Introduction
Attitude instrument flying in helicopters is essentially visual
flying with the flight instruments substituted for the various
reference points on the helicopter and the natural horizon.
Control changes, required to produce a given attitude by
reference to instruments, are identical to those used in
helicopter visual flight rules (VFR) flight, and pilot thought
processes are the same. Basic instrument training is intended to
be a building block toward attaining an instrument rating.
Helicopter Attitude
Instrument Flying
Chapter 8
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Start radial scan pattern
Figure 8-1. A radial scan pattern of the flight instruments enables the helicopter pilot to fully comprehend the condition and direction
of the helicopter.
Flight Instruments
When flying a helicopter with reference to the flight
instruments, proper instrument interpretation is the basis
for aircraft control. Skill, in part, depends on understanding
how a particular instrument or system functions, including
its indications and limitations (see Chapter 5, Flight
Instruments). With this knowledge, a pilot can quickly
interpret an instrument indication and translate that
information into a control response.
Instrument Flight
To achieve smooth, positive control of the helicopter during
instrument flight, three fundamental skills must be developed.
They are instrument cross-check, instrument interpretation,
and aircraft control.
Instrument Cross-Check
Cross-checking, sometimes referred to as scanning, is the
continuous and logical observation of instruments for attitude
and performance information. In attitude instrument flying,
an attitude is maintained by reference to the instruments,
which produces the desired result in performance. Due to
human error, instrument error, and helicopter performance
differences in various atmospheric and loading conditions,
it is difficult to establish an attitude and have performance
remain constant for a long period of time. These variables
make it necessary to constantly check the instruments and
make appropriate changes in the helicopter’s attitude. The
actual technique may vary depending on what instruments
are installed and where they are installed, as well as
pilot experience and proficiency level. This discussion
concentrates on the six basic flight instruments. [Figure 8-1]
At first, there may be a tendency to cross-check rapidly,
looking directly at the instruments without knowing exactly
what information is needed. However, with familiarity and
practice, the instrument cross-check reveals definite trends
during specific flight conditions. These trends help a pilot
control the helicopter as it makes a transition from one flight
condition to another.
When full concentration is applied to a single instrument, a
problem called fixation is encountered. This results from a
natural human inclination to observe a specific instrument
carefully and accurately, often to the exclusion of other
instruments. Fixation on a single instrument usually results
in poor control. For example, while performing a turn, there
is a tendency to watch only the turn-and-slip indicator instead
of including other instruments in the cross-check. This
fixation on the turn-and-slip indicator often leads to a loss of
altitude through poor pitch-and-bank control. Look at each
instrument only long enough to understand the information
it presents, and then proceed to the next one. Similarly, too
much emphasis can be placed on a single instrument, instead
of relying on a combination of instruments necessary for
helicopter performance information. This differs from fixation
in that other instruments are included in a cross-check, but too
much attention is placed on one particular instrument.
During performance of a maneuver, there is sometimes
a failure to anticipate significant instrument indications
following attitude changes. For example, during level off
from a climb or descent, a pilot may concentrate on pitch
control, while forgetting about heading or roll information.
This error, called omission, results in erratic control of
heading and bank.
In spite of these common errors, most pilots can adapt well to
flight by instrument reference after instruction and practice.
Many find that they can control the helicopter more easily
and precisely by instruments.
Instrument Interpretation
The flight instruments together give a picture of what is
happening. No one instrument is more important than the
next; however, during certain maneuvers or conditions,
those instruments that provide the most pertinent and useful
information are termed primary instruments. Those which
back up and supplement the primary instruments are termed
supporting instruments. For example, since the attitude
indicator is the only instrument that provides instant and
direct aircraft attitude information, it should be considered
primary during any change in pitch or bank attitude. After
the new attitude is established, other instruments become
primary, and the attitude indicator usually becomes the
supporting instrument.
Aircraft Control
Controlling a helicopter is the result of accurately interpreting
the flight instruments and translating these readings
into correct control responses. Aircraft control involves
adjustment to pitch, bank, power, and trim in order to achieve
a desired flight path.
Pitch attitude control is controlling the movement of
the helicopter about its lateral axis. After interpreting
the helicopter’s pitch attitude by reference to the pitch
instruments (attitude indicator, altimeter, airspeed
indicator, and vertical speed indicator (VSI)), cyclic control
adjustments are made to affect the desired pitch attitude. In
this chapter, the pitch attitudes depicted are approximate
and vary with different helicopters.
Bank attitude control is controlling the angle made by the
lateral tilt of the rotor and the natural horizon or the movement
of the helicopter about its longitudinal axis. After interpreting
the helicopter’s bank instruments (attitude indicator, heading
indicator, and turn indicator), cyclic control adjustments are
made to attain the desired bank attitude.
Power control is the application of collective pitch with
corresponding throttle control, where applicable. In straight-
and-level flight, changes of collective pitch are made to
correct for altitude deviation if the error is more than 100
feet or the airspeed is off by more than 10 knots. If the error
is less than that amount, a pilot should use a slight cyclic
climb or descent.
In order to fly a helicopter by reference to the instruments, it
is important to know the approximate power settings required
for a particular helicopter in various load configurations and
flight conditions.
Trim, in helicopters, refers to the use of the cyclic centering
button, if the helicopter is so equipped, to relieve all
possible cyclic pressures. Trim also refers to the use of pedal
adjustment to center the ball of the turn indicator. Pedal trim
is required during all power changes.
The proper adjustment of collective pitch and cyclic friction
helps a pilot relax during instrument flight. Friction should
be adjusted to minimize overcontrolling and to prevent
creeping, but not applied to such a degree that control
movement is limited. In addition, many helicopters equipped
for instrument flight contain stability augmentation systems
or an autopilot to help relieve pilot workload.
Straight-and-Level Flight
Straight-and-level unaccelerated flight consists of maintaining
the desired altitude, heading, airspeed, and pedal trim.
Pitch Control
The pitch attitude of a helicopter is the angular relation of
its longitudinal axis to the natural horizon. If available, the
attitude indicator is used to establish the desired pitch attitude.
In level flight, pitch attitude varies with airspeed and center of
gravity (CG). At a constant altitude and a stabilized airspeed,
the pitch attitude is approximately level. [Figure 8-2]
Attitude Indicator
The attitude indicator gives a direct indication of the pitch
attitude of the helicopter. In visual flight, attain the desired
pitch attitude by using the cyclic to raise and lower the nose
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Figure 6-3. The initial pitch correction at normal
cruise is one bar width. Figure 8-3. The initial pitch correction at normal cruise is one bar
width or less.
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Figure 8-2. The flight instruments for pitch control are the airspeed indicator, attitude indicator, altimeter, and vertical speed indicator.
of the helicopter in relation to the natural horizon. During
instrument flight, follow exactly the same procedure in
raising or lowering the miniature aircraft in relation to the
horizon bar.
There is some delay between control application and resultant
instrument change. This is the normal control lag in the
helicopter and should not be confused with instrument lag.
The attitude indicator may show small misrepresentations
of pitch attitude during maneuvers involving acceleration,
deceleration, or turns. This precession error can be detected
quickly by cross-checking the other pitch instruments.
If the miniature aircraft is properly adjusted on the ground, it
may not require readjustment in flight. If the miniature aircraft
is not on the horizon bar after level off at normal cruising
airspeed, adjust it as necessary while maintaining level flight
with the other pitch instruments. Once the miniature aircraft
has been adjusted in level flight at normal cruising airspeed,
leave it unchanged so it gives an accurate picture of pitch
attitude at all times.
When making initial pitch attitude corrections to maintain
altitude, the changes of attitude should be small and smoothly
applied. The initial movement of the horizon bar should not
exceed one bar width high or low. [Figure 8-3] If a further
adjustment is required, an additional correction of one-
half bar normally corrects any deviation from the desired
altitude. This one-and-one-half bar correction is normally the
maximum pitch attitude correction from level flight attitude.
After making the correction, cross-check the other pitch
instruments to determine whether the pitch attitude change
is sufficient. If additional correction is needed to return to
altitude, or if the airspeed varies more than 10 knots from
that desired, adjust the power.
Altimeter
The altimeter gives an indirect indication of the pitch
attitude of the helicopter in straight-and-level flight. Since
the altitude should remain constant in level flight, deviation
from the desired altitude indicates a need for a change in
pitch attitude and power as necessary. When losing altitude,
raise the pitch attitude and adjust power as necessary. When
gaining altitude, lower the pitch attitude and adjust power
as necessary. Indications for power changes are explained
in the next paragraph.
The rate at which the altimeter moves helps to determine pitch
attitude. A very slow movement of the altimeter indicates
a small deviation from the desired pitch attitude, while a
fast movement of the altimeter indicates a large deviation
from the desired pitch attitude. Make any corrective action
promptly with small control changes. Also, remember that
movement of the altimeter should always be corrected by
two distinct changes. The first is a change of attitude to stop
the altimeter movement; the second is a change of attitude to
return smoothly to the desired altitude. If altitude and airspeed
are more than 100 feet and 10 knots low, respectively, apply
power in addition to an increase of pitch attitude. If the
altitude and airspeed are high by more than 100 feet and 10
knots, reduce power and lower the pitch attitude.
There is a small lag in the movement of the altimeter;
however, for all practical purposes, consider that the altimeter
gives an immediate indication of a change or a need for
change in pitch attitude. Since the altimeter provides the
most pertinent information regarding pitch in level flight, it
is considered primary for pitch.
Vertical Speed Indicator (VSI)
The VSI gives an indirect indication of the pitch attitude of
the helicopter and should be used in conjunction with the
other pitch instruments to attain a high degree of accuracy
and precision. The instrument indicates zero when in level
flight. Any movement of the needle from the zero position
shows a need for an immediate change in pitch attitude to
return it to zero. Always use the VSI in conjunction with
the altimeter in level flight. If a movement of the VSI is
detected, immediately use the proper corrective measures
to return it to zero. If the correction is made promptly, there
is usually little or no change in altitude. If the needle of the
VSI does not indicate zero, the altimeter indicates a gain or
loss of altitude.
The initial movement of the vertical speed needle is
instantaneous and indicates the trend of the vertical movement
of the helicopter. A period of time is necessary for the VSI to
reach its maximum point of deflection after a correction has
been made. This time element is commonly referred to as
instrument lag. The lag is directly proportional to the speed
and magnitude of the pitch change. When employing smooth
control techniques and small adjustments in pitch attitude are
made, lag is minimized, and the VSI is easy to interpret.
Overcontrolling can be minimized by first neutralizing the
controls and allowing the pitch attitude to stabilize, then
readjusting the pitch attitude by noting the indications of the
other pitch instruments.
Occasionally, the VSI may be slightly out of calibration.
This could result in the instrument indicating a slight climb
or descent even when the helicopter is in level flight. If the
instrument cannot be calibrated properly, this error must be
taken into consideration when using the VSI for pitch control.
For example, if a descent of 100 feet per minute (fpm) is the
vertical speed indication when the helicopter is in level flight,
use that indication as level flight. Any deviation from that
reading would indicate a change in attitude.
Airspeed Indicator
The airspeed indicator gives an indirect indication of
helicopter pitch attitude. With a given power setting and
pitch attitude, the airspeed remains constant. If the airspeed
increases, the nose is too low and should be raised. If
the airspeed decreases, the nose is too high and should
be lowered. A rapid change in airspeed indicates a large
change in pitch attitude, and a slow change in airspeed
indicates a small change in pitch attitude. There is very little
lag in the indications of the airspeed indicator. If, while
making attitude changes, there is some lag between control
application and change of airspeed, it is most likely due to
cyclic control lag. Generally, a departure from the desired
airspeed, due to an inadvertent pitch attitude change, also
results in a change in altitude. For example, an increase in
airspeed due to a low pitch attitude results in a decrease
in altitude. A correction in the pitch attitude regains both
airspeed and altitude.
Bank Control
The bank attitude of a helicopter is the angular relation of
its lateral axis to the natural horizon. To maintain a straight
course in visual flight, keep the lateral axis of the helicopter
level with the natural horizon. Assuming the helicopter is in
coordinated flight, any deviation from a laterally level attitude
produces a turn. [Figure 8-4]
Attitude Indicator
The attitude indicator gives a direct indication of the bank
attitude of the helicopter. For instrument flight, the miniature
aircraft and the horizon bar of the attitude indicator are
substituted for the actual helicopter and the natural horizon.
Any change in bank attitude of the helicopter is indicated
instantly by the miniature aircraft. For proper interpretation
of this instrument, imagine being in the miniature aircraft. If
the helicopter is properly trimmed and the rotor tilts, a turn
begins. The turn can be stopped by leveling the miniature
aircraft with the horizon bar. The ball in the turn-and-slip
indicator should always be kept centered through proper
pedal trim.
The angle of bank is indicated by the pointer on the banking
scale at the top of the instrument. Small bank angles, which
may not be seen by observing the miniature aircraft, can
easily be determined by referring to the banking scale pointer.
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0° bank
30° bank
45° bank
60° bank
90° bank
Banking scale pointer
Banking pointer
Horizon
Miniature aircraft
Figure 8-5. The banking scale at the top of the attitude indicator indicates varying degrees of bank. In this example, the helicopter is
banked approximately 15° to the right.
Figure 8-4. The flight instruments used for bank control are the attitude, heading, and turn indicators.
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Figure 6-4. The Bank Instruments.
Pitch-and-bank attitudes can be determined simultaneously
on the attitude indicator. Even though the miniature aircraft
is not level with the horizon bar, pitch attitude can be
established by observing the relative position of the miniature
aircraft and the horizon bar. [Figure 8-5]
The attitude indicator may show small misrepresentations
of bank attitude during maneuvers that involve turns. This
precession error can be detected immediately by closely
cross-checking the other bank instruments during these
maneuvers. Precession is normally noticed when rolling
out of a turn. If, upon completion of a turn, the miniature
aircraft is level and the helicopter is still turning, make a
small change of bank attitude to center the turn needle and
stop the movement of the heading indicator.
Heading Indicator
In coordinated flight, the heading indicator gives an indirect
indication of a helicopter’s bank attitude. When a helicopter is
banked, it turns. When the lateral axis of a helicopter is level,
it flies straight. Therefore, in coordinated flight when the
heading indicator shows a constant heading, the helicopter is
level laterally. A deviation from the desired heading indicates
a bank in the direction the helicopter is turning. A small angle
of bank is indicated by a slow change of heading; a large angle
of bank is indicated by a rapid change of heading. If a turn
is noticed, apply opposite cyclic until the heading indicator
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Turn Indicator A - Power is added to increase airspeed, nose up and yaws to the right. Figure 8-6. Coordinated flight is indicated by centering of the ball.
indicates the desired heading, simultaneously ensuring the
ball is centered. When making the correction to the desired
heading, do not use a bank angle greater than that required
to achieve a standard rate turn. In addition, if the number
of degrees of change is small, limit the bank angle to the
number of degrees to be turned. Bank angles greater than
these require more skill and precision in attaining the desired
results. During straight-and-level flight, the heading indicator
is the primary reference for bank control.
Turn Indicator
During coordinated flight, the needle of the turn-and-slip
indicator gives an indirect indication of the bank attitude
of the helicopter. When the needle is displaced from the
vertical position, the helicopter is turning in the direction of
the displacement. Thus, if the needle is displaced to the left,
the helicopter is turning left. Bringing the needle back to
the vertical position with the cyclic produces straight flight.
A close observation of the needle is necessary to accurately
interpret small deviations from the desired position.
Cross-check the ball of the turn-and-slip indicator to
determine if the helicopter is in coordinated flight.
[Figure 8-6] If the rotor is laterally level and pedal pressure
properly compensates for torque, the ball remains in the
center. To center the ball, level the helicopter laterally by
reference to the other bank instruments, then center the ball
with pedal trim. Torque correction pressures vary as power
changes are made. Always check the ball after such changes.
Common Errors During Straight-and-Level Flight
1. Failure to maintain altitude
2. Failure to maintain heading
3. Overcontrolling pitch and bank during corrections
4. Failure to maintain proper pedal trim
5. Failure to cross-check all available instruments
Power Control During Straight-and-Level Flight
Establishing specific power settings is accomplished through
collective pitch adjustments and throttle control, where
necessary. For reciprocating-powered helicopters, power
indication is observed on the manifold pressure gauge.
For turbine-powered helicopters, power is observed on the
torque gauge. (Although most instrument flight rules (IFR)-
certified helicopters are turbine powered, depictions within
this chapter use a reciprocating-powered helicopter as this
is where training is most likely conducted.)
At any given airspeed, a specific power setting determines
whether the helicopter is in level flight, in a climb, or in a
descent. For example, cruising airspeed maintained with
cruising power results in level flight. If a pilot increases the
power setting and holds the airspeed constant, the helicopter
climbs. Conversely, if the pilot decreases power and holds
the airspeed constant, the helicopter descends.