Question About External Air/Power on the Queen

[Airline2Sim 489]

LP air is pretty weak, and as Dan mentioned, it isn't regulated very well. When I worked ramp at IAD, it was all we could do for the SAABs (they didn't have APUs), but they would either be awfully hot or frigid cold, depending on the outside air.

 

HP air is pretty rare. It's mainly used for engine starts where the APU is inop.

 

With that in mind, it's best to get the APU running to get better air on the plane. If you're using the APU, then you might as well switch power over to it. Rampies have been known yank ground power without confirmation from the crew (at some locations more than others). The earlier you get away from the GPU, the better chance you have at avoiding that mess.

 

Our guy from a well know sandy airline related a tale of one their 777s readying for departure at a regional UK airport. The APU hadn't yet been started and the Skipper in a moment of brain fartedness just reached up and selected Primary (they didn't have secondary connected) to avail. Clunk, darkness. Apparently the SOP here is to not panic and select it back on instantly as that upsets all the CBs but quick as a flash he punched back to on. Clunk, myriad of yellow cautions that took an hour to clear. Happiness in a T7 is P-A-D-P-A. T7 Cadets will get that one  

[Qavion 19]

As I was reading more into the Pneumatic/Air Conditioning system, it would appear that in my manual, this particular airline's 747s only had high pressure external air receptacles aft of the Packs that would have to go through the packs in order to provide air conditioning. It makes no mention in both the textual description or the system diagram of low pressure external air connecting directly into the mix manifold.

 

 

Seems unlikely, but perhaps there is an option to have no conditioned air connectors. I'm only aware of an option to have two (on a Freighter) instead of the usual three. Experts?

 

Airlines do come up with many odd rules for APU usage. Whilst I agree that turning on the APU early avoids a lot of hassle in the final fraught stages of departure, the beancounters (company accountants) think they know better.

 

Hopefully they've factored these into the equation:

Sometimes external conditioned air is insufficient and pilots can stop passengers boarding if the aircraft isn't cool enough. Delays incur costs and make pax unhappy (and may switch to another airline)

Sometimes power transfers between ground and APU can cause electrical glitches which may need time to be sorted (delays and angry pax). Pax may stress out when the emergency exit lights come on.

Sometimes engineer or groundstaff time could be better spent fixing last minute defects than stowing electrical and conditioned air stuff (more delays and angry pax) Rushing can cause engineers to make maintenance errors or injure themselves. Costs variable (zero dollars to hundreds of millions).

If the APU doesn't start (delays and angry passengers).

 

The beancounters usually always win.

 

For the technically minded... There is a check valve on each conditioned air connector. You shouldn't have conditioned air on at the same time as the packs as the pressures from each source may vary and cause the check valve to oscillate... leading to premature check valve failure (or you may damage the conditioned air system. Sometimes these check valves can break away from their hinges. If the check valve breaks off completely (and disappears into the duct), the associated pack then discharges its air into the (honeycomb composite) wing to body fairings. At some point the pressurized wing to body fairings may decide to part company with the aircraft. They go off with a boom.   Since my old airline's fuel saving policy came into effect, I had never seen so many problems with this system.

 

Rant off.... :wink:

 

Cheers

John H Watson

[KORDATC 80]

Seems unlikely, but perhaps there is an option to have no conditioned air connectors. I'm only aware of an option to have two (on a Freighter) instead of the usual three. Experts?

 

Excellent post sir. So normally, the 744 has three external air connectors? I'm assuming 1 for low pressure pre-conditioned air and 2 for high pressure bleed air? I've found another manual, this one straight from Boeing instead of United's own training center. This one also only mentions high pressure bleed air on pg. 302 of the attached pdf and nothing about low pressure pre-conditioned air.

 

https://www.google.com/url?sa=t&source=web&rct=j&url=http://landrover.narod.ru/gershon/AIRPLANTANK/747-400_operations_manual.pdf&ved=0ahUKEwjs-vyEyMDRAhWDKyYKHUIHAhcQFggjMAQ&usg=AFQjCNFKpHVAA7BwrnsqWohx15ez4fNSPA&sig2=BO0Y9akj_8y4TIVmA6MFhg

 

This whole time, I've assumed the two air receptacles behind the packs were the only ones and that low and high pressure both connect to them. I've been driving myself nuts looking at the diagram trying to figure out how low pressure pre-conditioned air could go from these two receptacles to the mixing manifold with the packs off(which would close the pack control valve and restrict further flow to the mixing manifold unless it somehow went through the trim air manifold.) But knowing now, that there is a third receptacle for pre-conditioned air that is ahead of the packs and connects directly to the mixing manifold makes a lot more sense. So that must've been an option on earlier passenger 744s. I've seen some United 744s hooked up to external air, so maybe some of their earlier birds, elected for the 2 receptacle option, which would explain why my manual doesn't mention pre-conditioned air. Now that I think about it, didn't the first version of the PMDG 747 only offer pneumatic high pressure air?

[Qavion 19]

So normally, the 744 has three external air connectors?

 

 

My old airline had six.:

 

Three were for hp air (hot, high pressure bleed air). These all feed into the centre bleed manifold. I've seen KLM manuals with only 2 however.  I thought it might be an engine thing (some engines being harder to start than others), but I think all our aircraft had 3 (and we had RB211's and CF6's).

 

Three for cool/cold lower pressure conditioned air which were fed into the aircon manifold downstream of the individual packs. Freighters optionally have two (maybe less, but I don't know why they would). I wouldn't call it super low pressure. You would want to turn off the conditioned air supply before you disconnected the hoses.

 

Here's a pic of the high pressure connectors (bleed air supplied by a "start cart").... all go into the one spot behind some small panels.

 

The conditioned air connection points are in the same general area (forward of these) and are behind large hinged pack access panels (sorry, no pics)

 

Cheers

John H Watson

[KORDATC 80]

My old airline had six.:

 

Three were for hp air (hot, high pressure bleed air). These all feed into the centre bleed manifold. I've seen KLM manuals with only 2 however.  I thought it might be an engine thing (some engines being harder to start than others), but I think all our aircraft had 3 (and we had RB211's and CF6's).

 

Three for cool/cold lower pressure conditioned air which were fed into the aircon manifold downstream of the individual packs. Freighters optionally have two (maybe less, but I don't know why they would). I wouldn't call it super low pressure. You would want to turn off the conditioned air supply before you disconnected the hoses.

 

Here's a pic of the high pressure connectors (bleed air supplied by a "start cart").... all go into the one spot behind some small panels.

 

The conditioned air connection points are in the same general area (forward of these) and are behind large hinged pack access panels (sorry, no pics)

 

Cheers

John H Watson

 

Thanks, this helps a lot.  I know UAL has PW4056s on their 747s.  Maybe the connections are a little different as you had mentioned.  This leads to my eventual question - What configuration will the PMDG 747v3 have, or does this change with airline specific options liveries?

 

Regards,

Rob

[Qavion 19]

I don't think it's going to make much difference to what you would experience in the sim unless PMDG models air mass in the bleed ducts e.g. pressure is not always a good indication of what a system will do (e.g. the AUX hydraulic pumps output around 3000psi... but they are small pumps and would have problems doing the same job as Air-Driven Pumps such as retracting gear). I recall that the AUX pumps even move the flaps more slowly than the ADPs during ground tests. 

 

Assuming a perfect airport, the aircraft should be supplied with constant pressure at all 4~6 connectors. The hp external air all goes into the same part of the centre bleed duct. You can't change where it goes... Same for the cool conditioned air... it goes into the mix manifold.

 

I have a photo of an ECS synoptic showing EXT AIR providing pneumatic pressure to the packs and the  L duct pressure shows only 11psi and the R duct pressure shows 12psi. This is not enough to start an engine, but we could go into a "start mode" on the ground cart...  and hopefully this would provide enough pressure for engine start.

 

The logic behind the message EXT AIR on the synoptic is very complex. There are no microswitches to tell the airplane that the external air hoses are plugged in. The airplane has to see the effect of ext air before it will display EXT AIR.

 

Hope this makes sense.

Cheers

John H Watson