Helicopter Traversing Systems – The Last Mile Connectivity Problem!

One of the unique aspects about naval helicopters – one that often falls into a grey zone between helicopter manufacturer and shipbuilders – is the helicopter traversing system.

On a warship, landing of the helicopter marks the beginning of another important evolution that must be handled with equal precision and alacrity: the movement of helicopter from deck to hangar (known as ‘strike down’) and vice-versa (known as ‘ranging’ on deck). Why are these two evolutions important? Simple. The process of strike down and ranging imposes limitations on the warship’s manoeuvring, thereby rendering it vulnerable to both medium and adversary.

‘Manhandling’ a light helicopter on deck! (Picture courtesy USNTPS)

Consider a 6000-ton warship cutting through the waves with a 12-ton helicopter onboard. The only things holding the helicopter on deck, or in hangar, are the tie-downs or moorings. These must come off during traversing. Time, tide and formation wait for no one. Nobody in a seagoing fleet has abundant patience, neither is the medium (sea) always calm or forgiving. When the moorings are removed for traversing, something must hold ship and helicopter together. Once we have that figured out, a system should be in place that moves the helicopter through those few metres when everything else is conspiring to tip it overboard. That job is done by a Helicopter Traversing Systems (HTS).

Light helicopters like the 2-ton Alouette (Chetak) depend on a human HTS (call it ‘one-two-six-push’ or ‘heave-ho’ if you will). A steering arm is attached to the nose wheel, lashings are removed or shifted on the move (called ‘running lashings’), deck crew ‘manhandle’ the helicopter, while a supervisor barks “slight starboard”, “slight port”, “midships”, “on brakes”, “off brakes”, “stop pushing”, etc over the roar of machinery noise. This is HTS version 1.0!

Chetak off lashings and chocks!

Anything above this weight category will not manhandle kindly. It’s literally and metaphorically a slippery slope trying to push iron birds on a moving deck without the right implements. Naval aviators will recall at least a few occasions when ‘manhandle’ threatened to turn ‘man overboard!’

Evidently, the need for robust, heavy-duty helicopter traversing systems is very real onboard warships. The associated deck equipment and interfaces on helicopter must mate safely and expeditiously for achieving realistic reaction times in various degrees of readiness. This could range from a 5-10 minutes for ‘scramble’ (equivalent of ORP in air force) to 30 minutes or more when the bird is struck down in hangar with storm lashings. The system must deliver with equal precision and safety at night, low visibility and wet deck conditions. It is heavy engineering work and brooks no slippage or failure. If a helicopter gets stuck anywhere in this ‘last mile’ between deck and hangar, we have a real problem on our hands – one that may impose severe restrictions on the ship’s manoeuvrability.

In addition to traversing, some HTS also mate with an ‘arresting system’ that helps during launch and recovery in heavy seas. If you look closely at some naval decks, you may find a fixed helicopter landing grid or a moving ‘bear trap’. These are meant for taking the landing helicopter’s ‘Harpoon’ deck lock or ‘RAST’ probe. Such systems ensure stability of the helicopter on deck during the vulnerable period when chocks and lashings are not in place.

The delicate moments before ‘ON chocks & lashings’ (Picture courtesy USNTPS)

There are different types of HTS filling this challenging requirement.

The Recovery Assist, Secure and Traverse (RAST) system caters for recovery-assist and traversing. Curtiss-Wright’s Indal RAST is one such system that uses a single Rapid Securing Device (RSD) that remains attached to the helicopter after initial contact.

RAST in action on a rolling deck (Picture courtesy Curtiss Wright)

Aircraft Ship Integrated Secure and Traverse (ASIST) and Twin-Claw ASIST is another such system which may or may not employ a recovery probe. ASIST employs electro-optic tracking devices to detect, track and capture the helicopter during the deck launch/recovery.

Seahawk with ASIST (Picture courtesy Curtiss Wright)

The SAMAHE system, originally manufactured by French firm Naval Group (formerly DCNS), is a ‘traversing-only’ system for medium to heavy helicopters. In this system, the landing helicopter has to fire a ‘harpoon’ deck lock on to the landing grid soon upon landing. Once the helicopter is switched off, a handling interface attaches to strong points on the landing gear and traverses it into the hangar. This system and its variants have been in use on most Seaking-operating ships of Indian Navy for decades.

An Indian Navy Seaking helicopter on deck. The raised rail Samahe HTS can be seen in the foreground

Winch and wire-based systems that employ a set of winches, chains and pulleys to traverse helicopters in and out of hangar to the flight deck is yet another solution to the traversing problem. Such systems provide only traversing and have been used successfully for many years on Russian-origin ships like the ‘R-class’ destroyers and Krivak class frigates of the Indian Navy. Adequate caution needs to be exercised by deck crew, especially by night, to avoid injury since this system would invariably have multiple load-bearing wires running across the deck. Those who have operated 12-ton Kamov helicopters from small Kashin-class destroyer decks in pitch dark nights will agree that “it’s not over till the fat lady swings!”

At the lower end of spectrum are portable, remotely operated ‘aircraft tugs’. Rapid Aircraft Movers (RAM) such as INDAL’s MANTIS, Manual Aircraft Secure & Traverse (MAST) and Douglas RAM with Multi-aircraft Matrix Acquisition System (RAM MAMAS) are low-chassis, battery-operated towing systems suited for light helicopters and decks that play host to aircraft only infrequently. Although the OEM claims multi-aircraft compatibility in various conditions of deck motion, heavier helicopters can test the capacity of such systems (and the deck crew) if it catches a sudden ‘peak’ in ship motion. Decks that have protuberances may also pose challenges for such a system owing to its low chassis.

INDAL’s MANTIS aircraft tug (Image courtesy Curtiss-Wright)

Experienced seamen would quite understandably put their trust in fixed-deck systems that once mated with the helicopter will not allow it to meander with ship motion.

RAST & ASIST systems require rails on deck. Such rails could either be embedded (sunk) or run proud of the deck. Both present their own unique challenges. A sunken-rail system lends itself to water ingress, resultant corrosion and maintainability issues while rails running proud of deck may impede movement of helicopters with skid landing gear.

A RAST system with sunken rails (Picture courtesy Curtiss Wright)

Indian Navy has experimented with almost all types of HTS. Efforts to develop a Common HTS (CHTS) continue to elude us since a wide variety of landing gear designs are in use. CHTS is neither easy nor cheap for navies that buy helicopters from all over the world. Compromises have to be made that may eventually transfer the load onto deck crew operating hundreds of miles out at sea against vagaries of sea and weather.

Particularly in the case of Indian Navy, to this complex scenario will soon be added more types such as the MH-60 ‘Romeo’, Naval Utility Helicopter (NUH), Naval Multi Role Helicopter (NMRH) and maybe some unmanned rotorcraft too in future.

The obvious question weighing on readers’ minds must be: Does the type of recovery-assist and/or traversing system enhance Ship Helicopter Operating Limits (SHOL)? I posed this query to an experimental test pilot (ETP) who specialises in SHOL work. Craig Mathews is ex-Australian Defence Force and CEO of Prism Defence Pty Ltd based out of Adelaide, South Australia.

“The type of recovery system has a big influence on the ship motion limits as the aircraft is not limited by unrestrained toppling and sliding limits if a helicopter can be restrained up to the point of lift-off and immediately post touchdown”, Craig offers.

An unrestrained helicopter on deck is pretty much akin to riding a roller coaster without safety belts. As we add deck lock, chocks, lashings and then a traverse system, we enhance safety. This is part of an overall shipboard capability that includes traverse, launch and recovery.

Craig explains that “the traversing system influences overall capability, as the total capability includes the limits to move the aircraft to and from the hangar. This is not so critical as the launch and recovery tasks though, because benign ship motion can generally be achieved running down sea to range or stow the helicopter.”

“Recovery, or more correctly, post-recovery restraint systems, therefore, have a greater influence on overall capability than does the traversing system”, Craig opines.

So what would be a system of choice – a combined system such as the RAST or ASIST, or a combination of deck lock and separate traversing solution, I posed to Craig.

“My choice to maximise operational capability would be a deck lock system as it has no performance penalty during the recovery, has no ship maintenance overhead, and is pilot-controlled – meaning the pilot can release in the quiescent period and reapply if necessary during a delayed launch, and has rapid feedback of securing during a recovery”, Craig explains.

For all those who thought landing or taking off from a ship deck was tough enough, I hope through this article you have gained some insights into what happens before and after these evolutions and the solutions available to naval crew. Select the system carefully. Ask tough questions. Test it out from the most benign to most difficult situations. Safety of the helicopter and crew should remain the foremost consideration. Then comes operational capability and SHOLs.

Helicopter traversing system is like a baby caught between two parents: ship and helicopter. Many have been tested on that short strip of a few metres between flight deck and hangar. Stay secure, stay safe!

Cover story on Global Aviator Mar 2020 edition


©KP Sanjeev Kumar, 2020. All rights reserved. An edited version of this article featured as cover story for March 2020 issue of Global Aviator, an aerospace & defence magazine from South Africa.

3 thoughts on “Helicopter Traversing Systems – The Last Mile Connectivity Problem!

  1. Interesting, having grappled with a number of HTS , trying to prove their efficacy post repairs or during acceptance trials ; tend to agree with Curtis with the deck lock version being good. However, as technology develops maybe better options would be available. Also in my humble opinion, retrieval and paying out from hangar/ deck should also be part of the operating envelope ( if it already is not), that decides the overall efficiency of the chopper payload. What use is a million dollar chopper locked up in a hangar for want of a fully ops and capable HTS ? Operability at different ship motion modes must be studied during design. Have done some work on it, once !!

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