Memorandum

U.S. Department of Transportation

Federal Aviation Administration

Subject ACTION: Policy for Parts Manufacturer Approval (PMA)

of Reciprocating Engine Critical, Highly Stressed or

Complex Parts or Components

Date: 4/10/97

From: Manager, Engine and Propeller Standards Staff, ANE-110 Reply to 

Attn. of:

Mark

Rumizen,

ANE-110:

(781) 238-

7164

To Manager, Aircraft Engineering Division, AIR-100

Manager, Aircraft Manufacturing Division, AIR-200

Manager, Brussels Aircraft Certification Staff, AEU-100

Manager, Engine Certification Office, ANE-140

Manager, Engine Certification Branch, ANE-141

Manager, Engine Certification Branch, ANE-142

Manager, Boston Aircraft Certification Office, ANE-150

Manager, New York Aircraft Certification Office, ANE-170

Manager, Airframe and Propulsion Branch, ANE-171

Manager, Rotorcraft Directorate, ASW-100

Manager, Rotorcraft Standards Staff, ASW-110

Manager, Airplane Certification Office, ASW-150

Manager, Rotorcraft Certification Office, ASW-170

Manager, Special Certification Office, ASW-190

Manager, Small Airplane Directorate, ACE-100

Manager, Small Airplane Standards Office, ACE-110

Manager, Atlanta Aircraft Certification Office, ACE-115A

Manager, Propulsion Branch, ACE-140A

Manager, Chicago Aircraft Certification Office, ACE-115C

Manager, Propulsion Branch, ACE-118C

Manager, Wichita Aircraft Certification Office, ACE-115W

Manager, Propulsion Branch, ACE-140W

Manager, Anchorage Aircraft Certification Office, ACE-115N

Manager, Transport Airplane Directorate, ANM-100

Manager, Transport Standards Staff, ANM-110

Manager, Airframe and Propulsion Branch, ANM-112

Manager, Seattle Aircraft Certification Office, ANM-100S

Manager, Propulsion Branch, ANM-140S

Manager, Denver Aircraft Certification Office, ANM-100D

Manager, Los Angeles Aircraft Certification Office, ANM-100L

Manager, Propulsion Branch, ANM-140L
1. INTRODUCTION

The Engine and Propeller Directorate was recently requested to provide written guidance regarding the

requirements for PMA of reciprocating engine crankshafts. This memo establishes a standardized

policy for Aircraft Certification Offices (ACOs) to evaluate PMA applications for reciprocating engine

critical, highly stressed or complex parts, including, but not limited to crankshafts and cylinder heads. 

Critical parts are defined in FAA Order 8110.42, paragraph 5.c. PMA applications for these type of 

parts are considered significant projects in accordance with the intent of Order 8100.5 and Order

8110.42, and a Project Officer will be assigned by this Directorate to review the compliance program

relative to the requirements specified in this memo. These requirements should be conveyed to the

applicant as early as possible in the PMA process to allow the applicant ample opportunity to define an

acceptable compliance program.

2. PMA BY IDENTICALITY

Approval of aircraft engine replacement parts based on identicality requires that the applicant

substantiate that the design of the replacement part is identical to the design of the part covered under

the type certificate (TC) and defined by FAR 21.31. The design of the PMA part is defined by FAR

21.303(c)(3)(i) to include the drawings and specifications, as well as subparagraph (ii) to include

information on dimensions, materials, and processes necessary to define the structural strength of the

part. For simple, not highly stressed, or non-critical parts, the data evaluation typically focuses only on

subparagraph (c)(3)(i), and is usually accomplished by comparing the PMA applicant’s drawings,

material and process specifications to the TC holder’s corresponding drawings and specifications issued

in accordance with FAR 21.31(a) and (b). However, for critical, highly stressed and/or complex parts

and components (e.g., crankshafts, cylinder heads) additional emphasis on the requirements of

subparagraph (c)(3)(ii) is necessary. This would require the applicant submit detailed information on

the materials and processes as described by Order 8110.42, paragraphs 8 c. through 8.c.(3). This data

should include, but not be limited to: (1) all elements of the manufacturing cycle (raw material purchase,

material chemistry and grain structure, fabrication, melt, forging, machining, surface treatments, other

material properties, required inspections, etc.); (2) all the necessary characteristics required of the

design in its intended application; and 3) any other data required to show identicality to the type design

of the part covered under the TC as defined in FAR 21.31 (a) through (e). Therefore, the applicant

would need to provide appropriate details and compare every aspect of the manufacturing process, from

raw material procurement and first ariticle testing, through finished part, to substantiate identicality. 

Also, the applicant would need to substantiate a level of engineering and manufacturing oversight of

suppliers and vendors that is consistent with the TC holder’s. This is a substantial task that requires

significant engineering, manufacturing, and quality control resources and the cognizant ACO should

ensure applicants seeking PMA by identicality for these types of parts possess these necessary

resources. Once approved under identicality, the part would be subject to the same inspection and

maintenance requirements as the part approved under the TC.

3. PMA BY TEST AND COMPUTATIONS

Applicable Airworthiness Requirements: Federal Aviation Regulation (FAR) 21.303 (c)(4) requires that

if the applicant cannot substantiate that the design of the PMA part is identical to the design of the part

covered by the type certificate, then he must provide test reports and computations necessary to show

that the design of the part meets the airworthiness requirements of the FARs applicable to the product on

which the part is to be installed. Order 8110.42 specifies that “the certification basis for the PMA part is

the same as that for the product on which the part is to be installed”. For engine parts, the applicable

airworthiness requirements for test and computations would be CAR 13 or the applicable amendment

level of FAR 33. Regardless of the certification basis, the compliance program should be established by
the cognizant ACO in conjunction with the applicant, the TC holder’s ACO, and with this office and

should substantiate a level of airworthiness that is consistent with the following requirements:

FAR Subsection Requirement

33.4 Instructions for Continued Airworthiness

33.15 Materials

33.19 Durability

33.33 Vibration

33.42 General

33.43 Vibration Test

33.45 Calibration Tests

33.49 Endurance Test

33.53 Component Test

33.55 Teardown Inspection

33.57 General Conduct of Block Tests

Concurrent Introduction of Multiple Part Numbers: If the applicant chooses to introduce several

different P/Ns of a common type of part concurrently (e.g., several different crankshaft configurations),

then a separate compliance plan must be developed for each these P/Ns describing the applicant’s

proposed method of compliance to each applicable airworthiness requirement. For each unique P/N, test

and computations must be submitted to substantiate compliance to each requirement, unless

substantiation can be accomplished by a similarity analysis. This analysis can be relative to either; 1)

one of the applicant’s multiple P/Ns; or 2) the part covered under the TC.

• Relative to One of the Applicant Multiple P/Ns: If the similarity analysis is relative to one of the

applicant’s multiple P/Ns that is being concurrently introduced, then it should be carefully scrutinized by

the cognizant ACO to ensure that it addresses all aspects of the differences in design, installation,

manufacturing, and operating environment of the parts. Generalized statements that relate qualitative

comparisons of geometric or material differences between the parts to the horsepower ratings should be

considered inadequate and not consistent with the comprehensive engineering analyses necessary

substantiate compliance with airworthiness requirements.

• Relative to the Part Covered Under the TC: Comparative analyses relative to the TC holders parts

are permitted in accordance with Order 8110.42, paragraph 8.d.2, but the applicant would be required to

analyze the effect of all differences in design, manufacturing, and operation to the level of detail

described in the Identicality discussion above. The cognizant ACO must ensure that the applicant

possesses the knowledge of the TC holders design and manufacturing process to the extent necessary to

make a valid comparative analysis. As stated above, generalized or simple statements comparing

features of the parts are inadequate.

In either case, the applicant’s similarity analysis would be much more compelling if only one part is

introduced initially, and the introduction of the remaining parts is delayed until a substantive level of

experience is accumulated.

Airworthiness Limitations: A thoroughly substantiated compliance program by test and computations

will assure all applicable regulatory requirements are met, but will not necessarily demonstrate the same

tolerance to failure modes, or durability, as the design of the part covered under the TC has shown in

service. The applicant must be able to substantiate that the PMA part is designed and constructed to
minimize the development of an unsafe condition between overhaul periods, as required by FAR 33.14

(Durability). However, Time Between Overhaul (TBOs) for reciprocating engines can be as lengthy as

2000 hours, which would be very difficult to substantiate by test and computations, or by comparative

analysis (see above discussion). Substantiation to such lengthy overhaul intervals would also require

that the applicant accumulate the service, test, and manufacturing experience necessary to develop

knowledge on the statistical variability associated with the operating environment (e.g., stress,

temperature, mission profile), material properties, and manufacturing processes, and establish a

comprehensive continued airworthiness program (e.g., records, acquisition and analysis of field data,

capability to support field corrective actions, etc.).

Therefore, critical, highly stressed and/or complex PMA parts (e.g., a crankshaft) to be installed in these

engines cannot automatically or by default be substantiated to these lengthy intervals. It can then be

concluded that the TC holder’s service instructions relative to overhaul intervals are not adequate for the

highly stressed and/or complex PMA part or component, and, in accordance with paragraph 8.g of 

Order 8110.42, the PMA applicant may be required to provide Instructions for Continued Airworthiness,

in accordance with FAR 33.4, regardless of certification basis. The instructions for a more frequent

inspection interval (relative to the engine TBO) should take the form of mandatory Airworthiness

Limitations, to ensure compliance in accordance with FAR 43.16 and 91.403 (c). The FAA will,

however, consider relieving this requirement if the certification testing program is expanded to

substantiate the increased endurance and durability necessary for these longer TBOs.

All ACO’s with such programs should submit a Certification Program Notice (CPN) to this office and

review the project in detail with the assigned Project Officer.

Original Signed By:

Thomas A. Boudreau, Manager ANE-110