The new rule seeks to prevent widespread fatigue damage by requiring aircraft manufacturers and other certification applicants to establish a number of flight cycles...

In a continuing effort to address the safety issues posed by older aircraft, the Federal Aviation Administration (FAA) has finalized a rule designed to protect most of today’s commercial planes and those designed in the future from structural damage as they age.

The new rule seeks to prevent widespread fatigue damage (WFD) by requiring aircraft manufacturers and other certification applicants to establish a number of flight cycles or hours a plane can operate and be free from WFD without additional inspections for fatigue. Manufacturers have between 18 and 60 months to comply, depending on the particular aircraft type.

Once manufacturers establish these limits, operators of affected aircraft must incorporate them into their maintenance programs within 30 to 72 months, depending on the model of aircraft.

After the limit is in an operator’s maintenance program, the operator cannot fly any aircraft which has reached the specific limit for its model unless the FAA approves an extension of the limit. An FAA spokesman confirms that after an individual aircraft reaches the specific WFD limit, the operator would be required to conduct WFD-related inspections of the aircraft at specified, regular intervals in order for the FAA to approve an extension of the limit.

“Safety is our highest priority. This rule provides a comprehensive approach to the problem of widespread fatigue in aging aircraft,” says U.S. Transportation Secretary Ray LaHood. “Requiring carriers to regularly inspect their aircraft for possible fatigue is essential to ensuring the highest levels of safety.”

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“We’ve addressed the problem of aging aircraft with numerous targeted regulations and 100 airworthiness directives over the years,” says FAA Administrator Randy Babbitt. “This rule is a comprehensive solution to ensure the structural safety of today’s airliners and the airplanes of tomorrow.”

According to the FAA, an aircraft’s metallic structures are stressed and can develop cracks when they experience repeated loads such as the pressurization and depressurization that happens on every flight. While airlines regularly inspect aircraft for cracks exceeding a certain size, WFD involves aircraft developing numerous tiny cracks, none of which would have raised concerns individually but which together run the risk of joining up and impairing the structural integrity of the plane.

This phenomenon may have been at work in two non-fatal incidents in recent times. One involved a Southwest Airlines Boeing 737-300 on July 13, 2009 during a Baltimore-Nashville flight which had to divert to Charleston, West Virginia after the aircraft’s cabin depressurized when a 1 foot-by-2-foot hole appeared in its upper fuselage near its vertical stabilizer (commonly known as the tail). The incident caused no serious injuries.

Subsequently the U.S. National Transportation Safety Board (NTSB) found that the hole was caused by longitudinal cracking of the fuselage skin caused by metal fatigue. According to the NTSB’s report on the incident, highly magnified inspections of the longitudinal crack revealed continuous fatigue thumbnail cracks propagating outward (through the thickness of the aircraft’s skin) from multiple origins at the inner surface of the skin.

The second incident happened on October 26, 2010 when an American Airlines Boeing 757 flying from Miami to Boston had to return to Miami when it, too, depressurized after a 1 foot-by-1-foot hole opened in the upper part of the fuselage near a cabin door toward the front of the plane. There were no serious injuries in this second incident, which the NTSB is also investigating.

The Southwest Airlines 737-300 was about 15 years old and the American 757 20 years old at the times of the incidents. The Southwest 737-300 had accumulated 42,500 take-off and landing cycles and 50,500 hours of flight time; and the American Boeing 757 had accumulated some 22,000 take-offs and landings. (American did not provide the amount of flight hours the Boeing 757 had accumulated, but Boeing 757s tend to operate longer flights than do 737-300s as they are larger and offer much longer range.)

The FAA’s new WFD regulation applies to airliners with a takeoff weight of 75,000lb and heavier. It also applies to all transport designs certificated in the future.

According to the FAA, the affected models include a total of 4,198 U.S.-registered aircraft. They include all in-service Airbus, Boeing and McDonnell Douglas jets, as well as the Lockheed TriStar and Hercules, the Embraer 170 and 190, the Bombardier CRJ900 and the Fokker 100. The Boeing 787, 747-8 and Airbus A350 XWB are not covered by the current FAA rule, as they are not yet in service; but the FAA will require that these aircraft be covered by the rule too when they begin commercial operations.

The FAA says it is working closely with the European Aviation Safety Agency (EASA) and other national authorities to harmonize this rule with their regulations as much as possible. The FAA adds that the EASA is now developing rulemaking to address WFD and the FAA is participating in that process.