SAFETY OF FLIGHT

Icing accidents persist even for experienced IFR pilots

Accretion vulnerabilities pose dangers that require awareness both on ground and aloft.




Remote sensing systems that map aircraft icing conditions in the flightpath would allow icing to be avoided and exited. However, airborne systems to display conditions with icing potential are currently only vaguely promising.
NASA proposes the use of advanced ground radars to remotely-sense icing conditions and then broadcast their location so that pilots would have the same display as the ground operator.

Structural icing must be removed before flight. While failing to deice when mandated is regrettable, it happens.

In Siberia recently, an ATR­72 crashed minutes after takeoff from TJM (Tyumen, Russia), the cause being an alleged failure to deice.

Even aircraft equipped for flight into icing conditions are affected by structural icing on unprotected areas.

A NASA study showed that nearly 30% of the total drag associated with an ice encounter remained after clearing all protected surfaces.

(Nonprotected surfaces may include antennas, flap hinges, control horns, fuselage frontal area, windshield wipers, wing struts, fixed landing gear, etc.)

Given the same structural icing conditions, one cannot assume that accretion rates and aerodynamic effects are the same for all aircraft types.

It is equally imprudent to rely on pneumatic deicing systems for protection in exceptional icing conditions.

Propulsion system icing becomes more likely by failing to blend a fuel system icing inhibitor (FSII) correctly with jet fuel to stop ice crystal formation in the fuel tank.

Also, delayed activation of ice protection systems in flight can cause large pieces of ice to enter and damage the engine.

Threat and error management

Icing threats can only cause deviation or harm if, through errors, they are allowed to cause an undesired aircraft state. Returning to a desired aircraft state requires mitigation plans (in this case, through evolving technology and sound decisionmaking) meeting the needs of threat and error management.

Ground icing

The absence of consistent runway condition descriptors prompted the University of Waterloo (Ontario, Canada) to develop a braking availability tester (BAT) to validate using braking action in preference to friction measurement. The system provides pilots with a meaningful indication of the least available braking when maximum braking is applied (as during a rejected takeoff).

Similarly, the FAA Takeoff and Landing Performance Aviation Assess­ment Rule­making Committee (TALPA ARC) developed a reporting matrix which improves correlating reported runway condition with airplane performance (ukfsc.co.uk/files /External%20Meetings/ Specialist%20Subject/Int%20Winter%20Ops%20Conference%20C%20Collett %20presentation%20Oct%202011.pdf).

Related rulemaking will be rolled out over the next 3 years. The pilot version of the runway condition matrix is one of the TALPA ARC products that represent a significant improvement over current practices.

The main strategy for countering ground icing is the "clean airplane concept," which prohibits takeoff during conditions conducive to airplane icing when ice, snow, slush or frost is present or adhering to the wings, propellers, control surfaces, engine inlets or other critical surfaces.

Although specific regulations vary from country to country, all are founded on ICAO standards and recommended practices (SARPs) which require the following:

• A flight to be operated in known or expected icing conditions shall not be commenced unless the airplane is certified and equipped to cope with such conditions, and
• A flight to be planned or expected to operate in suspected or known ground icing conditions shall not take off unless the airplane has been inspected for icing and, if necessary, has been given appropriate deicing/anti-icing treatment.

NASA offers a free online course intended primarily for pilots who make their own operational deicing and anti-icing decisions. The website is icebox.grc.nasa.gov/education/products.html.

Structural icing

Battelle dispersed nanotubes into a coating solution to make it conductive. When power is applied, it heats up like a resistor. The coating has been tested in an icing tunnel using a scrap UAV wing.

The Commercial Aviation Safety Team (CAST) has 2 current goals to make airplanes more icing tolerant. The first regards basic airplane design (SE039), setting icing criteria to reduce fatal loss-of-control accidents by recommending amended icing certification criteria for new airplane designs not equipped with evaporative (ie, hot wing) systems.

The criteria would include performance and handling quality requirements for residual ice, intercycle ice, delayed anti-icing/deicing system activation, and anti-icing/ deicing system malfunction.

The 2nd related CAST initiative improves situational awareness during low-visibility operations and flight in icing conditions through the development and use of smart-pitch guidance systems (SE134) on all new type design airplanes to help prevent over-rotation in conjunction with a low-energy state or aerodynamic degradation due to the presence of ice on critical flight surfaces.

Detection refers to sensing the presence or potential of icing as well as assessing its severity and rate of accretion in order that appropriate avoiding action can be taken. Areas of icing activity are hidden from most detection systems because ice is a poor radar target above the freezing level, especially in precipitating clouds.

NASA has a mission to develop real-time, inflight, remote sensing of hazardous icing conditions, characterized by an SLW droplet environment several miles ahead of the aircraft. Two methods are currently in review. The first uses ground-based polarimetric radars optimized to detect SLW in cloud. The returns are suitably processed and broadcast to receiving aircraft as shown in the upper right illustration on this page.

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