'Light snowfall' Can Hurt Anti-Ice Efforts More Than Pilots Realize

Modern deicing and anti-icing methods have proven effective when properly applied and the limitations of the fluids are observed. However, the duration of that protection can be degraded by meteorological conditions including freezing rain, ice pellets and hail, heavy snow, high wind velocity, fast-dropping air temperatures or any time when freezing precipitation with high water content is present. Some other factors that can reduce the effectiveness of deicing and anti-icing procedures range from the inclination angle and contours to surface roughness and temperature of aircraft surfaces.

A team of researchers from the National Center for Atmospheric Research (NCAR) and United Airlines discovered several under-recognized conditions in which an aircraft might attempt to take off unknowingly with contaminated wings. The project was sponsored by the National Science Foundation through an interagency agreement with the FAA. Its results were reported as “Common Snowfall Conditions Associated With Aircraft Takeoff Accidents,” in the January-February 2000 issue of Journal of Aircraft.

In the study, the team examined five icing-related FAR Part 121 mishaps. The accidents occurred at New York LaGuardia (KLGA) on March 23, 1992 (USAir Flight 405, Fokker F-28); the former Stapleton International Airport (KDEN) on Nov. 15, 1987 (Continental Flight 1713, DC-9-14); Washington National (KDCA) on Jan. 13, 1982 (Air Florida Flight 90, Boeing 737-222); Logan International (KBOS) on Feb. 16, 1980 (RedCoat Air Cargo, Flight RY103, Bristol 175 Britannia) and Newark International (KEWR) on Nov. 27, 1978 (TWA Flight 505, DC-9-15).

While these accidents all occurred more than two decades ago, the detailed meteorological data attendant to them allowed the researchers to note key similarities among all five. For example, the temperatures were between 25F and 31F, the winds were 8-13 kt. and the accidents occurred during the peak snowfall period of a storm associated with snow bands.

There was, however, a large disparity between the reported visibilities that raised questions among investigators and others within the industry. The reported visibility varied from 0.25 mi. in the KDCA accident to 2 sm in the KBOS accident. That might lead one to conclude that the snow was falling heavily in the Washington accident versus rather lightly in the Boston crash. However, in the former, the first officer described the snowfall as “not heavy, no large flakes,” giving a false sense of security that the conditions were not too bad. Such a visual illusion can trick any flight crew into making the same wrong assessment.

This study, together with other work at the NCAR, determined that fluffy snowflakes can obscure visibility up to 10 times more than small, dense flakes, even though both contain the same amount of water. When researchers dived deeply into the meteorological data in these accidents, the snow was falling at a liquid-equivalent rate between 0.08 and 0.10 in./hr. in all five accidents.

This illusionary phenomenon can mislead pilots and air traffic controllers into interpreting relatively good visibility as “light” snowfall when in fact the snowfall consists of dense, compact flakes that are actually depositing substantial amounts of frozen water onto critical aircraft surfaces.

The types of snowfall that can result in deceptively high visibility during high rates of precipitation include wet snow, snow with rimed crystals (cloud droplets accreted onto ice particles, similar to rime ice on aircraft), snow consisting of compact crystals, and snow pellets. These can lead to high visibility because of their relatively small cross-sectional area and higher terminal velocity as compared to dry, fluffy snow. One of the important conclusions of this team’s project is that visibility is not a good indicator of liquid-equivalent snowfall rate.

As a result of this research, the FAA provides amended guidelines for the estimation of the prevailing visibility based upon snowfall intensity. Table 43 of the FAA’s Holdover Time (HOT) Guidelines, Winter 2019-2020, issued Aug. 6, 2019, provides guidance on the estimation of snowfall intensity. A pilot uses the reported visibility and the temperature to determine if the snowfall intensity falls within very light, light, moderate or heavy categories. A flight crew is then able to determine the HOT using this estimate of the snowfall intensity along with the air temperature and fluid concentration level.

The chart provides guidance for both day and night conditions since nighttime conditions increase the pilot’s perception of visibility by a factor of two. This is due to the different types of light scattering occurring during the day versus those at night.

Winds on Taxi

The NCAR/United team uncovered a secondary problem that can occur while waiting for takeoff. Wind can affect the snowfall accumulation rate on critical aircraft surfaces, especially those inclined into the direction of the wind. Let’s consider your aircraft is creeping along a taxiway parallel to the runway. The reported winds are 8-10 kt. straight down the runway. So, while slowly advancing, your aircraft is experiencing an 8- to 10-kt. tailwind. This matters because a tailwind will blow falling snow particles more directly onto the upper surface of the wing, which is typically angled at about 10 deg. from the true horizontal, and significantly increase accumulation. According to a lead NCAR researcher, “For the wind speeds observed during the aircraft accidents studied, the enhancement factor ranges from 1.75 to 2.0.” In other words, the accumulation rate of snow on a wing nearly doubled while waiting in line for takeoff.

The holdover times are tested to provide protection against accumulation of contamination under specific temperature and precipitation limits. That time calculation is negated when conditions fall outside of those limits. A two-fold increase in the accumulation of wet snow on a wing’s surface will most certainly degrade the protection provided by anti-icing fluids.

Another potential effect occurs during a crosswind since it can cause differential accumulation of frozen precipitation on the wing facing into the wind, leading to asymmetrical lift and drag during rotation for takeoff. During the KLGA accident, a crosswind was hitting the trailing edge of the right wing preferentially, with the left wing shielded from the wind by the fuselage. Upon rotation, the aircraft rolled while still in ground effect. The researchers opined that the roll may have been caused by one wing receiving more snow accumulation than the other.

Peak Snowfall = Higher Risk

As noted, the five accidents occurred during the peak snowfall period of storms associated with snow bands of heavier snowfall rates. Those rates at temperatures near freezing are also larger than at colder temperatures since cold air can’t contain as much moisture. Snowfall conditions near 32F are particularly hazardous due to the more-frequent occurrence of high snowfall rates and partial melting of the crystals, leading to the misleading condition of high visibility and high snowfall rates.

The research team provided a special warning to both flight crews and deicing operators: “The passage of potentially hazardous snow bands [which can be detected by Doppler radars by the NWS] should be of particular interest to deicing operators due to their potential hazard, and the resulting increased deicing operations likely to be required due to the higher snowfall rates associated with these bands.”

The study’s authors suggested that this particular combination of temperatures, wind and liquid-equivalent snowfall may be conducive to the buildup of hazardous ice accumulations on aircraft.

This research has contributed to essential information that is contained in special notes below the Holdover Time tables. For example, “The time of protection will be shortened in heavy weather conditions. Heavy precipitation rates or high moisture content, high wind velocity or jet blast may reduce holdover time below the lowest time stated in the range.”

Use the Right Charts

Deicing and anti-icing fluids are tested and qualified for operation within a specific temperature envelope and set of conditions. It is vital to use the right chart and to apply any corrections listed in the notes.

If critical aircraft surfaces are composed of composites, use the charts for composite surfaces. The difference is significant. For instance, the Holdover Time for Type I fluid used on an aluminum aircraft surface with temperatures at 27F or warmer is 6-11 min. In contrast, the Holdover Time with the same fluid on a composite aircraft surface is only 3-6 min. (Note: These numbers are quoted from the FAA’s Holdover Time Guidelines, Winter 2019-2020, issued Aug. 6, 2019. At the time of this article’s preparation, the 2020-2021 guidelines weren’t yet available.)

Holdover time may be reduced when aircraft skin temperature is lower than outside air temperature. The wings of an aircraft become “cold soaked” when they contain cold fuel as a result of flight at high altitude or from having been refueled with cold fuel.

Whenever precipitation falls on a cold-soaked aircraft when on the ground, clear icing may occur. Frost or ice on the lower surface of the wing in the area of the fuel tank is an indication of cold soaking. Even in ambient temperatures between -2C and +15C, ice or frost can form in the presence of visible moisture or high humidity if the aircraft structure remains at 0C or below. Clear ice is difficult to detect visually and may break loose during or after takeoff.

The following factors contribute to cold soaking: temperature and quantity of fuel in fuel cells; type and location of fuel cells; length of time at high altitude; temperature of refueled cell; and time since refueling.

The Association of European Airlines recommends using a strong mix (more glycol) to ensure a sufficient freeze point buffer. Its “Recommendations for Deicing/Anti-Icing of Aircraft on the Ground” from September 2008 states, “As fluid freezing may occur, 50/50 type II, III or IV fluid shall not be used for the anti-icing step of a cold-soaked wing.”

It is also necessary to adjust the holdover time when flaps or slats are extended to takeoff configuration before anti-icing fluid is applied and remain in those positions while taxiing. By the way, the fluid concentration may change if the fluid is subjected to sustained heating.

Applying Recent Findings

By combining the NCAR/United team’s findings with the results of other engineering studies on the performance of wings in wintry conditions, some clear guidelines emerge for winter operations.

Let’s presume you’ve dropped off your clients for their ski vacation at one of the uncontrolled airports in the Rocky Mountain region. While the passengers are delighted to deplane into a snowy scene, you have to reposition the aircraft for the next leg. As you perform your turn-around duties, a “light” snow begins to fall. AWOS reports a temperature of -3C and a visibility of 0.75 sm.

You note the conditions, consult Table 43 in the FAA’s Holdover Guidelines and apply the proper correction for the visibility. Doing so, you determine the proper estimation of the snowfall intensity as moderate,” and ask the FBO to apply Type I and Type IV fluids.

Since the FBO uses a “generic” Type IV fluid with a 75/25 fluid concentration ratio, the HOT obtained from Table ADJ-20: “Adjusted Generic Holdover Times for SAE Type IV Fluids” is 30 to 57 min. The deicing and anti-icing is done on the ramp. You then use the remote communications outlet frequency to get your IFR clearance and the controller asks you to call when you are sitting at the end of the runway ready for takeoff.

Unfortunately, there is a long line of inbound aircraft waiting to shoot the instrument approach, and since there is no radar coverage below the mountain top level, ATC can’t clear anyone for takeoff on the instrument departure procedure until the arriving aircraft have announced to ATC that they have safely landed. For the sake of our scenario let’s put an 8-kt. wind striking the right side of your aircraft as you patiently sit on the parallel taxiway near the end of the runway while awaiting release from ATC.

Your jet features a supercritical wing that produces remarkable cruise performance, but the downside is that high-performance airfoils exhibit markedly worse stall characteristics when contaminated by ice, snow, etc.

Clinton E. Tanner, Bombardier’s senior technical advisor in flight sciences, notes that high-performance jet wings tend to exhibit leading-edge stalling. In this phenomenon, the short bubble that naturally occurs along the airfoil’s leading edge “bursts” and the airflow detaches suddenly and completely from the leading to the trailing edge. A serious consequence is the lack of aerodynamic stall warning and an abrupt loss of lift.

NASA Glenn Research Center and the National Research Council of Canada have studied the aerodynamic characteristics of high-performance airfoils that have a coating of anti-icing fluid. The stall angle was reduced to 15.3 deg. compared to the clean value of 20 deg. The study concluded that secondary wave effects could have a significant impact on the maximum lift coefficient and stall angle for anti-icing fluid tests on the thin, high-performance wing.

Crosswinds can likewise create a stall at a lower angle of attack (AOA). During crosswind takeoffs and landings in a swept-wing jet, the “upwind” wing experiences airflow that is more direct (i.e., perpendicular) to the wing’s leading edge, and this generally improves the wing’s performance. Conversely, the “downwind” wing experiences the airflow at a greater angle (essentially increasing the “sweep” of the wing), which decreases its lift, increases drag, promotes the span-wise flow of air, and thereby reduces its stall AOA. Bombardier’s Tanner cites flight test results showing that sideslip reduces the stall AOA of the right wing by up to 3.5 deg. when it experiences a sideslip of 20 deg.

John O’Callaghan, a national resource specialist in aircraft performance at the NTSB, has warned that the stall of all types of aircraft occurs approximately 2-4 deg. AOA lower with the wheels of the aircraft on the ground. Flight test reports noted “post-stall roll-off is abrupt and will saturate lateral control power.” The catastrophic roll-off of the wing in the Roswell accident was due in part to no warning before stall in ground effect.

To sum up the threats to your repositioning flight:

(1) Your wings were cold soaked from the previous flight.

(2) The wind may have caused extra accumulation of wet snow on the right wing as you sat on the taxiway.

(3) You are flying a high-performance jet whose wing is especially prone to adverse stall characteristics when contaminated.

(4) A layer of anti-icing fluid was applied to give you protection within the Holdover Time, but anti-icing fluid itself extracts a stall margin penalty as the aircraft is rotated for takeoff.

(5) There is the negative influence of ground effect as well as crosswinds on the reduction in stall AOA, particularly on the right wing.

All of these reduce the margins from an actual aerodynamic stall during a takeoff that can occur without aerodynamic warning.

And lastly, (6) If there’s a notable delay between fluid application and takeoff, all Part 135 pilots must comply with procedures in their company’s “Ground Deicing and Anti-Icing Program” including actions required if an HOT is exceeded. This can include conducting a tactile check of the aircraft’s surfaces. And if operating under Part 91, and an excessive hold time is experienced, one of the pilots should conduct such a check even though a shutdown and then restart of the left engine will likely be necessary.

This hypothetical but not uncommon situation amply demonstrates the benefits when deicing and anti-icing operations are conducted close to the end of the runway and coordinated with air traffic control so that aircraft are promptly cleared for takeoff soon after being serviced. If your aircraft has been on the ground in conditions conducive to wing contamination and there is any doubt in your mind as to whether the lifting surfaces are free of ice or snow, get out and conduct a preflight tactile check to be sure.

Finally, the FAA’s Safety Alert for Operators 06002 (March 29, 2006) “Ground Deicing Practices for Turbine Aircraft in Nonscheduled 14 CFR 135 Operations and in Part 91” contains some solid safety information. Prepared by the General Aviation Joint Steering Committee, it recommends that directors of safety, operations and fractional ownership programs, along with flight crews of turbine aircraft, perform “a comprehensive review of current deicing policies and procedures” along with current winter weather operations training. Sound advice with winter’s chill in the air.