As if Boeing didn’t have enough troubles with its 737 MAX, new problems have risen with the company’s 737 NG aircraft.
Hairline cracks were detected on a crucial part of the plane called the “pickle fork,” the component that connects the wing structure, landing gear and fuselage. The NG has four pickle forks: two bracket the forward attachment frame and two bracket the rear attachment frame. The lower sections of the components are responsible for transferring most of the wing’s load into the fuselage.
The pickle fork helps manage the incredible torque, stress and aerodynamic pressures on the connection between the wings and the body. The cracks seem to have formed in the outer chord of the rear forks and the behind lying safety straps, just where they pass from the rear spar of the center wingbox to the fuselage side of the aircraft.
The first cracks were found on three Quantas aircraft in September, and other airlines have reported similar cracks since then. About 50 NGs have been grounded worldwide; about 7,000 are in active service and more are in production.
Boeing responded by informing the FAA of the cracks and is actively working with airlines to inspect and analyze the problems. The company said in a statement that just over 1,000 planes have been inspected with less than five percent needing repair. Boeing also specified that this problem does not affect the 737 MAX or the P-8 Poseidon, a variant of the NG.
The planemaker, the FAA and affected airlines don’t believe the cracks are an imminent threat to safety. While the stress of the flight cycle does eventually cause metal fatigue, aircraft can still fly with a damaged pickle fork thanks to redundant safety features such as redundant load paths, a “fail-safe” design, or a “safe life” design that minimizes stress levels. However, the pickle fork is a load-bearing component of the wing. “(Failure) may occur in turbulence or heavy landing […] and this could lead to loss of control of an aircraft,” said Steve Purvinas, federal secretary of the Australian Licensed Aircraft Engineers Association.
The structures are considered “safe life” designs, meaning they are supposed to last well beyond the life of the aircraft itself. A pickle fork is designed to withstand more than 90,000 flights worth of takeoffs and landings without cracking.
The problem is, though, that the cracks have been showing up in planes that have carried out less than a third of those projected flights. The three Quantas planes that the Australian airline grounded had only completed around 27,000 cycles. Southwest Airlines found cracks in a plane with about 28,500 cycles, and Lion Air found the damage in two planes that had flown less than 22,000 flights.
An NG will fly about 2,000 flights annually, so the inspection covers planes that are roughly 11 years old or more—planes designed to fly for decades.
So why are these cracks appearing in a component that’s supposed to outlast the plane it’s installed in?
While it’s still too early to tell, one possible cause could be the addition of winglets—a modification made after the plane was designed. The winglets could be changing the way the wing handles pressure, which would force the pickle fork assemblies to bear the stress in a way that wasn’t originally intended. By adding winglets, the pressure distribution is spread further outboard, which results in an increase in the bending that occurs at the wing root—putting more strain on the pickle fork.
Other possible causes could be excessive hard landing, environmental factors, manufacturing faults, or even a flaw in the plane’s design. If it’s a problem with the aluminum alloy used, or the way the pickle fork is installed during production of the aircraft, those problems can be fixed. But if it’s a design flaw—that will be harder to resolve.
What seems clear is that the local stresses on the NG’s pickle fork are greater than the part was designed to withstand.
The FAA mandated in October that 737 NG planes with more than 30,000 takeoff and landing cycles be inspected within seven days of the order, and that aircraft with 22,600 or more flights be inspected within 1,000 additional flights—or another seven months. Any planes found with cracks were to be immediately grounded.
In addition, one inspection found a crack in an adjacent component. The FAA has since expanded its inspection mandate to include eight fasteners. And airlines around the world have committed to inspecting all of their NGs.
Boeing has committed to repairing damaged forks and manufacturing new ones at maximum production capacity at its Victorville, California facility—and aircraft have already started receiving the new parts. It will take two to three weeks to replace the component. The company also intends to open other repair lines in Europe and Asia.
The NG is the third generation of 737 and is the precursor to the MAX. The NG first came into service in 1997, and the passenger jet has become the backbone of many airline fleets around the world.
While the cracking for the NG’s pickle forks isn’t an emergency, it does cause concern for a company already under intense scrutiny over the 737 MAX disasters. This is the last thing Boeing needs when it’s still trying to get the MAX back into the air.
Read more about Boeing’s troubles at The Airbus A220 Takes Aim at the 737 MAX.