+--------------+-------------------------------------------------------+
| ![]          | Federal Aviation                                      |
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| 1.jpeg){widt | Administration                                        |
| h="1.1666666 |                                                       |
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+--------------+-------------------------------------------------------+
| Memorandum   |                                                       |
+--------------+-------------------------------------------------------+

Date: September 6, 2007

From: Manager, Engine and Propeller Directorate, Aircraft Certification
Service

To: SEE DISTRIBUTION

Prepared by: Mark Rumizen, ANE-111, 781-238-7113 or Mark.Rumizen@faa.gov

Subject: **[ACTION]{.underline}**: Policy for Diesel (Compression
Ignition) Engine Certification, 33.7 \[ANE-2006-33.7-4-1\]

### 1. Purpose.

The purpose of this memorandum is to clarify the certification
requirements for 14 CFR part 33, type certification of diesel
reciprocating engines. This memorandum provides compliance
interpretations necessary to accommodate the unique design features of a
diesel reciprocating engine. It also identifies areas of regulatory
compliance that may require equivalent level of safety findings (ELOS)
or special conditions (SC) to address design features of diesel engines
not envisioned when part 33 was created. However, an appropriate ELOS or
special condition for each engine model must be determined on a
case-by-case basis, in accordance with part 21, §§ 21.16, 21.17, and
part 11.

**2. Related Documents.**

Final Policy Statement, Diesel Engine Installation, PS-ACE100-2002-004,
dated May 15, 2003

**3. Background.**

Several diesel engine models were certified in the United States and
Europe before World War II. However, the development of higher
performance spark-ignition engines fueled by leaded aviation gasoline
(AVGAS) during that conflict resulted in a suspension of further
development of these engines. Interest in diesel aircraft engines has
recently been renewed in response to the demand for engines that do not
require leaded fuel for operation. Many new issues develop as modern
diesel engine technology is integrated into these aircraft engines.

**4. Policy.**

All proposed diesel engine type certification projects must be
coordinated with the Engine and Propeller Directorate, as directed in
Order 8100.5A, Section 3-3, and Order 8110.4C, Change 1, section 2-4.
These projects are categorized as significant by the EPD, and early
program coordination between the Standards Staff Office and the Aircraft
Certification Office (ACO) is necessary. The ACO is expected to notify
the Standards Staff Office of these projects by promptly submitting the
certification project notification (CPN) and associated certification
plans as soon as practical after the project application is received.
The ACO should identify technical areas of concern based on the guidance
contained in this policy statement, as well as any other identified
concerns. The ACO will then develop a G-1 issue paper to establish the
certification basis, including any anticipated ELOS findings or special
conditions.

a\. [Nomenclature]{.underline}. The following nomenclature will be used
to distinguish the two different types of reciprocating engines:

\(1\) Compression-Ignition (CI): The combustion process is initiated
solely by the injection of fuel into hot, compressed air, as in a diesel
engine. Diesel engines will be referred to as CI engines in this memo.

\(2\) Spark-Ignition (SI): An electrical spark applied to a compressed
fuel and air mixture begins the combustion process in most conventional
reciprocating aircraft engines. Therefore, conventional engines are
referred to as "spark ignition" (SI) engines.

####  b. [Part 33 Compliance for CI Reciprocating Engines]{.underline}.

#########  The regulations applicable to CI reciprocating engines include, but are not limited to, the following: 

Table 1. Affected Regulations

+----------+--------------------+----------+--------------------------+
| **PART   |                    |          |                          |
| 33 --    |                    |          |                          |
| AIRWO    |                    |          |                          |
| RTHINESS |                    |          |                          |
| ST       |                    |          |                          |
| ANDARDS: |                    |          |                          |
| AIRCRAFT |                    |          |                          |
| ENGINES: |                    |          |                          |
| Affected |                    |          |                          |
| Regul    |                    |          |                          |
| ations** |                    |          |                          |
+==========+====================+==========+==========================+
|          |                    |          |                          |
+----------+--------------------+----------+--------------------------+
| **S      | **Title**          | *        | **Diesel Engine          |
| ection** |                    | *Applica | compliance               |
|          |                    | bility** | issue/COMMENTS**         |
+----------+--------------------+----------+--------------------------+
|          |                    |          |                          |
+----------+--------------------+----------+--------------------------+
| *        |                    |          |                          |
| *SUBPART |                    |          |                          |
| A --     |                    |          |                          |
| G        |                    |          |                          |
| ENERAL** |                    |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.1     | applicability      | Yes      | None                     |
+----------+--------------------+----------+--------------------------+
| 33.3     | General            | Yes      | none                     |
+----------+--------------------+----------+--------------------------+
| 33.4     | Instructions for   | yes      | none                     |
|          | continued          |          |                          |
|          | airworthiness      |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.5     | instruction manual | Yes      | None                     |
|          | for installing and |          |                          |
|          | operating the      |          |                          |
|          | engine             |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.7     | engine ratings and | Yes      | ELOS for Fuel Flow/power |
|          | operating          |          | lever angle used as      |
|          | limitations        |          | engine operating         |
|          |                    |          | limitation in lieu of    |
|          |                    |          | map\*.                   |
|          |                    |          |                          |
|          |                    |          | Fuel Specification       |
|          |                    |          | Oversight.               |
|          |                    |          |                          |
|          |                    |          | CETANE NUMBER analysis.  |
+----------+--------------------+----------+--------------------------+
| 33.8     | selection of       | Yes      | None                     |
|          | engine power and   |          |                          |
|          | thrust ratings     |          |                          |
+----------+--------------------+----------+--------------------------+
|          |                    |          |                          |
+----------+--------------------+----------+--------------------------+
| *        |                    |          |                          |
| *Subpart |                    |          |                          |
| B --     |                    |          |                          |
| design   |                    |          |                          |
| and      |                    |          |                          |
| const    |                    |          |                          |
| ruction; |                    |          |                          |
| g        |                    |          |                          |
| eneral** |                    |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.11    | applicability      | yes      | none                     |
+----------+--------------------+----------+--------------------------+
| 33.14    | start-stop cyclic  | Yes      | None                     |
|          | stress             |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.15    | materials          | Yes      | None                     |
+----------+--------------------+----------+--------------------------+
| 33.17    | fire prevention    | Yes      | evaluation of high       |
|          |                    |          | pressure fuel system     |
+----------+--------------------+----------+--------------------------+
| 33.19    | durability         | Yes      | Containment Safety       |
|          |                    |          | analysis                 |
+----------+--------------------+----------+--------------------------+
| 33.21    | engine cooling     | Yes      | None                     |
+----------+--------------------+----------+--------------------------+
| 33.23    | engine moun ting   | Yes      | None                     |
|          | attachments and    |          |                          |
|          | structure          |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.25    | Accessory          | yes      | none                     |
|          | attachments        |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.27    | turbine,           | yes      | none                     |
|          | compressor, fan    |          |                          |
|          | and turbo          |          |                          |
|          | supercharger       |          |                          |
|          | rotors             |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.28    | electrical and     | yes      | See AC 33.28-2           |
|          | electronic engine  |          |                          |
|          | control systems    |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.29    | instrument         | yes      | none                     |
|          | connection         |          |                          |
+----------+--------------------+----------+--------------------------+
|          |                    |          |                          |
+----------+--------------------+----------+--------------------------+
| *        |                    |          |                          |
| *subpart |                    |          |                          |
| c --     |                    |          |                          |
| deisign  |                    |          |                          |
| and      |                    |          |                          |
| const    |                    |          |                          |
| ruction; |                    |          |                          |
| recip    |                    |          |                          |
| rocating |                    |          |                          |
| aircraft |                    |          |                          |
| e        |                    |          |                          |
| ngines** |                    |          |                          |
+----------+--------------------+----------+--------------------------+
| 33.31    | applicability      | Yes      | None                     |
+----------+--------------------+----------+--------------------------+
| 33.33    | vibration          | Yes      | evaluation of high       |
|          |                    |          | pressure fuel line       |
|          |                    |          | integrity                |
+----------+--------------------+----------+--------------------------+
| 33.35    | fuel and induction | Yes      | ELOS to use Turbine fuel |
|          | system             |          | system icing regulation  |
|          |                    |          | § 33.67                  |
+----------+--------------------+----------+--------------------------+
| 33.37    | ignition system    | no       |                          |
+----------+--------------------+----------+--------------------------+
| 33.39    | lubrication system | Yes      | None                     |
+----------+--------------------+----------+--------------------------+

[\*Manifold air pressure]{.smallcaps}

Table 1. Affected Regulations (continued)

  --------------------------------------------------------------------------------------
  **PART 33 --                                               
  AIRWORTHINESS                                              
  STANDARDS:                                                 
  AIRCRAFT ENGINES:                                          
  Affected                                                   
  Regulations**                                              
  ----------------- -------------------- ------------------- ---------------------------
                                                             

  **Section**       **Title**            **Applicability**   **Diesel Engine compliance
                                                             issue/comments**

                                                             

  **subpart d --                                             
  block tests;                                               
  reciprocating                                              
  aircraft                                                   
  engines**                                                  

  33.41             applicability        Yes                 None

  33.42             general              Yes                 None

  33.43             vibration test       Yes                 single cylinder inoperative
                                                             test applies

  33.45             calibration test     Yes                 None

  33.47             detonation test      YES                 Not applicable

  33.49             Endurance test       Yes                 None

  33.51             operation test       Yes                 evaluation of overspeed

  33.53             engine component     Yes                 None
                    tests                                    

  33.55             teardown inspection  yes                 none

  33.57             general conduct of   yes                 none
                    block tests                              

                    Engine Containment   Yes                 SC for containment safety
                                                             analysis

                                                             

  **subpart e -                                              
  deisign and                                                
  construction;                                              
  turbine aircraft                                           
  engines -- not                                             
  applicable**                                               

                                                             

  **subpart d --                                             
  block tests;                                               
  turbine aircraft                                           
  engines -- not                                             
  applicable**                                               

                                                             

  **appendix a to                                            
  part 33 --                                                 
  instructions for                                           
  continued                                                  
  airworthiness**                                            

  a33.1             General.             Yes                 None

  a33.2             format               Yes                 None

  a33.3             content              Yes                 None

  a33.4             airworthiness        Yes                 None
                    limitations                              

                                                             

  **appendix b to                                            
  part 33 --                                                 
  certification                                              
  standard                                                   
  atmospheric                                                
  concentrations of                                          
  rain and hail --                                           
  not applicable**                                           
  --------------------------------------------------------------------------------------

c\. [Manifold Air Pressure Indicator: (§ 33.7(b)(1))]{.underline}.

\(1\) Section 33.7(b)(1) requires that ratings and operating limitations
be established for "manifold air pressure", but this may not be a
relevant engine parameter for CI engines.

\(2\) For an SI engine, engine power output is typically represented by
manifold air pressure and engine RPM. Manifold air pressure is also used
to control turbocharger speed and boost. These two parameters allow the
pilot to control the engine within the approved operating limits.
However, CI engines typically do not throttle airflow into the engine
inlet, but rather allow the engine to pump as much air as it demands.
The airflow and manifold air pressure are simply a function of the
engine RPM and ambient conditions, and the engine power is modulated by

controlling fuel flow. At a given RPM and ambient condition, the engine
power varies as fuel flow is increased from the lean limit, where
combustion cannot be supported, to a "rich limit" where power begins to
drop off because of excess fuel. Between these lean and rich limits,
fuel flow is the only indicator of engine power output at a given RPM.
Fuel flow is typically controlled by a power lever angle input into the
high-pressure fuel pump. Therefore, the power level is best represented
by a power lever angle and its association to the fuel flow.

\(3\) For turbocharged engines, other means such as a turbocharger
pressure ratio or speed indication can be used to control turbocharger
speed.

\(4\) Therefore, the engine parameter specified in § 33.7(b)(1) is not
appropriate for CI engines, and a method of compliance based on an ELOS
to §33.7(b)(1) should be established for approval of fuel flow or
power-lever angle as indicators of power level in place of manifold air
pressure.

d\. [Fuels Approval: (§ 33.7(b)(2))]{.underline}.

\(1\) Fuel Specification Oversight:

\(a\) Section 33.7 requires that the engine operating limitation be
established for "fuel grade or specification". This limitation is
usually specified on the type certificate data sheet (TCDS). The
specific limitations have varied over the years, from only octane grade
(for example, 80/87 AVGAS), to identifying the industry or company
specification numbers (for example, 80/87 AVGAS per ASTM D910, JET-A per
ASTM D1655). The specific details are dependent on the operating
requirements of the engine. For example, more modern engines may require
tightly controlled fuel properties that are defined in the ASTM
specifications, other industry specifications, or company
specifications. However, industry specifications are typically revised
on an annual basis, causing the specification number to change with each
revision (for example, ASTM D1655-05 to D1655-06). The FAA acceptance of
these annual revisions without re-certification is based on oversight
from a committee of industry expert's knowledgeable in fuel, and the
products the fuel is intended for. The primary purpose of this oversight
is to ensure the fuel remains \"fit for purpose\" for the type of
engines referenced in the specification. As a result, this committee
consensus process negates the need for re-certification of every engine
and aircraft when a new revision is issued.

\(b\) It is anticipated that CI engines will use jet fuels, such as
JET-A, which are produced in accordance with the ASTM International
specification D1655, or automotive diesel fuels produced in accordance
with automotive fuel specifications. In either case, the CI engine TC
applicant should provide evidence that they are an active member of the
ASTM or other industry committee responsible for oversight of the
designated fuel specification. In addition, the TC applicant should
evaluate specification revisions on an annual basis for their impact on
"fit for purpose" relative to aviation CI engines.

\(2\) Cetane Number Analysis: Aviation jet fuel specifications such as
ASTM International D1655 do not include criteria for control of cetane
number because this is not a critical performance parameter for turbine
engines. The cetane number characterizes the ignition capability of a
diesel fuel, and is a critical parameter for assessing the acceptability
of a fuel for a particular engine design. Using the appropriate cetane
rated fuel in a diesel engine is critical to developing the appropriate
power. Therefore, the CI engine applicant will be required to provide an
analysis that substantiates operation with jet fuel with an inadequate
cetane number will not create an unsafe condition. The applicant's
analysis should consider the minimum cetane requirement of their CI
engine design, the probability that commercially available fuel may have
a cetane number below that minimum, and the consequences of operating
with a fuel with a cetane number that is below that minimum.

\(3\) Additives: Additives needed for turbine fuels that will also be
required by an aircraft CI engine (such as anti-icing and biocide
additives) should be specified on the engine TCDS and in the fuel
specification listed in the TCDS.

\(4\) Automotive Diesel Fuel: If automotive diesel fuel is to be
approved, appropriate specifications will need to be identified.
Approval of fuels that do not have a specification (such as protein or
plant-based (bio-diesel) fuels) should be handled on a case-by-case
basis.

\(5\) Materials Compatibility: In addition to engine performance
requirements, compatibility of the fuel system materials (for example:
elastomers, sealants, seals, liners, hoses, composite parts) with the
approved fuels for an aircraft CI engine has to be established.

e\. [High Pressure Fuel Lines: (§§ 33.17 and 33.33)]{.underline}.

Using high-pressure fuel lines is common in the automotive industry, but
it is limited in the aircraft piston engine environment. However,
aircraft CI engines typically require fuel under high pressure to
support the combustion process. Failure of an external fuel line that
carries this high-pressure fuel may create a fire hazard and needs to be
evaluated under § 33.17 (Fire Prevention) and § 33.33 (vibration). This
should be accomplished as part of the basic compliance effort.

#######  f. Engine Containment: (§ 33.19).

\(1\) The possibility that engine failures may lead to liberation of
high-energy engine fragments must be evaluated because much higher
cylinder pressures exist in CI engines. The applicant should perform a
safety analysis that addresses the engine construction and possible
failure modes.

\(2\) The engine manufacturer should perform the safety analysis and
submit it to the FAA. If the safety analysis indicates a possible engine
failure mode that will liberate engine parts, then the energy levels of
the anticipated fragments should be characterized and noted in the
Installation Manual. This information will then be used during the 14
CFR part 23 aircraft certification process. This should be accomplished
as part of the basic compliance effort.

g\. [Electronic Engine Controls: (§ 33.28)]{.underline}.

Many new technology aircraft CI engines use a Full Authority Digital
Engine Control (FADEC) or Electronic Engine Control (EEC). Such systems
are not unique to aircraft

CI engines and are increasingly common on conventional reciprocating
engines. FAA AC 33.28-2 addresses EECs in reciprocating engines, and
most of the guidance in that document is applicable to CI engines as
well as SI engines. Note that coordination with the aircraft
manufacturer and the FAA ACO responsible for the aircraft certification
is necessary to ensure the lightning, HIRF, and EMI testing levels used
for the engine-level part 33 compliance meet the corresponding
aircraft-level certification requirements of part 23.

h\. [Fuel System Icing: (§ 33.35(b) and]{.underline} §
33.67[)]{.underline}.

\(1\) JET-A fuel has the potential to absorb greater amounts of water
into the solution than AVGAS, making it more susceptible for icing. In
addition, the higher fuel system operating pressures of CI engines,
relative to spark ignition engines, also increases the risk of fuel
icing. Sustained operation of a CI engine using jet fuel at high
pressure flows could result in supercooled water adhering to fuel system
surfaces, freezing, and forming blockages in the fuel system. Therefore,
CI reciprocating engines are very similar to turbine engines, relative
to the fuel used and the potential of freezing occurring before mixing
with air. The current reciprocating engine fuel carburetor icing
regulation (§ 33.35(b)) is intended to address condensation of
moisture-laden air occurring in the carburetor throat area and forming
ice that blocks the airflow. Accordingly, CI engine manufacturers should
apply the turbine engine fuel icing requirements of § 33.67 to CI
engines instead of using § 33.35(b), which addresses the fuel system
icing requirements of reciprocating engines. Because of this unique
attribute of CI engines, the certification office must also ensure that
it evaluates CI engine fuel system icing using the turbine engine fuel
icing requirements of § 33.67, not the reciprocating engine fuel
carburetor icing requirements of § 33.35(b).

\(2\) Jet fuel icing protection is addressed for turbine engine fuel
systems in

§ 33.67(b)(4)(ii). That regulation requires the fuel system for a
turbine engine to sustain operation throughout its flow and pressure
range with fuel initially saturated with water at 80° F and having 0.025
fluid ounces of free water per gallon added and cooled to the most
critical condition for icing likely to be encountered in operation. Some
turbine engines have shown compliance with this requirement by mandating
the use of fuel anti-icing additives. Mandating the use of approved
anti-icing additives would be an acceptable method to show compliance;
however, the use of these additives is generally being decreased in some
countries for environmental reasons. As an alternative, the applicant
may show compliance by incorporating a fuel heater which maintains the
fuel temperature at the fuel strainer or fuel inlet above 32° F (0° C)
under the most critical conditions. The requirements of § 33.67 should
be applied as an ELOS to §33.35(b) for the CI engines.

####  i. [Vibration: (§§ 33.33 and 33.43)]{.underline}.

\(1\) Aircraft CI engines may yield a greater level of vibration than
current SI engines. As part of the basic compliance for aircraft CI
engine type certification, the effects of vibration levels higher than
those typical for conventional reciprocating engine powered airplanes
must be considered. In addition, the one-cylinder-inoperative vibration
test of §33.43(d) is applicable to CI engines and should be conducted by
deactivating a fuel injector.

\(2\) The vibratory loads imparted to the airframe by a CI engine may
exceed those of a conventional SI engine, and therefore need to be
defined during the engine certification process. This vibration data
should be included in the engine instructions for installation in
accordance with § 33.5 to establish it as required powerplant
installation design data

j\. [Detonation Test: (§ 33.47)]{.underline}.

Section 33.47 requires that applicants demonstrate that each engine is
"tested to establish the engine can function without detonation
throughout its range of intended conditions of operation." SI engines
rely on spark-initiated combustion timed to occur at a precise moment in
relation to piston travel. However, under excessive temperatures or
pressures, the fuel-air mixture can self-detonate without the aid of the
spark. This is an abnormal operating condition in SI engines. In CI
engines, combustion is initiated by injecting fuel into a compressed,
high-temperature chamber, without the aid of a spark. Therefore,
uncontrolled detonation is very unlikely. Accordingly, § 33.47 is not
applicable for CI engines.

k\. [Engine Overspeed: (§ 33.51)]{.underline}.

CI engines have a greater potential for destructive overspeed due to an
oversupply of fuel. The engine control system operation and the engine
structural integrity should be evaluated relative to overspeed
conditions. This should be accomplished as part of the basic compliance
effort.

####### 

## 5. Effect of Policy.

a\. The general policy stated in this document does not constitute a new
regulation nor create a "binding norm". Offices should follow this
policy when it is applicable to a specific project. Whenever an
applicant\'s proposed method of compliance is outside this or any
established policy, the proposed method must be coordinated with the
policy issuing office (for example, through the issue paper process or
equivalent).

b\. Applicants should expect the FAA will consider this policy when
making findings of compliance relevant to new certificate actions.

######## 6. Conclusion.

All proposed CI engine type certification projects should be coordinated
with the Engine and Propeller Directorate, as defined in Order 8100.5A,
Section 3-3, and Order 8110.4C, Change 1, Section 2-4. The ACO is
expected to notify the Standards Staff Office of such projects by
promptly submitting the CPN and associated certification plans as soon
as practical after project application. The ACO will identify the
technological areas of concern identified in this policy paper, as well
as any additional concerns and develop a G-1 issue paper to establish
the certification basis. Signature authority for certificate issuance on
these projects are retained by the Standards Staff Office, and will be
redelegated on a case-by-case basis as this new technology is understood
and integrated into aerospace products.

/signed by Peter A White -- for Francis Favara/

Francis Favara

Distribution:

Manager, Aircraft Engineering Division, AIR-100

Manager, Brussels Aircraft Certification Staff, AEU-100

Manager, Small Airplane Directorate, ACE-100

Manager, Rotorcraft Directorate, ASW-100

Manager, Transport Airplane Directorate, ANM-100

Manager, Boston Aircraft Certification Office, ANE-150

Manager, New York Aircraft Certification Office, ANE‑170

Manager, Ft. Worth Airplane Certification Office, ASW‑150

Manager, Special Certification Office, ASW-190

Manager, Atlanta Aircraft Certification Office, ACE-115A

Manager, Chicago Aircraft Certification Office, ACE‑115C

Manager, Wichita Aircraft Certification Office, ACE‑115W

Manager, Anchorage Aircraft Certification Office, ACE‑115N

Manager, Seattle Aircraft Certification Office, ANM-100S

Manager, Denver Aircraft Certification Office, ANM‑100D

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