01/15/21 Editor's note: The final Airworthiness Directive has been issued and differs from this proposed version. 
A proposed AD requires an inspection of the lower wing spar cap on airframes with high-load or unknown usage history (as determined by a formula). STEVE ELLS shows you how to calculate your airplane’s “factored service history” and details the compliance steps and costs involved.

Dec. 21, 2018, the Federal Aviation Administration published a Notice of Proposed Rulemaking (NPRM), to define the proposed protocol for an inspection process to address the possibility of cracks in the lower wing spar cap of Piper PA-28 and PA-32 series airplanes.

After the crash of an Embry-Riddle Aeronautical University (ERAU) Piper PA-28R Arrow due to a wing separation on April 4, 2018, I researched and wrote a story about the accident, and looked back at the history of PA-28 and PA-32 wing cracks. The story appeared in the July 2018 issue oraf Piper Flyer. (See Resources for more information. —Ed.)

The importance of this proposed eddy current inspection is detailed in this sentence from the NPRM:

We are issuing this AD to detect and correct fatigue cracks in the lower main wing spar cap bolt holes. The unsafe condition, if not addressed, could result in the wing separating from the fuselage in flight.

The NPRM process

In my experience, the FAA often issues important NPRMs and Airworthiness Directives (ADs) just before a long weekend. This NPRM, 2018-CE-049-AD, was published Friday, Dec. 21, 2018. (See “Aviation Safety Alerts” on page Page 54 of this issue. —Ed.)

The NPRM proposal specifies that the AD will apply to the following Piper single-engine aircraft:

Model PA-28-140, PA-28-150, PA-28-151, PA-28-160, PA-28-161, PA-28-180,

PA-28-181, PA-28-235, PA-28R-180, PA-28R-200, PA-28R-201, PA-28R-201T, PA-28RT-201, PA-28RT-201T, PA-32-260, and PA-32-300 airplanes.

An NPRM is a preview of a proposed AD. The NPRM is an opportunity for owners, operators and other interested parties to respond to the proposal with comments, corrections and suggestions. 

The comments must have depth, breadth and be constructive. It’s important that the comments and corrections be based in experience and be factual. Comments that amount to nothing more than raging about cost or how the AD will decimate the fleet are of scant value. 

The comment period is 45 days from the date of issuance. Feb. 4, 2019, is the end of the comment period for 2018-CE-049-AD. 

“Factored service hours”

This proposed AD is unusual in that it requires owners and technicians to calculate “factored service hours.” The NPRM says:

This proposed AD would require calculating the factored service hours for each main wing spar to determine when an inspection is required, inspecting the lower main wing spar bolt holes for cracks, and replacing any cracked main wing spar.

The NPRM cites the discovery of a crack in the lower wing spar cap of a Piper PA-28R-201 as the reason for the proposal. It goes on to say:

An investigation revealed that repeated high-load operating conditions accelerated the fatigue crack growth in the lower main wing spar cap. In addition, because of the structural configuration of the wing assembly, the cracked area was inaccessible for a visual inspection. Model PA-28-140, PA-28-150, PA-28-151, PA-28-160, PA-28-161, PA-28-180, PA-28-181, PA-28-235, PA-28R-180, PA-28R-200, PA-28R-201T, PA-28RT-201, PA-28RT-201T, PA-32-260, and PA-32-300 airplanes have similar wing spar structures as the model PA-28R-201.

100-hour inspections as an indicator of high-load operations

Factored service hours are derived by researching the aircraft records to determine (1) the number of 100-hour inspections and (2) the total airframe hours, also called time in service (TIS). 

The factored service hours for each airframe are calculated by plugging the number of 100-hour inspections and TIS hours an airplane has accumulated into an equation. 

The rationale for using factored service hours (rather than total airframe time) is because the FAA believes that PA-28 and PA-32 airplanes used in flight schools, for-hire operations and other high-load environments such as low-altitude pipeline patrol, for example, are the airplanes that are subject to the heavy loading necessary for cracking to occur.

The NPRM says further:

Only an airplane with a main wing spar that has a factored service life of 5,000 hours, has had either main wing spar replaced with a serviceable main wing spar (more than zero hours TIS) or has airplane maintenance records that are missing or incomplete, must have the eddy current inspection.

How to determine factored service hours

The following is a summary of the formula for determining an airplane’s factored service life, published in the NPRM.

Step 1: Review the maintenance records (logbooks) to determine: a) the number of 100-hour inspections and b) total hours on the airplane since new or since any new wing or new wing spar replacement. 

Note: If a used spar or wing has been installed; or if the aircraft’s maintenance records are unclear as to the number of hours on the airplane, the bolt hole eddy current inspection must be done since it is impossible in those cases to determine how long the wing has been in service.

Step 2: Calculate the factored service hours for each main wing spar using the following formula: (N x 100) + [T-(N x 100)]/17 = Factored Service Hours, where N is the number of 100-hour inspections and T is the total hours TIS of the airplane. 

Thereafter, after each annual inspection and 100-hour TIS inspection, recalculate the factored service hours for each main wing spar until the main wing spar has accumulated 5,000 or more factored service hours.

The same formula is used to determine the factored service hours for all PA-28 and PA-32 airplanes. It works for those that have had only 100-hour inspections, those that have had no 100-hour inspections and airplanes that had some (but not all) 100-hour inspections over the life of the airplane.

Factored service hour calculations

Now, let’s do a few. Remember N is the number of 100-hour inspections and T is the total hours TIS of the airplane.

Picking numbers out of the air, let’s say our sample airplane has been used exclusively as a trainer for a well-known flight school for 4,662 hours and has had 46 100-hour inspections. What are the factored service hours of this airplane?

The formula for factored service hours is given in the NPRM as (N x 100) + 

[T – (N x 100)]/ 17 

For this airplane, that’s (46 x 100) + [4,662 – (46 x 100)]/17 

Simplified, (4,600) + [4,662 – (4,600)]/17

And finally, 4,600 + 3.657, which means this airplane has 4,603.65 factored service hours.

The inspection isn’t due yet, but will be soon, once the airplane reaches 5,000 factored service hours.

What about a privately-owned Piper PA-28-180 Cherokee 180 with complete maintenance records that has never had a 100-hour inspection?

Here’s an example straight out of the NPRM for determining factored service hours for an airplane with no 100-hour inspections. 

The airplane maintenance records show that the airplane has a total of 12,100 hours TIS, and only annual inspections have been done. Both main wing spars are original factory-installed. In this case, N = 0 and T = 12,100. 

Use those values in the formula as follows: (0 x 100) + [12,100 - (0 x 100)]/17 = 711 factored service hours on each main wing spar.

Despite the high number of airframe hours, this airplane has relatively few factored service hours and thus won’t need the inspection for quite some time.

Then, there are airplanes that have been used by a flight school, yet are now privately-owned. Here’s an example for an airplane that has 5,500 hours TIS and 25 100-hour inspections.

Use the same formula: (25 x 100) + [5,500 – (25 x 100]/17 equals 2,676 factored service hours.

This airplane is a little more than halfway to needing the inspection.

Math whizzes will recognize that the factored service hours formula is written based on an engineering calculation that wing spars in airplanes used for hire are 17 times more likely to have a spar crack than those that haven’t been flown for hire. 

My friend Mike Busch remarked:

The idea is that factored service hours are the sum of “abusive hours” and one-seventeenth of “non-abusive hours,” where “abusive hours” are defined as those hours during which the airplane was engaged in operations requiring 100-hour inspections (i.e., ops that included carrying passengers for hire and/or giving flight instruction for hire).

The only gotcha is for airplanes that have incomplete or approximated airframe hours instead of actual airframe hours. For instance, if an aircraft maintenance record (logbook) was lost or if one of the continuous record logs is missing, that airplane must have the wing spar bolt hole eddy current inspection specified in Paragraph (h) (1) and (2) of the NPRM and the inspection protocol in Appendix 1 of the AD. 

Inspection timeline and ongoing inspection requirements

The AD, as proposed, will require each airplane affected to have its number of inspections and TIS hours recalculated using the formula in the AD at each annual or 100-hour inspection to determine if it has gotten to the 5,000-hour factored service time point. 

Airplanes that get to 5,000 factored service hours per the formula, or airplanes with unknown airframe or wing hours TIS must have the eddy current inspection done within the next 100 hours time in service or 60 days, whichever occurs later.

According to figures in the NPRM, the eddy current inspection should take 1.5 man-hours. 

Reporting inspection results

The AD will require a written report within 30 days following each inspection. Here’s how it’s explained:

Within 30 days after completing an inspection required in Paragraph (h) of this AD, using Appendix 2, “Inspection Results Form,” of this AD, report the inspection results to the FAA at the Atlanta ACO Branch. Submit the report to the FAA using the contact information found in Appendix 2 of this AD.

Interim action

We consider this proposed AD interim action. The inspection reports will provide us additional data for determining the cause of the cracking. After analyzing the data, we may take further rulemaking action.

Based on these calculations, affected airframes that have never been operated where 100-hour inspections were required, seem to have little to be concerned about.

Airframes that have a factored service life of 5,000 hours or more will need to find a facility that can do a bolt hole inspection in accordance with the guidelines in Appendix 1 of the AD. 

If cracks are found, the wing spar will need to be replaced. The AD estimates that that repair will take 32 work hours and estimates that, at a labor cost of $85/hour the total cost will be $2,720 in labor. The FAA projects the part cost at $5,540, for a total cost of $8,260. 

However, since many of affected airframes are approaching 60 years’ time in service, I suspect that there will be owners and operators that elect to get the bolt hole eddy current inspection done regardless of the number of factored service hours on the airframe. It’s the only way to make sure there are no cracks. 

Know your FAR/AIM and check with your mechanic before starting any work.

Steve Ells has been an A&P/IA for 45 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Templeton, California, with his wife Audrey. Send questions and comments to editor [AT] piperflyer [DOT] com.




“PA-28 and PA-32 Wing Spar Cracks: What You Should Know”
by Steve Ells, July 2018


NPRM 2018-CE-049-AD
Federal Aviation Administration