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

[744]

Engine Crossbleed Start or Engine Ground Pneumatic Start Duct Pressure Issue only 24psi.

According to RR (RB211), the minimum recommended duct pressure for a crossbleed or ground pneumatic start is 30psi, less 1 psi per 1,000 feet of pressure altitude.

During a Cross Bleedstart,  even closing the opposite isolation valve and increasing N3 to 70% on the operating engine, I can still only get 24 - 25 PSI at best.

EXT AIR for a ground pneumatic start, again 24psi maximum.

Is their a way to increase duct pressure from within the aerowinx programme?

Thanks in advance,

Steve.

[Hardy Heinlin]

I think I can increase the engine crossbleed for engine start, but I'm not sure if should increase the airport's external bleed pressure as well.

As far as I understand the text, 30 psi is just a recommendation, not a requirement; I think it should also start with 24 psi.

PSX is intentionally programmed so that you can start 2 engines at the same time when using APU bleed pressure which goes up to 43 or 44 psi during engine start. With EXT bleed alone it provides pressure for just one engine start, unless the packs or any LE flap drives are running additionally, in which case the pressure will drop below 10 psi and stop the starter motor. I've made the EXT bleed weaker because I've learned that at most airports the external pressure has such low reserves that you can't even drive the LE flaps and the packs at the same time.

So my suggestion for the next update:

Crossbleed: Increase by 5 psi
EXT bleed: No change (keep it weak)
APU bleed: No change

What do you think?


Cheers,

|-|ardy

[744]

Crossbleed: Increase by 5 psi
EXT bleed: No change (keep it weak)
APU bleed: No change

Yes, that sounds perfect! Thats probably the most realistic presentation as the quality of airport ground equipment around the world is variable at best. If Aerowinx could be tweaked to show an increase in duct pressure by closing the isolation valve (opposite side) and increasing the N3 on the operating engine to 70% N3 and achieving 29-30psi that, in my experience would be correct.

Please accept my apologies if I appear to be finding fault, Im not - it's a truly fantastic simulation. I feel quite privileged to be able to access it.

Kind regards

Steve.

[Hardy Heinlin]

Thank you for the tips and the kind words!

The inrease to 70% N3: Do you mean the increase is automatic, or the pilot should advance the throttles out of idle?


Cheers,

|-|ardy

[744]

The Pilot should advance the thrust lever out of Idle. The book says.......
"If duct pressure is low, close opposite isolation valve during start, to reduce duct leakage. Re-open the isolation valve after the engine is running"

Personally I'd only autostart one engine at a time for a crossbleed start. Typical format would be to close the opposite isolation valve, get engine 4 going with a pneumatic start at the gate. Disconnect EXT AIR, and push back. Set the park brake, ask ground to verify the area behind the airplane is clear of equipment and personnel prior to increasing thrust on the operating No 4 engine.

Advance the No 4 Engine to 70% N3, check the duct pressure is @ 30%psi and start Engine No 3. Then Re set the closed isolation valve, and autostart engines 2 & 1 (one at a time). The item that always gets missed is remembering to retard the thrust lever No 4 back to ground idle (Human Factors).

From memory the MEL may suggest starting two engines on stand, not above ground idle but quite often the Tower get a bit sniffy about this and give permission for only one to be started before push back.

As the Aerowinx programming currently stands, closing the isolation valve on the opposite side and advancing the operating engine to 70% N3 - the duct pressure remains at about 24-25% psi.  This should increase to 30psi to meet the recommended duct pressure for a cross-bleed start.

Hope its of help,

ATB

Steve.

[Hardy Heinlin]

OK, thanks.

What if you set N3 higher than 70%? Will the pressure rise above 30 psi?

There must be some conditions that reset the pressure to 24 psi (or at least not higher than 30) when cruise power etc. is set.

I would expect that the engine bleed, for this matter, rises only when the start air valve opens. E.g. the APU bleed pressure rises to 40+ only during engine start.


|-|ardy

[744]

Good question. Well you've got me on that one. For an Engine Ground Pneumatic Start, the FCOM states that the duct pressure should be a minimum of 30 PSI (less 1 psi per 1,000 feet of pressure altitude). The inference being that the ground cart for EXT AIR is capable of delivering more and therefore the system capable of handling it. 

For an Engine Crossbleed Start, FCOM recommends a minimum duct pressure of 30psi, so yes as the N3 increases and corresponding thrust is increased, duct pressure will increase with it. What the typical or maximum operating duct pressure is I'm struggling to find. I'll look into it next week when I'm back at work. However I appreciate you need to know the correct figure before you can update the software. 

ATB

Steve.

[John H Watson]

I would expect that the engine bleed, for this matter, rises only when the start air valve opens. E.g. the APU bleed pressure rises to 40+ only during engine start.

No, the engine will behave normally with normal rpm rises. The engine bleed system (above a certain engine rpm will try to remain constant) by modulating the bleed valves. If you remember, bleed increases with rpms (and would blow up the bleed system if the High Pressure (HP) valve didn't close at higher rpms, allowing only the Intermediate Pressure (IP) Valve to supply the bleed air)

Here is the maintenance test (RB211) for the HP valve (paraphrased). "Switchover" refers to the point where the HP valve closes automatically. Note that it has a great deal of latitude:

With all other engine bleed valves (and APU valve closed) and one pack operating (optional these days), check that duct px is around 30psi (at idle).
Increase thrust and check that (prior to "switchover") duct pressure rises smoothly and stabilises between 32 and 43psi.
After "switchover" check that pressure stabilises at 20 +/- 5 psi (the PRSOV modulates to keep the pressure relatively constant).

From this, you can see that to get the maximum amount of pressure for engine start, you have to crossbleed prior to the (running) engine reaching the "switchover" value. Above this value, not only do you get less pressure, but risks of stalling the engine (if the wind is not right), yawing the aircraft, etc.

rgds
JHW

[John H Watson]

If Aerowinx could be tweaked to show an increase in duct pressure by closing the isolation valve (opposite side)

Do you really see this in reality? I see no reason why it should increase unless you have a maintenance issue.

If duct pressure is low, close the opposite isolation valve during start to reduce duct leakage. Re-open the isolation valve afer the engine is running

Boeing recommendation? I'm not sure what they had in mind when they said this. One would hope there would be no leaks at all. I can't imagine there would be any significant leaks which would affect engine start. Occasionally, engine bleed valves don't quite close properly when the engine is shut down, but bleed switches are usually cycled during preflights if the OFF light doesn't show (to close the valve properly). Then there are the devices which can't be turned off (in the left duct), such as aspiration plumbing for TAT probes (which is no bigger than a finger in diameter). I would expect no packs to be running during an external air start.
On aircraft with no AUX pump on the #1 Hydraulic system, I would assume you would turn off the ADP for a crossbleed start?

Rgds
JHW

[744]

If Aerowinx could be tweaked to show an increase in duct pressure by closing the isolation valve (opposite side)

Do you really see this in reality? I see no reason why it should increase unless you have a maintenance issue.

Yes you do. From memory, - it's been a long time since I last did a crossbleed start, you get another 2-3 psi. If your trying to achieve a minimum of 30psi then that can make all the difference. The procedure is predicated on the first running engine, typically No 4 running at 70% N3 in the manner I described in my previous post.

[Hardy Heinlin]

It might have something to do with the compressibility of the air. The larger the manifold, the greater the air mass that needs to be compressed. This requires more work.

[744]

Hi Hardy.

Just a quick update vis-a-vis APU delivered duct pressures. I've now done two sectors with very differing pre-start "APU Air" delivered duct pressure.

Sector One.
APU Duct Pressure, prior to engine start 14 & 15 psi respectively. Closing the left isolation valve increased the pressure to 24psi. Initiating the autostart procedure on Engines 3 & 4 in normal IV configuration caused the pressure to momentarily increase to 44psi but settle on about 35psi during the start sequence. After start it settled on about 30psi.

Sector 2
APU Duct Pressure, prior to engine start 22 & 23psi. During a two engined autostart this increased to 40psi but again fell back to about 35psi.

Regards

Steve.

[John H Watson]

Sector one sounds like an aberration to me.

A sample of one or two is really not enough to base PSX on. Real world APU bleed values are based on altitude & air temperature, bleed demand, APU age/serviceability, time between configuration changes, etc. There are a lot of variations in the real world.

In theory, the less the bleed demand, the lower the APU bleed pressure production. With no demand at all (APU bleed OFF), the APU produces the lowest bleed air pressure: I don't mean an indicated "0", I mean the APU compressor is adjusted to give the lowest pressure and with the most air being dumped to atmosphere via the APU Diverter Valve. Those people plane-spotting in the terminal area may hear large variations in APU noise when demand is reduced quickly. This will be bleed air being dumped.
When I used to turn on packs for a departure, I would switch the packs on one at a time (with a pause between each pack at least long enough to give the pack enough time to show HI flow on the ECS Synoptic). Note that PSX is a little fast to show this. The pause was to help prevent large pressure changes (best for APU, best for bleed system).

Steve, what is your normal configuration for engine start? All packs off, one pack on (which pack?). ADPs running if no AUX on #1 system? Do you have ADPs on #2 & #3?

Rgds
JHW

[Hardy Heinlin]

Sector one sounds like an aberration to me.

You mean it's unusual that "Closing the left isolation valve increased the pressure ..."?

(I'm referring to the increase in general, not the value in particular.)

It might have something to do with the compressibility of the air. The larger the manifold, the greater the air mass that needs to be compressed. This requires more work.

|-|

[John H Watson]

(I'm referring to the increase in general, not the value in particular.)

Just the value.

I'll have to do some research into the triggers for the APU bleed system load control. e.g. Does the APU controller react to pack switch position or pack valve position. Does the APU controller look at Isolation Valve circuits or isolation valve position... or not at all?  etc.

I think we discussed earlier that the APU controller doesn't react to engine start valve position, but to the start/autostart electrical circuit. Also, we discussed that automatic bleed pressure control may be slow to respond sometimes.. and the pneumatic bleed pressure drops before it increases.

[John H Watson]

For bleed control, my manuals say the APU Controller has the following inputs:

*APU Bleed Air Switch [I couldn't find a direct link between the switch and the APU Controller in the Wiring Schematics: The switch does input to the ASCTU via relays. The ASCTU may talk to the APU Controller (?). Also, the APU controller knows the physical position of the Bleed Air Valve (see notes about packs below)]

*Main Engine Start discrete (Start Switch) [not directly, according to the schematics: Start and Autostart relays are involved]

*ADP Pump discrete (ADP Switch) [On aircraft with 2 ADPs, the APU Controller gets 28volts from an APU Start CB on P83 (via two possible other CBs on P83 and P6) when either ADP is turned to ON or AUTO. I haven't looked at aircraft with 4 ADPs yet]

Environmental Control System discretes (Pack switches) [Not directly, but via relays: Ref Wiring Schematic 21-51-01. The APUC knows if the packs have been selected OFF (or not off). The relays lose power if the packs are not off or if the respective Pack Flow Control CBs on P180 are depowered. These relays also have an input from the Cargo Fire Cutoff relays  Even with the pack switches are not OFF, if these fire relays are activated, the APU Controller thinks the applicable packs have been selected off] (EDIT: This fire relay input may only be on pack #3)
 
In the text, there is no mention of a L/R Isolation Valve input to the APUC, but Wiring Schematic 49-52-01 shows bleed isolation circuits in parallel with the pack switch circuits. The physical positions of the L/R and APU Isolation valves are sent to the APUC controller to negate/override the signal from the pack switch circuits if the pack can't possibly operate with the isolation valves closed (even though the pack has been commanded on by the pack switches). The circuits have been physically simplified:
Pack 1 commanded on and Left Isolation Valve sensed closed = Pack 1 off
Pack 2 commanded on and APU Bleed Isolation Valve closed = Pack 2 off
Pack 3 commanded on and Right  Isolation Valve closed = Pack 3 off

The CMC APU Maintenance Page lists various dispayed bleed modes (not sure about submodes):

MIN (minumum): APU Bleed Valve closed, APU compressor inlet guide vales to minimum and diverter valve to full open.
DPZ (duct pressurisation): APU Bleed valve open, no pneumatic load and diverter valve commanded to close slightly
ADP (air driven hydraulic pump): Bleed valve open and an ADP selected to ON
ECS 1, 2 or 3. Bleed valve open and number of packs selected on.
FLIGHT: Airplane in air with ADP and/or pack selected on.
MES (main engine start) Bleed Valve position open and the airplane main engine to start.

The APUC also receives an ECS demand (0%~100%) from the ASCTU for APU air control.

On the one hand, the large pressure rise seems too much to be explained by compressibility and, on the other hand, unlikely to be caused by the system thinking that something is operating when it isn't (the system sounds quite smart*).

[John H Watson]

P.S. I just had a look at the wiring schematics for an aircraft with 4 ADPs. The APU Controller seems to have no idea how many ADPs are running (other than none).... There could be one or four ADPs running.

[John H Watson]

Random thought....If MES mode only looks at APU bleed valve position and the engine start circuits (as mentioned above), then perhaps the system isn't so smart. If you had the Isolation Valves closed, you would get a huge rise in bleed air pressure during (attempted) start with the bleed air not being used for start. Generally, however, the text in the manuals seems simplified, so maybe isolation valve position is also considered ... and the pressure in the ducts won't rise during (attempted) start.

Perhaps someone could try a start in a big sim with the isolation valves closed. I wouldn't try it in real life. It might put a lot of strain on the ducts. Unfortunately, you never know how accurate even a big sim is with regards to this kind of thing.

[744]

Hello John. The next time I'm in the company Sim, I'll have a look.

ATB

Steve

[John H Watson]

Thanks, Steve.
I'd be curious to know if the big sims even model these (duct volume) subtleties. Unfortunately, no two real aircraft are alike when it comes to bleed pressures, so the sim can only model averages or theoretical behaviour.

Whilst my notes do say that the APU bleed output is adjusted for demand, I don't know if the APU continually adjusts the bleed output to meet that demand. e.g. during engine start, the pressure is ramped up. If you then pulled ALL the start switches together, does the APU realise the pressure in the ducts (or at the compressor outlet) is dropping and try to compensate or does it just keep outputting the expected demand of a single engine start? Is the fact that the duct pressure does drop with multiple engine starts because the APU can't physically keep up with the increased demand or because it is not programmed to do so.
If there was, say, an APU bleed duct failure, would the APU go nuts trying to keep up the pressure ... or just calmly keep outputting "X".