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## 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. 

## 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 an attempt 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 (V$_{FE}$). 

## 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 pitot tubes and static ports for normal operation. 

