Helicopter Ditching Limitations
The European Aviation Safety Agency (EASA) is proposing introducing Emergency Floatation System ditching limitations into Rotorcraft Flight Manuals (RFMs) and that these are then a limitation for operations. This is in broad alignment with a UK CAA Safety Directive effective 1 September 2014.
On 30 May 2014 EASA issued a Proposed Airworthiness Directive (PAD) for public comment. That PAD (14-089) focused on EU helicopter Type Certificate Holders (TCHs). On 4 August 2014 EASA issued a PAD (14-130) to address non-EU TCHs. These will support European rulemaking on offshore helicopter operations (on which EASA published the Comment Response Document on 15 August 2014). EASA note that in accordance with “Paragraph 4.a of the Essential Requirements for air operations, aircraft must be operated within their certification limits”.
The ADs introduce a limitation in the RFMs based on the Sea State that the ditching performance (i.e. the water entry and floatation stability) the helicopter has been certified to. Some more recent helicopters do include this information, but the ADs ensure consistent data in all RFMs.
EFS are certificated against airworthiness standards (for large helicopters EASA CS-29 and US FAA FAR-29) that require demonstrating helicopter ditching performance under “reasonably probable water conditions”. What is “reasonably probable” of course depends on the operating environment. With the growth of offshore helicopter operations in hostile environments, such as the North Sea, Norwegian Sea and North Atlantic, increasingly what is “reasonably probable” is more severe than assumed when this rule was first drafted. Unfortunately, the advisory material defined “reasonably probable” as “at least sea state 4” and in a number of cases Sea State 4 is all the EFS has been substantiated to. More recent designs have tended to have demonstrated higher performance levels, mindful that the oil and gas market for large helicopters is substantial.
EASA have chosen to continue to use the World Meteorological Organisation (WMO) scale of Sea State, although also stating the associated wave height range.
The sea state basically specifies wave height. Wave height depends on local winds but on remote winds also (swell).
As far as weather observation is concerned, while sea state reporting remains a legal international practice, with modern in situ observing techniques we try to avoid using sea state or Beaufort scale as we prefer direct readings from appropriate instruments…
Swell is created by winds as much as a hundred miles away or more, is relatively unidirectional and results in wavelengths of a hundred meters or more and wave periods of 9 seconds or more. The distance the winds travelled to create swell is known as the fetch.
Local winds create more varied waves, with shorter wavelengths and periods.
Depending on the local geography and weather conditions, waves can be created by different combinations of local winds and swell. So for example in the Southern North Sea, fetch is limited by the adjacent land masses and waves are predominately driven by local winds. West of Shetland, in the Atlantic, the fetch is great and swell predominates.
The WMO Sea State definitions are as follows (Sea State / Description / Wave Height in metres):
0 Calm (glassy) 0
1 Calm (rippled) 0 – 0.1
2 Smooth (wavelets) 0.1 – 0.5
3 Slight 0.5 – 1.25
4 Moderate 1.25 – 2.5
5 Rough 2.5 – 4
6 Very rough 4 – 6
7 High 6 – 9
8 Very high 9 – 14
- 9 Phenomenal Over 14
You will notice that peak wave height of Sea State 6 is 240% greater than for Sea State 4.
UK Civil Aviation Authority CAA Paper 2005/06 (Summary Report on Helicopter Ditching and Crashworthiness Research) gives some statistical data on sea states along a series of North Sea and West of Shetland helicopter routes. According to CAA figures, a Sea State 4 capability would be exceeded 27.7% of the time, averaged over six North Sea routes, compared to just 1.4% with a Sea State 6 capability.
In the CAA CAP1145 report, which followed the recent CAA Offshore Helicopter Safety Review, Action 6 related to a restriction on operation in high seas states. This was ultimately turned into regulation in Safety Directive SD-2014/001, with an adjustment be based on significant wave height to avoid any uncertainty related to the actual size of the waves. From 1 September 2014 offshore flights can only be conducted in the UK if:
…the significant wave height of the sea over which the flight will be conducted to or from an offshore location does not exceed the certificated ditching performance of the helicopter.
The EASA ADs contain proposed limitations based on the ‘factory option’ EFS. The proposed limits are as follows:
Sea State 4
- Airbus Helicopters AS332C, AS332C1 & AS332L
- Airbus Helicopters AS332L1 (without MOD OP.26277)
- Airbus Helicopters SA365 Dauphin & EC155 (UPDATE March 2015: a SS6 modification for the EC155 is now certified & UPDATE March 2016: a SS6 modification for the AS365 is now also certified)
- Airbus Helicopters BK117
- AgustaWestland AB212 & AB412
- Bell B212, B214, & B412
- Sikorsky S-76
- Sikorsky S-92A (with MTOW < 8.4t with 3 or 5 floats)
Sea State 5
- Sikorsky S-92A (with MTOW > 8.4t with 3 floats)
Sea State 6
- Airbus Helicopters AS332L1 (with MOD OP.26277)
- Airbus Helicopters AS332L2
- Airbus Helicopters EC225
- Bell B222, B230 & B430
- Sikorsky S-92A (with MTOW > 8.4t with 5 floats)
EFS installed under Supplemental Type Certificates may differ but the ADs require appropriate limits to be included in the RFM supplement for the STC with Sea State 4 to be assumed unless proven otherwise.
UPDATE: 14 October 2014 CAA issued a new Operational Directive. Note this has since been revised and issued several times.
UPDATE: 8 December 2015 The latest directive is SD-2015/005: Offshore Helicopter Operations
UPDATE 24 January 2016: CAP1145 Helicopter Water Impact Survivability Statistics – A Critique
UPDATE 23 March 2016: EASA published NPA 2016-01 Helicopter ditching and water impact occupant survivability on a 3 month public consultation period.
Previous studies on and accident investigations into helicopter ditchings and water impact events have highlighted inadequacies in the existing certification specifications (CS-27, CS-29) and in the rules governing offshore operations. In particular, it has been established that in an otherwise survivable water impact, most fatalities occurred as a result of drowning because the occupants were unable either to rapidly escape from a capsized and flooded cabin, or to survive in the sea for sufficient time until rescue. Furthermore, the testing environment in which helicopters are type-certified for ditching bears little resemblance to the sea conditions experienced in operation.
In order to thoroughly address these and other ditching-related issues, and due to the nature of ditching-related hazards, this rulemaking task (RMT.0120 (27&29.008)) has taken a holistic approach to the problem, which crosses traditional airworthiness/operational boundaries. A detailed risk assessment has been undertaken that reflects both certification and operational experience and builds upon data extracted from accident reports and previous studies.
The specific objective of this NPA, however, is to propose changes to CS-27 and CS-29… Retroactive rules are to be considered in a second phase of this RMT.
…the primary change proposed aims to establish a new ditching certification methodology by which a target probability of capsize following a ditching can be determined based on the level of capsize mitigation applied to the design.
Aberdeen based aviation consultancy Aerossurance has extensive helicopter safety and operations experience. For advice you can trust on helicopter airworthiness, safety, regulatory or survivability matters, contact us at firstname.lastname@example.org
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