Ground De-Icing and Anti-Icing - ATPL 070 — Operational Procedures

The previous lesson filed the ground de-icing and anti-icing policy under Part A of the operations manual; this lesson opens that chapter. Its logic starts from one idea, the clean aircraft concept, and everything else, the fluids, the holdover clock, the checks and the commander's veto, exists to make that idea survive a freezing ramp.

The clean aircraft concept

The guidance points to ICAO's manual of ground de-icing and anti-icing operations, Doc 9640, and lists the concept's stakes: any deposit of frost, ice, snow or slush on the external surfaces may drastically affect flying qualities, through reduced aerodynamic lift, increased drag and modified stability and control; freezing deposits may jam elevators, ailerons and flap mechanisms; propeller, engine and APU performance deteriorates with frozen contaminants on blades and intakes, and ingested snow or ice can cause engine stall or compressor damage. Cold-soaked fuel can grow ice on wing surfaces at ambient temperatures well above 0 C. EU law compresses all of that into the commander's rule: take-off is commenced only when the aircraft is clear of any deposit that might adversely affect its performance or controllability, except as the operator's procedures under the rule permit, in accordance with the AFM.

The syllabus asks, in so many words, why safety must come before commercial pressure here, and the arithmetic answers itself: a treatment costs minutes and fluid, while a contaminated take-off wagers the aircraft, its passengers and the operator's existence against a saved turnaround slot. The clean aircraft concept admits no negotiation, and the commander's veto is written as a prohibition, not a preference.

The vocabulary of the ritual

De-icing eliminates frozen contamination from the surfaces, typically with heated fluid whose freezing point sits at or below the outside air temperature. Anti-icing protects clean surfaces against contamination from existing or expected weather, typically with unheated thickened fluids. Done together, the one-step treatment does both at once with a heated fluid and water mixture, while the two-step treatment de-ices first, heated water or a heated mix, and then sprays the anti-icing layer, the second step following before the first-step fluid freezes, typically within 3 minutes and if necessary area by area. Holdover time is the period during which the anti-icing fluid protects the treated surfaces, and it depends on the type and intensity of the precipitation, the outside air temperature, the wind, the fluid used and the aircraft's design and configuration.

Around those five terms sit the supporting cast: clear ice, smooth, transparent and very difficult to detect visually; cold-soaked surface frost from icy fuel; the lowest operational use temperature, the coldest temperature at which a fluid is certified, keeping a freezing-point buffer of 10 C for Type I and 7 C for the thickened types; the post-treatment check by qualified staff from suitably elevated points; and the anti-icing code passed to the flight crew after treatment: fluid type, name and concentration for the thickened types, the local time at the start of the anti-icing step, the date when given, and the statement that the check is complete, with a two-step treatment coded by its second-step fluid.

The fluids

The fluid list runs from plain to engineered. Heated water de-ices and protects nothing. Type I forms a thin liquid-wetting film with a very limited holdover, and with a property the exam loves: increasing the concentration of Type I in the mix does not extend the holdover time; for anti-icing its mixture keeps a freezing point at least 10 C below the outside air temperature, and applied as heated mix it reaches at least 60 C at the nozzle. Types II and IV carry thickeners, forming a deeper film with a generally longer holdover; Type III is a thickened fluid intended for aircraft with low rotation speeds. The fluids conform to the SAE specifications, AMS1424 for Type I and AMS1428 for the thickened types, and the thickened types repay respect: bad storage, wrong equipment, age or chemically incompatible layering degrades both their protection and their aerodynamic flow-off. A separate caution keeps excessive thickened fluid off the horizontal tail of aircraft with unpowered elevator controls, so the fluid flows off cleanly in the take-off.

Holdover time

The clock starts when the protection starts: with a one-step treatment, at the commencement of the treatment; with a two-step treatment, at the commencement of the second, anti-icing step. It runs out at the commencement of the take-off roll, when aerodynamic forces shed the fluid, or earlier, the moment frozen deposits start to form on treated surfaces. The operator publishes holdover times as tables or diagrams in the operations manual, and the guidance is blunt about their standing: the times are guidelines only, used together with the pre-take-off check, and an unexpired holdover does not by itself make flight safe, since conditions like freezing drizzle or freezing rain may lie beyond the aircraft's certification envelope altogether. Automated liquid-water-equivalent systems may stand in for the tables where the means exist.

The protection shortens with everything the tables cannot see: the exact type and rate of precipitation, wind direction and velocity, humidity and solar radiation, the surface's inclination, contour, roughness and temperature, and jet or propeller blast from nearby aircraft and equipment. For some conditions, freezing rain, ice pellets, hail, snow beyond certain intensities, high winds or a fast-dropping temperature, no holdover guidance exists at all. And when the holdover time has been exceeded, two doors remain: a pre-take-off contamination check of the treated surfaces, normally external and performed just before the take-off run, or re-treatment, and a re-treatment is never a top-up: any residue of the previous treatment is removed and a completely new de-icing and anti-icing treatment applied. If the check cannot be made just before take-off, re-treatment it is.

The checks and the commander

The operator shall establish procedures for whenever ground de-icing, anti-icing and the related inspections are necessary, and the guidance fills the frame: contamination checks that hunt clear ice and under-wing frost against the AFM's thickness and area limits, procedures for interrupted or unsuccessful treatment, the post-treatment check, the recording of incidents, the responsibilities of everyone involved, and the entry in the technical log. The wing-skin temperature is weighed against the outside air, since a cold-soaked wing changes both the need for treatment and how the fluids perform, and the training programmes gain additional content whenever a new method, a new fluid or equipment, or a new aircraft type arrives. In conditions conducive to ground icing, no aircraft is dispatched without a contamination or post-treatment check by a trained and qualified person, tactile where clear ice is suspected. The communication drill wraps the treatment: fluid, extent and procedures confirmed before it starts, and an all-clear confirmation that personnel and equipment are away before the aircraft is reconfigured or moved. Contracting the work to a handler changes none of it: the operator's training and procedures still govern.

After the treatment the burden crosses the cockpit sill. The commander continually monitors the weather, and within the holdover time and prior to take-off performs the pre-take-off check: an assessment of whether the applied holdover is still appropriate, weighing precipitation, wind and temperature, and a look at the wings or representative surfaces for frozen contaminants. Any doubt buys a re-treatment or the external contamination check. And the treatment itself is applied symmetrically: both wings, both tailplane sides, treated alike, because every effect the clean aircraft concept lists, lost lift, added drag, changed control response, becomes a rolling and handling asymmetry when only one side carries it, and an asymmetry discovered at rotation is discovered too late.

Into icing conditions

The airborne rule completes the ground story and hands the next lesson its subject. The operator shall establish procedures for flights in expected or actual icing conditions, and the commander only commences a flight, or intentionally flies, into such conditions if the aircraft is certified and equipped to cope with them. If icing exceeds the intensity for which the aircraft is certified, or a non-certified aircraft meets icing at all, the commander exits the icing conditions without delay, by a change of level or route, declaring an emergency to ATC if that is what it takes. What ice does to the aeroplane once airborne, and what a contaminated runway does to the numbers, is the next lesson entire.