The flight instructor told investigators that he and the student pilot had been practicing maneuvers in the Piper J-3 Cub before they proceeded toward Bremerton National Airport (KPWT) in Washington.
The student pilot performed the before-landing checklist, which included checking fuel and turning on carburetor heat, when they were about two miles from Runway 02.
Shortly after, they heard a loud noise, followed by a total loss of engine power.
The flight instructor took control of the airplane and initiated a forced landing to an open area near Belfair, Washington.
During the landing roll, the airplane hit rough and rising terrain. The left wing partially separated and landing gear collapsed, which resulted in substantial damage to the fuselage.
The flight instructor and the student pilot were seriously injured in the crash.
A post-accident examination of the airframe and engine revealed that, when the propeller was rotated, neither magneto coupling could be heard firing. The right magneto was removed, and remnants of the crankshaft accessory drive gear were observed in the accessory case. No additional, non-impact related anomalies were noted with the engine or airframe.
The engine was disassembled, and the crankshaft and the gear remnants were removed and sent to the National Transportation Safety Board Materials Laboratory for further examination.
The fracture faces of the gear contained mechanical damage. Two of the fracture faces contained fatigue striations typical of fatigue cracking that emanated from the root of the gear. The fatigue crack in these two gear fragments extended through the entire width of the gear. The terminus of each fatigue crack was not defined because of the severe mechanical damage on the fracture faces.
The engine had accumulated about 278 hours since last overhaul and about 20 hours since the last annual inspection. The total number of hours on the engine could not be determined.
Probable Cause: The total loss of engine power due to the fatigue failure of the crankshaft accessory drive gear.
To download the final report. Click here. This will trigger a PDF download to your device.
This June 2023 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
I am curious about how high they were cruising along.
If I get with in 2 miles of an airport I will be on the runway if the engine goes toes up.
read the 6120 form….
1,400 agl, 2 miles out…a Cub could make the runway, no wind….maybe.?
Good analysis!
In a proper design the dowel pins are a close/press fit with no play,they take the torque/shear loads, the flange bolts keep the gear aligned on the shaft.
Failure would occur if just bolts and no pins were used. The holes and bolts aren’t accurate enough on center. Consequently
one bolt would be carrying the shear load until it broke then the next one would carry the load til it broke and so on.
The gear tooth edges and bolt holes should have been chamferred to reduce stress risers. I’d be inclined to do this at o/h after consulting the oem.
A consideration on your bolt strength calc is that 4k psi at the 1/4″ shank or at the minor diameter of the threads?
I tend to agree with the idea that there was a problem with prior installation with the caveat that these old engines have never been 70 years old before.
The dowel pin is there primarily to get the timing marks aligned with the crank.[ 2 ‘dots’ on the gear to align with the dot on the cam gear]
Also, from the parts manual, the gear has a recess which goes over the crank end, hopefully with a tight fit. This would take a lot of any shear forces.
So, with the overhaul 270+ hours old….I will stick with an incorrect assembly of the gear to the crank. …impossible to see after the accessory case was assembled.!!
If nothing else broke inside the engine , it should easily to rebuild…again.!!
Looking at the pics in the docket…. 2 bolts found loose, obviously backed out of the crank. And, 2 of the bolts sheared at the crank flange, with part of the safety wire still attached.
My take is that the bolts were not torqued properly, and/or not safety wired correctly.
So, an error of some type in reassembling the crank gear to the crank.
Note that this gear not only drives all the accessories, but also has the starter motor drive this gear in starting the engine. So it gets substantial torque applied to those 4 little 1/4-28 screws, and the alignment dowel .!!
Thanks for your analysis, JimH. This accident started at an engineer’s drawing board when he/she specified woefully inadequate bolts to hold that gear and the duties and responsibilities it has, such as cranking plus accessories. Just shows to go: “If the ‘skeeters don’t get you, the ‘gators will” in GA airplanes.
/J
The AN-74 aircraft grade screw is more than capable of handling the shear applied to it in this application. The 1/4-28 screw has a shear strength of 96,000 psi. , or over 4,000 lb.
A rough calculation of the starter torque applied to these screws is about 70 ft-lb, which is about 800 lb force applied at the 1 inch radius of the screw location .
There is some impulse torque when the starter engages . If that is 10x the running torque, it’s still 1/2 the capabilities of the 4 screws.
There is also the shear strength of the alignment pin, to add to the shear strength.
So, with properly installed and torqued screws, it appears well designed for the application.