Continued Airworthiness (CAW) – Aircraft Structure Edgevarsity Blog Edgevarsity
Continued Airworthiness (CAW) – Aircraft Structure

Continued Airworthiness (CAW) – Aircraft Structure

Principal Structural Element (PSE) is an element of the aircraft that contributes significantly to the carrying of flight, ground, or pressurization loads and whose integrity is essential in maintaining the overall structural integrity of the airplane. 

It also includes those structures that are susceptible to fatigue cracking, which could contribute to a catastrophic failure. 

One can find PSEs on-

(1) Wing and Empennage

(2) Fuselage

(3) Landing gear and their attachments.

(4) Engine mounts

While designing the aircraft structure, the designers are given a mandate that, even if fatigue or corrosion or accidental damage occur within the limit of validity of the airplane structure, the remaining structure should withstand reasonable loads without failure or excessive structural deformation. 

Damage Tolerance Evaluation (DTE) is a process to determine maintenance actions necessary to detect or prevent fatigue cracking that could contribute to any catastrophic failure. It would mean that this is a process of evaluation of the fatigue critical structure affected by that repair.

Damage Tolerance Inspection (DTI) is an inspection developed post DTE. A DTI includes the areas to be inspected, the inspection method, the inspection procedures, including acceptance and rejection criteria, the threshold, and any repeat intervals associated with those inspections. The DTI specifies time limits for repairs. 

Baseline Zonal Inspection (BZI) is a term used to define normal versus supplemental inspections for repair assessment.

Any repair done on the aircraft requires some level of inspection to maintain its airworthiness. The level of inspection varies and is dependent on various governing parameters. Couple of such governing parameters are (1) original design of the structure and (2) repair design (3) Location of repair etc.

The inspection interval will have a threshold and repeat intervals and can vary from an external visual inspection to Non-Destructive Testing (NDT) inspections. 

The BZI enables operators to use their maintenance or inspection program to determine which repairs are damage-tolerant for their fleet. 

Category A. 

After a structural repair, if the BZI is adequate to ensure continued airworthiness equal to the unrepaired surrounding structure, then such a repair is called Category A repair.

Category B. 

If a permanent repair requires supplemental inspections to ensure continued airworthiness, then it is called Category B repair.

Category C. 

If a temporary repair that will need to be reworked or replaced before an established time limit and require supplemental inspections to ensure continued airworthiness before this limit, then such a repair is called a Category C repair.

Fatigue critical structure is a subset of principal structural elements that are susceptible to fatigue cracking which when ignored can contribute to a catastrophic failure. 

Fatigue-critical structure also includes structures which, if repaired or altered, could be susceptible to fatigue cracking and contribute to a catastrophic failure. 

Structure may be susceptible to fatigue cracking when subjected to tension-dominated repeated loads during operation. Such structure may be part of the baseline structure or part of an alteration. 

Baseline structure means structure that is designed under the original type certificate or amended type certificate for that airplane model (i.e., the as-delivered airplane model configuration).

Type Certificate (TC) Holders must identify and make available a list of baseline structure that are susceptible to fatigue cracking that could contribute to a catastrophic failure. The term “baseline” refers to the structure that is designed under the original type certificate or amended type certificate for that airplane model (that is, the as delivered airplane model configuration).

The purpose of requiring identification and listing of Fatigue Critical Structure (FCS) under the Design Approval Holder Data Rule is to provide operators with a tool that will help in evaluating existing and future repairs or alterations. 

Aircraft manufactured / operating under Federal Aviation administration (FAA) must comply with the FAA’s Aging Airplane Safety Rule (AASR). This must include damage-tolerance-based inspections to address the effects that repairs, alterations, and modifications may have on fatigue-critical structure and these inspections and a means by which all changes to the maintenance program receives FAA approval.

An evaluation of the strength, detail design, and fabrication must show that catastrophic failure due to fatigue will be avoided throughout the operational life of the airplane. 

When identifying Fatigue Critical Baseline Structure (FCBS), it is not sufficient to consider only that structure identified in the Supplemental Structural Inspection Document (SSID) or Airworthiness Limitation Section (ALS). Some SSIDs or ALSs might only include supplemental inspections of the most highly stressed elements of the FCBS. If repaired, other areas of structure not identified as a PSE in the SSID or ALS may require supplemental inspections.

Online aircraft maintenance engineering programs and CAMO training programs are expected to teach CAW principles and Structural requirements as a part of their training program.


  1. Damage-tolerance (DT) is the attribute of the structure that permits it to retain its required residual strength without detrimental structural deformation for a period of use after the structure has sustained a given level of fatigue, corrosion, and accidental or discrete source damage.
  2. For incorporated SBs that require DTI, the MRO / Airline Technical Service Engineer (TSE) is expected to verify that DTI has been included in the operations specification and include it if it is missing. 

by Sumanth Eswar

Ex Assistant Manager-Technical Services, Jet Airways

14 years of industry experience in various sections under Engineering and Maintenance department such as Engineering Planning, Technical Services Engineering including Structures, Cabin Maintenance, Airframe and Systems, Base Maintenance, Powerplant, Reliability, Aircraft Weighing, Rotable Management. Years of Safety Office experience and was a surveyor for Aircraft Insurance