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Running the packs and ground aircon at the same time

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Moderator
Registered: May 2009
Posts: 5140
John H Watson wrote
Britjet wrote
The main problem was that often the ground aircon wasn't as good as having the packs on, but there was no-one to remove the ground aircon in the early stages of a typical turnround , so we ran the packs anyway.

The engineers weren't best pleased! So then switched to a compromise system whereby the packs could be operated for a max of 30 mins with aircon air attached.


I can't even believe you got 30 minutes. There were two problems at our port. One was that the aircraft packs were sometimes damaging the ground air equipment with the back pressure (The airport authority was complaining about this so Engineering had to heed their request not to run the packs at the same time). Secondly, the check (non-return) valve didn't seem to be robust enough and the flapper would break off and in some cases disappear into the aircraft ducting). I think in some cases, the ground air and pack air would have similar output pressure and the flapper would oscillate violently leading to premature failure. When the bean counters and greenies told Engineering to start using ground air whenever they could, they soon ran out of spare check (non-return) valves. They had to fabricate parts in the Sheet Metal shops. In some cases, this caused departure delays.

If a broken check valve is not discovered prior to departure, you not only lose the use of that pack, you run a risk of pressuring area between the aircraft skin and the wing to body fairing panels (which will blow out the panels). Fortunately, check valves further down the line prevent you from losing cabin pressure.

Rgds
JHW.

Hi,

thinking about adding this to PSX ...

Will it be realistic -- and educational --, or rather annoying and users will complain? :-)

For example: When running the ground aircon together with more than 1 pack for over 20 minutes, a random generator will be executed every 3 seconds. Every time it is executed, the chance of getting a non-resettable pack fault will be 1:50 when 3 packs are runing, and 1:300 when 2 packs are running (and no chance when just 1 pack is running).

So when the packs and ground aircon have been running for 30 minutes, the random generator will have been executed 200 times (starting at the 20th minute, executed 20 times a minute), i.e. at the 30th minute the risk of getting a fault will be 1:4 (1:50 / 200). The risk will rise more and more.


Cheers,

|-|ardy
« Last edit by Hardy Heinlin on Sun, 05 Jul 2015 03:15:57 +0000. »
Member
Registered: May 2009
Posts: 958
Location: Chicago
I think it's a great idea. Maybe the risk could be cumulative between turns, too?
_______________
Will /Chicago /USA
Moderator
Registered: May 2009
Posts: 5140
I wouldn't store this specific risk accumulation in the situ file; the user cannot know for how long this stuff has been running in previous flights. So I would reset the timer to zero when a situ is loaded.

In similar cases, like nacelle icing and the respective risk of engine failure on takeoff, the user can see the OAT and precipitation, and thus is able to recognize the risk.


Cheers,

|-|ardy
Member
Registered: May 2010
Posts: 99
Location: Brussels
Would be great to add this IMO!
S.
Member
Registered: May 2010
Posts: 843
I think it would be too complex to model realistically. The external cooling air is only hooked up to the 1 & 3 pack outlets, so it would only affect two pack systems.

What would be the symptoms of this random pack fault? Running both Ext and packs together either results in the Ext Air supply failing*or the mechanical flapper in the check valve failing (in somewhat rarer cases). In these somewhat rarer cases, the total pack output would be very much reduced (cabin temperature and pressure change rate reduced). Depending on which packs were running, there might be no temperature or pressure rate change (e.g if pack 2 was not selected on) i.e. after the flapper failure. There should be no messages or fault lights illuminated in any situation, since the pack itself shouldn't fail.

In theory, the flapper should let the stronger source win. Perhaps the most damage is not done when they are competing (when the pressure is roughly equal during pack spool-up), but when one source is turned off causing the flapper to shut quickly leading to premature wear/tear.

These effects may be accentuated by sticky valves.

The external air, in theory, has the ability to increase cabin pressure if the outflow valves and cabin doors were closed (I mean with the packs off), but Ext air is not as powerful as the packs, so the Ext Air will probably trip off at some point.

Failure rates may also vary depending on which is turned on first (Pack or Ext).

(EDIT) * external air failure may affect both air hoses, irrespective of the pack/ext air combo in use.
« Last edit by John H Watson on Mon, 06 Jul 2015 02:22:13 +0000. »
Moderator
Registered: May 2009
Posts: 5140
Considering the educational aspect only ...

An educational "What am I doing wrong" effect regarding "Thou shalt not run the packs and the external aircon simultaneously" ...

How about failing pack 1 or 3, or disconnecting the external aircon? (At random after a random time.)
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
I suppose, realistically, that the flight deck effect would be about zero.
Yes, there is a potential for damage, which maintenance would detect and fix at an early stage, hopefully, but in all the time that my airline did this I never heard of a reactive failure as far as flight crew ops were concerned.

Misuse of any piece of equipment is eventually going to lead to failure, for example extending flaps at very high speeds, etc etc...

Peter
Member
Registered: May 2010
Posts: 843
Quote
Yes, there is a potential for damage, which maintenance would detect and fix at an early stage,


Unfortunately, the desire to save fuel has some airlines disconnecting the ext air/ext power at the last moment (prior to departure). i.e. delaying APU start as long as possible. A broken flapper valve can be easily overlooked in the rush (especially when Engineering is not doing the disconnecting... as seems to be the current trend). The potential for problems in flight increases. If panels blow off in flight, you can end up with airframe damage and, perhaps, damage to things (and people) on the ground.

My old airline now turns on the three packs after takeoff which might further delay the detection of problems.

Quote
How about failing pack 1 or 3, or disconnecting the external aircon? (At random after a random time.)


I'm for disconnecting the external aircon... as long as it's not interpreted by the user as some kind of automated switchover from ext to aircraft aircon :P
Member
Registered: Dec 2009
Posts: 334
Location: KTPA
FWIW, I'm a bit wary of interjecting this sort of thing simulator wide. I am very glad to read it will reset each flight... could it be added as an user selectable option instead, perhaps in Failures?

Best- C
Moderator
Registered: May 2009
Posts: 5140
OK, I drop the idea of adding any failure.

Thanks for sharing your thoughts.


|-|ardy
Member
Registered: May 2009
Posts: 958
Location: Chicago
The reason that the idea appealed to me, especially with risk accumulation, was the idea that the aircraft would age into a more precarious state the longer it is used before a full inspection. Fluid levels would go down, rate of oil consumption might increase, drag might increase, and in general the aircraft would slowly age. And then all of that could be restored to factory settings by a "heavy checks" item on the Maintenance page or else reverting to a situ derived from a factory new aircraft. Like in the days of Hoppie's Flight Line, when the aircraft slowly gathered a lived-in feel over time.

But there's not much educational value in that, so I see the other side too.
_______________
Will /Chicago /USA

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