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Topic-icon Cherokee Wing Spar Proposed AD: The other shoe drops

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4 months 3 weeks ago #1615

It would be great if other members of this forum could comment or provide contacts if they know about facilities that provide the proposed Eddy current testing. It appears that may be the difficult part of this AD.

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4 months 3 weeks ago #1628

Here is my take on the NPRM on the PA28/P32 bolted spar joint.
I'm a bit surprised that the FAA is not taking a more proactive posture on the spar failure that brought death to two very nice individuals. If another wing or two come off causing more fatalities, the NPRM will likely become an emergency AD. It is conceivable that this failure could have been prevented had there been some good type support. But by whom? Piper is not interested since the liability protection by the 18 year rule limits their exposure.
It is unknown what design/stress methodology Piper used when the airplane was designed. CAR 3 did not require a fatigue analysis or testing. Bolted joint analysis is actually quite complicated. The loads in the bolts are not uniform, but rather peak at the two ends of the joint, similar to a bonded joint. That is, the highest loaded bolt in the joint is the first and last bolt in the row. The bolts in the center of the joint see very little load. Today, sophisticated analysis methods have been developed and used in modern aircraft stress analysis, which were not available to Piper engineers in their time.
Since this NPRM will become an AD in time, what are the options? Replacing the spar will be an expensive solution. Who will take the role of developing an AMOC or total fix? The total fix will require extensive analysis, followed by expensive fatigue testing, likely more involved than what Piper did when the airplanes were certified.
The formula that Piper has developed to determine eligibility for the inspection is a bit bizarre. Since cracks develop inside out, I would expect that mechanics/inspectors would not have removed the required bolts to check for cracks since the bolt head would hide the start of cracking. Is there a Piper Service Letter that previously required this inspection?
To get into the ballpark, the FAA has developed software to determine the stress in the spar cap, factored by aircraft usage. Stress concentration for this configuration is 3, modified to a correction value based on bolt interaction. There are fatigue curves in MMPDS which can be used as an initial point to determine the extent of internal bearing stress. In the formula, if N = 0, then T = 85000, and TIS becomes 5000.
For the PA28/PA32 owners, lets hope that rational thinking comes to the forefront.
Hans Neubert
FAA Structures DER 23/25/27/29

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4 months 3 weeks ago #1629

I have a PA-32 301 Saratoga I just purchased and am surprised that it is not on the list as well. Does anyone know what the structural difference is that makes it fall outside the scope of the NPRM? Even though it is not included, I intend to pursue eddy current testing in the near term as it is relatively inexpensive and will provide a baseline for future inspections.

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4 months 1 week ago #1650

Here's a copy of Piper Aircraft's comment on the NPRM:
In response to your specific question, the first point of contact regarding any Piper work should be a Piper Authorized Service center. Those can be found on the Piper website: www.piper.com under the “Support” tab.

As it relates to this specific NPRM from the FAA there is no immediate regulatory or recommended action needed. The current situation is one where operators and owners of PA-28 and PA-32 aircraft are invited to comment on a proposed AD that would involve an inspection of the spar in a specific area at a particular point in time to be calculated based on the use and maintenance intervals of the aircraft. The purpose of the NPRM is to gather necessary data in order to determine a path forward for a potential AD. Again, at this point in time, there isn’t any specific inspection or maintenance required.

With that being said, it is important for you and your members to know that Piper disagrees with several aspects of the NPRM pertaining to PA-28 and PA-32 aircraft, including, 1) the proposed aircraft inspection criteria of 5,000 hours and 2) proposed aircraft model applicability.

Piper has been working closely with the FAA and NTSB in providing volumes of technical data and subject matter experts addressing how PA-28 and PA-32 aircraft not only meet, but exceed all required levels of safety.

In many ways the numbers speak for themselves.
- To date Piper aircraft has produced nearly 40,000 PA-28 aircraft of all models and nearly 7,500 PA-32 aircraft.
- Those aircraft have amassed 186 million flight hours - providing safe and efficient personal/business transportation and pilot training.
- Literally, hundreds of thousands of pilots have learned to fly in Piper PA28 aircraft since the first aircraft was produced 57 years ago.

Given the above information, we ask you and your members, specifically Piper PA-28 and PA-32 owners and operators to take advantage of the opportunity a NPRM affords and speak up, by providing written relevant data, views or arguments. Comments may be submitted, using the procedures found in 14CFR 11.43 and 11.45 by any of the following methods: Federal eRulemaking Portal: Go to www.regulations.gov . Follow the instructions for submitting comments.
- Fax: 202-493-2251
- Mail: U.S. Department of Transportation, Docket Operations, M-30 West
Building Ground Floor, Room W12-1430, 1200 New Jersey Avenue SE,
Washington, DC 20590.

You can also forward any pertinent comments and/or information directly to Piper at .

Piper will continue to work with the FAA and NTSB to ensure any future inspection criteria impacted by this NPRM is properly scoped and reflects the proven strength and integrity of the robust PA-28 and PA-32 aircraft design. In the meantime, we invite our owners to provide their input and comments.

Please let me know if you have any questions in the meantime.

Best regards,

Last edit: 3 months 2 weeks ago by Jen D. Reason: attachment not available

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4 months 1 week ago #1651

The article by Steve Ells was very interesting and highlights the whole wing spar story very well. After reviewing all the SB from Piper and my logs I decided to actively drive the eddy current inspection per FAA 2018-1046 even though my 1968 PA28R-180 has only 3300h. The reason is that the plane has 3 wing repairs after forced landings in its 50 years old history and I couldn’t ensure compliance with either 886 or 978 out of the logs. I am very happy since nothing was found during the inspection and I can continue to fly with confidence everything is ok. I am wondering that the FAA proposal does not include birds with a damage history independent from the hours, since introducing of cracks are likely to happen exactly during this kind of incidences. For sure wing repair on those low level planes are not an exaption especially in a training environmental.

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3 months 2 weeks ago #1681

Steve Ells comment on the NPRM:
Docket No.FAA-2018-1046; Product Identifier 2018-CE-049-AD

Thank you for the opportunity to comment on this proposed airworthiness directive. Based on my research, I’m going to break this response down to comment points as follows:
1. A comment involving ambiguity regarding using 100-hour inspections to define “factored service hours,” as defined in the NPRM.

2. A comment involving the necessity of incorporating the requirements of Piper Service Bulletin (SB) 1244C, dated Dec. 3, 2018, titled “Aft Wing Fitting Inspection Requirements” into the NPRM, since wear allowing movement of the wing at the aft wing attachment joint would subject the main wing spar attachment to additional stresses that are not included in the “factored service hours” formula.
a. For further information on wear in this fitting, refer to 14 CFR Part 141 “Safety Summit Maintenance” PowerPoint presentation, dated May 10, 2018 by Tony Alfaya, Program Manager, Orlando FSDO FAASTeam.

3. A comment comparing real-world labor reports to the estimate of labor man-hours to replace a wing spar in the NPRM.

Discussion No. 1
The NPRM suggests that 100-hour inspections are the “rate-determining step” in determining when an aircraft needs to be inspected. There is a loophole here.
What about the airplane that flies 300 hours a year in a flight training or for-hire operation, and its third inspection is signed off as an “annual” inspection? The way the NPRM is written, the “factored service hours” for that airplane would be off by one-third.
For example, let’s create a PA-28 that had flown 4,660 hours. If the annual were counted as a 100-hour inspection, its factored service hours would be 4,603.5. If the annual were not counted as a 100-hour inspection, the factored service hours would be 3,285—or 29 percent less.
I recommend that this ambiguity be addressed by adding the following to the AD:
“Airplanes on 100-hour inspection schedules must treat each ‘annual inspection’ signoff as a 100-hour inspection to arrive at the ‘N’ total in the ‘factored service hours’ equation.”

Discussion No. 2
The main spar of PA-28 wings supports vertical positive and negative loads; a steel stub spar that is bolted to a steel fuselage fitting (Aft Wing Spar) is designed to prevent any pivoting of the wing around the main spar.
Piper issued Service Bulletin (SB) 1244C, dated Dec. 3, 2018, titled “Aft Wing Fitting Inspection Requirements.”
This bulletin requires the inspection of the aft side of the aft wing spar for corrosion and for the correct torque on the single bolt that secures the wing stub spar and the fuselage fitting (part Nos. 62448-02 and 62448-03).
See SB 1244C, Illustration A-A on page 7 for a clear illustration of the joint.

If corrosion is found, the bulletin requires the installation of Piper Service Kit 764 998 using drawing 87584 Revision E or later. The drawing is included in the kit.
Piper Service Bulletin (SB) 789B, also dated Dec. 3, 2018, titled “Aft Inboard Wing Access Panel Retrofit and Aft Wing Spar Modification,” calls for the installation of an access panel in the bottom wing skin aft of the aft spar on certain PA-28 aircraft to inspect the aft wing spar.
Since the integrity of the steel aft spar fitting and the torque of the bolt is the subject of both bulletins, it’s obvious that Piper is greatly concerned with the integrity of the aft spar and the steel fittings that connect the aft spar and the fuselage fitting for the aft spar.
Based on this concern, I suggest that the following sentence be added to the Airworthiness Directive (AD) procedures:
“Place one hand on the aft end and one hand on the forward end of the wingtip area of the wing and apply pressure in a forward direction and then in an aft direction. If any movement is felt or observed at the wing-to-fuselage junction, inspect the main wing spar in accordance with the eddy current protocol in this AD regardless of the ‘factored service hours’ calculation.”
“If movement is observed, comply with Piper Service Bulletin SB 789B (if necessary) prior to completion of Piper SB 1244C repair before returning the airplane to service.”

Discussion No. 3
Based on conversations with A&P mechanics that have experience in conducting eddy current inspections and in changing wing spars, I believe the estimates of the man-hours required to change a wing spar is woefully inadequate.
I contacted a Non-Destructive Testing (NDT) company that has developed a bolt hole testing protocol for this NPRM and was told that it estimated that 4 man-hours were required to remove the bolts, set up and administer the test, interpret the results and complete the maintenance record and FAA report paperwork.
Wing removal estimates range from 8 to 12 hours.
According to the operators I spoke with, the 32 man-hours estimate for a wing spar replacement is low by 8 to 18 man-hours, while one shop said it would provide a bid estimate to an owner of 80 man-hours to change the inner spar of a PA-28 wing.

Respectfully submitted,
Steve Ells

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