The amount of electricity passing through the sensor indicates the potential for corrosion on the bridge.

CNN: Is this the 'safest bridge in America?'

Then we look at more nebulous attributes, such as country of registration, past utilization (types of flights, average lengths of flights and frequency of use), corrosion potential based on the region where it was based, importation costs and regulatory compliance, to name a few.

FORBES: What's The Best Jet For You? (Part 3)

Even if corrosion, metal fatigue and parts obsolescence were not taking a toll on the planes, potential adversaries are buying better and better air defenses that will make it harder for the bombers to reach their targets in future wars.

FORBES: Air Force Wants A Bomber That Balances Cost With Capability