Boeing 777-9 Engine Mount Structural Failures

Now the Boeing 777-9 fleet has been grounded for thrust link failures. Think of thrust links as one part that attaches the engine to the wing and exerts the pulling force. Think in terms of a big eager dog on a leash. The number 20 in this diagram is pointing to the rear attachment of a typical thrust link.

One 777-9 link was severed & it looks like they found cracks in others. The closest I can come to good news it that each engine will typically have TWO thrust links for belt & suspenders redundancy. Even then, in failure mode of one link all the load will shift to a single link and can cause it to deal with asymmetric forces, possibly including bending, which I'm pretty sure they're not designed to take. There's plenty more structure and I'm not an expert in all the load paths for engine mounting, I'm pretty sure the whole system is pretty stout but a failed link will cause some part of the load to be redistributed assymetrically. Still, it's like breaking a leg. You have one leg left and now you have crutches but you're not going to win any races. According to an AvWeek article, one link was severed & it looks like they found cracks in others. The severed thrust link had 214 hours & 515 cycles which is ridiculously low. This is very, very bad news for Boeing.

Here's how a Q&A with a structures engineer might go:

Q: Since the thrust links are redundant, is the airplane safe to fly if one fails?

A: In a perfect world, probably

Q: How long will it be safe?

A: I don't have a F$%^&ING clue

I'm going to lay the background here for what follows. The AvWeek article mentioned above was an FAA Type Inspection Authorization (TIA), which is an internal document typically signed?off by EVERY ENGINEERING DISCIPLINE in the FAA. It declares the airplane in question is safe to fly and conforms to its type design (dimensions, tolerances, material & process specs). The TIA is owned by the FAA but generated and shared with the applicant. The FAA has lost pilots in the past so the TIA is intended to ensure flight safety AND confirm the airplane is 100% ready for FAA testing, to save Boeing and the FAA time & aggravation. Three years ago, the FAA reprimanded the Boeing?Organization Designation Authorization (ODA) for pushing for the TIA far sooner than the FAA would allow. The current TIA is very fresh, and it took 3+ years to generate after that major Boeing error that shook what little faith the FAA had in the ODA.

Thing is, the ODA is a representative of the FAA, essentially an FAA extension, and that letter to the ODA should have been from the ODA to the Boeing project leadership. In other words, the Boeing ODA pushed through an obvious conflict by representing Boeing and suggesting to the FAA that it was time for a TIA, they flipped the script, which should NEVER have happened. So now, three years down the road, the new & current TIA is only weeks old. That makes it especially meaningful that this failure of a safety-critical primary structural component puts the 777-9 project in a very tight spot.

Breaking a thrust link can bring an airplane down. Although it was a completely different cause, many of us remember American Airlines Flight 191 in Chicago (link below), when they lost the left engine on takeoff, which took out the hydraulics which in turn took out the wing leading edge slats. The thrust link is a massive tubular titanium beam that transfers engine thrust to the wing strut. It's supposed to last thousands of hours rather than the 214 hours & 515 cycles this plane has accumulated. In many airplanes either flight time or cycles will dominate the design priorities but a thrust link has to deal with both:

• Cycles: Generally, each take-off will use full throttle. The 777-9 engine certainly cranks out 100,000+ lbs of thrust and the thrust links must take that huge load several times a day, so the link is a wee bit on the critical side. The pull exerted on the thrust link is far more than it will experience in cruise, of course, so that high thrust will be a major workout.
• Flight time: The engine is out there dealing with turbulence & bumps, plus pretty constant low inflight level movement, and the thrust link feels every single rut in the road on every flight. So here we have a fatigue problem - full throttle will also add to any fatigue cracks but every bump also contributes to crack growth. I might expect this constant movement under flight loads to be the primary design consideration for fatigue.
• A much, much lower contributor might be temperature changes from ground level temps to maybe? -45° F at 5,000 ft.? Expansion & contraction can contribute to crack growth. The United Airines Flt 232 DC-10 accident in Sioux City IA (link below) was caused by thermal cycles, but in the fan hub where the temperature swings are more dramatic.

I'd say Boeing's best hope to fix the problem is the possibility of titanium flaws, which takes us back to the UAL Flt 232 problem, caused by fan hub fatigue. Another possible cause might include a basic design problem like a moved decimal point, which would explain the failures at such low cycles & hours, and THAT would be devastating to Boeing. Problem there is, the 777 fleet is nearly 30 years old and if the design was so far off the requirement, as in a full order of magnitude, I would expect it to be noticed immediately, almost intuitively. Mechanics on the factory floor touch these things all the time and they're pretty sharp at finding problems - let's hope Boeing has moved on past "Just shut up & do your job."? So I'm sure as hell not going to point to THE problem, whatever that is, because it's way too early for that. I can definitely say Boeing has been on the phone to their supplier(s) asking about flaws in materials and conformity.

And let’s not forget that both Boeing and Airbus have recently been affected by questionable titanium with fraudulent documentation. Here’s a link to a June 14 CNN story about that, it was one of many:

• https://www.cnn.com/2024/06/14/business/faa-probe-counterfeit-titanium-boeing-airbus/index.html

I Googled for 777 thrust link news & found a BUNCH of articles, many, many people now know "Boeing has done it again." Both Boeing and the FAA are likely to be wire brushed again, Boeing by the FAA and the FAA by Congress. I’ve often stated that "The FAA is a technical organization in a political world". With so much coverage I'm thinking the FAA has been chewing on Boeing full time, in shifts. And Congress is doing the same to the FAA because in the political world the FAA is expected to CONTROL Boeing even though in our engineer & regulatory world it's 100% on Boeing based on a Supreme Court decision 40 years ago.

FAA Lessons Learned Library

The FAA “Lessons Learned” library is an outstanding source of information for the more critical accidents in the “Jet Age”. Every article is deeply vetted, and all details are compared to NTSB reports to avoid conflicts with factual findings. The site was started by FAA employees Dan Cheney and Steve O’Neal because they saw a continuous loss of corporate knowledge as industry Old Goats retired.

These two gentlemen were in the Seattle Aircraft Certification Office (ACO) and started the Lessons Learned library with transport category airplane accidents; that was their world. The library was so useful that rotorcraft and small airplane accident histories were added later. As a result, those libraries are still being built, but they’re off to a great start.

American Airlines Flight 191, DC-10 Chicago IL Accident:

• https://www.faa.gov/lessons_learned/transport_airplane/accidents/N110AA

United Airlines Flight 232, DC-10 Sioux City Accident:

• https://www.faa.gov/lessons_learned/transport_airplane/accidents/N1819U

?(Note, edited to clear out typos & add FAA Lessons Learned information)