In aviation, ground surfaces may be covered by meteorological contaminants resulting from precipitation.
This article will provide various definitions of these contaminants, enabling the possibility to sort them depending on their characteristics.
It is particularly critical to identify the correct contaminants when dealing with aircraft performances on contaminated runways as this will directly impact the braking action capability of the aircraft.
See Runway Condition Assessment Matrix - RCAM for more information on the subject.
Water is present as contaminant whenever a runway is stated as:
- flooded (more than 3 millimetres of standing water)
The main risk with water is hydroplaning, especially at high speed and during sharp turns.
In the very specific case of flooded runways, engine flame-out is a possibility and safeguarding the ignition becomes a priority during landing and takeoff.
Rime or Frost is a less-than-one-millimetre-deep thin ice layer.
Landing on such covered runways must be considered like a normal landing on a wet runway as the risks are the same: slippery at high speed or during sharp turns. Therefore, no particular action is required and the runway is not even considered contaminated as per ICAO definitions.
Snow is ice crystals falling as frozen water precipitation.
Depending on the ratio of liquid water within a snow layer due to a slightly positive outside temperature, the snow is classified as:
- dry snow, commonly seen as powdery, when this ratio is low due to snow falls during very cold outside air temperatures
- wet snow, commonly seen as sticky snow, when this ratio is high due to snow falls during outside temperature between 0°C and +2°C
- compacted snow, commonly seen as crusty snow, when a layer of snow has melted internally and frozen again overnight
Landing on snow-contaminated runways requires a very careful handling. It is recommended to perform a firm touchdown to break the contaminant layer enabling then to use the full potential of braking systems.
Asymmetrical thrust and/or crosswind is a huge risk component as directional control can be affected negatively. In some cases, it is recommended not to use thrust reversers at higher thrust than idle.
Slush is melting snow due to positive outside air temperature and results in a mix of water and snow.
Depending on the depth of slush, more than 3 millimeters will drastically affect the braking action as well as the directional control of the aircraft.
Above 3 millimeters, exercise caution as asymmetrical slips could happen. Moreover, even if no solid contaminant layer is to be found, a firm touchdown should be aimed for to ensure rapid braking in a very adverse environment.
Ice is a smooth surface of frozen water.
Given the density of ice, there is very little chance that the touchdown will break through ice.
Exercise the highest awareness when landing on icy runways, as directional control is expected to be dramatically reduced.
Any other contaminants on the runway such as water (wet ice), slush orsnow will render the trajectory of the aircraft completely unpredictable.
Frozen ruts or ridges are snow areas deeper than 60 millimeters but less than 60 centimeters.
Most aircraft are not fitted to operate on runways with snow depth greater than 60 millimeters
Snowbanks are snow areas deeper than 60 centimeters.
No snowbank shall be found on an active runway. Snow banks can be found near taxiways. In that case, the height of the snow banks must be reported as well as the lateral distance to the runway centerline (NOTAMs, SNOWTAMS, ATIS...).
Sand is the result of rock erosion and results in small particles being carried away by wind.
Sand will reduce the braking action capability of the aircraft.
In addition to this aspect, pilots must consider the possible consequences of sand ingestion by engines (abrasion, sand melting into glass, flame-out, deterioration of performance...)
Landslide will result in mud being deposited on the runway surface. Mud is unpredictable in terms of braking action and direction control capabilities depending on its content.
The runway is to be considered not usable.
Chemicals are fluids being deposited on a runway. Depending on the fluid characteristics, they can make the runway slippery.
Directional control and braking action cannot be predicted.
Examples of fluid contamination can include: