Recently, I’ve had wing bolts on the brain. I got wind of a King Air 200 getting a five-year wing bolt inspection that didn’t end up being the routine maintenance activity it should have been. On reinstallation, the technician installed the nut backward and torqued it down. He did this on both sides and damaged both spars in the process.
I’m still in disbelief that this happened. In 50 years of maintenance on King Airs, I’ve never seen or heard of this situation. The nut on this style wing bolt (see the top bolt/nut in the photo on the opposite page) is a unique design with an unusual half-moon shape. The convex curvature of the nut nestles into a fitting on the spar (the bathtub fitting), which has a corresponding concave curvature.
I always considered this peculiar nut to be a “Murphy-proof” design. Murphy’s law says: “Anything that can go wrong, will go wrong.” In cases where there is clearly one way to do a task, it is considered Murphy-proof, or mistake-proof. Unfortunately, the mechanic in this instance had other ideas. When the factory was consulted, the damage was deemed not repairable. Both spars must be replaced.
In tension or in shear
The wing bolts for King Airs are either in tension or in shear. In the above incident, the wing bolt/nut assembly is of the in tension variety. The orientation of wing bolts in tension is perpendicular to the fuselage and parallel to the leading edge. If you grasped the head of the bolt with one hand, and the nut with your other hand and tried to pull your hands apart as if to stretch the bolt lengthwise, that would be a crude example of a bolt in tension.
Older King Airs had in tension wing bolts installed in all positions – lower forward, lower rear, upper forward and upper rear. The maintenance manual is very specific on how the nuts should be torqued because of the stress being placed upon them. The 200 manual discusses placement of the bolt and nut in detail, with diagrams. It clearly specifies that the radiused surfaces of the nut and the wing fitting should be married together.
Beginning with BB-1193, the lower forward wing bolts in King Air 200 aircraft were changed to an in shear design. A wing bolt in shear lies parallel to the fuselage and is clasped by evenly spaced, curved fingers that are mounted on the center section and on the wing. Much like a piano hinge, these fingers mesh from opposite sides; their curved shape allows for the wing bolt to slide into the middle like a hinge pin. I used to keep a collection of old wing bolts in my office. It’s easy to tell the in shear bolts because the fingers have worn a stripe on the bolt shaft at even intervals. The bottom bolt in the photo on the right is of the in shear variety.
Only the lower forward wing bolts on 200s from BB-1193 and later are the in shear style. These bolts must be replaced if removed for any reason; they are never reinstalled. So, when the five-year wing bolt inspection comes around for these model 200 aircraft, all the wing bolts are removed for inspection, but only the bolts that are in tension can be reinstalled. New in shear bolts are procured and installed in the lower forward position on both sides.
All wing bolts, whether they are in tension or in shear, are subject to periodic replacement. Wing bolts in tension, regardless of position, used to be a 15-year item, but now their replacement has been extended to 20 years. As just mentioned, if the lower forward wing bolts are in shear, they are replaced at the five-year inspection, or any time the bolt is removed for any reason.
Lubrication and inspection
Preventive maintenance is vital to all aircraft. Many, many years ago news surfaced of a King Air wing bolt that was severely corroded. The 12-month wing bolt lube was added to the inspection regimen and has proven a very effective remedy. Even though wing bolts are now made with Inconel instead of steel, lubing the wing bolts every year is a small price to pay for the peace of mind it provides.
The various King Air models have different calendar requirements for inspection and replacement of wing attachment hardware. The inspection (a three-year or five-year occurrence, depending on the model) includes not just the hardware itself, but also the bathtub fittings, counter bores and flat surfaces. It can be a dye penetrant or an eddy current inspection.
When I worked at Beech, and when I had my own shop, I always used a non-destructive testing, or NDT, specialist for these inspections. In all the wing bolt inspections I’ve supervised in my career, I never had a major anomaly found on King Air wing attachment components. In very few instances, where the NDT results were inconclusive, I replaced the bolt to be on the safe side. And there were a few cases where damage induced by improperly installed washers was found and repaired.
That said, the NDT professionals I’ve known through the years always had their share of horror stories. Whenever they find bad news it always involves an aircraft that has been sorely neglected. Not surprisingly, the price required to bring things back up to par is a hefty one. It’s the mantra of maintenance: Do it right and pay a little now or ignore it and pay a lot more later.
Spar straps
Many years ago, a wing came off a King Air 90 that had been operated in a foreign country for some time. The wing bolt – made of steel back in those days – was found to be 70% corroded. Clearly, lack of maintenance was the culprit. But not long after, a cracked wing bolt nut was found on a 200, giving rise to concerns about King Air wing attachments.
Around this time, Dave Saunders of Aviadesign (who had designed spar straps for a variety of aircraft) came up with a spar strap STC for older King Airs. It was marketed as an added safety feature and became popular enough to prompt Beech to design their own spar strap. Bear in mind, it was not required equipment, it was an option. There are plenty of vintage era King Airs still flying safely without spar straps.
Like any STC, spar straps come with Instructions for Continued Airworthiness that specify periodic inspections. From a maintenance perspective, spar straps are annoying; the Beech spar strap, in particular, is a real pain to deal with. Inspections are labor-intensive. Fortunately, there are not a lot of Beech spar straps around. If your King Air has one, you are probably acutely aware of it.
Wing bolt worries
in the past
In response to the cracked nut found on the 200 mentioned earlier, there was an Emergency Airworthiness Directive issued in 1980 that grounded all King Air 200s where they sat if they did not comply. Back then, ADs were distributed by snail mail and the time frame for compliance for this AD was extremely tight. Once that AD hit, it was a mad scramble for model 200 owners to get their King Airs to a shop in time.
Compliance required removal of the lower forward wing bolts (all in tension at that time), along with the corresponding nuts and washers, and replacing with new. There weren’t enough new parts to go around, so there was an alternate method of compliance. This involved a heat treatment (baking at 350 to 400 degrees for 23 hours), followed by a prolonged cooling process (like annealing) and then putting the parts through an NDT protocol. If they passed, they could be reinstalled.
I was at BeechWest in Van Nuys, California, at the time. In addition to my many customers flying 200s, we reached out to every 200 owner we could find within an hour’s flight to Van Nuys (KVNY). That airport was clogged with King Airs, parked nose to tail, and we worked around the clock to take care of everyone and get them back in the air.
Interestingly, I never found any problems with the hardware we removed, nor did I hear that anyone else found any problems either. The lower forward wing bolts on the 200s were changed from in tension to in shear in 1984, just a few years after that AD. Since then, King Air wing bolts have remained out of the limelight.
Wing bolt inspections should be routine
Think of how many King Airs there are in existence. They all have their wing bolts inspected every three or five years. Thankfully, I don’t hear a lot of horror stories involving improperly installed wing bolt assemblies. The unfortunate situation with the 200 is the exception and far from the rule. Like I said, this was the first time I ever heard of that wing bolt nut with the radiused shape being put in backward and torqued down.
I do know of one other instance where an improperly done wing bolt inspection damaged the spars so severely that they had to be replaced. A maintenance technician had reinstalled the hardware in the wrong order on a King Air C90. The manual illustrates exactly how the washers should be installed – we call this the hardware stack-up – some of the washers have a shouldered (rounded) edge and others are square-edged. When reinstalling the wing bolt assembly (clearly we are dealing with wing bolts in tension here), damage will result if one or more of those washers is facing the wrong way and torqued down.
There is only one correct way to arrange the hardware. In my vast experience with King Airs, I’m sure I’ve seen every wrong-way stack-up imaginable with wing bolt hardware. Sometimes there is induced damage and it’s repairable. But in the case of this C90, the damage was severe enough that the spars had to be replaced. I think Carl Davis (of Davis Aviation and King Air Nation) was able to come up with the wings from a C90 with a serial number very close to the subject aircraft. The replacement spars came out of those wings, and the problem was ultimately resolved. Like the 200, this should never have happened in the first place.
Mechanics: You need to A) PAY ATTENTION and B) READ THE MANUAL. Study it carefully. Make sure the hardware you are removing was properly installed to begin with. Do not assume the guy before you did the job correctly. You don’t want to perpetuate a mistake made previously. If you ensure that everything you are taking apart was correctly installed, then there will be no hiccups on reinstallation.
Reputation for safety
Safety has long been a hallmark of the Beechcraft King Air. The King Air’s safety record was a key selling point in 1980, and it remains so to this day. Proper and prudent operation coupled with competent and conscientious maintenance is a winning combination that will keep any King Air flying for many more years to come.
As always, fly safely!