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Ashatsea

APU ON at gate?

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I am at VHHH waiting to fly back to the states and I noticed some of the 777's were plugged into GPU'S and Ground Air but the APU's were on the intake was open on the right side of the tail and you could see exhaust gas coming out of the left side. My question is the position lights were off (cockpit empty) and the plane was empty. Why would the APU be running? I don't have an aviation background and was just curious if anyone knew the reason.

 

P.S. VHHH is a great place to plane spot!

 

Ash Keelson,

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Maybe some of the rampies around here can answer that better than I could. But can be for various reasons - EXT Power cord broken, airline prefer pay for the fuel to run the APU than the cost for GPU..

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Maybe some of the rampies around here can answer that better than I could. But can be for various reasons - EXT Power cord broken, airline prefer pay for the fuel to run the APU than the cost for GPU..

Rgr That! Thanks

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My understanding is that the APU can provide much better a/c than obtained from an a/c on the ground which is piped to the a/c. Did you happen to see a thick yellow tube connected to the a/c somewhere in the vicinity of the wing root? If not that would confirm that they are using the APU for a/c... possibly.

 

Thanks, Bruce.

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I am at VHHH waiting to fly back to the states and I noticed some of the 777's were plugged into GPU'S and Ground Air but the APU's were on the intake was open on the right side of the tail and you could see exhaust gas coming out of the left side. My question is the position lights were off (cockpit empty) and the plane was empty. Why would the APU be running? I don't have an aviation background and was just curious if anyone knew the reason. P.S. VHHH is a great place to plane spot! Ash Keelson,

 

Most ground air conditioning systems are worthless so if its a hot and humid day you are better off leaving the APU running or that 777 will turn into an unbearable steam oven real quick, one time while working on a 777 we had an APU quit on us during our searing summer last year and all the PCA did was move the hot air around. The problems is that while the air comes out real cold from the unit the hoses are poorly insulated so they warm up real quick while traveling to the plane, AA doesn't even bother with GPU here at ORD for the 777 let alone PCA since the APU's are on the whole time. 

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The 777 APU runs at about 200kg's of fuel burn per hour. As stated above the air-conditioning on pier at Hong Kong is pretty poor with the PCA (Pre Conditioned Air) failing to cool the cabin sufficiently prior to boarding.

 

We, as a crew, have a duty of care to our passengers plus we don't wish them to enter a boiling tube, so we quite often either power up the APU to cool the cabin and disconnect the PCA or just leave the APU running and not bother connecting PCA.

 

Both the GPU and PCA are expensive so it very much depends as to how long the down time is for many companies.

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It depends on the airport too.

At BRU, you're not allowed to power up the APU until 15 minutes before Estimated Chocks Off, and untilf 5 minutes after Chocks On.

This is of course on stands having PCA and a GPU.

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Most ground air conditioning systems are worthless so if its a hot and humid day you are better off leaving the APU running or that 777 will turn into an unbearable steam oven real quick

 

Bingo.

 

Looking at the weather there right now, it being 28 degrees at 10PM would indicate that it's rather hot.  PCA is good to keep the cabins regulated within a band of temperatures (depending on the unit, hose, etc), but outside of that range, they're worthless.  The APU must be used.

 

Heck, some of those units were quoted as being good for E170s, but could barely cool a SAAB on a warm day.  When it was really hot, we'd hook one up to the normal receptacle, and also snake one into the door until boarding time (if we had an extra hose available).

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It depends on the airport too.

At BRU, you're not allowed to power up the APU until 15 minutes before Estimated Chocks Off, and untilf 5 minutes after Chocks On.

This is of course on stands having PCA and a GPU.

Very true, this is normally down to noise and environmental issues.

 

 

Something Hong Kong has never been too bothered about as far as the new airport is concerned!!!!

 

:-D

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I surprises me that seeing as Hong Kong is a fairly recent airport and the prevailing environmental conditions that the PCA is not good enough. Surely operators can't be happy about that?

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I'm not sure why but I know from friends that the Jumbo is a nightmare to cool on PCA and, when you add all the gated aircraft together, the PCA system probably simply isn't up to the heat and humidity.

 

Just my thoughts

 

(PCA doesn't work very well anywhere in my opinion)

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It depends on the airport too.

At BRU, you're not allowed to power up the APU until 15 minutes before Estimated Chocks Off, and untilf 5 minutes after Chocks On.

This is of course on stands having PCA and a GPU.

 

That has to suck on a hot and sticky day, I'm sure there are exceptions if the cabin is really hot and the PCA cannot keep up.

 

 

 

I'm not sure why but I know from friends that the Jumbo is a nightmare to cool on PCA and, when you add all the gated aircraft together, the PCA system probably simply isn't up to the heat and humidity.

 

Its the hoses, the air coming out of the actual unit is very cold but it gains a lot of heat while traveling in the hose which is poorly insulated. I think that if the APU is inop you can use the air start cart to feed the AC packs if you are really in a bind.

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Its the hoses, the air coming out of the actual unit is very cold but it gains a lot of heat while traveling in the hose which is poorly insulated. I think that if the APU is inop you can use the air start cart to feed the AC packs if you are really in a bind.

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!

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Thank you Ash for starting a very interesting topic, as the replies from the 'pros' has been quite revealing. Something new learned with this topic.

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That has to suck on a hot and sticky day, I'm sure there are exceptions if the cabin is really hot and the PCA cannot keep up.

 

 

 

 

Its the hoses, the air coming out of the actual unit is very cold but it gains a lot of heat while traveling in the hose which is poorly insulated. I think that if the APU is inop you can use the air start cart to feed the AC packs if you are really in a bind.

 

I had a flight back in August of 2010 at O'Hare where the APU on a CRJ-200 was inoperative. Suffice it to say the cabin was pretty hot until after we got airborne.

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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.

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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

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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  ^_^

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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.

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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.

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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.

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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).  

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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:

schemeaircon300small.gif

 

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

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