HiCould somebody explain me which is the function of the packs and when are used for takeoff and landing?Thanks in advanceAsier

Please use your full name on this forum otherwise you risk having your posts deleted. Packs are short for air conditioning packs. They take hot bleed air from the engines and mix it with cold air from outside to produce conditioned air for the cockpit, cabin and cargo areas. Have a look at the 'Air' synoptic for details. Because it takes air from the engine, it reduces the max available thrust, so needs to be considered when planning takeoffs. It is all in the manuals.

Paul: Does it use heat exchangers? I assumed it used air cycle machines... just wondering.

Paul: Does it use heat exchangers? I assumed it used air cycle machines... just wondering.

It uses heat exchangers.Kevin W.

A few links to peruse:http://en.wikipedia.org/wiki/Environmental_Control_Systemhttp://www.boeing.com/commercial/cabinair/...entcontrol.htmlhttp://en.wikipedia.org/wiki/Air_cycle_machineInteresting stuff.Regards,Pete Smith, UK

Reads like it uses both.. interesting tidbit on the new 787 is that the cabin uses no bleed air. The only bleed taken from the engines is for the inlet nacelle heating. The airpacks have powerful blowers to pressurize the cabin.I've re-read the AIR chapter in the system manual.. it doesn't have a schematic description of the use of either exchangers or air cycle machines.

I've re-read the AIR chapter in the system manual.. it doesn't have a schematic description of the use of either exchangers or air cycle machines.

I took this directly from the AAL AOM. AIR 3. The air conditioning system uses engine compressor bleed air from the main engines during flight and the engines, apu, or ground source during ground ops. The air is routed through a combination pressure regulator and shutoff valve that limits the supply pressure of air. After leaving the regulator the air passes thru a flow control valve. This valve limits the amount of air drawn from the air manifold by each of the three packs. If one pack fails to operate, the other packs will attempt to make up the difference in flow. After leaving the flow control valve, the air enters the pack compressor where it is compressed to a higher temp and pressure. The air then enters the pack heat exchanger. Low temp control is provided by ambient ram air. Ram air inlet and exit doors open and close to optimize cooling and minimizing drag in flight. Operation on the ground requires a fan to force air through the heat exchanger. This fan is powered by pack turbine wheel energy. A fan bypass check valve operates in parallel to the fan and will open to allow more air thru the heat exchanger in flight. The air within the pack is then divided. A portion flows thru the turbine blades and the rest bypasses the turbine depending on temp demand. The amount of air that bypasses the turbine is controlled by the turbine bypass valve. This valve modulates to control the temp of the air out of the pack. The air leaving the pack turbines can reach temps below dewpoint, causing ice to form in the pack. For this reason and anti ice valve and turbine outlet sensor are installed. The sensor will sense flow temp out of the turbine. The anti-ice valve opens to allow hot air from upstream of the compressor to mix with the cooled air to prevent formation of ice. The air then flows through a water seperator to remove any water vapor. The water seperator consists of a coalescer bag that extracts the water as air is forced thru it. The water is collected and sprayed into the face of the heat exchanger resulting in energy gain due to latent heat evaporation. The conditioned air then passes thru a check valve and into the conditioned air manifold. The air is then ducted into the cockpit and cabin compartments.Hope this helpskevin W.