The effect of frozen contamination on aircraft take-off performance is unpredictable. Allowable contamination is therefore limited by Airbus to the following cases:
[ fcom pro.nor.sup.advwxr ]
Airframe contamination is primarily removed using de-icing fluid.1 The main exception is the areas forward of the cockpit windows which should be de-iced mechanically.2 Note also that Airbus recommends removing contamination from the windshield and upper cockpit fuselage before turning on the window heat so as to avoid contamination of critical areas by re-frozen run off.
The airframe de-icing procedure is available in the supplementary information section of the QRH. This procedure prevents the ingress of de-icing fluid through the use of the ditching button. As this closes the outflow valve, all air sources, including ground air sources, must be turned off or disconnected during de-icing. It is permissible to de-ice the aircraft with the engines and/or APU running. With the engines running it is particularly important to maintain communications with the ground personnel so that they can be co-ordinated should an evacuation become necessary. If the APU is running and the fuselage has been sprayed, the APU bleed should remain off for approximately 5 minutes after de-icing to prevent the ingestion of de-icing fluid into the air conditioning system.
Once contamination has been removed, the airframe must remain uncontaminated until airborne. If conditions are such that re-contamination may occur, viscous anti-icing fluids that remain attached to the aircraft are used to provide protection until they are sheared off by high speed airflow during the take-off roll.
Anti-icing fluids have the potential to fail, either by freezing or losing their viscosity and flowing off the aircraft. A fluid’s freezing point and viscosity is determined by its chemical makeup and its dilution, dilution being a function of initial dilution3 and the amount of water absorbed in the process of protecting the aircraft. “Hold over time” tables are provided to allow estimation of the amount of time available before fluid failure occurs for a given combination of fluid type, temperature, intial dilution and precipitation type. The time is given as a range, the shorter time corresponding to “medium“ precipitation and the longer time corresponding to “light” precipitation. It is possible that the wing may be colder than its surroundings due to cold soaked fuel contained within. Therefore if the fuel temperature is below the ambient temperature, the fuel temperature should be used in hold over time calculations.
As anti-ice fluids are designed to shear off the aircraft as airspeed increases they are also susceptible to failure due to high winds and jet blast.
De-icing may be combined with anti-icing or carried out as distinct steps. In a two step process, hold over time begins at the commencement of the anti-icing step.
When taxiing over contaminated areas there is a risk that slush will contaminate the flap mechanisms. For this reason the flaps are kept retracted under these circumstances and will usually also be retracted during de-icing. Extending the flaps will therefore expose unprotected areas to precipitation and should therefore be left until just before takeoff in these conditions.
[ fcom pro-nor-sup-advwxr, eoma 8.2.4 ]
The engines must not be started until all contamination has been removed. Removal of this contamination is an engineering function, usually involving the use of hot air blowers.
When operating for extended periods in severe ground icing conditions (OAT≤3°C with visible moisture or ground contamination) it is possible that the fan blades will become contaminated by ice. Airbus provides a procedure for shedding this ice.
For CFM56 engines, every 30 minutes accelerate the engines to 70%N1 for 30 seconds, and do another such run up immediately before takeoff. In freezing rain, freezing drizzle, freezing fog or heavy snow, an additional run up to 70% with no dwell time should be made every 10 minutes.
For LEAP-1A engines, every 60 minutes accelerate the engines to 50%N1 for 5 seconds, and, again, do another such run up immediately before takeoff. Also apply this procedure if significant engine vibrations occur in icing conditions on the ground. If icing conditions are encountered for >120 minutes, an engineering inspection is required.
[ eomb 2.3.9 ]
Runway contamination affects aircraft operations in three ways:
Each contaminant type results in a different mix of these factors.
Where less than 25% of the runway is contaminated and/or the contaminant is water, slush or snow with a depth of 3mm or less the effects are negligible enough to simply consider the runway wet. This may, however, not be appropriate if contamination is localised to critical areas of the runway.
Where the contamination exceeds 12.7mm of water or slush, 25.4mm of wet snow or 100mm of dry snow, take-off is prohibited. Both takeoff and landing are prohibited if there is a layer of contaminant on top of a layer of ice or compacted snow since no performance data is available for combinations of contaminants. In addition EOMB 2.1 prohibits takeoff or landing on wet ice.
Between these extremes, take-off and landing are permitted so long as the effects of the contamination are mitigated:
In addition, snow clearing operations may have resulted in the build up of snow banks in the proximity of the runway. A diagram in EOMB 4.6.8 defines the maximum snow bank height against distance from the runway that is permitted for take-off.
The captain must be PF for all contaminated runway operations, and all contaminated take-offs should use TOGA thrust.
[ eomb 4.6, eomb 4.13, eomb 2.1, qrh per.a ]
1“Forced air” de-icing has recently been introduced at certain stations. This may be used in addition to or instead of de-icing fluid under certain circumstances.
2De-icing fluid remaining on these areas will run back over the windows and obscure vision during the take-off roll.
3Fluids may be applied pre-diluted with water to save expense.