Thursday, December 22, 2011

The Aircraft Structural Mechanic (why you deserve a pay raise)

"you spend too much time lookin" was the latest comment from the boss; a reflection of a time long gone when aircraft mechanics were first fixit men, and then parts changers. Aircraft Mechanic's lookin is what keeps the airlines flying; it's how transport class airplanes are designed; it's designed into the structure; easy access for lookin, lookin tools, lookin techniques, and methods. Does the organization understand this? If not -- and the aircraft is designed to be damage tolerant --it's not tolerant but a time bomb.

It is expected that the structure overtime will develop damage but that it will be found by the lookin mechanic before it becomes fatal. Damage tolerant structures must have a "high probability of detection" The organization needs to provide the personal, tools, environment to make this possible.

Large aircraft are designed to be damage tolerant 1. -- there is no limit to service life. Aircraft are kept in service by a partnership between the structural designer and the structural mechanic. This is no "remove-and-replace" maintenance; "structural maintenance is the cornerstone for ensuring continued airworthiness of damage tolerant structures." 2. 

Damage tolerant allows for cracks in Structurally Significant Items (SSI); it requires timely inspections to detect such damage with a high probability BEFORE residual structural strength falls below specified values.   There are no "standard" repairs or inspections. Each inspection and repair to a SSI is designed by the structural engineer, communicated to the structural mechanic without ambiguity, and performed as the engineer designed. When this does not happen, all hell breaks lose and you get Japan 123 type accidents.

"Inspectability" Where and When to Inspect is a key element
Damage tolerant only works when you know where and when to inspect. Fatigue cracking is cumulative with respect to aircraft usage so it is a straight-forward process to monitor. What is not so easy to predict is corrosion damage; both from standard corrosion and stress corrosion. Stress corrosion reduces fatigue life. A damage tolerant structure must include a Comprehensive Corrosion Prevention  Program(s). If this program is deficient then fatigue life estimates are not accurate and the whole concept of damage tolerance goes out the window and leaves the structure weaker than anticipated. (Aloha Airlines 243 for example) Corrosion prevention, detection, and removal is required for a damage tolerant structure and the execution of this program is part of the Structural Mechanic's job.

This is the concept and it has worked well. To give an example of just how well: Boeing's 737 had a minimum service design objective of 75,000 flights but high-time aircraft have achieved 90,000 flights. Exceeding the design objective occurs across the Boeing fleet including the 707, 720, 727, 737, 747. 2., 3.

This is not only a Boeing accomplishment but also reflects the performance of the aircraft structural mechanic in meeting the engineering expectations in regards to inspecting, detecting, and repairing. If the organization don't have good lookers then they shouldn't be flying damage tolerant aircraft.  You can't attract and keep the best and most responsible aircraft mechanics unless you provide the best wages, benefits, and working conditions.

Other Articles on this subject

Aircraft Structural Integrity - The 5 Threats

Damage Tolerance doesn't work without Detection

1.In 1978, the FAA adopted “damage tolerance” as the preferred choice for managing fatigue in civil airliners.
2. Fatigue Issues in Aircraft Maintenance and Repairs, Ulf G. Goranson, Boeing Commercial Airplane Group.
3. Damage Tolerant works only for defects that are detectable. Adhesive bonded structures may have structurally significant defects that are not detectable.

1 comment:

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