0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
Region of disk
vortex interference
10 knots
360°
15 knots 20 knots
315°
285°
Wind
Figure 11-9. Main rotor disk vortex interference.
Region of possible yaw introduction by weathercock stability
0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
10 knots
360°
5 knots
15 knots
17 knots
Wind
Figure 11-10. Weathercock stability.
airflow in order to provide a steady and constant antitorque
reaction. The pitch and AOA of the individual blades will
determine the thrust. A change to either of these alters the
amount of thrust generated. A pilot’s yaw pedal input causes
a thrust reaction from the tail rotor. Altering the amount of
thrust delivered for the same yaw input creates an imbalance.
Taking this imbalance to the extreme will result in the loss
of effective control in the yawing plane, and LTE will occur.
This alteration of tail rotor thrust can be affected by numerous
external factors. The main factors contributing to LTE are:
1. Airflow and downdraft generated by the main rotor
blades interfering with the airflow entering the tail
rotor assembly.
2. Main blade vortices developed at the main blade tips
entering the tail rotor disk.
3. Turbulence and other natural phenomena affecting the
airflow surrounding the tail rotor.
4. A high-power setting, hence large main rotor
pitch angle, induces considerable main rotor blade
downwash and hence more turbulence than when the
helicopter is in a low power condition.
5. A slow forward airspeed, typically at speeds where
translational lift and translational thrust are in the
process of change and airflow around the tail rotor
will vary in direction and speed.
6. The airflow relative to the helicopter;
a. Worst case—relative wind within ±15° of the
10 o’clock position, generating vortices that
can blow directly into the tail rotor. This is
dictated by the characteristics of the helicopters
aerodynamics of tailboom position, tail rotor size
and position relative to the main rotor and vertical
stabilizer, size and shape. [Figure 11-9]
b. Weathercock stability—tailwinds from 120° to
240° [Figure 11-10] , such as left crosswinds,
causing high pilot workload.
c. Tail rotor vortex ring state (210° to
330°). [Figure 11-11] Winds within this region
will result in the development of the vortex ring
state of the tail rotor.
7. Combinations (a, b, c) of these factors in a particular
situation can easily require more antitorque than the
helicopter can generate and in a particular environment
LTE can be the result.
Certain flight activities lend themselves to being at higher
risk of LTE than others. For example, power line and pipeline
patrol sectors, low speed aerial filming/photography as well
as in the Police and Helicopter Emergency Medical Services
(EMS) environments can find themselves in low-and-slow
situations over geographical areas where the exact wind speed
and direction are hard to determine.
0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
Region of roughness due to tail rotor vortex ring state
10 knots
360°
5 knots
15 knots
17 knots
Wind
Figure 11-11. Tail rotor vortex ring state.
Unfortunately, the aerodynamic conditions that a helicopter
is susceptible to are not explainable in black and white terms.
LTE is no exception. There are a number of contributing
factors, but what is more important in preventing LTE is to
note them, and then to associate them with situations that
should be avoided. Whenever possible, pilots should learn
to avoid the following combinations:
1. Low and slow flight outside of ground effect.
2. Winds from ±15º of the 10 o’clock position and
probably on around to 5 o’clock position [Figure 11-9]
3. Tailwinds that may alter the onset of translational lift
and translational thrust, and hence induce high power
demands and demand more anti-torque (left pedal)
than the tail rotor can produce.
4. Low speed downwind turns.
5. Large changes of power at low airspeeds.
6. Low speed flight in the proximity of physical
obstructions that may alter a smooth airflow to both
the main rotor and tail rotor.
Pilots who put themselves in situations where the combinations
above occur should know that they are likely to encounter
LTE. The key is not to put the helicopter in a compromising
condition, while at the same time being educated enough
to recognize the onset of LTE and being prepared to react
quickly to it before the helicopter cannot be controlled.
Early detection of LTE, followed by the immediate flight
control application of corrective action, applying forward
cyclic to regain airspeed, applying right pedal not left as
necessary to maintain rotor rpm, and reducing the collective
(thus reducing the high-power demand on the tail rotor), is the
key to a safe recovery. Pilots should always set themselves
up when conducting any maneuver to have enough height
and space available to recover in the event they encounter
an aerodynamic situation such as LTE.
Understanding the aerodynamic phenomenon of LTE is by
far the most important factor in preventing an LTE-related
accident, and maintaining the ability and option either to go
around if making an approach or pull out of a maneuver safely
and re-plan, is always the safest option. Having the ability to
fly away from a situation and re-think the possible options
should always be part of a pilot's planning process in all phases
of flight. Unfortunately, there have been many pilots who
have idled a good engine and fully functioning tail rotor disk
and autorotated a perfectly airworthy helicopter to the crash
site because they misunderstood or misperceived both the
limitations of the helicopter and the aerodynamic situation.
Main Rotor Disk Interference (285–315°)
Refer to Figure 11-9. Winds at velocities of 10–30 knots from
the left front cause the main rotor vortex to be blown into the
tail rotor by the relative wind. This main rotor disk vortex
causes the tail rotor to operate in an extremely turbulent
environment. During a right turn, the tail rotor experiences a
reduction of thrust as it comes into the area of the main rotor
disk vortex. The reduction in tail rotor thrust comes from the
airflow changes experienced at the tail rotor as the main rotor
disk vortex moves across the tail rotor disk.
The effect of the main rotor disk vortex initially increases the
AOA of the tail rotor blades, thus increasing tail rotor thrust.
The increase in the AOA requires that right pedal pressure
be added to reduce tail rotor thrust in order to maintain the
same rate of turn. As the main rotor vortex passes the tail
rotor, the tail rotor AOA is reduced. The reduction in the
AOA causes a reduction in thrust and right yaw acceleration
begins. This acceleration can be surprising, since previously
adding right pedal to maintain the right turn rate. This thrust
reduction occurs suddenly, and if uncorrected, develops
into an uncontrollable rapid rotation about the mast. When
operating within this region, be aware that the reduction in
tail rotor thrust can happen quite suddenly, and be prepared
to react quickly to counter this reduction with additional left
pedal input.
Weathercock Stability (120–240°)
In this region, the helicopter attempts to weathervane,
or weathercock, its nose into the relative wind.
[Figure 11-10] Unless a resisting pedal input is made, the
helicopter starts a slow, uncommanded turn either to the right
or left, depending upon the wind direction. If the pilot allows
a right yaw rate to develop and the tail of the helicopter moves
into this region, the yaw rate can accelerate rapidly. In order
to avoid the onset of LTE in this downwind condition, it is
imperative to maintain positive control of the yaw rate and
devote full attention to flying the helicopter.
Tail Rotor Vortex Ring State (210–330°)
Winds within this region cause a tail rotor vortex ring state to
develop. [Figure 11-11] The result is a nonuniform, unsteady
flow into the tail rotor. The vortex ring state causes tail
rotor thrust variations, which result in yaw deviations. The
net effect of the unsteady flow is an oscillation of tail rotor
thrust. Rapid and continuous pedal movements are necessary
to compensate for the rapid changes in tail rotor thrust when
hovering in a left crosswind. Maintaining a precise heading
in this region is difficult, but this characteristic presents
no significant problem unless corrective action is delayed.
However, high pedal workload, lack of concentration, and
overcontrolling can lead to LTE.
When the tail rotor thrust being generated is less than the
thrust required, the helicopter yaws to the right. When
hovering in left crosswinds, concentrate on smooth pedal
coordination and do not allow an uncommanded right yaw to
develop. If a right yaw rate is allowed to build, the helicopter
can rotate into the wind azimuth region where weathercock
stability then accelerates the right turn rate. Pilot workload
during a tail rotor vortex ring state is high. Do not allow a
right yaw rate to increase.
LTE at Altitude
At higher altitudes where the air is thinner, tail rotor thrust
and efficiency are reduced. Because of the high-density
altitude, powerplants may be much slower to respond to
power changes. When operating at high altitudes and high
gross weights, especially while hovering, the tail rotor thrust
may not be sufficient to maintain directional control, and
LTE can occur. In this case, the hovering ceiling is limited
by tail rotor thrust and not necessarily power available. In
these conditions, gross weights need to be reduced and/
or operations need to be limited to lower density altitudes.
This may not be noted as criteria on the performance charts.
Reducing the Onset of LTE
To help reduce the onset of LTE, follow these steps:
1. Maintain maximum power-on rotor rpm. If the main
rotor rpm is allowed to decrease, the antitorque thrust
available is decreased proportionally.
2. Avoid tailwinds below airspeeds of 30 knots. If loss
of translational lift occurs, it results in an increased
power demand and additional antitorque pressures.
3. Avoid OGE operations and high-power demand
situations below airspeeds of 30 knots at low altitudes.
4. Be especially aware of wind direction and velocity
when hovering in winds of about 8–12 knots. A loss
of translational lift results in an unexpected high power
demand and an increased antitorque requirement.
5. Be aware that if a considerable amount of left pedal
is being maintained, a sufficient amount of left pedal
may not be available to counteract an unanticipated
right yaw.
6. Be alert to changing wind conditions, which may be
experienced when flying along ridge lines and around
buildings.
7. Execute right turns slowly. This limits the effects of
rotating inertia, and decreases loading on the tailrotor
to control yawing.
Recovery Technique (Uncontrolled Right Yaw)
If a sudden unanticipated right yaw occurs, the following
recovery technique should be performed. Apply full left
pedal. Simultaneously, apply forward cyclic control to
increase speed. If altitude permits, reduce power. As recovery
is affected, adjust controls for normal forward flight. A
recovery path must always be planned, especially when
terminating to an OGE hover and executed immediately if
an uncommanded yaw is evident.
Collective pitch reduction aids in arresting the yaw rate but
may cause an excessive rate of descent. Any large, rapid
increase in collective to prevent ground or obstacle contact
may further increase the yaw rate and decrease rotor rpm.
The decision to reduce collective must be based on the pilot’s
assessment of the altitude available for recovery.
If the rotation cannot be stopped and ground contact is
imminent, an autorotation may be the best course of action.
Maintain full left pedal until the rotation stops, then adjust to
maintain heading. For more information on LTE, see Advisory
Circular (AC) 90-95, Unanticipated Right Yaw in Helicopters.
Main Drive Shaft or Clutch Failure
The main drive shaft, located between the engine and the main
rotor transmission, provides engine power to the main rotor
transmission. In some helicopters, particularly those with
piston engines, a drive belt is used instead of a drive shaft.
A failure of the drive shaft clutch or belt has the same effect
as an engine failure because power is no longer provided to
the main rotor and an autorotation must be initiated. There
are a few differences, however, that need to be taken into
consideration. If the drive shaft or belt breaks, the lack of any
load on the engine results in an overspeed. In this case, the
throttle must be closed in order to prevent any further damage.
In some helicopters, the tail rotor drive system continues to
be powered by the engine even if the main drive shaft breaks.
In this case, when the engine unloads, a tail rotor overspeed
can result. If this happens, close the throttle immediately and
enter an autorotation. The pilot must be knowledgeable of the
specific helicopter’s system and failure modes.
Pilots should keep in mind that when there is any suspected
mechanical malfunction, first and foremost they should
always attempt to maintain rotor rpm. If the rotor rpm is at the
normal indication with normal power settings, an instrument
failure might be occurring, and it would be best to fly the
helicopter to a safe landing area. If the rotor rpm is in fact
decreasing or low, then there is a drive line failure.
Hydraulic Failure
Many helicopters incorporate the use of hydraulic actuators to
overcome high control forces. A hydraulic system consists of
actuators, also called servos, on each flight control; a pump,
which is usually driven by the main rotor transmission;
and a reservoir to store the hydraulic fluid. A switch in the
cockpit can turn the system off, although it is left on during
normal conditions. A pressure indicator in the cockpit may
be installed to monitor the system.
An impending hydraulic failure can be recognized by a
grinding or howling noise from the pump or actuators,
increased control forces and feedback, and limited control
movement. The required corrective action is stated in detail
in the RFM. In most cases, airspeed needs to be reduced in
order to reduce control forces. The hydraulic switch and
circuit breaker should be checked and recycled. If hydraulic
power is not restored, make a shallow approach to a running
or roll-on landing. This technique is used because it requires
less control force and pilot workload. Additionally, the
hydraulic system should be disabled by placing the switch
in the off position. The reason for this is to prevent an
inadvertent restoration of hydraulic power, which may lead
to overcontrolling near the ground.
In those helicopters in which the control forces are so high
that they cannot be moved without hydraulic assistance, two
or more independent hydraulic systems are installed. Some
helicopters use hydraulic accumulators to store pressure that
can be used for a short time while in an emergency if the
hydraulic pump fails. This gives enough time to land the
helicopter with normal control.
Governor or Fuel Control Failure
Governors and fuel control units automatically adjust engine
power to maintain rotor rpm when the collective pitch
is changed. If the governor or fuel control unit fails, any
change in collective pitch requires manual adjustment of
the throttle to maintain correct rpm. In the event of a high
side failure, the engine and rotor rpm tend to increase above
the normal range due to the engine being commanded to
put out too much power. If the rpm cannot be reduced and
controlled with the throttle, close the throttle and enter an
autorotation. If the failure is on the low side, the engine
output is allowed to go below the collective and normal
rpm may not be attainable, even if the throttle is manually
controlled. In this case, the collective has to be lowered to
maintain rotor rpm. A running or roll-on landing may be
performed if the engine can maintain sufficient rotor rpm. If
there is insufficient power, enter an autorotation. As stated
previously in this chapter, before responding to any type of
mechanical failure, pilots should confirm that rotor rpm is
not responding to flight control inputs. If the rotor rpm can
be maintained in the green operating range, the failure is in
the instrument, and not mechanical.
Abnormal Vibration
With the many rotating parts found in helicopters, some
vibration is inherent. A pilot needs 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, a pilot learns
what vibrations are normal and 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.
Low-Frequency Vibrations
Low-frequency vibrations (100–500 cycles per minute) usually
originate from the main rotor disk. The main rotor operational
range, depending on the helicopter, is usually between 320
and 500 rpm. A rotor blade that is out of track or balance will
cause a cycle to occur with every rotation. The vibration may
be felt through the controls, the airframe, or a combination
of both. The vibration may also have a definite direction
of push or thrust. It may be vertical, lateral, horizontal, or
even a combination of these. Normally, the direction of the
vibration can be determined by concentrating on the feel of
the vibration, which may push a pilot up and down, backwards
and forwards, or in the case of a blade being out of phase, from
side to side. The direction of the vibration and whether it is
felt in the controls or the airframe is important information for
the mechanic when he or she troubleshoots the source. Out-
of-track or out-of-balance main rotor blades, damaged blades,
worn bearings, dampers out of adjustment, or worn parts are
possible causes of low frequency vibrations.
Medium- and High-Frequency Vibrations
Medium-frequency vibrations (1,000–2,000 cycles per
minute) range between the low frequencies of the main rotor
(100–500 cycles per minute) and the high frequencies (2,100
cycles per minute or higher) of the engine and tail rotor.
Depending on the helicopter, medium-frequency vibration
sources may be engine and transmission cooling fans, and
accessories such as air conditioner compressors, or driveline
components. Medium-frequency vibrations are felt through
the entire airframe, and prolonged exposure to the vibrations
will result in greater pilot fatigue.
Most tail rotor vibrations fall into the high-frequency range
(2,100 cycles per minute or higher) and can be felt through
the tail rotor pedals as long as there are no hydraulic actuators
to dampen out the vibration. This vibration is felt by the pilot
through his or her feet, which are usually “put to sleep” by
the vibration. The tail rotor operates at approximately a 6:1
ratio with the main rotor, meaning for every one rotation
of the main rotor the tail rotor rotates 6 times. A main
rotor operating rpm of 350 means the tail rotor rpm would
be 2,100 rpm. Any imbalance in the tail rotor disk 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 rpm. Turbine engine
vibration can be at 30,000 rpm internally, but common
transmission speeds are in the 1,000 to 3,000 rpm range for
the output shaft. The vibrations in turbine engines may be
short lived as the engine disintegrates rapidly when damaged
due to high rpm and the forces present.
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.
The use of a system such as a health and usage monitoring
system (HUMS) provides the operator the ability to record
engine and transmission performance and provide rotor track
and balance. This system has been around for over 30 years
and is now becoming more affordable, more capable, and
more commonplace in the rotorcraft industry.
Multiengine Emergency Operations
Single-Engine Failure
When one engine has failed, the helicopter can often maintain
altitude and airspeed until a suitable landing site can be
selected. Whether or not this is possible becomes a function
of such combined variables as aircraft weight, density
altitude, height above ground, airspeed, phase of flight,
and single-engine capability. Environmental response time
and control technique may be additional factors. Caution
must be exercised to correctly identify the malfunctioning
engine since there is no telltale yawing as occurs in most
multiengine airplanes. Shutting down the wrong engine
could be disastrous!
Even when flying multiengine powered helicopters, rotor rpm
must be maintained at all costs, because fuel contamination has
been documented as the cause for both engines failing in flight.
Dual-Engine Failure
The flight characteristics and the required crew member
control responses after a dual-engine failure are similar to
those during a normal power-on descent. Full control of the
helicopter can be maintained during autorotational descent.
In autorotation, as airspeed increases above 70–80 KIAS, the
rate of descent and glide distance increase significantly. As
airspeed decreases below approximately 60 KIAS, the rate
of descent increases and glide distance decreases.
Lost Procedures
Pilots become lost while flying for a variety of reasons, such
as disorientation, flying over unfamiliar territory, or visibility
that is low enough to render familiar terrain unfamiliar. When
a pilot becomes lost, the first order of business is to fly the
aircraft; the second is to implement lost procedures. Keep
in mind that the pilot workload will be high, and increased
concentration will be necessary. If lost, always remember to
look for the practically invisible hazards, such as wires, by
searching for their support structures, such as poles or towers,
which are almost always near roads.
If lost, follow common sense procedures.
• Try to locate any large landmarks, such as lakes, rivers,
towers, railroad tracks, or Interstate highways. If a
landmark is recognized, use it to find the helicopter’s
location on the sectional chart. If flying near a town or
city, a pilot may be able to read the name of the town
on a water tower or even land to ask for directions.
• If no town or city is nearby, the first thing a pilot should
do is climb. An increase in altitude increases radio and
navigation reception range as well as radar coverage.
risk associated with being lost is waiting too long to land in a
safe area. Helicopter pilots should land before fuel exhaustion
occurs because maneuvering with low fuel levels could cause
the engine to stop due to fuel starvation as fuel sloshes or
flows away from the pickup port in the tank.
If lost and low on fuel, it is advisable to make a precautionary
landing. Preferably, land near a road or in an area that would
allow space for another helicopter to safely land and provide
assistance. Having fuel delivered is a minor inconvenience
when compared to having an accident. Once on the ground,
pilots may seek assistance.
VFR Flight into Instrument Meteorological
Conditions
Helicopters, unlike airplanes, generally operate under Visual
Flight Rules (VFR) and require pilots to maintain aircraft
control by visual cues. However, when unforecast weather
leads to degraded visibility, the pilot may be at increased
risk of Inadvertent flight into Instrument Meteorological
Conditions (IIMC). During an IIMC encounter, the pilot
may be unprepared for the loss of visual reference, resulting
in a reduced ability to continue safe flight. IIMC is a life-
threatening emergency for any pilot. To capture these IIMC
events, the Commercial Aviation Safety Team (CAST) and
International Civil Aviation Organization (ICAO) Common
Taxonomy Team (CICTT) categorizes this occurrence as
Unintended flight in Instrument Meteorological Conditions
(UIMC). This term is also recognized by the National
Transportation Safety Board (NTSB) and Federal Aviation
Administration (FAA). It is used to classify occurrences
(accidents and incidents) at a high level to improve the
capacity to focus on common safety issues and complete
analysis of the data in support of safety initiatives.
The onset of IIMC may occur gradually or suddenly, has
no simple procedural exit, and is unlike flight training by
reference to while in Visual Meteorological Conditions
(VMC). Most training helicopters are not equipped or
certified to fly under Instrument Flight Rules (IFR).
Therefore, General Aviation (GA) helicopter pilots may not
have the benefit of flight in actual Instrument Meteorological
Conditions (IMC) during their flight training. Helicopter
pilots that encounter IIMC may experience physiological
illusions which can lead to spatial disorientation and loss of
aircraft control. Even with some instrument training, many
available and accessible helicopters are not equipped with
the proper augmented safety systems or autopilots, which
would significantly aid in helicopter control during an
IIMC emergency. The need to use outside visual references
is natural for helicopter pilots because much of their flight
training is based upon visual cues, not on flight instruments.
This primacy can only be overcome through significant
instrument training. Additionally, instrument flight may be
• Navigation aids, dead reckoning, and pilotage are
skills that can be used as well.
• Do not forget air traffic control (ATC)—controllers
assist pilots in many ways, including finding a lost
helicopter. Once communication with ATC has been
established, follow their instructions.
These common-sense procedures can be easily remembered by
using the four Cs: Climb, Communicate, Confess, and Comply.
• Climb for a better view, improved communication and
navigation reception, and terrain avoidance.
• Communicate by calling the nearest flight service
station (FSS)/automated flight service station (AFSS)
on 122.2 MHz. If the FSS/AFSS does not respond,
call the nearest control tower, center, or approach
control. For frequencies, check the chart in the vicinity
of the last known position. If that fails, switch to
the emergency radio frequency (121.5 MHz) and
transponder code (7700).
• Report the lost situation to ATC and request help.
• Comply with controller instructions.
Pilots should understand the services provided by ATC and
the resources and options available. These services enable
pilots to focus on aircraft control and help them make better
decisions in a time of stress.
When contacting ATC, pilots should provide as much
information as possible because ATC uses the information
to determine what kind of assistance it can provide with
available assets and capabilities. Information requirements
vary depending on the existing situation, but at a minimum
a pilot should provide the following information:
• Aircraft identification and type
• Nature of the emergency
• Aviator’s desires
To reduce the chances of getting lost in the first place, use
flight following through active contact with an aircraft during
flight either by radio or through automated flight following
systems when it is available, monitor checkpoints no more
than 25 miles apart, keep navigation aids such as Very
High-Frequency Omni-Directional Range (VOR) tuned in,
and maintain good situational awareness. Flight following
provides ongoing surveillance information to assist pilots in
avoiding collisions with other aircraft.
Getting lost is a potentially dangerous situation for any
aircraft, especially when low on fuel. Due to the helicopter’s
unique ability to land almost anywhere, pilots have more
flexibility than other aircraft as to landing site. An inherent
intimidating to some and too costly for others. As a result,
many helicopter pilots choose not to seek an instrument
rating.
While commercial helicopter operators often prefer their
pilots to be instrument rated, fatal accidents still occur as
a result of IIMC. Many accidents can be traced back to
the pilot’s inability to recover the helicopter after IIMC
is encountered, even with adequate equipment installed.
Therefore, whether instrument rated or not, all pilots should
understand that avoiding IIMC is critical.
A good practice for any flight is to set and use personal
minimums, which should be more conservative than those
required by regulations for VFR flight. In addition, a thorough
preflight and understanding of weather conditions that may
contribute to the risk of IMC developing along a planned
route of flight is essential for safety. Pilots should recognize
deteriorating weather conditions so the route of flight can be
changed or a decision made to terminate the flight and safely
land at a suitable area, well before IIMC occurs. If weather
conditions deteriorate below the pilot’s personal minimums
during flight, a pilot who understands the risks of IIMC
knows that he or she is at an en route decision point, where
it is necessary to either turn back to the departure point or
immediately land somewhere safe to wait until the weather
has cleared. Pilots should recognize that descent below a
predetermined minimum altitude above ground level (AGL)
(for example, 500 feet AGL) to avoid clouds or, slowing
the helicopter to a predetermined minimum airspeed (for
example, slowing to 50 KIAS) to reduce the rate of closure
from the deteriorating weather conditions, indicates the
decision point had been reached. Ceilings that are lower than
reported and/or deteriorating visibility along the route of
flight should trigger the decision to discontinue and amend
the current route to avoid IIMC.
If the helicopter pilot is instrument rated, it is advisable to
maintain instrument currency and proficiency as this may
aid the pilot in a safe recovery from IIMC. A consideration
for instrument rated pilots when planning a VFR flight
should include a review of published instrument charts for
safe operating altitudes, e.g. minimum safe altitude (MSA),
minimum obstruction clearance altitude (MOCA), minimum
in VMC throughout a flight: off-route altitude (MORA),
etc. If IIMC occurs, the pilot may consider a climb to a safe
altitude. Once the helicopter is stabilized, the pilot should
declare an emergency with air traffic control (ATC). It is
imperative that the pilot commit to controlling the helicopter
and remember to aviate, navigate, and finally communicate.
Often communication is attempted first, as it is natural to
look for help in stressful situations. This may distract the
pilot from maintaining control of the helicopter.
If the pilot is not instrument rated, instrument current nor
proficient, or is flying a non-IFR equipped helicopter,
remaining in VMC is paramount. Pilots who are not trained
or proficient in flight solely by reference to instruments have
a tendency to attempt to maintain flight by visual ground
reference, which tends to result in flying at lower altitudes,
just above the trees or by following roads. The thought process
is that, "as long as I can see what is below me, I can continue
to my intended destination." Experience and statistical data
indicate that attempting to continue VFR flight into IMC can
often lead to a fatal outcome as pilots often fixate on what
they see below them and are unable to see the hazards ahead
of them (e.g., power lines, towers, rising terrain, etc.). By
the time the pilot sees the hazard, it is either too late to avoid
a collision, or while successfully maneuvering to avoid an
obstacle, the pilot becomes disoriented.
Flying at night involves even more conservative personal
minimums to ensure safety and avoidance of IIMC than
daytime flying. At night, deteriorating weather conditions
may be difficult to detect. Therefore, pilots should ensure
that they not only receive a thorough weather briefing, but
that they remain vigilant for unforecasted weather during
their flight. The planned route should include preselected
landing sites that will provide options to the pilot in the
event a precautionary landing is required to avoid adverse
weather conditions. As a pilot gains night flight experience
their ability to assess weather during a flight will improve.
Below are some basic guidelines to assist a pilot to remain
in VMC throughout a flight:
1. Slowly turn around if threatened by deteriorating
visual cues and proceed back to VMC or to the first
safe landing area if the weather ahead becomes
questionable. Remember that prevention is paramount.
2. Do not proceed further on a course when the terrain
ahead is not clearly discernible.
3. Delay or consider cancelling the flight if weather
conditions are already questionable, could deteriorate
significantly based on forecasts, or if you are uncertain
whether the flight can be conducted safely. Often, a
gut feeling can provide a warning that unreasonable
risks are present.
4. Always have a safe landing area (such as large open
areas or airports) in mind for every route of flight.
There are five basic steps that every pilot should be familiar
with, and which should be executed immediately at the onset
of IIMC, if applicable. However, remember that if you are
not trained to execute the following maneuvers solely by
reference to instruments, or your aircraft is not equipped
Food cannot be subject to deterioration due to heat or cold. There
should be at least 10,000 calories for each person on board, and it
should be stored in a sealed waterproof container. It should have
been inspected within the previous 6 months, verifying the amount
and satisfactory condition of the contents.
A supply of water
Cooking utensils
Matches in a waterproof container
A portable compass
An ax weighing at least 2.5 pounds with a handle not less than 28 inches
in length
A flexible saw blade or equivalent cutting tool
30 feet of snare wire and instructions for use
Fishing equipment, including still-fishing bait and gill net with not more
than a two-inch mesh
Mosquito nets or netting and insect repellent sufficient to meet the
needs of all persons aboard, when operating in areas where insects
are likely to be hazardous
A signaling mirror
At least three pyrotechnic distress signals
A sharp, quality jackknife or hunting knife
A suitable survival instruction manual
Flashlight with spare bulbs and batteries
Portable emergency locator transmitter (ELT) with spare batteries
Stove with fuel or a self-contained means of providing heat for cooking
Tent(s) to accommodate everyone on board
Additional items for winter operations:
• Winter sleeping bags for all persons when the
temperature is expected to be below 7 °C
• Two pairs of snow shoes
• Spare ax handle
• Ice chisel
• Snow knife or saw knife
EMERGENCY EQUIPMENT AND SURVIVAL GEAR
Figure 11-12. Emergency equipment and survival gear.
with such instruments, this guidance may be less beneficial
to you and loss of helicopter control may occur:
1. Level the “wings” – level the bank angle using the
attitude indicator.
2. Attitude – set a climb attitude that achieves a safe
climb speed appropriate to your type of helicopter.
This is often no more than 10° of pitch up on the
attitude indicator.
3. Airspeed – verify that the attitude selected has
achieved the desired airspeed. It is critical to
recognize that slower airspeeds, closer to effective
translational lift, may require large control inputs and
will decrease stability, making recover impossible
while in UIMC.
4. Power – adjust to a climb power setting relative to the
desired airspeed. This should be executed concurrent
with steps 2 and 3.
5. Heading and Trim – pick a heading known to be free
of obstacles and maintain it. This will likely be the
heading you were already on, which was planned and
briefed. Set the heading bug, if installed, to avoid over-
controlling your bank. Maintain coordinated flight so
that an unusual attitude will not develop.
Try to avoid immediately turning 180°. Turning around is
not always the safest route and executing a turn immediately
after UIMC may lead to spatial disorientation. If a 180° turn
is the safest option, first note the heading you are on then
begin the turn to the reciprocal heading, but only after stable
flight is achieved (items 1 through 5 above) and maintain
a constant rate of turn appropriate to the selected airspeed.
Each encounter with UIMC is unique, and no single
procedure can ensure a safe outcome. Considerations in
determining the best course of action upon encountering
UIMC should include, at a minimum, terrain, obstructions,
freezing levels, aircraft performance and limitations, and
availability of ATC services.
There are new technologies being developed regarding
aircraft design, enhanced and lower-cost technologies,
and aircraft certification. Because of this promising future,
much of the discussion and guidance in this chapter may
one day become irrelevant. As helicopters integrate more
into the National Airspace System, the IFR infrastructure
and instrument training will become more prevalent. In the
future, UIMC may no longer be the emergency that ends
with a fatality but rather associated with proper prevention,
skilled recovery techniques along with the aid of emerging
new life saving avionics technology. A helicopter instrument
rating may be a life-saving addition to a pilot’s level of
certification. Please refer to the Instrument Flying Handbook
(FAA-H-8083-15, as revised); Advanced Avionics Handbook
(FAA-H-8083-6, as revised); and the Pilot’s Handbook of
Aeronautical Knowledge (FAA-H-8083-25, as revised) for
further exploration of IFR operations and how to obtain an
instrument rating.
When faced with deteriorating weather, planning and
prevention, not recovery, are the best strategies to eliminate
UIMC-related accidents and fatalities.
Emergency Equipment and Survival Gear
Both Canada and Alaska require pilots to carry survival
gear. Always carry survival gear when flying over rugged
and desolate terrain. The items suggested in Figure 11-12
are both weather and terrain dependent. The pilot also needs
to consider how much storage space the helicopter has and
how the equipment being carried affects the overall weight
and balance of the helicopter.
Chapter Summary
Emergencies should always be anticipated. Knowledge
of the helicopter, possible malfunctions and failures, and
methods of recovery can help the pilot avoid accidents and
be a safer pilot. Helicopter pilots should always expect the
worse hazards and possible aerodynamic effects and plan for
a safe exit path or procedure to compensate for the hazard.
Introduction
Pilots rely more on vision than on any other sense to orient
themselves in flight. The following visual factors contribute
to flying performance: good depth perception for safe
landings, good visual acuity to identify terrain features and
obstacles in the flightpath, and good color vision. Although
vision is the most accurate and reliable sense, visual cues can
be misleading, contributing to incidents occurring within the
flight environment. Pilots should be aware of and know how
to compensate effectively for the following:
• Physical deficiency or self-imposed stress, such as
smoking, which limits night-vision capability
• Visual cue deficiencies
• Limitations in visual acuity, dark adaptation, and color
and depth perception
For example, at night, the unaided eye has degraded visual
acuity. For more information on night operations, reference
Chapter 17, Aeromedical Factors, of the Pilot’s Handbook
of Aeronautical Knowledge (FAA-H-8083-25, as revised).
Night Operations
Chapter 12
Figure 12-1. Effects of dimming cockpit lighting during night flight
to better see surrounding terrain.
Visual Deficiencies
Night Myopia
At night, blue wavelengths of light prevail in the visible
portion of the spectrum. Therefore, slightly nearsighted
(myopic) individuals viewing blue-green light at night may
experience blurred vision. Even pilots with perfect vision find
that image sharpness decreases as pupil diameter increases.
For individuals with mild refractive errors, these factors
combine to make vision unacceptably blurred unless they
wear corrective glasses. Another factor to consider is “dark
focus.” When light levels decrease, the focusing mechanism
of the eye may move toward a resting position and make the
eye more myopic. These factors become important when
pilots rely on terrain features during unaided night flights.
Practicing good light discipline is very important and helps
pilots to retain their night adaptation. Keeping the cockpit
lighting on dim allows the pilot to better identify outside
details, unmarked hazards such as towers less than 200'
AGL, and unimproved landing sites with no hazard lighting.
A simple exercise that shows the effect of high versus low
light contrast would be to go out to a very dark road and
turn the dash board lights down very low or off and let your
eyes adjust to the ambient light level. Then, turn the dash
board lights up and note how the outside features disappear.
The same concept applies to cockpit lighting and being able
to see the surrounding terrain and obstacles. [Figure 12-1]
Special corrective lenses can be prescribed to pilots who
experience night myopia.
The eye automatically adjusts for the light level experienced.
During night flight, the cockpit and instrument lights should
be as dim as possible. The eye can then adjust for the outside
lighting conditions (ambient lighting) to see outside. The
dimmer the inside lighting is, the better you can see outside.
Hyperopia
Hyperopia is also caused by an error in refraction. In a
hyperopic state, when a pilot views a near image, the actual
focal point of the eye is behind the retinal plane (wall),
causing blurred vision. Objects that are nearby are not seen
clearly; only more distant objects are in focus. This problem,
is referred to as farsightedness.
Astigmatism
An unequal curvature of the cornea or lens of the eye causes
this condition. A ray of light is spread over a diffused area
in one meridian. In normal vision, a ray of light is sharply
focused on the retina. Astigmatism is the inability to
focus different meridians simultaneously. If, for example,
astigmatic individuals focus on power poles (vertical),
the wires (horizontal) are out of focus for most of them.
[Figure 12-2]
Presbyopia
This condition is part of the normal aging process, which
causes the lens to harden. Beginning in the early teens, the
human eye gradually loses the ability to accommodate for
and focus on nearby objects. When people are about 40 years
old, their eyes are unable to focus at normal reading distances
without reading glasses. Reduced illumination interferes with
focus depth and accommodation ability. Hardening of the lens
may also result in clouding of the lens (cataract formation).
Aviators with early cataracts may see a standard eye chart
clearly under normal daylight but have difficulty seeing under
bright light conditions. This problem is due to light scattering
as it enters the eye. This glare sensitivity is disabling under
certain circumstances. Glare disability, related to contrast
sensitivity, is the ability to detect objects against varying
shades of backgrounds. Other visual functions decline with
age and affect the aircrew member’s performance:
• Dynamic acuity
• Recovery from glare
• Function under low illumination
• Information processing
Vision in Flight
The visual sense is especially important in collision
avoidance and depth perception. Due to the structure of the
human eye, illusions and blindspots occur. The more pilots
understand the eye and how it functions, the easier it is to
compensate for these illusions and blindspots. Figure 12-3
shows the basic anatomy of the human eye and how it is like