REDUCING THE VULNERABILITY OF MILITARY HELICOPTERS TO COMBAT DAMAGE

Pat Collins, BTech Hons, CEng, FRAeSMissiles & Countermeasures Department,Defence Science & Technology Laboratory

FarnboroughHampshireGU14 OLX

United Kingdompwcollins@dstl.gov.uk

Campbell McAulay, MRAeSAirspace Weapons Systems,Future Systems Technology,

QinetiQ ltd.FarnboroughHampshireGU14 OLX

United KingdomCJMcaulay@QinetiQ.com

Abstract

Military helicopters are highly susceptible toattack by a wide spectrum of battlefieldweapons, from small arms to man portablesurface to air missiles. In addition, a wide rangeof “non-conventional” threats such as rocketpropelled grenades; anti-armour guidedweapons and main battle tank main armamentare also significant.

Considerable effort has been devoted toreducing the susceptibility of combat helicoptersto these threats. A review of combat datareveals, however, that many helicopters are stilllost to these less conventional or low technologythreats.

“Cheap kills” which result from engagements bythese less conventional threats tend to occurdespite the presence of sophisticated defensiveaids suites and hardening techniques that arerelied upon by helicopter designers today.

This paper presents a brief review of helicoptercombat loss data covering the period from theVietnam conflict to recent operations inAfghanistan. This is supported by keyconclusions from a number of helicoptervulnerability assessments performed on behalfof the UK MOD.

The paper provides a description of the basicvulnerability reduction measures applicable to allcombat aircraft, and describes practical

applications of these measures to the design ofmilitary helicopters.

Definitions

Any item of military equipment, including themilitary helicopter, must be survivable - it shouldbe able to withstand a man-made hostileenvironment [Ref.1]. The inverse of survivability,"probability of kill" (Pk), is a function of twoproperties, susceptibility and vulnerability. Thesurvivability paradigm is illustrated at Figure 1.

Figure 1: Survivability, a function of susceptibilityand vulnerability

Susceptibility is the inability of the helicopter toavoid being hit by a threat weapon, measured asprobability of hit, or Ph. Various steps may betaken to reduce it's susceptibility, including:

� the management and reduction of it'selectromagnetic (RF & IR) and acousticsignatures;

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� the use of active countermeasures such asjammers, decoys and flares;

� tactical flying to prevent detection bypotential threats;

� suppression of enemy weapons.

Susceptibility reduction measures will contributesignificantly to the survivability of the combathelicopter. However, it is inevitable that, at somepoint during operations the battlefield helicopterwill find itself exposed to hostile fire. Many of themeasures listed above are likely to be ineffectualagainst unsophisticated or novel weaponry, atwhich stage it is only effective vulnerabilityreduction measures that will allow the helicopterto avoid the "cheap kill".

Vulnerability is the inability of the helicopter towithstand the damage caused by a threat, oncea hit has occurred. The term “hit” should betreated with a degree of caution. In the case ofbullets and impact fuzed projectiles the term isused to denote a physical impact with the target.For proximity fuzed weapons a near miss issufficient to trigger the warhead resulting inmultiple (fragment) impacts with the target.Vulnerability is measured as probability of killgiven a hit, or Pk/h.

The helicopter may initially withstand thisdamage by preventing the threat damagemechanisms from penetrating its external skin.While this particular criterion is more suited toarmoured fighting vehicles, externally mountedarmour is occasionally used in helicopters (agood example being the Mil. 8AMTSh. Ifpenetration is unavoidable, various othermeasures may be taken to mitigate thedamaging effects of the threat; these arediscussed later in this paper.

Introduction

Helicopters have been used in war fighting sincethe latter stages of the Second World War,however it was not until the early 1960s inVietnam that their full potential was recognised.Small numbers of UH-1 helicopters were used inVietnam from 1962 onwards initially to transportsmall groups of soldiers and later as improvisedgun ships.

The US Army formed their first helicopter AirAssault Division in early 1963, tasked withdeveloping the necessary tactics to allowhelicopters to deliver large numbers of troops to

the battle. This allowed the basic tempo of warfighting to be increased dramatically as the troopcommanders were no longer constrained by thetopography of the battlefield. At the end of 1964the US 11th Air Assault (Test) Division conducteda major exercise which proved the theory ofmodern helicopter warfare and presaged theincorporation of helicopters into the regular army[Ref. 2]. With the escalation of hostilities in 1965the stage was now set for helicopters to play amajor part in any future conflicts. At this time thehelicopter fleet had little or no protection againsthostile fire, initial vulnerability reduction featuresbeing limited to armoured panels around thecrew seats.

Initially, tactics decreed that helicopters would flyat reasonably high altitude (above 3000 ft) inorder to avoid the majority of machine gun firecoming from the ground. This was a reasonablysuccessful tactic, although the helicopters werevulnerable when delivering their troops intolanding zones, in these instances coming underalmost constant small arms fire [Ref. 3]. With theadvent of the 9M32/SA-7 missile the tactics weremodified by flying at consistently low altitude inorder to place the helicopters below the lowerkinematic boundary of the missile’s engagementenvelope. This was successful to an extent butforced the helicopters to fly where small armsfire was at its heaviest.

In 1965, the US Army recognised a need for adedicated attack helicopter. At the time it wasbelieved that producing a vehicle that wastolerant to a degree of damage from thesignificant threats of the day was a better way ofensuring survivability than relying upon highspeed and agility. A solution to the requirementwas developed rapidly using the basic dynamicsystem of the original UH-1 troop-carryinghelicopter but with a new fuselage configurationfeaturing a tandem cockpit. This conceptualdesign developed into the AH-1 Huey Cobra andcan be considered to be the forerunner of allcurrent attack helicopters; many remain inservice today.

The AH-1 incorporated a number of designfeatures to provide a high degree of ballistictolerance to the most widely encountered threatof the time (the .30cal/7.62mm bullet) [Ref. 4].

The remainder of this paper provides a briefoverview of those weapons, which areconsidered to pose a particular threat to

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battlefield helicopters and a review of helicopterlosses in combat from Vietnam to the presentday. By reference to studies conducted in the UKon behalf of MOD it identifies those componentsof the modern combat helicopter that are mostvulnerable and describes the techniques thatcan be employed to reduce their vulnerability.

Threats to Combat Helicopters

Since the Vietnam war era, the sophistication ofanti-air weapons has increased dramatically andit has to be recognised that, in general, ahelicopter will remain very vulnerable to a largerange of these weapons. The probability ofsurvival of a combat helicopter in an “all out” warsituation relies heavily on the avoidance of thoseweapons by the use of sophisticated DefensiveAids Suites (DAS) rather than the inherenthardness of the helicopter to surviveimpact/engagements from them. However, withthe increasing frequency of peace-keeping/operations-other-than-war (OOTW), thelikelihood of encountering the high technologythreats, which the majority of DAS are designedto counter, has diminished. This leads to therequirement to provide a reasonable level ofprotection against what are generally termed“low technology” threats. These typically consistof hand-held rifles (e.g. AK-47), heavy machine-guns (e.g. ZPU-2/4), small/medium calibrecannon (e.g. ZU23/2) and shoulder launchedrocket propelled grenades (characterised by theRPG-7).

At the next level of sophistication are theshoulder launched MANPAD (MAN Portable AirDefence) SAM systems, these include theRussian SA-7/14/16/18 family of missiles, theUS Stinger and French Mistral. Providingprotection against these and larger systems isparticularly difficult for a helicopter designer. Themajority of MANPAD systems utilise IR seekersand consequently the best form of protection isto avoid being engaged. Combinations of IRJammers, thermal flares and reduced IRsignature from “hot spots” on the helicopter go along way to reducing the risk from this class ofweapon. Anti Tank Guided Weapons (ATGW)can also pose a viable threat although they aregenerally limited in kinematic performance andhave poor performance against a maneuveringtarget.

For larger SAM systems the only viable form ofprotection is the use of a sophisticated DAS.

This paper will concentrate on the lower levelthreats where designed in ballistic tolerance cancontribute significantly to the overall survivabilityof the helicopter.

Combat data

Considerable evidence exists to provide anindication of the vulnerability of helicopters tocombat damage. The following paragraphsprovide a broad overview of helicopter lossessince the Vietnam war.

Early US experience in Vietnam, 1966 to 1975

Between 1962 and 1973, the US military(principally the US Army) lost approximately2600 helicopters to hostile action in Vietnam[Ref. 5]. The majority of these losses can beattributed to small/medium, calibre threats(7.62mm-23mm). On average, the US lostapproximately one helicopter every two days tocombat damage. Of course, it is not possible toinfer the number of helicopters that survivedcombat damage, but the loss rate is stillconsiderable.

It should be remembered that, despite thereliance placed on the helicopter during thisconflict, combat helicopter design and tacticswere still in their infancy, and these statisticsundoubtedly reflect this. However the US rapidlybecame aware that certain parts of the helicopterwere particularly vulnerable, detailed analysesbeing carried out as early as 1965. Practicalefforts were made to provide protection to thesecomponents, initially through the use ofstrategically placed lightweight armour.

Soviet lessons learned in Afghanistan, 1979 to1989

Several sources provide differing statistics forthe loss of helicopters during this conflict [Ref. 6,7 and 8]. A report by the US Directorate ofIntelligence available on the internet assessesthat between 1980 and 1985 some 640 Soviethelicopters were lost from all causes (althoughonly around 300 of these have been confirmedby the intelligence community). A more recentreport, also freely available, produced by the USForeign Military Studies Office suggests that thetotal figure for the full ten years is 333helicopters lost, but it is not clear whether thisincludes operational losses as well as combatkills.

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Figure 2: Mil.24 Hind, Afghanistan. Note the IRsuppressor mounted on the exhaust

A third source suggests that between 600 and800 helicopters were lost due to combatdamage. Although many of these would beattributed to small arms fire up to 20mm calibre,a significant proportion of losses were caused byshoulder launched guided weapons includingSA-7 and (particularly) Stinger. It is interesting tonote that the first operational success of Stingerwas in September 1986, with three Hindhelicopters being destroyed in one operation,and that during 1987 Soviet helicopter losseswere assessed at 1.2 to 1.4 per day.

It is also well documented that, once the SAMthreat had been identified, changes in Sovietaviation tactics, and the introduction ofvulnerability reduction measures (principally theincorporation of IR “hot brick” type jammers andIR suppressors on the engine exhausts) to thehelicopters were instrumental in reducing losses.

Falkland islands 1982

Helicopter losses during the Falklands conflictwere not high. Six Argentinean Puma were lost;one each as a result of small arms fire, surfaceto air missile and air to air missile. Two wereshot down by a Sea Harrier using 30mm cannon.

The majority of UK helicopter losses were adirect result of operational accidents or the lossof the Atlantic Conveyor logistics ship. However,one Scout was destroyed in the air by a Pucaraground attack aircraft, one Gazelle wasdestroyed by a surface to air missile and twoGazelles were lost to small arms fire.

“Black Hawk Down!” Somalia 1992 to 1994

During Operation Restore Hope US forces inSomalia lost two Blackhawk helicopters [Ref. 9]during an air assault operation in Mogadishu. Itis noteworthy that both losses were caused byrocket propelled grenades (RPG-7). A thirdhelicopter was badly damaged in the same wayduring this operation, but managed to return tobase. This is the first widely documentedoccasion where “non-conventional” weapons areknown to have been used successfully againsthelicopters. The extremely short range (less than500m) and poor kinematic performance of anRPG-7 effectively limit it as a threat to all butvery close combat or urban warfare situations.

Chechnya

Between August 1999 and September 2001, 15helicopters were recorded as having beendamaged by ground fire, of which nine werelosses. 40 fatalities occurred as a result,although some of these are attributable to theengagement itself rather than the subsequentcrash. It is worth noting that all but two of thelosses were caused by small arms fire, theremainder being attributed to RPG and shoulderlaunched SAM.

Although the Mil.8 Hip represents a significantproportion of these losses, it is also interesting tonote that the more heavily armoured (andarmed) Mil.24 Hind has also incurredconsiderable losses. No statistics have beencompiled for subsequent years, but helicoptercombat losses have continued in the theatre.Notable amongst these is the loss of a Mil.26Halo transport helicopter, reportedly to ashoulder launched SAM, resulting in the death ofapproximately 120 passengers and crew.

Enduring Freedom, Afghanistan 2001 -

During Operation Anaconda (March 2002) [Ref.10], an AH-64 Apache and an UH-60 Blackhawkwere both hit by RPG and a further five Apachewere reported to have been damaged by smallarms fire. All aircraft are believed to havesurvived these encounters. Reports [Ref. 11]suggest, however, that one CH-47 Chinook wasshot down with the loss of several crew andpassengers and a second seriously damaged, inanother operation in this theatre.

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Operation Iraqi Freedom, 2003

It was reported [Ref. 12] that 31 out of 32 AH-64Apache engaged in operations near Karbalasustained combat damage although only onefailed to return. All of the helicopters wereoperational within 96 hours despite having atleast 6 bullet holes in each. The damage wasreported to have been as the result of gunfire (upto 30mm) and RPG impacts but none from radarguided SAMs.

This is, perhaps, a testament to the value ofsound vulnerability reduction measures designedinto Apache, rather than (as has occasionallybeen suggested) an indication of poor battlefieldsurvivability.

Discussion The examples presented here areinsufficient to draw detailed statisticalconclusions. However, a number of importantpoints may be inferred, the most significant ofwhich may be that, despite improved survivabilitydesign and tactics since their early deploymentin Vietnam, helicopter combat losses, althoughreduced, remain at a substantial level.

Against an opponent with a low level of weapontechnology, losses remain inevitable. The lack ofsophistication of the opponent merely serves toprompt the development of more effectivemethods of bringing weapons to bear, or morenovel methods of attack, i.e. the "cheap kill".With the current state of world affairs it wouldperhaps be wise for the designer and operator ofcombat helicopters to consider potential threatswhich increase in novelty rather than in level oftechnology.

The nature of helicopter combat operations issuch that significant loss of life is possible whena helicopter is successfully engaged. A cheap killis no less likely to result in a significant highhuman and materiel loss than a high-tech kill.

As evidenced by the Falklands conflict losses,even a technologically advanced opponent willfind the cheap kill a potent counter to the combathelicopter. However, high-tech countermeasureswill have little or no effect on the cheap kill. Thiscan only be countered by tactics (susceptibilityreduction) and hardening the platform(vulnerability reduction).

In-theatre modifications to the helicopter mayprove effective, but will often adversely affect its

performance. The impact on performance maybe reduced, and survivability will be increased, ifthese measures are driven by an analysis of thecharacteristics and capabilities of the threat, andimplemented as a part of the aircraft design (orupgrade) process. The USSR implemented anumber of vulnerability reduction designmeasures to the Su25 Frogfoot ground attackfighter as a direct result of experience during thewar in Afghanistan. These measures reportedlyreduced combat losses by a significant margin,admittedly in concert with tactical adaptations.

Vulnerability Assessment

Analysts have been assessing the vulnerabilityof combat vehicles since at least the early1940s. Initially the assessments consisted ofsimple judgements of which parts werevulnerable to particular threats and estimates ofvulnerable areas from a range of aspect angles.The probability of kill given a hit was simplyobtained by dividing the estimated vulnerablearea by the measured presented area from anygiven approach direction.

In the early to mid 1960s, the US started todevelop vulnerability assessment tools such asVAREA (Vulnerable Area) and HART (Helicopter(vulnerable) Area and Repair Time) codes [Ref.13]. These two codes evolved into the COVARTmodel (Computation of Vulnerable Area andRepair Time) which became “operational” in themid to late 1970s. The latest iteration of thiscode is still used extensively in the US for theassessment of fixed and rotary wing aircraftvulnerability.

In the UK a similar capability has beendeveloped commencing in the late 1970s. TheINTAVAL (Integrated Air target VulnerabilityAssessment Library) suite of computerprogrammes has been used for the past 20years in the assessment of air target vulnerabilityand anti-air weapon lethality. Separate modulesexist to assess the effects of inert projectiles(fragments and bullets) and shells (bothinternally and externally bursting). A shot lineapproach is employed where individualfragments are passed through a geometricrepresentation of the target and damage toindividual systems assessed.

A typical target will consist of between 1500 and2000 components which are either critical tomaintaining flight or mission capability or provide

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shielding to those components. Figure 3illustrates a section of a typical targetdescription. Each component is modeled interms of its physical dimensions, location withinthe target and its material composition.Component damage algorithms are assigned toeach vulnerable component, which express thedegradation to its functionality as a result offragment mass and (typically) impact velocity.Failure logic (fault tree) analysis is used tocalculate whole target vulnerability by summingthe individual component vulnerabilities whilsttaking account of the duplication andredundancy present within the system.

Figure 3: Detail from a typical INTAVAL targetdescription of a combat aircraft

System and component Vulnerability

Detailed vulnerability analyses of battlefieldhelicopters will reveal the critical componentsand their contribution to overall helicoptervulnerability. The analysis may be performed ina variety of ways, ranging from a visualinspection of the helicopter or its design to adetailed assessment using vulnerability analysissoftware.

The analysis should consider the range ofthreats, be they small arms fire or surface to airguided weapons, each of which will becharacterised by a particular damagemechanism. The analysis should also accountfor the required residual capability of the

helicopter after receiving damage. Should thehelicopter simply remain capable of maintainingcontrolled flight for sufficient time to reach asuitable landing position, or should it remaincapable of performing a weapons deliverymission?

A detailed, software driven assessment mayproduce images similar to the one at figure 4.This provides a reasonable indication of thevulnerability of a large, generic assault helicopterto attack by various inert shot - small armsbullets or AP shell. It has been assumed that thehelicopter need only remain controllable afterbeing damaged.

In this particular case, the pilot represents alarge proportion of the helicopter's vulnerablearea - he is both relatively large in area andhighly likely to be incapacitated if hit. It has beenassumed here that the second crewman doesnot have the ability to take control of the aircraftin the event that the pilot is incapacitated. If theopposite were the case, then the pilot would notshow as vulnerable in the figure (unless a singlebullet could kill both, e.g. for an attack fromahead). However, if it were required that thehelicopter should be able to deliver it's weaponsafter being damaged then both the pilot andsecond crewman would show as vulnerable.

A similar logic may be applied to the engines.Figure 4 assumes that both engines are requiredfor controlled flight, although if only one survivingengine were sufficient then the overall helicoptervulnerability would be somewhat lower thanshown. The drive shaft to the main rotor gearboxhas been assumed to be too large or robust tobe severed by a small arms projectile.

The hydraulic lines provide power to the mainand tail rotor controls. Normally this system isduplicated, and it might be expected that thiswould render it invulnerable. However, in thecase above it has been assumed that theduplicated lines are not separated and,consequently, a single shot could overcome theredundancy. Hydraulic fluid is a potential fireraiser, and this may also contribute to the overallvulnerability of the helicopter.

The controls transmit the pilot's commands tothe rotor blades, and if severed, the helicopterwill become uncontrollable. They show amedium to high level of vulnerability because ithas been assumed that the projectile is only just

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Figure 4: Assault Helicopter Component Vulnerability to Small Arms Fire

large enough to sever a rod, and that certainimpacts may fail to achieve this. The helicopter'sweapons are also assumed to be particularlyvulnerable; if hit they are likely to detonatesympathetically, with potentially catastrophicconsequences.

Other components although critical arenevertheless far less vulnerable. The main fueltank is the largest single critical component inthe helicopter, but a single bullet (or even alarger calibre inert shell) is unlikely to dosufficient damage to deny a supply of fuel to theengines within a short period after attack. If thehelicopter were required to survive for longerthan it took for the tank to leak dry, then thatcomponent would also show as highlyvulnerable. Furthermore, if there was a highprobability of the damage resulting in a fire, thenthe fuel tank would show as far more vulnerable.

Similarly, the gearboxes and driveshafts (main,tail rotor and intermediate) are highly critical, butare probably too robust to be significantlydamaged by a small projectile, although a largerprojectile would have a better chance of causingcritical damage.

Helicopter vulnerability

Many vulnerability assessments have beenperformed on a wide range of battlefieldhelicopters, considering various threat weapons.Whilst these cannot be discussed in detail, it ispossible to identify generic trends.

Three classes of helicopter are considered;

� Small utility helicopter (SUH), with a MTOWof around 5,000kg, (e.g. Lynx, UH-1);

� Large utility helicopter (LUH), with a MTOWof around 10,000kg to 15,000kg, (e.g. Mil.8,Merlin);

� Dedicated attack helicopter (AH), (e.g.Apache, A129, Mil.24). These are variable insize but are typified by designed-insurvivability features.

Figure 5 provides a general indication of thelevel of vulnerability each class of helicopter willhave for the various, less sophisticated, threatsthat were discussed earlier in this paper.

It can be seen that single hit Pk/h (SSPk/h) isunsurprisingly low for small arms and largercalibre AP rounds, although a SSPk/h of 5%translates to a much higher 20% if, for instance,five bullets hit the helicopter. Perhaps moresurprising is the observation that there is littledifference between SSPk/h for a highly survivableattack helicopter and that for the LUH. However,it should be remembered that these figures arebased on the precondition of a threat hitting thehelicopter. The probability that the AH will be hit(Phit) is bound to be considerably lower than Phitfor the LUH, by virtue of both the size andstealthiness of the former.

Notwithstanding the accuracy of the SSPk/hfigures given above it is clear that the probabilityof surviving a successful engagement by anyexplosive threat is likely to be low. However,there are means by which vulnerability may bereduced, and the SSPk/h for the AH is based onthe assumption that some of these have been

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Probability of kill given a single random hit (SSPk/h)AH LUH SUH

Small arms (7.62mm - 14.5mm) 1% >1% 5%Inert cannon (20mm - 40mm) 2% 1% 10%HE cannon (20mm - 40mm) 20% 40% 60%Rocket Propelled Grenade 30% 60% 90%Shoulder launched SAM 50% 80% 100%

Figure 5: Estimated Helicopter Vulnerability Data, note thatthe figures relating to RPG are based upon estimates of HE

shell performance

implemented in the design of the helicopter.While 50% may appear to signify a highprobability of loss, this should be contrasted withthe SSPk/h for the SUH and LUH.

Principles of Vulnerability Reduction

There are four basic methods of reducing targetvulnerability, these are:

� duplication of critical components orsystems;

� reducing the presented area of criticalsystems by concentration or miniaturisation;

� internal (and external) armour or shielding;� the use of damage tolerant component

designs.

The last of these, damage tolerant componentdesign, is simply the application of the previousthree principles at component level, and will notbe discussed further in this section.

Duplication

The duplication of components or systems is acommon design feature in most helicoptersgenerally employed as a safety measure againstreliability failures, rather than specifically as avulnerability reduction measure. The distinctionis an important one. To provide a true reductionin vulnerability, a duplicated system must alsoinclude the separation of the two redundant sub-systems to ensure that a single threat has noopportunity to damage both. This requirement isoften overlooked; indeed the co-location of tworedundant components is often a specificdesigned feature, employed to easemaintenance.

It is clear that the “separation” of two duplexcomponents is only effective from particular

attack directions. This implies the need tounderstand the nature of the threat and considervery carefully its expected direction of attack.This understanding can subsequently be used toensure adequate separation for likely attackscenarios.

It is also important to consider the nature of thethreat; if the primary threat is a single warheadfragment or inert projectile then the level ofduplication required to provide protection needonly be limited. If the primary threat isconsidered to be from, for example, an internallydetonating cannon shell, then to be trulyeffective, the two components must be morewidely separated.

Vulnerable area reduction

This design goal is driven by the fact that Pk/h isa function of the sum of presented areas of allcritical components in the helicopter, and theminiaturisation of critical components will reducetheir chance of being hit. However,miniaturisation generally carries with it acommensurate reduction in damage tolerance(robustness) that must also be considered.

Alternatively, the concentration or grouping ofcritical components will also reduce theircumulative presented area and hence the Pk/h ofthe helicopter. At first sight, this would appear tocontradict the requirement for separationdiscussed above. However, while separation is amethod applicable to duplicated criticalcomponents, grouping is applicable to singularlyvulnerable or non-duplicated components, suchas the cyclic and collective pitch and yaw controlrods. The loss of any one of these three controlaxes might be expected to result in the loss ofthe helicopter, so separating them would conferno survivability benefits. However, by grouping

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them together, a reduction in vulnerable areamay be achieved.

Again, it can be seen that this reduction is onlyeffective from one particular attack direction -from all other directions no benefit has beenachieved. Once more, the designer must beadvised by an understanding of the threatagainst which he is attempting to counter.

Shielding

The use of armour, either parasitic or integral,implies considerable weight, cost andperformance penalties and should be treated asa last resort, at which stage it should be usedsparingly. A detailed vulnerability analysis of thehelicopter will allow the designer to position thearmour to its best effect.

One potential method for reducing mass ofarmour is to increase the line of sight thicknessby inclining the armour, as per the glacis plateon an armoured fighting vehicle. This is arelatively simple prospect for an AFV designerwho will have a good idea as to the likelydirection of the threat. For an aircraft designer,this may present more of a problem.

An alternative to parasitic armour is, at thedesign stage, to make use of robust, non-criticalcomponents to provide shielding.

Vulnerability reduction measures forhelicopters

The previous section describes the basicprinciples of vulnerability reduction, applicable toany military vehicle. These principles may beapplied specifically to the components andsystems within a battlefield helicopter, and avariety of such applications are discussed below.

Analysis of data gathered from Vietnam [Ref. 10]allows the most frequent causes of loss to beidentified, these are listed in figure 6 in thegeneral order of frequency of occurrence.

It is not the intention of this paper to provide acomprehensive description of all possiblemeasures, rather a selection of potentialmethods. Their application will depend verymuch on the specific design and role of aparticular combat helicopter, and also on theexpected threats against which the helicoptermust be hardened. In general those causes of

loss identified in figure 6 have beenconcentrated upon.

Crashes ForcedLanding

Mission Abort

EngineF/ControlsCrewFuel

FuelLubricationEngineMech. ControlsMain Rotor

CrewPrecautionaryFuelMain RotorHydraulics

Figure 6: Causes of helicopter losses/missionaborts in Vietnam conflict

Powerplant

The design of small gas turbine engines withhigh inherent ballistic tolerance is beyond thescope of this paper. However a few design rulesto minimise engine vulnerability are presentedbelow. The positioning of engines can have asignificant effect on the overall survivability of thehelicopter however. Most purpose-designedcombat helicopters of recent years incorporatewidely spaced twin engines in their design (forexample Ka.50 Hokum, Rooivalk, YAH-63, AH-64, Tiger, etc). The separation of the enginesprovides a degree of protection to one engineshould the other one catastrophically break up.Some designs also incorporate armouredfirewalls between the engines to provideadditional protection, the A-129 being a notableexample. The introduction of armoured panelswithin the engine bay of the (fixed wing) Su-25Frogfoot as a result of combat experience inAfghanistan reportedly led to a significantreduction in combat losses due to small/mediumcalibre impacts.

In terms of the engines themselves, location of(vulnerable) accessories on the top or insidefaces of the engines will provide a degree ofprotection to those components. Centrifugalcompressors are generally more robust thanaxial ones, etc. If all else fails the designer cansimply armour the engine bay in its entirety, thisbeing the case for the Mil. 24 Hind.

Control system

The impracticality of totally duplicatingmechanical control runs makes it necessary toincrease the ballistic tolerance of the flyingcontrol signaling system. To some extent thecontrol runs in the cockpit will receive a degreeof shielding from armour placed around the

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cockpit. This is particularly noticeable in caseslike the AH-64 Apache (shown in figure 7) wherethere are several armour panels mounted awayfrom the immediate vicinity of the crewmembers.The survivability of the control rods can beimproved significantly by increasing theirdiameter such that they retain a minimum levelof stiffness despite ballistic damage. A roddiameter of 40mm is considered as a minimumdiameter to provide tolerance to a tumbled12.7mm bullet. Sufficient clearance needs to beallowed between the control rods and adjacentstructure such that petalled sections, due toballistic damage, of the rod do not foul thestructure.

Figure 7: Armour protection in and around thecockpit of AH-64 Apache

The use of Carbon Fibre Reinforced Plasticcontrol rods has been introduced in somehelicopters over the past 10 years. These rodsoffer benefits in terms of increased stiffness,allowing rods, which are longer than theirmetallic counterparts and are also lighter. Interms of tolerance to damage they do avoid theproblems of petalled damage described above

and exhibit reasonable levels of tolerance tosmall threats, however they are prone todelamination causing considerable reduction inbuckling stiffness and they can be seriouslyeffected by fire damage. This can also be true ofaluminium rods and generally controls routingthrough the engine/transmission bays should bemanufactured from either steel or titanium.

Although the tubular section of the rods makesup a large percentage of the total area of theflying control system, the tolerance of the endfittings and flying control levers can also beincreased with a corresponding reduction invulnerability. During the design of the UH-60Blackhawk, Sikorsky developed a tri-pivotconcept for rod end attachments which replacedthe single bolted joint at the rod/bellcrankinterface with no less than three. Any singlepivot could be damaged without loss of functionresulting [Ref. 14].

An alternative route to reducing the vulnerabilityof the bellcrank is shown in figure 8. In thisconcept the usual “L” shaped lever has beenreplaced by a triangular componentincorporating a redundant loadpath. Thebellcrank has sufficient stiffness to survive lossof one arm whilst still transmitting the (generallylow) control forces. The inset in figure 8illustrates a damaged component having beenimpacted by a 12.7mm AP projectile.

Figure 8: Damage Tolerant Flying Control Lever(Bellcrank) with Redundant Loadpath, (inset

view of damaged section)

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In certain older helicopter designs, control of thetail rotor pitch is accomplished by pairs of cablesrather than rods, the Mil. 24 Hind falls into thiscategory. Whilst the reduced diameter of thecables makes them less likely to be hit, animpact will almost always result in loss of yawcontrol. The replacement of these cable systemswith a series of conventional flying control rods islikely to reduce vulnerability significantly.

In future it is likely that more use will be made ofFly By Wire (FBW) or Fly By Light signalingsystems. These have the added advantage ofallowing multiplexed control routes to beintroduced although it is important to ensure thatsufficient separation between routes ismaintained. A study conducted in the UKsuggests that in many cases a proximity burstingHE round is likely to result in a kill due todefeating multiple control lanes.

Crew

The effect of armoured crew seats on helicoptervulnerability is debatable. Under certaincircumstances, a single shot incapacitation ofboth crew is possible, and crew armour willprevent this. However, from the majority ofattack directions a single inert projectile isincapable of incapacitating both crewmen.Consequently, the use of individual armour couldbe seen as a waste of valuable armour massbudget.

Of course, crew armour is much more thansimply "protecting the system" and it isacknowledged that the lives of two trained pilotsmay be more important than the survival of thehelicopter. However, a limitation of armouredcrew seats is that they provide protection only tothe crew, and a significant volume of criticalcontrol components remain outside the envelopeprotected by the seat armour. A more efficientdesign would protect not only the crew, but alsothese exposed control components.

Explosive projectiles (HE shell) have a greaterprobability of incapacitating both pilot andcopilot, and individual protection is morereasonable. However, a similar level ofprotection could be achieved by introducing anarmoured barrier between the two crewmen.Figure 8 illustrates the armour protection fitted toAH-64, including a blast screen betweencockpits providing protection from internallydetonating 23mm HE shell [Ref. 15].

Fuel system

The fuel system represents possibly a largerpresented area than any other system in thehelicopter and, as such, will contributesignificantly to vulnerability. The fuel systempresents a twofold hazard; the helicopterdepends on an uninterrupted supply of fuel inorder to maintain flight. Secondly, the fuelpresents a significant fire risk to other systems inthe event that it is released into the airframe.

Fuel supply protection The use of self-sealingfuel tank and fuel pipe liners will prevent fuel lossresulting from damage caused by small arms APprojectiles. As a minimum, this should be appliedto the engine feed tank and the pipe(s)connecting it to the engines. It may only benecessary to protect the lower part of the tankwalls in this way in order to provide a “get homecapability”. (Figure 9).

Figure 9: Damage Tolerant Fuel System

If the helicopter is equipped with several fueltanks, a cross-feed mechanism will allow anyone tank to provide a supply in the event ofdamage to another tank. However, it should berecognised that, in this case, the cross-feedvalve will represent a region of high vulnerabilitywhereby the entire fuel system can be cut off bya single projectile impact.

Fire/explosion prevention measures Foam fillingin dry bays adjacent to the fuel tanks will preventthe accumulation of large quantities of spilt fuel,as will dry bay drain holes or vents. Bulkheads orbaffles in the dry bays will also prevent leakedfuel migrating towards components that mightignite the escaped fuel. Hot metal and electricalcomponents should be removed from bays likelyto accumulate escaped fuel or fuel vapour, andthe ullage in the tank itself should be inerted toprevent explosion of fuel/air mixture in the tank.

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Inerting of the tank was originally achieved byfilling the tanks with a low-density reticulatedfoam. This method had the benefits of beingsimple and quick/easy to accomplish, howeverthe filling of fuel tanks with foam led to significantlonger term maintenance problems together witha not inconsiderable weight penalty. In futurethere is likely to be more reliance on systemswhich introduce an inert gas into the ullagespace, these can use either bottled gas sourcesor an “On Board Inert Gas Generating System”(OBIGGS).

A tank mounted fuel pump will continue to pumpfuel in the event of a fuel pipe rupture, unless itis switched off, leading to the escape of largequantities of fuel into the airframe. This may beprevented by using engine-mounted pumps,which suck fuel from the tanks. If the fuel line isdamaged the flow of fuel will be halted.Although beneficial to reducing helicopter lossesthe use of suction fuel tanks may be limited inhot and high conditions due to fuel vapourisationeffects.

Lubrication

The majority of current helicopters have adegree of run dry capability in all gearboxes, ingeneral a minimum of 20 minutes runningwithout oil is required.

For the YAH-64 grease lubricated gearboxeswere specified for the first time. This providedthe ability to sustain ballistic damage withvirtually no loss of either lubricant orperformance. A (known) impact on a gearboxwould not necessarily result in a mission abort.

Drive train

The tail drive shafts associated with mostconventional helicopters, the Boeing NOTARdesigns being a notable exception, areparticularly prone to ballistic damage. Themajority of “older” designs generally have smalldiameter drive shafts, which although unlikely tobe susceptible to damage from fully alignedsmall calibre bullets and fragments could suffercomplete severance following an impact from atumbled round. To reduce the susceptibility ofthe drive shaft to this and larger threatsmanufacturers have tended to introduce verylarge diameter shafts, the requirement to toleratea fully tumbled impact from a 12.7mm rounddictates a diameter of at least 115mm [Ref. 16].

The ballistic tolerance of these components toimpacts from inert rounds up to 23mm is high.

In addition to reducing the vulnerability of theshafts themselves the various couplings andbearings are also hardened to provide a highdegree of tolerance. The majority of westerndesigned helicopters use a “Thomas” coupling tojoin drive shafts, in general these employ anumber of redundant fixings such that the loss of2 bolted joints (out of 6) can be tolerated.

The loss of drive to the tail rotor does notautomatically result in the loss of the helicopterproviding there is sufficient forward speed tomaintain directional stability. An often seenserious consequence of the severance of a taildrive shaft is the loss of all control signaling andhydraulic power to the tail rotor gear box and inmany circumstances structural failure of thetailcone. The use of anti-flail bearings to preventlarge movement of the tail drive shaftsignificantly reduces the likelihood of majorfailures occurring.

Main Rotor

Considerable effort has been expended byhelicopter designers over the past 30 years ormore on the development of ballistically tolerantmain rotor blades. In itself this is probably not amajor challenge, however, a very highperformance blade similar to the WestlandBERP 3 (British Experimental Rotor Programme)design but with high levels of tolerance to largecalibre threats is somewhat more difficult toachieve.

Tests have been conducted in the UK to quantifythe performance of the BERP blades fitted to theLynx helicopter fleet, it is not possible to revealthe results of those tests here but it is sufficientto say that, against small arms projectiles, theblades performed well.

For large calibre projectiles, the most effectivemethod of providing a high degree of tolerance isto utilise a large cross sectional area spar (orspars) and use a ductile material. For this reasonolder designs of metallic blade are generallymore tolerant than composite ones. Figure 10shows a section of (metallic) blade from a SeaKing helicopter that was impacted (in flight) by a20mm round. The outcome of this engagementwas a slightly higher than normal level ofvibration.

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Figure 10: Damaged Main Rotor Blade

Hydraulic systems

The duplication of hydraulic systems iscommonplace in helicopter design, with mostmodern helicopters having at least two separatesystems. However, this is generally done toimprove the overall reliability of the hydraulicsupply rather than to increase the level oftolerance to battle damage.

In order to maximise survivability where duplexsystems exist it is important to ensure thatwherever practical hydraulic lines follow differentroutes from reservoirs/pumps to the controlactuators making maximum use of robuststructure and systems as shielding.

Over recent years considerable effort has beenexpended in the development of non-flammablehydraulic fluids to reduce the risk of fire given animpact on part of the system. Although thesefluids are less susceptible to fire than earlierones they do still pose a significant fire risk.

Conclusions

Early use of helicopters on the battlefieldinevitably resulted in a high attrition rate. Theearly tactics to counter this were to fly high

enough to be effectively out of small arms range,UK studies indicate that this is generally around3000 ft.

With the advent of large numbers of shoulderlaunched missiles in the late 1960s revisedtactics were developed, these resulted inhelicopters flying at low level and inevitablybeing impacted by large numbers of small armsprojectiles. To counter the effects of this damagea high degree of ballistic tolerance wasincorporated into the design requirements.

As a result of this experience a new “breed” ofbattlefield helicopters were developedcommencing with the US AAH (AH-64 Apache)and UTTAS (UH-60 Blackhawk) helicopters,both of which exhibit low levels of vulnerability.

This paper has provided a brief overview ofhelicopter operations in conflicts over the past 40years and has outlined some of the methods thatcan be incorporated into the design to enhanceballistic tolerance and hence platformsurvivability.

It is important in this age of sophisticatedDefensive Aids Suites that future helicopters atleast maintain the currently attainable levels ofballistic tolerance. Military operations in supportof peace keeping or enforcement involvehelicopters coming into contact with significantnumbers of low technology threats which cannoteasily be countered by electronic means.

The only sure counter to single or multipleimpacts from typical threats widely utilised byterrorist/freedom fighter organisations is to eitheravoid being hit through tactical flying or toprovide the helicopter with a high degree oftolerance to their effects.

It is essential that efforts to incorporate ballistictolerant design in future helicopters is done earlyin the design stage. The difficulties in retrofittingsuch features are generally too great to warrantgoing back and changing an already frozendesign.

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References

1. The fundamentals of aircraft combatsurvivability analysis and design, RobertE Ball, AIAA 1985

2. We Were Soldiers Once … And Young,Major General H G Moore, RandomHouse 1992

3. Chickenhawk, Robert Mason, Corgi1984

4. The Huey and Huey Cobra, W Siuru,TAB 1987

5. Vietnam: the helicopter war, PhilChinnery, Airlife 1991

6. The cost of Soviet involvement inAfghanistan, US Directorate ofintelligence report SOV87-10007, 1987

7. The Soviet war in Afghanistan: Historyand harbinger of future war?, M YNawroz and L W Grau, Foreign MilitaryStudies Office, 2002

8. The lessons of modern war. Vol. 3 TheAfghan and Falklands conflicts, A HCordesman and A R Wagner, Westview1990

9. Black Hawk Down, Mark Bowden,Penguin, 2001

10. Choppers in the coils, D Billingsley, 200211. Allied Press report 05 March 200212. Coming under fire, Aviation Week &

Space Technology 12 May 200313. COVART II User Manual, US JTCG/ME-

84-3, 198514. Survivability of the Army/Sikorsky YUH-

60A Helicopter, James Foulk, SikorskyAircraft, 1976

15. Minigraph 18 AH-64 Apache, LyleMonson and Kenneth Peoples, Aerofax,1986

16. Damage Tolerant design of the YAH-64Drive System, Albert Edwards and AlanNeugebauer, Hughes Helicopters, 1978

Acknowledgements

Midland Counties Publications for providingfigure 7, which was taken from Reference 15.

Figure 2 was obtained from Venik’s Aviation viawww.aeronautics.ru The preparation of this paper was funded by theUK MOD Defence Applied Research Programme(DSTL/CP07498, QINETIQ/FST/CP033133).

� Crown Copyright 2003. Published with thepermission of the Defence Science andTechnology Laboratory on behalf of theController of HMSO, © 2003 QinetiQ ltd

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