Getting Helicopter TAWS Right

By John Croft
Source: Business & Commercial Aviation
May 01, 2013

A light single-engine turbine helicopter buzzes a lake and pulls up to strafe the nearby craggy mountain peaks on moonless January night. Conditions that might otherwise be ingredients of an all-too-familiar chain of events leading to a crash site, on this particular night were rendered harmless by the 3-in., brightly colored back-lit display on the bottom right side of N547SA's panel.

The differentiating instrument on the Eurocopter AS350B2 in this case was a Sandel ST3400H “HeliTAWS” terrain awareness and warning system. HeliTAWS shows the location of terrain, obstacles and power lines relative to the helicopter's position on a color-coded 2-D moving map display. Perhaps more importantly, pilots with the retrofitted instrument can concentrate on flying, secure that the technology will give them 10-20 seconds warning via cockpit lights and urgent-sounding voices before the helicopter's path is projected to hit terrain or obstacles. The technology is similar to fixed-wing TAWS, which has been widely adapted for commercial aircraft by regulation starting in the 1970s, but must also work properly in the low-altitude regimes where helicopters tend to live.

While HTAWS is not new — some helicopters have been carrying the equipment since 1997 — what is new is that avionics providers are enhancing the databases and logic in the units both for the alerts and 2-D maps, and in some cases, for 3-D versions of terrain and obstacle alerts with synthetic vision. The action is being driven in part by a continued problem with controlled-flight into terrain (CFIT) accidents but also by customer requests for HTAWS that will allow them to do specialized work without unnecessary terrain alerts, aka nuisance alerts. Regulators are also driving the changes by progressively tightening guidance materials to cut down on nuisance alerts and increase the reliability of the systems, which some say is thwarting installations on low-end helicopters due to excessive costs.

My night demonstration flight on January 10 with Sandel's HeliTAWS test pilot, Gary Roesink, who's day job is chief pilot for San Diego-based Corporate Helicopters, showed off Sandel's trademark approach to heightened situational awareness without nuisance alerts in the rugged Lake Hodges area north of San Diego. By definition, a nuisance alert is an alert “that occurs when there is no threat or is unnecessary for the intended operation,” says the FAA. In practice, pilots tend to disable systems that generate too many nuisance alerts, a result that could have deadly consequences when those warnings are real.

These days, Sandel uses N547SA for demonstration flights, but the aircraft several years ago was the testbed for HeliTAWS development work, for which the company received a Technical Standard Order (TSO) approval in August 2010. Sandel followed the TSO with supplemental type certificates (STCs) to install the ST3400H in the Eurocopter AS350B2 and Bell 412EP. The company won't say how many HeliTAWS units it has in the field since first offering the ST3400H two years ago. For the fixed-wing world, Sandel builds compact integrated display systems, including primary flight displays and TAWS units, for customers in the business aviation, airline and regional airline sectors.

An FAA HTAWS advisory circular (AC 27-1B) update likely to come out later this year will raise the bar for gaining STC approval, in part by recommending a pilot-controlled reduced-protection mode for low-altitude operations and off-airport landings to reduce nuisance alerts. The AC will also recommend that FAA inspectors test the reduced-protection mode inflight.

“Applicants should consider providing a mode that will account for off-airfield operations that will still provide the pilot with essential alerts regarding terrain without nuisances alerts,” says the FAA. “Without a reduced protection or similar mode, nuisance alerts may lead to pilots ignoring or inhibiting the HTAWS at inappropriate times.”


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