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.
"Damage tolerance is a property of a structure relating to its ability to sustain defects safely until repair can be effected. In aerospace engineering, structure is considered to be damage tolerant if a maintenance program has been implemented that will result in the detection and repair of accidental damage, corrosion and fatigue cracking before such damage reduces the residual strength of the structure below an acceptable limit." From Wikipedia
|Lycoming cylinder gas leakage|
Damage Tolerant is illustrated by the concept of "Leak Before Break" which aims to demonstrate that a leak provides a visual clue and time duration before the catastrophic failure. Damage Tolerance failed in this example because the leak was not detected. Detection is the Achilles heal of "Damage Tolerance"
Safety of flight depends on continuing in service, aircraft with known and unknown damage (cracks, corrosion, etc.) until detection and repair. Since detection comes before any repair, the continued safety of flight depends on the mechanic's ability to detect damage, usually using a combination of formal NDT processes and gut instinct/random looking.
|Same Lycoming cylinder showing prior evidence of leakage damage.|
"Damage Tolerance" requires near-perfect detection since the cost of non-detection is non-safety. Antidotal evident shows that damage detection is common and has prevented many accidents; but is far from perfect. All of the engineering that goes into a Damage Tolerant aircraft fails catastrophically if the detection process is not also approached with the same analytical and management skills as engineering. How can the detection process be improved?
1. Mechanic training should move from a "systems learning" approach to detection and repair. Unfortunately, I don't see the vast changes in the aircraft mechanic curriculum.
2. Optimize the working environment. The working environment needs to be re-managed and engineered to optimize the needs of good inspection. Lighting system should be engineered to produce the most optimum level of lighting for detection. Detection tools, such as boroscopes, flash lights, NDT, should be easily available. Temperature control and its effects on mechanic's ability to detection damage needs to studied and applied to the workplace; an assumption that if the work environment is too hot or too cold then the inspection and detection is going to be less than optimal. Temperature and other environmental controls needs to be included in the formal inspection documents just as it is for a tool calibration process.
3. Management recognition that detection is a distinct and separate activity than repair. Illustrated by a comment received by one mechanic: "You spend too much time lookin". Aircraft repair schedules and time allocated, needs to consider "detection and repair." Including the word "detection" into the management system serves as a reminder of what the process actually is. An example of the different mind-set that needs to be developed is when we have our car "repaired" or we have the mechanic "fix" out refrigerator. This is not the process that works with aircraft built to Damage Tolerant standards. Damage tolerant aircraft are inspected for damage -- a different mindset that many in management do not fully appreciate, and won't appreciate without training and embedding "detection" into the management process.
4. What Damage Looks Like. In the pictures above the damage was readily visible but ignored. The common response received is "doesn't look too bad to me". A chest X-ray showing cancer doesn't look to bad to me, but of course I don't know what a chest X-ray is suppose to look like because I have never been trained in the subject. Training materials should include recognizing damage when you see it.
The recognition that Detection is a separate and vital component to any "aircraft repair" process is a top down approach developed by the military, then adopted and improved on by the air transport industry. Although aircraft inspection has always been an integral part of aviation maintenance, the emphasis was on prevention of defects and the removal of time-limit components from service before damage can occur. Damage Tolerance is the adoption of "if it's not broke -- don't fix it" concept. The engineers design the broke to be benign and detectable until the next scheduled inspection. We had better well detect it!
Aging Aircraft. Although not designed to a formal "Damage Tolerant" system, aging aircraft all have some damage such as corrosion. One can "time-life" expire these aircraft or apply Damage Tolerant concepts by developing more formal and rigorous Detection and Repair processes. It appears that Cessna has taken this approach with their Supplemental Inspection Program. “The supplemental inspection program we've developed is primarily a visual process aimed at supporting the continued airworthiness of aging airframes,” said Beth Gamble, Cessna's principal engineer for airframe structures. I would encourage readers who are responsible for maintaining these aircraft to adopt Cessna's program as I see no other alternative for keeping aging aircraft flying while maintaining safety to the flying public.
Additional Articles on this subject
The Aircraft Structural Mechanic (why you deserve a pay raise)
Aircraft Structural Integrity - The 5 Threats