The Dangers of Airframe Icing


Over the years, Slack & Davis attorneys have investigated and successfully resolved several cases in which airframe icing was a significant factor in the crash. The issues related to airframe icing are complex and typically involve questions about piloting procedures, aircraft characteristics, weather and pilot training. Some aircraft, such as the Cessna Caravan, have proven to be more susceptible to airframe icing than other aircraft and are susceptible to sudden loss of control in benign icing conditions.

What is Airframe Icing?
Airframe icing occurs when water droplets freeze on the external surfaces of an aircraft. In general, airframe icing is a potential flight hazard since it adds unintended weight and can significantly disrupt and destroy the functions of the aerodynamic surfaces and degrade performance of the aircraft’s propulsion system. Airframe icing alters the airplane geometry and destroys the intended aerodynamic functions of the wings, tail, ailerons, rudder, elevators and flaps. Airframe icing also affects propellers and significantly reduces engine power by blocking or clogging engine inlets. In short, nothing good occurs when ice accumulates on an aircraft, especially when it is in flight.

Airframe icing can occur while the aircraft is on the ground and frost forms on the airframe. This type of icing contamination can usually be removed on larger aircraft by spraying the aircraft, before take-off, with a de-icing chemical. Frost is removed from smaller aircraft by mechanically cleaning the affected areas.

In-flight airframe icing is more problematic. When icing occurs in flight the pilot must first recognize the potential hazard and react in a timely and appropriate fashion. Even though the aircraft may be certified to fly in icing conditions and is equipped with a de-icing or anti-icing system, the aircraft’s de-icing or anti-icing systems must be able to prevent or remove sufficient ice such that the functions of the aerodynamic surfaces and the engine are preserved and protected. Also, sharper surfaces accrete ice contamination at a faster rate than blunt surfaces, so the leading edges of the horizontal tail, which cannot be seen by pilots on most aircraft, typically gather ice at a faster rate than the leading edges of the wings.

When Does Icing Occur in Flight?
Airframe icing can occur at any phase of flight. It can occur during take-off and climb, during cruise flight and during approach and landing. A loss of control due to icing in any of these phases of flight is usually fatal to all of the aircraft occupants. Any time an aircraft encounters an area with sufficient water content at a sufficiently low temperature, icing is possible. The amount of time an aircraft is exposed to icing conditions is important since most mechanical de-icing systems, such as the pneumatic boot system, do not remove all of the ice. Therefore, ice accumulations will continue behind the areas protected by boots or on residual ice, which remains in place between inflation cycles of the boots.

Piloting Issues
Pilots must be able to recognize icing problems and deal with them immediately. On aircraft not certified to fly in icing, the pilot must exit the icing immediately or risk loss of control. In aircraft certified for flight in icing conditions, the pilot must avoid extended use of the autopilot. If activated, the autopilot will compensate for the loss of lift and will attempt to maintain attitude and altitude of the aircraft by adjusting the angle of attack and engine power. If the autopilot is engaged during icing conditions the pilot will not appreciate the extent of handling and performance degradation caused by the icing. Once there is a loss of control, the pilot is usually unable to restore control due to the contaminated aerodynamic surfaces.

Aircraft Characteristics and Certification Issues
Many aircraft are certified to fly in icing conditions. Despite the certification, the aircraft’s actual capability to perform in the icing environment can present serious safety issues. While the aircraft is technically certified for flight in icing by the Federal Aviation Administration (FAA), the certification process is handled by the aircraft manufacturer under a delegated authority procedure from the FAA. Thus, the manufacturer, in essence, certifies its own aircraft. The quality of icing certification can be very suspect and can raise serious questions about the adequacy of a particular aircraft to fly safely in even benign icing conditions. The Cessna Caravan is an example of an aircraft which is very susceptible to loss of control in icing conditions, but which is nonetheless certified for flight in icing conditions.

Training Issues
When it comes to icing training, it is much safer and less expensive to train pilots in simulators rather than in large, expensive aircraft as long as the simulator provides a realistic presentation of the flight conditions or emergencies being practiced. Unfortunately, most modern simulators do not present the trainee with a realistic feel for icing. Most simulators simply add a weight component and a drag component to account for the effect of icing. They do not emulate the true aerodynamic degradation of icing on the aircraft, which is frequently asymmetrical and induces yaw and roll tendencies and which can produce non-linear aerodynamic effects as irregular shapes develop span wise on the wing and tail surfaces. Most simulators in use today do not sufficiently account for the effect of icing on the horizontal tail nor do they expose the trainee pilot to the effect of the tail stall. Most training on icing focuses solely on wing stall even thought the tail is actually at much greater risk of stalling due to the more rapid rate of ice accretion.

Cessna Caravan 208B
The Cessna 208B Caravan equipped with pneumatic de-icing boots is very susceptible to airframe icing, especially in-flight airframe icing. This is due to several factors that include inadequate pneumatic de-icing coverage and capacity (due to inadequate engine power at higher altitudes), a susceptibility to pitch instability at higher altitudes, an airfoil that has a very sharp and unforgiving stall characteristic, and a high drag profile in icing resulting from a large, unprotected frontal surface area that accretes large quantities of ice contamination.

The Caravan is certified to fly in icing conditions but documents and testimony obtained by Slack & Davis, L.L.P. in connection with the November 2002 crash of aircraft N514DB near Parks, Arizona, contains substantial evidence that the icing certification of the 208B model was inadequate and does not meet minimum certification standards. Both the National Transportation Safety Board, in late 2005, and the Canadian Transportation Safety Board, in early 2006, issued safety recommendations and expressed concerns about the safety of the Caravan flying under icing conditions. The FAA however has muddled around with the Caravan problem for years without taking appropriate action. The NTSB has characterized the FAA’s action, or inaction, on the NTSB’s list of icing safety issues as “unacceptable.”

American Eagle Flight 4184 (Roselawn, Indiana)
On October 31, 1994, American Eagle Flight 4184 crashed near Roselawn, Indiana. The resulting litigation, in which Slack & Davis participated, developed strong evidence that, as designed and certified, the ATR aircraft was susceptible to a loss of control in icing conditions. The control problems faced by the pilots included an aerodynamic phenomena called “control reversal” in which certain aerodynamic characteristics of the aircraft in icing were opposite to what the pilots expected. There was also evidence developed that pointed to a delay in the pilots’ recognition of and failure to appreciate the severity of the icing.