DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2020-0692; Project Identifier MCAI-2019-00140-E;
Amendment 39-22016; AD 2022-08-13]
RIN 2120-AA64

Airworthiness Directives; Pratt & Whitney Canada Corp. Turboprop
Engines

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: The FAA is adopting a new airworthiness directive (AD) for all
Pratt & Whitney Canada Corp. (P&WC) PT6A-34, -34B, -34AG, -114, and -
114A model turboprop engines. This AD was prompted by several reports
of low-time fractures of compressor turbine (CT) blades resulting in
loss of power or in-flight shutdown (IFSD) of the engine. This AD
requires replacement of certain CT vanes. This AD also requires removal
from service of certain CT blades when these blades have been operated
with certain CT vanes. The FAA is issuing this AD to address the unsafe
condition on these products.

DATES: This AD is effective May 27, 2022.

ADDRESSES:

Examining the AD Docket

You may examine the AD docket at https://www.regulations.gov by
searching for and locating Docket No. FAA-2020-0692; or in person at
Docket Operations between 9 a.m. and 5 p.m., Monday through Friday,
except Federal holidays. The AD docket contains this final rule, the
mandatory continuing airworthiness information (MCAI), any comments
received, and other information. The address for Docket Operations is
U.S. Department of Transportation, Docket Operations, M-30, West
Building Ground Floor, Room W12-140, 1200 New Jersey Avenue SE,
Washington, DC 20590.

FOR FURTHER INFORMATION CONTACT: Barbara Caufield, Aviation Safety
Engineer, ECO Branch, FAA, 1200 District Avenue, Burlington, MA 01803;
phone: (781) 238-7146; fax: (781) 238-7199; email:
barbara.caufield@faa.gov.

SUPPLEMENTARY INFORMATION:

Background

The FAA issued a notice of proposed rulemaking (NPRM) to amend 14
CFR part 39 by adding an AD that would apply to all P&WC PT6A-34, -34B,
-34AG, -114, and -114A model turboprop engines. The NPRM published in
the Federal Register on August 17, 2020 (85 FR 49981). The NPRM was
prompted by several reports of low-time fractures of CT blades
resulting in loss of power or IFSD of the engine. In the NPRM, the FAA
proposed to require replacement of certain CT vanes. The NPRM also
proposed to require the removal from service of certain CT blades when
these blades have been operated with certain CT vanes. The FAA is
issuing this AD to address the unsafe condition on these products.
Transport Canada, which is the aviation authority for Canada, has
issued Transport Canada AD CF 2019-30R1, dated December 17, 2019
(referred to after this as ``the MCAI''), to address the unsafe
condition on these products. The MCAI states:

There have been several reported events of low time CT blade
fractures resulting in power loss/In-flight shutdown (IFSD) on post
P&WC Service Bulletin (SB) 1669 configured PT6A-114 engines,
featuring new CMSX-6 CT blades. In addition, relatively low time
failures of Non-P&WC CT blades have also been reported on PT6A-34
and -114 series engines.
In service data shows that these low time failures were reported
on engines that had CT vanes installed that were repaired in
accordance with repair specification number STI 72-50-254 held by
Southwest Turbine Inc. (STI). Most of the affected engines are
installed on single-engine powered aeroplanes and some events have
resulted in the loss of the aeroplane and fatalities.
Dimensional checks and operational testing of the subject STI
repaired CT vane removed from an incident engine, revealed that it
did not conform to the engine manufacturer's CT vane type design
criteria. The noted variations and features in the STI repaired CT
vane can cause airflow distortion and subsequent aerofoil excitation
of the CT blades resulting in High Cycle Fatigue (HCF) failure of
the CT blades. Test data indicates that the stress levels induced in
CT blades by the adverse effect of subject airflow distortion
exceeds the design requirements for CMSX-6 CT blades.
An IFSD or loss of power on a single-engine powered aeroplane
under certain conditions can lead to an unsafe condition as seen in
some past events. [Transport Canada] AD CF-2019-30 was issued on 19
August 2019 to address the potential hazard of power loss/IFSD as a
result of CT blade failures on engines with CT vanes installed that
were repaired in accordance with repair specification number STI 72-
50-254.
This [Transport Canada] AD revision, CF-2019-30R1, is issued to
update the background information and to clarify the affected P&WC
CT blade Part Numbers (P/Ns).

You may obtain further information by examining the MCAI in the AD
docket at https://www.regulations.gov by searching for and locating
Docket No. FAA-2020-0692.

Discussion of Final Airworthiness Directive

Comments

The FAA received comments from 13 commenters. The commenters were
Southwest Turbine Inc. (STI), an individual commenter, and 11 anonymous
commenters. The following presents the comments received on the NPRM
and the FAA's response to each comment.

Request To Revise Required Actions

STI requested that the FAA revise the reference in paragraph
(g)(1)(i), Required Actions, of this AD from ``. . . non-STI-repaired
CT vane'' to ``. . . non-STI 72-50-254 repaired CT vane. . .'' The
commenter reasoned that this AD specifically addresses CT vanes
repaired using STI Repair Specification STI 72-50-254 (STI 72-50-254).
Therefore, operators should be allowed to install CT vanes not repaired
using STI 72-50-254 and repaired within STI's current FAA rating.
Additionally, STI reasoned that this change would mirror the language
in the Corrective Actions, paragraph 1, of Transport Canada AD CF-2019-
30R1.
The FAA agrees and updated paragraph (g)(1)(i) of this AD to refer
to CT vanes not repaired using STI 72-50-254. This change places no
additional burden on operators who are required to comply with this AD.

Request To Remove Engines From Applicability

STI requested that the FAA remove P&WC PT6A-34, -34B, and -34AG
model turboprop engines from paragraph (c), Applicability, of this AD.
The commenter reasoned that of the 20 P&WC CMSX-6 CT blade failures,
only six blade failures occurred with STI 72-50-254 repaired CT vanes
installed. Those six blade failures occurred on engines with CT vanes
repaired using STI 72-50-254, which were installed in P&WC PT6A-114A
model turboprop engines.
In addition, the commenter suggested that the only STI-repaired CT
vane installed on engines that has experienced CT blade failures was CT
vane part number (P/N) 3029051. The commenter continued that CT vane,
P/N 3029051, is not eligible for installation in P&WC PT6A-34, -34B,
and -34AG model turboprop engines, and therefore, these model engines
should be removed from the applicability of this AD.
STI cited Docket No. FAA-2013-0766 (AD 2014-17-08, 79 FR 52172,
September 3, 2014), which was superseded by AD 2014-17-08R1, (80 FR
24791, May 1, 2015), and the FAA's responses to public comments in the
preamble of these ADs. While referencing these ADs, STI underlined
specific portions of public comments involving the P&WC CMSX-6 CT blade
being an unproven CT blade replacement that has experienced low-time
failures and has been identified for removal in P&WC PT6A-34 model
turboprop engines. STI indicated that the FAA acknowledged the failure
mode in other P&WC engines, specifically including PT6A-34 turboprop
engines, was well understood and stated there have been no failures of
P&WC CMSX-6 CT blades in PT6A-34 turboprop engines with STI 72-50-254
repaired CT vanes. STI commented that, for these reasons, the STI 72-
50-254 repaired CT vane rings cannot be the cause of PWC CMSX-6 CT
blade failures in PT6A-34 series engines.
The FAA disagrees. The FAA recognizes that STI-repaired CT vane P/N
3029051 is not eligible for installation in P&WC PT6A-34, -34B, and -
34AG model turboprop engines. The FAA notes, however, that an
additional affected part-numbered STI-repaired vane is eligible for
installation in P&WC PT6A-34, -34B, and -34AG model turboprop engines.
In addition, CT blade failures have occurred with STI-repaired CT vanes
installed in P&WC PT6A-34, -34B, and -34AG model turboprop engines and
are susceptible to the unsafe condition of this AD. Further, Table 2 of
Southwest Turbine Repair, Inc., STI 72-50-254, Revision 08, dated April
14, 2019, lists P&WC PT6A-34, -34B, -34AG model turboprop engines as
eligible for this repair.
The FAA issued AD 2014-17-08 (79 FR 52172, September 3, 2014) and
AD 2014-17-08R1 (80 FR 24791, May 1, 2015) to require replacement of
P&WC IN100 CT blades with P&WC CMSX-6 CT blades. Although there have
been failures of the P&WC CMSX-6 CT blades with CT vanes not repaired
by STI, the FAA has found the failure rate of CT blades with CT vanes
not repaired by STI to be approximately one-tenth of those that were
repaired by STI.

Request To Restrict Applicability to Certain CT Blades

STI requested that the FAA revise paragraph (c), Applicability, of
this AD to indicate removal of STI 72-50-254 repaired CT vanes should
not apply to engines operating with pre-P&WC SB PT6A-72-1669 and pre-
P&WC SB PT6A-72-1690 IN100 CT blades. The commenter reasoned that the
MCAI and the NPRM addressed the unsafe condition of failure of P&WC
CMSX-6 CT blades. The commenter stated that the AD should not require
STI 72-50-254 repaired CT vanes to be removed when operated with P&WC
IN100 CT blades.
The FAA disagrees with revising paragraph (c), Applicability, of
this AD to limit the AD applicability to turboprop engines with certain
CT blades installed. Although most CT blades failures have occurred
with P&WC CMSX-6 CT blades installed, the FAA's data indicate that
several P&WC IN100 CT blade failures occurred with STI-repaired CT
vanes before incorporating procedures in P&WC SB PT6A-72-1669 and P&WC
SB PT6A-72-1690. Consequently, this AD requires that any CT vane with
P/N 3029051, 3032151, or 3123001 repaired in accordance with STI 72-50-
254 be removed from service.

Request To Restrict Applicability by CT Vane Part Number

STI requested that the FAA update paragraph (c), Applicability, of
this AD to indicate that only STI 72-50-254 repaired CT vanes P/N
3029051 or P/N 3123001 are affected by this AD. STI reasoned that all
the P&WC CMSX-6 CT blade failures that they are aware of occurred in
PT6A-114A engines operating with STI 72-50-254 repaired CT vane P/N
3029051. STI continued that there is no evidence that identifies
discrepant conditions or CT blade failures with any other part numbered
STI 72-50-254 repaired CT vanes.
The FAA disagrees that only STI 72-50-254 repaired CT vanes P/N
3029051 or 3123001 are affected by the unsafe condition addressed by
this AD. The FAA has reviewed data that shows failures of another CT
vane P/N in addition to the two P/Ns referenced by the commenter. In
response to this comment, the FAA updated paragraph (g)(1)(i), Required
Actions, of this AD to require the removal from service of any affected
CT vane, P/N 3029051, 3032151, or 3123001, repaired in accordance with
STI 72-50-254.

Request To Require Installation of Dampers/Dampeners

STI, an individual commenter, and two anonymous commenters
suggested that the FAA require operators install under platform seals
(dampers or dampeners) introduced by P&WC SB PT6A-72-1769, dated
December 21, 2015. One commenter reasoned that of the 20 CMSX-6 CT
blade fatigue failures that have occurred, none had occurred when
dampeners were installed. Based on a study and testing by P&WC, the
commenter determined that the dampeners appeared to have solved the
ongoing problem of P&WC CMSX-6 CT blade failures, regardless of which
CT vane was installed. The commenter suggested that the FAA withdraw
the NPRM and replace it with an AD requiring the installation of the
dampeners.
An anonymous commenter and an individual commenter referred to P&WC
documentation in which P&WC indicated that failures of P&WC CMSX-6 CT
blades in normal operation were caused by vibratory stress, and the
previous generation of CT blades did not exhibit this problem. To
reduce these vibratory stresses, P&WC introduced dampers. The two
commenters suggested requiring dampers and CT vane clocking to reduce
vibratory stresses.
The FAA disagrees with the suggestion to require operators to
install under platform seals to address the unsafe condition. Although
data suggest dampeners and clocking reduce vibratory stresses,
dampeners and clocking do not eliminate the unsafe condition caused by
the installation of the STI-repaired CT vanes.

Comments on Root Cause of CT Blade Failure

An individual commenter questioned whether Transport Canada CF-
2019-30R1, the MCAI on which the FAA's NPRM is based, tested a
representative sample of affected CT blades and
whether the root cause of the unsafe condition was determined
accurately. The commenter suggested that the MCAI is based on testing
of a single STI-repaired CT vane from an engine that suffered
catastrophic CT blade failure during an engine test run following an
overhaul. The commenter stated that P&WC engineers documented that the
root cause of the CT blade failure was undetermined and that the STI-
repaired CT vane was not a representative sample due to sustained
damage.
An anonymous commenter noted that of the 16 P&WC CMSX-6 blade
failures, 11 had P&WC CT vanes installed. The commenter stated that
this equates to an approximate 70% failure rate with the P&WC CT vanes.
The commenter questioned how a CT vane made by an alternate supplier
can be blamed as the cause of these failures.
The FAA disagrees with these comments. The MCAI and this AD are not
based exclusively on testing of a single STI-repaired CT vane.
Transport Canada and the FAA reviewed data from 38 CT blade failure
events prior to issuance of the MCAI and this AD. The relative rates of
CT blade failure are not simply the ratio of the number of events, but
also includes the number of engines with each part type installed.
Although there have been failures of the P&WC CMSX-6 CT blades with
non-STI repaired CT vanes installed, the FAA has found the failure rate
of CT blades with non-STI repaired CT vanes to be approximately one-
tenth of the failure rate of those that were repaired by STI.

Comments That the P&WC CMSX-6 CT Blades Are the Cause of Failures

Several anonymous commenters and an individual commenter cited the
history of P&WC CMSX-6 CT blade failures and the resulting P&WC service
bulletins involving procedures to inspect and replace the CT blades.
The commenters stated these failures occurred with factory manufactured
zero-time P&WC model engines and engines in operation with both P&WC CT
vanes and STI-repaired CT vanes installed. According to an individual
commenter, the evidence to condemn the STI-repaired CT vane would also
apply to the P&WC CT vane. Considering that factory manufactured, zero-
time P&WC engines have experienced CT blade failures, the commenters
concluded that unsafe condition with these blades cannot be the result
of a repair process.
Further, an anonymous commenter referenced a 2018 case in Dallas
County, Texas involving P &WC. The commenter summarized the case to
include blade development and problems encountered from coating cracks
migrating into the base material, gap platform, vibratory stress near
the operating rotational speed of the engine and other areas of concern
with the CT blade development. The commenter recommended that the FAA
review Analytical Summary D9297 (P&WC 008643-008680), and Analytical
Summary E7739, dated September 24, 2013 (P&WC 008599-008617), which,
the commenter states, both determined the problem to be the CT blade.
An anonymous commenter suggested that the FAA demand all documents
relating to the process and development of the P&WC CMSX-6 CT blade to
include testing, emails, minutes of meetings, and any sworn testimony
given, prior to deciding on the proposed AD. The commenter suggested
that the CT blade is the root cause of the failures, the manufacturer
is dictating the AD, and the manufacturer is going after a competitor.
STI cited National Transportation Safety Board (NTSB) Report, No.
WPR14FA024, dated October 14, 2015, which detailed an October 21, 2013
failure involving an STI 72-50-254 repaired CT vane. STI commented that
NTSB made no findings that indicated STI 72-50-254 repaired CT vane
contributed to the event.
An anonymous commenter stated they had a P&WC PT6A-114A model
turboprop engine undergoing overhaul and 18 P&WC CMSX-6 CT blades
failed the process compensated resonance testing per P&WC SB PT6A-72-
1762. The commenter suggested that these failures indicate that there
is a design flaw or quality escape with P&WC CMSX-6 CT blades.
The FAA does not agree with the commenters that there is an unsafe
condition affecting the P&WC CT blades. The FAA has reviewed event
reports, analyses, and test reports to make this determination.

Request To Consider Inaccuracy Tolerance

STI requested that the FAA consider an inaccuracy tolerance of 30%
when reviewing test data. STI cited P&WC report E8093 that indicates a
30% variance in repeatability of non-intrusive stress measurement
(NSMS) CT blade tip deflection of a P&WC CT vane. STI suggested that
P&WC retest prior configurations to determine the cause of variation in
repeatability.
The FAA disagrees to consider inaccuracy tolerance. P&WC examined
three STI-repaired CT vanes via dimensional inspection, one of which
was also tested using Non-intrusive Stress Measurement System (NSMS),
and determined the STI-repaired CT vanes did not meet P&WC's type
design criteria. The STI-repaired CT vane that P&WC tested had
scratches not exceeding a depth of 0.5 mils that did not alter the
dimensional aspects of the CT vane casting and assembly when measured
and did not preclude the engine from running during the NSMS testing.

Question About the Number of CT Blade Failures

An anonymous commenter asked how many CT blade failures have
occurred after incorporating P&WC SB PT6A-72-1768 and P&WC SB PT6A-72-
1769. Another anonymous commenter asked how many CT blade failures on
turboprop engines, equipped with CT vane P/N 3079351-01, which is the
third generation of single crystal blade used by P&WC in the affected
engines, have occurred after incorporating P&WC SB PT6A-72-1749.
The FAA notes that no known failures of CT blades have occurred
after incorporating P&WC SB PT6A-72-1768, P&WC SB PT6A-72-1769, or P&WC
SB PT6A-72-1749.

Comment About Repair Variation in P&WC CT Vanes

STI commented that P&WC regularly returns to service overhauled CT
vanes that exhibit greater variation in repair than that of STI-
repaired CT vanes. STI stated that P&WC's inspection requirements for
new CT vanes are different than overhauled CT vanes, and deviating
features found on P&WC's overhauled CT vanes are not inspected prior to
release.
The FAA cannot confirm STI's comment regarding P&WC's returned-to
service part variation. Most engine new-part inspection specifications
differ from those for used or overhauled parts. As stated in an earlier
comment reply, the FAA reviewed data from 38 CT blade failure events to
address the unsafe condition in this AD. Although there have been
failures of CT blades with CT vanes not repaired by STI, the FAA has
found the failure rate of CT blades with CT vanes not repaired by STI
is approximately one-tenth of those that were repaired by STI.

Comment About Original Equipment Manufacturer (OEM) Regulating the
Regulators

Two anonymous commenters suggested that this AD is an example of
the OEM regulating the regulators.
The FAA disagrees. The data reviewed by the FAA shows that CT blade
stresses are significantly higher in engines with STI-repaired CT
vanes, compared to those with P&WC CT vanes installed. In addition, event data
reviewed by the FAA shows that CT blade failure events are
approximately 10 times greater in engines equipped with STI-repaired CT
vanes as opposed to P&WC CT vanes.

Conclusion

The FAA reviewed the relevant data, considered any comments
received, and determined that air safety requires adopting this AD as
proposed. Accordingly, the FAA is issuing this AD to address the unsafe
condition on these products. Except for minor editorial changes, and
any other changes described previously, this AD is adopted as proposed
in the NPRM. None of the changes will increase the economic burden on
any operator.

Related Service Information

The FAA reviewed Southwest Turbine Repair, Inc., STI 72-50-254,
Revision 08, dated April 14, 2019. This service information describes
procedures for repair of the compressor turbine vane ring assembly.

Costs of Compliance

The FAA estimates that this AD affects 907 engines installed on
airplanes of U.S. registry. The FAA estimates that 63 engines will need
to replace the CT vanes and CT blades.
The FAA estimates the following costs to comply with this AD:


Estimated Costs

Authority for This Rulemaking

Title 49 of the United States Code specifies the FAA's authority to
issue rules on aviation safety. Subtitle I, section 106, describes the
authority of the FAA Administrator. Subtitle VII: Aviation Programs,
describes in more detail the scope of the Agency's authority.
The FAA is issuing this rulemaking under the authority described in
Subtitle VII, Part A, Subpart III, Section 44701: General requirements.
Under that section, Congress charges the FAA with promoting safe flight
of civil aircraft in air commerce by prescribing regulations for
practices, methods, and procedures the Administrator finds necessary
for safety in air commerce. This regulation is within the scope of that
authority because it addresses an unsafe condition that is likely to
exist or develop on products identified in this rulemaking action.

Regulatory Findings

This AD will not have federalism implications under Executive Order
13132. This AD will not have a substantial direct effect on the States,
on the relationship between the national government and the States, or
on the distribution of power and responsibilities among the various
levels of government.
For the reasons discussed above, I certify this AD:
(1) Is not a ``significant regulatory action'' under Executive
Order 12866,
(2) Will not affect intrastate aviation in Alaska, and
(3) Will not have a significant economic impact, positive or
negative, on a substantial number of small entities under the criteria
of the Regulatory Flexibility Act.

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Incorporation by
reference, Safety.

The Amendment

Accordingly, under the authority delegated to me by the
Administrator, the FAA amends 14 CFR part 39 as follows:

PART 39--AIRWORTHINESS DIRECTIVES

1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. 106(g), 40113, 44701.

Sec. 39.13 [Amended]

2. The FAA amends Sec. 39.13 by adding the following new airworthiness
directive: