Helicopter operations in the maritime environment

Citation

(2001), "Helicopter operations in the maritime environment", Aircraft Engineering and Aerospace Technology, Vol. 73 No. 4. https://doi.org/10.1108/aeat.2001.12773dac.001

Helicopter operations in the maritime environment

Helicopter operations in the maritime environment

For some time helicopters have been operated in both the civil and military roles in a wide range of important maritime duties. This conference at the Royal Aeronautical Society concentrated on the various operational aspects of, for example, the helicopter's key role in the North Sea industry and their use in search and rescue as well as the increasing military emphasis on anti-submarine warfare and other uses.

The first contribution came from WS Atkins Science and Technology UK, and described research into the development of a system to indicate to helicopter pilots, during their approach to helidecks located on ships and floating offshore platforms, the likelihood that their aircraft would tip, slide or otherwise lose equilibrium whilst on deck. This investigation was commissioned by the CAA during the 1990s and had the objectives set out here.

First, to identify which parameters would be required to quantify the severity of helideck motion, independent of vessel type and helideck location on the vessel. Next, to establish how these parameters might be consolidated to form a single index of helideck motion and indicate how appropriate limits would be established for a given helicopter type in terms of this index. Then to establish the minimum period of observation required to achieve sufficient statistical accuracy in the measured parameters for engineering purposes. Also, to demonstrate the practicality of providing a low cost instrumentation package to automatically measure the required parameters, carry out data processing, and present the information in a suitable form to the HLO and pilot.

The paper showed how it has been demonstrated that the ratio of lateral deck acceleration to vertical acceleration (including gravity), provides a meaningful measure of the inertia loads applied to an aircraft, which is directly linked to its stability. This ratio, the measure of motion severity, can be processed statistically from time history records gathered over periods as short as ten minutes prior to the arrival of the aircraft. These statistics can give a reliable forward prediction of most likely maximum values of deck motion severity for the time that the aircraft is on the deck, the MSI.

The MSI can be directly compared with an aircraft type specific limit of operability, which is itself governed by the wind speed and direction over the flight deck. This limit of operability can be reliably calculated from considerations of aircraft equilibrium, and based upon specific tipping or sliding failure modes. Further work is however, necessary, and is proceeding.

Helicopter deck handling systems

This was considered by Lockheed Martin and the initial view was given that launch and recovery of a helicopter from a small deck in adverse conditions is paramount to the basic concept of anti-submarine warfare. It extends the ship's fighting effectiveness by orders of magnitude. Recovery of the aircraft safely is very necessary for continual operation and the widest operational envelope is required; who flies first has the tactical advantage. The ship/aircraft interface needs clear definition and ownership. The concept of operation should be defined before a design is set. Big decks and fine weather would be the option of choice.

Free landing is preferred by UK and most European countries. This is an aircraft carried harpoon or deck lock fired into a ship secured grid. Secondary mechanisms used to subsequently restrain and move, lashings used to secure. Assisted landing is preferred by USA and many Pacific rim countries. In this the aircraft probe is engaged into rail mounted carriage built into ship, movement and restraint by primary mechanism, lashing used to secure. Both systems impact ship and helicopter design. Interoperability is a real problem.

A braced deck lock may be used which can also serve as a towing attachment. There is an interface problem for aircraft structural loading through attachment points, and aircraft stability and security during ranging and stowing, and rotor run up and stopping.

The MOD study of 1995 identified three distinct handling attachment options for Merlin:

1. 1.

   continued use of existing or modified spurs (eight solutions);

2. 2.

   introduce probe (three solutions);

3. 3.

   extend the deck lock capability (one solution).

Option (1) required no major modification of the aircraft whilst options (2) and (3) both required considerable aircraft modification.

Solutions are being worked on and meanwhile, for the future, unmanned air vehicles (UAVs) (are coming, which are seen as a solution (smaller aircraft off a small ship)). Simplicity of operation continues to be a goal and concept of use and total product must be considered simultaneously to produce the most effective system.

Lighting and guidance

Two papers from DERA followed, the first concerning helideck lighting. DERA Bedford has been asked to conduct a series of flight trials on behalf of the Safety Regulation Group of the CAA, to develop and demonstrate improved offshore methods. The results of the flight trials are detailed.

The final approach and landing phases of all offshore helicopter operations are carried out by reference to visual cues that are mainly derived from the destination platform. Even of those occasions when an instrument approach procedure is in use, the latter stages of the operation remain a visual task.

For successful approach and landing, the following visual tasks are necessary:

• •

  platform location;

• •

  platform identification;

• •

  helideck acquisition;

• •

  final approach; and

• •

  hover and landing.

Between November 1998 and March 2000, flight trials took place at the NAM K14 complex in the Dutch Sector of the North Sea. Equipment evaluated on the flight trials included green helideck perimeter lighting; green perimeter electro-luminescent panels; and floodlights and hoods. Equipment configuration on K14B helideck is shown in Figure 1. Full results are not yet completed, but useful information has been obtained. It is recommended that ICAO adopt:

Figure 1 Experimental equipment locations on K14B

• •

  green perimeter lighting;

• •

  illuminated H, using luminescent panel lighting; and

• •

  illuminated aiming circle, using LED, or similar strip lighting technology.

Also, that an in-service trial is conducted to demonstrate the applicability of the proposed visual aids in a wider range of meteorological conditions, and to expand the pilot sample size. The requirements for floodlighting should be reviewed, and test methods to assess the photometric characteristics of LED strips should be developed to enable specifications to be published and equipment compliance verified.

Another contribution, also from DERA, was on deck landing guidance systems, which took a whole systems approach to the challenges at the helicopter ship dynamic interface. It examined the problems of operating large helicopters from small ships in adverse weather conditions, with due emphasis on the recovery to the ship after the mission. It addressed both manual and automatic operations, including flight path management and control, flight control systems, and visual and non-visual guidance systems. Research at DERA has been undertaken in support of naval helicopter operation, with particular reference to large helicopters such as the Merlin HM Mk 1, operating from small ships.

The problems include approach to the ship and ship and deck environment, particularly in adverse conditions. After a successful formation with the ship, the final stages of recovery, the transition over the deck and the land on are affected by the pilot's cueing environment in poor weather and at night when there is no external horizon reference. Other difficulties are also present. The limits imposed on deck landings and take-offs by wind condition and ship motion are expressed in terms of ship helicopter operating limits or SHOLs, which are usually expressed in diagrammatic form. Integration of human factor considerations into the specification, design and testing of a new system is critical.

The key technology for the recovery of both rotary and fixed wing aircraft to offshore platforms is that which provides accurate relative positioning information such that aircraft can be guided along pre-defined trajectories using information displayed in the cockpit. For single pilot aircraft, such as the Navy's Merlin, ensuring that the pilot's workload is maintained at safe levels is a key requirement.

The ship aircraft integrated landing system (SAILS) research programme at DERA Bedford is based on GPS technology. The main element of this is the high integrity GPS guidance enhanced receiver, or HIGGER, based on the Raytheon STR2515 GPS receiver. This exists in two forms, HIGGER I and II. The HIGGER I receiver is currently in use for flight tests. It can support a number of advanced modes requiring datalink capability. The HIGGER II system supports additional features and provides four position outputs.

The initial work was performed using the DERA trials Wessex, and included the development of approach guidance systems and pilot displays to aid in the recovery task. The objectives of the second phase of the work were to produce a piloted flight simulation of the approach and landing guidance system in a realistic environment. The Merlin flight simulator at GKN-Westland, Yeovil, was used. Work included the dynamic-interface environment, airwake and its impact on SHOLs, wind tunnel investigations, and the role of computational fluid dynamics (CFD) in airwake investigations. Other subjects included:

• •

  helicopters operating close to hangars, islands, etc.;

• •

  augmentation for manual recoveries;

• •

  visual aids for manual recoveries;

• •

  luminescent panels;

• •

  ship helicopter approach recovery kit (SHARK);

• •

  landing period designator;

• •

  quiescent period predictor; and

• •

  various aspects of flight control system augmentation.

Overall, the paper provided a review of future systems, and particularly visual and non-visual guidance systems aimed at the recovery of helicopters to ships at sea.

Helideck environment hazards

From BMT Fluid Mechanics came this paper which describes results from a major research project on the environment around offshore helidecks, and the way in which helicopters respond to the aerodynamic disturbances. This work was begun in 1997 and is expected to be completed early in 2002. It addressed various issues but is particularly concerned here with the response of the helicopter and pilot to the aerodynamic effects.

Accidents and incidents have highlighted the principal salty hazard and source of highest workload which, according to a survey of pilots, has been cited as turbulence within the wake region downwind of an offshore installation. Improvement of helicopter airworthiness has been accomplished and been made through health and usage monitoring (HUM) systems. The feasibility of applying operational monitoring techniques on a routine basis to offshore helicopter operations has been studied. Another facet of the safety issue is examined here; identifying the aerodynamic disturbances that occur in close proximity to offshore platforms.

The aerodynamic hazards around such a platform are:

• •

  wind flow round the platform and helideck;

• •

  hot exhaust plumes (power generation);

• •

  release of process gas; and

• •

  flared gas (flare towers).

The responses of the helicopter can vary in these differing circumstances, from loss of height due to down draft, hot gas plumes potentially inducing engine surge and possible flame-out, to loss of engine control and again, possible flame-out, due to process gas releases.

Helicopter response to turbulence is more difficult to quantify due to the range of frequencies (or eddy sizes) that can be encountered. No guidance criteria have, to date, been developed to deal with turbulence. The greater the severity of the turbulence experienced in the final stages of landing, then the harder the helicopter pilot must work. The harder the pilot has to work then the less attention he will have for other important tasks. The Cooper-Harper rating is an established method of evaluating aircraft handling qualities. In Figure 2, the acceptability of rotorcraft flying qualities for mission tasks is quantified in terms of three levels shown alongside the C-H rating. Level 1 corresponds to good flying qualities, Level 2 to those with tolerable deficiencies, and Level 3 to those with major deficiencies.

Figure 2 The Cooper-Harper rating

A study has been made of the complex response of a helicopter to disturbed air flow, and the stabilising and guidance actions of the pilot. Flight simulation has been used extensively, comparison of results with extensive flight trials have been encouraging, but further work requires to be done.

Military trials

From DERA came an explanation of the UK approach to deriving ship helicopter operating limits (SHOLs). Generally, this is aimed at providing the widest possible envelopes in terms of wind speed and direction relative to the ship together with maximum deck motion limits. Trial prerequisites include performance and handling data and it is necessary to also have a means of assessing the acceptability of each landing, and this is achieved using rating scales. This is the deck interface pilot effort scale (DIPES), with 1-3 being acceptable and 4-5 not acceptable.

The output from any trials programme needs to be wind envelopes for use by day and by night – together with associated deck motion limits. Aircraft and ship instrumentation requirements need to be established with sufficient time available to install such equipment. Based on the trials results the operational limits are published to the fleet on a polar diagram covering wind speed and direction for each particular mass band. A read across philosophy has been established over the years, enabling aircraft data derived on one ship type to be used to provide SHOLs either for another ship type for that same aircraft or a clearance for another aircraft type on the original ship.

Simulation trials have also taken place and it has been found feasible to use modelling and simulation in this scenario and potentially produce operating envelopes. Future developments include international collaboration and further advances in the integration of flight test and simulation.

Application of HOMP

Difficulties can be mitigated though the use of data generated by a helicopter operations monitoring programme (HOMP). Flight data monitoring (FDM) for fixed wing aircraft has been well known for some considerable time, and HOMP trials have been proceeding since the beginning of 1999 and will continue until the end of August this year.

Consideration is given to structure-induced turbulence and downdrafts, hot exhaust gas plumes, gas, flame and smoke from flare stacks, unburned hydrocarbon gas discharge and obstacles on the final approach or take off/go around path, as well as poor meteorological and installation data reports from unqualified observers.

The HOMP system performs two types of analysis:

1. 1.

   Event analysis, which detects exceedances of pre-defined operational envelopes and provides information on the extremes of the operation.

2. 2.

   Measurement analysis, which takes a set of measurements on every flight and provides information on the whole operation.

A wide range of events have been detected during the HOMP trial, providing information on a large variety of potential operational risks. Examples include turbulence; flight through hot gas; overtorque on landing on platform with wind in the turbulent sector; deck out of limits for landing; aircraft hit by a line squall on an offshore platform; go-around following an offshore airborne radar approach; and take-off with cabin heater on.

The HOMP measurements allow the transparent gathering of vast amounts of operational data, and the measurements allow a hypothesis about how a particular issue has arisen to be tested. The trial has demonstrated a near term, low risk and cost effective solution for making pro-active use of the flight data to bring about significant improvements in the safety of offshore operations.

Ditching

Two papers followed, one from the CAA on certification for ditching and the other on the RN approach to crew safety following ditching. The first of these aimed to review the airworthiness standards currently associated with both intentional ditching and unintentional water impact, in the light of service experience and ongoing research into occupant safety and survivability, and to recommend where improvements should be made.

The requirements specify that flotation and stability must be demonstrated in reasonably probable sea conditions. Also, that measures must be taken to minimise the probability that in an emergency landing on water, the behaviour of the rotorcraft would cause immediate injury to the occupants or would make it impossible for them to escape. Experience suggests that the greatest risk to the occupants in a ditching is drowning due to inability to evacuate to aircraft following a capsize and subsequent flooding of the hull. Although sea keeping qualities vary from one helicopter type to another, most types currently in use will capsize in sea states in the range 4-5 and above.

Various means have been considered and as part of its ongoing investigation of the stability of ditched helicopters, the UK CAA instigated research into novel flotation devices, the two most promising considered to be buoyant cowling panels in areas close to the main rotor, and additional flotation units high up on the side of the fuselage. This would have the effect of a side floating attitude and an accessible air gap.

Various other suggestions were made to improve survivability in view of service experience and associated research. The review of certification requirements is currently being pursued within JAA and involves both FAA, European and US industry representatives.

The second paper from the Royal Navy set out to describe the Naval Air Command's (NAC) aircrew survival training policy and describe the RN approach to crew survival after ditching. The aircraft of NAC are liable to be deployed on amphibious operations worldwide and the aircrew must be trained and equipped to meet the challenge of underwater escape and survival at sea in a wide range of maritime environments, in a variety of aircraft. An extensive range of protective clothing and survival equipment is provided for aircrew. Controlled ditching at sea can be a gentle affair with the helicopter issuing a Mayday call and then alighting gently upon a calm sea. Uncontrolled ditching is invariably violent and occurs quickly and without warning. In the seconds following impact. it is vital that crews react swiftly and automatically. In reality egress attempts are rarely as ordered as those experienced in the controlled training environment. Many factors such as the physical and mental shock of impact will impede underwater escape in some way. Any significant delay during the escape attempt would necessitate use of the short term air supply or STASS underwater breathing system.

To provide realistic underwater escape training and instil the required instinctive reaction to escape all RN helicopter aircrew and the majority of RAF and AAC aircrew carry out underwater escape and STASS training every two years. The helicopter underwater escape trainer (HUETU) or dunker as it is known is at RNAS Yeovilton. Completion of the training is mandatory in the RN and failure leads to removal from the course. Survival drills are carried out at six-monthly intervals. Under development is a range of fire retardant protective clothing designed to be thermally efficient and comfortable to wear for extended periods, and also a STASS system for passengers in military helicopters, as well as improved emergency locator beacons.

Navy Lynx helicopter

Design and engineering aspects of this helicopter were detailed by GKN Westland. The Navy Lynx has been designed for ship-borne operations against the most stringent operational requirements and environmental conditions. These include a semi-rigid rotor head which is machined from a one-piece titanium alloy forging. The main rotor gearbox is driven by two Rolls-Royce Gem 1120 shp gas turbine engines. The drive direction is turned through 90 achieved by a two stage reduction incorporating a conventional spiral bevel gear first stage and a conformal gear second stage. The basic oleo unit in the undercarriage is common to all three vertical members of the landing gear, and was designed specifically for flight deck operations on small ships in severe sea conditions. The unit comprises a two-stage air spring and a three-stage hydraulic damper (see Figure 3).

The Navy Lynx decklock system comprises a hydraulically operated "harpoon" mounted in the underside of the aircraft's structure. The decklock incorporates a swivelling head which enables the helicopter to be rotated either manually or with rotors running, by using the pitch directional controls, whilst still securely attached. The helicopter can undertake spot turns about the harpoon, which allows into wind take-offs irrespective of the ship's heading. High standard corrosion protective measures are incorporated in order to minimise the effects of the maritime environment.

Figure 3 Main rotor gearbox layout

The central tactical system (CTS) is incorporated, the prime requirements being to reduce the work load of the crew and enhance the tactical capability of the aircraft. The CTS collects, integrates and processes sensor information and displays it in such a way to allow the aircrew to rapidly assess a tactical situation. The system also provides flight management facilities for the navigation and communication needs. The current development is the Super Lynx 300 which incorporates a new 10,000 hour airframe and LHTEC CTS800-4N engines, digital core avionics and an integrated glass cockpit, as well as other features.

Non-maritime helicopters in the maritime role

A shift in emphasis has resulted in the need for shipborne deployment of military helicopters not specifically designed for this environment. An example of this is the RN requirement to embark the Apache AH Mk 1 in support of amphibious operations. Corrosion is a major problem in airframe structures in such an environment, influenced by two factors. The first is whether it is wet assembled, which is far from universally adopted with, sometimes, just a bead of sealing compound round the edge of joints, which has disadvantages leading to serious corrosion problems. Proper wet assembly involves the application of a jointing compound to the whole of mating surfaces before they are assembled together.

Magnesium components are still subject to galvanic corrosion, even with improved corrosion resistance. Fatigue failures initiated by corrosion are also a problem. Better oils have been formulated for helicopter gearboxes and should provide improved protection. Water absorption by composites in the maritime environment is a particular problem with deleterious effects, with particular reference to water in a composite skinned honeycomb trailing edge. Porosity is more likely to occur if the skins and core are co-cured during manufacture. A better method is to pre-cure the skins on hard tooling and then assemble them to the core.

Although much depends on the type of ship and the operational flexibility required, vertical and lateral velocities encountered during landing are of concern if the helicopter has not been designed for maritime operations. High winds can be encountered and wheel brake capability is important. A host of underwater escape issues must be addressed. Electromagnetic compatibility is also critical, with particular concerns related to safety critical systems such as engine and flight controls, weapon control systems and other inputs to flight management such as radalts.