Airports may use a variety of methods to maintain the surface friction, and therefore the breaking action and directional control, on runways and on taxiways and aprons if they are contaminated by snow and/or ice.
Most airports aim to clear frozen deposits completely but where this is impractical because of low temperature and persistent snow conditions, treatment of a frozen surface to achieve adequate braking action may be undertaken. In the more usual case, mechanical methods are used to clear snow and ice and when necessary, de/anti icing chemicals applied in solid or liquid form are used too. The purpose of these products, often referred to as Pavement De-icing Products (PDP) or Runway De-icing Fluids (RDF), is to cause frozen deposits to melt or to prevent freezing or re-freezing of liquid on the surface by lowering its freezing point.
Types of Runway De-icing Product
The products most commonly used for this purpose are:
- Ethylene/Propylene Glycol-based fluids
- Sodium Acetate
- Potassium Acetate (KAc)
- Sodium Formate
- Potassium Formate (KF)
Continuing Airworthiness Concerns
Because the chemicals used to de-ice runways can pose an environmental hazard, there has been a move from urea and glycol-based de-icing products to alkali-metal-salt-based products such as Potassium Acetate and Potassium Formate. However, these alkali metal products have been associated with damage to carbon brakes through catalytic oxidation of the carbon which can lead to brake failure and corrosion of Cadmium plated airframe components. Impingement of frozen or semi-frozen deposits on carbon brake assemblies is more likely to lead to frozen brake units and progressively affect their function over time due to their greater porosity compared to the older steel brakes. In 2013, EASA issued a second revised version of a Safety Information Bulletin originally issued in 2008 Safety Information Bulletin No: 2008-19R2 and discussing these safety concerns. Another Safety Information Bulletin No: 2018-01 recommends operators of aerodromes, that regularly conduct de/anti-icing operations of the aircraft movement area(s), to publish information on the generic fluids and/or solid materials they are using, in a SNOWTAM, when it is issued, or to insert such information in the remarks column of part AD 2.7 of the AIP.
Interaction Between Runway and Airframe De-icing Products
When runway de-cing products (mainly but not only Potassium Acetate or Potasssium Formate) alone or mixed with water or semi frozen deposits splash onto control surfaces during taxi or take-off run, they may affect the performance of an applied aircraft de-icing product. The effects may include a reduction in Holdover Time (HOT) or, in thickened fluids (Type II and IV), may accelerate the precipitation and build up of thickener residues encouraging greater moisture update on the rehydration of those residues, which may cause handling problems in flight. A separate challenge associated with the use of runway de icing chemicals is that melting snow and ice results in wet and mixed contamination so that friction may be reduced until the frozen contaminant is fully melted. It is also possible for the water from melted snow and ice to dilute the liquid de-icing agent and enable it to freeze and form invisible ice (’black ice’).