SEATTLE — The Federal Aviation Administration says it will mandate a fix for a new 737 MAX design problem discovered by Boeing that, although it’s a remote possibility, could theoretically disable the jet’s engine anti-ice system.
A different flaw in the MAX’s engine anti-ice system design drew scrutiny in January and forced the company to drop a request for an exemption from key safety regulations.
And now, it’s not just the MAX with an engine anti-ice system problem.
Airlines have reported a separate issue with a similar system on Boeing’s 787 Dreamliner that has caused what the FAA calls “relatively minor” damage to the engine inlets on some two dozen of these widebody jets in service.
Though the FAA considers neither problem to be an immediate risk to flight safety, in February it issued separate notices of two proposed airworthiness directives to mandate the fix for the engine anti-ice system on the MAX and to lay out inspection and repair procedures for that system on the 787.
Boeing previously issued guidelines that recommended airlines do what the FAA will require within three years in the case of the MAX and within 30 months for the 787.
When there is an immediate safety risk, the FAA issues a more urgent, emergency directive that must be acted upon before further flight. Jets are grounded until it’s dealt with. That’s not the case with these two proposed airworthiness directives.
Indicating that the risk is considered slight, both of the proposed directives will be open for public comments until April. Only after that will action be mandated; in each case, probably in the time frame Boeing recommended.
The FAA said the amount of time given to address an identified problem depends on “the risk associated with the unsafe condition.”
A single point of failure
On the MAX, the proposed FAA directive states that Boeing identified a potential single point of failure when it reviewed the internal design of the unit that provides a backup power supply to aircraft systems if the primary electrical system fails.
Such a failure could potentially result in the loss of the anti-ice systems on both engines, with no indication or warning that would alert the pilots, the FAA directive states.
When switched on, the engine anti-ice system blows hot air through ducts from the engine core onto the inlet at the front of the engine to prevent ice from building up in moist, cold air.
“The unsafe condition, if not addressed, could result in loss of thrust on both engines … and can result in loss of continued safe flight and landing,” the airworthiness directive states.
If such a scenario were to happen, it would clearly be very serious and dangerous. But Boeing spokesperson Jessica Kowal said this is a “theoretical issue that has never been seen in service.”
She said Boeing discovered the issue during a comprehensive review of flight deck systems in 2021.
That review was initiated after Boeing in April 2021 discovered a separate electrical grounding problem on the flight deck that caused the backup power supply to fail. MAX deliveries were halted for about five weeks as Boeing developed and installed a fix.
The new flaw is located in the engine anti-ice system controller which is in the panel of switches located in the cockpit above the heads of the pilots.
Kowal said Boeing identified the potential issue through computer modeling analysis that looked at possible risk scenarios, however unlikely.
In November 2022, Boeing sent a service bulletin alerting airlines and describing the required fix, which the FAA will now mandate.
It requires installing four diodes in the panel and changing some wire bundles to control the electrical flow, as well as performing installation and power tests. Boeing recommended airlines make the fix within 36 months.
“This is a remote concern that has never been seen during decades of service from the 737 NG [the model that entered service in 1997] to the 737 MAX,” Kowal said.
She added that, besides the engine anti-ice system, Boeing’s engineering experts have analyzed every other airplane system powered by the backup power system and have concluded that even if the backup power system failed, they could continue to operate to ensure the safety of the airplane.
FAA spokesperson Ian Gregor agreed via email that “there have been no reported failures of this in the service history of the 737 NG/MAX fleet.”
He added that “although the failure is a potential single point of failure within the component, other factors would need to present themselves for the potential condition to occur.”
Damage to 787 engine inlets in service
Unlike this MAX issue, the fault discovered on the 787 Dreamliner has resulted in actual damage to engines on passenger aircraft.
The FAA airworthiness directive on the 787 states that “damage was found during overhaul on multiple inlets around the Engine Anti-Ice duct within the inlet aft compartment.”
The ducting that delivers the hot air to the engine inlet to prevent ice buildup has seals at key connecting points. Those seals are installed by the supplier of the inlets: Collins Aerospace, now part of RTX, formerly known as Raytheon.
Boeing’s Kowal said the duct seals on the damaged 787s had degraded over time.
Collins spokesperson Lori O’Donley said the problem was not a production quality lapse.
“All inlets were delivered as designed with all required hardware,” she said.
Rather than a production issue, it was a matter of the seals being insufficiently durable.
Even when the plane was flying in dry air and the anti-ice system was not switched on, the seal degradation led to hot air leaking into the inlet compartment, “exposing inlet components to high temperatures,” the FAA states.
Boeing said this resulted in “thermal damage and discoloration to a limited area of the surrounding composite and metallic structure inside the inlet.”
“Fewer than two dozen inlets have been identified” with this damage, said Kowal.
The FAA’s proposed airworthiness directive warns that heat damage to the inlet structure could lead to “reduced structural strength and departure of the inlet from the airplane.”
“Departure of the inlet” is a bland way of describing the front of the pod around the engine fan detaching, potentially striking the jet’s wing, tail or fuselage. Such disintegration could result in “subsequent loss of continued safe flight and landing or injury to occupants,” the airworthiness directive states.
It was exactly such an accident in 2018 that produced the single U.S. airline passenger fatality in the last 15 years, when an engine inlet broke off on a Southwest Airlines 737 en route from New York to Dallas and struck the fuselage.
Despite this alarming potential outcome, once again the FAA doesn’t consider this likely and assesses the risk as low enough to allow airlines to take their time to fix it.
In an email, the FAA’s Gregor said the damage found on the in-service 787s was “relatively minor” and that other factors besides the heat damage “would need to present themselves” for the catastrophic scenario to happen.
Likewise, Kowal said Boeing has done extensive engineering analysis and determined “it’s not an immediate safety of flight issue, and the fleet can continue operating safely.”
Boeing issued a service bulletin to MAX operators in September informing them of the issue and recommending inspection of the inlet cowl assembly on both engines of every 787 for signs of heat damage around the engine anti-ice duct.
The bulletin also provided instructions to install or replace seals on the ducts, repairing any damage, and replacing the engine inlet if necessary. These actions were to be accomplished within 30 months.
The proposed FAA airworthiness directive will make those instructions mandatory.
Meanwhile, Kowal said, at Boeing “redesign activities are underway for a long-term solution” to fix the durability problem with the duct seals.
Flaws missed during certification
A separate question is how this flaw with the 787 anti-ice duct seals and the single point of failure in the backup power supply on the MAX slipped through the FAA’s original certification of these aircraft.
The FAA said it “certifies aircraft based on available information at the time” and “takes appropriate action when it becomes aware of new information.”
On the 787 in-service damage, Boeing’s Kowal said the composite and metallic structure that makes up the engine inlet “was properly tested to temperature levels that are expected in service” during certification.
However, she said the gradual degradation of the duct seals during service “is new information and that’s the focus of the redesign.”
On the failure of the backup power supply on the 737 MAX, Kowal said the comprehensive review that discovered this potential problem through computer modeling in 2021 was “a reevaluation over 20 years after the original certification of the 737 NG.”
She acknowledged that the continuous revelation of newly discovered design flaws or quality lapses over the past year looks bad for Boeing. “It is one thing after another, and that’s not good,” Kowal said.
The loss of the MAX door plug on the Alaska Airlines flight in January was clearly a reputation-shredding quality lapse.
But Kowal said Boeing alerting airlines and the FAA to the kind of potential risks flagged in the latest directives instead reflects Boeing’s firm intent following the MAX crashes to examine every possible risk, however small, and to take appropriate action and be transparent about it.
