APU LOW FREQ AND SURGE VALVE

[lookilook]

Hi Guys,

737-400 APU GTCP85-129H....starts normal but frequency is constant 385hz. What is causing this?
we also want to verify that the surge vave and FCU work correctly on ground, anyone knows the procedure to do this?
Thanks.

[Jeddy22]

Is the Freq @ 385Hz after loading the APU Gen ? I believe 385 is the low end where the APU Gen will trip. My suggestion would be to check Both LP/HP Fuel Filters & then adjust the Governor on the FCU to bring your Speed up & to attain 405Hz. Load up the APU to get at least 80-100 Amps, while monitoring EGT….between 620-640 Deg C. To Chk the SBV oper on ground, quickly pull the Indicator Light CB (Acft goes to Air Mode) on the P6 Panel Behind the F/O Seat & have someone hear out the APU increase Speed (Noise Level Incr) while you Montor the APU EGT which should show a small rise.....that indicates the SBV has opened on Grd.....it is normally closed on the Grd....when the APU Bleed Valve is Open. Hope this helps....Jeddy22

[lookilook]

Hi Jeddy22 , thanks for your reply.

We performed TASK 49-52-41-705-001-D00 and pulled out the LDG light c/b but we cannot hear and verify what the test says so i assume Surge valve might not be perating correctly. At the moment we are at a stage where the APU starts normal and freq is 402 steady and still steady when galley load ,hydrau pump and fuel pump is on.
When we put bleed on then it drops to 390 and when turn the engine for start up it drops to 382hz and bleed pr = 22 psi but engine starts.
I could not find a verification test of the FCU itself neither how to adjust the governor as said .

[Jeddy22]

Hello Lookilook.......this is in response to very original query.........SBV working & FCU question.....
Why couldn't you find out if the APU SBV operated when CB Ldg GR Ind Lts was pulled to get Acft into Airmode....Make sure the APU Bleed Air S/w is in the OFF Posn i.e. LCV Closed/No Duct Px indicated before this Test...else get someone get to the E11 Shelf & hit the Grd Sense, if pulling the CB in the Cockpit does not help.The APU will get "noisier" more than usual & the EGT should rise a tad indicating the SBV opened on Grd. The SBV also dumps on the RHS of the APU (viewed thru the APU Door Cut-out at the bottom). The Gov Adj on the APU FCU is on the top of its body while the bottom is for Accel Limiter valve Cracking Px adj. APU Freq is low, you want to adjust the Gov Adj CW to increase your Freq between 405-410Hz, with NO load on the APU.......i.e. APU Bleed Air OFF & Gen OFF LIne. Ref to T/C49-11-00-705-035-D00 (MM49-11-00, Pgs 521, 524, 527).......Good Luck !
Rgds
Jeddy22.

[lookilook]

Hi Jeddy22,

Thanks for the info, for the surge valve the test we carried out and heard nothing neither the EGT changed as per MM 49-52-41-705-001-D00. even when we put bleed air as per test procedure we did not hear any change in sound or indications in the cockpit. I will have someone to go to to the E11 Shelf & hit the Grd Sense.
I will have a look also on the FCU for the adjustment.
Thanks again.

[Jeddy22]

Lookilook:
Is this a Hi-Time, tired APU ?
Sometimes, it’s better & economical to R/R APU (if Available) than throwing Components (Shot-gunning) it without achieving positive Results.
Anyway, Suggest carry out (1) Wiring Chks at 3 Way-Solenoid Vlv Connx D1574. Measure Resistance @ Connx, from Pin A to B. You should see anywhere from 35-110 Ohms, if not R/R the 3 way Solenoid Vlv.
(2) Also Chk Pin B to Grd. Repair CCT if not there.
(3)With 3 Way Sol Vlv Connx D1574 disconnected, Start & run APU. When APU on-Speed, (after 95%+) Chk for 28V DC @ Pin A. If not present, Chk wiring from Connx D1574 Pin A to APU Connx D452 Pin S….Note: this Connx is on the APU Shroud.
(4)It doesn’t hurt to Chk FCU Cracking Px – around 60+/-5 psi. Adjust if low.
(5)Check & R/R Proportional Control Valve (if Available). Watch APU EGT indication & Duct Px Flux & see if APU is “Load Topping”.
(6)Also doesn't hurt to going splits…….i.e. Use APU Gen on Line, with Bleed Air OFF, Use a Reliable External Air Start unit (hook up @ RH AIrcon Bay Fwd), Bleed Air ISO Vlv Closed & start #2 Eng. If normal Start, with no APU Gen drop-off, eliminate APU Gen/Electrical Problems……Hope you try that out with a good Electr fella with ya……….Good Luck.
Jeddy22

[chal45]

if pulling the CB in the Cockpit does not help.The APU will get "noisier" more than usual & the EGT should rise a tad indicating the SBV opened on Grd. The SBV also dumps on the RHS of the APU (viewed thru the APU Door Cut-out at the bottom). The Gov Adj on the APU FCU is on the top of its body while the bottom is for Accel Limiter valve Cracking Px adj. APU Freq is low, you want to adjust the Gov Adj CW to increase your Freq between 405-410Hz, with NO load on the APU.......i.e. APU Bleed Air OFF & Gen OFF LIne. Ref to T/C49-11-00-705-035-D00 (MM49-11-00, Pgs 521, 524, 527).......Good Luck !

[lookilook]

Guys,

Thank you very much for the input on the subject.
However i left this company only after 2 weeks with them as i could not agree with their procedures and way of acting on the aircraft.
I tried to indicate a proper way of doing troubleshooting but i was banging on walls.
A month after i left the local caa revoked their aoc and after a while they shutdown.

Thank you