Okay, I guess the suspense kept everybody awake all night, so I won’t torture you anymore (oh, won’t that sentence draw some trenchcoated traffic from google--can’t wait).
The Whatziss is the Shaft Pinion and Reduction (SPR--pronounced “spur” and so I shall refer to it as such in the narrative) gear from an OH-6A.
Background
Now, because the Loach was intended for scouting in Vietnam and aeroscouts in Vietnam got shot down a lot, the aircraft was designed to be as light, as maneuverable and as inexpensive as possible. Basic flyaway cost of the ’65 model was about $12,000, which was cheap enough to put it into the “disposable” category in the Army’s mind.
Heh—the Air Force dropped *bombs* that cost more than that.
However, in addition to being light, maneuverable and inexpensive, the OH-6 was also overpowered, overbuilt and rugged as all get-out. Repairing them wasn’t always cost-effective, though, since the Hughes Tool and Die Company (don'tcha just *love* that name!) never did ramp up to producing a lot of spare parts, so, with limited production runs coupled with the usual boondoggles, it was sometimes cheaper to buy a new helicopter than repair the old one. So the Army bought a * lot * of them--over 1,400.
In 1968, the Army decided that Howard Hughes had gotten rich enough off the gummint, ceased purchases of the OH-6 and fielded Bell’s OH-58. Or, as we called it, the OH-Five-point-Eight--because it wasn’t quite as good as a Six.
Then, in 1975, the Army decided to dump allocate anything even remotely reminding it of Vietnam the remaining OH-6s to the National Guard—and for the first time ever, the Guard had a better piece of equipment than the Active Army. So, for twenty-plus more years, the little disposable helicopter soldiered on. Found a niche in the civilian world as the Hughes 500, too. But helicopters don’t age gracefully--vibration, tension, torsion and corrosion take their toll and metal fatigue inevitably sets in and weakens critical components.
The Problem
Since the shaft pinion and reduction gear was part of the mechanism that reduced 6,000 engine rpm to 497 main rotor rpm, it was a critical component. And one that the book said didn’t have to be inspected more often than every 500 flight hours. The Loach’s engine was mounted diagonally and the spur gear connected directly from the accessory gearbox to the transmission driveshaft--so in addition to vibration, tension, torsion and corrosion, the gear was also subjected to temperature extremes. But it was tough--remember what I said about the Loach being overbuilt? Unfortunately, it was only overbuilt for a “disposable” helicopter.
Enter metal fatigue. The steel crystallizes at stress points, the crystals shatter and hairline cracks develop in the shaft walls. The hairline cracks become * big * cracks and, if not discovered, become mini San Andreas faults. Here’s where the problems were developing:
Then, one day, the San Andreas goes *ka-rack!*, and you get the following:
The engine, suddenly unloaded from the task of turning the massive gears in the transmission, overspeeds and overheats, in excess of, respectively, 30,000 rpm and 1,000 degrees Fahrenheit. The pieces of spur gear start rattling around internally and the accessory gearbox eats itself alive. Soon thereafter, the fuel and oil lines surrounding and feeding the engine warp and crack due to the excessive heat and spray aromatic hydrocarbons all over. If you’re lucky, it doesn’t explode immediately, it just sort of catches fire. And, since the Falklands War pretty much established the fact that aluminum *will* burn, your fuselage joins the action, too.
Meanwhile, the transmission, since it’s not being powered anymore, decides to take a rest, and the main rotor rpm decays--rapidly. Which means, if you’re caught by surprise by the noise in back and the realization that you can now see the individual blades, you’ve got about a second to enter autorotation, retard the throttle to the idle detent, honk the nose back into a hairy assed pronounced flare to attempt to regain rotor rpm, and pray you’re directly above an open field that won’t mind hosting the burning, disabled helicopter that just dropped in unannounced.
I just hate it when that happens.
So, for about a month, us Instructor Pilot types did nothing but teach the other guys the symptoms and the cure and practice, practice, practice the emergency procedure. Finally, some bright light said, “Hey—the civilian models don’t have this problem. I wonder why?” The answer was simple. Because the Hughes Aircraft and Missile Corporation anticipated that the Hughes 500 would last a good, long while and so made the upper portion of the spur gear wall one millimeter thicker. As in, 1mm.
Great, said the Guard Bureau’s Aviation Directorate. We’ll just install the Hughes 500 spur gear in the OH-6 and everything will be hunky-dory. So Guard Bureau shelled out seven grand apiece for about four hundred civilian spur gears.
Non-milspec spur gears.
The Dénouement
You can see this coming already, can’t you? Yup. Some Log Colonel who didn’t care *why* Aviation bought the civvie gears had a fit because they were “non-standard,” snagged the shipment before it could be distributed and had a machine shop mill one millimeter from the thickness of the wall. He thereby turned the new, non-milspec, *good* gears into new, milspec *bad* gears and released them for distribution.
Without telling the Aviation guru what he’d done.
Fortunately, the Quality Control guys in hangars around CONUS noticed the shiny, freshly-milled band (Hi-Rez!), broke out the micrometers and quarantined the lot of ‘em.
The word reached the top of the heap and, that afternoon, there was a fresh scalp dangling from a Two-Star’s lodge pole.
And, since an IP never passes up a chance to add a busted part to his collection of training aids, I snagged a couple for the IP office before they got sent to the recycle bin. Casual visitors used to spot the one on my desk and ask, “Whazzat?”
So, I labeled it for their benefit.
I can get downright artistic with White-Out…
* * * * * * * * * * * *
Epilogue
From HomefrontSix:
Mac sez: The cut into the valleys is too deep. This is evident by the machining grooves that can be seen in the buttress. ie the bevel at the bottom of the top gear. If the cut was made to the proper depth, machine marks would not be seen below the bottom of the top gear. The depth of this cut has negated any additional strength that the buttress might have provided.
Heh. MacGyver twigged that it was too thin and he spotted the extraneous milling--not *perfect*, but I'd say it's close enough for an FFE. I'm quietly proud that a fellow brooding, introverted, anticipator of trouble got it before one of the buoyantly extroverted stiff-wing polishers.
And, as usual, Boq cut right to the chase.
Discoloration on The Vanes??? Why, I think that those polychromatic shades of patina confers an august je-ne-sai-quois to that paper weight ;)
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