“It was a loud rattling sound. Like straining and shaking metal…”
This draws attention to one of the engine fans. While the original jet engine produces all the forward thrust from a hot exhaust, modern high-bypass jet engines contain something resembling an airplane propeller, called the fan. Like a room fan, it moves cool air, both through the combustion chamber, and around it. You don’t see the resemblance to an airplane propeller, because the fan is hidden by a shroud.
Because the fan moves cool air, it can be made from a variety of materials. The traditional choice has been titanium. In the past 5 years, carbon fiber, with titanium facings, has been taking over, because it is lighter, and more resistant to bird strikes.
Bird strikes are simulated by firing a dead chicken into the fan at high speed. Most of the time, an engine withstands a bird strike with minor damage. Sometimes, it causes major damage. Over 60 years, thousands of bird strikes have provided a huge amount of experimental data, mostly with titanium fans.
Both titanium and carbon fiber are quirky materials. But carbon fiber is a composite of multiple substances. Without titanium facings, early carbon fiber fans disintegrated on bird strikes.
The combination of fiber and titanium is an even more complicated mix. While apparently stronger, the ways it can fail may not have been completely anticipated. After all, it hasn’t been around for 60 years of bird strikes.
Now we add a complication. The engine in question is controlled by BAE’s FADEC-2 “Full Authority Digital Engine Control-2.” If FADEC-2 realized an engine fan had a problem it should have shut it down. But FADEC may have had no programming for the particular case of a facing strip that was torn away from the fan, raking the shroud at high RPM (the noise.)
The mix becomes more lethal when the pilots, distracted by the pitch control problem, leave FADEC-2 to handle the engine by itself. They can’t be expected to kill the engine manually when they are fighting for control. When the titanium facing is ingested further into the engine, the turbine blades are cut apart, causing an uncontained “explosion.”
Here two systems intended to enhance the safety margin over human control combine in an unanticipated way to create a lethal event.
With the enthusiasm about A.I., much more of this lurks in our future.