Ice and Rain Control, and Cabin Atmosphere

Ice destroys lift and adds weight, and clear ice is more dangerous than frost because it forms where the airflow is fastest. De-icing boots inflate to shed accumulated ice and deflate so the ice can fall away, so their duty cycle is intermittent. Anti-icing is preventive — heated surfaces, chemical or electric systems switched on before entering the conditions — and the two are not interchangeable.

Pitot and static ports are heated to keep their openings clear, and windshields are heated to prevent an optical failure of the laminate; an over-heated windshield delaminates. Rain systems use repellent or a clear-view coating, and each is a consumable that is checked for quantity and effectiveness.

Pressurisation is a closed control loop: a pressure controller commands an outflow valve to hold the scheduled cabin altitude, with an independent safety valve to protect the aircraft if the primary control fails. Pack air is conditioned by pre-coolers and an air cycle machine, and ground bleed or engine bleed supplies it. Any discrepancy in cabin altitude is treated as a grounding item until resolved.

Oxygen is life support and is treated as such. Crew and passenger systems are checked for flow and for the required service pressure, and oxygen fittings, lines and tooling must be scrupulously free of oil and grease because oxygen in contact with hydrocarbons ignites violently. A pressure drop or a leak is a finding, not a nuisance.