Airworthiness Limitations: EASA Vs FAA
The Aircraft Type Certificate (TC) is supplemented by aircraft Type Design along with other documents. According to EASA (Part-21B, 21.A.31) and FAA (FAR 21.31), the Type Design must include the Airworthiness Limitations Section (ALS) in the Instructions for Continued Airworthiness (ICAs). The requirements for the ALS in the ICAs originate from the EASA Certification Specifications CS 25-1529 and FAA Airworthiness Standards FAR 25.1529. Let’s formally define the ALS before delving deeper into the topic.
Airworthiness Limitations Section (ALS):
Airworthiness Limitations Section (ALS) is an EASA-approved (or FAA-approved, as applicable) document as part of the Instructions for Continued Airworthiness (ICA) and is referenced in the Aircraft Type Certificate Data Sheet (TCDS). It defines mandatory maintenance tasks and intervals including Life-Limited Parts (LLPs) necessary to ensure the continued airworthiness of the aircraft. As per EASA CS-25, ALS mainly includes:
Non-compliance with the requirements of ALS may result into “Unsafe Conditions” in aircraft.
So, as per above definition, ALS can be mainly divided into Structure ALS, Systems ALS and Fuel Tank Safety ALS.
EASA VS FAA REGULATIONS:
EASA and FAA both mandate the establishment and implementation of ALS in a similar manner, as illustrated in the infographic below. The FAA further specifies requirements through Special FAR 88 pertaining to Fuel Tank Safety. In contrast, EASA includes additional guidelines for Fuel Tank Safety and Electrical Wiring Interconnection System (EWIS) in AMC 20-21. The infographic depicts the regulatory requirements of EASA and FAA regarding ALS and its connection to the Type Design.
Now that we have covered the regulatory background and definition of ALS. Let’s explore how the Airbus and Boeing manage the ALS in their ICAs.
AIRBUS AIRWORTHINESS LIMITATIONS SECTION (EASA ENVIRONMENT)
Airbus issues ALS in a stand-alone document. Airbus has divided ALS into four categories i.e. Structures, Systems, Fuel and Aircraft Information System Security (AISS).
(note: in some literature, fuel-ALS & AISS are considered as part of System ALS).
Structure ALS:
ALS Part-1 (Safe-Life Airworthiness Limitations)
The Safe-life is a point in the airplane’s operational life at which the structure must be taken out-of-service or replaced. The structure is retired or replaced at the safe-life to prevent the structure from developing fatigue cracks (FAA AC 23-13A).
In safe-life components, Damage-tolerant inspections are impractical, and components will be retired from service at fixed interval (for example: Life-Limited Parts of Landing Gears are Safe-Life components). So, all the Airworthiness Limitations of the Safe-Life components are given in Airbus ALS Part-1.
ALS Part-2 (Fail-Safe / Damage-Tolerant Airworthiness Limitations)
The ALS Part 2 provides mandatory instructions and airworthiness limitations arising from fatigue and damage tolerance evaluation of damage tolerant structural elements whose failure could contribute to catastrophic failure of the aircraft. It also contains airworthiness limitations arising from the evaluation of Widespread Fatigue Damage (WFD) and give the Aircraft Limit-of-Validity (LoV).
Systems ALS:
ALS Part-3 (Certification Maintenance Requirements- CMR)
The ALS Part 3 provides Certification Maintenance Requirements (CMR) which are system related latent defects finding periodic tasks established during the Aircraft Type Certification. ALS Part-3 is based on the aircraft pre-Design Service Goal (Pre-DSG) System Safety Analysis and Maintenance Steering Group (MSG)-3 analysis.
ALS Part-4 (System Equipment Maintenance Requirements- SEMR)
SEMR are sometimes referred to as Ageing System Maintenance (ASM). Establishment of SEMR involves post-DSG System Safety Analysis, MSG-3 analysis and System Life-Limits. All the requirements of SEMR are covered in Airbus ALS Part-4.
Fuel ALS:
ALS Part-5 (Fuel Airworthiness Limitations)
The ALS Part 5 provides airworthiness limitations, also called Fuel Airworthiness Limitations Items (Fuel ALI), related to the fuel tank ignition prevention and fuel tank flammability reduction. It includes maintenance tasks to prevent any degradation of fuel tank and inerting system, which could lead to an unacceptable risk. It also provides Critical Design Configuration Control Limitations (CDCCL) to preserve ignition source prevention and flammability reduction design features needed to prevent the development of an unsafe condition.
ALS Part-6 (Applicable on A380 aircraft):
Airbus has now introduced ALS Part-6 which is applicable on A380 aircraft type, regarding the Aircraft Information System Security (AISS) such as, but not limited to, secure and reliable aircraft communication system.
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BOEING AIRWORTHINESS LIMITATIONS SECTION (FAA ENVIRONMENT)
The format of Boeing ALS is different than the Airbus. Boeing typically incorporates the ALS in the “Maintenance Planning Document (MPD)”. The ALS is organized into different sections of MPD. For instance, for Boeing 777 aircraft, ALS requirements are covered in the MPD Section-9. The following flowchart gives an example of B777 where Boeing has divided ALS into Airworthiness Limitations (AWL) and Certification Maintenance Requirements (CMRs). Boeing further divides AWLs into four sections as depicted in the flowchart. “System AWL” contains the CDCCL and Airworthiness Limitations Items (ALI) requirements related to Fuel tank, Nitrogen Generation system, Engine drain lines, engine thrust reverser thermal protection system and engine suction feed system etc.
Approval of ALS and subsequent Revisions:
ALS and its subsequent revisions are approved by Primary Certification Authority (i.e. EASA for Airbus & FAA for Boeing). Whenever, EASA or FAA wants to mandate the restrictive interval / threshold of ALS maintenance tasks, EASA / FAA issues an Airworthiness Directive (AD) pertaining to the amendments in ALS.
Aircraft Operators responsibility:
Airlines and maintenance organizations are responsible for incorporating the ALS requirements into their maintenance programs. They must ensure that the latest revisions and updates are implemented to maintain the airworthiness of their aircraft.
Can Aircraft Operators / Owners / CAMO organizations escalate or delete ALS maintenance requirements?
ALS requirements cannot be escalated or deleted without the approval of or inputs from Primary Certification Authority / National Aviation Authority (NAA) as applicable. For example:
Boeing AWLs / Airbus ALS Part-2: Thresholds and intervals of “DT-ALI” cannot be increased, unless advised by the manufacturer following approval by the Authority of the State of aircraft registry.
Boeing CMR / Airbus ALS Part-3:
One Star CMRs (*) – The tasks and intervals specified are mandatory and cannot be changed, escalated, or deleted without the concurrence of the Primary Certification Authority (EASA for Airbus, FAA for Boeing).
Two Star CMRs (**) – Task intervals may be adjusted in accordance with each operator’s approved escalation program or an approved reliability program in a like manner for any MRB Report task, but may not be deleted without prior approval of Primary Certification Authority (EASA for Airbus).
ALS Part-4: Exceptional short-term extensions are permissible. Approval of such extension rests with the Authority of the State of aircraft registry in the same way as it does for any other scheduled maintenance task interval extension.
ALS Part-5: Unless otherwise stated, the tasks and intervals ALS Part 5 are mandatory and cannot be changed, escalated or deleted without the concurrence of the Primary Certification Authority (EASA for Airbus).
To wrap up:
The Airworthiness Limitations Section (ALS) is an EASA/FAA-approved document within the Instructions for Continued Airworthiness (ICA) and is referenced in the aircraft Type Certificate Data Sheet (TCDS). Compliance with ALS is mandatory, as non-compliance may lead to unsafe conditions. Both EASA and FAA have similar regulations concerning ALS and issue Airworthiness Directives (AD) when any restrictive interval of ALS needs to be mandated. ALS are typically categorized into Structural, Systems, and Fuel Tank Airworthiness Limitations. While Airbus and Boeing manage ALS documents differently, both adhere to all regulatory requirements outlined in FAR/CS-25 and FAR/Part-21, among others. ALS requirements cannot be exceeded or deleted unless explicitly specified or approved. Operators must incorporate ALS into their aircraft maintenance programs.
Definitions of terms used above:
Type Design: A collection of drawings and specifications necessary to define the configuration and the design features of the product shown to comply with the applicable type-certification basis and environmental protection requirements. It also includes an approved airworthiness limitations section of the instructions for continued airworthiness as defined by the applicable certification specifications. (EASA Part-21)
Type Certificate: A type certificate is issued by the?National Aviation Authority (NAA)?of the?State of the Operator?stating the airworthiness standard for the aircraft type, model, aircraft engine or aircraft propeller. The initial Type Certificate is likely to be obtained in the?State of Manufacture. A key annex to any Type Certificate is a?Type Certificate Data Sheet (TCDS) (skybrary.aero)
Instructions for Continued Airworthiness (ICA). The methods, techniques, and practices for performing maintenance, preventive maintenance, and alterations, which are provided by the design approval holder or its component manufacturers. For example, under Part 25, appendix H, the ICA includes an airplane maintenance manual or section, maintenance instructions, and an Airworthiness Limitations section. (FAA AC 120-77)
Unsafe Condition: If there is factual evidence (from service experience, analysis or tests) that an event may occur that would result in fatalities, usually with the loss of the aircraft, or reduce the capability of the aircraft, or there is an unacceptable risk of serious or fatal injury to persons other than occupants, or Design features intended to minimise the effects of survivable accidents are not performing their intended function. (EASA Part-21)
CMR: a required periodic task, established during the design certification of the airplane as an operating limitation of the type certificate. CMRs usually result from a formal, numerical analysis conducted to show compliance with catastrophic and hazardous failure conditions. A CMR is intended to detect safety-significant latent (hidden) failures that would, in combination with one or more specific failures or events, result in a hazardous or catastrophic failure condition. (FAA AC 25-19)
Limit of Validity: is the period of time (in flight cycles, flight hours, or both), up to which it has been demonstrated that WFD is unlikely to occur in an airplane’s structure by virtue of its inherent design characteristics and any required maintenance actions”. (FAA AC 120-104)
Design service goal (DSG): is the period of time (in flight cycles or flight hours, or both) established at design and/or certification during which the aircraft structure is reasonably free from significant cracking. (EASA CS-25)
Widespread fatigue damage (WFD): in a structure is characterised by the simultaneous presence of cracks at multiple structural details that are of sufficient size and density whereby the structure will no longer meet the residual strength requirement (EASA CS-25).
CDCCLs: are a means of identifying certain design configuration features intended to preclude a fuel tank ignition source for the operational life of the airplane. (B737 document D6-38278-CMR)
Airworthiness Limitation Item (ALI): ALI identify inspection tasks related to fuel tank ignition source prevention which must be done to maintain the design level of safety for the operational life of the airplane. (B737 document D6-38278-CMR)
Further Reading:
References:
Aeronautical engineer/ Aviation
5 个月Thank you Sir, for your valuable information.
Quality Engineering | Airworthiness | Manufacturing Engineering | Pakistan Air Force | Project Management | Supply Chain Management |
6 个月Very informative, well explained and wonderful effort Muhammad Inayat . Great share.
A/C Maintenance Program and Task Card Engineer at Ethiopian Airlines
8 个月Thanks for sharing
Airworthiness Inspector at UAE General Civil Aviation Authority (GCAA)
8 个月Interesting!
A/C Maintenance Program and Task Card Engineer at Ethiopian Airlines
8 个月I'll keep this in mind. Its very improtant