**4.2 Helicopter Operations:**

Special nature of helicopter flying

Vibration exposure

Noise exposure

Flicker vertigo

Break-off phenomenon

Spatial disorientation

Crash survival and crash worthiness

Emergency evacuation (land and water)

Helicopters are classified as a rotorcraft, certified under 14 CFR,
which includes the autogyro. Autogyros are not commonly seen and are
distinguished from helicopters, in that while they do have a rotary
wing, unlike a helicopter, the autogyro's rotary wing is unpowered. It
swings freely, and its motion is due to the forward motion of the
aircraft. The autogyro power source is usually a pusher propeller.

While there are few autogyros today, they were very popular in the 1930s
and the design features of helicopters, save for the anti-torque tail
rotor, were all pioneered with the autogyro. However, the advent of the
helicopter in the mid-1940s quickly relegated the autogyro to a
curiosity, because the autogyro is capable of short field takeoffs and
landings, but cannot perform vertical takeoffs and landings, and more
importantly, cannot hover.

![](media/image1.png){width="3.9298611111111112in"
height="2.1479166666666667in"}

JT-5 Autogyro

Helicopters have directly powered main rotors, usually a jet engine with
a transmission linkage to the rotor, but some helicopters have internal
combustion engines as their power supply. The flight characteristics of
the helicopter permit it to hover or move in any direction. Helicopters,
in principle, use either a single main rotor combined with tail rotor
that counteracts the torque of the main rotor, or two main rotors that
rotate in opposite directions, thus canceling out the torque.

Most helicopters hover within "ground effect". This is defined as a
height above ground equivalent to the rotor diameter, that is, if the
span of the tip of one rotor to the other is 100 feet, then the
helicopter is capable of hovering in ground effect up to 100 feet. The
importance of ground effect is that up to that limit, the air is
physically compressed beneath the helicopter, between the helicopter and
the ground, and a cushion of the air is created. At altitudes above
ground in excess of one rotor diameter the ground effect is lost, with
the result is that much more power must be provided by the rotors and
more fuel burned as the cushion dissipates, forcing the helicopter to
hover, by brute force. This also means there is an altitude limit to
hovering, even with ground effect.

The highest helicopter hovering capability, under the most unusual
circumstances, has been at altitudes of approximately 15,000 to 18,000
feet. Translational flight, or normal forward flight without hovering,
is limited to about 30,000 feet, but rarely do helicopters operate at
such altitudes. Helicopters are also subject to an aerodynamic speed
limit. This is because as a helicopter flies forward, one blade is also
moving forward relative to the airflow and meets the air at a much
higher velocity and provides lift. The opposite blade, known as the
retreating blade, however, is moving much slower than the helicopter,
and as its speed drops relative to the air, its angle of attack, or the
angle in which the rotor blade is going through the air relative to the
flight path, must be increased in order to provide the same amount of
lift as the faster moving, advancing blade. However a limit is reached
when a helicopter is flying at the same speed as the rotors are turning.
This effectively creates a zero air speed of the retreating blade. It
provides no lift, and the retreating blade then stalls. The retreating
blade stall actually occurs below this velocity and retreating blade
stall causes the helicopter to fall off towards the side of the loss of
lift.

![](media/image2.png){width="3.9319444444444445in" height="2.6625in"}

Highest helicopter rescue in history took place in 1995, a Bell 206L4 at
24,000 feet on a glacier in Pakistan. Helicopter wreck in the
background.

In theory, it would be possible to increase the speed of the rotor
blades, but there is a point at which the blades would become supersonic
and the incredible racket of several thousand sonic booms per minute
striking the pilots has been demonstrated in experimental (XF-94)
aircraft to be completely intolerable, not to mention structurally very
demanding on the rotor blades and the vehicle. Thus, for all practical
purposes, the top speed limit for helicopters is in the order of 250
knots, and most helicopters cruise in the 100 to 150 knot range.
Autogyros are similarly limited.

Operations of rotorcraft are essentially identical to fixed wing
aircraft for private, commercial and air transport operations. A
separate pilot's license for rotorcraft is required even if the pilot
already holds a pilot's license for other kinds of aircraft. The same
medical certification for the operation of rotorcraft applies for
conventional aircraft with a Third Class for private, Second Class for
commercial, and First Class for airline transport operations.

Because of their utility, helicopters are extremely versatile, and are
not only used for transporting passengers, but are seen in
law-enforcement, news reporting, cargo operations, agriculture, and
rescue and medical operations.

![](media/image3.png){width="4.0569444444444445in"
height="2.3201388888888888in"}

Television News Helicopter, Robertson R-44

The nature of the rotary blade system causes considerable low-frequency
vibration and higher noise levels. Typically, helicopter pilots are
required to wear helmets with high quality hearing protection, and the
seats in many helicopters are designed to absorb more of the low
frequency vibration exposure. Overall, the cabin environment is more
similar to a heavy, over-the-road truck than other forms of aircraft. In
longitudinal studies, noise-induced sensory-neural hearing loss is the
most common problem encountered with helicopter crew members.

As with any other aircraft, spatial disorientation can be a problem, but
helicopter operations provide two additional problems. One is known as a
flicker vertigo. In this situation, a light source, usually above the
rotor blades, such as an overhead sun or moon, will be observed blinking
off and on, and at certain frequencies of flicker, angular vection
illusions occur. This may result in motion sickness or become a source
of irritation and distraction. However, one should be aware that photic
stimulation and frequencies in the 8 to 14 hertz range is that of the
encephalographic alpha rhythm, and can produce seizures in rare
individuals who are susceptible to flicker-induced vertigo. The
prevalence of this condition is very low (less 1 in 20,000), and the
number of pilots affected are very few, although some helicopter crashes
are felt to have been caused by this.

In addition, during hover flight, at low altitude, a visual obscuration
may occur when flying over ground material that can be blown up into the
rotors. This causes a "white out" with snow, or a "brown out" with dust
and sand. The result is that the helicopter is engulfed in the cloud of
a suspended particulate material and the pilot will lose outside
reference and horizon cues. Most helicopters are equipped with radar
altimeters that permit them to determine their above-ground height, even
at one or two feet above ground level, as well as transitional motion.
It is important to avoid sideways motion while landing, as a phenomenon
known as dynamic roll over may occur: When drifting sideways, sometimes
due to a cross wind, a helicopter may touch down with a wheel while
moving sideways. This torque will cause the entire helicopter to rotate
around its longitudinal axis until the rotor strikes the ground, and the
vehicle rolls up in a ball. Maintaining height above ground and ground
position is extremely important and crew coordination can help prevent
this, particularly when flying over poor reference surfaces, such as
water.

Because helicopters often operated near the ground, crash worthiness and
crash survival are important features to helicopter design and
operations. The controls of a helicopter are mounted somewhat
differently from a fixed wing aircraft. The pilot typically flies with
the right hand on the control stick (which tilts the rotor mast) and the
left hand on a "collective". The collective is a combined throttle and
pitch control. By twisting the throttle, engine power is controlled, and
by pulling back and forth on the collective, the pitch angle of attack
for the rotor blades is changed. Due to the fact that the helicopter is
unstable in flight and must be flown constantly, the pilots cannot take
their hands off the control stick. If the pilot needs to manipulate
controls on the center console, it must be done with the left hand.
Therefore, unlike conventional aircraft, a helicopter pilot will be
seated in the right seat, whereas in conventional aircraft, the pilot in
command is seated in the left seat.

Additional modifications of a helicopter are required for crash
worthiness. Because the helicopter's descent is usually vertical rather
than in a longitudinal axis, the stress loading for crashing tends to be
in the vertical axis, with energy absorbing seats and crushable material
in the belly of the aircraft.

Also, unlike fixed wing aircraft, ejection/bailout is not an option. The
rotor also creates an additional problem because of the blade's inertia
upon crashing may cause them to bounce and shatter and enter the
cockpit. Special rotors with frangible tips have been built into modern
helicopters, and the transmission has been moved from out of the cockpit
and tends now to be mounted above the crew compartment. Fuel lines are
usually changed to quick disconnects and new experimental helicopters,
using engine thrust, instead of an anti-torque tail rotor are being
evaluated to reduce the potential problems of striking objects in flight
with the tail rotor, or more importantly, on the ground. All too
frequently, ground personnel have accidentally walked into a tail rotor
with fatal results.

Post accident evacuation of a helicopter is extremely difficult, due to
its tendency to roll over, both from the dynamic roll over effect, as
well as torque transfer from the rotating blades. Therefore, part of
aircrew training for helicopters requires specialized training for
emergency evacuation, on land and water, including "worst-case" scenario
is inverted underwater.

Increasing demands on helicopter pilots involve night and all-weather
operations. Although this has become routine with military personnel,
the use of night vision devices, for light amplification, and infrared
detectors are now entering civilian, police and law-enforcement
departments. These create new problems with limited field of vision and
decreased acuity at night. Both require extensive training and safety
procedures to avoid accidents.

Reference: Parmet, A.J., and Gillingham, K., Chapter 8, [Spatial
Orientation in Fundamentals of Aerospace Medicine]{.underline}, 3^rd^
Edition, DeHart and Davis.