Hi,Can any expert let me know exactly what the `PACKS` are for on the747. When are they used, when turned on etc.Thanks for any replyJohn

The packs (simplified) are part of the HVAC system of the aircraft, as well as providing controlled airflow for pressurization. For the 74x generally at least 1 is turned on when the crew arrives for the pre-flight, all but one are turned off when starting engines, all are turned on during taxi (unless you need extra duct pressure for anti-ice, but all are turned on after liftoff regardless because they do the pressurization and you need them for climb), and all but 1 are turned off after parking at the destination (so you keep the cabin comfortable for disembarking the passengers).Course, this is highly simplified...Take care!P

Check out wwww.aerowinx.com for a good tutorial on packs.