SAFETY OF FLIGHT
Icing accidents persist even for experienced IFR pilots
Accretion vulnerabilities pose dangers that require awareness both on ground and aloft.
By Don Van Dyke
ATP/Helo/CFII, F28, Bell 222
Pro Pilot Canada Technical Editor
NASA is to investigate engine core icing, wherein ice crystals enter a turbine engine core, melt and then refreeze, causing loss of power or shutdown.
Aircraft icing is a known hazard to all types of flying. Advances in icing detection include not only technology but also expanded awareness of vulnerabilities. Pilots may take comfort in thinking that the problem of icing is reduced to manageable status given modern aircraft equipment, weather forecasting and reporting. This is wrong.
Ground and airborne icing are complex issues involving aspects of meteorology, aircraft design and flight phase that determine the type and severity of accretion and its effect on aerodynamic performance and handling. A systems approach to icing encounters involves consideration of associated threats and errors as well as their management to improve margins of safety.
From 2000–10, NTSB investigated more than 50 accidents involving aircraft icing and causing over 200 fatalities. Icing accidents continue to attract worldwide attention.
Ground icing is accumulation on airport maneuvering areas and runways. Procedures to clear such accumulation are costly and often time-consuming, frequently inducing delays at larger airports. However, NTSB reports that roughly 18% of runway excursions involving corporate/business aircraft are due to ineffective braking on contaminated runways.
Propulsion system icing is accretion on an engine nacelle, induction system, core or fuel system, threatening to block air, fuel or both. All aviation fuels retain various amounts of dissolved water, in spite of precautions adopted by refineries, transportation/distribution facilities and aircraft servicing stations. Fuel system icing occurs when liquid water in the fuel system freezes into ice crystals.
Ice can also cause engine stoppage by choking the carburetor, blocking a fuel filter or a fuel-injected engine's air source. The Transportation Safety Board of Canada (TSB) has determined that the amount of ice derived from as little as 2 droplets of water may affect most fuel injection systems.
NASA will investigate another variant—engine core icing, in which ice crystals (formed in high-altitude, warm-weather storms) enter a turbine engine core, melt and then refreeze, causing loss of power or shutdown. (See photo above.)
Structural icing is accretion on exposed surfaces of the airplane. Wind tunnel and flight tests have shown that accumulations (on the leading edge or upper surface of the wing) no thicker or rougher than a piece of coarse sandpaper can reduce lift by 30% and increase drag by up to 40%. Larger accretions reduce lift even more and can increase drag by 80% or more.
On-ground structural icing includes frost, freezing rain, ice, snow and slush as well as the following recently identified variants:
• Active frost, which can form when airflow is nullified by wind (eg, when tailwind and taxi speed are roughly equal) or is calm (eg, when holding).
• Supercooled dew, which is an outcome of rapid (under 2 sec) conversion from a liquid state to frost triggered by a disturbance such as a pushback jolt.
Inflight structural icing can accumulate rapidly on aircraft surfaces (wing, tail, nacelle, etc), destroying smooth air flow, interfering with instruments, increasing drag and decreasing lift. Effects on aerodynamic performance include deteriorated speed, power, climb and control, all possibly accompanied by vibration or buffet.
While severe icing can destabilize an aircraft, effective anti/deicing equipment gives the pilot the opportunity to combat ice buildup and fly out of icing conditions. However, freezing drizzle and freezing rain, even if light, are extremely hazardous conditions that lie outside FAA certification requirements for flight into icing.
According to AC 00-24B 5.d—Icing—supercooled liquid water (SLW) droplets freeze on impact with an aircraft. Clear icing can occur at any altitude above the freezing level. At high levels, icing from smaller droplets may be rime or mixed rime and clear. The abundance of large SLW droplets makes clear icing very rapid between 0°C and –15°C, and encounters can be frequent in a cluster of cells.
Thunderstorm icing can be extremely hazardous. Critical conditions for inflight ice formation include the presence of SLW droplets, high humidity and freezing temperatures.
Errors are actions or inactions by crew or personnel or design deficiencies which tend to reduce margins of safety and increase the probability of an undesirable event. The following scenarios illustrate ways in which operational or design errors compromise safety margins.
Ground icing may allow taxiing aircraft to create ice lumps which could become projectiles on first acceleration. Also, runway conditions are currently reported in one of 3 ways—braking action reports based on Pireps, reports on type and depth of contamination, and runway friction reports.
This lack of consistency prompted ICAO to call for a "single overarching source ... for the promulgation of runway condition information." Flight Safety Foundation (FSF) warns that, because of these conflicting systems, slush may induce poor/nil aircraft braking action, contrary to runway friction readings provided by certain friction measuring systems.