Weight  Icing  CG  Aircraft nose pitches down Figure 4-23.  Ice on the tailplane. Piper PA-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. 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. Any of the following symptoms, occurring singly or in  combination, may be a warning of tailplane icing: Tailplane Stall Symptoms Effects of Icing on Critical Aircraft Systems Propeller Icing Flight Instruments If any of the above symptoms occur, the pilot should: 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 (V$_{FE}$). 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. 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. Various aircraft instruments including the airspeed indicator,  altimeter, and rate-of-climb indicator utilize pressures  sensed by pitot tubes and static ports for normal operation.  •  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. •  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 an attempt to clear the tailplane  of ice.  4-16