Appendix A A-57
The following are examples of NOT SIGNIFICANT changes for Propellers (Parts 35):
Description of change Is there a change to the general configuration?
14 CFR §
21.101(b)(1)(i) Is there a change to the principles of construction?
14 CFR §
21.101(b)(1)(i) Have the assumptions used for certification been invalidated?
14 CFR §
21.101(b)(1)(ii) Notes Change in the material of a blade bearing No No No Change to a component in the control system No No No Change to a propeller de-icer boot No No No Changes to the operational design envelope such as an increase in power No No No Propeller's operating characteristics and inherent strength require re-evaluation.
Change to the intended usage such as normal to acrobatic category No No No Propeller's operating characteristics and inherent strength require re-evaluation.
Appendix B B-1 Appendix B. Procedure for Evaluating Impracticality of Applying Latest Requirements to a Changed Product
1. Introduction.
a. The basic principal of enhanc ing the level of safety of changed aeronautical products is to apply the latest regulations for significant design changes, to the greatest extent practical. In certain cases, the cost of complying fully with a later regulation may not be commensurate with the small safety benefit achieved. It is recognized that the existing fleet and newly produced airplanes, engines and propellers are safe, and any unsafe condition is immediately addressed through the airworthiness directive process. These factors form the basis where compliance with the latest standard may be considered impractical, thereby allowing compliance with an earlier regulation. This appendix gives one method of determining if compliance with a later regulation is impractical, however, this does not preclude the use of other methods for improving the safety of aeronautical products.
b. This AC recognizes that other procedures can be used and have historically been accepted on a case-by-case basis. The acceptance of results through the use of these procedures may vary from state to state. Consequently, they may not be accepted through all bilateral certification processes. Regardless of which method is used, the process must show that a proposed certification basis is able to achieve a positive safety benefit for the overall product.
c. In this regard, any method used must encourage incorporating safety enhancements that will have the most dramatic impact on the level of safety of the aircraft while considering effective use of resources. This important point is illustrated graphically in the accompanying figure. This figure notionally shows the interrelation between the total resources required for incorporating each potential safety enhancement with the corresponding net increase in safety benefit.
Figure 2. Safety Benefits vs. Resources
Safety benefit of the regulatory requirement Resources to implement the regulatory requirement Potential safety enhancements
Appendix B B-2
d. Typically, one will find that th ere are proposals that can achieve a positive safety benefit that are resource effective. Conversely, there are proposals that may achieve a small safety benefit at the expense of a large amount of resources to implement. Clearly, there will be a point where a large percentage of the potential safety benefit can be achieved with a reasonable expenditure of resources. The focus of the methods used should be to determine the most appropriate regulatory standards relative to the respective cost to reach this point.
e. This appendix provides procedural guidance for determining the practicality of applying a requirement at a particular amendment level to a changed product. This guidance can be used to evaluate the safety benefit and resource impact of implementing the latest airworthiness requirements in the certification basis of a changed product. The procedure is generic in nature and describes the steps and necessary inputs that any applicant can use on any project to develop a position.
f. The procedure is intended to be used , along with good engineering judgment, to evaluate the relative merits of a changed product complying with the latest regulations. It provides a means, but not the only means, for an applicant to present its position in regard to impracticality.
g. The certification basis for a change to a product will not be at an amendment level earlier than the existing certification basis or any requirement found in 14 CFR §§ 23.2, 25.2, 27.2,
29.2 or part 26 that is related to the change. Therefore, when determining the impracticality of applying a requirement at the latest amendment level, only the increase in safety benefits and costs beyond compliance with the existing certification basis should be considered.
2. Procedure for Evaluating Impracticality of Applying Latest Requirements to a Changed Product. The following are steps to determine the impracticality of applying a requirement at a particular amendment level.
a. Step 1: Identify the Regulatory Change Being Evaluated. In this step, document:
(1) The specific requirement (for example, 14 CFR § 25.365);
(2) The amendment level of the existing certification basis for the requirement; and
(3) The latest amendment level of the requirement.
b. Step 2: Identify the Specific Ha zard that the Requirement Addresses.
(1) Each requirement and subsequent amendments are intended to address a hazard or hazards. In this step, the specific hazard(s) is/are identified. This identification will allow for a comparison of the effectiveness of amendment levels of the regulation at addressing the hazard.
(2) In many cases, the hazard and the cause of the hazard will be obvious. When the hazard and its related cause are not immediately obvious, it may be necessary to review the
Appendix B B-3 preamble of the regulation. It may also be helpful to discuss the hazard with the responsible FAA office.
c. Step 3: Review the Consequences of the Hazard(s).
(1) Once the hazard has been identified, it is possible to identify the types of consequences that may occur because of the presence of the hazard. More than one consequence can be attributed for the same hazard. Typical examples of consequences would include, but are not be limited to:
Incidents where only injuries occurred;
Accidents where less than 10 percent of the passengers died;
Accidents where 10 percent or more passengers died; and
Accidents where a total hull loss occurred.
(2) The preamble to the regulation may pr ovide useful information regarding the consequences of the hazard the requirement is intended to address.
d. Step 4: Identify the Historical and Predicted Frequency of Each Consequence.
(1) Another source for determining impracti cality is the historical record of the consequences of the hazard that led to a requirement or an amendment to a requirement. From these data, a frequency of occurrence for the hazard can be determined. It is important to recognize that the frequency of occurrence may be higher or lower in the future. Therefore, it also is necessary to predict the frequency of future occurrences.
(2) More than one consequence can be attr ibuted for the same hazard. Therefore, when applicable, the combination of consequences and frequencies of those consequences should be considered together.
(3) The preamble of the regulation may provide useful information regarding the frequency of occurrence.
e. Step 5: Determine How Effective Full Compliance with the Latest Amendment of the Requirement Would Be at Addressing the Hazard.
(1) When each amendment is promulgated, it is usually expected that compliance with the requirement would be completely effective at addressing the associated hazard. It is expected that the hazard would be eliminated, avoided, or mitigated. However, in a limited number of situations, this may not be the case. It is also possible that earlier amendment levels may have addressed the hazard, but were not completely effective. Therefore, in comparing the benefits of compliance with the existing certification basis to the latest amendment level, it is useful to estimate the effectiveness of both amendment levels in dealing with the hazard.
Appendix B B-4
(2) It is recognized that the determination of levels of effectiveness is normally of a subjective nature. These are relative assessments of a qualitative nature that should not be treated as absolute determinations. Therefore, exercise good judgment when making these determinations. In all cases, it is necessary to document the assumptions and data that support the determination.
(3) The following five levels of effec tiveness are provided as a guideline:
(a) Fully effective in all cases. Compliance with the requirement eliminates the hazard or provides a means to avoid the hazard completely.
(b) Considerable potential for elimina ting or avoiding the hazard. Compliance with the requirement eliminates the hazard or provides a means to completely avoid the hazard for all probable or likely cases, but it does not cover all situations or scenarios.
(c) Adequately deals with the hazar d. Compliance with the requirement eliminates the hazard or provides a means to avoid the hazard completely in many cases.
However, the hazard is not eliminated or avoided in all probable or likely cases. Usually this action only addresses a significant part of a larger or broader hazard.
(d) Hazard only partly addressed. In some cases compliance with the requirement partly eliminates the hazard or does not completely avoid the hazard. The hazard is not eliminated or avoided in all probable or likely cases. Usually this action only addresses part of a hazard.
(e) Hazard only partly addressed, but action has negative side effect.
Compliance with the requirement does not eliminate or avoid the hazard or may have negative safety side effects. The action is of questionable benefit.
f. Step 6: Determine Resource Costs and Cost Avoidance.
(1) There is always cost associated with complying with a requirement. This cost may range from minimal administrative efforts to the resource expenditures that support full scale testing or the redesign of a large portion of an aircraft. However, there are also potential cost savings from compliance with a requirement. For example, compliance with a requirement may avoid aircraft damage or accidents and the associated costs to the manufacturer for investigating accidents. Compliance with the latest amendment of a requirement may also help a foreign authority certificate a product.
(2) When determining the impracticality of applying a requirement at the latest amendment level, only the incremental costs and safety benefits from complying with the existing certification basis should be considered.
(3) When evaluating the incremental cost, it may be beneficial for the applicant to compare the increase in cost to comply with the latest requirements to the cost to incorporate the
Appendix B B-5 same design feature in a new airplane. In many cases an estimate for the cost of incorporation in a new airplane is provided in the regulatory evaluation by the FAA, which was presented when the corresponding regulation was first promulgated. Incremental costs of retrofit/incorporation on existing designs may be higher than that for production. Examples of costs may include, but are not limited to:
(a) Costs: The accuracies of fleet si ze projections, utilization, etc. may be different than that experienced for derivative product designs and must be validated.
Labor: Work carried out in the design, fabrication, inspection, operation or maintenance of a product for the purpose of incorporating or demonstrating compliance with a proposed action. Non-recurring labor requirements, including training, should be considered.
Capital: Construction of new, modified or temporary facilities for design, production, tooling, training, or maintenance.
Material: Cost associated with product materials, product components, inventory, kits, and spares.
Operating Costs: Costs associated with fuel, oil, fees, and expendables.
Revenue/Utility Loss: Costs resulting from earning/usage capability reductions from departure delays, product downtime, capability reductions of performance loss due to seats, cargo, range, or airport restrictions.
(b) Cost Avoidance:
Avoiding cost of accidents, including investigation of accidents, lawsuits, public relations activities, insurance, and lost revenue.
Foreign Certification: Achieve a singular effort that would demonstrate compliance to the requirements of most certifying agencies, thus minimizing certification costs.
g. Step 7: Document Conclusion. Once the information from previous steps has been documented and reviewed, the applicant’s position and rationale regarding practicality can be documented. Examples of possible positions would include but are not limited to:
(1) Compliance with the latest requirement is necessary. The applicant would pursue the change at the latest amendment level.
(2) Compliance with an amendment level be tween the existing certification basis and the latest amendment would adequately address the hazard at an acceptable cost, while meeting
Appendix B B-6 the latest amendment level would be impractical. The applicant would then propose the intermediate amendment level of the requirement.
(3) The increased level of safety is not commensurate with the increased costs associated with meeting the latest amendment instead of the existing certification basis.
Therefore, the applicant would propose the existing certification basis.
(4) The results of this analysis were inc onclusive. Further discussions with the FAA are warranted.
Note: This process may result in a required certification basis that renders the proposed modification economically not viable.
3. Examples of How to Certify Changed Aircraft. The following examples are for transport airplanes and illustrate the typical process an applicant follows. The process will be the same for all product types.
a. Example 1: 14 CFR § 25.963, Fuel Tank Access Covers.
(1) This change is part of a significant transport airplane change that increases passenger payload and gross weight by extending the fuselage 20 feet. To accommodate the higher design weights and increased braking requirements, and to reduce runway loading, the applicant will change the landing gear from a two-wheel to four-wheel configuration; this changes the debris scatter on the wing from the landing gear. The new model airplane will be required to comply with the latest applicable regulations based on the date of application.
(2) The wing will be strengthened locally at the side of the body and at the attachment of engines and landing gear, but the applicant would not like to alter wing access panels and the fuel tank access covers. Although the applicant recognizes that the scatter pattern and impact loading on the wing from debris being thrown from the landing gear will change, he proposes that it would be impractical to redesign the fuel tank access covers.
(3) Step 1: Identify the Regulatory Change Being Evaluated.
(a) The existing certification basis of the ai rplane that is being changed is part 25 prior to amendment 25-69 (25-40).
(b) Amendment 25-69 added the requirement that fuel tank access covers on transport category airplanes be designed to minimize penetration by likely foreign objects, and be fire resistant.
(4) Step 2: Identify the Specific Hazard th at the Regulation Addresses. Fuel tank access covers have failed in service due to impact with high-energy objects such as failed tire tread material and engine debris following engine failures. In one accident, debris from the runway impacted a fuel tank access cover, causing its failure and subsequent fire, which resulted
Appendix B B-7 in fatalities and loss of the airplane. Amendment 25-69 will ensure that all access covers on all fuel tanks are designed or located to minimize penetration by likely foreign objects, and are fire resistant.
(5) Step 3: Review the History of the Consequences of the Hazard(s). Occurrences with injuries and with more than 10 percent deaths.
(6) Step 4: Identify the Historical and Predicted Frequency of Each Consequence.
(a) In 200 million departures of large jets:
One occurrence with more than 10 percent deaths; and
One occurrence with injuries.
(b) There is no reason to believe that the future rate of accidents will be significantly different than the historical record.
(7) Step 5: Determine How Effective Full Compliance with the Latest Amendment of the Regulation Would Be at Addressing the Hazard.
(a) Considerable potential for eliminating or avoiding the hazard.
(b) Compliance with amendment 25-69 eliminates the hazard or provides a means to avoid the hazard completely for all probable or likely cases. However, it does not cover all situations or scenarios.
(8) Step 6: Determine Resource Costs and Cost Avoidance.
(a) Costs:
For a newly developed airplane, there would be minor increases in labor resulting from design and fabrication.
There would be a negligible increase in costs related to materials, operating costs, and revenue utility loss.
(b) Cost Avoidance:
There were two accidents in 200 million departures. The applicant believes that it will manufacture more than 2,000 of these airplanes or derivatives of these airplanes. These airplanes would average five flights a day.
Therefore, statistically there will be accidents in the future if the hazard is not alleviated. Compliance will provide cost benefits related to avoiding lawsuits, accident investigations, and public relation costs.
Appendix B B-8 There are cost savings associated with meeting a single certification basis for FAA and foreign regulations.
(9) Conclusion. It is concluded that compliance with the latest regulation increases the level of safety at a minimal cost to the applicant. Based on the arguments and information presented by the applicant through the issue paper process, the FAA determined that meeting the latest amendment would be practical.
b. Example 2: 14 CFR § 25.365 P ressurized Compartment Loads.
(1) This example is a passenger to freighter conversion STC.
(2) This change affects the floor loads on the airplane as well as the decompression venting.
(3) Step 1: Identify the Regulatory Change Being Evaluated.
(a) The existing certification basis of the airplane that is being changed includes 14 CFR § 25.365 at amendment 25-00. The initial release of 14 CFR § 25.365 required that the interior structure of passenger compartments be designed to withstand the effects of a sudden release of pressure through an opening resulting from the failure or penetration of an external door, window, or windshield panel, or from structural fatigue or penetration of the fuselage, unless shown to be extremely remote.
(b) Amendment 25-54 revised 14 CFR § 25.365 to require that the interior structure be designed for an opening resulting from penetration by a portion of an engine, an opening in any compartment of a size defined by 14 CFR § 25.365(e)(2), or the maximum opening caused by a failure not shown to be extremely improbable. The most significant change is the “formula hole size” requirement introduced into 14 CFR § 25.365(e)(2) at amendment 25-
54.
(c) Amendment 25-71/72 (amendments 25- 71 and 25-72 are identical) extended the regulation to all pressurized compartments, not just passenger compartments, and to the pressurization of unpressurized areas. Pressurization of unpressurized areas had previously been identified as an unsafe feature under 14 CFR § 21.21(b)(2).
(d) Amendment 25-87 redefined the pressure differential load factor that applies above an altitude of 45,000 feet. Compliance with amendment 25-87 is not affected since the airplane does not operate above an altitude of 45,000 feet. The applicant proposes to meet the “pressurization into unpressurized areas” requirement introduced in amendment 25-71/72. The applicant does not propose to comply with the formula hole size requirement introduced in 14 CFR § 25.365(e)(2) at amendment 25-54.
(4) Step 2: Identify the Specific Hazard that the Regulation Addresses. The hazard is a catastrophic structure and/or system failure produced by a sudden release of pressure through an opening in any compartment in flight. This opening could be caused by an uncontained engine
Appendix B B-9 failure, an opening of a prescribed size due to the inadvertent opening of an external door in flight, or an opening caused by a failure not shown to be extremely improbable. The opening could be produced by an event that has yet to be identified.
(5) Step 3: Review the History of the Consequences of the Hazard(s). Occurrences with injuries, less than 10 percent deaths, and more than 10 percent deaths.
(6) Step 4: Identify the Historical and Predicted Frequency of Each Consequence.
(a) In 200 million departures of large jets:
Two occurrences with more than 10 percent deaths;
One occurrence with less than 10 percent deaths; and One occurrence with injuries.
(b) There is no reason to believe that the future rate of accidents will be significantly different than the historical record.
(7) Step 5: Determine How Effective Full Compliance with the Latest Amendment of the Regulation Would Be at Addressing the Hazard.
(a) Compliance with the latest amendmen t eliminates the hazard or provides a means to avoid the hazard completely.
(b) Design changes made to the proposed de rivative airplane bring it closer to full compliance with 14 CFR § 25.365 at amendment 25-54. The original airplane was shown to meet the requirements for a hole size of 1.1 square feet. Amendment 25-54 would require a hole size of 5.74 square feet, and the current reinforcements for the converted airplane can sustain a hole size of 3.65 square feet in the forward area and 2.65 at the aft area. This is 3.1 and 2.4 times, respectively, better than the original design condition of amendment 25-0 and is a significant improvement over the world wide passenger fleet in service.
(8) Step 6: Determine Resource Costs and Cost Avoidance.
(a) Costs: There would be savings in both labor and capital costs if compliance were shown to amendment 25-0 instead of amendment 25-54. Major modifications to the floor beams would be necessary to meet the formula hole size requirement in amendment 25-54.
(b) Cost Avoidance:
There were four accidents in 200 million departures. The applicant believes that it will manufacture more than 2,000 of these airplanes or derivatives of these airplanes. These airplanes would average two flights a day. Therefore, statistically there will be accidents in the future if the
Appendix B B-10 hazard is not alleviated. Compliance will provide cost benefits related to avoiding lawsuits, accident investigations, and public relation costs.
There are cost savings associated with meeting a single certification basis for FAA and foreign regulations.
(9) Step 7: Document Conclusion Regard ing Practicality. The design complies with 14 CFR § 25.365 at amendments 25-0, 25-71/72, and 25-87, and is nearly in full compliance with amendment 25-54 (and certain aspects of amendments 25-71/72 and 25-87). The design would adequately address the hazard at an acceptable cost. Therefore, based on arguments of impracticality discussed in an issue paper, the FAA accepts the applicant’s proposal to comply with 14 CFR § 25.365 at amendment 25-0.
09/03/2010 AC21.101A Appendix C C-1
Appendix C. The Use of Service Experience in the Certification Process
1. Introduction. Service experience may support the application of an earlier regulatory standard if, in conjunction with the applicable service experience and other compliance measures, the earlier standard provides a level of safety comparable to that provided by the latest requirements. The applicant must provide sufficient substantiation to allow the FAA to make this determination. A statistical approach may be used, subject to the availability and relevance of data, but sound engineering judgment must be used. For service history to be acceptable, the data must be both sufficient and pertinent. The essentials of the process involve:
a. A clear understanding of the requirement ch ange and the purpose for the change and hazard addressed;
b. A determination based on detailed k nowledge of the proposed design feature;
c. The availability of pertinent and su fficient service experience data; and
d. A comprehensive review of that service experience data.
2. Guidelines. The issue paper process (either a standalone issue paper or included in the G-1 issue paper) would be used, and the applicant should provide documentation to support the following:
a. The identification of the differences between the requirement in the existing certification basis and the requirement as amended, and the effect of the change in the requirement.
b. A description as to what aspect(s) of the latest requirements the proposed changed product would not meet.
c. Evidence showing that the proposed certi fication basis for the changed product, together with applicable service experience, relative to the hazard, provides a level of safety consistent with complying with the latest requirements.
d. A description of the design feat ure and its intended function.
e. Data for the product pert inent to the requirement.
(1) Service experience from such data sources as the following:
(a) Accident reports;
(b) Incident reports;