Freighter M/D Flow

[Spiderschwein]

The FCOM states that all packs should configure to high flow when setting the M/D Flow selector to HIGH.

Also they should configure to high flow when switching off the recirc fans.

I detected that there is no difference in air flow noise or duct pressure when i switch the M/D Flow between NORM and HIGH.

Also there is no change in duct pressure when i switch off the recirc fans, only the air flow noise reduces.

Is this the correct behaviour?

[scandinavian13]

 

 

Also there is no change in duct pressure when i switch off the recirc fans, only the air flow noise reduces.

 

While I know this is not a 737, I couldn't find information specific to the 777 and how the system is laid out, but on the 737, the duct pressure is determined prior to where the recirc air enters the system.  As such, I don't think there'd be a changed in indicated pressure.  The indicated pressure is that between the engine bleed and pack.  Recirc fans are post-pack implements.

 

Not sure about the other issue, though.

[Spiderschwein]

I know that the recirc fans themself have nothing to do with the duct press, but a duct pressure change (and a change in pack noise) should be created by the pack flow switching between high/norm i think.

[scandinavian13]

I know that the recirc fans themself have nothing to do with the duct press, but a duct pressure change (and a change in pack noise) should be created by the pack flow switching between high/norm i think.

 

...then why did you say "[a]lso there is no change in duct pressure when i switch off the recirc fans?"

 

If your only concern was the other issue - high/norm - then that's all that should've been stated.

[Fabo]

Kyle,

 

 

...then why did you say "[a]lso there is no change in duct pressure when i switch off the recirc fans?"

 

I suppose because of

 

 

 

Also they should configure to high flow when switching off the recirc fans.

[scandinavian13]

Kyle,

 

 

 

I suppose because of

 

Aaaahhh...good point.  For the life of me, I couldn't dissect all of that.  My inner grammarian gets in the way when different paragraphs get involved.  It all looked like separate thoughts to me.

[mfrantz]

The FCOM states that all packs should configure to high flow when setting the M/D Flow selector to HIGH.

Also they should configure to high flow when switching off the recirc fans.

I detected that there is no difference in air flow noise or duct pressure when i switch the M/D Flow between NORM and HIGH.

Also there is no change in duct pressure when i switch off the recirc fans, only the air flow noise reduces.

Is this the correct behaviour?

 

Yes, the packs do configure for high airflow when the M/D switch is set. Unless if they air already operating in high airflow, which is the normal mode of operation in most cases.

 

There are 4 pack flow schedules and the Air Supply Controller selects the appropriate one for the conditions. Schedule 1 gives the greatest airflow and is the usual flow schedule. Schedules 2, 3 and 4 give less air to the packs and are selected under some specific conditions (like during take-off, single engine operation, wing anti-ice on, cargo fire switches armed, available bleed air less than usual, more then the normal number of bleed air users systems etc,)

 

Setting the M/D Flow selector will override the selected schedule and set schedule 1.

 

Duct pressure has nothing to do with packs airflow. Duct pressure is regulated to a fixed target by bleed valves regardless of selected pack flow schedule.

 

Packs schedule selection and actual airflow is continuously calculated by our simulation of the pneumatic system and the airflow volume level is accordingly adjusted.

However, it may be very hard to hear the differences because the changes are usually small (for example schedule 2 flow is 15% less than schedule 1 flow) and are masked by the high level of the recirculation and the equipment cooling fans volume.

 

And of course, if packs are already operating at schedule 1 (which happens most of the time) there will be no difference at all.

[Spiderschwein]

Hello Michael,

thanks for the detailed explanation of the logics that are behind the system.

Very interesting to see how incredibly deep and detailed the systems are modeled.

I did some further experiments and saw that the M/D Flow switch influences pack flow only under some certain conditions, as you explained.