U.S. Department 

of Transportation 

Federal Aviation 

Administration

Advisory 

Circular

 

Subject: SYSTEM SAFETY 

ANALYSIS AND ASSESSMENT FOR 

PART 23 AIRPLANES

Date: 11/17/2011 

Initiated by: ACE-100 

AC No: 23.1309-1E

This advisory circular (AC) sets forth an acceptable means of showing compliance with Title 14 

of the Code of Federal Regulations (14 CFR), § 23.1309, through Amendment 23-62 for 

equipment, systems, and installations in 14 CFR part 23 airplanes. 

This AC is not mandatory and does not constitute a regulation. It is issued for guidance purposes 

and to outline a method of compliance with the rules. An applicant may elect to follow an 

alternate method, provided the FAA finds it to be an acceptable means of complying with the 

applicable requirements of 14 CFR. However, if the applicant uses the means described in the 

AC, they must follow it in all important respects. 

s/ 

Earl Lawrence 

Manager, Small Airplane Directorate 

Aircraft Certification Service 
11/17/2011 AC 23.1309-1E 

iii (and iv) 

CONTENTS 

Paragraph Page 

1. What is the purpose of this AC? ................................................................................................ 1 

2. Who does this AC apply to? ...................................................................................................... 1 

3. Cancellation. .............................................................................................................................. 1 

4. Related regulations and documents. .......................................................................................... 1 

5. Applicability. ............................................................................................................................. 3 

6. Regulations and AC background. .............................................................................................. 4 

7. Acronyms................................................................................................................................... 6 

8. Definitions. ................................................................................................................................ 8 

9. Application of § 23.1309(a), (a)(1), (a)(2), and (a)(3), as adopted by Amendments 23-41 and 

23-49. ............................................................................................................................................ 14 

10. Showing compliance with the requirements of § 23.1309 (a) through Amendment 23-49... 14 

11. Application of § 23.1309(a)(4), as adopted by Amendment 23-49. ...................................... 18 

12. Application of § 23.1309(a)(1) and (a)(2), as adopted by Amendments 23-62..................... 18 

13. Application of § 23.1309(b), as adopted by Amendment 23-62............................................ 20 

14. Application of § 23.1309(b), as adopted by Amendments 23-41 and 23-49 and § 23.1309(c), 

as adopted by Amendment 23-62. ................................................................................................ 21 

15. Four certification classes of airplanes.................................................................................... 22 

16. Safety assessments................................................................................................................. 25 

17. Failure conditions. ................................................................................................................. 29 

18. Assessment methods. ............................................................................................................. 30 

19. Assessment of failure condition probabilities and analysis considerations........................... 32 

20. Operational and maintenance considerations. ....................................................................... 33 

21. Software and complex hardware DALs for airborne system and applications...................... 34 

APPENDIX 1. PARTIAL LIST OF FUNCTIONAL HAZARD ASSESSMENT (FHA) FOR 

CONSIDERATION TO MEET 14 CFR PART 23 REQUIREMENTS FOR IFR CLASS I 

AIRPLANES ................................................................................................................................A1 

APPENDIX 2. SAMPLE FUNCTIONAL HAZARD ASSESSMENT (FHA) FORMAT ........A2 

APPENDIX 3. CALCULATION OF THE AVERAGE PROBABILITY PER FLIGHT HOUR

......................................................................................................................................................A3 
11/17/2011 AC 23.1309-1E 

1 

1. What is the purpose of this AC? 

a. This AC provides guidance and information for an acceptable means, but not the only 

means, for showing compliance with the requirements of § 23.1309 (Amendment 23-62) for 

equipment, systems, and installations in 14 CFR part 23 airplanes. 

b. This material is neither mandatory nor regulatory in nature and does not constitute a 

regulation. It describes acceptable means, but not the only means, for demonstrating compliance 

with the applicable regulations. We will consider other methods of demonstrating compliance 

that an applicant may elect to present. While these guidelines are not mandatory, they are 

derived from extensive FAA and industry experience in determining compliance with the 

relevant regulations. Whenever an applicant’s proposed method of compliance differs from this 

guidance, the proposal should be coordinated with the Small Airplane Directorate Standards 

Staff, ACE-110. In addition, if an office believes that an applicant’s proposal that meets this 

guidance should not be approved, that office should coordinate its response with the Small 

Airplane Directorate Standards Staff, ACE-110. 

c. Terms such as “must” are used in this AC only in the sense of ensuring applicability of 

this particular method of compliance when the acceptable method of compliance described 

herein is used. The word “must” is also used in this AC when referring to a specific regulation 

or guidance that is essential when the applicant uses this AC for the means of compliance. In 

this case there is no deviation. The word “should” is used to express a recommendation. 

Deviation from the specified recommendation may require justification. 

2. Who does this AC apply to? 

The guidance provided in this document is directed to airplane manufacturers, modifiers, foreign 

regulatory authorities, and Federal Aviation Administration (FAA) personnel. This AC is 

applicable only to the original applicant seeking issuance of a Type Certificate (TC), an 

Amended Type Certificate (ATC), a Supplemental Type Certificate (STC), or a Parts 

Manufacturer Approval (PMA) for the initial approval of the new type design or a change in the 

approved type design. 

3. Cancellation. 

 

This AC cancels AC 23.1309-1D, System Safety Analysis and Assessment for Part 23 Airplanes, 

dated January 16, 2009. 

This AC supersedes PS-ACE100-2005-50001, “Applying AC 20-152, ‘RTCA, Inc., Document 

RTCA/DO-254, Design Assurance Guidance for Airborne Electronic Hardware,’ to Title 14 

Code of Federal Regulations, Part 23 Aircraft”; dated January 26, 2007 

4. Related regulations and documents. 

a. Regulations. Sections 23.1301 and 23.1309 of part 23 (through Amendment 23-62). 
11/17/2011 AC 23.1309-1E 

2 

b. ACs, orders, and policy. You may access the latest version of ACs, notices, orders, and 

policy on the FAA website: www.faa.gov. 

AC 20-115B RTCA, Inc., Document RTCA/DO-178B 

AC 20-136A Protection of Aircraft Electrical/Electronic Systems Against the 

 Indirect Effects of Lightning 

AC 20-138B Airworthiness Approval of Positioning and Navigation Systems 

AC 20-152 RTCA, Inc., Document RTCA/DO-254, “Design Assurance 

 Guidance for Airborne Electronic Hardware” 

AC 20-158 The Certification of Aircraft Electrical and Electronic Systems for 

 Operation in the High-Intensity Radiated Fields (HIRF) 

 Environment 

AC 21-16F RTCA, Document DO-160 version D, E, and F , “Environmental 

Conditions and Test Procedures for Airborne Equipment” 

AC 21.101-1A Establishing the Certification Basis of Changed Aeronautical 

Products 

AC 23-17C Systems and Equipment Guide for Certification of Part 23 

Airplanes and Airships 

AC 23.1311-1C Installation of Electronic Displays in Part 23 Airplanes 

AC 25.1309-1A System Design and Analysis 

AC 33.75-1A Guidance Material for 14 CFR 33.75, Safety Analysis 

Order 8110.4C Type Certification 

Order 8110.105 Simple and Complex Electronic Hardware Approval Guidance 

c. Industry documents. You may obtain copies of current editions of the following 

publications as listed. These documents are excellent resource materials. 

(1) RTCA documents. The following RTCA documents are available from RTCA, 

Inc., Suite 805, 1828 L Street NW, Washington, DC 20036-4001 or at their website at 

www.rtca.org. 

RTCA/DO-160G Environmental Conditions and Test Procedures for Airborne 

 Equipment 
11/17/2011 AC 23.1309-1E 

3 

RTCA/DO-178B Software Considerations in Airborne Systems and 

 Equipment Certification 

RTCA/DO-254 Design Assurance Guidance for Airborne Electronic 

 Hardware 

(2) Society of Automotive Engineers (SAE), Inc. The following SAE, Inc., Aerospace 

Recommended Practice (ARP) documents are available from SAE, 400 Commonwealth Drive, 

Warrendale, PA 15096-0001 or from their website at www.sae.org. 

ARP 4754A Guidelines for Development of Civil Aircraft and Systems 

ARP 4761 Guidelines and Methods for Conducting the Safety Assessment 

 Process on Civil Airborne Systems and Equipment 

Note: ARPs 4754A and 4761 provide guidelines and methods of performing the safety 

assessment for certification of civil aircraft. The guidelines in ARP 4754A were 

developed in the context of 14 CFR part 25. It may be applicable to other 14 CFRs, such 

as parts 23, 27, 29, 33, and 35. 

This AC is not intended to constrain the applicant to the use of these documents in the definition 

of their particular methods of satisfying the objectives of this AC. However, these documents 

contain material and methods of performing the System Safety Assessment (SSA) that an 

applicant may choose to use. The guidance in this AC takes precedence over the recommended 

practices in these ARPs if there is a conflict. (See paragraph 21 for more guidance) Contact the 

Small Airplane Directorate if there are conflicts with other guidance or ACs and this AC. 

5. Applicability. 

a. In addition to specific part 23 design requirements, § 23.1309 requirements, except as 

identified below, are applicable to any equipment or system installed in the airplane. This 

section addresses general requirements and does not supersede any specific requirements 

contained in other part 23 sections. New advance technology in electrical, electronic, and 

mechanical systems designs that include complex electronics with software, complex hardware, 

HIRF, and/or lightning requires a § 23.1309 analysis. An SSA is required to determine the level 

of certitude for the processes in standard and guidance documents such as RTCA/DO-178B, 

RTCA/DO-254, AC 20-136A, and AC 20-158 or equivalent. Section 23.1309 should be used to 

determine failure condition, probability of failure condition, software Development Assurance 

Level (DAL), and complex hardware DALs shown in Figure 2. The safety assessment process is 

used to determine the failure condition classification, which determines the HIRF and lightning 

protection levels (reduced DALs in Figure 2 are not used). For simple and conventional 

mechanical or analog electromechanical systems, or both, with well-established design and 

certification processes (where the installation is not complex), safety analysis may be satisfied by 

a qualitative assessment such as the single-failure concept and experience based on serviceproven designs and engineering judgment. In this case, a FHA, a design appraisal, and an 

installation appraisal addressed in this AC may satisfy § 23.1309 as shown in Figure 3. 
11/17/2011 AC 23.1309-1E 

4 

b. Section 23.1309 does not apply to the performance, flight characteristics requirements of 

subpart B, and structural loads and strength requirements of subparts C and D. However, except 

as noted below, § 23.1309 does apply to systems that comply with subparts B, C, D, and E 

requirements. The flight structure such as wing, empennage, control surfaces and their simple 

systems; the fuselage, engine mounting, and landing gear and their related primary attachments 

are excluded. For example, § 23.1309 does not apply to an airplane's inherent stall 

characteristics or their evaluation of § 23.201, but does apply to a stick pusher (stall barrier) 

system installed to attain compliance with § 23.201. We will determine additional exceptions in 

the future. Until then, contact the Small Airplane Directorate for determination and approval of 

proposed exceptions not included in this AC. 

c. Experienced engineering and operational judgment should be applied when determining 

whether or not a system is complex. Comparison with similar, previously approved systems is 

sometimes helpful. All relevant systems attributes should be considered. For example, the 

design may be complex, such as a satellite communication system used only by the passenger, 

but its failure may cause only minor safety effects.

6. Regulations and AC background. 

a. Regulation. 

(1) Amendment 23-14 (effective December 20, l973) adopted the original airworthiness 

standards in § 23.1309(a). Before amendment 23-14, neither the Civil Air Regulations (CAR) 

part 3, nor 14 CFR part 23, contained safety requirements in § 23.1309 for equipment, systems, 

and installations in small airplanes. In 1968, the FAA instituted an extensive review of the 

airworthiness standards of part 23. Because of the increased use of part 23 airplanes in all 

weather operations and the pilot’s increased reliance on installed systems and equipment, the 

FAA issued § 23.1309 to provide an acceptable level of safety for such equipment, systems, and 

installations. When the FAA adopted § 23.1309 (Amendment 23-14), it did not envision 

installation of systems that perform critical functions in small airplanes; therefore, before 

Amendment 23-41, this section did not contain safety standards for evaluating critical functions. 

When such equipment, systems, and installations were included in the airplane design, they were 

evaluated under special conditions in accordance with the procedures of 14 CFR part 21. 

(2) Amendment 23-34 (effective February 17, 1987) expanded § 23.1309 to include 

certification of commuter category airplanes. This expansion added a requirement to ensure 

applicable systems and installations are designed to safeguard against hazards. It also added 

requirements for equipment identified as essential loads and the affected power sources. 

(3) Amendment 23-41 (effective November 26, 1990) retained in § 23.1309 the existing 

safety requirements adopted by amendment 23-14 for airplane equipment, systems, and 

installations that are not complex and that do not perform critical functions. For those cases 

where the applicant includes complex systems, or systems that perform critical functions, 

Amendment 23-41, § 23.1309, provides additional requirements for certification and identifying 

such equipment, systems, and installations. This amendment permitted the approval of more 

advanced systems having the capability to perform critical functions. 
11/17/2011 AC 23.1309-1E 

5 

(4) Amendment 23-49 (effective March 11, 1996), amended § 23.1309(a)(4) to correct 

Amendment 23-41, which inadvertently removed the commuter category requirement originally 

added by Amendment 23-34 as § 23.1309(d). 

(5) Amendment 23-62 consolidated and revised the existing requirements to reduce the 

certification burden. The FAA removed § 23.1301(d) and clarified the requirement in 

§ 23.1309(a) to improve standardization for systems and equipment certification, particularly for 

non-required equipment and non-essential functions embedded within complex avionic systems. 

Section 23.1309(b) requires minor, major, hazardous, or catastrophic failure condition(s) that 

occur during certification testing have a root cause analysis and corrective action. Section 

23.1309(c) updates the safety assessment process terminology. Amendment 23-62 also made 

§ 23.1309(d) compatible with § 23.1322 (Warning, caution, and advisory lights) for the design of 

systems and controls, including indications and annunciations. The power source capacity and 

distribution requirements, which are not directly related to the other safety and analysis 

requirements from § 23.1309, Amendment 23-49, were moved to a new section in § 23.1310 

with clarification. 

(6) Qualitative and quantitative analyses are often used in assessing the acceptability of 

complex designs that have a high degree of integration, use new technology, are new or different 

applications of conventional technology, or are designs that perform critical functions. These 

assessments lead to the selective use of quantitative analyses to support experienced engineering 

and operational judgment and to supplement qualitative analyses and tests. Numerical 

probability ranges associated with the terms used in § 23.1309 are accepted for evaluating 

quantitative analyses that have a logical and acceptable inverse relationship between the 

probability and severity of each failure condition. 

b. AC. 

(1) AC Revisions. 

(a) The revision from AC 23.1309-1B to AC 23.1309-1C on March 12, 1999, 

provided the four-tier certification classes with different criteria for probability of failures and 

software levels for systems. The purpose of this certification approach is to increase safety by 

enhancing equipment on General Aviation (GA) airplanes that facilitate new technologies. 

(b) Since the issuance of AC 23.1309-1C, there has been a large number of 

electronic displays and electronic systems installed on part 23 airplanes, especially Primary 

Flight Displays (PFD), Multifunction Flight Displays (MFD), Integrated Flight Systems, and 

Synthetic Vision Systems (SVS). A study of the FAA Alaska Capstone demonstration program 

for new avionics systems technology determined that four-tier certification classes demonstrated 

significant operational safety benefit and reduced accident rates. These installations, especially 

on Class I and II airplanes, would have been too costly without the establishment of the four-tier 

certification classes of airplanes as shown in paragraph 15. 

(2) Broad causes of fatal accidents. Accident rate is a function of many factors. These 

factors include human performance, weather, design, operation, training, maintenance, and 
11/17/2011 AC 23.1309-1E 

6 

airspace system infrastructure. For all airplanes, but particularly GA airplanes, pilot decisionmaking causes most accidents. Pilot decision-making accidents are often the result of a lack of 

situational awareness relative to terrain or weather, or to a loss of control due to excess 

workload. Correct pilot interventions and actions have prevented some of these accidents. 

Increases in avionics equipage rates that improve pilot situational awareness or simplify the task 

have a significant positive impact on the GA accident rate. The Aircraft Owners and Pilots 

Association, Air Safety Foundation, conducted a study of safety effects of glass cockpits and 

concluded that technologically advanced aircraft provide added situational awareness tools that 

have dramatically improved aspects of GA safety. Technologically advanced aircraft deliver 

multiple safety benefits to GA pilots, but pilot training tied to experience has to evolve with it. 

(3) Installing affordable systems.

(a) Enhancing the quantity, quality, and presentation of situational data in the cockpit 

can improve pilot situational awareness, efficiency, and safety. Many studies have shown that 

equipping these airplanes with safety devices such as Terrain Awareness Warning Systems 

(TAWS), Graphical Weather Displays, Map Displays, Integrated Flight Systems, SVS, and 

Enhanced Vision Systems may dramatically reduce a number of accident types. Pilots have 

reported that integrated flight displays help reduce workload, improve situational awareness, and 

increase safety. 

(b) The aviation industry as a whole is on the threshold of a revolutionary change in 

communication, navigation, and surveillance of aircraft operations. The Next Generation Air 

Transportation System will overhaul the National Airspace System (NAS) to take advantage of 

new technology. It will also likely result in the long-term replacement of many avionics and 

instrument equipment in the existing fleet as well as in new production aircraft. Facilitating 

safety equipment installation should enhance the NAS efficiency and safety. If GA is to operate 

within a revised NAS, new technologies should be available and affordable for GA aircraft. 

With the four-tier certification class criteria, new technologies are affordable for GA. If GA had 

only one class for certification, due to the cost of equipment for the NAS architecture, 

implementation would be incomplete or exclude large portions of the GA fleet from the NAS 

system. Neither situation is desirable or acceptable. 

7. Acronyms. 

14 CFR Title 14 Code of Federal Regulation 

AC Advisory Circular 

ACO Aircraft Certification Office 

AFM Airplane Flight Manual 

AFMS Airplane Flight Manual Supplement 

ARP Aerospace Recommended Practice 

ATC Amended Type Certificate 

CAR Civil Air Regulations 

CFR Code of Federal Regulations 

CHT Cylinder Head Temperature 

DAL Development Assurance Level 
11/17/2011 AC 23.1309-1E 

7 

EEC Electronic Engine Control 

EGT Engine Gas Temperature 

EPR Engine Pressure Ratio 

FAA Federal Aviation Administration 

FHA Functional Hazard Assessment 

FMEA Failure Modes and Effects Analysis 

FTA Fault Tree Analysis 

GA General Aviation 

GNSS Global Navigation Satellite System 

HW Hardware 

HIRF High Intensity Radiated Fields 

ICA Instructions for Continued Airworthiness 

ICAO International Civil Aviation Organization 

IFR Instrument Flight Rules 

ILS Instrument Landing System 

IMC Instrument Meteorological Conditions 

MFD Multifunction Flight Display 

MRE Multiple Reciprocating Engine 

MTE Multiple Turbine Engine 

MTBF Mean Time Between Failures 

NAS National Airspace System 

P Primary System 

PFD Primary Flight Display 

PMA Parts Manufacturer Approval 

PSSA Preliminary System Safety Assessment 

R Reserved 

S Secondary System 

SAE Society of Automotive Engineers 

SRE Single Reciprocating Engine 

SSA System Safety Assessment 

STE Single Turbine Engine 

STC Supplemental Type Certificate 

SVS Synthetic Vision Systems 

SW Software 

TCAS Traffic Collision Avoidance System 

TIA Type Inspection Authorization 

TAWS Terrain Awareness Warning System 

TC Type Certificate 

TIT Turbine Inlet Temperature 

TSO Technical Standard Order 

VFR Visual Flight Rules 

WAAS Wide Area Augmentation System 
11/17/2011 AC 23.1309-1E 

8 

8. Definitions. 

a. Adverse effect. A response of a system that results in an undesirable operation of an 

airplane system, or subsystem. 

b. Analysis. An evaluation based on decomposition into simple elements. 

c. Adverse operating condition. A set of environmental or operational circumstances 

applicable to the airplane, combined with a failure or other emergency situation that results in a 

significant increase in normal flight crew workload. 

d. Assessment. An evaluation based upon engineering judgment. 

e. Attribute. A feature, characteristic, or aspect of a system or a device, or a condition 

affecting its operation. Some examples would include design, construction, technology, 

installation, functions, applications, operational uses, and environmental and operational 

stresses. It would also include relationships with other systems, functions, and flight or 

structural characteristics. 

f. Average probability per flight hour. A representation of the number of times the 

subject failure condition is predicted to occur during the entire operating life of all airplanes of a 

type, divided by the anticipated total operating hours of all airplanes of that type. 

Note: The average probability per flight hour is normally calculated as the probability of a 

failure condition occurring during a typical flight of mean duration divided by that mean 

duration. See Appendix 3. 

g. Caution. A clear and unambiguous indication to the flight crew or pilot of a failure that 

requires subsequent crew action. An inherent characteristic of the airplane or a device that will 

give clearly distinguishable indications of malfunction or misleading information may provide 

this caution. 

h. Complex hardware item. All items that are not simple are considered to be complex. 

See definition of simple hardware item. Source: RTCA/DO-254, Appendix C and Order 

8110.105. 

i. Complex system. A system is “complex” when its operation, failure modes, or failure 

effects are difficult to comprehend without the aid of analytical methods or structured assessment 

methods. FMEA and FTA are examples of such structured assessment methods. Increased 

system complexity is often caused by such items as sophisticated components and multiple 

interrelationships. For example, for these types of systems, a portion of the compliance may be 

shown by the use of DALs such as by processes in RTCA/DO-178B or RTCA/DO-254 or 

equivalent. See the definitions for “conventional” and “simple” for more information. 

j. Continued safe flight and landing. This phrase means that the airplane is capable of 

continued controlled flight and landing, possibly using emergency procedures, without requiring 
11/17/2011 AC 23.1309-1E 

9 

exceptional pilot skill or strength. Upon landing, some airplane damage may occur as a result of 

a failure condition. 

k. Conventional system. A system is considered “conventional” if its function, the 

technological means to implement its function, and its intended usage are all the same as, or 

closely similar to, that of previously approved systems that are commonly used. The systems 

that have established an adequate service history and the means of compliance for approval are 

generally accepted as "conventional." Normally conventional and simple systems may be 

analyzed by qualitative assessments as shown in Figure 3. See the definitions for complex and 

simple systems for more information. 

l. Critical function. A function whose loss would prevent the continued safe flight and 

landing of the airplane. 

Note: The term “critical function” is associated with a catastrophic failure condition. Newer 

documents may not refer specifically to the term “critical function.” 

m. Design appraisal. A qualitative appraisal of the integrity and safety of the system 

design. An effective appraisal requires experienced judgment. 

n. Design assurance level. All of those planned and systematic actions used to substantiate, 

at an adequate level of confidence, that design errors have been identified and corrected such that 

the items (hardware, software) satisfy the applicable certification basis. This term may be used 

in some SAE and RTCA documents, but in this AC it is intended that design assurance levels 

will correlate to the same levels as the DALs for the safety assessment process. See section 21 

for more information. 

o. Development Assurance Level (DAL). All those planned and systematic actions used to 

substantiate, to an adequate level of confidence, that errors in requirements, design, and 

implementation have been identified and corrected such that the system satisfies the applicable 

certification basis. 

Note: For this AC, DALs in figure 2 and throughout this AC are also intended to correlate to 

software levels in RTCA/DO-178B and complex hardware design assurance levels in 

RTCA/DO-254 for the system or item. See section 21 for more information. 

p. Equipment essential to safe operation. Equipment installed in order to comply with the 

applicable certification requirements of part 23 or operational requirements of parts 91, 121, and 

135. 

q. Error. An omission or incorrect action by a crewmember or maintenance personnel, or a 

mistake in requirements, design, or implementation. 

r. Essential function. A function whose loss would reduce the capability of the airplane or 

the ability of the crew to cope with adverse operating conditions. 
11/17/2011 AC 23.1309-1E 

10 

Note: The term “essential function” is associated with failure conditions between major and 

hazardous. Newer documents may not refer specifically to the term “essential function.” 

s. Event. An internal or external occurrence that has its origin distinct from the airplane, 

such as atmospheric conditions (for example, gusts, temperature variations, icing, and, runway 

conditions, conditions of communication, navigation, and surveillance services, bird-strike, fire, 

leaking fluids, tire burst, HIRF exposure, lightning, uncontained failure of high energy rotating 

machines, etc.). The term is not intended to cover sabotage. 

t. Essential load. Equipment essential to safe operation that requires a power source for 

normal operation. 

u. Extremely remote failure conditions. Those failure conditions not anticipated to occur 

to each airplane during its total life but which may occur a few times when considering the total 

operational life of all airplanes of this type. For quantitative assessments, refer to the probability 

values shown for hazardous failure conditions in figure 2. 

v. Extremely improbable failure condition. For commuter category airplanes, those 

failure conditions so unlikely that they are not anticipated to occur during the entire operational 

life of all airplanes of one type. For other classes of airplanes, the likelihood of occurrence may 

be greater. For quantitative assessments, refer to the probability values shown for catastrophic 

failure conditions in figure 2. 

w. Failure. An occurrence that affects the operation of a component, part, or element such 

that it can no longer function as intended (this includes both loss of function and malfunction). 

Note: Errors may cause failures but are not considered failures. 

x. Failure conditions. A condition having an effect on either the airplane or its occupants, 

or both, either direct or consequential, which is caused or contributed to by one or more failures 

or errors considering flight phase and relevant adverse operational or environmental conditions 

or external events. Failure conditions may be classified according to their severity as follows: 

(1) No safety effect. Failure conditions that would have no effect on safety (that is, 

failure conditions that would not affect the operational capability of the airplane or increase crew 

workload). 

(2) Minor. Failure conditions that would not significantly reduce airplane safety and 

involve crew actions that are within their capabilities. Minor failure conditions may include a 

slight reduction in safety margins or functional capabilities, a slight increase in crew workload 

(such as routine flight plan changes), or some physical discomfort to passengers or cabin crew. 

(3) Major. Failure conditions that would reduce the capability of the airplane or the 

ability of the crew to cope with adverse operating conditions to the extent that there would be a 

significant reduction in safety margins or functional capabilities. In addition, the failure 

condition has a significant increase in crew workload or in conditions impairing crew efficiency; 
11/17/2011 AC 23.1309-1E 

11 

or a discomfort to the flight crew or physical distress to passengers or cabin crew, possibly 

including injuries. 

(4) Hazardous. Failure conditions that would reduce the capability of the airplane or 

the ability of the crew to cope with adverse operating conditions to the extent that there would be 

the following: 

(a) A large reduction in safety margins or functional capabilities; 

(b) Physical distress or higher workload such that the flight crew cannot be relied 

upon to perform their tasks accurately or completely; or 

(c) Serious or fatal injury to an occupant other than the flight crew. 

(5) Catastrophic. Failure conditions that are expected to result in multiple fatalities of 

the occupants, or incapacitation or fatal injury to a flight crewmember normally with the loss of 

the airplane. 

Notes: (1) The phrase “are expected to result” is not intended to require 100 percent 

certainty that the effects will always be catastrophic. Conversely, just because the effects 

of a given failure, or combination of failures, could conceivably be catastrophic in 

extreme circumstances, it is not intended to imply that the failure condition will 

necessarily be considered catastrophic. (2) The term “catastrophic” was defined in 

previous versions of advisory materials as a failure condition that would prevent 

continued safe flight and landing. 

y. Function. The lowest defined level of a specific action of a system, equipment, and flight 

crew performance aboard the airplane that, by itself, provides a completely recognizable 

operational capability (e.g., an airplane heading is a function). One or more systems may 

contain a specific function or one system may contain multiple functions. 

z. Functional hazard assessment. A systematic, comprehensive examination of airplane 

and system functions to identify potential minor, major, hazardous, and catastrophic failure 

conditions that may arise as a result of a malfunction or a failure to function. 

aa. Hazard. A potentially unsafe condition resulting from failures, malfunctions, external 

events, errors, or combinations thereof. This term is intended for single malfunctions or failures 

that are considered probable based on either past service experience or analysis with similar 

components in comparable airplane applications, or both. There is no quantitative analysis 

intended in this application. 

Note: There is a difference between “hazardous” as used in general policy or regulations 

and “hazardous failure condition” as used in an FHA. When the term "hazard" or 

"hazardous" is used in general policy or regulations, it is generally used as shown in this 

definition. A hazard could be a failure condition that relates to major, hazardous, or 

catastrophic. 
11/17/2011 AC 23.1309-1E 

12 

bb. Improbable failure conditions. Those failure conditions unlikely to occur in each 

airplane during its total life, but that may occur several times when considering the total 

operational life of a number of airplanes of this type. Also, those failure conditions not 

anticipated to occur to each airplane during its total life but that may occur a few times when 

considering the total operational life of all airplanes of this type. For quantitative assessments, 

refer to the probability values shown for major and hazardous failure conditions in figure 2. For 

more specific guidance, see definitions of “remote failure conditions” and “extremely remote 

failure conditions” 

cc. Item. One or more hardware and/or software elements treated as a unit.

dd. Installation appraisal. A qualitative appraisal of the integrity and safety of the 

installation. Any deviations from normal industry-accepted installation practices should be 

evaluated. 

ee. Latent failure. A failure is latent until it is made known to the flight crew or 

maintenance personnel. 

ff. Malfunction. Failure of a system, subsystem, unit, or part to operate in the normal or 

usual manner. The occurrence of a condition whereby the operation is outside specified limits. 

gg. Minimize. To reduce, lessen, or diminish a hazard to the least practical amount with 

current technology and materials. The least practical amount is that point at which the effort to 

further reduce a hazard significantly exceeds any benefit in terms of safety derived from that 

reduction. Additional efforts would not result in any significant improvements to safety and 

would inappropriately add to the cost of the product without a commensurate benefit. 

hh. Power source. A system that provides power to installed equipment. This system 

would normally include prime mover(s), required power converter(s), energy storage device(s), 

and required control and interconnection means. 

ii. Primary function. A function installed to comply with applicable regulations for the 

required function and provides the most pertinent controls or information instantly and directly 

to the pilot. For example, the PFD is a single physical unit that always provides the primary 

display and complies with the requirements of all the following: altitude, airspeed, aircraft 

heading (direction) and attitude. The PFD is located directly in front of the pilot and used 

instantly and first by the pilot. A standby or another display intended to be used in the event of 

failure of the PFD or as a cross reference is an example of a secondary system. For example, a 

brake control system normally uses the electronic brake system most of the time because of its 

better performance, but it does not comply with all the requirements. In this case, the 

mechanical brakes are used as the backup systems; yet, it is consider the primary with regard to 

meeting the requirements and the electronic brake system is the secondary. 

jj. Primary system. A system that provides the primary function. 

kk. Probable. Probable as defined for § 23.1309(a) through Amendment 23-49, as a 

probable malfunction or failure, is any single malfunction or failure that is considered likely on 
11/17/2011 AC 23.1309-1E 

13 

the basis of either past service experience or analysis with similar components in comparable 

airplane applications, or both. 

Note: Normally, there is no quantitative analysis intended in this application. This should 

not be confused with a probable failure condition when used for a safety assessment process.

ll. Probable failure conditions. Those failure conditions anticipated to occur one or more 

times during the entire operational life of each airplane. These failure conditions may be 

determined on the basis of past service experience with similar components in comparable 

airplane applications. For quantitative assessments, refer to the probability values shown for 

minor failure conditions in figure 2. 

mm. Qualitative. Those analytical processes that assess system and airplane safety in an 

objective non-numerical manner. 

nn. Quantitative. Those analytical processes that apply mathematical methods to assess the 

system and airplane safety. 

oo. Redundancy. The presence of more than one independent means for accomplishing a 

given function. Each means of accomplishing the function need not be identical. 

pp. Reliability. The determination that a system, subsystem, unit, or part will perform its 

intended function for a specified interval under certain operational and environmental conditions. 

qq. Remote failure conditions. Those failure conditions that are unlikely to occur to each 

airplane during its total life but that may occur several times when considering the total 

operational life of a number of airplanes of this type. For quantitative assessments, refer to the 

probability values shown for major failure conditions in figure 2. 

rr. Secondary system. A redundancy system that provides the same function as the primary 

system. 

ss. Similarity. The process of showing that the equipment type, form, function, design, and 

installation have only minor differences to previously approved equipment. The safety and 

operational characteristics and other qualities of the new proposed installation should have no 

appreciable effects on the airworthiness of the airplane. 

tt. Simple hardware item. An item with a comprehensive combination of deterministic 

tests and analyses appropriate to the design assurance level that ensures correct functional 

performance under all foreseeable operating conditions, with no anomalous behavior.--Source: 

RTCA/DO-254, paragraph 1.6 and Order 8110.105.

uu. Simple system. Usually a system that can be evaluated by only qualitative analysis and 

it is not complex. Functional performance is determined by combination of tests and analyses. 

See the definitions for “conventional” and “complex” systems for more information. 
11/17/2011 AC 23.1309-1E 

14 

vv. Single failure concept. The objective of this design concept is to permit the airplane to 

continue safe flight and landing after any single failure. Protection from multiple malfunctions 

or failures should be provided when the first malfunction or failure would not be detected during 

normal operations of the airplane, which includes preflight checks, or if the first malfunction or 

failure would inevitably cause other malfunctions or failures. 

ww. System. A combination of components, parts, and elements that are interconnected to 

perform one or more functions. 

xx. Warning. A clear and unambiguous indication to the flight crew or pilot of a failure 

that requires immediate corrective action. An inherent characteristic of the airplane or a device 

that will give clearly distinguishable indications of malfunction or misleading information may 

provide this warning. 

9. Application of § 23.1309(a), (a)(1), (a)(2), and (a)(3), as adopted by Amendments 23-41 

and 23-49. 

If the certification basis for the airplane is Amendment 23-14, § 23.1309(a) (See Note in 

paragraph 10.) is appropriate to use for systems in airplanes approved to fly either VFR or IFR, 

or both. With the certification basis at Amendment 23-14, systems that must meet the singlefailure concept with the requirements of § 23.1309(a) should comply if the guidance in 

paragraph 10 of this AC is used. Under the certification basis at Amendment 23-14, compliance 

with § 23.1309(b) is not required and a safety assessment is not necessary, but it may be used. 

For complex systems, the requirements of Amendment 23-14 may not provide an adequate level 

of safety. Then, the certification basis should be Amendment 23-41 or 23-49 as appropriate. In 

accordance with AC 21.101-1, in cases where no regulatory standards are defined in the existing 

certification basis for the design change, but applicable regulatory standards exist in a 

subsequent amendment to the regulations, the subsequent amendment will be made part of the 

certification basis. Therefore, the change must comply with later appropriate regulations. 

10. Showing compliance with the requirements of § 23.1309 (a) through Amendment 23-

49. 

Note: The requirements of paragraphs (a), (a)(1), (a)(2), and (a)(3) of § 23.1309, as 

amended by Amendments 23-41 and 23-49, are the same requirements as paragraphs (a), (b), 

and (c) of § 23.1309, as amended by Amendment 23-14. These same requirements in 

paragraphs (a), (a)(1), (a)(2), and (a)(3) (above) were deleted in § 23.1309 by Amendment 

23-62 because there was a significant revision of § 23.1309. 

a. In order to show compliance with the requirements of § 23.1309(a), (a)(1), (a)(2), and 

(a)(3), it will be necessary to verify that the installed systems and each item of equipment will 

cause no unacceptable adverse effects and to verify that the airplane is adequately protected 

against any hazards that could result from probable malfunctions or failures. Analyze, inspect, 

and test equipment, systems, and installations to ensure compliance with the requirements of 

§ 23.1309(a), (a)(1), (a)(2), and (a)(3). 
11/17/2011 AC 23.1309-1E 

15 

b. A step-by-step diagram to comply with § 23.1309(a), (a)(1), (a)(2), and (a)(3) is shown in 

figure 1. These steps are described below. 

(1) Evaluate all airplane systems and each item of equipment in order to determine 

whether they are the following: 

(a) Essential to safe operation; or 

(b) Not essential to safe operation. 

(2) Determine that operation of installed equipment has no unacceptable adverse effects. 

Verify this by applicable flight or ground checks, as follows: 

(a) If it can be determined that the operation of the installed equipment will not 

adversely affect equipment essential to safe operation, the requirements of § 23.1309(a)(1)(i) 

have been satisfied; and 

(b) If it is determined that the operation of the installed equipment has an adverse 

effect on equipment not essential to safe operation and a means exists to inform the pilot of the 

effect, the requirements of § 23.1309(a)(1)(ii) have been met. An acceptable means to inform 

the pilot that the affected system is not performing properly would include any visual or aural 

method (flags, lights, horns, loss of display, etc.). 

(3) Determine that failure or malfunction of the installed equipment could not result in 

unacceptable hazards. 

(a) Each item of equipment must be evaluated for general installation hazards. These 

types of hazards would normally include those hazards that would directly compromise the 

safety of the airplane or its occupants, such as fire, smoke, explosion, toxic gases, 

depressurization, etc. A hazard could also result from loss of equipment or systems essential to 

safe operations when the minimum required functions are lost. Individual failure of redundant 

equipment would not necessarily be considered a hazard. For example, the single failure of 

either a communication transceiver or a navigation receiver (but not both) during IFR operation 

is not considered a hazard; however, a single failure of a common power supply to those systems 

would be considered a hazard. 

(b) Systems and equipment essential to safe operation must also be assessed for 

probability of malfunction or failure. Where the installation is conventional, and where there is a 

high degree of similarity in installations and a significant amount of service history is available 

for review, this determination can be an engineering judgment. Service history should show that 

past malfunctions or failures have not resulted in hazards and there are no unresolved problems. 

(c) Hazards identified and found to result from probable failures are not acceptable in 

multiengine airplanes. In these situations, a design change may be required to remove the hazard 

or to reduce the probability of failure, such as increasing redundancy, substitution of more 

reliable equipment, annunciation, etc. 
11/17/2011 AC 23.1309-1E 

16 

(d) If it has been determined that a probable failure or malfunction could result in a 

hazard to a single-engine airplane, that hazard must be minimized or prevent hazards in a 

multiengine airplane. To minimize is to reduce, lessen, or diminish a hazard to the least practical 

amount with current technology and materials. Design features should be taken into account to 

prevent hazards either by ensuring that the failure condition will not occur or by having 

redundancy or annunciation with the associated flight crew’s corrective action. In either case, 

the hazards should be addressed to the least practical amount to the point at which the effort to 

further reduce a hazard significantly exceeds any benefit in terms of safety derived from that 

reduction that is practical for this type of airplane. Additional efforts would not result in any 

significant improvements of safety and would inappropriately add to the cost of the product 

without a commensurate benefit. This determination should come from an experienced 

engineering judgment based on the criticality of the hazard and the intended kinds of operation. 
11/17/2011 AC 23.1309-1E 

17 

FIGURE 1. METHOD OF COMPLIANCE DIAGRAM OF § 23.1309(a) 

THROUGH AMENDMENT 23-49 

Start

Assessment

All Airplanes

Corrective

Measures

Unacceptable

Will Operation

of this Equipment have

Adverse Effect on Equipment

Essential to Safe 

Operation?

Any Adverse

Effect on Other

Equipment?

Is there a means

to Inform the Pilot

of Effect?

Meets Requirements

of § 23.1309(a)(1)

Will Any

Probable Failure or Malfunction 

Result in a Hazard?

Corrective Measures

Unacceptable Single or Multiengine?

Has Hazard Been Minimized?

Meets Requirements of 

§ 23.1309(a)(2) & (3)

Yes

Yes

No

No

No

Yes

Yes No

Yes

No Yes

Single 

Engine

Multiengine
11/17/2011 AC 23.1309-1E 

18 

11. Application of § 23.1309(a)(4), as adopted by Amendment 23-49. 

a. For those commuter airplanes that include the certification basis of Amendments 23-34, 

23-41, or 23-49, § 23.1309(a)(4) requires all applicable systems and installations to be designed 

to safeguard against hazards to the airplane in the event of their failure. This requirement in 

§ 23.1309(a)(4) for commuter airplanes was introduced into part 23 airplanes by Amendment 23-

34 before the safety assessment process was included by Amendment 23-41. 

b. Design features should be taken into account to safeguard against hazards either by 

ensuring that the failure condition will not occur or by having redundancy or annunciation with 

the associated flight crew’s corrective action. The reliability should be such that independent 

failures of the redundant systems are not probable during the same flight. If a redundant system 

is required, a probable failure in one system should not adversely affect the other system’s 

operation. No probable failure should result in a “safe” indication of an “unsafe” condition so 

that the flight crew would incorrectly assume the system is available or functional. When the 

unsafe condition is annunciated or detected, the AFM should have clear and precise corrective 

procedures for handling the failure without an excessive increase in workload. 

c. Service history for similar installations may be utilized to meet part or all of this 

requirement if a system or installation has a significant and favorable service history in 

environments similar to the airplane. The claim of similarity should be based on equipment type, 

function, design and installation similarities, and other relevant attributes. It is the applicant’s 

responsibility to provide accepted/approved data that supports any similarity claims to a previous 

installation. More information is available in Order 8110.4C. 

12. Application of § 23.1309(a)(1) and (a)(2), as adopted by Amendments 23-62. 

a. Section 23.1309(a) requires the airplane equipment and systems be designed and installed 

so that: 

(1) Those required for type certification or by operating rules perform as intended under 

the airplane operating and environmental conditions, including the indirect effects of lightning 

strikes. 

(2) Any equipment and system does not adversely affect the safety of the airplane or its 

occupants, or the proper functioning of those covered by paragraph (a)(1) of this section. 

b. Section 23.1309(a) has requirements for different classes of equipment and systems 

installed in the airplane, that is, those that are required and not required. Section 23.1309(a)(1) 

covers the equipment and systems installed to meet a regulatory requirement. Such systems and 

equipment are required to “perform as intended under the airplane’s operating and environmental 

conditions.” 

c. Section 23.1309(a) gives the conditional qualifiers “under the airplane operating and 

environmental conditions.” This section describes two actions for the applicant. First, the 
11/17/2011 AC 23.1309-1E 

19 

applicant must consider the full normal operating envelope of the airplane, as defined by the 

AFM, with any modification to that envelope associated with abnormal or emergency procedures 

and any anticipated crew action. Second, the applicant must consider the anticipated external 

and internal airplane environmental conditions, as well as any additional conditions where 

equipment and systems are assumed to “perform as intended.” Although certain operating 

conditions are foreseeable, achieving normal performance when they exist is not always possible 

and may not need to be considered. For example, you may foresee ash clouds from volcanic 

eruptions; however, airplanes with current technology cannot safely fly in such clouds. 

d. Other external environmental conditions such as atmospheric turbulence, HIRF, lightning, 

and precipitation, which the airplane is reasonably expected to encounter, must be considered. 

These severities of the external environmental conditions to be considered are limited to those 

established by certification standards and precedence. Also, the environmental effect within the 

airplane must be considered. These effects should include vibration and acceleration loads, 

variations in fluid pressure and electrical power, and fluid or vapor contamination due to either 

the normal environment or accidental leaks or spillage and handling by personnel. 

e. We accept equipment susceptible to failures if these failures do not contribute 

significantly to the existing risks (e.g., some degradation in functionality and capability is 

routinely allowed during some environmental qualifications, such as HIRF and lightning 

testing). For example, system lightning protection allows momentary lost or upset of specific 

functions of electrical/electronic systems. These functions are for failure conditions that are 

hazardous or major. But, the function must recover in a timely manner after the airplane is 

exposed to lightning. See AC 20-158 and AC 20-136B for more specific guidance. The safety 

assessment process of § 23.1309 does not supersede either the HIRF or lightning specific 

requirements. Environmental effects such as HIRF and lightning should not be considered in 

combination with another single failure or pre-existing latent failure. 

f. Using § 23.1309(a)(2), we must analyze any installed equipment or system that has 

potential failure condition(s) that are catastrophic, hazardous, major, or minor to determine their 

impact on the safe operation of the airplane. Usually, normal installation practices can be based 

on a relatively simple qualitative installation evaluation. If the possible safety impacts, including 

failure modes or effects, are questionable, or isolation between systems is provided by complex 

means, more formal structured evaluation methods or a design change may be necessary. 

Operational and environmental qualification requirements for those equipment, systems, and 

installations are reduced to the necessary tests that show their normal or abnormal functioning 

does not adversely affect the proper functioning of the equipment, systems, or installations under 

§ 23.1309 (a)(1) and does not otherwise adversely influence the safety of the aircraft or its 

occupants. Examples of adverse influences include fire, explosion, exposing passengers to high 

voltages, etc. 

g. Section 23.1309(a)(2) requires the applicant to show that all required and non required 

equipment and systems (including approved “amenities,” such as a coffee pot and entertainment 

systems) have no safety effect on the operation of the airplane. Section 23.1309(a)(2) does not 

require non-required equipment and systems to function properly during all airplane operations 

once in service if analysis shows that all potential failure condition(s) have no adverse safety 
11/17/2011 AC 23.1309-1E 

20 

effects on safe operation of the airplane. The equipment or system must function in accordance 

with the manufacturer’s operating manual or specification. An applicant’s statement of intended 

function must be sufficiently specific and detailed so the FAA can evaluate whether the system is 

appropriate for the intended function(s) and the associated flight crew tasks. However, we 

would require equipment or systems to function when they are tested to verify that they do not 

interfere with the operation of other airplane equipment and systems and do not pose a hazard 

themselves. The normal operation of non-required systems should not interfere with the proper 

operation of any required systems or present a hazard themselves. Non-required systems are not 

required to perform their intended function throughout the aircraft operating and environmental 

conditions. However, in situations where the non-required system has failed, there can be no 

adverse safety effect to the aircraft, its occupants, or any adverse effect on required equipment 

and systems. Malfunctioning and erroneous behavior of all systems, including non-required, 

should be addressed under § 23.1309(c). 

13. Application of § 23.1309(b), as adopted by Amendment 23-62. 

a. Section 23.1309 (b) requires for minor, major, hazardous, or catastrophic failure 

condition(s) which occur during TIA or FAA flight-certification testing must have root cause 

analysis and corrective action. Testing is an important aspect of the overall compliance 

processes with §§ 23.1301 and 23.1309. The applicant should conduct bench, ground, and/or 

flight testing when necessary to validate hazard classifications, acceptability of crew procedures, 

human factors, and other assumptions made during the root cause analysis processes and 

corrective actions. The applicant must also discuss with the project ACO what aspects of this 

testing will need to be included in the FAA certification testing. Those aspects required for 

formal certification testing must be included in the appropriate FAA approved test plans and 

conducted on an FAA conformed test article in the presence of the FAA or delegated FAA 

witness in accordance with Order 8110.4C. Before receiving TIA, the applicant should be able 

to show qualitatively that the proposed design change will meet the requirements of section 

23.1309. 

b. The FAA will typically conduct some level of function and qualitative reliability testing 

during certification to ensure required functions demonstrate an acceptable level. The FAA will 

also conduct other required certification tests and analyses. These tests are meant to verify 

availability, accuracy, and qualitative reliability of the system. The FAA expects the applicant to 

show that the system does not exhibit unintended or undesirable functionality failure conditions 

that are minor, major, hazardous, or catastrophic. The FAA also expects that failures, 

malfunctions, and design errors with potential safety hazards have a full assessment of the 

problem, root cause analysis processes, and corrective action. 

c. It is not intended for the probability requirements based on random distribution across a 

fleet of aircraft be applied on the beginning phase and to be fully complaint with this 

requirements. It is not appropriate to apply probability values to the typical certification flight 

test because the sample is too small. Failures during TIA and FAA flight-certification testing 

must have root cause analysis and corrective action (include traceability to production within the 

change) with robust corrections and substantiation of the corrections. The regulations do not 

required FAA approval for the root cause analysis. 
11/17/2011 AC 23.1309-1E 

21 

14. Application of § 23.1309(b), as adopted by Amendments 23-41 and 23-49 and 

§ 23.1309(c), as adopted by Amendment 23-62. 

a. The installed systems should be evaluated by performing a safety assessment as shown in 

this AC. The depth and scope of the safety assessment depends on the types of functions 

performed by the systems, the severity of the failure conditions, and whether the system is 

complex. For instance, the safety assessment for a slightly modified single-engine airplane with 

simple systems might consist only of an FHA with a design and installation appraisal. This FHA 

will be much less extensive than the FHA for a commuter category or a multiple turbine-engine 

airplane with more complex systems. The types of analyses selected by an applicant and 

approved by the certification authority should be based on factors such as the system 

architecture, complexity, particular design, etc. 

b. The safety assessment objective is to ensure an acceptable safety level for equipment and 

systems installed on the airplane. A logical and acceptable inverse relationship should exist 

between the average probability per flight hour and the severity of failure conditions effects (as 

shown in figure 2). This figure defines the appropriate airplane systems probability standards for 

four certification classes of airplanes designed to part 23 standards. The relationship between 

probability and severity of failure condition effects are as follows: 

(1) Failure conditions with no safety effect have no probability requirement. 

(2) Minor failure conditions may be probable. 

(3) Major failure conditions must be no more frequent than remote. 

(4) Hazardous failure conditions must be no more frequent than extremely remote. 

(5) Catastrophic failure conditions must be extremely improbable. 

c. Compliance with § 23.1309(c) may be shown by analysis and, where necessary, by 

appropriate ground, flight, or simulator test. The analysis should consider— 

(1) Possible modes of failure, including malfunctions and damage from external sources; 

(2) The probability of multiple failures and the probability or undetected faults; 

 

(3) The resulting effects on the airplane and occupants, considering the stage of flight 

and operating conditions; and 

(4) The crew warning cues, corrective action required, and the crew's capability of 

determining faults. 
11/17/2011 AC 23.1309-1E 

22 

15. Four certification classes of airplanes. 

a. The four-certification classes of airplanes for this AC are shown in figure 2 and the 

acronyms are defined in paragraph 7. They are Class I (Typically SRE 6,000 pounds (lbs.) or 

less (Maximum Certificated Gross Takeoff Weight)), Class II (Typically MRE, MTE and STE, 

6,000 pounds or less), Class III (Typically SRE, STE, MRE, and MTE greater than 6,000 

pounds), and Class IV (Typically Commuter Category). 

b. Numerical values are assigned for use in cases where the impact of system failures is 

examined by quantitative methods of analysis. Also, the related software and complex hardware 

DALs for the various failure conditions are part of the matrix in figure 2 for most systems. 

These levels should be used unless there are some unique architecture considerations. For these 

unusual situations there should be specific policy, guidance, or approval by the Small Airplane 

Directorate. (See paragraph 21 for more information) The probability standards are based on 

historical accident data, systems analyses, and engineering judgment for each class of airplane. 

c. In assessing the acceptability of a design, the FAA recognized the need to establish 

rational probability values. Historically, failures in GA airplanes that might result in 

catastrophic failure conditions are predominately associated with the primary flight instruments 

in IMC. Historical evidence indicates that the probability of a fatal accident in restricted 

visibility due to operational and airframe-related causes is approximately one per ten thousand 

flight hours or 1 x 10-4 per flight hour for single-engine airplanes under 6,000 pounds. 

Furthermore, from accident databases, it appears that about 10 percent of the total was attributed 

to failure conditions caused by the airplane's systems. It is reasonable to expect that the 

probability of a fatal accident from all such failure conditions would not be greater than one per 

one hundred thousand flight hours or 1 x 10-5 per flight hour for a newly designed airplane. From 

past service history, it is also assumed that there are about ten potential failure conditions in an 

airplane that could be catastrophic. The allowable target average probability per flight hour of 1 x 

10-5 was thus apportioned equally among these failure conditions, which resulted in an allocation 

of not greater than 1 x 10-6 to each. The upper limit for the average probability per flight hour for 

catastrophic failure conditions would be 

1 x 10-6, which establishes an approximate probability value for the term "extremely improbable." 

Failure conditions having less severe effects could be relatively more likely to occur. Similarly, 

airplanes over 6,000 pounds have a lower fatal accident rate; therefore, they have a lower 

probability value for catastrophic failure conditions. 
11/17/2011 AC 23.1309-1E 

23 

FIGURE 2. RELATIONSHIP AMONG AIRPLANE CLASSES, PROBABILITIES, 

SEVERITY OF FAILURE CONDITIONS, AND 

SOFTWARE AND COMPLEX HARDWARE DAL 

Classification of 

Failure Conditions 

No Safety Effect <----Minor-----> <----Major----> <--Hazardous---> < Catastrophic> 

Allowable 

Qualitative 

Probability 

No Probability 

Requirement 

Probable Remote Extremely 

Remote 

Extremely 

Improbable 

Effect on Airplane No effect on 

operational 

capabilities or safety 

Slight reduction in 

functional 

capabilities or safety 

margins 

Significant reduction 

in functional 

capabilities or safety 

margins 

Large reduction in 

functional 

capabilities or safety 

margins 

Normally with hull 

loss 

Effect on Occupants Inconvenience for 

passengers 

Physical discomfort 

for passengers 

Physical distress to 

passengers, possibly 

including injuries 

Serious or fatal 

injury to an occupant 

Multiple 

fatalities 

Effect on Flight 

Crew 

No effect on flight 

crew 

Slight increase in 

workload or use of 

emergency 

procedures 

Physical discomfort 

or a significant 

increase in workload 

Physical distress or 

excessive workload 

impairs ability to 

perform tasks 

Fatal Injury or 

incapacitation 

Classes of 

Airplanes: 

Allowable Quantitative Probabilities and Software (SW) and Complex Hardware (HW) Development Assurance 

Levels (Note 2) 

Class I 

(Typically SRE 

6,000 pounds or less) 

No Probability or 

SW and HW 

Development 

Assurance Levels 

Requirement 

<10-3 

Note 1 

P=D 

<10-4 

Notes 1 and 4 

P=C, S=D 

<10-5 

Note 4 

P=C, S=D 

<10-6 

Note 3 

P=C, S=C 

Class II 

(Typically MRE, 

STE, or MTE 6,000 

pounds or less) 

No Probability or 

SW and HW 

Development 

Assurance Levels 

Requirement 

<10-3 

Note 1 

P=D 

<10-5 

Notes 1 and 4 

P=C, S=D 

 

<10-6 

Note 4 

P=C, S=C 

<10-7 

Note 3 

P=C, S=C 

Class III

(Typically SRE, 

STE, MRE, and 

MTE greater than 

6,000 pounds) 

No Probability or 

SW and HW 

Development 

Assurance Levels 

Requirement 

<10-3 

Note 1 

P=D 

<10-5 

Notes 1 and 4 

P=C, S=D 

<10-7 

Note 4 

P=C, S=C 

<10-8 

Note 3 

P=B, S=C 

Class IV 

(Typically 

Commuter Category) 

No Probability or 

SW and HW 

Development 

Assurance Levels 

Requirement 

<10-3 

Note 1 

P=D 

<10-5 

Notes 1 and 4 

P=C, S=D 

<10-7 

Note 4 

P=B, S=C 

<10-9 

Note 3 

P=A, S=B 

Note 1: Numerical values indicate an order of probability range and are provided here as a reference. 

Note 2: The letters of the alphabet denote the typical SW and HW Development Assurance Levels for Primary System (P) and Secondary System 

(S). For example, HW or SW Development Assurance Level A on Primary System is noted by P=A. 

Note 3: At airplane function level, no single failure will result in a Catastrophic Failure Condition. 

Note 4. Secondary System (S) may not be required to meet probability goals. If installed, S should meet stated criteria. 
11/17/2011 AC 23.1309-1E 

24 

d. The criteria shown in figure 2 directly reflect the historical accident and equipment 

probability of failure data in the CAR 3 and 14 CFR part 23 airplane fleet. Characteristics of the 

airplane, such as stall speed, handling characteristics, cruise altitude, ease of recognizing system 

failures, recognition of entry into stall, pilot workload, and other factors (which include pilot 

training and experience) affect the pilot’s ability to safely handle various types of system failures 

in small airplanes. The criteria considered for all airplanes’ failure conditions is based on 

service experience, operational exposure rates, and total airplane system reliability. The values 

for individual system probability of failure could be higher than probability values shown in 

figure 2 for specific failure conditions because it considers the installed airplane systems, events, 

and factors. 

e. These classes were defined based on the way accident and safety statistics are currently 

collected. Generally, the classes deal with airplanes of historical equivalent levels of system 

complexity, type of use, system reliability, and historical divisions of airplanes according to 

these characteristics. However, these classes could change because of new technologies. The 

placement of a specific airplane in a class should be done in reference to all of the airplane’s 

missions and performance characteristics. The applicant should have the concurrence of the 

certification authority that is knowledgeable about the applicable airplane class early in the 

program. When unusual situations develop, consult the Small Airplane Directorate to obtain 

specific policy guidance or approval. 

f. For example, airplanes with considerably more than 10 catastrophic failure conditions, 

that have greater performance characteristics and incorporate many complex systems and 

advance technologies may have lower probability values and higher DALs. These airplanes’ 

probability values and DALs may fall between the classes of the airplanes. For instance, the 

performance characteristics of a complex airplane including airplane handling qualities and stall 

speed may be similar to existing Class II airplanes. However, this airplane’s mission and other 

performance characteristics including high speed, high altitude, and extended range operations 

may be similar to existing Class III airplanes. The major difference between the DALs for Class 

II and Class III airplanes is for primary systems whose failure would result in a catastrophic 

failure condition for the airplane. Since this complex airplane falls between these two classes, it 

is reasonable to choose the higher DAL and a lower probability level. 

g. For example, in part 23, turbine-engine airplanes traditionally have been subject to more 

stringent requirements than a single-engine reciprocating airplane. A single-engine reciprocating 

airplane generally has a wider stall-cruise speed ratio than traditional turbine-engine airplanes. 

Such an airplane with a stall speed under 61 knots with simple systems, and with otherwise 

similar characteristics to a traditional single-engine reciprocating airplane (except for a higher 

cruise speed and a more reliable engine that is simpler to operate), can be treated as a Class I 

airplane under this analysis. Conversely, if a single-engine reciprocating airplane has the 

performance, mission capability, and system complexity of a higher class (such as cabin 

pressurization, high cruise altitude, and extended range), then that type of airplane design may 

align itself with the safety requirements of a higher class (for example, Class II airplane). These 

determinations should be made during the development of the certification basis. 
11/17/2011 AC 23.1309-1E 

25 

h. This AC uses terminology similar to AC 25.1309-1A. However, the specific means of 

compliance for § 25.1309 are defined differently due to the higher safety level required for 

transport category airplanes. However, there are some similarities with part 23 commuter 

category airplanes. 

16. Safety assessments. 

a. The applicant is responsible for identifying and classifying each failure condition and 

choosing the methods for safety assessment. The applicant should then obtain early concurrence 

from the cognizant certificating authority on the identification of failure conditions, their 

classifications, and the choice of an acceptable means of compliance. Figure 3 provides an 

overview of the information flow to conduct a safety assessment. This figure is a guide and it 

does not include all information provided in this AC or the documents referenced in section 4 of 

this AC. 

b. Functional hazard assessment (FHA).

(1) Before an applicant proceeds with a detailed safety assessment, an FHA of the 

airplane and system functions to determine the need for and the scope of subsequent analysis 

should be prepared. This assessment may be conducted using service experience, engineering 

and operational judgment, or service experience and a top-down deductive qualitative 

examination of each function. An FHA is a systematic, comprehensive examination of airplane 

and system functions to identify potential no safety effect, minor, major, hazardous, and 

catastrophic failure conditions that may arise, not only as a result of malfunctions or failure to 

function but also as a result of normal responses to unusual or abnormal external factors. The 

FHA concerns the operational vulnerabilities of systems rather than a detailed analysis of the 

actual implementation. 

(2) Each system function should be examined regarding the other functions performed 

by the system because the loss or malfunction of all functions performed by the system may 

result in a more severe failure condition than the loss of a single function. In addition, each 

system function should be examined regarding functions performed by other airplane systems 

because the loss or malfunction of different but related functions, provided by separate systems, 

may affect the severity of failure conditions postulated for a particular system. 

(3) The FHA is an engineering tool that should be performed early in the design and 

updated as necessary. It is used to define the high-level airplane or system safety objectives that 

should be considered in the proposed system architectures. Also, it should be used to assist in 

determining the DALs for the systems. Many systems may need only a simple review of the 

system design by the applicant to determine the hazard classification. An FHA requires 

experienced engineering judgment and early coordination between the applicant and the 

certification authority. 
11/17/2011 AC 23.1309-1E 

26 

FIGURE 3. DEPTH OF ANALYSIS FLOW CHART 

Conduct Functional 

Hazard Assessment

Is there a safety 

effect?

Is the 

Failure Condition 

Minor?

Is the 

system 

and installation 

similar to a 

previous 

design?

Is the Failure 

Condition Major?

Is the system 

simple and 

conventional?

Conduct qualitative 

and quantitative 

assessments

Is the system 

simple?

Is the system 

redundant?

Note: FHA may be based on 

a design and installation 

appraisal for these systems

Verify by design and 

installation appraisal

Verify similarity

Conduct qualitative 

assessment

Conduct qualitative and 

quantitative assessments

Conduct qualitative 

assessments

NO

YES

YES

NO

NO

YES

NO

YES

YES

YES

NO

NO

YES

NO

Note: Numbers in 

parenthesis refer to the 

paragraphs in this AC

17(a) & (b)

17(c) & (d)

17(c)

17(c)

17(d)
11/17/2011 AC 23.1309-1E 

27 

(4) Depending on the extent of functions to be examined and the relationship between 

functions and systems, different approaches to FHA may be taken. Where there is a clear 

correlation between functions and systems, and where system and function interrelationships are 

relatively simple, it may be feasible to conduct separate FHA’s for each system. However, this 

is conditional providing any interface aspects are properly considered and easily understood. 

However, a top-down approach from an airplane level perspective should be taken in planning 

and conducting an FHA where system and function interrelationships are more complex. 

(5) After each failure condition is classified, refer to figure 2 to identify the failure 

condition probability and software and complex hardware DALs. For example, the probability 

requirement for a hazardous failure condition for a Class I airplane should be less than 1 x 10-5. 

In addition, the primary system for a Class I airplane should have software and complex 

hardware DALs of C and, if required, the secondary system for a Class I airplane should have 

software and complex hardware DALs of D. 

(6) The classification of failure conditions does not depend on whether a system or 

function is required by any specific regulation. Some systems required by specific regulations, 

such as transponders, position lights, and public address systems, may have the potential for only 

minor failure conditions. Conversely, other systems not required by any specific regulation, 

such as flight management systems and automatic landing systems, may have the potential for 

major, hazardous, or catastrophic failure conditions. 

(7) The classification of failure conditions should consider all relevant factors. 

Examples of factors include the nature of the failure modes, which includes common mode 

faults, system degradation resulting from failures, flight crew actions, flight crew workload, 

performance degradation, reduced operational capability, effects on airframe, etc. It is 

particularly important to consider factors that would alleviate or intensify the severity of a failure 

condition. An example of an alleviating factor would be the continued performance of identical 

or operationally similar functions by other systems not affected by a failure condition. Examples 

of intensifying factors would include unrelated conditions that would reduce the ability of the 

crew to cope with a failure condition, such as weather or other adverse operational or 

environmental conditions. The ability of a system to inform the pilot of potential or real failure 

conditions so that timely corrective action can be taken to reduce the effects of the combination 

of events is desirable. This approach may reduce the severity of the failure condition.

(8) Because of the large number of combinations of failures, various mitigating factors, 

airplane characteristic effects, and similar factors, a specific FHA and the related safety 

assessments may be significantly different for each evaluated airplane type and configuration. 

These factors preclude providing a concrete example of a FHA that applies across the board to 

every installation. However, general examples may be provided that illustrate the concepts 

involved in an FHA. It is critical to understand that significant engineering judgment and 

common sense are necessary to provide a practical and acceptable evaluation of the airplane and 

its systems. 

c. Appendix 1 provides a partial list of FHA for consideration for part 23 IFR Class I 

airplanes with typical functions and, in general, the related failure conditions are at the aircraft 
11/17/2011 AC 23.1309-1E 

28 

level. The criteria at the aircraft level are useful to derive the system FHA. The failure 

conditions for the examples in Appendix 1 cannot be applied indiscriminately to a particular 

airplane installation. This table is primarily for use to reduce the regulatory burden on applicants 

who are not familiar with the various methods and procedures generally used in industry to 

conduct safety assessments. It is only intended to be a guide, not a certification checklist, since 

it does not include all the information necessary for an FHA for a specific airplane with its 

various functions and its intended use. The functions listed in the partial FHA as a guide for the 

classification of failure conditions when the functions are installed. The list of functions is not 

intended to suggest that the functions are required for Class I airplanes. Even if there is guidance 

information in Appendix 1, the applicable regulations provide the requirements of the functions 

for installations. 

(1) The applicant should use Appendix 1 and the appropriate certification authority as a 

point of departure for the specific system or airplane assessment. It can be used to arrive at the 

appropriate failure conditions for a specific system by similarity to or by interpolating between 

the example systems. It does not necessarily provide, by itself, an answer for an applicant’s 

system unless that system is exactly as described. Its sole purpose is to assist applicants by 

illustrating typical functions and the related failure conditions. This appendix addresses general 

applicability, which is valuable for determining software and complex hardware DALs, and it 

should not be utilized to replace any specific guidance intended for individual types of 

equipment, systems, and installations. The FHA results are airplane characteristic and system 

architecture dependent. The examples in this appendix are based on traditional airplanes and 

traditional architectures. Section 23.1309 is a regulation of general requirements and should not 

be used to supersede any specific requirements of part 23. 

(2) In addition to the general technical guidance provided in Appendix 1, a sample of 

one suggested format is provided in Appendix 2 for documenting the results of an FHA. This 

format illustrates how factors other than those directly illustrated in Appendix 1 are pertinent. It 

also illustrates that failure conditions are not limited to only the three general types shown in 

Appendix 1. The actual data shown in Appendix 2 is only used to illustrate the typical approach 

and should not be viewed as technically representative of any particular airplane. A complete 

FHA could be comprised of the layout shown in Appendix 2 by utilizing pertinent technical 

considerations identified in Appendix 1, which are modified and expanded to reflect the specific 

proposed airplane design under consideration. 

d. Part 23 airplanes cover a wide range of airplane sizes and capabilities. These airplanes 

range from single-engine, single-seat, low-performance airplanes to complex multiengine, highspeed, high-performance airplanes. At the bottom end of these part 23 airplane types, there are 

several compensating characteristics that mitigate many of the effects of a failure. Docile 

handling characteristics, low stall speeds, spin resistant designs, lower probability of operation in 

extreme weather conditions, and the inherent design philosophies used to design single-engine 

airplanes are specific examples of characteristics to consider in an FHA for systems installed in 

this class of airplane. Usually, air traffic control support is not a mitigating factor. 
11/17/2011 AC 23.1309-1E 

29 

17. Failure conditions. 

a. Failure conditions with no safety effect. An FHA with a design and installation 

appraisal to establish independence from other functions is necessary for the safety assessment 

of these failure conditions. In general, common design practice provides physical and functional 

isolation from related components, which are essential to safe operation. If the applicant chooses 

not to do a detailed FHA, the safety effects may be derived from the design and installation 

appraisal performed by the applicant. 

b. Analysis of minor failure conditions. An analysis should consider the effects of system 

failures on other systems or their functions. An FHA with a design and installation appraisal to 

establish independence from other functions is necessary for the safety assessment of these 

failure conditions. In general, common design practice provides physical and functional 

isolation from components that are essential to safe operation. If the applicant chooses not to do 

a detailed FHA, the safety effects may be derived from the design and installation appraisal 

performed by the applicant. 

c. Analysis of major failure conditions. An assessment based on engineering judgment is 

a qualitative assessment, as are several of the methods described below: 

(1) Similarity allows validation of a requirement by comparison to the requirements of 

similar certified systems. The similarity argument gains strength as the period of experience 

with the system increases. If the system is similar in its relevant attributes to those used in other 

airplanes and if the functions and effects of failure would be the same, then a design and 

installation appraisal and satisfactory service history of either the equipment being analyzed or 

of a similar design is usually acceptable for showing compliance. It is the applicant’s 

responsibility to provide data that is accepted, approved, or both, and that supports any claims of 

similarity to a previous installation. 

(2) For systems that are not complex and where similarity cannot be used as the basis for 

compliance, then compliance may be shown by means of a qualitative assessment that shows that 

the major failure conditions of the system, as installed, are consistent with the FHA (for 

example, redundant systems). 

(3) To show that malfunctions are indeed remote in systems of high complexity without 

redundancy (for example, a system with a self-monitoring microprocessor), it is necessary to 

conduct a qualitative functional FTA or FMEA supported by failure rate data and fault detection 

coverage analysis. 

(4) An analysis of a redundant system in the airplane is usually complete if it shows 

isolation between redundant system channels and satisfactory reliability for each channel. For 

complex systems, where functional redundancy is required, a qualitative FMEA or FTA may be 

necessary to determine that redundancy actually exists (for example, no single failure affects all 

functional channels). 

d. Analysis of hazardous and catastrophic failure conditions. For these failure 

conditions, a thorough safety assessment is necessary. The assessment usually consists of an 
11/17/2011 AC 23.1309-1E 

30 

appropriate combination of qualitative and quantitative analyses. Except as specified in the next 

paragraphs below, a detailed safety analysis must be completed for each hazardous and 

catastrophic failure condition identified by an FHA. The analysis will usually be a combination 

of qualitative and quantitative assessments of the design. 

(1) For simple and conventional installations (that is, low complexity and similarity in 

relevant attributes), it may be possible to assess a hazardous or catastrophic failure condition as 

being extremely remote or extremely improbable, respectively, on the basis of experienced 

engineering judgment using only qualitative analysis. The basis for the assessment will be the 

degree of redundancy, the established independence and isolation of the channels, and the 

reliability record of the technology involved. Satisfactory service experience on similar systems 

commonly used in many airplanes may be sufficient when a close similarity is established 

regarding both the system design and operating conditions. 

(2) For complex systems where true similarity in all relevant attributes, including 

installation attributes, can be rigorously established, it may also be possible to assess a hazardous 

or catastrophic failure condition as being extremely remote or extremely improbable, 

respectively, on the basis of experienced engineering judgment using only qualitative analysis. 

A high degree of similarity in both design and application is required. 

(3) No catastrophic failure condition (Note 3 in figure 2) should result from the failure of 

a single component, part, or element of a system. Experienced engineering judgment and service 

history should show that a catastrophic failure condition by a single failure mode is not a 

practical possibility. The logic and rationale used in the assessment should be so straightforward and obvious that the failure mode simply would not occur unless it is associated with an 

unrelated failure condition that would, in itself, be catastrophic. 

18. Assessment methods. 

a. Assessment methods. Methods for qualitatively and quantitatively assessing the causes, 

severity, and likelihood of potential failure conditions are available to support experienced 

engineering and operational judgment. Some of these methods are structured. The various types 

of analyses are based on either inductive or deductive approaches. The applicant should select 

analyses to validate the safety of a particular design based on factors such as the system 

architecture, complexity, criticality of the function, etc. ARP 4761 has more details of the 

various methods. Descriptions of typical types of analyses that might be used are provided 

below. 

(1) Design appraisal. A qualitative appraisal of the integrity and safety of the system 

design. An effective appraisal requires experienced judgment. 

(2) Installation appraisal. This is a qualitative appraisal of the integrity and safety of 

the installation. Any deviations from normal, industry-accepted installation practices should be 

evaluated. An effective appraisal requires experienced judgment. 

(3) Failure Modes and Effects Analysis (FMEA). A structured, inductive, and bottomup analysis that is used to evaluate the effects on the system and the airplane of each possible 
11/17/2011 AC 23.1309-1E 

31 

element or component failure. When properly formatted, it should aid in identifying latent 

failures and the possible causes of each failure mode. ARP 4761 provides methodology and 

detailed guidelines that may be used to perform this type of analysis. An FMEA could be a 

piece-part FMEA or a functional FMEA. For modern microcircuit-based line replaceable units 

and systems, an exhaustive piece-part FMEA is not practically feasible with the present state of 

the art. In that context, an FMEA may be more functional than piece-part oriented. A 

functional-oriented FMEA can lead to uncertainties in the qualitative and quantitative aspects, 

which can be compensated for by more conservative assessments, such as the following: 

Assuming all failure modes result in failure conditions of interest, carefully choosing system 

architecture, and using lessons learned from similar technology. 

(4) Fault Tree Analysis (FTA). A structured, deductive, and top-down analysis that is 

used to identify the conditions, failures, and events that would cause each defined failure 

condition. FTAs are graphical methods of identifying the logical relationship between each 

particular failure condition and the primary element or component failures, other events, or 

combinations thereof that can cause it. The fault tree should be developed to the lowest level for 

which failure rates can be substantiated. Rates derived from applicable service experience, 

acceptable industry wide sources, manufacturer’s accelerating testing data, or from an FMEA 

may be used as inputs to the lowest level events. 

(5) Common cause analysis. The acceptance of adequate probability of failure 

conditions is often derived from the assessment of multiple systems based on the assumption that 

failures are independent. Therefore, it is necessary to recognize that such independence may not 

exist in the practical sense, and specific studies are necessary to ensure that independence can 

either be assured or deemed acceptable. The “common cause analysis” is divided into three 

areas of study: 

(a) Zonal safety analysis. This analysis has the objective of ensuring that the 

equipment installations within each zone of the airplane are at an adequate safety standard 

regarding design and installation standards, interference between systems, and maintenance 

errors. 

(b) Particular risk analysis. Particular risks are defined as those events or 

influences outside the systems concerned (e.g., fire, leaking fluids, bird strike, tire burst, HIRF 

exposure, lightning, uncontained failure of high energy rotating machines, etc.). Each risk 

should be the subject of a specific study to examine and document the simultaneous or cascading 

effects, or influences, which may violate independence. 

(c) Common mode analysis. This analysis is performed to confirm the assumed 

independence of the events that were considered in combination for a given failure condition. 

The effects of specification, design, implementation, installation, maintenance errors, 

manufacturing errors, environmental factors other than those already considered in the particular 

risk analysis, and failures of system components should be considered. 
11/17/2011 AC 23.1309-1E 

32 

19. Assessment of failure condition probabilities and analysis considerations. 

a. An assessment of the probability of a failure condition may be either qualitative or 

quantitative. An analysis may range from a simple report that interprets test results or compares 

two similar systems to a detailed analysis that may or may not include estimated numerical 

probabilities. The depth and scope of an analysis depends on the type of functions performed by 

the system, the severity of failure conditions, and whether the system is complex. A quantitative 

analysis is intended to supplement, but not replace, qualitative methods based on engineering 

and operational judgment. A quantitative analysis is often used for catastrophic or hazardous 

failure conditions of systems that are complex and major failure conditions that are complex 

without redundancy. For the cases were there is insufficient service experience to help 

substantiate their safety, or that have attributes that differ significantly from those of 

conventional systems. 

b. A probability analysis may be either an FMEA or an FTA, which also includes numerical 

probability information. Numerical values are assigned to the probabilistic terms included in the 

requirements for use in those cases where the impact of system failures is examined by 

quantitative analysis methods. 

c. The probabilities of primary failures can be determined from failure rate data and 

exposure times using failure rates derived from either service experience on identical or similar 

items, manufacturer’s accelerating testing data, or from acceptable industry standards. 

Conventional mathematics of probability can then be used to calculate the estimated probability 

of each failure condition as a function of the estimated probabilities of the various identified 

contributory failures or other events. See Appendix D of ARP 4761 for more information. 

d. When calculating the estimated probability of each failure condition, a margin may be 

necessary to account for uncertainty. A margin is not normally required for an analysis that is 

based on proven data or from operational experience and tests. Where data has limited 

background for substantiation, a margin may be required depending on the available 

justification. 

e. The applicant should obtain early certification authority concurrence for an acceptable 

classification of the failure conditions and probability for each minor, major, hazardous, and 

catastrophic failure condition. Early concurrence on the classification of the failure conditions 

may reduce the applicant’s efforts in determining the probabilities resulting from changes. 

f. The details on how to calculate the “average probability per flight hour” for a failure 

condition are given in Appendix 3. The “average probability per flight hour” is the probability of 

occurrence, normalized by the flight time of a failure condition during a single flight. If the 

probability of a subject failure condition occurring during a typical flight of mean duration for 

the airplane type, divided by the flight’s mean duration in hours, is likely to be significantly 

different from the predicted average rate of occurrence of that failure condition during the entire 

operational life of all airplanes of that type, then a risk model that better reflects the failure 

condition should be used. The single flight is analyzed to be representative of an average over 
11/17/2011 AC 23.1309-1E 

33 

all possible flights of the fleet of airplanes to be certified. The calculation of the “average 

probability per flight hour” for a failure condition should consider the following: 

(1) The average flight duration and the average flight profile for the airplane type to be 

certified. A common assumption for part 23 airplanes is that the average flight duration is 1 

hour; 

(2) All combinations of failures and events that contribute to the failure condition; 

(3) The conditional probability if a sequence of events is necessary to produce the failure 

condition; 

(4) The relevant "at risk" time if an event is only relevant during certain flight phases; 

and 

(5) The average exposure time if the failure can persist for multiple flights. 

20. Operational and maintenance considerations. 

a. Alerts 

(1) Section 23.1309(d) requires information concerning unsafe system operating 

condition(s) must be provided in a timely manner to the crew to enable them to take appropriate 

corrective action. An appropriate alert must be provided if immediate pilot awareness and 

immediate or subsequent corrective action is required. The particular method of indication 

depends on the urgency and need for flight crew awareness or action necessary for the particular 

failure. Inherent airplane characteristics may be used in lieu of dedicated indications and 

annunciations if they can be shown to be timely and effective. However, the use of periodic 

maintenance or flight crew checks to detect significant latent failures when they occur should not 

be used in lieu of practical and reliable failure monitoring and indications. 

(2) Section 23.1309(d) specifies that the design of systems and controls, including 

indications and annunciations, must be design to minimize crew errors, which could create 

additional hazards. The additional hazards to be minimized include those caused by 

inappropriate actions by a crewmember in response to the failure, or those that could occur after 

a failure. 

b. Flight crew and maintenance task. These tasks, which relate to compliance, should be 

appropriate and reasonable. Quantitative assessments of the probabilities of flight crew and 

maintenance errors are not considered feasible. Reasonable tasks are those for which full credit 

can be taken because the flight crew or ground crew can realistically be anticipated to perform 

them correctly when they are required or scheduled. For the purposes of quantitative analysis, a 

probability of one can be assumed for flight crew and maintenance tasks that have been 

evaluated and found to be reasonable. In addition, based on experienced engineering and 

operational judgment, the discovery of obvious failures during normal operation and 
11/17/2011 AC 23.1309-1E 

34 

maintenance of the airplane may be considered, even though such failures are not the primary 

purpose or focus of the operational or maintenance actions. 

c. Flight crew action. When assessing the ability of the flight crew to cope with a failure 

condition, the information provided to the crew and the complexity of the required action should 

be considered. 

(1) If the evaluation indicates that a potential failure condition can be alleviated or 

overcome in a timely manner without jeopardizing other safety related flight crew tasks and 

without requiring exceptional pilot skill or strength, correct crew action may be assumed in both 

qualitative and quantitative assessments. 

(2) Annunciation that requires flight crew actions should be evaluated to determine if the 

required actions can be accomplished in a timely manner without exceptional pilot skills. If the 

evaluation indicates that a potential failure condition can be alleviated or overcome during the 

time available without jeopardizing other safety related flight crew tasks and without requiring 

exceptional pilot skill or strength, credit may be taken for correct and appropriate corrective 

action for both qualitative and quantitative assessments. Similarly, credit may be taken for 

correct flight crew performance if overall flight crew workload during the time available is not 

excessive and if the tasks do not require exceptional pilot skill or strength. 

(3) Unless flight crew actions are accepted as normal airmanship, the appropriate 

procedures should be included in the FAA approved AFM or in the AFM revision or 

supplement. The AFM should include procedures for operation of complex systems such as 

integrated flight guidance and control systems. These procedures should include proper pilot 

response to cockpit indications, diagnosis of system failures, discussion of possible pilot-induced 

flight control system problems, and use of the system in a safe manner. 

d. Maintenance actions. Credit may be taken for correct accomplishment of maintenance 

tasks in both qualitative and quantitative assessments if the tasks are evaluated and found to be 

reasonable. Required maintenance tasks, which mitigate hazards, should be provided for use in 

the FAA approved maintenance programs such as the ICA. Annunciated failures will be 

corrected before the next flight or a maximum duration will be established before a maintenance 

action is required. If the latter is acceptable, the analysis should establish the maximum 

allowable interval before the maintenance action is required. A scheduled maintenance task may 

detect latent failures. If this approach is taken, and the failure condition is hazardous or 

catastrophic, then a maintenance task should be established. Some latent failures can be 

assumed to be identified based upon a return to service test on the equipment following its 

removal and repair (component MTBF should be the basis for the check interval time). 

21. Software and complex hardware DALs for airborne system and applications. 

a. Background. AC 20-115B discusses how RTCA/DO-178B provides an acceptable 

means for showing that software complies with pertinent airworthiness requirements. AC 20-

152 and Order 8110.105 provide acceptable means for showing that complex hardware complies 

with the pertinent airworthiness requirements. 
11/17/2011 AC 23.1309-1E 

35 

b. Acceptable application of software and complex hardware DALs. It is necessary to 

consider the possibility of requirement, design, and implementation errors in order to comply 

with the requirements of § 23.1309. Errors made during the design and development of systems 

have traditionally been detected and corrected by exhaustive tests conducted on the system and 

its components. These tests used direct inspection and other direct verification methods capable 

of completely characterizing system the performance. These direct techniques may still be 

appropriate for simple systems, which perform a limited number of functions and which are not 

highly integrated with other airplane systems. 

(1) For more complex or integrated systems, exhaustive testing may either be impossible 

because all of the systems states cannot be determined or it may be impractical due to the 

number of tests that must be accomplished. For these types of systems, compliance may be 

shown by the use of software and complex hardware DALs. The software and complex 

hardware DALs should be determined by the severity of potential effects on the airplane in case 

of system malfunctions or loss of functions. 

c. Software and complex hardware DALs criteria for part 23 airplanes. The DALs in 

figure 2 and throughout this AC correlate to the software level in RTCA/DO-178B and the 

complex design assurance level in RTCA/DO-254 documents. The classification of the failure 

condition and airplane class must be determined before figure 2 is used to determine these levels. 

These levels in figure 2 are acceptable for part 23 airplanes instead of software levels in 

paragraph 2.2.2 in RTCA/DO-178B and of the complex hardware design assurance levels 

defined in paragraph 2.2 in RTCA/DO-254. 

d. Complex hardware level D. AC 20-152 provides an exclusion from FAA review for 

complex hardware design assurance level “D” developed under DO-254. The exclusion from 

FAA review of life cycle data only applies for minor failure conditions and will not apply to 

level D for the reduced levels shown for major and hazardous failure conditions identified in 

figure 2. 

e. System architecture for determination of the appropriate DALs. 

(1) SAE S-18, Airplane Safety Assessment Committee, revised ARP 4754 to ARP 

4754A and is revising ARP 4761. The committee developed new concepts for DAL and Design 

Assurance Levels in ARP 4754A. These SAE documents are guidelines for assigning the DALs 

that start from the aircraft/system level and end at the item/component level. ARP 4754A 

addresses the development phase in two phases: Function Development Phase and Item 

Development Phase, with two different types of development processes. 

(2) These assignments depend on the failure condition classification, the number of 

independent failure paths, and their associated independence attributes. The independence 

attributes are the functional independence, design independence, and physical independence. In 

essence, functional independence ensures that the functional requirements that are implemented 

in the design are different, whereas design independence ensures that the hardware or software 

design, in which the functions are implemented, is different. 
11/17/2011 AC 23.1309-1E 

36 

(3) There may be significant difference in the guidance provided on the use of system 

architecture for appropriate determination of the DALs between this AC and guidelines 

contained in ARP 4754A. Where apparent differences exist between this AC and ARP 4754A, 

this AC takes precedence. The FAA recognizes that consideration of system architecture for 

determining DALs is appropriate and may lead to lower levels in some specific cases. Figure 2 

in this AC already allows reduction of software and complex hardware DALs for Class I, II, and 

III airplanes; therefore, no additional reductions from these levels are permitted without the 

Small Airplane Directorate approval or there is established specific policy or guidance. If the 

Small Airplane Directorate has established specific guidance or policy for these levels, then the 

approval can made by the ACO. For commuter category airplanes, the guidance in ARP 4754A 

is more likely to be appropriate since its DALs are higher. There may be acceptability only if 

additional credit for architecture is requested for hazardous or catastrophic failure conditions in 

Class IV, commuter category airplanes. 

f. Equipment installed in part 23 airplanes that performs functions addressed by TSO 

standards. Equipment installed in part 23 airplanes that performs functions addressed by TSO 

standards should meet applicable TSO standards. FAA prefers the equipment meet design 

approval of the functional TSO. However, the equipment is not required to have TSO 

authorization, but it would need to meet other equivalent minimum performance standards 

acceptable to the Administrator. The TSO data should include the equipment complex hardware 

and software DALs. For both TSO and non-TSO equipment, the safety assessment and figure 2 

should be used to check the complex hardware and software DALs against the installation 

requirements. 

22. Electromagnetic protection for electrical/electronic systems. 

Current trends indicate increasing reliance on electrical/electronic systems for safe operations. 

Electromagnetic effects, environmental effects, and environmental qualifications should be 

considered for systems that perform flight, propulsion, navigation, and instrumentation 

functions. The software and complex hardware DALs shown in figure 2 are not applicable for 

HIRF and lightning protection levels. For guidance for the protection against these effects, refer 

to the latest version of AC 21-16F, AC 23-17C, AC 20-136A, and AC 20-158.