OT -pilots - plane question

Probably just turned off while idling to save power and fuel.

Planes on the ground have to either have the engine running, the APU running or be plugged into ground power and air.
Some APU's can't handle the load for an A/C so have to have a ground A/C unit plugged in. There were probably no ground units available and you can't have the engines running while boarding but the engines would be running the A/C system while in flight.

Should have been a towed power unit providing AC on the ramp.

At altitude, the air is recirculated just like at home. It is probably heated at that point but controlled in the cockpit along with pressure.
I am not a pilot but I was trained on commercial jet's APU way back when I was on a ground crew.

Air is bled off the engines to pressurize the cabin and provide heating and cooling it's called bleed air. The cabin crew selects the temperature. Cabin outflow valves allow for the pressurized air two exit the cabin so it doesn't blow up like a balloon that you put too much air into. These outflow valves are controlled by the pressurization controller in the cockpit which allows the captain to select what altitude your cabin will be at.

At altitude, farts are converted to A/C.

Genius!

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When we are at altitude does it need AC

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When the engines are running they "bleed" very hot air off the compressor sections for pressurization and environmental air. It's put through an air cycle machine that rapidly expands the air a few times(makes it colder), adds in a little moisture and mixes some hot air back in based on what the controller asks for.

It's a constant inflow of air, and there are usually 2 or 3 outflow valves that meter the outflow to control pressure.

None of the airliners you ride on have AC like freon air, some smaller private jets do.

An exception to all this is the 787 that runs all of this, including brakes and hydraulics electrically, no bleed systems, no Hydraulic accessories dragging on the motors. Just electric stuff. Pretty amazing airplane

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Auxiliary Power Unit or APU, which is a small jet engine used for electrics and pneumatic functions on the ground and in some cases, electrics airborne, was most likely “deferred” or not working properly.

Most aircraft use the bleed air from the APU to start the remaining engines or if the APU is “deferred” the crew will use an air cart to start one engine and then crossbleed air to start the other engine(s).

Once the engines are started, the APU is generally shutdown and the engines provide electrics and pneumatics for the aircraft.

Pressurization and air conditioning while airborne is done by pulling air out of the engine at one of the various stages(with each engine model, it happens at different stages). The air is used to pressurize, control cabin air temp, and provide anti-ice, if needed

The hot air is routed through a “pack” that mixes cold outside air and hot air from the engine to meet the desired temperature for the cabin. It goes thru a filtration system and then to the front of the cabin and out the back thru an “outflow valve”.

To my knowledge, cabin air is not recirculated on many, if any, aircraft.