Examination of the wreckage revealed heavy impact
damage to the airplane's forward fuselage, engines, and
wings. Approximately one-half inch of rime ice was
observed adhering to the leading edges of the left and right
horizontal stabilizers and along the leading edge of the
vertical stabilizer.

The National Transportation Safety Board (NTSB)
determined the probable cause of the accident was the pilot's
failure to use the airplane's deicing system, which resulted
in an accumulation of empennage ice and a tailplane stall.
Factors relating to this accident were the icing conditions and
the pilot's intentional flight into those known conditions.

Tailplane Stall Symptoms
Any of the following symptoms, occurring singly or in
combination, may be a warning of tailplane icing:

• Elevator control pulsing, oscillations, or vibrations;
• Abnormal nose-down trim change;
• Any other unusual or abnormal pitch anomalies
  (possibly resulting in pilot induced oscillations);
• Reduction or loss of elevator effectiveness;
• Sudden change in elevator force (control would move
  nose-down if unrestrained); and
• Sudden uncommanded nose-down pitch.

if any of the above symptoms occur, the pilot should:

• Immediately retract the flaps to the previous setting
  and apply appropriate nose' up elevator pressure;
• Increase airspeed appropriately for the reduced flap
  extension setting;
• Apply sufficient power •for aircraft configuration
  and conditions, (High engine power settings may
  adversely impact response to tailplane stall conditions
  at high airspeed in some aircraft designs. Observe the
  manufacturer' s recommendations regarding power
  settings.);
• Make nose-down pitch changes slowly, even in
  gusting conditions, if circumstances allow; and
• If a pneumatic deicing system is used, operate the
  system several times in no at tempt to clear the tailplane
  of ice

Once a tailplane stall is encountered, the stall condition
tends to worsen with increased airspeed and possibly may
worsen with increased power settings at the same flap
setting. Airspeed, at any flap setting, in excess of the airplane
manufacturer's recommendations, accompanied by uncleared
ice contaminating the tailplane, may result in a tailplane stall
and uncommanded pitch down from which recovery may not
be possible. A tailplane stall may occur at speeds less than
the maximum flap extended speed (VFE).

Propeller Icing
Ice buildup on propeller blades reduces thrust for the same
aerodynamic reasons that wings tend to lose lift and increase
drag when ice accumulates on them. The greatest quantity
of ice normally collects on the spinner and inner radius of
the propeller. Propeller areas on which ice may accumulate
and be ingested into the engine normally are anti-iced rather
than deiced to reduce the probability of ice being shed into
the engine.

Effects of Icing on Critical Aircraft Systems
In addition to the hazards of structural and induction icing,
the pilot must be aware of other aircraft systems susceptible
to icing The effects of icing do not produce the performance
loss of structural icing or the power loss of induction icing
but can present serious problems to the instrument pilot.
Examples of such systems are flight instruments, stall
warning systems, and windshields.

Flight Instruments
Various aircraft instruments including the airspeed indicator,
altimeter, and rate-of-climb indicator utilize pressures
sensed by pilot tubes and static ports for normal operation.
When covered by ice these instruments display incorrect
information thereby presenting serious hazard to instrument
flight. Detailed information on the operation of these
instruments and the specific effects of icing is presented in
Chapter 3, Flight Instruments.

Stall Warning Systems
Stall warning systems provide essential information to pilots.
These systems range from a sophisticated stall-warning vane
to a simple stall warning switch. Icing affects these systems
in several ways resulting in possible loss of stall warning to
the pilot. The loss of these systems can exacerbate an already
hazardous situation. Even when an aircraft's stall warning
system remains operational during icing conditions, it may
be ineffective because the wing stalls at a lower angle of
attack due to ice on the airfoil.

Windshields
Accumulation of ice on flight deck windows can severely
restrict the pilot's visibility outside of the aircraft. Aircraft
equipped for flight into known icing conditions typically have
some form of windshield anti-icing to enable the pilot to see
outside the aircraft in ease icing is encountered in flight. One
system consists of an electrically heated plate installed onto
the airplane's windshield to give the pilot a narrow band of
clear visibility. Another system uses a bar at the lower end
of the windshield to spray deicing fluid onto it and prevent
ice from forming, on high performance aircraft that require
complex windshields to protect against bird strikes and
withstand pressurization loads, the heating element often is
a layer of conductive film or thin wire strands through which
electric current is run to heat the windshield and prevent ice
from forming.