Page 71 - UK Regulation Part 21 Initial Airworthiness Annex I (consolidated) March 2022

PART 21 - INITIAL AIRWORTHINESS (ANNEX I)

with the ignition prevention requirements of Amendment 25-102, together with
the fuel tank ignition prevention airworthiness directives issued as a result of
the Special Federal Aviation Regulation number 88 reviews, resulted in the
range of effectiveness in preventing fuel tank explosions between 25 to 75 per
cent with a median value of 50 per cent (73 FR 42449).
E.3.3.6 Step 6: Determine resource costs and cost avoidance. Costs.
- For newly developed designs, there would be minor increases in costs
resulting from the identification and implementation of fuel tank system
airworthiness limitations.
- There would be no increase in costs related to materials, operating
costs, and revenue utility loss. Cost avoidance. There were 18
accidents in 200 million departures. The applicant believes that it will
manufacture more than 2 000 of these aeroplanes or derivatives of
these aeroplanes. These aeroplanes would average 5 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 fatalities and injuries.
E.3.3.7 Step 7: Document the conclusion. It is concluded that compliance with the
latest certification specification 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. The following is additional
background on the specific hazard that the certification specification
addresses: As stated in the 2001 FTS rule under ‘Changes to Part 25’, §
25.981(a)(3) was adopted because the previous regulations (§§ 25.901 and
25.1309) were not always properly applied. Section 25.901(b)(2),
Amendments 25-40 through 46, requires in part preventative maintenance as
necessary to ensure that components of the powerplant installation, which
includes the fuel tank system, will safely perform their intended function
between inspections and overhauls defined in the maintenance instructions.
When demonstrating compliance with the requirements of § 25.901(b) for
maintenance of fuel tank ignition prevention features, the policy has been that
the applicant identify critical features as critical maintenance requirements
using the guidance in AC 25-19A.

21.A.101 APPF to GM Appendix F to GM 21.A.101 The use of service experience in the exception process
F .1 Introduction.
Service experience may support the application of an earlier certification specification
pursuant to point 21.A.101(b)(3) if, in conjunction with the applicable service experience
and other compliance measures, the earlier certification specification provides a level of
safety comparable to that provided by the latest certification specification. The applicant
must provide sufficient substantiation to allow the CAA 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 clear understanding of the certification specification change and the purpose for
the change,
- A determination based on detailed knowledge of the proposed design feature,
- The availability of pertinent and sufficient service experience data, and
- A comprehensive review of that service experience data.
F .2 Guidelines.
The CRI process (either as a standalone CRI or included in the CRI A01) would be used,
and the applicant should provide documentation to support the following:
F.2.1 The identification of the differences between the certification specification in the
existing basis and the certification specification as amended, and the effect of the
change to the specification.
F.2.2 A description as to what aspect(s) of the latest certification specifications the
proposed changed product would not meet.
F.2.3 Evidence showing that the proposed certification basis for the changed product,
together with applicable service experience, relative to the hazard, provides a level
of safety that approaches the latest certification specification, yet is not fully
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