Sofema Aviation Services (SAS) www.sassofia.com considers the development of the Supplemental Structural Inspection SSI requirements
Introduction
On 14 May 1977, a Boeing 707 (Cargo Aircraft) operated by UK Operator Dan Air Services Ltd. crashed some 3 miles short of the runway at Lusaka International Airport.
The Boeing 707 was certified failsafe, however, the Model 707-300 flown by Dan Air did not undergo a fatigue test program.
- The aeroplane crashed due to an inflight separation of the right-hand horizontal stabilizer and elevator as a result of metal fatigue.
- A failsafe design is one that has one or more redundant structural elements which are capable of carrying the flight loads in the event of a failure of one of the primary load members.
- A safe-life design is one that has an established life limit based on fatigue testing.
- Boeing performed an extensive structural fatigue test on the Model 707-100 which was an earlier variant of the Model 707-300. It was thought that a new fatigue test wasn’t necessary for the Model 707-300 and the regulations didn’t require it for failsafe designs.
The Model 707-300 was considered a failsafe and therefore met the regulatory requirements
FAR – Part 25, Amendment 45 – The Dan Air crash showed that total reliance on the failsafe design approach may not meet the intended minimum level of safety & illustrated shortcomings in failsafe analysis assumptions and associated inspections.
Damage Tolerance Requirements was introduced into FAR Part 25 at Amendment 45 on 1 December 1978
Note: While the new rule was beneficial to the safety of new designs, there wasn’t any guidance regarding ageing aeroplanes certified prior to Part 25 Amendment 45.
Supplemental Structural Inspection Program
On 6 May 1981 AC 91-56 was published which created the expectation that manufacturers develop an SSIP (Supplemental Structural Inspection Program) which is contained in a SID (Supplemental Inspection Document) for their ageing aeroplanes.
The guidance of AC 91-56 was originally intended for large transport airplanes which were certified prior to Part 25 Amendment 45. FAA mandated SSIPs through ADs on 11 different transport category models. With the introduction of damage tolerance for new designs and SSIPs for ageing aeroplanes, it seemed the shortcomings of the past requirements had been fixed.
However, in 1988 the Aloha accident happened
The Boeing Model 737 was certified in 1967 which was prior to Part 25 Amendment 45. Just like the Dan Air Boeing 707, the 737 was certified failsafe.
- Boeing did have a published SSIP which was being used to inspect a fleet of high-time 737s which included the Aloha Airlines airplane.
- Boeing’s approach for the 737 SSIP was to reassess the airplanes using the new damage tolerance approach which was in the Part 25 Amendment 45 rule.
- During the program formulation, a structural classification system was created to determine which inspections should be included in the SSIP.
- One of the classifications that was excluded from the directed inspections was that of “damage obvious or malfunction evident”.
- Fuselage minimum gauge skin that fails by controlled decompression and flapping was considered “obvious”.
- All of the failsafe testing that Boeing had done up to this point showed that controlled decompression and flapping was the failure mode of the fuselage.
That failure mode turned out not to be representative of the Aloha failure since the failure was explosive and uncontrolled
On 29 April 1998, the FAA published a revision to AC 91-56.
- The guidance provided in this AC was still applicable to large transport aeroplanes certified under the failsafe and fatigue requirements of Part 25 prior to Amendment 45.
- An evaluation for Widespread Fatigue Damage (WFD) was added to this revision.
- Widespread fatigue damage is characterized by the simultaneous presence of cracks at multiple structural details that are of sufficient size and density such that the structure will no longer meet damage tolerance requirements and could catastrophically fail.
- Consider that uniformly loaded structures may develop cracks in adjacent fasteners or in adjacent similar structural details.
- These cracks can interact and reduce the damage tolerance of the structure in a manner that may not be readily detectable.
While AC 91-56A only applied to aeroplanes certified prior to Amendment 45, the requirement for a WFD evaluation was eventually written into Part 25 at Amendment 132
FAA started working on the Aging Aircraft Safety Rule (AASR)
- There were two NPRMs (1993 and 1998),
- The interim final rule (2002),
- and the final rule (2005).
Repair Assessment Guidelines Development
After the 1998 NPRM of the AASR, the FAA moved forward with a rulemaking proposal which would require operators of the 11 aeroplanes with FAA-mandated SSIPs (through previous ADs) to incorporate repair assessment guidelines for the fuselage pressure boundary into their FAA-approved maintenance programs.
- The final rule was published on 25 April 2000. In the final rule, the FAA sited the structural issues regarding the Aloha accident as the justification for rulemaking.
Aging Aircraft Safety Rule, 2005
- Changed the operational requirements for Parts 121 and 129 and created a December 20, 2010 deadline for implementing damage tolerance-based inspections and procedures.
- Included a requirement for addressing the adverse effects that repairs and alterations may have on fatigue critical structure and on required inspections. With the following Applicability:
o Type certificated passenger seating capacity of 30 or more;
o A maximum payload capacity of 7,500 pounds or more.
FAA Part 26 December 12, 2007
Mandated the Design Approval Holders (DAHs) to generate the data necessary for the operators to meet these new requirements.
Part 26, 2007 required DAHs to make available to operators the damage tolerance data for repairs and alterations to fatigue-critical aeroplane structures.
- It is still the responsibility of the operator to come up with a means to address the adverse effects of repairs and alterations; however, the DAH is required to develop the damage tolerance data which supports an operator’s means to comply.
- The FAA recommends in AC 120-93 that operators create an Operator Implementation Plan (OIP) which contains the means to comply, and data from the DAH is used to develop the OIP.
Operators must comply both for new repairs and alterations, as well as to review existing repairs throughout the fleet.
AC 91-56B – Guidance for showing compliance with the applicable regulations in order to satisfy the AASR.
- Damage tolerance SSIPs are required for all aeroplanes operated under subpart D of part 121 and 129.
o 30 passengers or more
o Payload of 7,500 pounds or more.
AC 120-93 – Guidance for developing and incorporating a means for addressing adverse effects that repairs or alterations might have on fatigue critical structure.
- Applicable to Operators of transport category aeroplanes who are complying with the requirements of the Aging Airplane Safety Rule AASR.
o 30 passengers or more.
o Payload of 7,500 pounds or more.
AC 91-82A Guidance for developing and implementing a fatigue management program for metallic fatigue critical structure.
- An applicant must develop a fatigue management program as one method to address an unsafe condition when the FAA determines an aeroplane has a demonstrated risk of catastrophic failure due to fatigue. (Not mandatory unless there is a demonstrated risk of catastrophic fatigue.)
Note: This applies to Part 23 certificated aeroplanes, Part 25 certificated aeroplanes not covered under AC 91-56B or AC 120-93, and aeroplanes certificated in the primary and restricted categories.
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Tags:
Compliance, FAA, EASA Type Certificate, Airbus Repairs, Aeroplane, Widespread Fatigue Damage (WFD), Design Certification, Supplemental Structural Inspection Programme (SSIP), Design Approval Holder (DAH), FAR, Part 25, Aging Aircraft Safety Rule (AASR), Aircraft Safety
