Aeronautical Information Manual (AIM), page 571

Index   570 -- Page 571 -- 572

Chapter 10. Helicopter Operations
Section 1. Helicopter IFR Operations

10-1-1. Helicopter Flight Control Systems

a. The certification requirements for helicopters to
operate under Instrument Flight Rules (IFR) are
contained in 14 CFR Part 27, Airworthiness
Standards: Normal Category Rotorcraft, and 14 CFR
Part 29, Airworthiness Standards: Transport
Category Rotorcraft. To meet these requirements,
helicopter manufacturers usually utilize a set of
stabilization and/or Automatic Flight Control
Systems (AFCSs).

b. Typically, these systems fall into the following
1. Aerodynamic surfaces, which impart some
stability or control capability not found in the basic
VFR configuration.

2. Trim systems, which provide a cyclic
centering effect. These systems typically involve a
magnetic brake/spring device, and may also be
controlled by a four-way switch on the cyclic. This
is a system that supports "hands on" flying of the
helicopter by the pilot.

3. Stability Augmentation Systems (SASs),
which provide short-term rate damping control
inputs to increase helicopter stability. Like trim
systems, SAS supports "hands on" flying.

4. Attitude Retention Systems (ATTs), which
return the helicopter to a selected attitude after a
disturbance. Changes in desired attitude can be
accomplished usually through a four-way "beep"
switch, or by actuating a "force trim" switch on the

cyclic, setting the attitude manually, and releasing.

Attitude retention may be a SAS function, or may be

the basic "hands off" autopilot function.

5. Autopilot Systems (APs), which provide for
"hands off" flight along specified lateral and vertical
paths, including heading, altitude, vertical speed,
navigation tracking, and approach. These systems

typically have a control panel for mode selection, and
system for indication of mode status. Autopilots may
or may not be installed with an associated Flight
Director System (FD). Autopilots typically control
the helicopter about the roll and pitch axes (cyclic
control) but may also include yaw axis (pedal control)
and collective control servos.

6. FDs, which provide visual guidance to the
pilot to fly specific selected lateral and vertical modes
of operation. The visual guidance is typically
provided as either a "dual cue" (commonly known as
a "cross-pointer") or "single cue" (commonly known
as a "vee-bar") presentation superimposed over the
attitude indicator. Some FDs also include a collective
cue. The pilot manipulates the helicopter's controls to
satisfy these commands, yielding the desired flight
path, or may couple the flight director to the autopilot
to perform automatic flight along the desired flight
path. Typically, flight director mode control and
indication is shared with the autopilot.

c. In order to be certificated for IFR operation, a
specific helicopter may require the use of one or more
of these systems, in any combination.

d. In many cases, helicopters are certificated for
IFR operations with either one or two pilots. Certain
equipment is required to be installed and functional
for two pilot operations, and typically, additional
equipment is required for single pilot operation.
These requirements are usually described in the
limitations section of the Rotorcraft Flight Manual

e. In addition, the RFM also typically defines

systems and functions that are required to be in

operation or engaged for IFR flight in either the single

or two pilot configuration. Often, particularly in two
pilot operation, this level of augmentation is less than
the full capability of the installed systems. Likewise,
single pilot operation may require a higher level of

Helicopter IFR Operations 10-1-1

Page 571 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   570 -- Page 571 -- 572