Helicopter Survival: Fly or Die? AUTHOR Major Jordan D. Yankov, USMC CSC 1991 SUBJECT AREA - Aviation EXECUTIVE SUMMARY TITLE: HELICOPTER SURVIVAL: FLY OR DIE? Improvements in helicopter capabilities and tactical application have led to the development of an array of anti- helicopter weapons systems. Helicopter design must take into account the requirement to survive and continue to operate as an effective combat platform in a hostile environment. Expanding the profiles and capabilities of modern combat helicopters is within technological reach. Cost ultimately drives compromises made to achieve a viable balance between helicopter design and the ability to survive. Survivability of combat aircraft is directly related to minimizing the likelihood of being detected. A helicopter has five distinct signatures by which its presence can be detected: visual, acoustic, radar, infrared, and electronic. Ultimately, despite all efforts to minimize detection, helicopters on the modern battlefield can expect to be sensed, acquired, and engaged by enemy weapons systems. However, much can be done tactically to prevent sustaining life threatening and mission aborting damage. The ability of a helicopter to survive an enemy attempt to engage with weapons may be built into the aircraft's comprehensive maneuver quality and structural design. Acquisition and engagement may further be reduced through the use of an integrated early warning system that is tailored to sense specific threat systems. Receiving timely warning of a threat through the use of sensors is one thing, but the consequent reactive use of countermeasures, when required, is what's important. The use of radar or infrared jammers, and chaff or decoy flares could cause sufficient threat system degradation to keep the helicopter from being engaged by enemy weapons. Inevitably, helicopters will be drawn into close combat and will be engaged by anti-aircraft weapons systems. The ability of an aircraft to withstand battle damage is referred to as tolerance to fire. Tolerance to fire is best considered during the design stage when ballistic tolerance, redundancy and separation, armor and fire suppression can be assessed. Modern materials can achieve ballistic immunity from a variety of calibers for many parts of a helicopter. A great deal of effort has been devoted to saving the crew and limiting damage to the helicopter in the event of a crash. Improvements in crew and passenger compartment protection, post- crash fuel containment, and emergency egress systems enhance helicopter survivability suites. An optimum balance between cost, performance, and survivability must be established to design and field a combat helicopter capable of successfully flying into harms way. HELICOPTER SURVIVAL: FLY OR DIE? OUTLINE Thesis Statement. Helicopter design must take into account the requirement to survive and to continue to operate as an effective combat platform in a hostile environment. I. Survivability Requirements A. Balance Armament, Mobility and Protection B. Trade-offs/Cost Potential II. Detection A. Tactical Measures B. Positioning of Sights C. Reduction of Visual Signature D. Acoustic Signature E. Radar Signature F. Infrared Signature G. Miscellaneous Electromagnetic Emitters III. Survival on the Ground A. Concealment B. Camouflage IV. Engagement A. Tactical Considerations B. Agility 1. Power 2. Speed 3. G Capability C. Warning Sensors 1. Radar Warning Receiver 2. IR Missile Detection System 3. Acoustic Sensors 4. Wire Detection D. Countermeasures 1. Chaff 2. IR Decoy Flares 3. IR Jammer 4. Radar Jammer 5. Smoke E. Tolerance to Fire and Helicopter Design 1. Destruction Priority 2. Component Ballistic Material Design 3. Redundancy 4. Fire Detection/Extinguishing Systems F. Crashworthiness 1. Crew Protection 2. Cockpit Design 3. Occupant G Load 4. Post-Crash Fire 5. Aircraft Egress Considerations Helicopter Survival: Fly or Die? The Vietnam War was the spark that caused a virtual explosion in the use of combat helicopters in tactical support of ground forces. This war brought American weaponry up to the standard known as "high technology." Helicopter technology advances and refinements in tactical application have continued to afford the ground commander speed, flexibility, and surprise to successfully prosecute his mission. Improvements in helicopter capabilities and tactical application have led to the development of an array of anti- helicopter weapons systems. Aircraft combat survivability is defined as the capability to avoid and/or withstand a man-made hostile environment. Helicopters operating on the modern battlefield will be exposed to anti-aircraft artillery, surface- to-air missiles, small arms fire, enemy armed helicopters, massed artillery, and multiple rocket fire. Fixed-wing aircraft of various design also pose a threat. The shift from a low level threat environment to higher intensities indicates the need for more advances in survivability technology. Helicopter design must take into account the requirement to survive and continue to operate as an effective combat platform in a hostile environment. Until recently, prohibitive cost, design oversights, and production deficiencies have limited the ability to field an integrated weapons system built around the concept of survivability. Traditionally, helicopter power and design limitations restricted available payload to fuel and mission specific equipment at the expense of ballistic hardening and survivability suites. Expanding the profiles and capabilities of modern combat helicopters is within technological reach. What is required are helicopters that are difficult to detect, difficult to hit when detected, capable of continuing the mission after sustaining a hit, and crashworthy if shot down. Improvements on typical helicopter designs currently fielded should include a full suite of warning devices, countermeasures sub-systems, and protective measures including ballistic hardening of critical components. These basic improvements translate into a requirement for more powerful engines and transmission systems. A complete survivability package is very expensive. Cost ultimately drives compromises made to achieve a viable balance between helicopter design and the ability to survive. (2:10) Survivability of combat aircraft is directly related to minimizing the likelihood of being detected. Aside from exposure to enemy indirect fire, something an aircrew can do little about except to depart the area under attack, a helicopter has to be seen to be hit. Successful acquisition requires direct line of sight between the observer or sensor and the target. Direct line of sight acquisition requirements can be exploited tactically by training aircrew to fly at very low level using terrain masking techniques to minimize detection. Unfortunately, very few combat environments afford terrain that allows masking to be used as a continuous means to avoid detection. A helicopter has five distinct signatures by which its presence can be detected: visual, acoustic, radar, infrared, and electronic.(1:229-243) All signatures must be managed or suppressed to minimize exposure to threat systems. Reduction of visual signature is most critical as evidenced by the fact that helicopters have been most successfully engaged by small arms fire and optically sighted anti-aircraft artillery. Visual detection is achieved by the unaided eye, optical instruments such as magnifiers and image intensifiers, or electronic enhancement such as television. (2:24) Success of these systems depends on size and shape of the helicopter, its contrast with the background and movement.(1:286) Small helicopters are more difficult to detect than large helicopters. Paint schemes should enhance the machine's ability to blend into the background of its surroundings.(2:98) Movement across country tends to attract attention, but even when in a hover, sun glint off a canopy or rotor can give away a helicopter's position. Canopy glass construction that reduces glint and appropriate paint composition that minimizes rotor flicker are desirable. Obviously, visual detection capabilities are degraded at night or during weather that produces marginal visibility. Since the battlefield is perceived as being a noisy environment, the acoustic signature of a combat helicopter has not been given the attention it requires. There are occasions, such as during covert infiltration operations, insertion and extraction of ground reconnaissance units, and certain counter- insurgency operations for example, when noise reduction is essential. Some of the noise can be attenuated by terrain features, but usually the noisy presence of a nearby helicopter gives more than adequate warning to allow a trained observer to point his weapon in the correct direction before the helicopter becomes visible. The biggest contributors to noise are main rotors, tail rotors, and engines. Slowing down main rotor speed, improvements in blade tip design, and no-tail rotor (NOTAR) designs reduce inherent acoustic signatures.(3:123) A diverse family of combat radar systems are prevalent on the modern battlefield. A target illuminated by radar produces a signature known as radar cross-section. Size, material composition, and design configuration (shape) are all factors contributing to the amount of radar cross-section associated with an aircraft.(3:11) Radar cross-section also depends on the electronic characteristics of the radar itself. Detection of an aircraft by radar depends on the target's signature and the ability of the radar and its operator to process the information it "sees." Low altitude flight operations afford ground clutter that can degrade a radar system's capability to track and distinguish targets. Helicopter design shape and use of structural materials that absorb or deflect radar energy away from the tracking source can further reduce detection. Helicopters do not necessarily need to be in forward flight in order to be tracked by radar. Helicopters have many moving parts, such as the main rotor and tail rotor, which render the airframe susceptible to radar detection, even while hovering. Energy conductive coatings and broad-band radar absorbing materials reduce the risk of detection during this flight profile. (1:229) Turning to the question of infrared energy detection, there is little chance of a combat aircraft ever being designed that does not emit heat. Heated metal on the exhaust section of a helicopter engine produces a low-band infrared signature. The hot exhaust gases emitted by the engine produce a middle-band infrared signature.(2:103) Infrared suppressors may be designed and used to vent hot exhaust gases into the rotor system. Blower fans may also be used to introduce ambient air into the engine exhaust housing. This allows the rotor/fan system to mix and dilute the hot gases with cooler air, thereby reducing the helicopter's infrared signature. An infrared suppressing system, combined with infrared low reflective paint to resist detection by thermal imagery, reduces the acquisition range of heat-seeking missiles and enhances the capabilities of on-board infrared jammers and decoy flares. In addition to the aircraft signatures already discussed, any intentional or inadvertent electromagnetic emission from the aircraft, such as the aircraft's navigation radar, radar altimeter, laser range-finder/designator, or any other active system, is liable to detection. (1:228) Active electromagnetic systems should be directional and shielded by design, and their use must be kept to a minimum. Non-emitting, on-board, passive systems, such as inertial navigation, should be the design of choice. Radio communications should be infrequent, using burst transmission technology to further reduce the detectable, electromagnetic foot-print. Before leaving the matter of detection it is well to remember that combat helicopters spend more time on the ground than they do in the air. Survival on the ground is possible by denying the enemy information on where the aircraft are based. Concealment and camouflage are essential to prevent enemy reconnaissance pilots and sensors from discovering hidden aircraft.(3:10) Hardened ramp areas, and dispersed parking plans lessen the likelihood of sustaining collateral damage during an attack. Ultimately, despite all efforts to minimize detection, helicopters on the modern battlefield can expect to be sensed, acquired, and engaged by enemy weapons systems. However, much can be done tactically to prevent sustaining life threatening and mission aborting damage. Good tactics, such as minimizing helicopter exposure time, may not give the enemy time to aim their weapon. Moving to a new position immediately after attacking a target from ambush will also reduce enemy engagement capabilities. Flight profiles that off-set enemy detection capabilities often require advanced aircrew skills. Training programs that support tactical survivability concepts are essential. The ability of a helicopter to survive an enemy attempt to engage with weapons may be built into the aircraft's comprehensive maneuver quality and structural design. Design criteria that provides for agility enhances the aircraft's ability to remain an elusive target. While speed is very important, the concept of agility in describing the flight qualities of a helicopter simply means the ability to accelerate and decelerate in all dimensions. A power margin that allows for rapid rate of climb, and the ability to arrest high rates of descent so as not to strike the ground, is required. In addition, a helicopter must be able to rapidly displace laterally in order to avoid engagement by enemy weapons systems. The combination of power margin, flight control response, and structural strength, allows a helicopter to fly a high G profile, when required, to reduce susceptibility to enemy weapons systems once acquired and engaged. Acquisition and engagement may further be reduced through the use of an integrated early warning system that is tailored to sense specific threat systems. To see without being seen is the ideal state - the reverse may be disastrous. Warning the crew of the presence of enemy weapons systems plays a large part in the avoidance of being engaged. A radar warning receiver which can provide immediate indication of a radar, which is associated with a weapon that threatens the helicopter, will alert the crew to the potential danger and help them decide what to do. For example, if a pilot knew that a missile was approaching from a particular direction, he could make an evasive maneuver, at the appropriate moment, that would increase the miss distance.(1:271-276) Future enhancements of the radar warning receiver will allow the system processor to detect and identify laser threats such as range finders, designators, and beam-riding missiles. The need for an infrared sensing device to automatically dispense decoy flares led to the development of an infrared- homing, missile detection system. Any missile with the proper angle-intercept velocity will trigger the helicopter's decoy flares automatically. Wires are a hazard to low-flying helicopters during peace and war. Thorough flight planning and continuous lookout, especially while flying during periods of low visibility or during night vision goggle operations, are essential.(8:4-47) Electronic means of detecting wires have been under study for a number of years and millimeter wave radars and lasers seem to be most promising. However, simple wire cutters and deflectors may be the solution. Receiving timely warning of a threat through the use of sensors is one thing, but the consequent reactive use of countermeasures, when required, is what's important. An evasive maneuver may be all that is needed, but active countermeasures may be more effective. The most direct method is to kill the offending system. When the direct method is not feasible, the use of radar or infrared jammers and chaff or decoy flares could cause sufficient threat system degradation to keep the helicopter from being hit.(2:100-105) Chaff is one in the family of countermeasures known as expendables. Chaff consists of amass of radar reflectors, that is launched as soon as a tracking radar locks on to a helicopter. The chaff has the effect of breaking lock which forces the radar system or its operator to re-establish lock. This allows the aircraft to take evasive action or to fire upon the radar system. Combined with a radar warning receiver, chaff may be dispensed automatically, or manually, and in the correct direction. A typical dispenser found in the United States inventory can carry 60 chaff units. The same dispenser can be programmed to fire another member of the expendable family, the decoy flare. This system offers an infrared missile an alternate target by creating a second, hotter heat source to home in on. As an example, in 1986, televised video footage showed Soviet transport aircraft landing in Afghanistan regularly dispensing flares to decoy heat-seeking missiles. Infrared missiles can also be decoyed through the use of an infrared jammer. The jammer is omnidirectional and projects modulated, infrared energy away from the helicopter. The projected energy is several times greater than the aircraft's heat signature. A secondary infrared source at an alternate position has the effect of confusing the missile's homing system as to the true location of the helicopter.(1:285-286) Radar jammers have been in use since the Second World War, but until recently have been too large, heavy, complex, or impractical for employment on helicopters. Radar jammers deny threat radar systems the information required to develop tracking and targeting information.(1:276-282) This system simply "outshouts" or masks the electronic echo from the aircraft. Several systems are currently being tested and hold promise for the future. Finally, a helicopter may use a variety of smokes in self- defense. Smoke could provide ordinary screening from enemy view or could be used to degrade laser beams and infrared seekers. In Sweden, the FFV company is marketing (initially for hovering helicopters) a self-screening smoke launcher.(2:99) The launcher can fire ten rockets over an arc of 120 degrees which burst 330ft ahead of the helicopter to create an instant smoke screen as an optical countermeasure to direct fire weapons. Inevitably, helicopters will be drawn into close combat and will be engaged by anti-aircraft weapons systems. Helicopters that have been designed to absorb enemy fire and remain functional allow the combat commander to continue to prosecute his mission with less reduction in combat power. The ability of an aircraft to withstand battle damage is referred to as tolerance to fire. Aircraft that are more vulnerable to fire are softer, whereas aircraft that are less vulnerable to fire are harder or tougher. The more vulnerable an aircraft is, the more likely it will be killed when hit by enemy fire. When designing a helicopter which is to be tolerant to fire, it is vital to consider the caliber of the weapon against which the protection is required and its purpose. Because of the relatively large warhead found in missiles, hit survivability for the helicopter and its crew is essentially a matter of luck. Historically, most damage to low flying aircraft is sustained from anti-aircraft artillery. The high rates of fire associated with anti-aircraft artillery often inflict multiple hits, especially to a slow moving target. Protection can be provided for strikes by anti-aircraft artillery from 7.62mm to 23mm in caliber.(3:108) However, multiple hit protection is far from simple or cheap. In decreasing order of priority the enemy intends to destroy the helicopter, kill the crew, force it to land, or prevent it from continuing on its mission. Helicopter designers plan for the requirement to maximize crew survivability and to keep systems operational for mission accomplishment. Inevitably, trade-offs will arise in the degree of protection against cost, space, complexity, and weight.(2:9-10) Designers try to keep the most vulnerable areas as compact and protected as possible. Designers also attempt to disperse duplicated components and systems throughout the helicopter to reduce chances of total failure caused by a centralized hit. Tolerance to fire is best considered during the design stage when ballistic tolerance, redundancy and separation, armor and fire suppression can be assessed. Modern materials can achieve ballistic immunity from a variety of calibers for many parts of a helicopter. Lightweight, composite, epoxy-resin materials, with good ballistic tolerance, can be used to protect dynamic components such as rotor systems and drive shafts.(4:2) Epoxy-resin materials may also be used as armor protection for the crew compartment.(2:141) A great deal of effort has been devoted to the problems of saving the crew and limiting damage to the helicopter in the event of a crash. Most helicopters produced in the United States since the early 1980's incorporate crashworthy design criteria. To maintain a protective, crashworthy shell around crew and passenger compartments, airframe components such as engines and transmissions must be anchored to be able to withstand high G loads under the stress of crash impact. Unrestrained, massive components become crushing projectiles under the force of impact with the ground. Crash-attenuating seats that "stroke" or compress during impact, limit G loads on helicopter occupants. Containment of fuel is the only practical method of preventing post-crash, helicopter fires. Impact resistant fuel tanks that are positioned away from ignition sources and penetrating objects are basic considerations. Many helicopters today are built with self-sealing fuel tanks and breakaway fuel line connection valves. Fuel is contained upon crash impact, reducing the possibility of fire, which gives the passengers and crew a chance to successfully egress the aircraft. Inertial reel, five-point safety belts with a single-point release mechanism, allow for rapid helicopter egress. Doors and windows that jettison further enhance egress requirements. Emergency breathing devices provide oxygen in the event of forced water landing that submerges the crew. Ejection seats are feasible, but the problem of interfacing seat ejection with main rotor clearance requirements has proven to be too complicated and cumbersome to incorporate into system design. The consideration of survivability in the design of aircraft has been important in past conflicts and will be more so in future conflicts. Survivability in a combat helicopter is a function of many interrelated factors which include threat, tactics, training, aircraft performance, target acquisition and engagement, and survivability equipment. The cost of procuring and operating modern aircraft makes it imperative to study each factor to ensure that no weaknesses exist that might surface in combat. Survivability should be considered during the early design phase of an aircraft. An optimum balance between cost, performance, and survivability must be established to design and field a combat helicopter capable of successfully flying into harms way. Just as in the last war, the next war will require us to fly the aircraft we have at hand. There will be no time to train new aircrews or to build new aircraft. What we have has to survive.
