737 Pack switches at the gate ???

[sire]

So you are at the gate, APU is off and there is no conditioned air connected.
You are doing a 40 minute turn and don't plan on using any air until you start the APU about 5 prior to push time.

Had another pilot tell me that he was told to turn the PACK switches to off in that scenario as it would "burn up the fans".
Now, I have always left the packs in AUTO until ready to start engines. Couldn't find anything in our books either as to where the switches were supposed to go. The only reference was to turn them off if conditioned air is connected at the gate.

I haven't talked to our Mx department yet, but as far as I know, the pack switch just moves the pack valve, or lets it.
The pilot insisted that it also turns on the fan that draws air over the packs and he could hear that when doing the walk-around.
I say that fan comes on when the pack is operation only and it does not matter where the Pack switch is at the gate with nothing running?

Any ideas or explanation?

[STBYRUD]

There is no fan that 'draws air over the packs', what you must mean is the fan that is part of the air cycle machine that draws air through the heat exchangers and that is mounted on the same spool as the compressor and turbine. If there is no bleed air then that air cycle machine won't run, and neither will the fan. Your SOPs should normally dictate what you should be doing with the pack switches on shutdown, but usually they stay on. Back in the day it used to be best practice to keep the APU running for a minute without bleed load (either with packs off or APU bleed off), the NGs do this automatically, so there is no reason to keep them off.

[TriStar_drvr]

Without conditioned air or APU air supplying the packs you are doing your passengers a great disservice. It gets very stuffy in a hurry with only the recirc fan providing ventilation.

[ImbracableCrunk]

The Classic 737 have a pneumatically operated turbofan for the heat exchangers. Those won't work without a source either, so no pressure - no burning out.

[yotty]

Is it possible they are referring to the recirc fan(s)?

[fruitloop]

The Classic and the Jurrasic both had turbo fans to draw air over the packs..

[NSEU]

Maybe he's confusing it with the sound of a hydraulic pump or some other device. I have heard a Ram Air Inlet actuator on one aircraft type making a buzzing sound (can't remember the circumstances though).

Of course, it's easy enough to check on your next walkaround. Is there any airflow from the ram air exit vanes with no bleed air (the answer should be no).

Having said that, to me it makes sense to switch something off when you're not using it (unless something is energised by turning off the device, like an "OFF" solenoid that might get burned out). You don't normally turn on all the electrical devices in your house at the same time by turning on the main switch at the fuse box. Similarly, on, say, a 747-400, I would turn on the APU bleed after the APU had warmed up and then turn on the 3 packs individually and monitor the duct pressures whilst doing it.

However, if your SOP's say otherwise...

[Xiamen]

Follow the check list. Packs to off on the secure check list only.

[FlyingStone]

One reason to put packs OFF on Classic is to prevent excessive APU EGT if you would put just the APU bleed without selecting one pack OFF - you can use only one pack with APU on the Classic unless it's very cold (-30°C if I remember correctly).

[Too Few Stripes]

Most airports now prohibit the use of APU on stand for more than a few minutes. Many 'helpful' ground agents will connect conditioned air without consultation, for this reason it would be prudent to select the pack switches to off as per the warning in the supplementary procedures -

Ground Conditioned Air Use

Before connecting ground conditioned air:

PACK switches ............................................................ ................OFF Packs can be damaged if they are operated while ground conditioned air is connected.

After disconnecting ground conditioned air:

PACK switches ............................................................ ...... As needed

[framer]

Most airports now prohibit the use of APU on stand for more than a few minutes.
Is that accurate? I regularly fly to ten or so International Airports and none of them prohibit APU use for any length of time.

[MD80767 Driver]

Framer,

Most European airports have a restriction on APU use. Like, 5 min after docking, 10 min before push.

[Too Few Stripes]

Perhaps I should have been more specific - most of the airports which I operate to/from have this restriction (European holiday destinations). That aside, the point regarding ground conditioned air damaging the packs remains.

[FlyingStone]

Too Few Stripes: Packs can be only damaged if they are operating. They are not operating if the switches are in AUTO mode if you don't have any pneumatic source from engines/APU/HP ground source.

Conditioned air from external air conditioning unit is a LP source and is connected downstream of the packs - into the mix manifold to be exact. Also, there is a check valve located after the pack itself to prevent reverse flow.

[Too Few Stripes]

Flying stone - my text is a copy and paste from Boeing's supp procedures. Boeing are clear, if you are using external conditioned air then the pack switches should be off. As to whether damage would occur, well if the check valve were to fail then yes it could - nonetheless it would be prudent not to second guess Boeing's published procedure regardless of my belief of the chance of damage. The original question was whether pack switches should be off or not during a turnaround - I have merely highlighted a situation which I have regularly experienced where the pack switches should be off.

[Skyjob]

whether pack switches should be off or not during a turnaround

This is airline and type dependent, thus your SOP.
Some operators in transit checks turn them OFF, to enable Ground Air Source (LP) to be connected by default.
Other operators in transit checks turn leave them AUTO, enabling APU Conditioning when APU BLEED is selected ON, yet switching them OFF immediately prior to engine start, not using them prior to such event when OAT is above or below a certain minimum or maximum value (Never understood the reason why SOPs would dictate this method of using them, but anyhow...).

Either case is/can be correct,d spending on your type of operation and associated SOPs.

Furthermore, there is a potential difference between NG's with multiple zones (800/900) and the single zone cabins (600/700), again SOPs define their use.

There are many way str skin a cat.
In this case there is no requirement for either position, merely a defined SOP advising you what to do in your type of operation.
Remember an operator generally operating in moderate altitudes to large well equipped airports with APU restrictions and Ground Conditioned Air Sources will have a different view as another operator flying in areas with mostly isolated stands and higher then average temperatures.

[NSEU]

Boeing are clear, if you are using external conditioned air then the pack switches should be off.

We (Engineering/Maintenance) were told that there was a possibility of damaging the external conditioned air compressors with both running. There is a check valve on the external connection, but it is a simple hinged flap on Boeing aircraft. When we started using more external conditioned air for enviromental reasons, we noticed that more and more of these flaps were getting damaged and even detaching (ending up further down the pack outlet duct)

When you restart your APU, do you normally ask for ground clearance? (I guess not). However, you should ask for ground clearance if you start the packs (if you are not sure if ground air is connected). To me, leaving the pack switches on just seems like another hole in the cheese.

[sire]

Thanks everyone.
I always love to come on here to get different opinions and answers.
And just to clarify, the question was about being at the gate without the external air connected. Our manuals also state that in that case to leave the pack switches in OFF.
The whole thing seems to be "technique" being tought in the training center.
They still fly it like a 200. And we all can thank them for the overhead panel.
New everyone should know where that is......