FAA Orders Angle Of Attack Sensor Upgrades To Older Boeing Aircraft

Nuisance stall warnings on certain older Boeing aircraft have prompted the FAA to issue a Notice of Proposed Rulemaking in regards to servicing the planes. The angle of attack (AOA) sensor vanes were found to be suffering from a buildup of ice, causing the stick shaker to activate. If the rule is accepted, it will become an Airworthiness Directive, mandating all operators of these aircraft to undertake remedial action.

Delta Boeing 757
Delta’s 757s will require inspection. Photo: Wikimedia Commons

The NPRM

In the Notice of Proposed Rulemaking (NPRM) the FAA explains why the Airworthiness Directive (AD) is being issued. They say that,

This proposed AD was prompted by reports of nuisance stick shaker activation while the airplane accelerated to cruise speed at the top of climb. This proposed AD was also prompted by an investigation of those reports that revealed that the angle of attack (AOA) (also known as angle of airflow) sensor vanes could not prevent the build-up of ice, causing the AOA sensor vanes to become immobilized, which resulted in nuisance stick shaker activation. This proposed AD would require a general visual inspection of the AOA sensors for a part number, and replacement of affected AOA sensors. We are proposing this AD to address the unsafe condition on these products.

The AD will apply to certain Boeing aircraft, including the 727, 757, 767-300, -300F and 400ER series.

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The 727, although still in use by some freighter companies and military operations, has been retired from commercial service worldwide. The 757 is still in operation by a number of US carriers, as is the 767. We’re guessing Drake will need to have his private jet inspected too, although being registered in Canada, maybe not?

Air Drake
Will Air Drake be inspected? Photo: Steelhead 2010 via Flickr

What happens next?

If the ruling is finalized, it would require all operators of US registered aircraft of these types to be inspected. ATWOnline estimate that there are around 1,300 aircraft which could require inspection. The AOA vane would need to be located and checked for a specific part number, and if identified as a potentially problematic part, would require the operator to swap them out for other components.

B727
The 727 ceased operating commercially at the start of this year. Photo: Iberia

In the NPRM, the FAA estimate that swapping the vanes would take around three hours per aircraft. That’s not including the time it takes to inspect and identify the part. If all the aircraft affected by this AD require the part to be replaced, that would total around 4,000 hours of maintenance across the fleets.

Boeing previously issued service bulletins

This issue was identified some time ago by Boeing, who issued service bulletins in relation to the problem. The alert service bulletins for each model were issued within the last six months. Clearly, the servicing has not been carried out by all operators, which is why the FAA has now sought to mandate the part replacement, so that operators have no choice to comply.

767 Delta
The 767 is widely used in the US. Photo: Wikimedia Commons

The FAA’s set compliance time for this AD is 36 months, considerably shorter than Boeing’s original compliance timelines. In the service bulletins, compliance for the 757 was set at 9,960 hours, for the 767 it was 3,470 hours, and for the 727, 2,750 hours. The FAA told ATW,

“We have determined that this compliance time will not ensure that the identified unsafe condition is addressed in a timely manner. In developing an appropriate compliance time for this AD, we considered the degree of urgency associated with addressing the subject unsafe condition, the average utilization of the affected fleet, and the time necessary to perform the modifications.”

Boeing’s take

Boeing have been in hot water lately with a slew of bad press regarding the 737 MAX and questions over the manufacturing of their highly regarded 787 Dreamliner. However, in this situation it appears that the FAA are simply helping the US manufacturer to encourage action by operators of the aircraft.

In a message to Simple Flying, Boeing said,

“In the past year Boeing recommended inspections and replacement of certain Angle of Attack (AoA) sensors on Boeing 727, 757 and 767 model airplanes following reports from operators of nuisance stick shaker activation while the airplane is accelerating to cruise speed at the top of a climb.

“A review indicated that the AOA sensor vanes were frozen and had become immobilized because the heaters in the AOA sensor were not sufficient to prevent ice build-up in the faceplate and vane. This can be caused during heavy moisture conditions leading to water entering the AOA vane pivot and freezing during takeoff.

“The FAA has published an airworthiness directive (AD) mandating these actions to operators. Boeing’s recommendations are not binding on operators. Only a regulatory agency has the authority to require them.

“Boeing works closely with the FAA to monitor the fleet for potential safety issues and take appropriate actions. This is an ongoing and continuous process.

“Airworthiness directives are part of the long-standing rulemaking process by which airplane manufacturers, operators and regulators work together to ensure that the safety of the world’s commercial jetliners continues at the highest levels.”

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Nigel

The FAA looks like it may be stiffening its stance toward Boeing. If that is indeed the case, then it’s a very welcome and necessary development.

kilonzo

they want to get back their trust after the speculations that boeing commands them….I too love how they are now correcting Boeing

Nigel2

Boeing made the decision to address these AoA vanes a few years ago. This is not an FAA driven activity.

Eugene Zawadzki

It is crucial to aviation safety that Boeing not be permitted to “fix” the 737 MAX with software. The plane was made unstable due to new, larger and heavier engines which were stupidly located upward and further forward for ground clearance. This causes the 737 MAX to often pitch up at max thrust on take-off. Basic aerodynamic stability was compromised. MCAS is compounding the error: for an airplane that lacks essential stability, software only “papers over” the underlying problem, and Boeing management knows this. But they decided to allow a flawed fifty year old design decision to continue rather than… Read more »

Natalie

While I am not an expert by any means, I am not sure the 737 MAX is considered aerodynamically unstable. Yes, the placement of the engines has reduced the margin of stability in the pitch axis, but what I have read so far does not suggest that this has made the aircraft unstable. The MCAS was implemented to produce the artificial feel of the control forces a pilot would experience in an NG or Classic 737, but is not as I understand a full fly-by-wire system to make an unstable aircraft flyable. It is a stability augmentation system, yes, but… Read more »

Nigel

If the MCAS software has been well executed and implemented, we’d probably never have known it was there (which, of course, was Boeing’s intention). What’s different about MCAS compared to other aviation software is that MCAS is designed to circumvent the effects of a fundamental aerodynamic instability…and that’s a no-no in civil aviation. It’s been quite normal in military aircraft for years (which often have metastable designs in order to achieve high performance) — but, remember, that the pilot in a military aircraft has an ejection seat if something goes wrong. The civil aviation industry is ultra-conservative (for good reason),… Read more »

Gretna

If a Notice of Proposed Rulemaking (NPRM) is accepted, then it will become is far short of an absolute order to accomplish it now. Your copy editor who wrote the headline ‘FAA Orders Angle Of Attack Sensor Upgrades To Older Boeing Aircraft’ needs to talk to the writer Joanna Bailey. Her first paragraph states, “the FAA to issue a Notice of Proposed Rulemaking in regards to servicing the planes. If the rule is accepted, it will become an Airworthiness Directive, mandating all operators of these aircraft to undertake remedial action.” A NPRM is a public notice issued in the Federal… Read more »

Al Peterson

Case heaters on the AOAs are wired in parallel to the vane heaters are not monitored on a B757 and B767 EICAS system. The circuit for the Vane is monitored by the EICAS system. A frozen vane could be caused by a case heater not working and the crew and maintenance get no messages. The case heater also keeps the dampening fluid from getting cold soaked in flight to allow the AOA to react quickly. Need the AD to reflect this and check the case heater as well . When I worked checks in maintenance at Delta airlines we had… Read more »