The FAA is proposing changes to installation guidance (AC 27-1B) regarding helicopter terrain awareness and warning systems (HTaws). HTaws is required for air ambulance operations under 14 CFR Part 135 Subpart L. Functionality and performance requirements for HTaws were previously established in TSO-C194 and this new proposed advisory circular does not change that. However, it does suggest several changes to make HTaws more effective in the operational environment.
These changes include a “reduced-protection mode” for off-airfield operations as a way of minimizing nuisance alerts without compromising essential alerts. The FAA notes that “without a reduced-protection or similar mode, nuisance alerts may lead to pilots’ ignoring or inhibiting the HTaws at inappropriate times.”
The proposed AC also provides guidance with regard to airworthiness and certification requirements. The FAA notes that an integrated HTaws installation in legacy aircraft may trigger a “significant change” under 14 CFR 21.101 and require meeting current regulations regarding new technologies. It also suggests flight-testing that checks both the interface, functionality and visibility of the system as well as a system safety assessment that takes into account the probability of various failures, including false cautions. Environmental testing should take into account the increased structural vibration of helicopters compared with fixed-wing aircraft, temperature and electromagnetic interference. Installation should not conflict with navigation displays, but “HTaws status and mode selection annunciation should be as close as possible to the pilot’s primary field of view to enable rapid assessment of HTaws status and configuration.”
Ground-testing criteria are also suggested and include location of controls, displays and annunciators; readability of same in all lighting conditions; evaluations of failure modes and interfaces; compatibility with night-vision imaging system lighting and NVGs; electromagnetic compatibility; and harmonic tests. Flight-testing should assess off-airfield operations, performance of displays and audio in all flight and lighting conditions and performance of the reduced-protection mode when flown against obstacles and terrain.