The National Transportation Safety Board followed its urgent safety alert last week about a potential Boeing 737 rudder failure by criticizing the Federal Aviation Administration for a desultory response to the risk and Boeing for not fully informing airlines in advance about the system that failed.
In an unusually strong public rebuke, NTSB Chair Jennifer Homendy in a letter Monday to FAA Administrator Mike Whitaker expressed concern that the FAA “did not take this issue more seriously” after an incident on a United Airlines 737 Max in February.
Homendy added she’s “disappointed” the FAA did not initiate “urgent actions to address the risk of jammed rudder controls” after the NTSB preliminary report on the incident was released six months ago.
“Equally concerning,” Homendy said, is that Boeing didn’t provide detailed information about the component that failed to United Airlines when it delivered the nine affected Maxes to its fleet.
United and its pilots were unaware that this component — an option the airline had not ordered — was nevertheless installed and mechanically connected to the rudder on the tail fin, which is used to steer the jet down the runway on landing.
Homendy seemed to imply a parallel with how Boeing actively hid from airlines and pilots the flight control system — the Maneuvering Characteristics Augmentation System, or MCAS — that was a primary cause of the two fatal Max crashes five years ago.
“Not making operators fully aware of the installed systems and equipment on the airplanes delivered to them is unacceptable and cannot continue to be tolerated,” Homendy wrote.
Her letter emphasized that although United has replaced the parts on its nine Maxes so that no airplanes flown by U.S. airlines are now at risk, 271 other potentially faulty parts “may be installed on aircraft in service operated by at least 40 foreign air carriers.”
In addition, she wrote, a further 75 potentially bad parts were sent directly to airlines as spares and “may currently be on airplanes in service” with foreign airlines.
The FAA in a statement said it is taking last week’s NTSB recommendations seriously.
“The corrective action review board met, and the FAA is moving quickly to convene a call with the affected civil aviation authorities to ensure they have the information they need from the FAA including any recommended actions,” the federal aviation regulator said, adding that it will conduct “additional simulator testing” this month.
Boeing declined to respond to questions about Homendy’s letter, citing the “ongoing investigation” into the incident on Feb. 6 when United Flight 1539 from the Bahamas landed in Newark, New Jersey, and the pilot found the foot pedals used to move the rudder were stuck.
How United got an optional system it didn’t order
The Boeing system that failed is designed to help steer the airplane straight on the centerline as it decelerates down the runway upon landing, a phase of the flight operation after touchdown called “rollout.”
It’s part of an automated landing system intended to help land the jet precisely using instruments in very low visibility conditions.
With fog obscuring the runway, such technology allows pilots to get as low as 50 feet above the ground before they must decide if the runway is clear to land. These are known in aviation as Category IIIB landings.
After touchdown, the system keeps the plane straight on the runway with small adjustments to the rudder, a movable panel on a jet’s vertical tail fin that can deflect to one side so the air pressure on the panel turns the plane’s nose right or left.
NTSB tests after the February incident showed the component that failed was the “rollout guidance actuator,” which has a servo motor that moves the rudder. It’s made by Collins Aerospace, a unit of aerospace giant RTX, formerly known as Raytheon.
However, United doesn’t allow its pilots to do Category IIIB landings, only Category IIA landings where the pilots must see the runway no lower than 100 feet above the ground. Consequently, United did not order this system.
How the system nevertheless ended up on United’s planes is explained in a footnote to the NTSB report on the February incident.
It notes that United’s nine Max jets “had initially been configured for another operator” and that when transferred to United the “actuators were electrically disabled,” which Boeing had noted in a 2022 service bulletin to the airline.
Critically though, the actuators were still “mechanically connected” to the rudder, the NTSB noted.
In late February, the NTSB tested two of these actuators at a Collins facility in Cedar Rapids, Iowa, and discovered that when operated in very cold conditions “the actuators’ function was significantly compromised.”
Investigators found evidence of moisture inside the actuators.
Collins later determined that a sealed bearing was incorrectly assembled during production of the actuators, “leaving the unsealed side susceptible to moisture that can freeze and limit rudder system movement.”
On the United flight in February, even though the Category IIIB autoland system was disabled, the faulty actuator mechanically connected to the rudder jammed. That’s why the rudder wouldn’t move when the pilot tried the foot pedals.
The NTSB said Boeing failed to inform United that “the 737 airplanes it had delivered to the airline were equipped with these actuators — and that the actuators were mechanically connected to the rudder control system.”
Only on May 6, three months after the United incident, did Boeing provide instructions that allowed United to permanently remove the rudder rollout guidance actuators from its 737s.
“We are concerned of the possibility that other (foreign) airlines are unaware of the presence of these actuators on their 737 airplanes,” Homendy wrote in the letter to Whitaker. “Consequently, their flight crews may not know what to expect if the rollout guidance actuator fails at low altitude or during landing.”
In a statement Tuesday, major Boeing customer All Nippon Airlines of Japan said it would act immediately on the NTSB advice.
“As a precautionary measure, ANA is in the process of removing the identified parts,” the airline said. “There will be no impact on operations.”
How many planes are at risk?
The stuck rudder on the United flight in February did not cause a serious problem. The captain was able to keep the airplane near the center of the runway using the nose wheel tiller, and 30 seconds after touchdown the rudder pedals began to operate normally.
The malfunction was not comparable to that on two major fatal 737 accidents, in Colorado Springs in 1991 and near Pittsburgh in 1994. On those flights, the rudder control power unit jammed and the rudder fully deflected to one side midflight.
A rudder that’s temporarily frozen in the neutral position, as in the United incident in February, is not so dangerous.
Nevertheless, a pilot needs a functioning rudder in case of an engine failure to counteract the turning effect of the asymmetric thrust, or in the case of a crosswind pushing a jet off course when close to the ground.
Homendy’s letter notes that Boeing told 737 airlines in August that Collins had delivered more than 353 actuators with this defect since February 2017. (New Boeing CEO Kelly Ortberg was then CEO of Collins Aerospace.)
Almost 300 were installed on planes registered to foreign carriers and a further 75 were delivered as spare parts to various airlines.
“Some of these may be spares, while others may currently be on airplanes in service,” Homendy wrote.
Boeing declined to say how many airplanes could be affected worldwide, citing the ongoing investigation.
“We are working with our supplier to develop additional guidance to address the potential condition,” Boeing said last week.
