Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
2022-20
October 19, 2022
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin (SAIB) alerts all
owners, operators, maintenance technicians, and inspectors of Piper
Aircraft, Inc. (Piper) PA-28 and PA-32 airplanes of information
gathered as result of a fatal accident of a Piper Model PA-28R-201
airplane in 2018 and the inspection findings that followed. The
accident, determined to be caused by fatigue cracks in the lower spar
cap, resulted in the Federal Aviation Administration (FAA) issuing
Airworthiness Directive (AD) 2020-26-16, which requires several actions
including a one-time inspection of certain lower spar cap bolt holes.
AD 2020-26-16 was issued as an interim action and requires reporting
certain inspection information to the FAA to help determine the number
of cracks present in the fleet. The inspection reports received by the
FAA thus far indicate the presence of numerous cracks and hole-quality
issues in a significant number of airplanes.
The FAA is concerned that the presence of cracks and other hole-quality
issues could lead to additional in-flight wing separations if
additional actions or modifications are not taken. Analysis of the AD
inspection data along with additional analysis by various contributors,
including Piper and the U.S. Air Force, is guiding the FAA to develop
further corrective action. These analyses are indicating a possible
need for frequent inspections, and inspections of additional airplanes
beyond those initially inspected per AD 2020-26-16, to ensure proactive
detection of fatigue cracks.
The FAA has determined that this airworthiness concern is an unsafe
condition that warrants AD action under Title 14 of the Code of Federal
Regulations (14 CFR) part 39. The action s specified by this SAIB are
not mandatory, however, the information requested by this SAIB will
help the FAA develop an AD action to address this unsafe condition.
Background
As stated above, the FAA issued AD 2020-26-16 due to a 2018 in-flight
wing separation on a Piper Model PA-28R-201 airplane caused by fatigue
cracks located in a visually inaccessible area of the lower main wing
spar cap. Inspection of another airplane in the same fleet of the
accident airplane (manufactured at a similar time and operated in a
similar training mission) was also found to have cracking in the wing
spar. The National Transportation Safety Board (NTSB) found fatigue
cracking across an extensive cross-section of the lower spar cap. The
crack and failure occurred across the T - section at the two most
outboard bolt holes attaching the lower spar to the wing box (see
Figure 1).
AD 2020-26-16 requires a one-time eddy current inspection of certain
bolt holes of spars that have reached 5,000 factored service hours
(FSH), and a reporting of the inspection results to the FAA and Piper.
The inspections were required for the most outboard set of bolt holes,
both forward and aft (see Figure 2). The eddy current inspection method
is a non-destructive technique that utilizes electromagnetism to detect
flaws on and under the surface of a part in question. The inspection
method has certain limitations including the depth in the material
where a flaw can be detected, the size of the flaw, and that a highly
skilled and properly trained inspector is required, but it is a proven
tool that is commonly used for inspections of this nature. The eddy
current inspection method is capable of detecting cracks, but is also
sensitive to surface anomalies that may cause cracks to form in the
future, such as corrosion or gouges. Visual inspections are not
sufficient to detect such cracks.
The FAA and Piper received reports on approximately 2,880 airplanes.
Out of these, 115 airplanes were reported as having crack indications.
Approximately 25 percent of these indications were later confirmed to
be hole damage or corrosion and not a crack. The remaining indications
include a mix of unconfirmed indications, but fatigue cracks were also
identified, including some examples shown in this SAIB. Unconfirmed
crack indications were found in airplanes with as little as 3,264 hours
time in service (TIS) on a Piper Model PA-28R-180 airplane, but has not
yet been confirmed. Figure 3 shows a summary of the positive indication
inspection results received by the FAA and airplane models involved in
accidents attributed to or potentially related to the spar cracking
issue. In addition, some owners and operators of airplanes not included
in the AD’s applicability voluntarily inspected and reported findings
of crack indications in the wings of their airplanes (not shown).
While there are a large number of crack indications in the inspection
results, it is possible the inspection can result in a crack indication
due to something other than a confirmed crack, such as mechanical
bolt-hole damage, corrosion, or some other type of discrepancy. Some
examples of the types of spar damage findings are shown in Figures 4
through 9. AD 2020-26-16 eddy current inspections successfully
identified bolt-hole mechanical damage allowing for corrective action
to be taken before that damage could lead to cracking, which could lead
to a wing separation. Multiple NTSB laboratory examinations have shown
that cracks have formed due to internal hole anomalies as well
anomalies adjacent to the hole. The 2018 accident showed multiple crack
origins at the hole inside diameter.
A challenge of the eddy current inspection is that corrosion or damage
may indicate a crack when no crack exists. However, it is important to
note that a concerning number of confirmed cracks have been found that
were fatigue related and not related to mechanical damage. The FAA has
not confirmed all of the crack findings, but believes there would still
be a mix of confirmed cracks and other bolt-hole discrepancies. The FAA
believes that the sensitivity of the eddy current inspection is
necessary in detecting cracks because of the small critical crack size
in this spar as calculated by the U.S. Air Force and documented in NTSB
reports.
The AD 2020-26-16 inspection reports indicate that additional
corrective action beyond that required by AD 2020-26-16 is likely
needed. While AD 2020-26-16 addresses the immediate safety concern, due
to aging, more airplanes will need to be inspected, and the airplanes
already inspected will need additional inspections. Additionally, the
FAA may consider establishing a life limit on the wing spar for the
high-time severe usage airplanes. This life limit may be similar to the
life limit imposed on Piper PA-44 airplanes. The Piper PA-44 airplanes,
which have a similar wing spar attachment design to the Piper PA-28 and
PA-32 airplanes, have a life limit on the spar based on normal usage of
14,663 or 16,462 hours TIS, depending on the model.
Aircraft aging, due either to hours TIS or usage, is an issue that
needs to be addressed for every fleet at some point. Small
imperfections can exist in aircraft structure from an early age;
however, through operation, these imperfections can slowly grow into
fatigue cracks. Crack development is a function of many factors,
including design of the structure, how severely the aircraft is flown,
and manufacturing processes. Fatigue cracks have the effect of
weakening the structure and its ability to support the stresses the
aircraft is originally designed to handle.
The 2018 accident, and the AD 2020-26-16 inspection reports indicate an
aging fleet that requires intervention to ensure the damage does not
reach a critical state prior to being detected. At a minimum, this
typically takes the form of repetitive inspections to be able to
capture the formation of a detectable crack, requiring repair or
replacement.
Including the most recent accident in 2018, three accidents have
occurred where fatigue cracking in the lower wing spar was the cause or
was found during the accident investigation. These accidents and the
inspection results from AD 2020-26-16 provide valuable knowledge that
can be applied to determine appropriate corrective actions. As a result
of the 2018 accident, the FAA is studying the inspection results of AD
2020-26-16 and other field data to determine if additional spar
inspection criteria are needed to address the aging fleet.
Accident No. 1, described in the next paragraph, resulted in the FAA
issuing AD 87 -08-08. This AD required several actions including
removing and dye penetrant inspecting the wing for cracks. Similar to
AD 2020-26-16, AD 87-08-08 resulted in many inspections; however,
unlike recently, only a few cracks were found, likely due to the low
hours TIS of the fleet at this time. At the time, the FAA determined
the cracks found as a result of the inspections required by AD 87
-08-08 only developed due to the airplane’s severe flying usage or
previous repairs, and as a result the AD was rescinded in May 1989.
While this AD rescission may have been appropriate at that time given
that this was a low hours TIS fleet, these assumptions cannot be
applied equally now. Given the current age of the fleet, both aircraft
usage and hours TIS must be considered, and aircraft must be inspected
for fatigue cracking.
The first related accident occurred in 1987 in Marlin, TX. The accident
airplane was a Piper Model PA-28-181 airplane with 7,490 hours TIS.
Nearly all of this airplane’s flight time was known to be spent in a
"Pipeline Patrol" mission. This type of mission is characterized by
very low altitude and terrain following flight profiles. These flying
characteristics degrade an aircraft’s structure faster than most other
missions. See Figure 10 for a view of the accident airplane’s failed
spar, where "O" indicates fatigue crack origins and "W" indicates a web
fatigue crack.
The second of these accidents occurred in 1993 in Provincetown, MA. The
accident airplane was a Piper Model PA-28-181 airplane with 11,683
hours TIS. This accident was attributed to weather with signs of
structural overloading, but a fatigue crack was also found in the
failed spar. The crack is beyond the critical crack size later
calculated for that spar. While the NTSB did not identify the fatigue
crack as the probable cause of the accident, its presence may have
contributed to the effect of the structural overload. At the time of
the accident, the airplane was being flown for personal use; however,
the airplane was operated by a company providing a mix of training and
charter flights. The training mission type of flying is characterized
by low altitudes and many takeoff and landing cycles. While less severe
than a "Pipeline Patrol" mission, the training mission would contribute
more towards fatigue than personal use. See Figure 11 for a view of the
accident airplane’s failed spar near a bolt hole, where "O" indicates
fatigue crack origins. The origin in this case was adjacent to the
hole, not in the hole, as in the 2018 accident.
The third and most recent of these accidents occurred in 2018 in
Daytona Beach, FL. The accident airplane was a Piper Model PA-28R-201
airplane with 7,691 hours TIS. This airplane was used almost
exclusively for flight training throughout its lifetime. This type of
usage is expected to reduce the fatigue life of the airplane more than
an airplane used as a mixture of personal and training, while still
less damaging than use exclusively in the "Pipeline Patrol" mission.
See Figure 12 for a view of the accident airplane’s failed spar.
The FAA is recommending that owners and operators provide the following
information. Reporting is voluntary. The FAA will analyze the
information received to determine further action. As part of its
commitment to continued operational safety, the FAA is monitoring the
Piper PA-28 and PA-32 airplanes for issues related to lower main wing
spar cap cracks and requests submission of such reports to the person
listed under For Further Information Contact.
1) Availability of Eddy Current Inspectors
The FAA is concerned about the availability of eddy current inspectors
to perform inspections of the high-time airplanes. The FAA recommends
providing information regarding the number of inspectors, including
Level 2 and Level 3, available to perform inspections similar to those
required by AD 2020-26-16.
2) Factored Service Hours (FSH) Feedback
The FAA has received both positive and negative comments about the FSH
approach to setting the inspection threshold for AD 2020-26-16. By
using a factor, the intent was to target inspections to the airplanes
that were most likely to be engaged in training, which was one of the
risk factors identified. While this necessitated some complexity in the
calculation of inspection times, it significantly reduced the cost of
inspection for personal use airplanes. EASA AD 2021-0107R2 uses a
different method for determining factored service hours because in the
European Union all aircraft are required to complete 100-hour
inspections regardless of the operational requirements. In the United
States 100-hour inspections are only required when operating for hire.
In the FSH calculation, the EASA method accounts for age of the
airplane whereas the FAA AD 2020-26-16 method accounts for a
combination of age and the number of 100-hour inspections. The EASA
method was developed to account for the FAA’s FSH method under the
European Union regulatory system. We appreciate any feedback on the
practice of using an FSH, and what the factors should be in the
equations embedded in any future inspection thresholds and intervals.
The EASA AD is available at ad.easa.europa.eu.
3) Cracks or Damage Not Already Reported
The FAA recommends reporting any crack(s) or damage found in the wing
spar bolt holes if not already reported to the FAA or Piper per AD
2020-26-16 or other means. Please provide the airplane model, serial
number, hours TIS, number of 100-hour inspections, photographs and a
description of the damage found in any wing spar bolt hole.
4) Other Feedback
The FAA welcomes any additional feedback on what has been discussed in this SAIB.
Paperwork Reduction Act Burden Statement
A federal agency may not conduct or sponsor, and a person is not
required to respond to, nor shall a person be subject to a penalty for
failure to comply with a collection of information subject to the
requirements of the Paperwork Reduction Act unless that collection of
information displays a currently valid OMB Control Number. The OMB
Control Number for this information collection is 2120-0731. Public
reporting for this collection of information is estimated to be
approximately 1 hour per response, including the time for
reviewing instructions, searching existing data sources, gathering and
maintaining the data needed, completing and reviewing the collection of
information. All responses to this collection of information are
voluntary; the nature and extent of confidentiality to be provided, if
any. Send comments regarding this burden estimate or any other aspect
of this collection of information, including suggestions for reducing
this burden to: Information Collection Clearance Officer, Federal
Aviation Administration, 10101 Hillwood Parkway, Fort Worth, TX
76177-1524.
For Further Information Contact
Fred Caplan, Aviation Safety Engineer, Atlanta ACO Branch, FAA, 1701
Columbia Avenue, Atlanta, GA 30337; phone: (404) 474-5507; email:
frederick.n.caplan@faa.gov.