thanks for the informative input...

Yep, that and the fact that the hoses are usually twisted and kinked therefore only letting about 25% of their potential throughput into the aircraft!

We try to untwist them but the kinks can't be helped if the hose is making that 90* turn upwards towards the jet's belly.

I wonder what the cost difference is between APU and GPU/PCA anyway . Even the 737 Taking 400v at 90 amps can't be cheap on the meter.

 

Eric W

The 777 APU runs at about 200kg's of fuel burn per hour.

 

Thanks for that. Couldn't seem to locate it in my manuals. 

 

Don't suppose you're doing BAW195 tomorrow? A friend is SFO on it  ^_^

Related topic- when accessing external air (on arrival) from the PCA, and before turning off the APU, how is the external air switched into the a/c manifolds?

 

I have noted that while external power is often connected to a forward location on the fuselage or on the forward landing gear strut (if the fuselage is too high to access from the ground), the air connection is often at the wing root area, where the packs are located and close to the engine bleeds.

 

Thanks, Bruce.

Love having to use an air start at CYOW when an APU's inoperative.

... Not so much.

Related topic- when accessing external air (on arrival) from the PCA, and before turning off the APU, how is the external air switched into the a/c manifolds?

 

There is no switching for PCA (low pressure) connections.  It is "selected" simply by connecting it and turning the PCA unit on.

 

If you had high pressure air (from the same cart used for starting the engines if the APU is inop), you can use that and the packs to provide air.  The preferred connection is the low pressure, however, to save wear on the packs.

Thanks Kyle,

 

Is there some sort of one-way valve then that would prevent air escaping when at altitude. Just trying to visualize what this PCA connection is all about.

 

Thanks, Bruce.

Thanks Kyle,

 

Is there some sort of one-way valve then that would prevent air escaping when at altitude. Just trying to visualize what this PCA connection is all about.

 

Thanks, Bruce.

 

Yep there is a one way flapper valve and the connection is merely a hole with the flapper that is surrounded by a metal collar that the hose attaches to. Once its hooked up you simply hit the on button on the jetbridge and no the ramper has no control over the temperature (at least at our station we have no control over it).  

 

 

Is there some sort of one-way valve then that would prevent air escaping when at altitude.

 

Yeah, there must be something in the mix manifold preventing that.  Here's a schematic:

 

From http://www.b737.org.uk/pneumatics.htm - it's a great site for 737 info.

 

Once its hooked up you simply hit the on button on the jetbridge and no the ramper has no control over the temperature (at least at our station we have no control over it).

 

@JFK we have a temperature probe that looks like a little hand mic that is put into the cabin from the Jetway. Most of the time, the CS Agent puts it in a cool place, like right at the doorway where a little breeze could give a false reading, so the air-conditioning pushes in warmer air than it should.

 

Sent from my Sony Xperia TL

 

 

Awesome schematic, Kyle. Still trying to get my head around it

 

Thanks for all the replies, really useful info.

 

Bruce.

@JFK we have a temperature probe that looks like a little hand mic that is put into the cabin from the Jetway. Most of the time, the CS Agent puts it in a cool place, like right at the doorway where a little breeze could give a false reading, so the air-conditioning pushes in warmer air than it should.

 

Sent from my Sony Xperia TL

Some of the agents at ORD tuck the probe into the jumpseat and that helps and on Qatar tonight the GPU mercifully failed which forced them to turn on the APU bringing much needed relief from the humidity.

Generally it is advised that if you have PCA and the APU running, as is often the case, the packs will be selected off to avoid chattering the flapper valves due to dual input.

 

As far as I am aware the APU generally has more oomph than PCA and, if the packs are left on, will override the PCA.

 

Airbusman330, I've seen the dispatcher at ORD huffing onto the probe or cupping it around a cup of tea to try and get the PCA to blow colder!!!!!

 

From the Fcom:

 

Pack Ground Operation

Both air conditioning packs are normally selected to AUTO for ground operations.

Fuel consumption is about the same for single pack and two pack operation, and

single pack operation causes higher flight line noise levels.

A ground source of conditioned air may be used to supply conditioned air directly

to the cabin distribution system, eliminating the need for pack operation.

So, as the PCA supplies the cabin distribution system directly there is no chattering and a NRV protects back pressuring of the PCA system if the packs are left in Auto.

 

A self governing system. :-)