July 30th, 2017

Helicopter Terrain Awareness Warning System (HTAWS) - What Next?

HTAWS began to be fitted to the newer generation of helicopters, such as the S92, H225 and AW139 when they were introduced into the market. At the time, the principle HTAWS available was the Honeywell Enhanced Ground Proximity Warning System (EGPWS) MKXXII. The thinking behind the fitting of HTAWS was that rotary wing operations could benefit from the use of this equipment that had been so successful in fixed wing operations.
 
However, a direct read-across is probably not valid as aeroplanes operate from airfields where the local terrain in well mapped in the TAWS database and so the Ground Proximity element can be desensitised to reduce false alerts.
 
The FAA saw a need for improved terrain warning for helicopters, in particular for HEMS operations where a number of well publicised accidents occurred in the USA. I was a member of RTCA Special Committee 212 which had the mandate to produce MOPS for a Class B HTAWS, i.e. a system without a radio altimeter input which derived altitude information from GPS: DO 309 was the resulting standard.
 
For offshore operations, a different approach was needed to warn crews of an unsafe descent rate or proximity to the surface as accident had occurred which had not been, or in the case of legacy helicopters would not have been, prevented by the extant HTAWS warning envelopes. A UK CAA group was formed using industry funding to further enhance the capabilities of HTAWS for offshore operations. As well as Flight Data Monitoring (FDM) and accident data, we had some AAIB recommendations to consider.
 
UK AAIB issued a number of Safety Recommendations regarding H-TAWS:

• Timeliness of warnings (2011-061);
• Frequency of nuisance warnings (2011-062);
• Quality of obstacle database (2011 -063);
• Improved warnings (2016-012);
• Mandate H-TAWS (2016-013).

 
We were ready to finalise our recommendation in 2013 when the accident to G-WNSB occurred during at approach to Sumburgh airport. During our analysis we confirmed that our new envelopes would not have provided a sufficient alert to the crew and so further analysis was conducted. During this additional analysis we obtained FDM data on several serious incidents where a loss of airspeed had been the precursor to a loss of control. Using data from over 100,000 approaches on various helicopter types we produced a new warning envelope which alerts the crew to a lack of power applied to maintain airspeed. In early 2017 the CAA issued CAP 1538 and CAP 1519 which document the work undertaken and the proposed offshore warning envelopes.
 
CAP1538 -
http://publicapps.caa.co.uk/modalapplication.aspx?appid=11&mode=detail&id=7887
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CAP1519 -  
https://publicapps.caa.co.uk/modalapplication.aspx?appid=11&mode=detail&id=7886
 
HeliOffshore has been supportive in aligning the equipment and airframe manufactures, as well as the operators and oil companies. It is intended that the first aircraft to be fitted with the new certified warning envelopes will be in early 2018.
 
Of course, generating the alerts is only part of the story, as the crews must also perceive the warnings and react appropriately. For example, the incident where G-WIWI nearly came to grief during a night approach to a private house at Peasmarsh in Sussex. Phase 2 of this project, funded by HeliOffshore, is looking at the optimum way to warn the crew. This work has been undertaken by experts from Cranfield University and Royal Holloway University and should report soon.
 
So, what is next? EASA is likely to start a EUROCAE group to address the issues raised and the CAA recommendations – we are meeting EASA on the 26th October to discuss the next steps.