Two thirds of the aircraft’s value can be attributed
directly to the chronological age of the aircraft. The remainder becomes the value of the engines
and maintenance. At the 20 year point plus
or minus, most planes are at the portion of their life that a majority of their
value is in the last third category of engine and maintenance being the main
driver of value. A well maintained plane
therefore brings top dollar and continues to operate. But is that the end of the story?
There is another portion of the story and that is in the
science of predicting failures. The
inspection process is designed around the philosophy of finding issues prior to
them becoming flight critical. In the
most simplistic form, if a crack forms at a certain point in time during
testing or actual in-service use, inspect a proper interval prior to the crack
to ensure no safety of flight issues. There
are hard time inspections, on condition and those that are monitored.
The bathtub curve for failures becomes of interest. Basically, it states that after the burn in
rate/infant mortality, that products fail at a normal rate up until the wear
out phase. Modern production methods
have significantly reduced the burn in/infant mortality portion of the
curve. All components do not wear out at
the same time leaving multiple components having different aging curves.
However, ultimately, all components are subject to wearing out.
But, as history has proven, can we really predict all of the
possibilities? Even with fail safe
design philosophies, there are numerous accident and incidents that have failed
to predict a critical failure due to the location of the issue. Some are related to hours and cycles, some
are related to the environment the aircraft has been operated in, and a few are
related to the aircraft just being old.
The FAA, as part of its aging aircraft program, has stated that
inspection programs may not prevent some of these issues.
Unfortunately, many of the items that can become flight
critical are hidden and can be in areas that we do not have good methods of
detecting. Corrosion in certain areas
deep inside joints or fatigue cracks that promulgate quicker than previously
thought are just a couple of examples.
Perhaps one of the harder ones to predict is the health of
the wiring in the plane. Studies have
shown that vibration and jostling of wire bundles over time can cause cracking
in wires and their protective casings.
Common areas are around clamps and in areas where after time the wire
bundle starts to rub against the airframe or another component. (Note, wires rubbing against these places are
not by original design but could result in years of bumps and bundle movement.)
Inspections may actually have an influence
on the increase of wire issues as the bundles are moved around inspecting other
areas of the aircraft. There are also
those wire bundles deep in the plane that would be nearly impossible to
inspect. There are efforts to find
methods of determining the health of wires but so far none are accurate enough.
So, how old is
too old? No one really knows. It can vary within the same
aircraft model and type. There are many variables and many of them
outside of detection that can influence the reliability of the plane. I
would suggest that when the plane’s intrinsic value is primarily in the
engines and maintenance portion of its life that you have reached the
point of diminishing returns.
Mike McCracken
President
Hawkeye Aircraft Acquisitions
Office 727 796 0903
"Jets without Regrets"
Mike,
ReplyDeleteGood article and I like your assessment but I have a few questions.
1. Having witnessed and practiced aircraft maintenance and inspection programs for years, Regarding airframe stress and fatigue what is your position on the value of aircraft X-Rays? Does the use of this type of inspection mitigate or acknowledge the life limit of an aircraft?
2. Working in avionics maintenance and modifications I have witnessed first-hand the issues with age and environment on aircraft wiring. At what point or age is the value of an aircraft diminished to the point that avionics upgrades are not a value to the purchase or retention of an aircraft?
First, I am not an expert in maintenance but an observer in trends that effect values. However, I believe eddy current and X-rays where applicable have proven their value. As to wiring, I suspect there are many variables like cycles, how well the original wires were routed, environmental concerns etc. It just appears to me that spending serious amounts of money for an upgrade should also include upgrading other wires in the process.
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