Eng 1 MES Bleed Duct Press low while ENG 2 ok (APU source)

[maro12]

Good Day All,

737-800

I am facing following situation:

Starting Eng#2 as first (APU Bleed Source, APU Bleed Press without loads around 17psi): -> Engine 2 Start Sw to GND -> observing IGV position on APU input monitoring going to 90deg -> Bleed Duct Press rising to around 35psi and N2 rising, reaching max motoring around 25%N2 quite quickly (around 22 sec) = All Normal

Eng#2 running, Starting Eng#1 (APU Bleed Sourc, Bleed Duct press around 17psi): -> Engine 1 Start Sw to GND -> observing IGV position on APU input monitoring going to 90deg -> Bleed duct press not rising staying around 17psi, N2 rising, struggling to reach max motoring 22% -> LOW DUCT PRESS FOR MES

What could be the cause that when starting Eng#1 with APU Bleed, Duct Pressure is not increasing while IGV pos is open? can air press go out through Bleed Air Regulator if ACAU would not command it close, or through 450F thermostat if faulty? Or APU check valve partially closing?

Anyone experienced similar situation?

Thank You in advance for Your thoughts on the issue.

Best Regards,
Maro12

[Nation]

Hi Maro12
We experienced the same issue.My suggestion is check the following components suspected to have leakage on ENG 2.
1、HIGH STAGE REGULATOR 2、HIGH STAGE VALVE 3、FLEXIBLE LINE P/N:16135-81
Hope useful to you!
Nation

[maro12]

Hello Nation,

Today we started Eng#1 first with Isolation Valve closed, Duct Pressure from APU (Left needle) during MES was satisfactory, reached around 37psi after Start Switch to GND, max motoring over 25%N2 in good time (around 24 sec) - All OK.

That means problem is on Eng#2 side. I will definitely take Your advice, and check items You listed on Eng#2 with next opportunity.

Thank You very much for Your feedback, I appreciate it a lot.

Best Regards,
Maro12

[tweakjr]

So what fixed it?

[maro12]

Hi Tweakjr,

Aircraft had to depart so I did not have time to do any additional inspections/testing. Since that time the aircraft is in operation, probably will be looking on this issue during next longer maintenance input.

Best Regards,
Maro12

[maro12]

Hi All,

I have the aircraft in my hands again for some time. What I could manage to do so far is swap High Stage Regulators, no change, problem still exists.

What we could notice, is that with ENG 2 running, Isol Vlv Open and ENG 2 Bleed Switch in OFF, problem is not there, Engine 1 starts OK.

Now I am thinking that it can not be the leak in ENG 2 High Stage Regulator/High Stage Valve when with ENG 2 PRSOV closed (ENG 2 Bleed Sw Off) problem is not there

Reverse flow protection in High Stage regulator should close High Stage Valve (as APU bleed for MES shall go up to around 37 psi).

But I am also trying to find the answer what is happening with ENG 2 PRSOV when ENG 2 is running, ENG 2 Bleed Sw ON, Isolation Vlv OPEN and we are starting ENG 1 from APU

From what I see on the schematic and from reading, PRSOV on ENG 2 (running), during ENG 1 start from APU with Isol Vlv open, shall also close as downstream sense line will supply APU MES press to chamber B of PRSOV against BAR control pressure which shall be 20-28 psi (according to health check).

Anyone?

Thank You in advance.

Best Regards,
Maro12

[maro12]

Hello All,

Yesterday I leak checked ENG 2 Precooler Downstream sense line (line which actually sends precooler downstream pressure to High Stage Regulator and PRSOV chamber B for reverse flow protection). Part of this line is Flexible Line P/N: 16135-81 mentioned previously by Nation, I found this line leaking a lot.

For what it looks from schematics, if this line is leaking, during ENG 1 start ENG 2 reverse flow pressure coming to High Stage Regulator and PRSOV is bled, and in consequence High Stage Valve nor PRSOV would not close letting ENG 2 Reverse Flow Condition.

I will not be able to do Engine Run for some time, but I hope replacing Flexible Line will fix the problem.

@Nation - Thank You very much for sharing Your experience - spot on! I appreciate it a lot!

Best Regards,
Maro12