My understanding of the air conditioning system is that very hot air from engines is cooled down by packs, and used as air conditioning through out the aircraft. The trim air system then uses the hot air from engines to carefully inject hot air to meet a desired temp between 18-28 degrees. When selecting AIR on EFIS, I did not really understand how the behaviour related to my understanding. Firstly, while taxiing I noticed the trim air valves were randomly opening and closing (even though set as ON). Why is this (Air system was setup normally, with engines and packs all on)? II thought maybe the trim air valves open and close to do the heat monitoring (stop flow when temp is met, allow flow when too cold). However, I then turned the heating down in the cabin and flight deck, to see if the trim air valves would cut flow, and they didn't (remained green). So then I realised the trim air flow remains on at all time but just changes level depending on on temp request. So, this does not explain the random opening and closing of trim valves during taxi. Note all demand hydraulics was set to auto. Thanks if anyone can clear this confusion up.
Having come from X-Plane's Flight Factor 777, I wanted to get set up with Prepar3d so I can finally check out PMDG. It's clearly even more realistic, because I seem to be less capable of operating it :) Here are some questions I have, even after researching for ages: 1. When loading fuel through FMC GROUND OPERATIONS page, the center fuel tank never loads. All the fuel just goes into the left and right tank. Why is, and how would I load it into all 3 tanks evenly? 2. Does PMDG GROUND OPERATIONS not actually have any visuals (push trucks, fuel trucks, etc), or could I just have a setting set incorrectly? It is weird being pushed back by an invisible push truck. If not, I have seen GSX, which apparently does not work well with PMDG. 3. When being pushed back, I cannot steer it. My joystick controls ailerons and auto-rudder (it does not have a twist axis for rudder). How can my cheap joystick be linked to the nose wheel to steer? Thanks to anyone that can answer any of the above.