De-Icing /Anti-Icing Procedures

Sofema Online (SOL) www.sofemaonline.com considers typical aircraft ground De-Icing /Anti-icing procedures

Introduction

The necessary inspections and communications can be made beforehand at the gate whereas, at remote/centralised de-icing, the necessary information must be informed to the de-icing crew in another way (e.g. coordinator communication).

The determining of the need for de/anti-icing can be made by other qualified persons, not necessary the de-icing crew.

Pilots must have a description of any local differences they will be exposed to (e.g., de/anti-ice location, fluids and mixtures available, start clearance for de-icing, checks, etc.)

Determining the need for De/Anti-Icing

The need for de/anti-icing is usually determined well beforehand by the trained and qualified ground crew or flight crew.

Certain aspects must be considered, such as,

» What are the A/C specific requirements and precautions,
» Is the de-icing operation performed at the gate or remote, can the aircraft start the engines and taxi to a remote de-icing fully contaminated,
» Who makes the request for the de-icing,
» Verification of proper procedure with all parties involved (ground crew/flight crew / de-icing), should air-blower/brushes be used beforehand etc.
» The contamination check shall cover all critical parts of the aircraft and shall be performed from points offering sufficient visibility of these parts (e.g. from the de-icing vehicle itself or any other suitable piece of equipment).
» Any contamination found, except frost allowed in certain areas, shall be removed by a de-icing treatment followed by an anti-icing treatment if required.

Some inspected areas can be cleaned manually during the inspection and a de-icing procedure is not necessary.  (Such procedure must be confirmed with the flight crew.)

The captain has the final authority of the procedure but the safer option should always be considered, whether it is the opinion of the flight crew or ground crew (company and A/C limits to be noted). There are some areas to include in the inspection while waiting for instructions from the flight crew.

Areas to check to include:

» Wings (upper and lower)
» Vertical and horizontal tail surfaces (upper and lower horizontal surfaces)
» Fuselage
» Engine inlets and fan blades (front and backside of fan blades)
» Control surfaces and gaps
» Pitot heads and static ports
» Landing gear and landing gear doors
» Antennas and sensors
» All other aerodynamic surfaces
» Propellers

After checking these areas, a decision with the flight crew on de-icing procedures can be made accordingly.

The weather elements and taxi distances will affect the choice of type and mixture of fluid to use.

One-step/two-step de/anti-icing

When aircraft surfaces are contaminated by frozen moisture, they shall be de-iced prior to dispatch.

When freezing precipitation exists and there is a risk of contamination of the surface at the time of dispatch, aircraft surfaces shall be anti-iced.

 If both de-icing and anti-icing are required, the procedure may be performed in one or two steps. The selection of a one- or two-step process depends upon weather conditions, available equipment,

available fluids and the holdover time to be achieved.

One Step Procedure - Some contamination, such as frost, can be removed and the surface protected from refreezing, all at the same time using the same fluid and same mixture.

This is called a one-step procedure.

» One-step de/anti-icing is generally performed with a heated un thickened fluid.
» Thickened fluid can and is in some cases used for this one-step process.

Caution must be taken for the dry-out characteristics and gel residue problems of this particular

scenario.

The mixture to choose for this step is the mixture that gives a protective cover; in other words, the

De-icing is performed with an anti-icing mixture, which protects the surface at the same time.

» The correct fluid concentration shall be chosen with regard to desired holdover time and is dictated by outside air temperature and weather conditions.
» Wing skin temperatures may differ and, in some cases, be lower than OAT.
» A stronger mix (more glycol in the glycol-water mixture) can be used under these conditions.

Note  - The stronger mix will not improve the holdover time but it will lower the freezing point of the mixture.

Two-step de/anti-icing (when the first step is performed with de-icing fluid) is a procedure performed whenever the contamination demands a de-icing process separately.

» The correct fluid(s) shall be chosen with regard to ambient temperature.
» After de-icing, a separate over-spray of anti-icing fluid shall be applied to protect the relevant surfaces thus providing maximum possible anti-ice capability.
» The second step is performed with anti-icing fluid.
» The correct fluid concentration shall be chosen with regard to desired holdover time and is dictated by outside air temperature and weather conditions.
» The second step shall be performed before the first step fluid freezes (typically within 3 min) if necessary area by area.
» A two-step procedure is common during freezing precipitation.
» The second step shall be applied in such a way that it gives a complete, sufficient and even layer of anti-icing fluid on the treated surfaces

Critical Common Elements

» De/anti-icing fluids shall not be sprayed directly on wiring harnesses and electrical components (receptacles, junction boxes, etc.), onto brakes, wheels, exhausts, or thrust reversers.
» De/anti-icing fluid shall not be directed into the orifices of pitot heads, static ports or directly onto airstream direction detectors probes/angle of attack airflow sensors.
» All reasonable precautions shall be taken to minimise fluid entry into engines, other intakes/outlets and control surface cavities.
» Fluids shall not be directed onto flight deck or cabin windows as this can cause crazing of acrylics or penetration of the window seals.
» Any forward area from which fluid can blowback onto windscreens during taxi or subsequent takeoff shall be free of residues prior to departure.
» If Type II, III or Type IV fluids are used, all traces of the fluid on flight deck windows should be removed prior to departure, particular attention being paid to windows fitted with wipers.
» Landing gear and wheel bays shall be kept free from the build-up of slush, ice or accumulations of blown snow.
» When removing ice, snow, slush or frost from aircraft surfaces care shall be taken to prevent it from entering and accumulating in auxiliary intakes or control surface hinge areas, e.g. manually remove snow from wings and stabilizer surfaces forward towards the leading edge and remove from ailerons and elevators back towards the trailing edge.

Clean Aircraft Concept Consideration

A clean aircraft is considered to be either totally clean or cleaned and protected with de/anti-icing fluids that still protect the surface and are able to perform aerodynamically correct.

» Contaminated fluid on the surface must not be misunderstood as a clean aircraft; this contamination must be removed.
» Under no circumstances shall an aircraft that has been anti-iced received a further coating of anti-icing fluid directly on top of the contaminated film.
» If additional treatment is required before the flight, a complete de/anti-icing shall be performed. Ensure that any residues from previous treatment are flushed off. Anti-icing only is not permitted.

Spray Areas

» Areas to spray on any aircraft are in most cases the upper surfaces. However, underwing de-icing may for some A/C types be very common.
» When talking about upper surfaces, it is referred to as the wings, tail (including vertical stabilizer) and fuselage.
» As a rule of thumb, the de/anti-icing procedure should be performed from the top-down, leading-edge towards trailing edge and from the A/C front parts backwards.
» On most aircraft, start at the wingtip and work towards the wing root. Areas to protect from refreezing depend on the aircraft limitations but in general, the upper surfaces of the wings and the tail section should be anti-iced. The fuselage may also need anti-icing but underwings are not generally anti-iced with thickened fluid.

Aircraft surfaces

» There is no single rule of spray order that can be applied to all aircraft. It is, however, recommended to start with the fuselage (front part covering the wing area) whenever it needs treatment (spray along the top centre-line and then outboard).
» After the fuselage comes the wings and the way to treat the wings depends on the aircraft and the place where de-icing is performed (gate vs. remote). The wing should always be treated from the highest part towards the lowest part (generally wingtip inboard).
» Some aircraft have wingtips lower than the wing root and in that case de-icing should be performed from the wing root outboard.
» The tail should be performed from the vertical stabilizer downward and the aft-fuselage part before the horizontal stabilizer (excluding T-tail A/C).

Note - It is recommended that anti-icing fluid be sprayed within 3 min. after de-icing but in no case can it be sprayed after the de-icing fluid freezes.

If the wing area is large and the contamination is heavy, previously de-iced parts should be considered to be de-iced again before anti-icing.

The following surfaces shall be protected:

» Wing upper surface and leading edges
» Horizontal stabilizer upper surfaces including leading edges and elevator upper surfaces
» Vertical stabilizer and rudder
» Fuselage upper surfaces including VHF-antenna depending upon the amount and type of precipitation (required on centre-line engine aircraft).

Note - Underwings do not need anti-icing since the precipitation cannot reach there. However, a sufficiently high (concentrated) mixture must be used so as not to cause ice formation after the de-icing.

» Gate de-icing is somewhat different from remote/centralised de-icing and local settings and precautions should be noted.
» Using multiple de-icing vehicles at one aircraft may change the spray order but the same concept (high-low, front-back) should be applied.
» Different vehicles may also be needed for different de-icing work (e.g. underwing) in this case the procedure should be coordinated accordingly.

Considering the Need to Address other Areas

» Other areas may need special attention for example flight deck windows to need de-icing but there is no need for anti-icing.
» It should be noted that drain off the water may freeze elsewhere on the fuselage if using water for de-icing  flight deck windows
» The radome needs de-icing. Caution must be taken so that fluids would not flow in large quantities on the flight deck windows during take-off
» Static ports and pitot tubes may need an inspection. (Any contamination like, e.g. ice and drain off fluid, shall be removed from these areas.)

Note - The application of de-icing- fluid in landing gear and wheel bay areas shall be kept to a minimum.

» De-icing fluid shall not be sprayed directly onto brakes and wheels.
» Accumulations such as blown snow should be removed by other means

Next Steps

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