Duct Pressure Issue - Engine Crossbleed Start & Engine Ground Pneumatic Start

[John H Watson]

I think it would be difficult to design experiments to figure out how the aircraft is programmed.

This might be an interesting test:

Check bleed duct pressure with no load (APU providing bleed).
Run an ADP. Check the new duct pressure (after the bleed pressure has stabilised).
Close the isolation valve associated with that ADP (leaving the ADP switch in Auto).

In the long term...
Does the duct pressure go up because the APU controller doesn't know the isolation valve has closed and is not designed to keep duct pressures constant?
Does the pressure remain rougly the same because the APU controller tries to keep the duct pressure at a fixed value for one ADP running?
Does the duct pressure go down because the APU controller knows the isolation valve is closed (and knows that the ADP can't run).

I think doing tests with packs would be less conclusive because of the number of variables. The APU Controller knows which packs are switched on and the external wiring circuits show that it has some idea of the position of the bleed isolation valves (in relation to pack ops) and it will compensate for this, but the APU also knows how hard the packs are working and is programmed to respond to this (on a continuous basis). e.g. on a temperate day with the cabin intially hot, the packs will work a lot harder, initially, to achieve the target temperature than they would if the cabin was already at the target temperature. The APU Controller knows what the Pack Controller/s are trying to do. Unless cabin temperatures are completely stable, it would be difficult to ascertain APU Controller behaviour.

[Hardy Heinlin]

Modifications discussed in this thread are now implemented in PSX version 10.0.7-beta13:

http://aerowinx.com/board/index.php?topic=3388.0

Refer to items:
0.7.0061
0.7.0062
0.7.0063


Regards,

|-|ardy

[John H Watson]

0.7.0062. Crossbleed start on ground: Duct pressure is now commanded to rise when start is detected

I don't understand why this should happen. 

With a start valve open, there will be more demand on the bleed system (and other started engines). During main engine operation (non-APU), the PRSOVs control air to the bleed system. Each PRSOV autonomously regulates the bleed flow to the duct based on local engine bleed pressure. The bleed temperature is also monitored locally and the PRSOV will close the valve if the bleed air gets too hot. The only input from the aircraft systems to the PRSOV is for reverse flow (during respective engine start) and full closed signals.

The PRSOV starts to open at 10psi, is fully open at less than 30psi. If engine rpm increases, the pressure increases. The PRSOV will limit the pressure to around 45psi (if the engine produces more than 45psi) by partially reclosing.

I can only see the duct pressure going down (at low rpms) with this kind of bleed control. 

Rgds
JHW

[Hardy Heinlin]

Perhaps the PRSOV is not fully open at 24 psi, and fully opens only when a crossbleed start is detected, which will increase it to 30+ psi.


|-|

[John H Watson]

Has this been seen in real life?

The PRSOV regulator has only springs and pressure bellows to mechanically influence valve position. Surely a regulator of this nature would be designed to try to maintain a fixed pressure differential between pressure upstream of the valve (a fixed value for a given engine rpm) and pressure downstream of the valve (the latter being the only source of data the PRSOV has on system demands).

The PRSOV regulator should behave in a similar manner for all loads (be it packs, ADPs or starter motors) because it has no way of identifying what is generating these loads (Even a duct leak could be seen as a load*). With a single engine running at idle does the duct pressure go up if you start turning on packs? (APU off).

*a starter motor is basically an air leak (it gets very windy in the area of the starter with the cowls open)

[John H Watson]

For the engine/airframe experts...

Here's a schematic of the PRSOV...

http://www.members.iinet.net.au/~b744er@ozemail.com.au/PSX/PRSOV2.GIF

Perhaps someone can explain the finer details of operation. I'm not 100% sure how the valve works. The description of operation in the manuals is not entirely helpful.

The spring in the servo (marked in green) keeps the butterfly valve spring-loaded closed via the linkages. Strangely, upstream pressure from the engine on the left side of the bellows seems to assist in this closure. As far as I can see, the right side of the bellows has the same pressure (or less).... unless you are starting that engine... or pressure from the other engines is greater. On the other hand, the surface area of the diaphragm on the right may provide a greater mechanical advantage ().

Is the red item at the bottom of the pressure regulator a cam which forces the butterfly valve open when the  pressure regulator mechanism pushes down on the cam? I don't understand how the double-diaphragm in the pressure regulator operates. Are the blue and yellow pressures added or subtracted to produce downward motion?

On the right side of the pressure regulator is a regulator spring which seems to resist the motion of the balance lever. If the balance arm goes up more on the right, it will open the bleed poppet a certain amount (bleeding air to atmosphere, reducing the yellow pressure). There seems to be a screw adjustment at the top (allowing for calibration)

The valve position switch is activated with the valve fully closed (This puts on the OFF light in the cockpit engine bleed switch)

Thanks,

Rgds
JHW