Sounds like some control system or regulation valve was not working properly or at least not working to maintain a set pressure. Maybe by design, maybe a fault.Cheers,

Don't really know why this would happen, Andrew, although I know what you mean.. I have noticed it, as a passenger, on other aircraft types.On the NG, there is a pack flow control valve which should regulate the flow of bleed air going into a pack even though the bleed duct pressures may be changing with engine rpms. In normal flow mode, my manuals quote a flow rate of about 55lbs of air per minute. I'm wondering if idling NG engines are unable to supply this 55ppm flow rate to the pack under all ambient temperatures? If it can't then the packs may output less air at idle and more at higher thrust.As I understand it, a Pack increases/decreases cabin air temperature by running its turbine slower/faster. When warm temperatures are required, some of the bleed air is diverted around the turbine, missing the turbine (and slowing it down). This may affect the air flow in the cabin... but I can't see how cabin temperatures would change rapidly by taxying an aircraft faster/slower.A simple question... but there may not be a simple answer to it ;-)Cheers.Ian.