Just one more for FYI - Why air conditioning goes off during engine start on many planes.

[benhasajeep52 50]

This is the reason the air conditioning goes off when they start jet engines during push back. I see all you people who reach up thinking turning the gasper (what the air vent is called) will do something.

Except for the newest planes like the 787 the engines are started with an air driven starter. The 787 is electric. But these starters are mini turbines, so a lot of air is needed to get them going, and then speed up enough for the main engine to compress air enough to put fuel to it and start the ignition process. These starters will be on until the motor reaches 40-45% of engine rpm. From there the motors have enough going they can continue and finish the start process. If they don't it's called a hung start. That can be bad juju if not stopped. Most of the time if they don't get it going its not a hung start but no light off. The fuel didn't catch fire, or did some but not enough.

The blue line points to the flange where a large air duct will connect to supply air to the starter. The yellow line pointing to the turbine blades that catch the air to start the spinning. That starter weight about 15#'s, And can start that big motor. BTW the 777 is one of the few planes that can start both motors at the same time. Normally they don't, but it is easily capable of it.

 

[Hardus Nameous121 69]

I can see diverting all bleed air to the starter but are you saying it's bleed air coming out of the vents in the cabin? Wouldn't that require a gynormous heat exchanger?
I thought the cabin vents had little local recirc fans for every few rows of seats.

 

[benhasajeep52 50]

I can see diverting all bleed air to the starter but are you saying it's bleed air coming out of the vents in the cabin? Wouldn't that require a gynormous heat exchanger?
I thought the cabin vents had little local recirc fans for every few rows of seats.

They do have giant heat exchangers (4 to be exact, 2 on each side) because the a/c is an air cycle machine no chemicals or gasses just air (2 on most planes). But the actual reason for the air conditioning loss is the air conditioning packs are powered by air (except the 787 again, electric). They need a lot of air to work, like the starters there is a turbine to spin up with the air, the other side of the turbine is a compressor, the compressed air gets hot as it's compressed, hot air goes through the mentioned heat exchangers, the air cools some, then goes through expansion where the temp drops alot as the pressure goes much lower, wella you have cold air. Takes a lot of air to power the air cycle machines. The 787 actually has no bleed air system. It's truly an electric plane.

It's why the planes get cooler after engines are running. The APU (auxiliary power units) in the tale can't put out as much air as the engines do while at power. Also the reason the a/c goes off during engine start, apu can't handle the starter and a/c pack at same time. And on some planes you will notice when they go to take off power the a/c stops again. This is because bleed valves close so more air pressure goes through the motors for more power. Some planes they don't some they do, all the older planes closed the bleeds for takeoff. Now the engines are so efficient they can have bleeds on during takeoff, and in flight the bleeds are constantly moving and some air is dumped into the air stream.

 

[AlanKlein86 3]

I once flew with my wife in a seaplane with a Rolls-Royce prop engine from Juneau, the capital of Alaska, to a lodge 50 miles away on a river near a glacier. We went there to eat baked salmon and see some bears as well as the glacier. One of the passengers sat in the co-pilot's seat. The rest of us sat in the passenger section that had one seat on each side of the single aisle. No business or first class.

For fresh air, there was this little straw-shaped metal tube bent into a square letter "S". One part stuck out through the fuselage facing into the wind. The other end was inside the plane aimed at your face so the air would blow on you. You could move the tube so you could aim the air to different parts of your head. Ingenious. No motors, no filters, no nothin".

 

[Hardus Nameous121 69]

They do have giant heat exchangers (4 to be exact, 2 on each side) because the a/c is an air cycle machine no chemicals or gasses just air (2 on most planes). But the actual reason for the air conditioning loss is the air conditioning packs are powered by air (except the 787 again, electric). They need a lot of air to work, like the starters there is a turbine to spin up with the air, the other side of the turbine is a compressor, the compressed air gets hot as it's compressed, hot air goes through the mentioned heat exchangers, the air cools some, then goes through expansion where the temp drops alot as the pressure goes much lower, wella you have cold air. Takes a lot of air to power the air cycle machines. The 787 actually has no bleed air system. It's truly an electric plane.

It's why the planes get cooler after engines are running. The APU (auxiliary power units) in the tale can't put out as much air as the engines do while at power. Also the reason the a/c goes off during engine start, apu can't handle the starter and a/c pack at same time. And on some planes you will notice when they go to take off power the a/c stops again. This is because bleed valves close so more air pressure goes through the motors for more power. Some planes they don't some they do, all the older planes closed the bleeds for takeoff. Now the engines are so efficient they can have bleeds on during takeoff, and in flight the bleeds are constantly moving and some air is dumped into the air stream.

I had no idea the AC had its own turbine powered compressor; I'll have to look this up sometime, thanks.