# U.S. Department of Transportation![](media/image1.wmf){width="0.5506944444444445in" height="0.5708333333333333in"}

## Federal Aviation Administration

+-----+-----------------------------------------+----+----------------+
| Su  | **[INFORMATION]{.underline}**: Final    | D  | May 15, 2003   |
| bje | Policy Statement; Diesel Engine         | at |                |
| ct: | Installation; PS-ACE100-2002-004        | e: |                |
+-----+-----------------------------------------+----+----------------+
|     |                                         |    |                |
+-----+-----------------------------------------+----+----------------+
| Fr  | Manager, Small Airplane Directorate,    | R  | Peter Rouse    |
| om: | ACE-100                                 | ep |                |
|     |                                         | ly | \(816\)        |
|     |                                         | to | 329-4135       |
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| To: | SEE DISTRIBUTION                        |    |                |
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### Summary

The purpose of this policy statement is to help identify appropriate
certification requirements for installation of a diesel engine into a
small airplane. It includes guidance related to methods of compliance as
well as potential equivalent level of safety findings (ELOS) and special
conditions.

Installation of diesel engines into part 23 airplanes may include design
features not envisioned when 14 CFR part 23 was created. This policy
highlights areas where special conditions may be appropriate for these
installations. However, appropriate special conditions for each
installation must be determined on a case-by-case basis in accordance
with 14 CFR part 21, §§ 21.16, 21.17 and 14 CFR part 11.

The installation of a diesel engine in a small certificated airplane
design is not considered a design change so substantial that it would
require a new airplane Type Certificate (TC) under 14 CFR part 21,
§ 21.19. Therefore, it is considered appropriate to install a
certificated diesel engine into a certificated airplane using the
Supplemental Type Certificate (STC) or Amended Type Certificate (ATC)
process.

All proposed diesel engine installations, whether supplemental, amended
or new TC projects, will be considered significant as defined in Order
8100.5. Given the significance of the change, early program coordination
between the Standards Office and the Aircraft Certification Office (ACO)
is necessary. The ACO is expected to notify the Standards Office of such
projects promptly and forward certification project notifications and
associated certification plans as soon as practical after project
application. The ACO will identify the technological areas of concern
identified in this policy statement as well as any additional concerns
and develop a G-1 issue paper to establish the certification basis.

II. **Discussion of Significant Issues**

#### General

Engines are being proposed to be certificated for installation in
aircraft, especially small airplanes and airships, that operate on
compression ignition power cycles rather than spark ignition power
cycles. Some aircraft diesel engines have been type certificated with
others expected in the near future. Diesel engines, however, have not
been utilized in type certificated airplanes in the United States for
over 70 years. While some historical experience may be useful, there are
many new and novel issues to be addressed as modern diesel engine
technology is integrated into modern aircraft.

#### Nomenclature

In 14 CFR part 23, the utilization of fuel requirements is often used as
a proxy for engine type (reciprocating versus turbine; AvGas versus Jet
A, respectively). The phrase "reciprocating engine" is often shorthand
for "operates on AvGas" and the phrase "turbine engine" is often
shorthand for "operates on kerosene-based fuels." To date, this approach
has been sufficient. However, an aircraft diesel engine is typically a
reciprocating engine that would operate on jet fuel, and it was not
envisioned by the 14 CFR part 23 rules. While this policy has been
developed to address most of the known technical issues where diesel
engine and gasoline engine requirements must be differentiated, there
may be other assumptions built into the regulations that will need to be
evaluated to determine the appropriate technical requirements.

For the purposes of this policy statement, the term \"conventional
reciprocating engine\" will be used to describe spark ignition,
reciprocating engines that run on AvGas. For the purposes of this policy
statement, the term \"aircraft diesel engine,\" or \"ADE,\" will be used
to describe compression ignition, reciprocating engines that run on
kerosene-based fuels.

#### Installation Manual

Title 14 CFR part 33, § 33.5, requires, as part of the engine type
certification effort, that an instruction manual for installation and
operation of the engine be prepared and approved prior to issuance of
the engine TC. 14 CFR part 23, § 23.901, requires that engine
installations comply with these instructions. The applicable regulations
identified in this policy statement (including potential special
conditions), combined with the engine installation instructions, should
provide a comprehensive list of installation requirements for most
diesel engine installations. In addition, the methods of compliance and
potential ELOS\'s described in this policy statement should assist
applicants and ACO engineers when assessing methods to demonstrate
compliance to applicable regulations.

#### Fuels

The FAA\'s current understanding is that Jet A is the leading candidate
for use in aircraft diesel engines; however, this does not rule out
other fuels such as #2 Diesel or some other alternative fuel
(bio-diesel). The engine Type Certificate Data Sheet (TCDS) identifies
the approved fuel(s) for use in an aircraft diesel engine. Any fuels to
be used in aircraft diesel engines will need a specification and Type
Certificate Data Sheet (TCDS) approval.

The fuel and additives required for the aircraft diesel engine will be
determined as part of the 14 CFR part 33 certification process and will
be specified on the engine TCDS and in the engine installation data. For
a small airplane powered with a reciprocating engine that operates on
kerosene-based fuels, such as an aircraft diesel engine, certain
"turbine engine" fuel-related requirements should be met in lieu of the
gasoline fuel related requirements. Additives needed for turbine fuels
that will also be required by an aircraft diesel engine (such as
anti-icing and biocide additives) will be called out on the engine TCDS
and in the fuel specification listed in the TCDS.

The appropriate fuels and additives, consistent with both the engine
certification and airframe certification, must also be specified on the
aircraft TCDS. However, when approaching an aircraft diesel engine
installation project, certain aspects of the diesel engine and its
certificated fuel requirements should be kept in mind. For instance, the
cetane number is the primary parameter that defines diesel engine fuel
that is similar to octane rating for conventional reciprocating engines.
Using the appropriate cetane rated fuel in a diesel engine is critical
to developing the appropriate power. Commercial turbine fuels will
probably not be given a cetane rating; therefore, aircraft diesel
engines will be approved and certificated to operate on a specific
turbine fuel(s). Any additional limiting parameters or specifications
for the fuel will need to be defined on the engine TCDS, in lieu of
specifying cetane requirements.

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 (biodiesel) fuels) is
anticipated; these will be handled on a case-by-case basis as approval
is requested. While the engine certification process addresses these
concerns, it is best to be aware of the possible complications of
specifying appropriate fuels for an aircraft diesel engine and the
associated airframe compatibility issues. Compatibility of the fuel
system materials (elastomers, sealants, seals, liners, hoses, composite
parts, etc.) with the approved fuels for an aircraft diesel engine has
to be established.

#### Engine Controls

Many new technology aircraft diesel engines utilize a Full Authority
Digital Engine Control (FADEC) or Electronic Engine Control (EEC). Such
systems are not unique to aircraft diesel engines and are increasingly
common on conventional reciprocating engines. Previous aircraft FADEC
installations have required a special condition addressing § 23.1309 \--
Equipment, Systems and Installation. This special condition is available
from the Small Airplane Directorate Standards Office. In addition, an
appropriate airplane level analysis must be performed to determine
appropriate levels of High Intensity Radiated Fields (HIRF) compliance,
and the HIRF special condition will most likely be applied. The software
certification level will need to be compliant with the appropriate RTCA
DO178 level as described in AC 23.1309-1C.

This policy statement briefly touches on the subject of FADEC
installation; however, it is not intended to cover all of the aspects of
FADEC installation. A future policy statement covering FADEC
installation into 14 CFR part 23 aircraft is currently being written.

#### Vibration

Aircraft diesel engines may yield a greater level of vibration than
current aircraft propulsion systems. This issue is based upon feedback
from industry during the development of aircraft diesel engines. As part
of the basic compliance efforts for an aircraft diesel engine airplane,
the effects of vibration levels higher than those typical for
conventional reciprocating engine powered airplanes must be considered.
In addition, the one cylinder inoperative condition will need to be
evaluated. The primary concern is that the vibratory loads imparted to
the airframe by an aircraft diesel engine may be in excess of the
experience of the general aviation fleet, thereby reducing the margin of
design safety. The goal of this policy statement is to ensure that the
aircraft fleet will have an equivalent or better level of safety without
unnecessary regulatory requirements.

#### Regulatory Review

The following table is taken from 14 CFR part 23 with an emphasis on the
sections that require more consideration when incorporating a diesel
engine for aircraft use. The following is the criteria for a special
condition as defined in § 21.16:

*"If the Administrator finds that the airworthiness regulations of this
subchapter do not contain adequate or appropriate safety standards for
an aircraft, aircraft engine, or propeller because of a novel or unusual
design feature of the aircraft, aircraft engine or propeller, he
prescribes special conditions and amendments thereto for the product.
The special conditions are issued in accordance with Part 11 of this
chapter and contain such safety standards for the aircraft, aircraft
engine or propeller as the Administrator finds necessary to establish a
level of safety equivalent to that established in the regulations."*

The policy statement makes several references to special conditions due
to the new and novel features of aircraft diesel engines. The existing
regulations lack the adequate or appropriate safety standards equivalent
to those currently utilized by aircraft propulsion systems. There are 14
potential special conditions identified in the regulations matrix that
starts on the next page. The sections annotated with an asterisk (\*)
highlight requirements that may be applicable via special condition or
may have additional requirements applied via special condition. See the
appropriate text paragraph describing the potential criteria for the
special condition.

  -------------------------------------------------------------------------------------
  PART 21 and 23 --                                       
  AIRWORTHINESS                                           
  STANDARDS:                                              
  NORMAL, UTILITY,                                        
  ACROBATIC, AND                                          
  COMMUTER CATEGORY                                       
  AIRPLANES:                                              
  Affected                                                
  Regulations                                             
  ----------------- --------------------- --------------- -----------------------------
                                                          

  Section           Title                 Applicability   Anticipated Differences and
                                                          Rationale

                                                          

  21.19             Issue of type         Yes             None
                    certificate: Normal,                  
                    utility, acrobatic,                   
                    commuter, and                         
                    transport category                    
                    aircraft; manned free                 
                    balloons; special                     
                    classes of aircraft;                  
                    aircraft engines;                     
                    propellers                            

  21.21\*           Certification         Yes             See comments regarding unsafe
                    Procedures For                        conditions. Special
                    Products and Parts                    Condition(s) may apply -- See
                                                          paragraph text.

                                                          

  23.1              Applicability.        Yes             None

  23.2              Special retroactive   Yes             None
                    requirements.                         

  23.3              Airplane categories.  Yes             None

                                                          

  Subpart B --                                            
  Flight                                                  

                                                          

  GENERAL                                                 

                                                          

  23.21             Proof of compliance.  Yes             None

  23.23             Load distribution     Yes             None
                    limits.                               

  23.25             Weight limits.        Yes             None

  23.29             Empty weight and      Yes             None
                    corresponding center                  
                    of gravity                            

  23.31             Removable ballast.    Yes             None

  23.33             Propeller speed and   Yes             None
                    pitch limits                          

                                                          

  PERFORMANCE                                             

                                                          

  23.45             General.              Yes             None

  23.49             Stalling period.      Yes             None

  23.51             Takeoff speeds.       Yes             None

  23.53             Takeoff performance.  Yes             None

  23.55             Accelerate-stop       Yes             None
                    distance.                             

  23.57             Takeoff path.         Yes             None

  23.59             Takeoff distance and  Yes             None
                    takeoff run                           

  23.61             Takeoff flight path.  Yes             None

  23.63             Climb: General.       Yes             None

  23.65             Climb: All engines    Yes             None
                    operating                             

  23.66             Takeoff climb:        Yes             None
                    One-engine                            

  23.67             Climb: One engine     Yes             None
                    inoperative                           

  23.69             Enroute               Yes             None
                    climb/descent.                        

  23.71             Glide: Single-engine  Yes             See comment concerning glide
                    airplanes.                            characteristics in Subpart B
                                                          General discussion.

  23.73             Reference landing     Yes             None
                    approach speed                        

  23.75             Landing distance.     Yes             None

  23.77             Balked landing.       Yes             None

                                                          

  FLIGHT                                                  
  CHARACTERISTICS                                         

                                                          

  23.141            General.                              

                                                          

  CONTROLLABILITY                                         
  AND                                                     
  MANEUVERABILITY                                         

                                                          

  23.143            General.              Yes             None

  23.145            Longitudinal control. Yes             None

  23.147            Directional and       Yes             None
                    lateral control                       

  23.149            Minimum control       Yes             None
                    speed.                                

  23.151            Acrobatic maneuvers.  Yes             None

  23.153            Control during        Yes             None
                    landings.                             

  23.155            Elevator control      Yes             None
                    force in maneuvers                    

  23.157            Rate of roll.         Yes             None

                                                          

  TRIM                                                    

                                                          

  23.161            Trim.                 Yes             None

                                                          

  STABILITY                                               

                                                          

  23.171            General.              Yes             None

  23.173            Static longitudinal   Yes             None
                    stability.                            

  23.175            Demonstration of      Yes             None
                    static longitudinal                   
                    stability                             

  23.177            Static directional    Yes             None
                    and lateral stability                 

  23.181            Dynamic stability.    Yes             None

                                                          

  STALLS                                                  

                                                          

  23.201            Wings level stall.    Yes             None

  23.203            Turning flight and    Yes             None
                    accelerated turning                   
                    stalls                                

  23.207            Stall warning.        Yes             None

                                                          

  SPINNING                                                

                                                          

  23.221            Spinning.             Yes             None

                                                          

  GROUND AND WATER                                        
  HANDLING                                                
  Characteristics                                         

                                                          

  23.231            Longitudinal          Yes             None
                    stability and control                 

  23.233            Directional stability Yes             None
                    and control                           

  23.235            Operation on unpaved  Yes             None
                    surfaces                              

  23.237            Operation on water.   Yes             None

  23.239            Spray                 Yes             None
                    characteristics.                      

                                                          

  MISCELLANEOUS                                           
  FLIGHT                                                  
  REQUIREMENTS                                            

                                                          

  23.251            Vibration and         Yes             See comment concerning
                    buffeting.                            vibration characteristics

  23.253            High speed                            
                    characteristics.                      

                                                          

  Subpart C --                                            
  Structure                                               

                                                          

  GENERAL                                                 

                                                          

  23.301            Loads.                Yes             None

  23.302            Canard or tandem wing Yes             None
                    configuration                         

  23.303            Factor of safety.     Yes             None

  23.305            Strength and          Yes             None
                    deformation.                          

  23.307            Proof of structure.   Yes             None

                                                          

  FLIGHT LOADS                                            

                                                          

  23.321            General.              Yes             None

  23.331            Symmetrical flight    Yes             None
                    conditions.                           

  23.333            Flight envelope.      Yes             None

  23.335            Design airspeeds.     Yes             None

  23.337            Limit maneuvering     Yes             None
                    load factors                          

  23.341            Gust loads factors.   Yes             None

  23.343            Design fuel loads.    Yes             None

  23.345            High lift devices.    Yes             None

  23.347            Unsymmetrical flight  Yes             None
                    conditions.                           

  23.349            Rolling conditions.   Yes             None

  23.351            Yawing conditions.    Yes             None

  23.361\*          Engine torque.        Yes             See comment concerning engine
                                                          torque characteristics. NOTE
                                                          sudden stoppage cases should
                                                          be considered. Special
                                                          Condition(s) may apply -- See
                                                          paragraph text.

  23.363            Side load on engine   Yes             None
                    mount                                 

  23.365            Pressurized cabin     Yes             None
                    loads.                                

  23.367            Unsymmetrical loads   Yes             None
                    due to engine failure                 

  23.369            Rear lift truss.      Yes             None

  23.371            Gyroscopic and        Yes             None
                    aerodynamic loads                     

  23.373            Speed control         Yes             None
                    devices.                              

                                                          

  CONTROL SURFACE                                         
  AND SYSTEM LOADS                                        

                                                          

  23.391            Control surface       Yes             None
                    loads.                                

  23.393            Loads parallel to     Yes             None
                    hinge line                            

  23.395            Control system loads. Yes             None

  23.397            Limit control forces  Yes             None
                    and torques                           

  23.399            Dual control system.  Yes             None

  23.405            Secondary control     Yes             None
                    system.                               

  23.407            Trim tab effects.     Yes             None

  23.409            Tabs.                 Yes             None

  23.415            Ground gust           Yes             None
                    conditions.                           

                                                          

  HORIZONTAL                                              
  STABILIZING AND                                         
  BALANCING                                               
  SURFACES                                                

                                                          

  23.421            Balancing loads.      Yes             None

  23.423            Maneuvering loads.    Yes             None

  23.425            Gust loads.           Yes             None

  23.427            Unsymmetrical loads.  Yes             None

                                                          

  VERTICAL SURFACES                                       

                                                          

  23.441            Maneuvering loads.    Yes             None

  23.443            Gust loads.           Yes             None

  23.445            Outboard fins or      Yes             None
                    winglets                              

                                                          

                                                          

                                                          

                                                          

  AILERONS AND                                            
  SPECIAL DEVICES                                         

                                                          

  23.455            Ailerons.             Yes             None

  23.459            Special devices.                      

                                                          

  GROUND LOADS                                            

                                                          

  23.471            General.              Yes             None

  23.473            Ground load           Yes             None
                    conditions and                        
                    assumptions                           

  23.477            Landing gear          Yes             None
                    arrangement.                          

  23.479            Level landing         Yes             None
                    conditions.                           

  23.481            Tail down landing     Yes             None
                    conditions                            

  23.483            One-wheel landing     Yes             None
                    conditions.                           

  23.485            Side load conditions. Yes             None

  23.493            Braked roll           Yes             None
                    conditions.                           

  23.497            Supplementary         Yes             None
                    conditions for tail                   
                    wheels                                

  23.499            Supplementary         Yes             None
                    conditions for nose                   
                    wheels                                

  23.505            Supplementary         Yes             None
                    conditions for                        
                    skiplanes                             

  23.507            Jacking loads.        Yes             None

  23.509            Towing loads.         Yes             None

  23.511            Ground load;          Yes             None
                    unsymmetrical loads                   
                    on multiple-wheel                     
                    units                                 

                                                          

  WATER LOADS                                             

                                                          

  23.521            Water load            Yes             None
                    conditions.                           

  23.523            Design weights and    Yes             None
                    center of gravity                     

  23.525            Application of loads. Yes             None

  23.527            Hull and main float   Yes             None
                    load factors                          

  23.529            Hull and main float   Yes             None
                    landing conditions                    

  23.531            Hull and main float   Yes             None
                    takeoff condition                     

  23.533            Hull and main float   Yes             None
                    bottom pressures                      

  23.535            Auxiliary float       Yes             None
                    loads.                                

  23.537            Seawing loads.        Yes             None

                                                          

  EMERGENCY LANDING                                       
  CONDITIONS                                              

                                                          

  23.561            General.              Yes             None

  23.562            Emergency landing     Yes             None
                    dynamic conditions                    

                                                          

  FATIGUE                                                 
  EVALUATION                                              

                                                          

  23.571            Metallic pressurized  Yes             None
                    cabin structures                      

  23.572            Metallic wing,        Yes             See comment concerning
                    empennage, and                        vibration characteristics
                    associated structures                 

  23.573            Damage tolerance and  Yes             See comment concerning
                    fatigue evaluation of                 vibration characteristics
                    structure                             

  23.574            Metallic damage       Yes             See comment concerning
                    tolerance and fatigue                 vibration characteristics
                    evaluation of                         
                    commuter category                     
                    airplanes                             

  23.575            Inspections and other Yes             None
                    procedures                            

                                                          

  Subpart D --                                            
  Design and                                              
  Construction                                            

                                                          

  23.601            General.              Yes             None

  23.603            Materials and         Yes             None
                    workmanship.                          

  23.605            Fabrication methods.  Yes             None

  23.607            Fasteners.            Yes             None

  23.609            Protection of         Yes             None
                    structure.                            

  23.611            Accessibility         Yes             None
                    provisions.                           

  23.613            Material strength     Yes             See comment concerning
                    properties and design                 vibration characteristics
                    values                                

  23.619            Special factors.      Yes             None

  23.621            Casting factors.      Yes             None

  23.623            Bearing factors.      Yes             None

  23.625            Fitting factors.      Yes             None

  23.627            Fatigue strength.     Yes             See comment concerning
                                                          vibration characteristics

  23.629\*          Flutter.              Yes             See comment concerning
                                                          vibration characteristics.
                                                          Special Condition(s) may
                                                          apply -- See paragraph text.

                                                          

  WINGS                                                   

                                                          

  23.641            Proof of strength.    Yes             None

                                                          

  CONTROL SURFACES                                        

                                                          

  23.651            Proof of strength.    Yes             None

  23.655            Installation.         Yes             None

  23.657            Hinges.               Yes             None

  23.659            Mass balance.         Yes             None

                                                          

  CONTROL SYSTEMS                                         

                                                          

  23.671            General.              Yes             None

  23.672            Stability             Yes             None
                    augmentation and                      
                    automatic and                         
                    power-operated                        
                    systems                               

  23.673            Primary flight        Yes             None
                    controls.                             

  23.675            Stops.                Yes             None

  23.677            Trim systems.         Yes             None

  23.679            Control system locks. Yes             None

  23.681            Limit load static     Yes             None
                    tests                                 

  23.683            Operation tests.      Yes             None

  23.685            Control system        Yes             None
                    details.                              

  23.687            Spring devices.       Yes             None

  23.689            Cable systems.        Yes             None

  23.691            Artificial stall      Yes             None
                    barrier system                        

  23.693            Joints.               Yes             None

  23.697            Wing flap controls.   Yes             None

  23.699            Wing flap position    Yes             None
                    indicator                             

  23.701            Flap interconnection. Yes             None

  23.703            Takeoff warning       Yes             None
                    system.                               

                                                          

  LANDING GEAR                                            

                                                          

  23.721            General.              Yes             None

  23.723            Shock absorption      Yes             None
                    tests.                                

  23.725            Limit drop tests.     Yes             None

  23.726            Ground load dynamic   Yes             None
                    tests                                 

  23.727            Reserve energy        Yes             None
                    absorption drop test                  

  23.729            Landing gear          Yes             None
                    extension and                         
                    restraction system                    

  23.731            Wheels.               Yes             None

  23.733            Tires.                Yes             None

  23.735            Brakes.               Yes             None

  23.737            Skis.                 Yes             None

  23.745            Nose/tail wheel       Yes             None
                    steering.                             

                                                          

  FLOATS AND HULLS                                        

                                                          

  23.751            Main float buoyancy.  Yes             None

  23.753            Main float design.    Yes             None

  23.755            Hulls.                Yes             None

  23.757            Auxiliary floats.     Yes             None

                                                          

  PERSONNEL AND                                           
  CARGO                                                   
  ACCOMMODATIONS                                          

                                                          

  23.771            Pilot compartment.    Yes             None

  23.773            Pilot compartment     Yes             None
                    view.                                 

  23.775            Windshields and       Yes             None
                    windows.                              

  23.777            Cockpit controls.     Yes             See comment concerning single
                                                          control levers.

  23.779            Motion and effect of  Yes             See comment concerning single
                    cockpit controls                      control levers.

  23.781            Cockpit control knob  Yes             See comment concerning single
                    shape                                 control levers.

  23.783            Doors.                Yes             None

  23.785            Seats, berths,        Yes             None
                    litters, safety                       
                    belts, and shoulder                   
                    harnesses                             

  23.787            Baggage and cargo     Yes             None
                    compartments                          

  23.791            Passenger information Yes             None
                    signs.                                

  23.803            Emergency evacuation. Yes             None

  23.805            Flightcrew emergency  Yes             None
                    exits.                                

  23.807            Emergency exits.      Yes             None

  23.811            Emergency exit        Yes             None
                    marking.                              

  23.812            Emergency lighting.   Yes             None

  23.813            Emergency exit        Yes             None
                    access.                               

  23.815            Width of aisle.       Yes             None

  23.831            Ventilation.          Yes             None

                                                          

  PRESSURIZATION                                          

                                                          

  23.841            Pressurized cabins.   Yes             None

  23.843            Pressurization tests. Yes             None

                                                          

  FIRE PROTECTION                                         

                                                          

  23.851            Fire extinguishers.   Yes             None

  23.853            Passenger and crew    Yes             None
                    compartment interiors                 

  23.855            Cargo and baggage     Yes             None
                    compartment fire                      
                    protection                            

  23.859            Combustion heater     Yes             None
                    fire protection                       

  23.863            Flammable fluid fire  Yes             None
                    protection                            

  23.865            Fire protection of    Yes             None
                    flight controls,                      
                    engine mounts, and                    
                    other flight                          
                    structure                             

                                                          

  ELECTRICAL                                              
  BONDING AND                                             
  LIGHTNING                                               
  PROTECTION                                              

                                                          

  23.867            Electrical bonding    Yes             None
                    and protection                        
                    against lightning and                 
                    static electricity                    

                                                          

  MISCELLANEOUS                                           

                                                          

  23.871            Leveling means.       Yes             None

                                                          

  Subpart E --                                            
  Powerplant                                              

                                                          

  GENERAL                                                 

                                                          

  23.901\*          Installation.         Yes             See comment concerning unique
                                                          installation concerns:
                                                          vibration, one cylinder
                                                          inoperative, catastrophic
                                                          failure. Special Condition(s)
                                                          may apply -- See paragraph
                                                          text.

  23.903            Engines.              Yes             See comment concerning unique
                                                          installation concerns:
                                                          starting and stopping,
                                                          restarting envelope.

  23.904            Automatic power       Yes             None
                    reserve system                        

  23.905            Propellers.           Yes             None

  23.907\*          Propeller vibration.  Yes             See comment concerning unique
                                                          installation concerns:
                                                          vibration characteristics and
                                                          effects on propellers.
                                                          Special Condition(s) may
                                                          apply -- See paragraph text.

  23.909            Turbocharger systems. Yes             None

  23.925            Propeller clearance.  Yes             None

  23.929            Engine installation   Yes             None
                    ice protection                        

  23.933            Reversing systems.    Yes             None

  23.934            Turbojet and turbofan Yes             None
                    engine thrust                         

  23.937            Turbopropeller-drag   Yes             None
                    limiting systems.                     

  23.939            Powerplant operating  Yes             None
                    characteristics.                      

  23.943            Negative              Yes             None
                    acceleration.                         

                                                          

                                                          

                                                          

                                                          

  FUEL SYSTEM                                             

                                                          

  23.951\*          General.              Yes             See comment concerning water
                                                          absorbtion characteristics of
                                                          fuel. Note: turbine engine
                                                          requirements are sometimes
                                                          more relevant to apply due to
                                                          the nature of the ADE fuel.
                                                          Special Condition(s) may
                                                          apply -- See paragraph text.

  23.953            Fuel system           Yes             None
                    independence.                         

  23.954            Fuel system lightning Yes             None
                    protection                            

  23.955\*          Fuel flow.            Yes             See comment concerning fuel
                                                          temperature and fuel pump
                                                          requirements. Note: turbine
                                                          engine requirements are
                                                          sometimes more relevant to
                                                          apply due to the nature of
                                                          the ADE fuel. Special
                                                          Condition(s) may apply -- See
                                                          paragraph text.

  23.957            Flow between          Yes             None
                    interconnected tanks                  

  23.959            Unusable fuel supply. Yes             None

  23.961            Fuel system hot       Yes             See comment concerning fuel
                    weather operation                     temperature.

  23.963            Fuel tanks: General.  Yes             None

  23.965            Fuel tank tests.      Yes             None

  23.967            Fuel tank             Yes             None
                    installation.                         

  23.969            Fuel tank expansion   Yes             None
                    space                                 

  23.971            Fuel tank sump.       Yes             See comment fuel Sump.

  23.973\*          Fuel tank filler      Yes             See comment concerning fuel
                    connection                            filler and also placard as
                                                          called out in 23.1557. Note:
                                                          turbine engine requirements
                                                          are sometimes more relevant
                                                          to apply due to the nature of
                                                          the ADE fuel. Special
                                                          Condition(s) may apply -- See
                                                          paragraph text.

  23.975            Fuel tank vents and   Yes             None
                    carburetor vapor                      
                    vents                                 

  23.977\*          Fuel tank outlet.     Yes             See comment concerning fuel
                                                          filtration and possibility of
                                                          injection blockage. Note:
                                                          turbine engine requirements
                                                          are sometimes more relevant
                                                          to apply due to the nature of
                                                          the ADE fuel. Special
                                                          Condition(s) may apply -- See
                                                          paragraph text.

  23.979            Pressure fueling      Yes             None
                    systems.                              

                                                          

  FUEL SYSTEM                                             
  COMPONENTS                                              

                                                          

  23.991            Fuel pumps.           Yes             See comment concerning fuel
                                                          delivery. Note: turbine
                                                          engine requirements are
                                                          sometimes more relevant to
                                                          apply due to the nature of
                                                          the ADE operation.

  23.993            Fuel system lines and Yes             None
                    fittings                              

  23.994            Fuel system           Yes             None
                    components.                           

  23.995            Fuel valves and       Yes             None
                    controls                              

  23.997            Fuel strainer or      Yes             See comment concerning fuel
                    filter                                filtration and possibility of
                                                          injection blockage. Note:
                                                          turbine engine requirements
                                                          are sometimes more relevant
                                                          to apply due to the nature of
                                                          the ADE fuel.

  23.999            Fuel system drains.   Yes             None

  23.1001           Fuel jettisoning      Yes             None
                    system.                               

                                                          

                                                          

                                                          

  OIL SYSTEM                                              

                                                          

  23.1011           General.              Yes             None

  23.1013           Oil tanks.            Yes             None

  23.1015           Oil tank tests.       Yes             None

  23.1017           Oil lines and         Yes             None
                    fittings                              

  23.1019           Oil strainer or       Yes             None
                    filter                                

  23.1021           Oil system drains.    Yes             None

  23.1023           Oil radiators.        Yes             None

  23.1027           Propeller feathering  Yes             None
                    system.                               

                                                          

  COOLING                                                 

                                                          

  23.1041           General.              Yes             None

  23.1043           Cooling tests.        Yes             None

  23.1045           Cooling test          Yes             None
                    procedures for                        
                    turbine engine                        
                    powered airplanes                     

  23.1047           Cooling test          Yes             None
                    procedures for                        
                    reciprocating engine                  
                    powered airplanes                     

                                                          

  LIQUID COOLING                                          

                                                          

  23.1061           Installation.         Yes             None

  23.1063           Coolant tank tests.   Yes             None

                                                          

  INDUCTION SYSTEM                                        

                                                          

  23.1091           Air induction system. Yes             None

  23.1093           Induction system      Yes             None
                    icing protection                      

  23.1095           Carburetor deicing    Yes             None
                    fluid flow rate                       

  23.1097           Carburetor deicing    Yes             None
                    fluid system capacity                 

  23.1099           Carburetor deicing    Yes             None
                    fluid system detail                   
                    design                                

  23.1101           Induction air         Yes             None
                    preheater design                      

  23.1103           Induction system      Yes             None
                    ducts.                                

  23.1105           Induction system      Yes             None
                    screens.                              

  23.1107           Induction system      Yes             None
                    filters.                              

  23.1109           Turbocharger bleed    Yes             None
                    air system                            

  23.1111           Turbine engine bleed  Yes             None
                    air system                            

                                                          

  EXHAUST SYSTEM                                          

                                                          

  23.1121           General.              Yes             None

  23.1123           Exhaust system.       Yes             None

  23.1125           Exhaust heat          Yes             None
                    exchangers.                           

                                                          

  POWERPLANT                                              
  CONTROLS AND                                            
  ACCESSORIES                                             

                                                          

  23.1141           Powerplant controls:  Yes             None
                    General.                              

  23.1142           Auxiliary power unit  Yes             None
                    controls                              

  23.1143           Engine controls.      Yes             None

                                                          

                                                          

  23.1145\*         Ignition switches.    Yes             See comment concerning usage
                                                          of \'ignition switch\' as a
                                                          means of engine shutdown; it
                                                          is realized that \'ignition
                                                          switch\' is a misnomer when
                                                          concerned with ADE. Special
                                                          Condition(s) may apply -- See
                                                          paragraph text.

  23.1147           Mixture controls.     Yes             None

  23.1149           Propeller speed and   Yes             None
                    pitch controls                        

  23.1153           Propeller feathering  Yes             None
                    controls.                             

  23.1155           Turbine engine        Yes             None
                    reverse thrust and                    
                    propeller pitch                       
                    settings below the                    
                    flight regime                         

  23.1157           Carburetor air        Yes             None
                    temperature controls                  

  23.1163           Powerplant            Yes             None
                    accessories.                          

  23.1165\*         Engine ignition       Yes             See comment concerning FADEC
                    systems.                              usage. Special Condition(s)
                                                          may apply -- See paragraph
                                                          text.

                                                          

  POWERPLANT FIRE                                         
  PROTECTION                                              

                                                          

  23.1181           Designated fire       Yes             None
                    zones; regions                        
                    included                              

  23.1182           Nacelle areas behind  Yes             None
                    firewalls                             

  23.1183           Lines, fittings, and  Yes             None
                    components                            

  23.1189           Shutoff means.        Yes             None

  23.1191           Firewalls.            Yes             None

  23.1192           Engine accessory      Yes             None
                    compartment diaphragm                 

  23.1193           Cowling and nacelle.  Yes             None

  23.1195           Fire extinguishing    Yes             None
                    systems materials.                    

  23.1197           Fire extinguishing    Yes             None
                    agents.                               

  23.1199           Extinguishing agent   Yes             None
                    containers.                           

  23.1201           Fire extinguishing    Yes             None
                    systems                               

  23.1203           Fire detector system. Yes             None

                                                          

  Subpart F \--                                           
  Equipment                                               

                                                          

  GENERAL                                                 

                                                          

  23.1301           Function and          Yes             None
                    installation.                         

  23.1303           Flight and navigation Yes             None
                    instruments                           

  23.1305\*         Powerplant            Yes             See comment concerning unique
                    instruments.                          instrumentation required for
                                                          ADE operation. Special
                                                          Condition(s) may apply -- See
                                                          paragraph text.

  23.1307           Miscellaneous         Yes             None
                    equipment.                            

  23.1309           Equipment, systems,   Yes             None
                    and installations.                    

                                                          

  INSTRUMENTS:                                            
  INSTALLATION                                            

                                                          

  23.1311           Electronic display    Yes             None
                    instrument systems                    

  23.1321           Arrangement and       Yes             None
                    visibility.                           

                                                          

  23.1322           Warning, caution, and Yes             None
                    advisory lights                       

  23.1323           Airspeed indicating   Yes             None
                    system.                               

  23.1325           Static pressure       Yes             None
                    system.                               

  23.1326           Pitot heat indication Yes             None
                    systems                               

  23.1327           Magnetic direction    Yes             None
                    indicator.                            

  23.1329           Automatic pilot       Yes             None
                    system.                               

  23.1331           Instruments using a   Yes             None
                    power source                          

  23.1335           Flight director       Yes             None
                    systems.                              

  23.1337           Powerplant            Yes             None
                    instruments                           
                    installation.                         

                                                          

  ELECTRICAL                                              
  SYSTEMS AND                                             
  EQUIPMENT                                               

                                                          

  23.1351           General.              Yes             None

  23.1353           Storage battery       Yes             None
                    design and                            
                    installation                          

  23.1357           Circuit protective    Yes             None
                    devices.                              

  23.1359           Electrical system     Yes             None
                    fire protection                       

  23.1361           Master switch         Yes             None
                    arrangement.                          

  23.1365           Electric cables and   Yes             None
                    equipment                             

  23.1367           Switches.             Yes             None

                                                          

  LIGHTS                                                  

                                                          

  23.1381           Instrument lights.    Yes             None

  23.1383           Taxi and landing      Yes             None
                    lights                                

  23.1385           Position light system Yes             None
                    installation                          

  23.1387           Position light system Yes             None
                    dihedral angles                       

  23.1389           Position light        Yes             None
                    distribution and                      
                    intensities                           

  23.1391           Minimum intensities   Yes             None
                    in the horizontal                     
                    plane of position                     
                    lights                                

  23.1393           Minimum intensities   Yes             None
                    in any vertical plane                 
                    of position lights                    

  23.1395           Maximum intensities   Yes             None
                    in overlapping beams                  
                    of position lights                    

  23.1397           Color specifications. Yes             None

  23.1399           Riding light.         Yes             None

  23.1401           Anticollision light   Yes             None
                    system.                               

                                                          

  SAFETY EQUIPMENT                                        

                                                          

  23.1411           General.              Yes             None

  23.1415           Ditching equipment.   Yes             None

  23.1416           Pneumatic de-icer     Yes             None
                    boot system                           

  23.1419           Ice protection.       Yes             None

                                                          

  MISCELLANEOUS                                           
  EQUIPMENT                                               

                                                          

  23.1431           Electronic equipment. Yes             None

  23.1435           Hydraulic systems.    Yes             None

  23.1437           Accessories for       Yes             None
                    multi-engine                          
                    airplanes                             

  23.1438           Pressurization and    Yes             None
                    pneumatic systems                     

  23.1441           Oxygen equipment and  Yes             None
                    supply                                

  23.1443           Minimum mass flow of  Yes             None
                    supplemental oxygen                   

  23.1445           Oxygen distribution   Yes             None
                    system.                               

  23.1447           Equipment standards   Yes             None
                    for oxygen dispensing                 
                    units                                 

  23.1449           Means for determining Yes             None
                    use of oxygen                         

  23.1450           Chemical oxygen       Yes             None
                    generators.                           

  23.1451           Fire protection for   Yes             None
                    oxygen equipment                      

  23.1453           Protection of oxygen  Yes             None
                    equipment from                        
                    rupture                               

  23.1457           Cockpit voice         Yes             None
                    recorders.                            

  23.1459           Flight recorders.     Yes             None

  23.1461           Equipment containing  Yes             None
                    high energy rotors                    

                                                          

  Subpart G --                                            
  Operating                                               
  Limitations and                                         
  Information                                             

                                                          

  23.1501           General.              Yes             None

  23.1505           Airspeed limitations. Yes             None

  23.1507           Operating maneuvering Yes             None
                    speed.                                

  23.1511           Flap extended speed.  Yes             None

  23.1513           Minimum control       Yes             None
                    speed.                                

  23.1519           Weight and center of  Yes             None
                    gravity                               

  23.1521\*         Powerplant            Yes             See comment concerning unique
                    limitations.                          operational requirements for
                                                          ADE. Special Condition(s) may
                                                          apply -- See paragraph text.

  23.1522           Auxiliary power unit  Yes             None
                    limitations                           

  23.1523           Minimum flight crew.  Yes             None

  23.1524           Maximum passenger     Yes             None
                    seating configuration                 

  23.1525           Kinds of operation.   Yes             None

  23.1527           Maximum operating     Yes             None
                    altitude.                             

  23.1529           Instructions for      Yes             None
                    Continued                             
                    airworthiness                         

                                                          

  MARKINGS AND                                            
  PLACARDS                                                

                                                          

  23.1541           General.              Yes             None

  23.1543           Instrument markings:  Yes             None
                    General.                              

  23.1545           Airspeed indicator.   Yes             None

  23.1547           Magnetic direction    Yes             None
                    indicator.                            

  23.1549           Powerplant and        Yes             None
                    auxiliary power unit                  
                    instruments                           

  23.1551           Oil quantity          Yes             None
                    indicator.                            

  23.1553           Fuel quantity         Yes             None
                    indicator.                            

  23.1555           Control markings.     Yes             None

  23.1557\*         Miscellaneous         Yes             See comment concerning fuel
                    markings and placards                 placard for ADE. Special
                                                          Condition(s) may apply -- See
                                                          paragraph text.

  23.1559           Operating limitations Yes             None
                    placard.                              

  23.1561           Safety equipment.     Yes             None

  23.1563           Airspeed placards.    Yes             None

  23.1567           Flight maneuver       Yes             None
                    placard.                              

                                                          

  AIRPLANE FLIGHT                                         
  MANUAL AND                                              
  APPROVED MANUAL                                         
  MATERIAL                                                

                                                          

  23.1581           General.              Yes             None

                                                          

  23.1583           Operating             Yes             See comment concerning unique
                    limitations.                          operational requirements for
                                                          ADE.

  23.1585           Operating procedures. Yes             See comment concerning unique
                                                          operational requirements for
                                                          ADE.

  23.1587           Performance           Yes             None
                    information.                          

  23.1589           Loading information.  Yes             None
  -------------------------------------------------------------------------------------

I.  **Section Discussions**

**Part 21**

## Primary Category

An aircraft diesel engine design cannot be approved as part of an
airframe TC under a primary category approval. The intent of primary
category is a simple aircraft, and the aircraft diesel engine does not
have any service history to warrant approval in the primary category
without prior engine certification. It would be allowable, per part 33,
to put a certified diesel engine in a primary category airplane. This
position may change once the service history of the aircraft diesel
engine is established.

#### Airships

The technical concerns mentioned in this policy should also be
considered with respect to airship projects.

**Legacy Aircraft Diesel Engines**

There are three certificated diesel airplane engines from the 1930\'s
and 1940\'s that the FAA is aware of:

Packard DR-980 ATC 43 1930

Guiberson A-980 ATC 79

Guiberson A-1020 TC 220

(ATC \-- Approved Type Certificate, TC \-- Type Certificate).

The Packard went into limited trial service, and there is no knowledge
if either of the Guiberson engines went into aircraft service. In the
unlikely event that one of these engines is proposed for installation,
existing changed product policy should be applied, depending on the
vintage of the airframe to be used. The reliability of the Packard is
considered substandard to conventional reciprocating engines of the same
period, and if any of the Packard engines is to be installed in an
airplane for other than experimental purposes, this must be considered.
It is extremely unlikely that any of the remaining Guiberson engines
could be brought into suitable condition for installation in an
aircraft, but similar considerations would apply.

Some of the original known airplane Packard diesel engine installation
combinations include the following (some of these may not have been
approved for standard operations): Army (Consolidated) primary trainer,
Bellanca \"six place,\" Bellanca Special, Buhl Air Sedan, Ford
Tri-Motor, Ryan \"four place,\" Stewart Twin, Stinson Detroiter, Stinson
Junior, Towle Amphibian, Verville Air Coach, and Waco Straight Wing.

[§ 21.19 \-- Changes Requiring a New Type Certificate]{.underline}

You may install a certificated aircraft diesel engine into a
certificated airplane using either the STC or ATC process. You may not
install a certificated diesel engine into a certificated airplane
through a field approval. The design requirements appropriate to the
aircraft diesel engine and its systems along with historical precedents
justify this rationale.

[§ 21.21(b)(2) Issue of Type Certificate; Normal, Utility, Acrobatic,
Commuter, and Transport Category Aircraft; Manned Free Balloons; Special
Classes of Aircraft; Aircraft Engines; Propellers (Special Condition \--
Engine Vibration)]{.underline}

§ 21.21(b) states:

*The applicant submits the type design, test reports, and computations
necessary to show that the product to be certificated meets the
applicable airworthiness, aircraft noise, fuel venting, and exhaust
emission requirements of the Federal Aviation Regulations and any
special conditions prescribed by the Administrator, and the
Administrator finds─*

§ 21.21(b)(2) states:

*For an aircraft, that no feature or characteristic makes it unsafe for
the category in which certification is requested.*

Given the lack of field service history on aircraft diesel engines, the
following special conditions will most likely be required for diesel
engine installations:

\(1\) No unsafe condition will exist in case of an inoperable cylinder
until the engine can be shut down and the resistance of the propeller,
engine mount, and airframe structure to shaking moment/vibration damage
is investigated. It must be shown that the shaking and vibration damage
from the engine (caused by an inoperative cylinder) will not cause a
catastrophic airframe failure and no unsafe condition will exist.

\(2\) The propeller utilized on the engine must be certificated to
survive such an event without catastrophic failure. The concern is a
catastrophic failure of the propeller in case of an inoperable cylinder.

**Part 23**

**Subpart B -- Flight**

General Comment: As with all air breathing engines, a diesel engine will
lose performance at lower intake air densities. The rate of change will
be different for a diesel than either a conventional reciprocating
engine (including turbocharged or supercharged) or a turbine engine. The
airplane performance effects need to be understood and documented in the
airplane flight manual. Airplane engine conversions should be evaluated
thoroughly because a diesel engine\'s standard day rating may not be
directly comparable to a conventional reciprocating engine. In addition,
the power-off glide characteristics with an aircraft diesel engine may
be substantially different due to the effect of a wind milling or
stopped propeller.

[§ 23.251 \-- Vibration and Buffeting]{.underline}

As part of the basic compliance efforts for aircraft diesel-powered
engine airplanes, the effects of vibration levels higher than those
typical for conventional reciprocating engine powered airplanes must be
considered. The concern is for engine excitation of the airframe within
the certification flight envelope, especially at the limits of the
flight envelope.

**Subpart C \-- Structure**

[§ 23.361 paragraphs (b)(1) and (c)(3) \-- Engine Torque]{.underline}

The sudden stoppage criterion used for turbine engines is considered
appropriate for aircraft diesel engines. The sudden stoppage criterion
is considered appropriate due to the much greater cylinder pressures and
the potentially more severe failure mode of an aircraft diesel engine.
This may be applicable via a special condition. The applicant can
develop, in lieu of this, data that would show that an ELOS would be
demonstrated with similar installations.

The factor of four, for aircraft diesel engines with four cylinders or
more, will be used to multiply the aircraft diesel engine\'s mean torque
to calculate the limit torque; this is due to the higher expected load
levels in the diesel engines. In the case of an aircraft diesel engine
with less than four cylinders, the FAA will consider this on a
case-by-case basis. This may be applicable via a special condition. The
applicant can develop, in lieu of this, data that would show that an
ELOS would be demonstrated with similar installations.

[§ 23.572 \-- Metallic Wing, Empennage, and Associated
Structure]{.underline}

As part of the basic compliance efforts for aircraft diesel engine
powered airplanes, the effects of vibration levels higher than those
typical for conventional reciprocating engine powered airplanes must be
considered. There are no additional tests other than the basic
compliance requirements. The vibration surveys would be conducted at
various points throughout the flight envelope and the data would be
analyzed against the structure fatigue characteristics. The concern is
for any adverse effects on fatigue characteristics of the airframe.

The added weight due to the increased density of diesel fuel compared to
AvGas will need to be evaluated when an aircraft diesel engine is being
installed via STC into an already certificated aircraft. AC 23-7
provides the appropriate guidance.

[§ 23.573 \-- Damage Tolerance and Fatigue Evaluation
Structure]{.underline}

As part of the basic compliance efforts for aircraft diesel engine
powered airplanes, the effects of vibration levels higher than those
typical for conventional reciprocating engine powered airplanes must be
considered. Our concern is for any adverse effects on fatigue
characteristics of the airframe.

[§ 23.574 -- Metallic Damage Tolerance and Fatigue Evaluation Structure
of Commuter Category Airplanes]{.underline}

As part of the basic compliance efforts for aircraft diesel engine
powered airplanes, the effects of vibration levels higher than those
typical for conventional reciprocating engine powered airplanes must be
considered. Our concern is for any adverse effects on fatigue
characteristics of the airframe.

**Subpart D \-- Design and Construction**

[§ 23.613 \-- Material Strength Properties and Design
Values]{.underline}

As part of the basic compliance efforts for aircraft diesel
engine-powered airplanes, the effects of vibration levels higher than
those typical for conventional reciprocating engine powered airplanes
must be considered. Our concern is any adverse effects on fatigue
characteristics of the airframe.

[§ 23.627 \-- Fatigue Strength]{.underline}

As part of the basic compliance efforts for an aircraft diesel engine
airplane, the effects of higher vibration levels that might be higher
than those typical for conventional reciprocating engine powered
airplanes must be considered. Our concern is for any adverse effects on
fatigue characteristics of the airframe.

[§ 23.629, paragraphs (e)(1) and (2) \-- Flutter]{.underline}

The vibration levels of the aircraft diesel engine will most likely
require the application of the turbopropeller requirements by a special
condition. Considerations for vibration levels, including those with an
inoperative cylinder, will need to be evaluated. Our concern is for
engine excitation of the airframe within the certification flight
envelope.

When installing an aircraft diesel engine in an airplane, the applicant
will show by test or analysis, or by a combination of methods, that, in
the event of an inoperative cylinder, the airframe can withstand the
shaking or vibratory forces imposed by the engine. Diesel engines of
conventional design typically have extremely high levels of vibration in
the case of an inoperative cylinder. The engine manufacturer should
provide data to the airframe installer/modifier so that either of those
appropriate design considerations or operating procedures, or both, can
be developed to prevent airframe damage.

Under § 21.21(b)(2), the following requirement appears: No feature or
characteristic makes the aircraft unsafe for the category in which
certification is requested. This is of particular concern during an
airplane engine conversion as the original design data is often not
available and the engine/propeller is apt to be heavier as well as being
subjected to higher vibratory loads. As described earlier in this policy
statement, there is a potential special condition applied to
§ 21.21(b)(2) concerning the one cylinder inoperative scenario and its
potential to be an unsafe condition.

[§ 23.629(i) \-- Flutter]{.underline}

Considerations for vibration levels, including those with an inoperative
cylinder, will need to be evaluated. Our concern is for engine
excitation of the airframe within the certification flight envelope.

Care must be taken for older aircraft, which will be changed and which
have been certificated against a lower level of airworthiness
requirements. Principal ground vibration tests will most likely be
necessary to show compliance with § 23.629 in terms of
symmetric/asymmetric coupling between engine mass and empennage,
especially elevator and horizontal stabilizer structure.

[§ 23.777(d) \-- Cockpit Controls]{.underline}

If a single power lever/single power control is used, the applicant will
need to request an ELOS per AC 23-17A guidance.

[§ 23.779(b) \-- Motion and Effect of Cockpit Controls]{.underline}

If a single power lever/single power control is used, the applicant will
need to request an ELOS per AC 23-17A guidance.

[§ 23.781 \-- Cockpit Control Knob Shape]{.underline}

If a single power lever/single power control is used, the applicant will
need to request an ELOS per AC 23-17A guidance.

**Subpart E \-- Powerplant**

[§ 23.901 \-- Installation]{.underline}

Based on limited historical precedent, but also on the fact that much
higher cylinder pressures exist in diesel engines, the possibility that
engine failures may lead to liberation of high energy engine fragments
must be evaluated. The applicant will submit a safety analysis that
addresses the engine construction and possible failure modes.

The engine manufacturer should perform the safety analysis and supply
the safety analysis as part of the engine design data. If the safety
analysis indicates that there may be an engine failure mode that will
liberate engine parts, then the design features of the airplane must be
configured to minimize the hazard to crew, controls, and passengers. If
the engine has failure modes that will liberate parts, then consequences
of the failure must be analyzed and minimized; i.e., the probability of
the failure will be 100 percent. Mitigation of the failure hazard must
include reasonable design features to achieve minimization and not
merely be an analysis of the final design. This requirement may be
applied with a special condition.

[§ 23.901(d)(1) \-- Installation]{.underline}

The vibration levels of the aircraft diesel engine, if greater than
those for conventional reciprocating engines, may require the
application of this turbine engine requirement by a special condition.
Considerations for vibration levels, including those with an inoperative
cylinder, will need to be evaluated.

When installing an aircraft diesel engine in an airplane, the applicant
will show by test or analysis, or by a combination of methods, that in
the event of an inoperative cylinder the engine and airframe can
withstand the shaking or vibratory forces imposed by the engine. Diesel
engines of conventional design typically have extremely high levels of
vibration in the case of an inoperative cylinder. The engine
manufacturer should provide data to the airframe installer/modifier so
that either of those appropriate design considerations or operating
procedures, or both, can be developed to prevent airframe damage.

Under § 21.21(b)(2), the following requirement appears: No feature or
characteristic makes the aircraft unsafe for the category in which
certification is requested. This is of particular concern during an
airplane engine conversion as the original design data is often not
available and the engine/propeller is apt to be heavier as well as being
subjected to higher vibratory loads. As described earlier in this policy
statement, there is a potential special condition applied to
§ 21.21(b)(2) concerning the one cylinder inoperative scenario and its
potential to be an unsafe condition.

There is no existing requirement for conventional reciprocating engines
to ensure that the installed engine does not exceed the engine\'s
vibratory limit under all operating conditions. This requirement is in
§ 23.901(d)(1) and is applicable to turbine engines only. A special
condition may be needed to impose this requirement on any airplanes that
use an aircraft diesel engine.

[§ 23.903(f) \-- Engines]{.underline}

Diesel engine starting systems may utilize unconventional starting
technology. Due to high compression levels of the diesel engine (or
possibly environmental conditions), air starters or other systems may be
used that may be expended or that have recharge times. Also,
high-pressure fuel systems are more susceptible to air lock conditions
during altitude restart situations. Limitations may have to be added
that address these design features. This would be considered a basic
compliance issue.

[§ 23.907 \-- Propeller Vibration]{.underline}

The power cycle of the diesel engine is fundamentally different from a
conventional reciprocating engine and may impose unacceptable loads,
vibration levels, and vibration modes on propellers that have previously
been approved and considered acceptable for gasoline or turbine engine
installations. Propeller vibration requirements must be substantiated by
testing, as conventional propellers might not be robust enough to use on
a diesel engine.

Aircraft diesel engine crankshafts typically have high first order
torsionals, the propeller is likely to be quite rugged and possibly
contain new and unusual design characteristics. For the initial
installations, the listing of propeller combinations approved vibration
wise in a propeller TCDS, Note 9, would not be considered to be a
showing of compliance to § 23.907. The applicant must perform a
vibration test and analysis to determine engine/propeller/airframe
compatibility prior to the approval of the (certificated) engine and
propeller installation in a certificated airplane.

§ 23.907(a) Fixed pitch wooden propellers may not be excluded from

the requirement. The requirement may be applied via

special condition.

§ 23.907(a)(1) Required.

§ 23.907(a)(2) Not acceptable for diesel engine installations at this
time.

§ 23.907(a)(3) Service history may not be acceptable for new technology.

§ 23.907(b) Fixed pitch wooden propellers will not be excluded. The

requirement will be applied via special condition.

[§ 23.951(c) \-- Fuel System \-- General]{.underline}

The airplane will comply with the requirements of part 23, § 23.951(c),
as kerosene-based fuels absorb greater amounts of water than gasoline
fuels and have a much greater potential for fuel system icing.

Some turbine engine powered airplanes have complied with this
requirement by mandating the utilization of fuel anti-icing additives.
Mandating the use of anti-icing additives would be an acceptable method
to show compliance; however, the use of these additives is generally
being curtailed in some countries for environmental reasons. A special
condition will likely be required to impose the requirements of
§ 23.951(c) on any airplanes that use an aircraft diesel engine and
operate on kerosene-based fuels.

[§ 23.955(a) \-- Fuel Flow \-- General]{.underline}

See the discussion concerning fuel temperature under § 23.961,
§ 23.1521(e) and fuel temperature limitations.

[§ 23.955, paragraphs (c) and (f) \-- Fuel Flow \-- Pump
Systems]{.underline}

The minimum flow requirements of § 23.955(c) apply to pump systems for
conventional reciprocating engines. The requirement was developed to
prevent detonation by ensuring that a rich mixture could always be
attained. The turbine engine requirements of § 23.955(f) are more
suitable to an aircraft diesel engine because a diesel engine, which
already operates lean of stoichiometric, does not have the same need to
ensure a rich operating condition to prevent engine damage. The
certificated engine will address the issue of "detonation margin" and,
if required, the 100 percent requirement of § 23.955(f) will be adjusted
accordingly. The requirement will be applied by a special condition if
applicable to the installation.

[§ 23.961 \-- Fuel System Hot Weather Operation]{.underline}

The engine fuel injection system will most likely provide a high volume
of return fuel. This may lead to an excessive temperature rise of the
fuel. The acceptable fuel temperature limitation required by shall not
be exceeded. A test has to be performed to determine the fuel
temperature under the worst condition. The hot fuel test has to be
performed under these conditions.

[§ 23.971(c) \-- Fuel Tank Sump]{.underline}

The purpose of the sediment bowl of the configuration specified in
§ 23.971(c) is to remove any water that separates from the gasoline.
Water is more soluble in kerosene fuels than in gasoline and a fuel
sediment bowl may not be necessary based upon the experience of turbine
powered airplanes. This section might be deemed "not applicable" for an
aircraft diesel engine.

[§ 23.973(f) \-- Fuel Tank Filler Connection]{.underline}

The fuel filler nozzle for turbine engine fuel (Jet-A) is larger than
the corresponding fuel filler nozzle for conventional reciprocating
engine fuel (100LL). In order to minimize the potential for mis-fueling,
the airplane should comply with the requirements of § 23.973(f) in lieu
of § 23.973(e). Also, see the § 23.1557 discussion in this policy
statement. The requirement may be applied by a special condition.

[§ 23.977(a)(2) \-- Fuel Tank Outlet]{.underline}

The strainer size must be appropriate for the aircraft diesel engine
fuel requirements, especially considering eliminating the possibility of
injector and pump contamination. The airplane should comply with the
requirements of § 23.977(a)(2) in lieu of § 23.977(a)(1). The
requirement may be applied by a special condition.

[§ 23.991 \-- Fuel Pumps,]{.underline}

[§ 23.993 \-- Fuel System Lines and Fittings,]{.underline}

[§ 23.994 \-- Fuel System Components,]{.underline}

[§ 23.995 \-- Fuel Valves and Controls,]{.underline}

[§ 23.997 \-- Fuel Strainer or Filter]{.underline}

Fuel system components designed for small airplanes may be optimized or
designed for low-pressure gasoline systems. Components that will be
exposed to the effects of high-pressure pumps will need to be designed
to account for the effects of high pressure and fatigue, perhaps similar
to requirements for hydraulic systems. Turbine fuels may have additives
added after they are refined that modify fuel properties associated with
airplane icing, biological contamination, static buildup, flow
improvement, weathering, and temperature stability. Frequently, it
cannot be determined if these additives are in the fuel that is
delivered to the airplane.

Over-wing fueling systems at airports may not have the same source as
pressure fueling systems for large transport airplanes and may contain
different additives. Determine whether the aircraft fuel system
components are compatible with the fuel and with any potential additive
that could be introduced into the fuel.

The military are utilizing some additives to increase the thermal
stability of jet fuels that are not commercially utilized. The potential
effects of these additives on the systems of small airplanes will need
to be examined if there is any possibility of the airplane being
operated on military fuels. In addition to material properties, the
effects on fuel gauging systems, and the effects on filtering
systems/media will need to be specifically evaluated.

Kerosene fuels can potentially produce an explosive or flammable mixture
in the fuel tanks at normal operating temperatures while gasoline
mixtures in the fuel tanks are typically beyond the rich limit. Fuel
tank ignition sources need to be eliminated over the total life cycle of
the airplane, and extreme care should be used when converting the fuel
system of an airplane from gasoline to kerosene-based fuels. Grounding
of all metal components within the fuel tank is perhaps more important
with diesel powered engines (relative to gasoline) to ensure common
potential (and, therefore, no arcing) between the metal components and
the inrushing fuel, which is something to consider in STC re-engine
projects. Also, re-evaluation of fuel quantity indicating systems for
compatibility, ignition source, and marking requirements should be
considered.

In some ground-based diesel engine designs, fuel line pressures have
been known to exceed 3,000 psi, which is significantly higher than
conventional turbine engine fuel systems. Extra attention should be
given to fuel system components and the instructions for continued
airworthiness for the system and its components. Basic compliance
addressing the specific design challenges of the aircraft diesel engine
systems will be adequate.

[§ 23.1145 \-- Ignition Switches]{.underline}

This requirement will have to be modified to incorporate an appropriate
means of stopping the engine. In place of the basic requirement, there
must be a means to stop the engine; this may mean shutting off the
engine control (FADEC, or similar) or appropriate components. Depending
on specific system details, a special condition, exemption or ELOS for
§ 23.1145 could be utilized.

As the diesel engine is a compression ignition engine, the ignition
source is the heat created by compressing the air-fuel mixture in the
cylinder. Section 23.1145(a) would not be applicable. Part of the intent
of § 23.1145(b) is to have a method available to the crew to quickly
stop an engine(s). As a diesel engine does not have an ignition system,
an alternate means of rapidly stopping an engine(s) would need to be
provided and addressed via a special condition, exemption or ELOS.

[§ 23.1165 -- Engine Ignition Systems (Permanent electrical
power)]{.underline}

Modern diesel engines will most likely be equipped with EEC or FADEC.
These systems need permanent electrical power. Battery ignition systems
are addressed in § 23.1165. The need for permanent electrical power is
identical to the EEC/FADEC Electrical Power requirement. The EEC/FADEC
system will have the same critical function as the battery ignition
system identified in § 23.1165. This requirement will need to be
addressed via a special condition or ELOS.

Since the electronic engine control without mechanical backup requires
permanent electric power, the criticality of the power supply system may
be higher than on a conventional magneto ignition system. The § 23.1309
assessment (FHA and FMEA) has to address this failure mode. If the
electric power after loss of generator is limited to a specific time
(e.g., if supplied by battery), this remaining time should be
demonstrated and the information should be in the POH. The time required
in § 23.1353(h) may not establish the same level of safety as a
conventional magneto ignition system. In any case, the pilot should be
informed that the engine will stop if the electric power is lost (POH).

[§ 23.1203(a)(1)(ii) \-- Fire Detector System]{.underline}

If the aircraft diesel engine is installed on a multi-engine airplane
and the engine utilizes turbochargers, this section will apply. Basic
compliance is adequate.

#### Subpart F \-- Equipment

[§ 23.1305 \-- Powerplant Instruments]{.underline}

The installation of an aircraft diesel engine will be in accordance with
the engine\'s installation manual and operated within the requirements
of its TC. The instrumentation requirements in § 23.1305 were written
for conventional reciprocating engines and turbine engines. The
requirement did not envision the use of aircraft diesel engines. In
addition, instrumentation may be determined necessary for a particular
installation that is not specified in the current requirements or the
engine\'s installation manual. Should the aircraft diesel engine
limiting parameters be different than those specified in § 23.1521,
special conditions will most likely be developed to require those
parameters to be used as limitations (see § 23.1521). The limiting
parameters will be displayed in the instrument panel as required by
§ 23.1305. Also, see § 23.1521 for fuel temperature limitations.

Special conditions will most likely be developed if any additional items
to those specified in § 23.1305(b), or items described under § 23.1305,
paragraphs (c), (d), or (e), are required.

Any deletions from the parameters required by § 23.1305(b) could be
administered via an exemption through the development of an equivalent
level of safety or in combination with any special conditions requiring
additional parameters.

**Subpart G \-- Operating Limitations and Information**

[§ 23.1521 \-- Powerplant Limitations]{.underline}

General Comment: An aircraft diesel engine may have different critical
parameters than conventional reciprocating engines. Should the aircraft
diesel engine limiting parameters be different than those specified in
§ 23.1521, special conditions will most likely be developed to require
those parameters to be used as limitations. Also see the discussion for
§ 23.1305.

Section 23.1521(d) requires minimum fuel grades in order for the engine
to be operated within the limitations of § 23.1521, paragraphs (b) and
(c). It is a general opinion that Jet A will be the fuel of choice. Fuel
grade designations are applicable to aviation gasolines and, therefore,
are not appropriate for aircraft diesel engines. The intent of the
requirement is clear; however, the turbine engine "fuel designation" is
more applicable for this section.

Fuel gelling or freezing has not been a concern with airplanes operating
on gasoline fuels; however, kerosene fuels will freeze or gel at ambient
temperatures much higher than gasoline fuels. The CRC fuel properties
handbook shows the specification limit freezing point for Jet-A fuel as
--40C, with the typical kinematic viscosity of 10 centistokes at the
freezing point. An airplane operating on kerosene fuels should have some
means of ensuring that the fuel in the tanks stays in the liquid state
during operations. Fuel gelling or freezing is a special concern for
small airplanes; smaller fuel tanks will cool much quicker than
transport-sized fuel tanks.

The most common method of heating turbine fuel has been through motive
fuel being returned into the fuel tanks. This method can often be shown
to heat the fuel to an adequate temperature so that the danger of fuel
freezing or gelling is avoided.

If the fuel in the tanks cannot be shown to flow suitably under all
temperature conditions, then fuel temperature limitations will be
required. For the different types of fuels (Kerosene (Jet) Fuel and
Diesel car fuel), different temperature limitations may be necessary. If
both fuels are used at the same time, the fuel temperature limit may be
unclear. This is the case if the pilot does not know the type of the
fuel; therefore, the worst limitation has to be used. Clear and
unambiguous information will have to be in the Airplane Flight Manual
(AFM). The temperature limitations would be considered as part of the
essential operating parameters for an airplane with an engine that
operates on kerosene-based fuels, and these limitations will be mandated
as operating limitations on the TCDS and in the Airplane Flight Manual
(AFM).

\(1\) The take-off temperature limitation will be determined by testing
to determine the minimum cold-soaked temperature at which the airplane
can operate.

\(2\) The minimum operating temperature limitation will be determined by
testing to determine the minimum operating temperature acceptable after
take off from the minimum take-off temperature established in (1).

Analysis should be avoided as testing is preferred in establishing these
limitations, similar to hot fuel testing in an overall systems test. If
low temperature operating limits are not established by testing, then
the airplane will be temperature limited. The temperature limit is set
by establishing the temperature where the specified fuel gels, by
evaluating any limiting viscosity effects of the fuel at the cold
temperatures, and by adding an appropriate temperature margin above any
limiting temperature. The temperature margin will be necessary to
account for the variations in fuel properties in order to allow adequate
fuel flow at the limiting temperature. This requirement (depending upon
how it is implemented) could also impose the need for a fuel temperature
display.

Section 23.1521(e) will probably need to be replaced by a special
condition requiring the establishment of maximum and minimum operating
temperatures, as discussed above, for any airplane powered by a diesel
engine.

[§ 23.1557(c)(1)(ii) \-- Miscellaneous Markings and
Placards]{.underline}

The airplane will comply with the provisions of § 23.1557(c)(1)(ii) in
lieu of § 23.1557(c)(1)(i). A special condition would most likely be
needed to require these changes.

Inadvertent over-wing fueling with gasoline is also a possibility that
should be minimized. An additional warning placard stating the following
(or similar) should be placed nearby or integrated with the placard
required by § 23.1557(c)(1)(ii), and the engine TCDS fuels should be
listed:

**[EXAMPLE:]{.underline}**

**"WARNING**

**THIS AIRPLANE IS EQUIPPED WITH A**

**COMPRESSION IGNITION (DIESEL) ENGINE**

###### SERVICE WITH APPROVED FUELS LISTED BELOW ONLY

**JET A**

**JET A1**

##### DIESEL #2"

A special condition will most likely be needed to require the placard.

## Effect of Policy

The general policy stated in this document does not constitute a new
regulation or create what the courts refer to as a \"binding norm\". The
office that implements policy should follow this policy when applicable
to the specific project. Whenever an applicant\'s proposed method of
compliance is outside this established policy, it must be coordinated
with the policy issuing office, e.g., through the issue paper process or
equivalent.

Applicants should expect that the certificating officials will consider
this information when making findings of compliance relevant to new
certificate actions. Also, as with all advisory material, this policy
statement identifies one means, but not the only means, of compliance.

**Conclusion**

The Small Airplane Directorate has determined that all proposed diesel
engine installations, whether supplemental, amended or new TC projects,
will be significant as defined in Order 8100.5. As such, the ACO is
expected to notify the Standards Office of such projects promptly and
forward certification project notifications 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 is retained by the Standards
Office and will be redelegated on a case-by-case basis as this new
technology is understood and integrated into aerospace products.

S/ Dorenda Baker

for

Michael Gallagher

Manager, Small Airplane Directorate

Aircraft Certification Service

> Distribution:
>
> Manager, Aircraft Engineering Division, AIR-100
>
> Manager, Brussels Aircraft Certification Staff, AEU-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