A little more detail on the Auto Pack Trip system.

The pack fans are big KW draw items. Their start-up draw is a big factor as is the running load. The Auto Pack trip system is designed to reduce the pack fan load and also give the pilots a quick warning of power loss. The pack fans run whenever the flaps are not up. On taxi out for takeoff, the checklist provides a place for arming the Auto Pack Trip system. As the pilot takes the runway and advances the power, as the EPR passes a certain reading (around 1.50) the system arms. If any engine EPR falls below a reading of (I do not remember the exact value, I think it is 1.60) the system goes into action. It sends a signal to trip both packs - this gives a bit more thrust during a critical time. It also kills both pack fans -RIGHT NOW! If you were to just turn off the pack switch, the valve takes many seconds to close. As long as the pack valve is still open, even a little, the fan continues to run and draw current. So this is the really big thing the system does - kill both fans now to avoid an electrical overload. It also turns on the pilots engine fail light and gives them a quick warning of failure.

Now, here is a trap. If the F/E is not paying very close attention, and there is a noise abatement procedure, or the tower or departure control asks for a level off and the pilot pulls the thrust back...you got it - Auto Pack Trip and loss of cabin pressure and a bunch of confusion in the cockpit. Most F/E's kept their finger on the arm switch to avoid just such an incident.

The system could not be used unless all EPR gauges were working.