Aircraft Ageing Program

In 1988, the Aloha Airlines Flight 243 (Boeing 737-200) has suffered an extensive damage after decompression in-flight. At 24,000 feet, the roof of this aircraft was torn off in midair. More than 90,000 flight cycles and 35,000 flight hours have been clocked on this 19-year-old aircraft. The aircraft's age, excessive usage, and insufficient maintenance program to detect the presence of fatigue cracking and disbonding in the structure are the primary causes of this accident.

In response to the above accident, the Federal Aviation Authority (FAA) has developed and recommended the implementation of an aircraft ageing program in collaboration with the global airlines, aircraft manufacturers, and other national aviation authorities (NAA) in order to maintain the structural integrity of the aged aircraft.?The maintenance program was developed constantly by taking inspection results feedbacks from the global operators.

The Type Certificate Holders (TCH) or Manufacturers are ultimate responsible for this aircraft ageing program and to develop an effective maintenance program. This maintenance programs are monitored by the state of design Aviation Authority constantly. The operators are responsible for the implementation of this maintenance program on their fleet. The repairs to Fatigue Critical Baseline Structures (FCBS) and Primary Structures are to be assessed and evaluated for the aged aircraft as the Widespread Fatigue Damage (WFD) on the FCBS will lead to catastrophic failure of the aircraft. There is high possibility of cracking due to ageing which may occurs on the intersections of frames and stringers, areas with high bending stress, Rib to skin attachments, window surrounding structure, etc..., due to aircraft ageing, improper repairs and usage of wrong materials or design errors at design stage.

Ideal time to send aircraft to boneyard or scrap?

The retirement age of the aircraft is generally known as Service Goal expressed in terms of Flight Hours and Flight Cycles (not in Calendar days). The Design Service Goal (DSG) is determined at design stage which is an initial operational life of the aircraft. During this service life, the aircraft primary structure or principle significant structural element are expected to be free from significant cracking including WFD that tends to happen because of aircraft ageing. High utilization of the aircraft accelerates the aircraft age and the service goal would be reached sooner.

The DSG will be determined by the Original Equipment Manufacturer (OEM) based on the aircraft type, modification status or Supplemental Type Certificates (STC) etc.. For example, the DSG of A320-200 series is 48,000FC or 60,000FH whichever comes first. However, the DSG of A320 with Sharklet installed is 48,000FC or 96,000FH (whichever occurs first).

The European Aviation Safety Agency (EASA) and Airbus uses the DSG idea. The Limit of Validity (LOV) of an aircraft is used by the Federal Aviation Administration (FAA) as a measure of its operational limit where WFD won't happen. For example, the LOV of Boeing 737 is 75,000 TAC.

Any possibility for life extension?

Yes, the Airbus aircraft can be used beyond DSG provided it undergoes a program called Extended Service Goal (ESG). All the publications data are valid until the Maintenance Publication Trigger Date (MPTD) and is equivalent to ESG limit. By which, the service limit of the aircraft can be extended to certain extend as determined by the OEM/State of Design. During this period, the aircraft is expected to be reasonably free from WFD.

The operators should contact their respective OEM to initiate this program well before the aircraft fleet reaches the DSG in order to avoid long term grounding of the aircraft. This ESG program requires aircraft to undergo through heavy maintenance check which involves reassessment of the historical structural repairs or production anomalies (generally called as Concessions) and accomplishment of various inspection & modification Service Bulletins (SB) that reinforce the aircraft structures, components and restores the strength lost in Flight Hours or Flight Cycles operated.

This program requires more ground time, heavy maintenance (includes Modifications and Repairs reassessment); which is going to be additional cost to the operators. The aircraft should be removed from service when it reaches the permitted service goal or LOV.

Corrosion Prevention and Control Program:

The aircraft structures are also tends to get corrosion over a period of time because of operational conditions and ages. To control and prevent this corrosion in the aircraft structure, Corrosion Prevention and Control Program (CPCP) was developed and incorporated into the Maintenance Program. The findings from this CPCP tasks are grouped as Level-1, Level-2, and Level-3 based on their severity and its effect on the aircraft structure.

The subsequent repeat inspections results reveals corrosion level 1, then no changes required in the maintenance program. In case of Level-2 and Level-3 findings on the subsequent repeat inspections, the existing maintenance program has to be revised and implemented on all similar age group aircraft in the operator fleet which are undergoing same operational conditions.

The Aviation Authority and Type Certificate Holders (TCH) are notified for Level-2 and 3 findings for further analysis and to develop the Maintenance Program in order to avoid these findings in future.

In addition, operators are also encouraged to develop their own CPCP maintenance program from their in-service experience in their fleet. The Level-2 and Level-3 Corrosion findings requires review of operator's maintenance program in order to control and maintain to Level-1 or better.

Structural Repairs and Modifications:

The operational and physical conditions of each aircraft may differs from one aircraft to other aircraft. Hence, in addition to CPCP, Structural Inspections, and Limit of Validity the damage tolerance of any repairs and modifications that have an impact on FCS should be considered. Damage tolerance evaluation (DTE) will be used in order to identify the effectiveness of the existing maintenance activities to identify the fatigue cracking on the structures earlier. Further supplemental structural inspections for the damaged region specific to that aircraft may be issued based on the DTE.

The repairs performed in accordance with the approved published data like SRM, SB, CMM, AMM, AD suggested repairs etc.. are generally evaluated for Damage Tolerance. The repairs performed outside these approved published data undergoes three stages of approval process where the static strength, and damage tolerance are evaluated and additional inspection requirements are determined to meet the Instructions for Continued Airworthiness (ICA) requirements as applicable.

To maintain the structural integrity of aircraft, the Inspection Service Bulletins (ISB) and Modification programs are reviewed to identify the cracking at its early stage. The findings tends to be more when the aircraft ages (usage). Since there is high possibility of missing these inspection findings because of human factors, and other reasons, the modification programs are developed and Modification Service Bulletins are issued to reinforce the structure in order to eliminate the crack source.

Closing Thoughts:

Early detection of the issues with the ageing fleet is crucial since there is a significant likelihood of major structure collapse, which might result in catastrophic breakdown of the aircraft and put flying lives at danger. Thus, it is imperative that the Aircraft Ageing Program be treated as essential on all the commercial and military aircraft.

For LOV details of other aircraft models, refer to https://www.govinfo.gov/content/pkg/CFR-2012-title14-vol3/xml/CFR-2012-title14-vol3-sec121-1115.xml

Appreciate your time on reading this article. Please share your views and thoughts in comment.

Thanks and Best Regards,

Saravana Kumar Marisvaran.