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an autorotation. If the governor fails on the low side,
normal r.p.m. may not be attainable, even if the throttle
is manually controlled. In this case, the collective has
to be lowered to maintain r.p.m. A running or roll-on
landing may be performed if the engine can maintain
sufficient rotor r.p.m. If there is insufficient power,
enter an autorotation.
ABNORMAL VIBRATIONS
With the many rotating parts found in helicopters, some
vibration is inherent. You need to understand the cause
and effect of helicopter vibrations because abnormal
vibrations cause premature component wear and may
even result in structural failure. With experience, you
learn what vibrations are normal versus those that are
abnormal and can then decide whether continued flight
is safe or not. Helicopter vibrations are categorized into
low, medium, or high frequency.
11-15
LOW FREQUENCY VIBRATIONS
Low frequency vibrations (100-500 cycles per minute)
usually originate from the main rotor system. The
vibration may be felt through the controls, the airframe,
or a combination of both. Furthermore, the vibration
may have a definite direction of push or thrust. It may
be vertical, lateral, horizontal, or even a combination.
Normally, the direction of the vibration can be determined
by concentrating on the feel of the vibration,
which may push you up and down, backwards and
forwards, or from side to side. The direction of the
vibration and whether it is felt in the controls or the
airframe is an important means for the mechanic
to troubleshoot the source. Some possible causes
could be that the main rotor blades are out of track or
balance, damaged blades, worn bearings, dampers out
of adjustment, or worn parts.
MEDIUM AND HIGH FREQUENCY VIBRATIONS
Medium frequency vibrations (1,000 - 2,000 cycles per
minute) and high frequency vibrations (2,000 cycles
per minute or higher) are normally associated with outof-
balance components that rotate at a high r.p.m., such
as the tail rotor, engine, cooling fans, and components
of the drive train, including transmissions, drive shafts,
bearings, pulleys, and belts. Most tail rotor vibrations
can be felt through the tail rotor pedals as long as there
are no hydraulic actuators, which usually dampen out
the vibration. Any imbalance in the tail rotor system is
very harmful, as it can cause cracks to develop and
rivets to work loose. Piston engines usually produce a
normal amount of high frequency vibration, which is
aggravated by engine malfunctions such as spark plug
fouling, incorrect magneto timing, carburetor icing
and/or incorrect fuel/air mixture. Vibrations in turbine
engines are often difficult to detect as these engines
operate at a very high r.p.m.
TRACKING AND BALANCE
Modern equipment used for tracking and balancing the
main and tail rotor blades can also be used to detect
other vibrations in the helicopter. These systems use
accelerometers mounted around the helicopter to detect
the direction, frequency, and intensity of the vibration.
The built-in software can then analyze the information,
pinpoint the origin of the vibration, and suggest the
corrective action.
FLIGHT DIVERSION
There will probably come a time in your flight career
when you will not be able to make it to your destination.
This can be the result of unpredictable weather conditions,
a system malfunction, or poor preflight planning. In any
case, you will need to be able to safely and efficiently
divert to an alternate destination. Before any crosscountry
flight, check the charts for airports or suitable
landing areas along or near your route of flight. Also,
check for navaids that can be used during a diversion.
Computing course, time, speed, and distance information
in flight requires the same computations used
during preflight planning. However, because of the
limited cockpit space, and because you must divide
your attention between flying the helicopter, making
calculations, and scanning for other aircraft, you should
take advantage of all possible shortcuts and rule-ofthumb
computations.
When in flight, it is rarely practical to actually plot a
course on a sectional chart and mark checkpoints and
distances. Furthermore, because an alternate airport is
usually not very far from your original course, actual
plotting is seldom necessary.
A course to an alternate can be measured accurately
with a protractor or plotter, but can also be measured
with reasonable accuracy using a straightedge and the
compass rose depicted around VOR stations. This
　
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