Saturday, June 02, 2012

Aircraft Structural Integrity - The 5 Threats

Since 2008 the role of the aircraft mechanic working under US FAR 26 or European EASA has dramatically changed. Maintenance responsibilities now cover the 5 threats listed below:
  1.  Conceptual failures: (Comet and B-707 at Lusaka)
  2. Unanticipated structural damage: (F-111, DC-10 at Sioux City, B-777)
  3.  Improper repairs or servicing: (DC-10 at Chicago, B-747 Japan Air Lines, 
    Grumman Mallard, B-747 China Airlines)
  4. Improper manufacturing and quality-control: (F-15C)
  5. Aging aircraft (MSD and WFD): (B-737 Aloha Airlines)

"FAR-26 Regulations: Prior to 2008, the FAA or EASA had very little control on repairs and modifications that were implemented by the manufacturer or aircraft operator. In 2008 the FAR-26 Regulations were adopted to require a damage-tolerance substantiation for all repairs, alterations or modifications that are performed by the operator or manufacturer, on fatigue critical structure. As a result of this damage-tolerance substantiation, special periodic inspections may be needed to verify that cracks have not formed and begun to grow at or under the repair."Repairs must satisfy damage tolerant requirements in addition to strength and aeroelastic requirements.

It's not a bad idea for the aircraft mechanic to become familiar with the repair concepts incorporated into "Damage Tolerance." Even if you don't work on pressurized aircraft, the repair concepts are valuable to all aircraft. Here is the short take-away from Threat 3 above:

Aircraft Mechanics who don't follow the manual, who don't use the prescribed tools, who substitute their own repair methods are not suitable for work on damage-tolerant aircraft.

Damage tolerant tightly couples design and maintenance thereby increasing system complexity. Normal Accident Theory (NAT) suggests that safety systems need to be adapted to deal with the tight coupling that exists between operation, management, maintenance, regulatory bodies, inspectors and others. This introduces my 6th threat to Aircraft Structural integrity: Too much coupling.

An interesting read on this subject:

Beyond Normal Accidents and High Reliability Organizations: The Need for an Alternative Approach to Safety in Complex Systems

The Lessons of ValuJet 592

Normal Accident Theory

Other Articles from this blog:

Damage Tolerance doesn't work without Detection

Resources on Damage Tolerance:


An excellent review of all of the design concepts used in aviation.

1. "Developing Strategies to Combat Threats against the Structural Integrity of


  1. Greetings,

    Just a heads up, the "Developing Strategies to Combat Threats against the Structural Integrity of
    Aircraft" link appears to be a bad URL.

    ~Jon Wanzer