Each winter season in Earth’s northern hemisphere creates linked challenges for aviation-focused meteorologists and an SAE committee of expert international stakeholders.1 The committee’s pre-season fluids testing, performance validation and technical guidance enable the publication of new aircraft ground deicing/anti-icing holdover time (HOT) tables and allowance times. The season’s updated technical standards are reviewed by the Association of European Airlines (AEA), the U.S. Federal Aviation Administration (FAA) and Transport Canada (TC), the principal entities that in turn develop authoritative derivative documents for the industry.
Their documents underscore the addition of newly approved fluids, the removal of obsolete fluids, changes in approved water dilutions and the effect, if any, of such changes on generic HOT table values. Most airport service providers and regional airlines in Europe, for example, have preferred deicing/anti-icing systems combining relatively low cost and HOTs that are suited to diverse airport environments in which frost is more prevalent than the ice/snow contaminants common to the United States and Canada.2,3
According to SAE, approved Type I fluids are “generally used heated, either diluted with water, or as supplied, for the removal of, and time-limited protection against, deposits of frost, ice and snow on exterior aircraft surfaces prior to takeoff.” The approved Type II, Type III and Type IV alternative fluids comply with one of the SAE specifications for non-Newtonian fluid, also called pseudoplastic fluid, which contains thickeners to improve HOT compared with Type I fluids. In a 2008 article, subject matter specialists from Europe told ASW that Type II and Type IV fluids are formulated primarily for anti-icing, keeping airplane surfaces free of frozen contaminants before takeoff, and also are approved for deicing.
Eurocontrol’s SKYbrary website describes AEA, FAA and TC as “the main practical sources of HOT information,” adding that “each issues [its] own version of the HOT tables and associated support publications independently of each other and SAE. The generic changes from one season to the next are usually relatively few. However, in recent years, issues with residues from thickened fluids have been the main driver for the appearance of product-specific HOT tables, which are increasingly used by operators.”4 Moreover, advisers to SKYbrary have identified, as a winter operations risk factor, the fact that other organizations publish HOT tables less frequently than AEA, FAA and TC. This leads to situations in which aircraft flight manuals go out of date relative to international best practices and safety knowledge (see “Generic Reminders”).
Beyond differences highlighted by the U.S. Federal Aviation Administration (FAA) in its holdover time (HOT) tables and guidance for winter 2014–2015, this official document — similar to those issued by the Association of European Airlines and Transport Canada — reiterates safety reminders critical to airport deicing/anti-icing service providers, aircraft operators and flight crews.
For example, the FAA cautions this community that fluids used during deicing/anti-icing do not provide in-flight icing protection. A cautionary note on tables warns, “This table is for departure planning only and should be used in conjunction with pre-takeoff check procedures.” Another reminder is that although U.S. air carriers may have approval of their winter operations plan for using handheld electronic devices for HOT/allowance time determinations, unreliable accuracy or failure of such devices requires reversion to the official HOT tables as backup.
The document adds, “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 [HOT table] range. Holdover time may be reduced when aircraft skin temperature is lower than [outside air temperature].”
Preparing for every winter flight also requires the operator/flight crew to recognize when the conditions to be encountered exceed those used in the HOT tables and/or exceed the performance of approved fluids or procedures. “Use light freezing rain holdover times in conditions of very light or light snow mixed with light rain,” FAA said. “Use light freezing rain holdover times if positive identification of freezing drizzle is not possible. No holdover time guidelines exist for this condition for 0 degrees C (32 degrees F) and below. [Regarding] heavy snow, ice pellets, moderate and heavy freezing rain, small hail and hail … no holdover time guidelines exist for this condition below [minus] 10 degrees C (14 degrees F).
A related Eurocontrol concern is assuring the safe performance of new devices that measure precipitation rate in real time and make the data available for flight operations. In this case, SKYbrary’s advice says, “These systems, referred to as liquid water equivalent systems (LWES), can be used by check-time determination systems (CTDS) and holdover time determination systems (HOTDS) to calculate more precise holdover times than can be obtained from the HOT tables. They do this by using the weather data they collect as the input to the underlying assumptions employed in calculating the times in the HOT tables.”
This article cites only the Official FAA Holdover Time Tables, Winter 2014-2015, Revision 1.1 — published Oct. 22, 2014 — which contains agency guidance, holdover tables and allowance times for use during this season. The document states, “It is the responsibility of the end user to periodically check the following website for updates: <www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/deicing/>.”
Winter operations specialists will find new holdover/allowance time adjustments built into this season’s tables because of early fluid failure recently observed when the slats and flaps of test aircraft had been deployed prior to deicing/anti-icing. The change was based on research that found that the steeper angles of the flaps/slats in the takeoff configuration accelerate the flow-off degradation of fluids, with “the degree of potential degradation … significantly affected by the specific aircraft design.”
The document also refers to new scientific research indicating that, in some conditions, HOTs of Type III fluid can be shorter when fluid is applied heated versus unheated. Therefore, a note stresses that generic HOT table values for Type III fluids are applicable only when the fluid is applied unheated.
The document points out specific adverse HOT effects from nonstandard dilutions of Type I, Type II, Type III and Type IV fluids. “When a Type II, III or IV fluid is diluted to other than the published 100/0, 75/25 or 50/50 dilutions, the more conservative holdover time [and lowest operational use temperature (LOUT)]5 associated with either the dilution above or below the selected dilution are applicable,” FAA said. “For example, the holdover time and LOUT of an 80/20 dilution would be the more conservative holdover time and LOUT [compared with] either the 100/0 or 75/25 dilutions. The holdover time and LOUT of a 60/40 dilution would be the more conservative holdover time and LOUT [compared with] either the 75/25 or 50/50 dilutions.”
Ice Pellets and Small Hail
Also new for this season are ice pellet and small hail allowance times. “Additional research has been conducted to provide guidance for aircraft operations during ice pellet conditions when operating with Type III undiluted (100/0) fluid applied unheated,” FAA said. “A separate ice pellet allowance time table has been developed for Type III fluids. … Small hail has been added to the allowance time tables as it has been determined to be meteorologically equivalent to moderate ice pellets.
“Research has indicated that Type IV propylene glycol (PG) fluids are removed less effectively during takeoff [by the airflow] when contaminated with moderate ice pellets at temperatures below [minus] 16 degrees C [3 degrees F]. Therefore, operations in these conditions are not recommended and no allowance times exist for PG fluids in conditions of moderate ice pellets at temperatures below [minus] 16 degrees C, irrespective of aircraft rotation speed. Research has provided data to support a new Type IV allowance time of seven minutes for light ice pellets mixed with moderate snow at temperatures below [minus] 5 to [minus] 10 degrees C [23 to 14 degrees F].”
Cues From Visibility
New guidance on airport surface visibility and snowfall visibility in relation to use of HOT tables also applies this season. “Whenever surface visibility is available from an official source, such as a METAR [aviation routine weather report], in either the main body of the METAR or in the Remarks (‘RMK’) section, the preferred action is to use the surface visibility value,” as opposed to tower visibility, if shown, FAA said. Operators are not permitted to apply runway visual range data for determining the visibility used with the HOT tables.
Although no changes were made to the standard table showing snowfall intensities as a function of prevailing visibility, FAA this season allows optional use of locally adapted summaries written by the operator. The guidance says that an air carrier’s winter operations plan simply could say, for example, “Since very light snow is being added to some of the Type II and Type IV [fluid HOT] tables, and since the METAR and the associated ATIS [automatic terminal information service broadcasts] do not report very light snow, a METAR-reported visibility of 2.5 mi [4 km] or higher can be used as an indication that the snowfall intensity is very light. An air carrier certainly would also have the option of providing a more detailed description utilizing lower METAR reported visibilities for specific day/night and temperature conditions.”
FAA policy on hail versus small hail, terms that are not equivalent in this HOT tables and guidance context, also is new. “No holdover times exist for either of these conditions; however, it has been determined that small hail is meteorologically equivalent to moderate ice pellets and therefore moderate ice pellet allowance times can be used in small hail conditions,” the agency said, pointing to relevant changes throughout the document.
Not all countries follow the World Meteorological Organization’s application of the METAR code GS to snow pellets and small hail, FAA found, adding that this creates a safety concern. “The use of the reported GS code can potentially lead to difficulties in determining which condition (snow pellets or small hail) is occurring and therefore in establishing the appropriate holdover time/allowance time,” the document said in providing updated guidance on which to term to use.
- Eurocontrol SKYbrary. “Holdover Time (HOT) Tables.” Accessed Sept. 29, 2014. The international body responsible for the annual testing and calculations is the SAE G-12 Aircraft Ground De-Icing Steering Committee.
- Six years ago, AeroSafety World reported on consensus-building difficulties among European regional airlines, scientists and deicing/anti-icing service providers regarding the selection of deicing/anti-icing fluids offered to operators, safe application procedures and related ground-handling practices. The objective was to reduce the risk of accumulated gel-like fluid residues freezing and impeding the concealed control linkages of susceptible aircraft types (ASW, 10/08 and ASW, 11/08). SAE G-12’s latest specification for Type II, Type III and Type IV fluids (issued in December 2010) responded to that safety issue and others by changing its fluid-qualification process; changing how the test laboratory inspects fluid-covered test plates for dry-out after exposure to cold dry air, and how it performs the thin-film thermal stability test; changing the hard-water stability test; changing the lot-acceptance test to allow measurement of fluid viscosity at 0 degrees C (32 degrees F) or 20 degrees C (68 degrees F); and changing the successive dry-out and rehydration test.
- European Aviation Safety Agency (EASA). “Ground De-/Anti-Icing of Aeroplanes: Intake/Fan-blade Icing and Effects of Fluid Residues on Flight Controls.” Safety Information Notice (SIN) no. 2008-29, April 4, 2008. EASA said, “Type II and Type IV [anti-icing] fluids contain thickeners which enable the fluid to form a thicker liquid-wetting film on surfaces to which it is applied. Generally, this fluid provides a longer holdover time than Type I [deicing] fluids in similar conditions. … Type III [is a] thickened [anti-icing] fluid intended especially for use on airplanes with low rotation speeds.”
- SAE G-12 subcommittees focus on aircraft deicing fluids, runway deicing fluids, deicing facilities, holdover testing, methods, equipment, ice detection, training, and future deicing. The most recent standards development or revision activities have focused on Type II, Type III and Type IV fluids; aircraft after-market surface-coating interaction with fluids; the processes for qualifying and testing the endurance of Type I fluids; ramp de-icing; design of aircraft deicing facilities; and the Weather Support to Deicing Decision Making Winter Weather Nowcasting System. A recent example of an addition to supporting documents for these activities is SAE AS5681A, “Minimum Operational Performance Specification for Remote On-Ground Ice Detection Systems.”
- FAA defines LOUT as the lowest temperature at which a deicing/anti-icing fluid will adequately flow off aircraft critical surfaces and maintain the required anti-icing freezing point buffer. This buffer is 10 °C (18 °F) below outside temperature (OAT) for SAE Type I fluids. The buffer is 7 degrees C (13 degrees F) below OAT for SAE Type II, Type III and Type IV fluids.