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Ditching: Rotary Wing Aircraft

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Category: General General
Content source: SKYbrary About SKYbrary
Content control: Air Pilots The Honourable Company of Air Pilots

Definition

The controlled emergency landing of a helicopter on water.

A Difficult and Demanding Manoeuvre

British Airways helicopter Sikorsky S-61N, G-ASNL ditched in the North Sea, 75 nm North-east of Aberdeen on 11 March 1983. Source: Wikicommons, Author: Morrispeter

Ditching an aircraft into the sea, a river, or a lake, is a manoeuvre that, by definition, cannot be practiced and may be extremely hazardous. It is only contemplated when no other realistic option remains in an emergency or, in the rotary case, imminent emergency situation.

For helicopters, ditching may be prompted by a sudden-onset emergency or where continued safe flight to the nearest designated landing location or even the nearest suitable land is not assured; it will often be accompanied by considerable structural conditions outside of the “norm", such as tail rotor failures or very high vibrations. All of these contribute to the already considerable pressure of getting the aircraft down and evacuating it.

Even if the Sea State is slight, there is a strong possibility that a helicopter will fail to remain upright and/or will sink rapidly. If the occupants survive the ditching then, even with the benefit of protective clothing and equipment, in cold waters they may succumb to cold shock or hypothermia before they are rescued.

Scenarios

A controlled ditching might be made in anticipation of a loss of control or its actual onset. Other potential causes of an impending loss of control which could prompt a decision to ditch include uncontrollable and developing fire or structural failure and, for rotary wing aircraft over water, indications of imminent power transmission failure or loss of rotor integrity for which the procedural response is 'Land Immediately'. Rotary wing aircraft may also need to 'land immediately' for other reasons, including the failure of a single engine.

Considerations when Ditching a Rotary Wing Aircraft ("Power On")

The majority of rotary wing ditchings, especially those carried out in hostile environments such as the North Sea or off Newfoundland in Canada will be those of transport helicopters or SAR support for those and other operations where the helicopter involved has type certification which relates to ditching. Such helicopters will include a number of Quick Reference Handbook (QRH) responses to malfunctions which, although power will still be available require “Land Immediately rather than “Land as soon as possible”. The following generic remarks are focused on this scenario rather than the wider case because it predominates in respect of both survivable and fatal accidents on account of the high proportion of risk exposure which it represents. Always bear in mind that there is much to be learned from the investigation reports which usually follow both ditchings and survivable water impacts. Some of these may be found in a listing of helicopter accident and serious incident reports.

Whilst there are many variables in any evolving incident, a controlled “power on” ditching will be considerably easier than a "power off" autorotation to the sea; an autorotation to an angry sea in the dark needs immediate transition to trained automatic actions, as time is extremely limited. Ultimately, success will be determined by the rigorous and comprehensive training that should be undertaken on a regular basis. It cannot be stressed enough that thorough and ongoing training is the key to survival in these (and most) circumstances, and that crews should regularly take the opportunity to discuss situations and procedures when possible.

Few emergencies ever go according the script, and ditchings are no different. The priority should be, as always, “fly the aircraft”; training and procedure will get you into the water in one piece. Helicopters often fly at relatively low altitudes, and generally a decision to ditch (apart from impending fuel starvation) tends to be reached fairly quickly as the aircraft malfunctions around us. Good CRM and repeated training, is the key.

As it is for crew, so it should be for our passengers. Inverted in a hull rapidly filling with water is not the time to ponder the correct way to activate one’s re-breather or open and exit with window/hatch. In an ideal world this should be done without conscious thought, as a direct result once again from good training and a positive will to survive.

Facilitating Rescue after ditching

  • Ensuring Awareness' of the Emergency. If in a radar environment, a MAYDAY transmitted on the ATC frequency in use should suffice. Otherwise, distress calls should be made 121.5 or allocated UHF or HF frequencies and (if time permits and not at the expense of more pressing actions) A7700 should be selected on the transponder. If feasible, operate Emergency Location Transmitters (ELT).
  • Position. Ensure that any agency which has responded to the MAYDAY declaration is made aware of the likely location of a ditching. (Bear in mind a helicopter committed to ditching will not travel very far from its initial descent point. Depending on altitude, the distance will only be a matter of a few miles at best.) Agencies include ATC, other aircraft or boats and, where possible, directly to the emergency services such as the Coastguard.
  • Shipping. Survival after a ditching will often depend on how quickly you are picked up so unless it is going to be possible near an offshore platform then ditching within line of sight of any vessel which can be located makes a lot of sense,
  • Ensure Floatation. The Helicopter Emergency Floatation Systems (EFS) must be armed as soon (within the limitations of the flight manual) as it becomes obvious that a ditching is inevitable.
  • Cabin Preparation. In the time available, passengers should be forewarned of the impending ditching as soon as possible to allow them a mental rehearsal of the evacuation and survival techniques which will be needed. This may be accomplished by the flight crew but cabin crew (if present) should be prepared to supplement any announcement and, if necessary, provide a complete briefing. Many (although not all) oil and gas support helicopters have push out windows as emergency exits. These plastic windows are held in place by a rubber tear out seal around the window that must be removed before exit. Once removed the windows are pushed out by hand. Consider briefing passengers to remove the seal (but not push out the windows) before touchdown, facilitating emergency exit should the aircraft capsize.

The Ditching

  • Final Approach and touchdown. Determine the best direction to approach the ditching location in relation to sea state and wind direction. Unless the wind is calm, the chances are that the direction of the wind will differ from that of the swell. Where the swell is more marked, it may be advisable to ditch along the swell accepting a crosswind component and the higher touchdown speed, thus minimising the potential for nosing into the face of the rising swell. Radio Altimeter height callouts from the PM are likely to be of assistance both by day and by night. The overriding objective should be to maximise the chances the helicopter remaining upright. Clearly a low forward speed, low rate of descent and appropriate pitch attitude will all be of importance. Monitoring the horizon if possible at touchdown, rather than the sea at one’s feet, should reduce the tendency to “hunt” the swell, facilitating the landing. In the dark, the extent to which external lighting may assist touching down will be a matter of judgement based on the prevailing circumstances. Since judging sea state at night may be difficult, a power-available ditching will allow a low-level pre-inspection of the sea surface and may be worthwhile.
  • Evacuation. Once on the surface, the engines should be shut down if running and the main rotor stopped. In most cases, Rotor Brake should not be used as the transfer of energy thorough the aircraft could be enough to capsize it. Once the rotor has stopped, the occupants should be instructed to begin evacuation. The primary objective will be to deploy and board the liferafts carried without undue delay. After a headcount, these should be detached from the helicopter to avoid being dragged down should the helicopter then capsize fully - but attached to each other if more than one is being used. Where a liferaft “long line” has a shear type connection that is designed to tear free if the helicopter sinks, remaining attached to the helicopter can be considered, to remain close to the scene of the accident where SAR (Search And Rescue) will begin its search and where surface oil/debris and aircraft provide added visual cues for the searchers. There will be a survival knife in the liferaft’s kitbag if it becomes necessary to cut free from the aircraft. In all cases, if there is more than one liferaft they should be attached to each other. Deploy the liferaft sea anchor to improve its stability and, as early as possible, administer seasickness tablets to everyone; it is almost inevitable that when one person vomits the rest will follow, increasing the survivors’ fatigue and dehydration and reducing their morale.

Post Ditching Survival

Many aspects of post ditching survival are covered by the actions described above. Training and procedure may get you down into the water safely; leadership and strong mental attitude is needed to get out. The environment the survivors will now find themselves in will certainly be unfamiliar and unexpected and may be extremely hostile. Continued survival will depend on the water and air temperature, the wind and sea state, the physical and mental condition of individuals, the clothing worn and the availability of useful survival equipment amongst many other things. Swift location and rescue may well be of crucial importance. For all occupants of rotary wing offshore flights, prior familiarisation with and recurrent training in survival equipment, evacuation procedures and maritime survival is of paramount importance; there will not be an opportunity to practice in a live accident. The references in 'Further Reading' below offer considerable discussion of the issues involved in survival following a ditching.

Training for Ditching and the Aftermath

Full Flight Simulator practice of ditching by pilots is impossible because there is no data with which to meaningfully program the simulator. For rotary wing crew regularly operating offshore and their passengers, realistic practice of some aspects of evacuation and post evacuation scenarios is available and usually is a significant part of training/familiarisation. Regular training in helicopter underwater escape should continue throughout aircrew’s careers and is equally important for passengers. Although all helicopters operating offshore public transport are likely to be equipped with emergency floatation systems and Automatic Deployment Emergency Locator Beacons and the occupants provided with some form of survival suit and emergency breathing equipment, this preparation for survival after ditching addresses the need for prompt evacuation before any capsize of the floating helicopter. The installation of floats towards the top of the fuselage could clearly sustain a helicopter that had rolled on the water 90° to the vertical. However, such modifications carry significant technical challenge to avoid the risk of an inadvertent inflation interfering with the main rotor and creating an accident in the first place.

As has already been stressed, for the rotary wing case, much of the training / familiarisation for the aftermath of ditching is of equal relevance to survivors of a water impact accident such as the unintended descent into the sea of a Super Puma Helicopter near Sumburgh, Shetland Isles in 2013 following which, UK CAA introduced new regulations regarding window size, the number and location of passengers and the placement of XBR (extra broad) passengers within the cabin.

Examples of Rotary Wing aircraft Ditchings

  • EC25, en-route, 20nm east of Aberdeen UK, 2012 - On 10 May 2012, the crew of a Eurocopter EC225 LP on a flight from Aberdeen to an offshore platform received an indication that the main gearbox (MGB) lubrication system had failed. Shortly after selecting the emergency lubrication system, that also indicated failure and the crew responded in accordance with the QRH drill to “land immediately” by carrying out a successful controlled ditching.
  • EC25, en-route, 32nm southwest of Sumburgh UK, 2012 - On 22 October 2012, the crew of a Eurocopter EC225 LP on a flight from Aberdeen to an offshore platform received an indication that the main gearbox (MGB) lubrication system had failed. The crew responded in accordance with the QRH drill by carrying out a successful ditching procedure.
  • AS3B, vicinity Den Helder Netherlands, 2006 - On 21 November 2006, the crew of a Bristow Eurocopter AS332 L2 making an unscheduled passenger flight from an offshore platform to Den Helder in night VMC decided to ditch their aircraft after apparent malfunction of an engine and the flight controls were perceived as rendering it unable to safely complete the flight. Despite extensive investigation, no technical fault which would have rendered it unflyable could be confirmed.

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Further Reading