Operational consequences of technical malfunctions in commercial aircraft:

ATA 21 - Air Conditioning

As promised in my first article ?from this series about how technical malfunctions are dealt with in aviation from an operational point of view, I will provide some examples per ATA (sub) system starting with a generic system description and then the? possible operational consequences when they fail.

General system description:

The air conditioning system manages both temperature control and ventilation. In short, we can say that aircraft must be equipped with heaters to prevent a too low cabin temperature when ambient temperature falls, either on the ground or due to the climb to the cruising altitude. Cooling systems are only necessary when the temperature of the ambient air or fuselage is high. This might be the case in tropical weather/climates.

Temperature Control

Like in modern cars, the operating principle of temperature control in large airplanes is based on mixing hot air with cold air. Hot bleed-air (extracted air from the compressor of the jet engine(s) or the auxiliary jet engine in the tail enters a cooling pack, which cools the hot air.

A pack (which stands for "pneumatic air cycle kit”) is a component of an aircraft's environmental control system (ECS) responsible for regulating the temperature and pressure of the air circulated within the cabin. Each pack usually consists of various components such as heat exchangers, compressors, and valves, that work together to provide conditioned air, to maintain a comfortable environment for passengers and crew during flight. A part of the hot bleed-air bypasses the cooling pack. The hot air is then mixed with the cold air to obtain the desired temperature.

Before we take an in-depth look at the temperature control system, we must first look at the cooling pack that supplies the cold air.

The cooling pack basically consists of two heat exchangers and an air-cycle machine. During flight, the heat exchangers are cooled by ram air; on the ground a cooling fan draws air across the heat exchangers. The ram-air inlet duct is controlled by a ram-air valve that ensures the flow is correct at high and low airspeeds. The air cycle machine consists of an air-driven turbine that drives a compressor.

Heat exchangers and air cycle machine.

Hot bleed air first passes through the primary heat exchanger, which removes some of the energy by reducing the temperature of the air. This warm air leaves the primary heat exchanger and enters the compressor of the air cycle machine. The compressor adds energy by increasing both temperature and pressure. High-temperature and high-pressure air now enters the secondary heat exchanger. Like the primary heat exchanger, the secondary heat exchanger also reduces the temperature. Warm, high-pressure air now enters the turbine of the air cycle machine. The turbine removes most of the energy from the air by converting temperature and pressure into motion (to drive the compressor). The resulting cold air flows into the mixing duct.

Water separator

Due to the drop in temperature and pressure in the turbine, the relative humidity of the air rises. When the relative humidity exceeds 100%, water droplets will form. The water separator forces the air to swirl around centrifugally. As a result, the heavy water droplets are separated from the air and dumped overboard.

Ice protection.

If the air leaving the cooling turbine contains supercooled water droplets, the water separator may become blocked by ice. When this occurs, the differential pressure across the water separator increases. This opens an ice protection valve, which bypasses warm air to the water separator to melt the ice.

Operational consequences.

The impact of inoperative A/C components may vary, depending on the aircraft type, the number of packs which are installed and unserviceable. These range from passenger/crew comfort (which can be a no-go decision by itself, for revenue flights), limitation of perishable and/or live cargo carried, to a limitation of the maximum flight level, (e.g., B737 21-01 Air Conditioning Packs 21-01-01 All Passenger Configuration 21-01-01-01 -700/-800/-900) which directly impacts on the required block fuel and therefore also the range and/or maximum payload that can be carried.

Aside from the latter, simplified by a reduction of the maximum amount of payload that can be carried due to the increase in required block fuel, also the aircraft performance itself can also be impacted , as A/C has a very close relationship with engine bleed air as its usual source for pressurized air. E.g. 21-51-01 Air Conditioning Packs B777-200/-300

And inoperative pack on an ETOPS -certified aircraft like the B777-200/300, as in the above example has a direct impact on this capability, making it effectively non-ETOPS which requires that the aircraft must remain within 60 min of a suitable airport to land. On long-haul operations over remote areas this can severely increase the flight time and thus reduce the effective range, due to a less efficient route as explained in the Wikipedia link.

Airconditioning pack malfunctions can go as far as prohibiting the use of the autothrottle in certain conditions, as the target thrust setting of the FMC might be too high, resulting in an engine overboost. E.g. B737 21-02 Pack Airflow / Shutoff Valves 21-02-04 Position Indicator Switch Discrete Signal 21-02-04B Failed Closed.

Note: I would like to thank my co-writer Terry Mitchell for his contribution to the design of the article and for ensuring readability also for people outside the aviation industry.

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