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.
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