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U.S. Department
of Transportation
Federal Aviation
Administration
Advisory
Circular
AC
90-87
DATE: l/27/86
ADVISORY CIRCULAR
AC No: 90-87
Date: 01/27/86
Initiated
by: AFS-840
Change:
Subject:
Helicopter Dynamic Rollover
1. PURPOSE.
To familiarize the helicopter pilot/instructor and pilot school personnel of
the hazards associated with dynamic rollover.
2. BACKGROUND.
An increasing percentage of helicopter accidents are being attributed to
dynamic rollover, a phenomenon that will, without immediate corrective action,
result in destruction of the helicopter and possible serious injury. This
advisory circular informs helicopter flightcrews of the cause of helicopter
dynamic rollover and
measures to take to prevent such occurrences.
3. DISCUSSION.
Helicopter pilots in general are required to be skillful in operations on
both improved or unimproved surfaces. During normal or slope takeoffs and
landings with same degree of bank angle or side-drift with tie skid/wheel an
the ground, the bank angle or side drift can place the helicopter in a
situation where it is pivoting (rolling) about a skid/wheel which is still
in contact with the ground. When this happens, lateral cyclic control
response becomes more sluggish and less effective than for a free hovering
helicopter. Consequently, if a roll rate is permitted to develop, a critical
bank angle (the angle between the helicopter and the horizon) my be reached
where roll cannot be corrected, even with full lateral cyclic, and the
helicopter will roll over onto its side. As the roll rate increases, the
angle at which recovery is still possible is significantly reduced. The
critical rollover angle is also reduced. The critical rollover angle is
further reduced under the following conditions:
a. Right side skid
down condition;
b. Crosswinds;
c.
Lateral center of gravity offset;
d. Main rotor thrust
almost equal to helicopter weight; and
e.
Left yaw inputs.
4. CRITICAL
CONDITIONS. When certain elements of helicopter operations are at or
near their
most
critical condition, such as high gross weight, right lateral center of
gravity, crosswind from the left, hovering with only the right skid/wheel in
contact with the surface and with thrust (lift) approximately equal to the
weight, very little right roll rate is correctable for any given bank angle.
(See Figure 1.)
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