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.
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.
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?
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.
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.
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 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.”