11

WhyWhy TRIGON TRIGON  has no rails has no rails

IntrIntroductioduction..................on............................................................................................................................................................................................................ 33BackgBackgrounroundd.............................................................................................................................................................................................................................. 33

TractoTractor...........r........................................................................................................................................................................................................................... 33RASRASTT ........................................................................................................................................................................................................................................ 44

SecurSecure e on on landinlandingg ............................................................................................................................................................................................ 44MoveMovement ment along the along the deck........................deck............................................................................................................................................ 44Good Good points............................points.................................................................................................................................................................................. 55Bad Bad pointspoints .................................................................................................................................................................................................................. 55

TRIGTRIGONON .............................................................................................................................................................................................................................. 66SecurSecure e on on landinlandingg ............................................................................................................................................................................................ 66MoveMovement ment along the along the deck........................deck............................................................................................................................................ 66Good Good points............................points.................................................................................................................................................................................. 66Bad Bad pointspoints .................................................................................................................................................................................................................. 77

DeveDevelopmenlopmentt .......................................................................................................................................................................................................................... 88MacTaMacTaggarggart t ScottScott .................................................................................................................................................................................................... 88TRIGON TRIGON ........................................................................................................................................................................................................................ 88PRISM PRISM .............................................................................................................................................................................................................................. 88CLAMPDOWN CLAMPDOWN .. .................................................................................................................................................................................................. 88 HELIOS  HELIOS .........................................................................................................................................................................................................1...............100

Indal.........................Indal.................................................................................................................................................................................................1................111 ASIST  ASIST .........................................................................................................................................................................................................................11.....11TC-ASIST TC-ASIST ....................................................................................................................................................................................................1..............111

Copies......................Copies...................................................................................................................................................................................................................11...11IssuIssues es raisraiseded .......................................................................................................................................................................................................................12...12

Ship Ship MotioMotion........n.......................................................................................................................................................................................................12.....12Pilot Pilot contrcontrol.....................ol.......................................................................................................................................................................................13........13StraigStraighteninhteningg ..................................................................................................................................................................................................1..............133 RAST  RAST .............................................................................................................................................................................................................................13...13 ASIST  ASIST .........................................................................................................................................................................................................................13.....13TC-ASIST TC-ASIST ....................................................................................................................................................................................................1..............144FHS.........................FHS.................................................................................................................................................................................................14..........14SAMAHE SAMAHE ......................................................................................................................................................................................................1..............144PRISM PRISM .........................................................................................................................................................................................................................14...14TRIGON TRIGON ........................................................................................................................................................................................................1..............144ConcluConclusionsion ....................................................................................................................................................................................................14..........14

SteeriSteeringng.............................................................................................................................................................................................................................15...15 RAST  RAST .............................................................................................................................................................................................................................16...16 ASIST  ASIST .........................................................................................................................................................................................................................17.....17TC-ASIST TC-ASIST ....................................................................................................................................................................................................1..............177FHS.........................FHS.................................................................................................................................................................................................17..........17SAMAHE SAMAHE ......................................................................................................................................................................................................1..............177PRISM PRISM .........................................................................................................................................................................................................................18...18TRIGON TRIGON ........................................................................................................................................................................................................1..............188ConcluConclusionsion ....................................................................................................................................................................................................18..........18

Attachment access & men.....................................................................................19Attachment access & men.....................................................................................19 RAST  RAST .............................................................................................................................................................................................................................19...19 ASIST  ASIST .........................................................................................................................................................................................................................19.....19

22

TC-ASIST TC-ASIST ....................................................................................................................................................................................................1..............199FHS.........................FHS.................................................................................................................................................................................................19..........19SAMAHE SAMAHE ......................................................................................................................................................................................................2..............200PRISM PRISM .........................................................................................................................................................................................................................21...21TRIGON TRIGON ........................................................................................................................................................................................................2..............211ConcluConclusionsion ....................................................................................................................................................................................................22..........22

Security, complexity and single points of failure...................................................23Security, complexity and single points of failure...................................................23 RAST  RAST .............................................................................................................................................................................................................................23...23 ASIST  ASIST .........................................................................................................................................................................................................................23.....23TC-ASIST TC-ASIST ....................................................................................................................................................................................................2..............244FHS.........................FHS.................................................................................................................................................................................................24..........24SAMAHE SAMAHE ......................................................................................................................................................................................................2..............255PRISM PRISM .........................................................................................................................................................................................................................25...25TRIGON TRIGON ........................................................................................................................................................................................................2..............255ConcluConclusionsion ....................................................................................................................................................................................................25..........25

SummSummary ary of of issuissueses ..............................................................................................................................................................................................26........26Ship Ship MotioMotion........n.......................................................................................................................................................................................................26.....26Pilot Pilot contrcontrol.....................ol.......................................................................................................................................................................................26........26StraigStraighteninhteningg ..................................................................................................................................................................................................2..............266SteeriSteeringng.............................................................................................................................................................................................................................26...26Security, complexity and single points of failure...................................................26Security, complexity and single points of failure...................................................26User User friendfriendlyly ..................................................................................................................................................................................................2..............266FutuFuture re prooproof..........f.......................................................................................................................................................................................................27...27VersaVersatiletile.........................................................................................................................................................................................................................27.....27Value Value for for money..........money..................................................................................................................................................................................27..........27DrainDrainage.....................age...........................................................................................................................................................................................2..............277MissMission ion capabcapabilityility .............................................................................................................................................................................................28...28ManniManningng.........................................................................................................................................................................................................................28.....28StealtStealthh ...............................................................................................................................................................................................................................28.....28

ExperExperienceience .........................................................................................................................................................................................................................29.....29

3

Introduction

This document explains why MacTaggart Scott do not normally supply rail based

helicopter handling systems. By looking at the background of early systems and

identifying how they have developed, the strengths and weaknesses of different

concepts can be considered.

It must be understood that helicopter handling as two phases

1. Secure on landing, strictly called helicopter handling

2. Movement along deck, strictly called helicopter deck handling

In this document helicopter handling will mean both secure on landing and movement

along the deck.

Background

When helicopter handling first started on small ships there were three systems:

1. a tractor

2.  RAST 

3. TRIGON 

Tractor

A tractor offers a simple way to move the helicopter along the deck after it has landed.

Apart from the weight of the tractor it offers no security against sliding or tipping. It

has no function for secure on landing.

Tractors are limited to large stable ships or small ships in calm weather.

4

RASTThe Indal RAST  was originally designed for Sea King and Seahawk, both of which are

tail wheel helicopters.

Secure on landing

With RAST  the helicopter is winched down to the deck by a ship mounted cable. The

cable runs through a probe underneath the helicopter. The probe is captured on

landing by a bear trap device (also called RSD).

Movement along the deck

The RSD runs in a rail.

After secure on landing the helicopter tail gear is aligned to the track by tail guide

winches. A probe in the tail landing gear drops into the track. This process is called

straightening.

The helicopter is moved up and down the deck by the RSD pulling and pushing probe.

The RSD is winched by under deck wires.

All USN ships use RAST , except for the new LCS where TRIGON  is being fitted.

5

Good points

1. In service for many years with over 200 supplied.

Bad points

1. Secure on landing. This is a very expensive exercise as you need two pilots.

You need one pilot to fly the helicopter and one on the ship to control the

winch.

2.  RAST  is only suitable for specially built variants of Sea King and Seahawk.

3. Helicopters must have been originally designed to take high forces, and

bending moments, where the probe fits in the fuselage.

4.  RAST  RSD is very complicated.

5.  RAST  needs men on deck to connect haul down cable.

6.  RAST   needs men on deck to connect tail guide winch cables.

7. The probe is not at the centre of rotation, so pulling on the tail guide winches

damages tail wheel tyres and fatigues the tail cone. To try and relieve this

stress, the jaws of the bear trap are released in the lateral direction whilst

camped about the probe, and the helicopter is yo-yoed backwards and

forwards whilst being straightened. This photograph was taken during

straightening and shows what happens when you have a single point failure.

6

TRIGONThe MacTaggart Scott TRIGON  system was originally designed for Wasp and Lynx,

which are four wheel and nose wheel helicopters respectively.

Secure on landing

TRIGON   systems have a landing grid on the ship and a decklock in the helicopter.

This is a simple system where the landing and take off is totally in the control of the

pilot.

Movement along the deck

The helicopter is simply winched up and down the deck by the TRIGON  winches.

Straightening is achieved by steering the nose gear.

Good points

1. In service for many years with over 200 supplied.

2. Secure on landing. Simple and in pilot’s control.

3. TRIGON  is suitable for any type of helicopter.

7

4. Helicopters do not need advance engineering to be suitable for TRIGON .

5. TRIGON  is very simple.

6. TRIGON  has no single point of failure.

7. Simple to install

Bad points

1. TRIGON  needs men on deck to connect cables.

2. TRIGON  is thought by some people to offer little security.

8

Development

MacTaggart Scott

TRIGON 

MacTaggart Scott has not developed the TRIGON   concept. Equipment is of course

continuously updated to gain the benefits of technology, but the basic concept never

changes. Like a bicycle – why reinvent it.

PRISM 

MacTaggart Scott has built a rail system, called PRISM , for the 16 tonne Merlin

helicopter on the Type23 frigate. This is a specialised 3 rail system connecting to the

helicopter through axle extensions.

Rails were required to meet classified Royal Navy requirements.

MacTaggart Scott was the first company to use axle extensions, since copied by Indal.

MacTaggart Scott is the only company to use a section of moving flight deck for

stress free straightening to the deck rails.

MacTaggart Scott the only company to handle such a large helicopter with a

traversing system.

Other ideas

Some potential customers think TRIGON   is less secure. Around 1982/3 MacTaggart

Scott investigated some alternative concepts.

CLAMPDOWN .

A decklock is ball jointed at the airframe. MacTaggart Scott, and also independently

AgustaWestland, considered a rigidly mounted decklock which could both secure on

landing and be used as a towing probe. This was rejected due to the inflexibility in

9

engaging the grid, loads in the airframe and the difficulty in designing a smooth sided

decklock.

The next idea, also reconsidered and rejected later by IBM ASIC, was to tow the

helicopter through a moving grid with the decklock engaged. This was abandoned

because the decklock would need to be rigid. FHS have used this idea in a modified

form by having the decklock engaged in a separate moving grid.

The final idea was the equivalent of a travelling decklock. MacTaggart suggested

using a block on the cargo hook to which was connected a cable coming up from a

shoe in the deck rail. A force on the cable in the rail would apply a down force, called

a clampdown force on the helicopter. The benefit was that the helicopter would drag

the shoe along with it, and it would allow for sideways misalignment. The concept

was to be used with TRIGON .

MacTaggart Scott showed the concept to Aerospatiale as we wished to know if it

could be used with the Super Puma (Cougar). The concept was dropped because of

the difficulty of getting under the helicopter to make the connection and the fact that

the rail could not pass through, or into, the landing grid. The concept was also

dropped because mathematical modelling showed no cost benefit.

Aerospatiale passed the MacTaggart Scott concept to Sofma (now DCN), who then

developed it into the SAMAHE  system.

So SAMAHE  is a copy of an idea rejected by MacTaggart Scott.

SAMAHE  no longer uses the clampdown for Lynx and Dauphin/Cougar although it is

retained for the Indian Sea King and will be used with NH90.

10

HELIOS 

MacTaggart Scott considered handling the helicopter through main wheel spurs. This

was first considered by MacTaggart Scott privately and then again later as part of a

Pathfinder project funded by DERA (UK defence Research Association). It was

found that very high twisting loads can be generated during straightening and so the

idea was dropped. The HELIOS  concept is the basis of the German FHS system and

the Indal TC-Asist.

11

IndalIndal were forced to develop the  RAST  system because it could not operate with other

popular helicopters like Lynx, NH90 and AB212, and it is expensive and heavy.

ASIST 

The first development led to the  ASIST  system where the RSD tracks the helicopter’s

probe as the helicopter makes its landing.

A secondary device has to be fitted by hand below the nose wheels to steer the

helicopter. The nose gear has to be manually lifted clear of the deck to position the

device.

TC-ASIST 

For customers who want to use a decklock, Indal has proposed the twin claw  ASIST .

This system uses axle extensions as pioneered by MacTaggart Scott. However, using

only the main wheels produces high twisting load in the main landing gears.

12

Copies

 Imitation is the sincerest form of flattery

• We have seen that SAMAHE  is based on MacTaggart Scott’s CLAMPDOWN 

concept.

• We have seen that the FHS system is based on MacTaggart Scott’s  HELIOS 

concept. The FHS system, only fitted to the German F124 Frigate is a very

poor system. FHS also copied the MacTaggart Scott concept of a travelling

grid.

• We have seen that Indal copied MacTaggart Scott’s use of axle extensions.

• Blohm und Voss, the German shipbuilder, made a copy of the TRIGON 

system (called Hercules) for the German F123 frigates. This was a very poor

system and is no longer manufactured.

• Larsen and Toubro (through Haean) have made a copy of the TRIGON  system

for the KCG. It does not work (as of summer 2009)

Issues raised

So far we have come across some rather important issues.

Ship MotionThe helicopter must be secure against sliding and toppling. But it is not Sea State, roll

or pitch which is most important here, it is ship acceleration. Definitive limitations

can only be stated after a seakeeping study which combines the ship characteristics,

the helicopter characteristics, the geometry of the handling system and a model of the

sea.

MacTaggart Scott has undertaken many such seakeeping studies in accordance with

STANAG 4194 [1]. The exact results are confidential to each customer as this is a

measure of the ship’s operational capability. However, it can be stated that every

1STANAG 4154 (Edition 3), Common Procedures for Seakeeping in the Ship Design Process, NATO,

December 2000

13

frigate and TRIGON  system combination analysed theoretically by MacTaggart Scott

shows that in general

1. Any helicopter can be safely handled in head or following seas in North

Atlantic Sea State 6 at 10 knots and above ship speeds.

2. Any helicopter can be handled at any wave encounter angle in North Atlantic

Sea State 5 at 10 knots and above

3. The worst condition is usually low ship speed in quartering seas.

Any claims of handling system capability in terms of particular Sea States, or in terms

of angular displacements, must also state the ship type, range of wave encounter

angles and ship speeds.

Pilot controlWhen a helicopter has decklock the pilot has full control of security on landing. The

pilot can choose the moment of landing and take off. This makes the operation as safe

as possible. The pilot knows if the decklock fails to engage, and so he can take off,

apply negative thrust or execute any planned emergency procedure.

When the security is provided by an active device on the ship, like  RAST or  ASIST ,

then the pilot requires a feedback to tell him he is secure. This is first problem. A

second problem is that of necessity the active capture devices are of necessity

complicated electro-mechanical devices which are unreliable.

StraighteningWhen a system uses a rail, then the helicopter has to be lined up with the rail. This

process is called straightening. Because the helicopter will never land repeatedly in

the same location, straightening will involve rotation and/or translation.

RAST

The helicopter is rotated by tail guide winches to get the tail landing gear over the rail.

To reduce stress the RSD jaws are laterally released as a pair. As the helicopter is yo-

yoed backward and forward, the main landing gear is influenced to crab port /

starboard.

ASIST

By a combination of towing forward and backwards at the probe, and port / starboard

motion of the capture device, and free castor of the nose/tail gear the helicopter is

aligned with the rail. This is like an XY plotter.

14

Note that helicopters like Lynx and NH90, which have no trailing castor, will require

manual steering from the pilot during this process and are therefore unsuitable.

TC-ASIST

The helicopter is manipulated into alignment by the linear motion of two main landing

gear axle extensions with their carrier beam. Once the centre of the MLG is over the

track centre, the carrier beam is rotated to rotate the whole aircraft into alignment.

Note that helicopters like Dauphin/Cougar, which have a trailing castor NLG, will

require manual steering from the pilot during this process and are therefore unsuitable.

FHS

The helicopter is manipulated into alignment by the differential motion of two

telescopic rams, one attached to each main landing gear axle extension.

Note that helicopters like Lynx, which have no trailing castor, will require manual

steering from the pilot during this process.

SAMAHE

For helicopters with a decklock, the helicopter is rotated about the decklock by the

pilot using tail rotor thrust. The helicopter is rotated until the nose wheels are over the

rail.

For no decklock helicopters like Seahawk, the helicopter is rotated in to position using

winches.

PRISM

Only used for Merlin. The nose gear is aligned with the centre rail by rotation about

the decklock using slewing winches. A nose gear shuttle is attached and the Merlin

pulled forward a short distance to a set position where the main landing gear is over

arcuate plates. These plates slide over the flight deck to rotate the main landing gears

into alignment about the nose landing gear.

TRIGON

The helicopter is straightened as it is pulled forward by steering.

Conclusion

 RAST   causes stress as the helicopter is not rotated about its centre of rotation. To

reduce stress the helicopter is yo-yoed.

 ASIST , TC-ASIST  and FHS may require pilot co-operation in steering.

15

TC-ASIST  and FHS have high twisting loads in the main landing gears.

SAMAHE  does not get the main landing gear centre aligned with the rail.

PRISM  aligns all landing gears in a stress free manner.

TRIGON  requires no straightening.

SteeringWhen a system uses a rail, then the helicopter should not require to be steered as it is

attached to the rail. This is not always exactly as good as it sounds.

To reduce fatigue loads on the airframe it is important that the helicopter wheels roll

along a path of true rolling radius. That is to say that at any instant in the curved

movement of the helicopter the centre of rotation for each wheel is at the same point

in space, or that all the wheels are lined up with each other for a straight motion.

If all the wheels roll along a true path, with no slide slip or tyre scrub, then the main

wheels will follow a path called a tractrix [2]. The tractrix is an exponential curve to

which the axis of the main wheels is always normal and the direction of travel

tangent.

If the distance between nose and main wheel centres is a, then the tractrix is defined in

Cartesian coordinates by

( ) c yaa ya yady ya y x +−++−−=−−=−−

∫22122221 ln.

2Tractrix is derived from the Latin trahere meaning to pull.

The tractrix was discovered by Sir Isaac Newton and is said to be the first known use of logarithms incalculus and the first curve determined by integration. ref. Newton's 2nd Epistle to Oldenburg 1676.Newton’s solution is for the ‘nose wheel’ moving in a straight line.Euler solved the case for purely circular motion of the ‘nose wheel’ 100 years later.There is no general solution for other motions of the ‘nose wheel’ because of elliptic integrals.W. G. Cady described a mechanical device for drawing the curve and gave this brief history in American Mathematical Monthly, Vol. 72, 1965.

16

It is thus proved that whilst towing a helicopter forward, from the nose, the main

wheels will tend to align themselves.

Conversely if the helicopter is pushed back, from the nose, the main wheels will tend

to run off to port or starboard.

The effect of the tractrix is self evident in every day life. You cannot park a car

alongside the kerb without some measure of over steer on 'the nose wheel' path to get

the 'main wheels' where you want. Similarly, it would be difficult to push a child's

tricycle backwards in a straight line by applying a light force, but no torque, at the

centre of the handle bars without jack-knifing.

In order to ensure stress free movement of the helicopter these effects must be

considered in both directions.

RAST

As the helicopter is pulled forward, the main landing gear is influenced to crab port /

starboard by applying a sideways force to the probe.

On a curved track the instantaneous centres of rotation of the main landing gears, tail

landing gear and probe will be different.

Consider the diagram above. If the main wheels roll on some true path then the path

of the  RAST  probe • is defined and hence the  RAST   track. However, the tail wheel

and tail wheel probe will describe a different path to the main probe. This results in

tyre scrub and airframe fatigue.

17

ASIST

 ASIST  requires that an additional shoe is fitted to connect the nose landing gear to the

rail after straightening. As the nose wheel has to be manually jacked clear of the deck

you loose nose wheel friction and the helicopter is free to slide in a rotary fashion

about the probe.

As with RAST  on a curved track, or if the probe is not perfectly aligned on a straight

track, the instantaneous centres of rotation of the main landing gears, tail landing gear

and probe will be different.

TC-ASIST

TC-ASIST  is not in service and the detail is not yet clear. However, if a nose landing

gear device is not used then as described above the main landing gears will require

perfect alignment for a straight rail, and for a curved path the nose and main landing

gears will have different centres of rotation.

FHS

In the FHS system all the helicopter wheels are off the deck and so steering presents

no problems.

SAMAHE

SAMAHE   pulls the helicopter forward through the nose landing gear. The main

wheels follow the tractrix. This means that the main wheels may not be aligned when

the helicopter is the hangar. Because the nose wheels are attached to the rail, the nose

gear can not be over steered to bring the main wheels into alignment. If the mains

wheels are substantially out of line then they must be aligned using a re-centring

device.

18

The helicopter is pushed back to the flight deck through the nose landing gear. This

means the main wheels will run along the tractrix to either port or starboard of the

required path. This is prevented by fitting a ‘MLG guiding device’ between the main

wheels with a connection into the deck. This means there will be a continuous side

force on the helicopter forcing to keep to the path.

PRISM

Only used for Merlin. Steering is not an issue as PRISM  only uses a straight track and

spring compliance is built in to the attachments. Even so, the reverse tractrix effect

can be seen when the helicopter is pulled back and the ship is l isted.

TRIGON

The TRIGON   deck man steers the helicopter naturally through the steering arm.

There are no external influences to stress the helicopter.

Note that the helicopter is pulled forward and he helicopter is pulled back. The

helicopter is never pushed.

Conclusion

 RAST ,  ASIST , TC-ASIST   and SAMAHE  all cause stresses in the airframe by forcing

the helicopter to roll along a single path.

PRISM  causes minor side loads when the ship is listed.

FHS and TRIGON  cause no stresses in steering.

19

Attachment access & menThe helicopter needs to be attached to the handling system. This is either automatic

or manual.

RAST

The RAST  probe is automatically attached to the RSD. However, men are required on

deck to attach the initial haul down cable when the helicopter is in hover, and to

operate the tail guide winches.

Access is clear.

ASIST

The ASIST  probe is captured automatically, but the nose landing gear shoe has to be

attached manually by lifting the nose landing gear.

Access is moderately restricted under the nose fuselage.

The shoe is carried on to the flight deck

TC-ASIST

TC-ASIST  is not in service and the detail is not yet clear. Theoretically it should be a

no men on deck system, but then so was ASIST  claimed to be.

Access is clear at the main landing gear.

FHS

FHS would appear to be theoretically a no men on deck system, but as it is so

unreliable in attaching itself men are required.

Access is clear at the main landing gear and moderately restricted under the nose

fuselage.

20

SAMAHE

SAMAHE   requires a man to attach the nose towing bar. The nose towing bar is

carried to the helicopter by hand. Connection is difficult to fit as the carriage must be

driven to an exact position relative to the tow bar. The tow bar must be held up

manually during this process (approximately 25 kg).

The underside of the helicopter requires a slotted hole for the spring loaded pin of the

swivel arm, shown here on a yellow test rig, or something similar on a real helicopter.

A man will have to crawl under the helicopter to make and check this connection.

Access is very restricted.

Fitting the ‘MLG guiding device’ between the main wheels for moving toward the

landing spot is very awkward has men must crawl under the tail of the helicopter.

21

Access is very restricted on Super Lynx because of the Doppler.

The ‘MLG guiding device’ must be manually removed at the end of the rail, before

the decklock is over the grid. After removal it must be carried back to the hangar

manually (approximately 28 kg).

It is very important for owners to check that helicopter has the correct mating part.

With this version of the design for Lynx there is no clampdown force.

The Dauphin/Cougar system is similar.

PRISM

Only used for Merlin. PRISM   was designed to be a no man on deck system, but

because of problems with the helicopter men are to attach the slewing cables and to

lock the nose wheel shuttle.

TRIGON

TRIGON  requires men to connect wires and steer.

22

Access is clear at the main landing gear and moderately restricted under the nose

fuselage.

Conclusion

Men on deck will always be required to fold blades, load stores and all the other tasks

associated with shipbourne helicopters.

Every workable system needs men on deck.

Access for manned intervention is reasonably easy except for SAMAHE .

23

Security, complexity and single points of failureThe helicopter requires to be secure against sling and toppling. Highly complex

systems are more likely to be unreliable and therefore increase the risk of an accident.

Systems with small, complex, parts exposed to the marine environment are likely to

corrode to failure. Additional security is given when there are multiple failure points.

RAST

The RAST  probe and the RAST  RSD are both single points of failure. There have been

many documented cases of loss of Seahawks were the RSD has failed, or the pilot has

been unaware that the jaws are open.

The complexity of the RAST  RSD makes it unreliable.

ASIST

The ASIST  probe is a single point of failure. Because of the complexity of the ASIST 

RSD, its associated control systems and sensors, the Chilean Navy report a target of

only 90% availability [3].

3 ASNE Launch & Recovery of Manned and Unmanned Vehicles - 2005, Annapolis, November 2005

24

TC-ASIST

As the system is not yet in service no conclusions can be drawn, but of necessity the

cross beam will be complex. To date (summer 2009) the system on the Andrea Doria

has not been brought into service.

The only single point of failure is the decklock on landing.

FHS

The FHS system uses very complex engineering and sensors. This has made the

system very unreliable.

25

The decklock is a single point of failure both on landing and during the movement to

and from the hangar.

SAMAHE

SAMAHE   is complex in that is difficult to attach. The first single point of failure is

spring loaded pin at the end of the curved beam. If the spring, or pin, fails then the

helicopter is only restrained at the nose. This will not prevent sliding or tipping.

The second single point of failure is the pin connecting the tow bar to the carriage. If

this fails the helicopter can roll backwards and be completely unrestrained.

PRISM

1. Only used for Merlin. PRISM   has undergone a full safety assessment by the

Royal Navy. The only single point of failure for movement of the helicopter is a

wire break, inside the rail, to the nose shuttle. This is same risk for all rail

handling systems as their various RSDs and carriages are moved by wires.

TRIGON

TRIGON  is simple by design to make it highly reliable. Because it has 5 cables it has

multiple points of failure.

Conclusion

TRIGON  is the only system which does not have a single point of fa ilure.

26

Summary of issues

The following is a summary of the reasons MacTaggart Scott do not normally provide

rail systems.

Ship MotionExtensive practical experience and dynamic simulations have shown TRIGON  to

perform adequately.

Pilot controlUse of TRIGON   with a landing grid gives the pilot full control of launch and

recovery.

StraighteningSkilled manipulation and stress inducing straightening is not required with TRIGON .

SteeringWith TRIGON  the helicopter moves with true rolling motion so no additional stresses

are induced.

Security, complexity and single points of failureTRIGON   has multiple points of failure. As far as is known no system has ever

suffered a wire break, but if it happenened with TRIGON   it would only be 1 of 5.

TRIGON  has multiple points of failure.

The simple, but effective, design of TRIGON  ensures high reliability.

User friendly

No heavy parts to be carried out to the deck in heavy wether.

27

Future proofTRIGON  will handle future helicopters and UAVs without expensive upgrading.

VersatileTRIGON   can handle all types of helicopter without modification to the aircaft or

TRIGON . You do not need to change the large and expensive moving carriage

Value for moneyCapital cost of a rail system is more expensive.

Rails are expensive to install in the ship.

TRIGON  equipment can be mounted anywhere to suit the shipbuilder and the Navy.

DrainageRails allow salt water to flow in to the hangar, and in a worst case situation allow

burning fuel in to the hangar.

Tricycle Tail dragger

Skid 4 wheel

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Mission capabilityWarships are warships, designed to operate a fighting asset in a theatre of war. The

worst possible situation is where a rail based carriage is stuck on the deck from ship

damage, blackout or breakdown. Such an obstacle prevents a helicopter being moved

by hand, or possibly landing. TRIGON  can not make such an obstruction.

ManningAll systems require men on deck for blade fold and/or tail fold and/or blade props, etc.

Finally, could you line up and attach the SAMAHE  arm, or tow bar, under the Lynx in

this weather.

StealthDeck rails can have a detrimental radar cross section unless their gap is manually

filled in with sealing strips.

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Experience

The engineers of MacTaggart Scott have many years of experience in designing,

developing, building and testing different types of helicopter handling systems for

many helicopter types.

Research and Development always continues with an aim to reduce costs or to

embrace new technologies, or in response to specific customer requests. For instance,

MacTaggart Scott is currently working on an electric drive version of TRIGON  as

many Navies have the aim of ‘the all electric ship’.

As well as the facility to land test systems, MacTaggart Scott can also perform

dynamic simulations of the ship/helicopter/handling system interface.

From time to time MacTaggart Scott make presentations or give papers at

international gatherings such as

• American Society of Naval Engineers

• AGARD

• The Embarked Aviation Seminar

• The MECON conference

• The Royal Aeronautical Society

It is with this wealth of experience, together with the ability to understand all  the

issues associated with helicopter handling, which leads MacTaggart Scott to still

propose TRIGON  as the most suitable system in the vast majority of cases.