Accident prevention is a big part of most pilots training. You learn whats most likely to go wrong, and then you try to figure out how to handle it with the highest probability of a safe landing.
Weather briefings, preflight inspections, recurrent training, annual inspections, proper maintenance and pilot proficiency are all important pieces in the risk management puzzle. Without all of them, the picture is ruined.
And so it was on one August morning that a Grumman Tiger was flying VFR from a private strip near Ketchum, Okla., to Oklahoma City. Skies were clear, the wind was calm and visibility was reported at 10 miles.
At the controls, and alone in the airplane, was a 66-year-old, 8,000-hour pilot with a commercial license and a host of ratings, including CFII. A few months before he had been issued a second-class medical with no limitations.
As the pilot cruised at 2,500 feet, he later said he noticed a vibration that appeared to come from the engine compartment. The pilot had not been in the air long. He was barely 40 miles from where he started.
The vibrations became more pronounced and suddenly the propeller assembly was flung from the airplane. The pilot began to maneuver for a deadstick landing to a field, but noted the airplanes glide ratio was so improved without the propeller that he decided he could make it to nearby Claremore Municipal Airport, just northeast of Tulsa.
As he slowed on the approach, however, he noted that the airplane was not as stable without the propeller. The Tiger began to oscillate in pitch, and the pilot decided to touch down on the grass next to the runway instead of aiming for the pavement. As the airplane touched down, the nose gear sheared, the airplane slid across the runway and came to rest between two runway lights.
The pilot, a U.S. Senator, was uninjured. The same could not be said for the airplane. The firewall was buckled, the landing gear attach points were structurally damaged and the wing roots showed evidence of structural damage.
The entire propeller assembly was found in one piece about three miles west of the airport. The assembly consisted of the spinner cone, the forward bulkhead, the fixed-pitch propeller, a spacer, and the aft bulkhead. The blades were damaged by contact with the airplane as the assembly was flung off and by the impact with the ground. But investigators found other damage that was much more interesting.
The airplane had originally been equipped with a McCauley propeller. Because of harmonics between the engine and propeller in this installation, a caution power range had been established for descents to prevent damage. In 1983, a Sensenich propeller was installed under a supplemental type certificate, which removed the engine RPM limitation.
That propeller was damaged in a runway overrun accident in 1995 and a new propeller was installed. The airplane had flown just over 200 hours with the new prop.
When the propeller was examined, investigators determined that one of the propeller bolts had sheared, flinging the bolt head through the spinner dome. Then all of the other bolts sheared, sending the prop on a flight path of its own. Some of the safety wire was still in place, but the threaded portions of each of the six propeller bolts were still firmly in place in the engine crankshaft.
A metallurgical analysis of the bolts concluded that they were not defective. The hunt for the culprit went on. An intake valve for the No. 1 cylinder had also been destroyed and ingested into the engine.
One potential scenario was that the intake valve on the 1,800-hour engine let go, starting a vibration that was amplified somehow by the propeller installation until the propeller assembly failed. Its also possible that the vibration caused by the propeller failure trashed the engine. Regardless of which came first, the propeller assembly was clearly a chief suspect.
Investigators found that the propeller bolt holes in the aft bulkhead featured sheared-off arcs that elongated the holes. Two of the holes were not elongated. They corresponded to places where the crankshaft flange bushings did not extend through the starter ring gear.
In examining the installation instructions for the propeller STC, it became obvious that the propeller had been incorrectly installed. The aft bulkhead holes are supposed to line up with the bushings so that, during installation, the bushings extend through the bolt holes. Because the bushings are fairly short, however, its easy for the bulkhead to slip off and rest on the bolts themselves.
When the bolts are tightened, the spacer pinches the bulkhead, punching out part of the bulkhead and elongating the hole. If the error is not noticed, the entire propeller assembly ends up being a bit loose.
The installation instructions for the STC installed and another STC for the same propeller on the same airplane contain numerous cautions that the bulkhead should be taped or held in place while the rest of the assembly is installed.
Investigators contacted the holder of the competing STC, who said he had performed hundreds of aft spinner bulkheads over the years, regardless of whether the airplane was equipped with the original propeller or one of the STCs. He estimated that about 40 percent of Tiger propellers are loose because of the installation error. He also said the error could be minimized by installing longer bushings in two spots to allow the aft bulkhead to rest in place more easily while the installation was completed.
Although the problem is well-known to mechanics and the American Yankee Association, the owners group for the type, it has been reported on the FAAs Service Difficulty Reporting System only twice. That leaves mechanics who are without the knowledge unable to determine if theres a problem they should look out for.
Certainly the accident outlines the necessity to be familiar with the idiosyncracies of your airplane. This Tigers propeller had been installed for five years and had been through four annual inspections. Although it didnt fly much, averaging only 40 hours a year for the past few years, it had certainly been through a few preflight inspections – or it should have been.
During that time, no one thought to ask if this common problem was one to investigate. No one apparently even grabbed the prop and gave the airplane a good shake.
Accidents traced to mechanical causes are fairly rare in general aviation. In 1998, there were only 236, with only 19 resulting in fatalities, according to AOPAs Nall Report.
The major mechanical issues appear to be solved, leaving us dealing with nuances.
This accident points out a couple more insidious problems that affect all airplanes.
Foremost is that mechanics often dont follow instructions. Perhaps they subscribe to the adage that instructions are for people who cant figure it out on their own. Perhaps the shops dont make installation maintenance instructions available or discourage their use to save time.
Of course, not all shops can inventory all instruction manuals for all airplanes, much less all STCs for those airplanes. It then falls on the airplane owner to make sure the STC instructions follow the airplane around – whether its from shop to shop or from owner to owner.
Most pilots like to think theres something special about their airplane. While there may be some pride in knowing thats true, it can also be a maintenance headache waiting to happen somewhere down the line.
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-by Ken Ibold
