Icing wind tunnel research commissioned by The Wolk Law Firm in connection with the case of Randolph vs.Cessna revealed serious flaws in the design and operation of the deicing system of that aircraft. Experts hired by Arthur Alan Wolk confirmed that the problems ranged from the choice of wing airfoil, design of the deicing boots, flaws in the design of the inflation hardware, inadequate stall warning, underpowered engine, and complete inadequacy of the pilot's operating handbook for safe flight into known icing conditions.
After a spate of icing accidents Cessna Aircraft Company, encouraged by the FAA and the NTSB revised the Pilot's Operating Handbook to prohibit the use of flaps when airspeed reductions occurred due to unshed ice accumulations.
One of the experts working for the Wolk Law Firm, Harry Riblet, a noted designer of general aviation airfoils openly criticized the Cessna, FAA, and NTSB action and wrote to them repeatedly warning that use of the flaps could help but not cure the controllability problems with the airplane when flying in icing conditions. Riblet was ridiculed by the Government and Cessna even though his airfoils adopted for use by homebuilders around the world had proven themselves stall spin proof.
This Airworthiness Directive, with the full force and effect of law, now removes any restriction for the use of flaps in icing conditions and instead requires their use when the aircraft airspeed is reduced to 110 knots or less. It also prohibits the flight of the aircraft in moderate icing conditions for which the aircraft was originally certificated and removes the words "certified for flight into known icing conditions" from the handbook without revoking the certification entirely.
The AD requires installation of a low speed warning in all Cessna Caravans and cautions pilots that the stall warning may be completely unreliable in icing conditions.
What is remarkable about the Airworthiness Directive is what it doesn't do.
- It does not revoke the "known icing certification" of the Caravan which means it may still be dispatched into conditions where ice is reported or forecast.
- It ignores the fact that moderate icing is unpredictable so it may still not be possible for pilots to safely exit those conditions.
- It ignores the bad design of the deicing boots and does not require the introduction of a water separator into the system to prevent ice boot inflation line icing.
- It does not require that all Caravan pilots be taught how to recognize and recover from a tail stall.
- It does not require the installation of vortex generators on the boots of the wing and the horizontal stabilizer to delay the onset of ice induced stall.
- It does not require Cessna to install a stall warning indicator that is impervious to ice induced errors.
- It defines moderate icing encounters as a reduction in airspeed to 120 knots in cruise flight which is already in most instances a state beyond which the aircraft will be recoverable once control is lost.
- It further defines moderate icing as an accumulation of 1/4 inch on the wing strut which is the amount of ice accumulation Cessna requires before operation of the deicing boots in the first place even in light icing conditions.
View wind tunnel test.