Ice can partially block or limit control surfaces, which
limits or makes control movements ineffective. At so, if the
extra weight caused by ice accumulation is too great, the
aircraft may not be able to become airborne and, if in flight,
the aircraft may not be able to maintain altitude. Therefore
any accumulation of ice or frost should be removed before
attempting flight.

Another hazard of structural icing is the possible uncommanded
and uncontrolled roll phenomenon, referred to as roll upset,
associated with severe in-flight icing. Pilots flying aircraft
certificated for flight in known icing conditions should be
aware that severe icing is a condition outside of the aircraft's
certification icing envelope. Roll upset may be caused by
airflow separation (aerodynamic stall), which induces self-
deflection of the ailerons and toss of or degraded roll handling
characteristics [Figure 2-20]. These phenomena can result
from severe icing conditions without the usual symptoms of
ice accumulation or a perceived aerodynamic stall.

Figure 2-20. Effect of Ice and Frost on Lift.

Most aircraft have a nose-down pitching moment from the
wings because the CO is ahead of the CP. It is the role of the
tailplane to counteract this moment by providing a downward
force. [Figure 2-21] The result of this configuration is

that actions winch move the wing away from stall, such
as deployment of flaps or increasing speed, may increase
the negative angle of attack of the tail. With ice on the
tailplane, it may stall after full or partial deployment of flaps.
[Figure 2-22]

Figure 2-21. Downward Force on the Tailplane.

Figure 2-22. Ice on the Tailplane.

Since the tailplane is ordinarily thinner than the wing, it is a
more efficient collector of ice. On most aircraft the tailplane
is not visible to the pilot, who therefore cannot observe how
well it has been cleared of ice by any deicing system. Thus, it
is important that the pilot be alert to the possibility of tailplane
stall, particularly on approach and landing.

Piper FA-34-200T (Des Moines, Iowa)
The pilot of this flight, which took place on January 9,
1996, said that upon crossing the runway threshold and
lowering the flaps 25°, "the airplane pitched down." The
pilot "immediately released the flaps and added power, but
the airplane was basically uncontrollable at this point." The
pilot reduced power and lowered the flaps before striking
the runway on its centerline and sliding 1,000 feet before
coming to a stop. The accident resulted in serious injury to
the pilot, the sole occupant.