I’m fascinated by the concept of tolerance in mechanical engineering. Tolerance is the permissible variation between components. In many forensic engineering cases, we see several instances of mechanical failure due to parts that weren’t made to spec.
Just how perfect do parts need to be? It depends on what we’re making. Variation in manufactured objects is unavoidable. But when the product under discussion is one of the greatest weapons of war ever produced, the margins for error are quite small.
A Forensic Engineering Case
The McDonnell Douglas F-15 Eagle entered service in 1976 as the world’s top air superiority fighter. When the sun finally sets on its production in 2022, the F-15 will have been in service for around 50 years. But it’s lucky to have lasted that long.
Things almost went south for the F-15 after a Missouri Air National Guard fighter crashed during an exercise in 2007. That was the fourth F-15 crash that year. What gives? The incident caused a global shutdown on F-15 flights while forensic engineers evaluated the problem.
A technical analysis of the wreckage determined that a longeron—a load-bearing beam of the fuselage frame, chassis, and cockpit—was a couple of millimeters off from the blueprint specifications. Some longerons were too thin or had rough surfaces that resulted in too much pressure. This led to cracks developing in the longeron, in time causing it to fail completely.
Structural Defects in the F-15 Eagle
Without precise measurement, this level of forensic evaluation would be difficult. Not all structural defects are even visible to the naked eye. Investigators used a surface scan that passes electricity through a coil. A crack in the structure interrupts the current, telling engineers to “look here.” They apply a dye to the area and view it with an ultraviolet light to confirm the crack.
After engineers had determined the cause of the accident, an Immediate Action Time Compliance Technical Order was issued. Maintenance crews thoroughly inspected all 700 of America’s F-15s to check the longerons.
About 40 percent of our F-15s had at least one longeron that wasn’t up to spec. Nine of them had developed cracks that would have led to mechanical failure. Since it costs more than $250,000 to replace each longeron, it didn’t make sense to do extensive repairs on a fighter near the end of its service life. Over 160 planes had to be grounded indefinitely.
Who Was Responsible for the Accident?
A longeron that wasn’t up to design specifications to begin with, and had seen more than 25 years of service, cracked under the stress of a 7G turn. This led to a chain of failures in other longerons, and the cockpit separated from the rest of the fuselage.
Major Stephen Stilwell, the pilot, was able to eject, but he was injured by the canopy as he did so. He suffers from chronic pain and was left with a 10-inch metal plate in his arm and shoulder. He sued Boeing, believing that the manufacturer was aware of the risk of structural failure and failed to warn the Air Force.
While navigating the labyrinth of legal issues is tough in a case that involves the government, Boeing, and a fleet of aircraft from a generation ago, at least we know what happened. The parties involved can litigate it out as newer aircraft are phased in.
The diligent work on this forensic engineering case likely prevented further crashes and saved the lives of American pilots.