FAA notice returns attention to TAWS nuisance alerts
By Elan Head | April 5, 2023
Estimated reading time 7 minutes, 13 seconds.
A new Information for Operators (InFO) letter from the U.S. Federal Aviation Administration addresses nuisance alerts from terrain awareness and warning systems (TAWS), encouraging operators to develop and train procedures for use of the terrain warning inhibit switch.
The letter, dated March 23, 2023, stems from the fatal 2016 crash of a Cessna 208B Grand Caravan operated by Hageland Aviation near Togiak, Alaska. While that accident involved a fixed-wing aircraft and TAWS, the FAA confirmed to Vertical that the InFO letter applies to helicopter terrain awareness and warning systems (HTAWS), too.
In the Togiak crash, the Caravan, which was operating under visual flight rules (VFR), collided with a mountain ridge after having likely entered instrument meteorological conditions. The aircraft was equipped with a Class B TAWS that provided en route terrain warnings within 700 feet of the ground, although Hageland was legally permitted to fly as low as 500 feet above ground level.
That altitude mismatch meant the TAWS often provided unwanted aural and visual warnings or “nuisance” alerts, which Hageland pilots routinely disabled using the terrain inhibit switch. The National Transportation Safety Board concluded this had likely been the case on the accident flight, with the result that the system did not provide warning of the impending controlled flight into terrain (CFIT).
The NTSB recommended that the FAA work with part 135 air carriers like Hageland to raise awareness of the risks associated with distraction from continuous nuisance alerts, as well as complacency brought about by routine use of the terrain inhibit feature. The board wanted to see operators develop procedures to specifically address when pilots should test, inhibit, and uninhibit TAWS alerts.
In 2021, the FAA told the NTSB it was in the process of finalizing a Safety Alert for Operators (SAFO) that would guide part 135 operators to develop and implement policies and procedures for proper use of the terrain inhibit feature. That SAFO, originally scheduled for publication in mid-2022, never appeared, and the FAA told Vertical that the new InFO letter represents its changed approach to communicating the information.
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With less than two full pages of text, the InFO letter is brief. It advises part 135 directors of operations, part 91 managers and fractional ownership program managers to “review their approved training programs to ensure procedures for the use of the terrain warning system inhibit switch is adequately addressed,” but it provides no guidance on what appropriate procedures might look like.
That may be because there is no clear agreement on how best to use TAWS in low-level VFR operations. The technology was initially developed for instrument flight rules (IFR) operations, where proximity to terrain always indicates a deviation from the proper flight path. By contrast, designing a system for VFR operations in which the aircraft is intentionally flown close to terrain is no simple task.
In 2017, prompted by another fatal airplane accident in Alaska, the NTSB recommended that the FAA implement ways to provide effective TAWS protections while mitigating nuisance alerts for single-engine airplanes that frequently operate at altitudes below their TAWS alerting thresholds. At the FAA’s request, the standards organization RTCA reconvened a special committee on TAWS to explore various possible approaches to the problem, which it summarized in a white paper published in 2020.
Each of the potential solutions explored by the committee had limitations, reflecting the inherent tension between providing pilots with enough advance warning of terrain, but not too much. The FAA ultimately decided to allow certain Alaskan airplane operators to fly with Class C TAWS, which has smaller alerting thresholds than the Class B TAWS typically mandated for single-engine turbine-powered airplanes with six to nine passenger seats. Standards for Class A and B TAWS alerting thresholds were also reduced from 700 to 500 feet, and the FAA said earlier this year that it continues to advance research into a ground collision avoidance system for aircraft operating at low altitude near proximate terrain.
The FAA has required HTAWS for helicopter air ambulance operators since 2017, but has resisted extending that requirement to all part 135 helicopter operators. In October 2022, the FAA submitted a report to Congress that concluded the costs of mandating HTAWS on all commercial helicopters — $134 million over five years — outweighed the estimated passenger and public safety benefit of less than $5 million over the same time period.
While HTAWS is widely recognized as a safety-enhancing technology, it has also been implicated in some accidents. Notably, the NTSB concluded the pilot in the fatal crash of an air ambulance in 2013 was likely responding to an HTAWS obstacle alert when he climbed into clouds, resulting in an in-flight loss of control. There have also been at least two accidents in which an HTAWS-equipped helicopter struck an obstacle that was not in its HTAWS database.