degrees. At the end of this gouge was a 40 foot wide and 10 foot deep ravine. The airplane was found on the bank of the ravine on a magnetic heading of 080 degrees.

The propeller assembly remained attached to the engine but one blade was broken off. All three blades exhibited chordwise scratches on the cambered surfaces and were S-curled midspan. The empennage was severed just aft of the baggage compartment. The left wing remained attached to the fuselage but had a circumferential crack at the inboard aileron hinge point. The flap was broken off. The right wing lay next to the fuselage but was twisted 180 degrees.

MEDICAL AND PATHOLOGICAL INFORMATION

An autopsy was conducted by the Jefferson County Coroner's Office, Golden, Colorado. In addition, FAA's Civil Aeromedical Institute (CAMI) conducted a toxicology screen. Trace amounts of pseudoephedrine was detected in blood and urine, and guaiphenesin was detected in urine. Acetaminophen was detected in blood (6.900 ug/ml, ug/g) and urine (152.600 ug/ml. ug/g). According to a CAMI toxicologist, acetaminophen is found to Tylenol; pseudoephedrine is a decongestant, and guaiphenesin is an expectorant. On his most recent medical examination dated October 13, 1993, the pilot indicated he was taking medications that included acetaminophen and pseudoephedrine. The toxicologist was unable to say definitively whether any of these medications, either taken singularly or together, may have been contributory to the accident.

TESTS AND RESEARCH

Engine disassembly revealed the crankshaft had failed at the crankcheek between numbers 3 and 4 connecting rod journals. The number 4 cylinder lower right base nut and the number 3 cylinder upper left base nut were finger tight. The upper right base nuts on numbers 2 and 4 cylinders broke free approximately 400 inch-pounds. The number 1 cylinder lower right and left base nuts and the number 3 cylinder lower right base nut loosened at less than 500 inch-pounds. All other base nuts loosened at less than 350 inch-pounds. Metal shavings were found inside the left magneto mounting on the accessory case, on the oil suction screen, and in the oil sump. There were two breaks on the bottom of the crankcase below the numbers 3 and 4 cylinders. The engine received a major overhaul on January 11, 1991. Total engine time was 4,412 hours: 266 hours since major overhaul and 11 hours since a 100-hour inspection.

The crankshaft was sent to NTSB's metallurgical laboratory for analysis. According to its report, the crankshaft failed as the result of "fatigue cracking that originated from the radius on the aft end on the number 3 connecting rod journal and propagated through most of the crankcheek before final separation." There was "no evidence of metallurgical or manufacturing defects" in the crankshaft.

The airplane had been modified by two Supplemental Type Certificates (STCs). The engine was equipped with a Rayjay turbocharger (SE32WE) and a McCauley "BlackMac" 3-blade propeller (SA704NE). According to a spokesman for U.S. Propeller Service in East Haddon, Connecticut, holder of STC SA704NE, the company will not install the "BlackMac" propeller on an airplane with a turbocharged engine because certification vibratory tests were made on normally aspirated engines only. No data exists for turbocharged engines.

Maintenance records indicate that after its installation on January 17, 1991, the propeller was removed on March 28, 1991, "for blade angle check," and on March 19, 1991, for "static balance" and "re-index prop #1 blade 180 degrees." On May 23, 1991, the propeller received a "dynamic balance --- before .55 IPS, after .1 IPS."

ADDITIONAL INFORMATION

The wreckage was released to the owner's representative on May 3, 1994. The crankshaft was released on August 15, 1994.

NTSB Probable Cause

FATIGUE FAILURE OF THE ENGINE CRANKSHAFT; AND FAILURE OF THE PILOT TO MAINTAIN ADEQUATE AIRSPEED, WHILE MANEUVERING TO LAND, WHICH RESULTED IN A STALL AND COLLISION WITH THE TERRAIN. FACTORS RELATED TO THE ACCIDENT WERE: THE UNDESIRABLE ENGINE/PROPELLER VIBRATION, AND THE LOOSE CRANKCASE AND CYLINDER BASE NUTS.