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NPRM FAA-2017-1059 – Checking for Main Wing Spar Corrosion in Cherokees 

NPRM FAA-2017-1059 – Checking for Main Wing Spar Corrosion in Cherokees 

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Over 10,000 Piper PA-28 and PA-32 series aircraft will be affected by a proposed Airworthiness Directive requiring inspection of the main wing spar for corrosion. Associate editor SCOTT KINNEY decides to act now to inspect his Cherokee and secure his peace of mind.

 

One of the benefits of hanging around type-specific flying forums on the internet is that you’ll often get wind of FAA Airworthiness Directives (ADs) before they’re made public. I’d chanced across just such a post on Nov. 6, 2017.

The poster claimed that Piper Service Bulletin (SB) No. 1304 was about to become an AD, affecting thousands of aircraft. SB 1304 mandates a “thorough one-time inspection of the wing root area for corrosion” and lays out steps to be taken if corrosion is found. To perform the inspection, an access panel must be installed in each wing if one does not already exist.

Sure enough, the next morning, my email inbox contained a confirmation of the rumor: the FAA was moving to adopt SB 1304 as an AD after a 45-day comment period. (See page 60 of this issue. —Ed.) The Notice of Proposed Rulemaking (NPRM) states that compliance will be required within the next 100 hours or 12 months’ time in service from the date of the AD.

Summary: 11,476 PA-28 aircraft owners will soon be getting potentially expensive news.

I’m one of them.

My 1963 Piper PA-28-180 Cherokee 180, N7294W, has served me well for the past few years. As with many older birds, she has a few negatives in the logs. Four Whiskey lived her first several years near the beach in Southern California. I’d guess she was parked outside too, as the logbooks show a few corrosion repairs in the mid-1970s. Since then, she’s been primarily a high desert airplane.


Why not wait?

I decided to move forward with the inspection right away. I suppose I could’ve held off until my July annual and/or until the AD wording was finalized. It may be that the final AD has other accepted methods for inspection (borescope?) that don’t require cutting large access panels in the lower wing skins.

However, I had never seen the spars on N7294W with my own eyes. I am not sure I would’ve been happy flying the airplane for many more hours knowing the potential consequences of wing spar corrosion. November is also a good time of year to get work done on an aircraft in Oregon—it’s not flying season.

And I kept coming back to the reasons behind the proposed AD. A failed main spar means that your wings may fall off. In my book, that’s a very bad day.


Checking for access panels

The first step was to check for existing access panels. I thought that perhaps the panels could have been installed as part of some of the previous repair work. Since I was at home and had the aircraft logs on hand, I checked for any mention of SB 1304, SB 1244B or SB 789A (the latter two Service Bulletins also recommend addition of the access panel kit). Nothing.

The previous owner did pull the fuel tanks about seven years ago to check for spar corrosion (Piper SB 1006), but that’s further outboard on the wing. I went to the airport and crawled under the wing. Maybe the work hadn’t been logged and the panels were already in place.

No joy. I fired up Google and went parts shopping.


Finding parts

The NPRM estimates the cost of Piper’s 765-106V kit “that contains provisions to install inspections access panels on both wings” at $175. I lucked out and found a new old stock kit for a little less than that.

Since the announcement on Nov. 7, 2017, these kits have gotten increasingly difficult to find. Many vendors sold out of 765-106V in the first two days after the announcement, though they have since restocked.

Current street price for P/N 765-106V is between $200 and $250; slightly higher than the $175 estimated in the NPRM. As of early December 2017, PFA supporter AirWard shows a dozen kits in stock at a price of $229; a Google search for “Piper 765-106V” will give the most current situation. I would expect these kits to become increasingly rare or backordered immediately after the final AD is announced.


Installing the access panels

I contacted PFA member Tony Hann at Infinite Air Center in Albany, Oregon to schedule the work. Tony and his lead A&P/IA, Robert Lind, operate several PA-28s out of Albany Municipal Airport (S12). Robert has been working with Piper aircraft for more than 30 years and their shop is just a short hop from my home base.

Once I had my parts in hand, I braved the stormy mid-November weather and flew Four Whiskey up to Albany in what I’ll generously call “imperfect VFR conditions.”


Nuts ‘n bolts

Robert, Tony and I unpacked the kit’s contents onto the wing of the aircraft.

They’d ordered a few kits from Aviall to service their PA-28s. We compared the Aviall kits with my kit from Piper. My kit—dated 1987—matched up parts-wise, meaning Piper hasn’t changed the kit contents in 30 years.

The kit consists of two reinforcing doubler plates and two inspection covers. There are also 40 AN426AD4-4 rivets, used to affix the plates to the lower wing skin and 16 MS24693-S48 machine screws for attachment of the inspection covers to the doubler plates.

The Piper instructions are skimpy, to say the least, and leave some room for imagination (or improvisation?):

1. Skin cutout to be located midway between ribs and midway between the main spar and stringer as shown in
Figure 1 (Sheet 4).

2. Locate and install doubler 38571-02 as shown and attach to skin using [P/N] 420 722 rivets. Dimple for C/S rivets.

3. Cover 38572-02 can be installed/removed as required, using [P/N] 414 761 fasteners.

Other vendors have been kind enough to include more detailed instructions and a tool list. I’ve seen the documentation AirWard supplies with its kit, and it’s a very helpful supplement.


Positioning the inspection panels

The Piper instructions that came with my kit, those in the new Aviall kits and the drawings in SB 1304 all specify slightly different placements of the access panel in relation to the main spar, ribs and stringers.

After some deliberation over the instructions, Robert, Tony and I positioned the cover and used it as a template to define the cutout area.

We marked the hole as specified in the new Piper instructions and SB 1304—approximately 2 inches aft of the main spar rivet line and 3 inches outboard of the rib at WS 24.240. The long and short of it is that you want to leave sufficient space on every side of the access hole to be able to rivet the doubler in place without getting too close to the spar or ribs.

It’s also important to understand that this is a recessed access plate; it’s different from those further out on the wing. Those are attached to the outside of the lower wing skin. When finished, the new inboard inspection cover will be flush with the wing skin.


Cutting access holes

Out came the power tools. I closed my eyes and turned the other way as Robert began the surgery. He drilled a 1-inch pilot hole with a step drill to provide a starting point.

For the primary cut in the skin, Robert chose a Dremel-like rotary tool with a fine tungsten carbide cutting bit. Smart choice. It allowed him to make a smooth radius cut in the thin aluminum skin.

It was helpful to have two sets of hands to finish the cuts—Robert on the Dremel and me holding the cutout piece in place to ensure it wouldn’t prematurely depart the wing. Wear eye/face protection and appropriate clothing when working with the Dremel as the hot aluminum shards fall straight down.

I cleaned up the edges with a half-round file while Robert moved on to the other wing and repeated the process. I held off from peeking inside until we were done cutting the panels.


The inspection

With the holes cut, it was time for the moment of truth. Robert asked, “Do you want to do the honors?” I meekly replied, “Uhh, I guess.” If the spars showed significant corrosion, it likely meant a repair bill of several thousand dollars.

I grabbed a flashlight and inspection mirror and rolled back under the right wing on a mechanic’s creeper. I poked the mirror up into the hole.

Oh, thank God. My wings will not fall off.

The main spar looked pristine. The aft spar was excellent as well. The WS 24.240 (inboard of the access panel) and 36.920 rib (outboard of the panel) showed some oxidation and very minor surface corrosion. Four Whiskey’s main spars had been treated with chromate at the factory, but the ribs hadn’t, so the corrosion on the ribs was no surprise.

Robert took a look and confirmed my initial thoughts. “That’s real clean. Great news!” The left wing looked the same.

It wasn’t all sunshine and rainbows, though. The inspection panel in the right wing allowed me to see the underside of the wing walk skin. A few minor cracks had developed in the reinforcing louvers—a common problem with PA-28s. I have felt a slight bit of oil-canning in the wing walk in the past, so I wasn’t shocked by the finding. Such is life with an old aircraft; one more thing to fix.

Cleaning and priming

Robert and I cleaned the interior of the inspection area with a degreaser spray per Part I, Step 3 of SB 1304. After 50 years, the wings had an impressive collection of dead bugs and grime. We reinspected the spar after cleaning and found no corrosion.

SB 1304 states that if corrosion is found in the main spar area, it must first be removed per FAA Advisory Circular AC 43.13-1B, Chapter 6. The affected areas then must be measured for minimum thickness. It is not possible to directly measure all dimensions, so nondestructive methods (ultrasound, eddy current, etc.) may need to be used.

If the thickness of the parts is greater than the limits specified in SB 1304 Part I, Step 5, the areas can be epoxy primed and the aircraft returned to service. The SB contains a list of approved epoxy primers.

If the thickness is below minimums, an FAA-approved structural repair must be performed. This is likely to be an expensive proposition.

We chose to clean and apply epoxy primer to the ribs to ensure no further corrosion on these surfaces. Though this action is not required by SB 1304, it made sense to do with the aircraft already opened up.


Affixing doublers and buttoning up

After the inspection and corrosion mediation steps were complete, Robert went to work on affixing the doublers. Riveting isn’t my strong suit, so I played the role of gofer.

Each doubler plate required 20 countersunk rivets. The rivets are equally spaced around the doubler plate, approximately 5/16 of an inch outside the cutout. Robert used a drawing compass, a slide rule and some mechanic’s magic to get the spacing right. The AirWard instructions contain an error here. They give a layout scheme for 24 rivets per plate, not the 20 per plate that is specified in the Piper documents. They are otherwise really helpful.

Drilling the holes for the rivets is a six-step process. First, Robert clamped the doubler in place. Next, he drilled 1/16-inch holes through the skin and doubler. He then enlarged the holes to 1/8 inch.

Once the holes were drilled, he removed the doubler and deburred the holes. The fifth step was to dimple the holes with a rivet squeezer and appropriate die. Finally, he used Clecos to hold everything in place while he set the rivets into the skin and doubler with the squeezer. The right tools made this job go quickly.

When he finished riveting, Robert made an entry in the logs noting compliance with SB 1304. All that was left was to install each cover with the eight machine screws. I managed this on my own. Four Whiskey needs a bit of paint touch-up in other spots, so I plan to paint the covers and rivet heads later on this winter as a part of that project.


Final thoughts

It took about eight hours of work for Robert to install the panels, clean the interior of the wing and perform the inspection. The NPRM estimates six hours’ labor for the panel installation and two hours for the inspection. For obvious reasons, the NPRM does not estimate labor time or parts costs for corrective actions, as these may range from a small area of sanding/priming all the way up to spar replacement. Nor does it account for cosmetics. Paint touch-up may take additional time.

I’d encourage those owners whose aircraft are affected to consider complying sooner rather than later. It may be that your aircraft already has the access panels, in which case it’s a quick inspection. Even if your aircraft doesn’t have the panels, the installation and subsequent inspection is time-consuming, but isn’t particularly complicated.

Installation of the panels can facilitate later inspections required by this or other ADs or SBs. Additionally, you’ll have better access to the inboard areas of the wing for future upgrades (pulling wires) or repairs (the wing walk comes to mind).

Now that I’ve seen the clean spar with my own eyes, I’m 100 percent confident that I have structurally sound wings holding me up in the air. It’s hard to put a price on that feeling of security.


Scott Kinney is a self-described aviation geek (#avgeek), private pilot and instructor (CFI-Sport, AGI). He is associate editor for Piper Flyer. Scott and his partner Julia are based in Eugene, Oregon. They are often found buzzing around the West in their Cherokee 180. Send questions or comments to .


RESOURCES >>>>>

CERTIFIED AIRCRAFT MAINTENANCE

Infinite Air Center
infiniteaircenter.com

 

FURTHER READING

Piper Service Bulletin No. 1304
“Main Wing Spar Inspection,”
published Aug. 23, 2017
https://s3.amazonaws.com/pipercrm/Solution/18990/SB_1304.pdf

 

Piper Service Bulletin No. 1244B

“Aft Wing Attach Fitting Inspection Requirements,” published Oct. 29, 2015

https://s3.amazonaws.com/pipercrm/Solution/18671/SB%201244B.pdf

 

Piper Service Bulletin No. 789A

“Aft Inboard Wing Access Panel Retrofit and Aft Wing Spar Modification”

published May 7, 1985
https://s3.amazonaws.com/pipercrm/Solution/17459/SB%20789A.pdf

Notice of Proposed Rulemaking (NPRM)

Docket No. FAA-2017-1059; Product Identifier 2017-CE-035-AD

https://www.federalregister.gov/documents/2017/11/07/2017-24083/airworthiness-directives-piper-aircraft-inc-airplanes (See page 60 of this issue. Comments closed Dec. 22, 2017. —Ed)


INSPECTION ACCESS HOLE KIT,
P/N 765-106V – VENDORS

AirWard, Inc.
– PFA supporter

airward.com/amelia/search.asp?store=airward&action=Search&ShowDetails=True&ShowImages=True&cat=10000368&subcat_12=10000368

 

Aviall

aviall.com/aviallstorefront/p/765-106=PI

 

Chaparral Parts

chaparralparts.aero/765-106-kit-inspection-Piper-Aircraft-NEW-p/765-106.htm

 

SkyGeek.com

 

 

skygeek.com/piper-765-106-kit-inspection-access-hole.html


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