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Rotary Aircraft Survivability

Aircraft survivability is a primary concern throughout planning and execution of all missions. Army aircrews operate in an extremely hazardous environment of highly lethal air defense threats. The array of enemy air defense systems includes radar, IR, EO, and directed-energy weapons. Proper use of Aircraft Survivability Equipment (ASE), combined with careful route planning and movement techniques, greatly reduces the enemy's ability to effectively engage helicopters.

Appropriate tactics are the most effective means of enhancing aircraft survivability. Mission planning begins with a detailed analysis of the enemy air defense threat. Planners then determine how to best avoid or degrade that threat. Flight planning incorporates a detailed flight route analysis, seeking to avoid known or suspected enemy air defense sites and reduce the chances of detection or engagement by the enemy. Flight modes (low-level, contour, and NOE) and techniques (traveling, traveling overwatch, and bounding overwatch) are selected. Effective terrain flight not only limits LOS exposure times but also places the aircraft's radar, IR, and visual signature in a cluttered environment.

Aviation elements choose the flight mode and movement technique based on available terrain and the probability of enemy contact.

Flight Modes

Flight modes include low-level, contour, and nap-of-the-earth (NOE). Because of the threat environment in which helicopters will operate, if deployed tactically, there exists the requirement to fly close to the earth. This type of flight has been segmented into these three primary profiles.

Nap-of-the-earth [NOE] entails flying as near the ground as possible, within the limits of flying safety, thus making use of all available cover and defilade and keeping exposure time to an absolute minimum. This technique is difficult and challenging. It requires maximum proficiency in both flying and adjustment skills, but it is being accomplished successfully. NOE requires flight as close to the earth's surface as vegetation or obstacles will permit, while generally following the contours of the earth. Air speed and altitude are varied as influenced by the terrain, weather and enemy situation. The pilot preplans a broad corridor of operation based on known terrain features which has a longitudinal axis pointing toward the objective. In flight, the pilot uses a weaving and devious route within his preplanned corridor while remaining oriented along the general axis of movement in order to take maximum advantage of the cover and concealment afforded by terrain, vegetation and man made features. By gaining maximum cover and concealment from enemy detection, observation and fire power, nap-of-the-earth flight exploits surprise and allows for evasive actions.

Contour flight is at low altitude conforming generally, and in close proximity to the contours of the earth. This type flight takes advantage of available cover and concealment in order to avoid observation or detection of the aircraft and/or its points of departure and landing. It is characterized by a constant air speed and a varying altitude as vegetation and obstacles dictate.

Low level flight is conducted at a selected altitude at which detection or observation of an aircraft is avoided or minimized. The route is preselected and conforms generally to a straight line and a constant air speed and indicated altitude. This method is best adapted to flights conducted over extended distances or periods of time.

The most demanding of these profiles is NOE flight because of its unique control and navigation requirements. The aviator who is flying NOE must maintain a high level of alertness to detect and avoid obstacles while maintaining maximum concealment and desired flight path. The aviator acting as navigator has the difficult task of determining aircraft position and giving navigation instructions based on recognition of marks and terrain feature, in a highly accelerated perceptual world. In many cases the pilot also has the responsibility for monitoring instruments and making necessary radio contacts.

The Soviets quickly embraced NOE tactics in Afghanistan. "In 1980, losses to SA-7 surface-to-air missiles (a hand-held, heat-seeking missile) led to a change in tactics at the end of 1980 or early 1981. Since then, the Hinds have used nap-of-the-earth flight patterns, for which the machines were not designed nor their crews properly trained. There have been reports of Hind rotors striking the rear of their own helicopters during some of these nap-of-the-earth flights. The wear on airframes and systems caused by these lower-altitude flights has also greatly increased rates of operational attrition.These nap-of-the-earth tactics are a significant change from those employed in 1979-80."

The foremost goal for both the UH-60 Black Hawk and AH-64, when it comes to survivability, was to be able to reach the target, perform the mission, and return home undetected. The Cold War threat scenario allowed detection visually, audibly, or by radar or infrared signature. The primary means these two craft have for avoiding detection is superior nap-of-the-earth performance. That is, the ability to fly among trees, popping up to perform its mission, and then returning nearer the earth where visual, radar, and infrared detection is more difficult, if not impossible.

Movement Techniques

Movement techniques include traveling, traveling overwatch, and bounding overwatch. Traveling is used for moving rapidly over the battlefield when enemy contact is unlikely, or the situation requires speed to evade the enemy. All aircraft move at the same speed. Units often employ low-level flight with the traveling movement technique.

Traveling overwatch is used when speed is essential and enemy contact is possible. Lead aircraft move constantly and trail aircraft move as necessary to maintain overwatch of lead. Units often employ contour flight with the traveling overwatch technique. Bounding overwatch is used when expecting enemy contact. It is the slowest movement technique. It uses alternate or successive bounds, with lead aircraft moving to a position while trail aircraft overwatch. The overwatching aircraft then bound to a position ahead of the lead aircraft. Each aircraft bounds separately while the other overwatches the movement. Length of the bound depends on the terrain, visibility, and the effective range of the overwatching weapon system. Units normally employ NOE flight with the bounding overwatch technique.

In urban areas, fields of fire are restricted, landing areas are limited, and buildings provide cover for enemy forces to engage helicopters with near impunity. The presence of noncombatants, protected structures, and important resources and facilities normally demands careful weapons and munitions selection to avoid collateral damage. The proximity of enemy and friendly ground forces increases the risk of fratricide. Communications may be degraded by many structures. Thermal effects from paved surfaces and the channeling effects of buildings can cause wind conditions to vary significantly from point to point. Special, restrictive ROE should be expected. Standoff is key to aviation survival.

Aircraft Survivability Equipment (ASE)

ASE systems can be categorized in three general areas: signature reduction (design passive), SA (electronic passive), and jamming and decoying (electronic active) countermeasures. ASE is used in combination with sound tactics to enhance survivability. ASE cannot compensate for dangers introduced by poor tactics.

Signature Reduction (Design Passive) measures are incorporated during manufacture or modification of aircraft. Examples include flat canopies, exhaust suppressors, and IR reflective paint. Aviators, in choosing how much signature to expose to the threat, also influence signature control. The IR and radar signatures are least when viewed from the front, while the maximum IR signature is from the rear quadrants. The maximum visual and radar signatures are from the sides. Aircrews reduce the signature by using terrain and changing the aircraft's physical orientation to the threat, based on the type of enemy system anticipated or encountered.

Studies and analysis of military helicopter operations have shown that the survivability and lethality of US helicopters can be increased by reducing their acoustic signature, which will make them more difficult to detect, track, and engage. The Helicopter Quieting Program is developing revolutionary new rotorcraft design tools that will enable the creation of novel rotor systems that can dramatically reduce the acoustic signature of a helicopter without sacrificing flight performance. Current rotor development is very costly, involving a time-consuming iterative, trial and error cycle of analysis and model wind tunnel tests, or occasionally, a faster but much riskier analysis path directly to full-scale wind tunnel/flight test. This program will leverage recent advances in computational fluid dynamics to develop physics-based predictive design tools that will enable helicopter rotor designers to explore the revolutionary potential of emerging new rotor noise-reducing technologies with a reduction in the cost and cycle time associated with iterative analysis and test.

Electronic passive ASE systems provide early warning to aircrews, allowing them reaction time. These systems include radar detecting sets, laser detecting sets (LDSs), and IR missile warning systems. Aircrews use cues from these systems to change modes of flight or increase vigilance (actively seek masking terrain features).

If detected, aircrews use electronic active countermeasures (jammers, chaff, and flares) to provide some protection while they maneuver to masking terrain or outside threat range. Radar threats can be decoyed by chaff. Other sets can jam radar and IR threats. Flares can decoy IR missiles.

Helicopters such as the UH-60 Black Hawk and AH-64 Apache have a variety of features that reduce detectability. Both have suppression devices that cool the hot exhaust gases from the turbine engines before they are exhausted to the atmosphere, for the exhaust plume is the primary source of infrared radiation. Radar signatures are reduced by controlling the shape of the fuselage and through proper attention to the engine inlet screening and rotor head. In addition, both craft have black boxes that provide a warning when enemy radar is beamed at them. They also have means to jam enemy detection systems, by dispensing flares and chaff, for example.

While the characteristic sound of a chopper can never be eliminated, it can be definitely reduced by careful design of the main and tail rotors. For example, the tail rotor of the AH-64 has its four blades not spaced perpendicular to one another purposely to reduce tail rotor noise. Visual detection can be reduced by low reflectance and camouflage paint. In the case of the AH-64, the nearly flat canopy glass goes a long way toward preventing reflections that could give away the aircraft's location.

But even if the Black Hawk and AH-64 are detected, they are designed to survive small arms fire. The Black Hawk is totally invulnerable to 7.62-mm rounds and has minimum vulnerability to 12.7-mm and 23-mm projectiles while in forward flight. The AH-64 can totally withstand the 12.7-mm threat and has minimum vulnerability to 23-mm high energy incendiary (HE I) rounds during hover and forward flight.



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