Design for Inspection (Aviation Norms and AWE)
Aircraft are sensitive fragile objects and even a small bump by ground
equipment or a wayward mud-dauber nest in a
pitot-tube can cause
a crash. Aircraft are typically inspected before each flight by at least a
walk-around Pre-Flight Inspection. NASA even learned by tragedy to
"Pre-Flight" the shuttle carefully in space before re-entry. By tradition
aircraft are given an intensive 100-hr (flight hours) Inspection. and a
rigorous Annual Inspection that "takes the plane apart" to visually
inspect every nook and cranny. The trend in commercial aviation is to make
inspection more flexible by squeezing sub-inspections into busy working
schedules to reduce downtime. Sensors are increasingly relied on to detect
fault conditions. Many aircraft mechanics and owners have varying
certification to perform limited inspections, with FAA Inspectors at the
top of the inspection hierarchy. The system works well enough that
accidents due to maintenance errors and inspection lapse are rare, but
extreme vigilance is an ongoing requirement.
We read many claims about utility-scale AWE aerial platforms that will
operate by themselves for even a year at a time. Its true that space
probes are designed for this sort of duty, but at fabulous expense and in
very limited numbers, with many failures overall. Despite some harsh
environmental factors, in many ways space is less dynamic and dangerous
than unattended operation on earth. Even nesting birds are a factor and
special cases like VTOL flight will throw gravel at high speed and
sandblast surfaces. We have to conclude that the loudest claims for
economically attainable acceptable reliability of complex AWE platforms
We should accept the pre-flight/100-hr inspection tradition as the initial
default, especially if we seek early FAA acceptance. Visual preflight
inspection might be done by cameras, with fault sensors relied on for
certain critical functions. The hundred-hour inspection standard will
require a human for all platforms of standard aircraft construction. Even
the most optimistic AWE claimants predict a rigorous "Annual", which
entails at least a few days downtime, likely in a hangar.
There are some ways to avoid the worst impacts of the inspection reality.
Smaller, lower-mass, lower-speed platforms have a lowered inspection
requirement due to lower consequence of mishaps. Redundant critical
features (like KiteGen's case for double tethers) also relax the
inspection need. Simplicity and easy inspectability really help. While
soft kites, especially the single-skin versions, are not the hottest wing
in the sky, they clearly are the most inspectable and repairable by easily
trained certified parachute riggers in the field.
~Dave Santos Jan.
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