HTAWS to be mandated on all EMS helicopters
FAA says only TSO'd equipment meets required specs.
Thales HTAWS is an example of a highly integrated system that incorporates traditional TAWS with newer concepts such as synthetic vision and other features that enhance a pilots situational awareness.
In Aug 2009, the Honeywell Mk XXI and XXII helicopter EGPWS/ HTAWS was the first system approved to carry the TSO C194 label. One year later, Sandel Avionics announced the TSO approval of its ST3400H HeliTAWS product.
This article focuses on equipment that meets current HTAWS TSO standards. Not all helicopter terrain systems are certified to the same standard—the term used for non-TSO compliant equipment is terrain advisory.
HTAWS with TSO markings provide many benefits to regulators, operators and pilots alike. The regulator ensures that the equipment meets the minimum performance standards identified by RTCA DO309, qualification and documentation requirements from TSO C194, and a level of quality assurance that will provide compliance with the meaning and intent of the proposed regulations.
Operators can use the TSO as a seal of approval to vet various TAWS equipment during the acquisition process, while ensuring regulatory compliance. Most importantly, pilots are provided with a guarantee of predictable system performance without exposure to false and nuisance alerts that diminish trust in these warning systems.
In general, helicopter systems are much more complex than those used in fixed-wing aircraft. Terrain and obstacle avoidance systems are no exception. Considering the operational envelope of a helicopter, TAWS equipment must have a very precise terrain database and employ very sophisticated algorithms and features to provide only valid terrain alerts.
For comparison, according to Boeing the average jetliner on a 1.5-hr flight will be exposed to the terrain threat (below 1000 ft AGL) for no more than 6% of the total flight time. In contrast, depending on the mission, the helicopter may never climb above 1000 ft AGL. HTAWS TSO C194 uses a cruise altitude of 500 ft AGL as its performance standard.
All TAWS terrain databases use a grid system.
HTAWS systems use highly refined databases based on a grid system to analyze and display terrain. Smaller grids increase the fidelity and accuracy of the systems. (L) Representation of a 6-arc-second grid. (R) Representation of a 3-arc-second grid.
These grids are used to display the terrain symbology or colors and as one element of the algorithms to trigger the terrain visual and aural alerts. Each grid segment is assigned a single value or height. The highest point or elevation within that segment establishes the height assigned to that area.
Arc second is the term used to describe the size of the terrain grid segment. Using smaller grid areas or arc seconds allows the alerted height to be triggered over a smaller area for more refined and precise alerting. (See diagrams above, which show arc second comparisons.)
Fixed-wing TAWS systems typically use a 15 to 30-arc-second database. HTAWS manufacturers, such as Honeywell, use a 6-arc-second grid. Sandel Avionics provides the greatest level of fidelity by using a 3-arc-second grid, which is 25 times smaller—ie, more precise—than a representative fixed-wing system.
Likewise, the performance envelope of a fixed-wing aircraft is very well defined—airplanes can only travel forward (above a stall speed), up, down or left or right, and they take off and land at a defined airport.
Helicopters, on the other hand, add to the complexity by hovering, flying backwards and sideways, and landing routinely in areas other than airports. Needless to say, defining the alerting algorithms for a helicopter is much more of a science than an art.
Revised AC 27-1B and 29-2C address some of the rotorcraft nuances by offering guidance to provide some consideration for a reduced protection mode specifically for off-airfield landings.
Although not required in TSO C194 or RTCA DO309, this reduced protection mode will provide the pilot with essential terrain alerts without nuisance alerts. According to the AC documents, the absence of this or similar modes "may lead to pilots ignoring or inhibiting the HTAWS at inappropriate times."
Furthermore, through flight evaluations, FAA found that "these modes usually decrease the vertical and horizontal alerting envelope over terrain and obstacles, thereby reducing time to collision alerts." FAA recommends that the HTAWS be so designed that only the obstacle—not terrain—alerting envelope remains the same as the normal mode when the reduced selection mode is selected.
Since 2001, Honeywell has sold more than 1600 Mk XXI and Mk XXII EGPWS systems and has logged more than 1 million HTAWS flight hours. Honeywell's HTAWS systems are adapted from its proven fixed-wing EGPWS technologies, which account for well over 800 million flight hours. Honeywell HTAWS are standard equipment in new Sikorsky S76s and S92s.
For rotorcraft operations, Honeywell EGPWS/HTAWS systems incorporate reduced sensitivity modes through either a low-altitude switch for operations below 500 ft AGL or a terrain-inhibit switch that disables alert functions when operating close to terrain or in urban areas.
The Mk XXII system provides the 6 Class A GPWS modes (Modes 1–6), with additional excessive-bank, tail-strike and autorotation features. Also included are terrain "pop-up" features if terrain mode is not displayed and an enroute terrain display (or peaks feature) that displays the highest terrain along the flightpath.
Both systems incorporate internal GPS cards and can interface with integrated avionics systems, weather radar indicators, MFDs or stand-alone displays.