B744 Air Systems

[Thunderbug]

Question for jumbo experts......

What difference does it make with APU/Engine air via pack being supplied compared with an external condtioned air source?

Particularly interested in how equipment cooling differs.

Background is that we had a couple of defects - Equip Cooling supply fan u/s and APU u/s. Our paperwork indicted that on the ground with high OAT we had to supply pack air for equipment cooling purposes. With the APU u/s, some people reckoned that an engine would have to be run, others reckoned that ground conditioning would do an adaquate job others

Any thoughts....:confused:

T'bug

[gas path]

What difference does it make with APU/Engine air via pack being supplied compared with an external condtioned air source?
The obvious answer is the APU will supply ample airflow to run all three packs and control (reduce!) the temp. in the cabin with a high OAT. An engine running at idle should be able to run a couple of packs reasonably efficiently, but you've got the added problem of safety on the ramp. The 'ground conditioned' air does seem to struggle especially on a 744.
Interestingly enough the AMM states that even with a fully serviceable equipment cooling system, supplemental cooling (pack or ground cart) should be provided if the OAT is >115f, and with one fan off the requirement drops to an OAT of >85f.
The cabin temperature is significantly higher than the OAT especially with all the IFE/lighting/galleys etc, and as the equipment cooling supply comes from cabin air it is a good idea to get some efficient cabin cooling running.

[Intruder]

Sufficient equipment cooling is provided with even a single pack running under normal conditions, and 2 packs run from the APU can cool the entire airplane on a reasonable day. though all 3 packs can be run from the APU, for some reason our engineers tell us it's not a good idea (there are no restrictions in our FHB or FOM).

There are several different configurations, so whether the cockpit and equipment cooling fans run automatically or are needed in any specific ground configuration varies.

With an u/s APU and equipment cooling fan, you will have to look at your DDPG to determine the requirements for your particular configuration. A single ground-source A/C hose may not provide enough airflow in MANY situations, and 2 hoses may be required on hot days. I've had widely varying luck at different airports with regard to performance of ground A/C units, so beware...

[SeldomFixit]

Intruder - one of the reasons for killing 1 pack on the ground is that during days with high humidity and temps, the fwd cabin tends to rain from the hatrack grilles and if you have too low a cooling demand dialed in, you may even hear what sounds like rats having a dance party behind the left hand side upperdeck stowages as ice pellets make their way through the cockpit delivery ducts. Apart from the above, it needs to be a VERY uncomfortable day before 2 packs on a B744 can't cope so why burn gas for the sake of a 3rd pack ? Have a look at your pack mass flows and you'll probably find that the total of 2 versus 3 isn't that much different. Look after the cents and the dollars look after themselves in this case as you save fuel as a bonus. :)

[spannersatcx]

With 2 packs only running they both go to hi flow, with 3 they don't, 2 packs running on hi flow on the ground is more effecient than running 3 on normal flow. This is on the ground using apu air not in the air.

[virginpaul]

Another reason to run 2 instead of 3 packs on the ground
APU turbine life extended due lower egt with 2 packs/hi flow versus 3 packs normal.
You don't get ought for naught!