I have the Air Condition on before start up to cool the cabin. Do I need to turn it to 'Fan Only' prior to engine start up or is the planes systems advanced enough to over ride the A/C while the engines are starting up?Reason why I ask is I left the A/C on by accident during start up and noticed that the air cond light went out on its own and turned back on after the engines spooled up and stablized.Any input on what s the normal procedure for this.ThanksNathan Palmer

Edited January 26, 201214 yr by Ident

The vapor cycle A/C is electrical, it's not the system you use in flight which is based on an air cycle machine. Not withstanding, best to remember to turn off the vapor cycle machine before start because it will give you heading errors. I think it's placarded.

Thanks. After reading your post I looked in the AOMA. Vapor-cycle Air Conditioning SystemChapter 3 - LimitationsAir-conditioning, Pressurization and OxygenAircraft Operating ManualAir-conditioning, Pressurization and Oxygen1.The Vapor-Cycle Air Conditioning System may only be used for taxi, takeoff, and initial climb (in this instance, initial climb is defined as 1500’ AGL);2.The Vapor-Cycle Air Conditioning System must not be used at a pressure altitude greater than 17,500 ft.NOTE:On certain aircraft the Vapor-Cycle Air Conditioning System was designed to operate as high as a pressure altitude of 17,500 ft., an amendment bulletin limitation restricts its use to taxi, takeoff, and initial climb. This is because the Vapor-Cycle Air Conditioning System, when in operation, may cause compass deviation errors on the No 1 and No. 2 EHSI.=============================================================================================So let me get this straight. The controls on the right side of the FO for A/C is for ground use only and when on, a green indicator is on the CAP. This needs to be off inflight.My follow up questions are:1) Is the compass deviation errors on the No 1 and No. 2 EHSI modeled in the PMDG? Cause I have recieved in the past problems with my EHSI where they stayed pointed North which would give me a bit of relief to know that its not a problem but a feature I have been seeing.2) What A/C do you use inflight? The controls on the center console?

Yes, the errors are modelled and in flight you use the center console control, which is for the air cycle machine (basically takes hi pressure hot bleed air and cools it by letting it do work and expand in a turbine and that cools the air, basic thermodynamics).