Helicopter Terrain Awareness and Warning System (HTAWS)

Helicopter Terrain Awareness and Warning System (HTAWS)


This article summarizes the current status and near-future trends of helicopter terrain awareness and warning systems (HTAWS).

Although many regional and national aviation authorities advocate wide voluntary adoption of HTAWS throughout most civilian flight operations, they primarily have required adoption only in two helicopter industry mission segments:

These mission segments were targeted for this safety intervention because of their track records of disproportionately high rates of crashes — categorised by accident investigators as Controlled Flight Into Terrain (CFIT). This type of accident occurs whether operating under Visual Flight Rules (VFR) or Instrument Flight Rules (IFR), often in hard-to-avoid environmental conditions and in mission situations that inherently are hazardous.

TAWS and HTAWS equipment computes the precise three-dimensional position and velocity of the aircraft — i.e., real-time monitoring — in relation to accurate databases of the terrain height and hazardous-obstacle locations. Like TAWS for fixed-wing aircraft, HTAWS automation continually monitors the terrain/object situation — with few distractions of the properly trained pilot’s attention — except when it annunciates one of the limited possible alerts. The system empowers the pilot to execute immediate changes to the flight trajectory to prevent CFIT.

The first generation of certified HTAWS software and databases, circa 2008, proved capable of reducing the CFIT accident rate in much the same way that fixed-wing aircraft escape imminent collisions. Pilots typically responded as expected to their ground proximity warning system (GPWS) and later to their enhanced ground proximity warning system (EGPWS), both introduced by predecessor companies acquired by Honeywell Aerospace. TAWS is a later term the encompassing products of all manufacturers.

In simple terms, RTCA special committees and European Organisation for Civil Aviation Equipment (EUROCAE) working groups establish minimum operational performance standards, and write TAWS technical standards and certification/upgrade methods for adding proprietary, patented functions.

As of early 2020, expert teams at these organisations are cooperating to produce what essentially will be the next generation of HTAWS (see References below). Manufacturers will continue to add complementary capabilities to address the needs expressed by the two mission segments mentioned, and by stakeholders in other helicopter segments.

HTAWS Benefits and Trends

One external research report, part of the U.K. Civil Aviation Authority’s (U.K. CAA’s) work on HTAWS enhancements, sought to mitigate human factors issues discovered in a few North Sea accidents where first-generation HTAWS were on board. The authors summed up: “Controlled flight into terrain is a major cause of accidents in helicopter operations which terrain awareness warning systems (TAWS) could help to address.

“However, existing HTAWS are not considered to be optimised for the offshore operations undertaken by the majority of the UK’s medium/large helicopter fleet, and would have offered little or no protection in the case of the accident scenarios that have been experienced in that environment. The objective of the research was therefore to seek to identify improvements to HTAWS to improve warning times for offshore operations without incurring an undue number of nuisance alerts.”

Projected HTAWS Capabilities

RTCA Special Committee 237, in collaboration with U.K. CAA and EUROCAE Working Group 110 counterparts, agrees that offshore safety could be enhanced with flight envelope protection. This idea is similar in concept to TAWS “classic modes” (also called Class A TAWS). As of mid-2019, classic modes for offshore operations were proposed for a new CAP 1519, a specification titled “Offshore Helicopter Terrain Awareness Warning System Alert Envelopes.”

RTCA SC-237 says, “RTCA Current Helicopter Terrain Awareness and Warning System (HTAWS) Minimum Operational Performance Standards (MOPS) — RTCA DO-309 — do not include the classic modes that are included in TAWS. Furthermore, the European Commission has mandated installation of a Class A HTAWS for new aircraft for offshore operations … however, there is no ‘acceptable standard’ or formal definition for Class A HTAWS provided to meet this requirement.”

The MOPS are necessary for creating a technical standard to support the European air operating rule mandate. The committee said, “HTAWS provided with the classic modes defined in UK CAA CAP 1519 can provide a significant (four major accidents in UK operations alone could have been avoided) and very cost-effective (on the order of $20k per aircraft) improvement in the safety of offshore helicopter operation, and would address a number of UK Air Accidents Investigation Branch safety recommendations. The MOPS will promote and facilitate the introduction of appropriate HTAWS.”

As of February 2020, RTCA SC-237 and EUROCAE WG-110 are preparing new or upgraded: RTCA DO-309, Minimum Operational Performance Standards (MOPS) for Helicopter Terrain Awareness and Warning System (HTAWS) Airborne Equipment; TSO/ETSO-C194, Helicopter Terrain Awareness and Warning System (HTAWS); UK CAA CAP 1519, Offshore Helicopter Terrain Awareness Warning System Alert Envelopes; and UK CAA CAP 1538, Class A Terrain Awareness Warning System (TAWS) for Offshore Helicopter Operations.

For this article, SKYbrary editors reviewed brief descriptions of specific HTAWS products. One or more of the manufacturers emphasise the following examples of today’s most advanced benefits or features:

  • Integrated three-dimensional (3D) visualization, displays, sensors and database components;
  • High-resolution terrain imagery;
  • Sourcing of multiple specialised databases for airports, obstacles and terrain;
  • Improved proprietary high-accuracy data;
  • Helicopter-specific alerting algorithms;
  • Forward-looking terrain and obstacle–avoidance capability;
  • Visual and aural advisories (including voice alerts about relative conflict threat level) designed for helicopter pilots and their mission profiles;
  • Integration of HTAWS data with 3D synthetic vision system, traffic-alerting and collision avoidance system, weather radar and a separate obstacle warning system;
  • Height-above-terrain voice callouts at pilot-selected height ranges;
  • Pilot-selectable modes for of minimising/suppressing nuisance alerts (enabling low-level operations and off-airport landings while keeping protection from terrain and obstacles); and,
  • Significantly more helicopter-relevant obstacle data compared with fixed-wing TAWS,

Accidents and Incidents

On 18 February 2009, the crew of Eurocopter EC225 LP Super Puma attempting to make an approach to a North Sea offshore platform in poor visibility at night lost meaningful visual reference and a sea impact followed. All occupants escaped from the helicopter and were subsequently rescued. The investigation concluded that the accident probably occurred because of the effects of oculogravic and somatogravic illusions combined with both pilots being focused on the platform and not monitoring the flight instruments.

On 12 March 2009, a Sikorsky S-92A crew heading offshore from St. John's, Newfoundland declared an emergency and began a return after total loss of main gear box oil pressure but lost control during an attempted ditching. The Investigation found that all oil had been lost after two main gear box securing bolts had sheared. It was noted that ambiguity had contributed to crew misdiagnosis the cause and that the ditching had been mishandled. Sea States beyond the capability of Emergency Flotation Systems and the limited usefulness of personal Supplemental Breathing Systems in cold water were identified as Safety Issues.

On 23 August 2013, the crew of a Eurocopter AS332 L2 Super Puma helicopter making a non-precision approach to runway 09 at Sumburgh with the AP engaged in 3-axes mode descended below MDA without visual reference and after exposing the helicopter to vortex ring conditions were unable to prevent a sudden onset high rate of descent followed by sea surface impact and rapid inversion of the floating helicopter. Four of the 18 occupants died and three were seriously injured. The Investigation found no evidence of contributory technical failure and attributed the accident to inappropriate flight path control by the crew.







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