Cracks Grow

This one strange fact determines the design and inspection of aircraft and almost all of our critical structures such as bridges, buildings, pressure tanks, and engines. Riding in my dad's car with a cracked windshield one day my dad happened to mention that the crack was "growing". No! Kids and plants grow but not cracks. Yet, over the months the crack grew until it stretched clear across the windshield. With aging aircraft and structures, crack growth is a hot topic in engineering and regulatory agencies - but it's the mechanic who will find those growing cracks and protect the public.

Growing cracks can be a liability to the mechanic. A broken part (such as a wing) is a design or operational flaw that the mechanic was expected to "uncover" or reveal before it broke. Just as engineers assume that all structures have cracks and flaws, it cannot be assumed that every one will be, or can be, found before failure. Fault tolerant design recognizes these limitations and designs around them. Fatigue life limits are applied to critical parts where fault tolerant design cannot be applied.

Many aging aircraft were not designed with Fault Tolerant Features, are in operation past their maximum anticipated life span, and have not a Supplemental Inspection Document based on "Damage Tolerant Analysis". Without adequate engineering support, failures can be assured regardless of the competency or thoroughness of the maintenance crew.

Anytime your working on an airplane -
no matter what the repair or inspection -
you should be looking for cracks

An airplane taxies up to the repair hangar while we're having lunch. One of the mechanics looks at the airplane taxing and says to us "I'll bet you $20.0 that there is a crack in the exhaust pipe where it's welded to the muffler". We took his bet and lost. How did he know it was cracked? "When I saw that long exhaust pipe vibrating and unsupported I knew that it would crack at weld". 1.
A crack is the warning sign before the part breaks. All airplanes have them. The goal is to find them before they become critical.
Steps that you can take during maintenance:

Take the time during each job to anticipate where they might be located and look for them.
Avoid heavy decorative paint films on critical parts that may hide a crack in a Principal Structural Element (something that holds the aircraft or engine together). Aircraft piston engine cylinders, crankcases, and engine mounts are all PSE's and prone to cracking. A thin enamel paint film has been traditionally used on these structures and has a long history of not hiding cracks. Modern epoxy or powder coat finishes prevent adequate inspection and should not be used.
Have a small dye penetrant inspection kit available for a quick check of suspect areas.