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B757-200 APU MX Question  
User currently offlineUAL Bagsmasher From , joined Dec 1969, posts, RR:
Posted (10 years 9 months 5 days 10 hours ago) and read 8129 times:

I have been helping out a few A&P friends of mine on a 757-200 and they have a problem with the APU that they have been unable to fix. Here is the scenario:

If you turn off both packs simultaneously, the APU does an auto-shutdown. If you turn off either pack individually, the whole plane shudders and the EGT spikes, but the APU keeps running. We get an APU BITE message each time it happens. We did the BITE, and got a "Reverse Flow" fault message. They have changed out the surge valve and the inlet temperature sensor per the FIM. They have watched the surge valve operation while the packs were cycled on and off. It appears to be trying to modulate the way it should be. But the problem continues. They are thinking about the IGV actuator, although the FIM doesn't point to it. In order to use the APU, the APU bleed has to be closed before you can shut the packs off. They are thinking of calling Garrett for advice, as they don't want to keep changing parts out for the sake of changing parts.

Any ideas as to what the problem could be? Thanks.

42 replies: All unread, showing first 25:
User currently offlineA/c train From , joined Dec 1969, posts, RR:
Reply 1, posted (10 years 9 months 5 days 10 hours ago) and read 8095 times:

Surge control valve

User currently offlineA/c train From , joined Dec 1969, posts, RR:
Reply 2, posted (10 years 9 months 5 days 10 hours ago) and read 8093 times:

aah ok, they've already changed that, stupid me for not reading that far.

User currently offlineA/c train From , joined Dec 1969, posts, RR:
Reply 3, posted (10 years 9 months 5 days 10 hours ago) and read 8092 times:

they could try changing the three components of the raft, it looks after bleed control and stops reverse flow and has things like delta P sensors on it, thats the usual suspect with bleed problems on our 75 APU's, it sounded like a surge control valve chnage straight away to me, I did the worse thing by jumping in to quickly there !
Could just change the APU if its on going and send the one coming off to a test rig. APU change in a couple of hours.

User currently offlineAuae From , joined Dec 1969, posts, RR:
Reply 4, posted (10 years 9 months 5 days 10 hours ago) and read 8088 times:

If the APU is not running the pacs, then where is the bleed air supposed to bypass to?? Not my area of knowledge here, but I would check the bleed air bypass.

User currently offlineMD11Engineer From , joined Dec 1969, posts, RR:
Reply 5, posted (10 years 9 months 5 days 9 hours ago) and read 8068 times:

I would start with the raft first. Another thing is, did they check the surge bleed valve filter? There is a little filter attached to the line of the surge bleed valve. We had occasional problems with it becoming clogged. Checking and changing takes about 10 minutes. But the IGV actuator could be at fault as well. Those are the things I would think of, beside the surge bleed valve. Oops, check the small air line to the surge bleed valve for leakage.


User currently offlineUAL Bagsmasher From , joined Dec 1969, posts, RR:
Reply 6, posted (10 years 9 months 5 days 8 hours ago) and read 8026 times:

I failed to mention we also changed that surge valve filter during a recent A check, which didn't help. We also inspected the line to the surge bleed valve, and it too was O.K.

User currently offlineUAL Bagsmasher From , joined Dec 1969, posts, RR:
Reply 7, posted (10 years 9 months 5 days 8 hours ago) and read 8019 times:

What exactly is this "raft" and where is it located. I am not too familiar with the 757 systems yet, as I am just hanging around to soak up a little knowledge Big grin

User currently offlineVC-10 From , joined Dec 1969, posts, RR:
Reply 8, posted (10 years 9 months 5 days 7 hours ago) and read 8013 times:
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I'm not surprised it ASD's when you turn both packs of simultaneously, it's a hell of a sudden flow change and not a very good practice.

What happens when you turn the packs off independantly?

User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 9, posted (10 years 9 months 5 days 7 hours ago) and read 8013 times:

The "raft" of "flow module" or "bleed module" is located at the 6 O'clock position. It consists of 3 components (The only component I remember of hand is the delta P sensor), but as A/c train said; it looks after the bleed system. The parts can be changed individually, but it is easier to replace them as an assembly.

We've had this problem before except we had a fluctuating APU whenever packs were on. But shutting them down usually put the APU into auto shutdown.

FYI, at the APU bleed duct in the left wheel well when turning off the packs. You will probably see the duct flex in the clamps. Pretty interesting to watch.

By the way, check the FIM under APU bleed flux, or what ever that heading is.

[Edited 2004-07-21 18:03:42]

User currently offlineUAL Bagsmasher From , joined Dec 1969, posts, RR:
Reply 10, posted (10 years 9 months 5 days 7 hours ago) and read 8007 times:

When we shut down the packs seperately, there is still a heavy shudder and loud thump and the plane shakes. It doesn't ASD, but something is still rotten in Denmark.

User currently offlineMD11Engineer From , joined Dec 1969, posts, RR:
Reply 11, posted (10 years 9 months 5 days 7 hours ago) and read 8001 times:

Sounds as if the load compressor is striking a wall of air. So either the surge valve doesn´t open to bleed the excess air or the inlet guide vanes didn´t close. Did you guys check the IGV actuastor for movement? As far as I remember you can remove a cover over the actuating rod clevis. It is on the 9 o´clock postion.


User currently offlineDc10hound From , joined Dec 1969, posts, RR:
Reply 12, posted (10 years 9 months 5 days 4 hours ago) and read 7949 times:

What exactly is this "raft" and where is it located.

B757 AMM 49-53-08-4


1. General

A. This procedure contains the removal and the installation tasks for the flow sensing module for the APU surge system. The module contains most of the components that are necessary for the surge bleed system on the APU.

B. Garrett SB GTCP331-49-5840 installs the flow sensing module on the APUs that do not have the module.

C. When you replace the flow sensing module, all of these surge system components are replaced:
(1) The total pressure sensor
(2) The total pressure transducer
(3) The differential pressure transducer
(4) The variable volume chamber
(5) The static pressure sensor.

D. The flow sensing module is on the bottom of the APU between the gearbox and the air inlet plenum. You can get access to the module through the APU access doors.

I hope this may help....

User currently offlineMender From , joined Dec 1969, posts, RR:
Reply 13, posted (10 years 9 months 5 days 2 hours ago) and read 7939 times:

A little bit of info I found out from Garrett. I don't think this is in the AMM or the FIM.

When you change the raft assy you need to do a reset of the APU control unit, the black box, the bite box.

Why? Because it "remembers" the pressures and values sensed by the original raft assembly and will continue to give failures because the "values" are now out of range. If you haven't done this already do a box reset and run the again.

Also have you checked to see if there are any mini flags shown on the APU control unit. Read the bite test procedure in the FIM/AMM to find out about these. (early control units won't show them so check the part number of the box.)

Let us know how you get on

User currently offlineAvioniker From , joined Dec 1969, posts, RR:
Reply 14, posted (10 years 9 months 5 days ago) and read 7919 times:

I would have bet on the filter if you already changed the surge valve. Be careful, I've gotten as many as three bad valves from stock. One other thing to consider, the valve may be damaged by repeated stalls or surges and the filter may get crudded up from the same thing.
If it isn't the flow sensor or the control unit reset and you've changed the filter I'm betting on the inlet temp sensor.
If the sensor (or wiring) is broken it's reporting colder air to the control unit so the load compressor will be providing a little less air than normal. (Does the APU run hotter than normal with packs on?) When you shut it down the system thinks the IGV's are already partly closed so the anticipator circuit isn't "preloaded" to close the IGV's as quickly and fully as it should on a hot day when they're full open.
When you shut down the pack(s) you cause a compressor stall or a series of stalls because the load compressor isn't unloading as quickly as it should.
BTW a broken wire to the sensor or the (g-d help you) ECU will cause this problem big time. (been there, not fun)
Best of luck.
BTW do you have a 131 APU?

User currently offlineMD11Engineer From , joined Dec 1969, posts, RR:
Reply 15, posted (10 years 9 months 5 days ago) and read 7918 times:


Acc. to my paperwork the miniflags only work from -19 onwards and almost all conditions shown by the miniflags (except some rare internal failures of the ECU) would also trigger a normal fault/LRU light on the box.


User currently offlineAvioniker From , joined Dec 1969, posts, RR:
Reply 16, posted (10 years 9 months 5 days ago) and read 7916 times:

The other answer is, of course, replace the thing and let the shop sort it out.
Like the man said, it's sometimes the best two hours you can spend before shift change...

User currently offlineMD11Engineer From , joined Dec 1969, posts, RR:
Reply 17, posted (10 years 9 months 4 days 23 hours ago) and read 7908 times:

Well, I ordered a new B757 APU once and I had to justify it to engineering, those things are not cheap. In my case it was spitting fire and smoke from the tail pipe. After a colleague and myself had it out, we discovered that the whole exhaust cone was gone, and when it went, it tore off the oil lube and scavenge pipes of the aft bearing, essentialy it was just pumping it´s oil into the tail pipe.

I know I would have a hard time to explain to engineering and the beancounters that I need a new APU without being exactly able (preferably with pictures) to tell WHAT is wrong with it.

It is the same when I do a boroscope. If I find something, the first thing engineering and maintenance control demand are pictures. They try to tell me my job, but in all cases I insist on a written engineering order to refer to (I don´t want to be the scape goat if something goes wrong after they told me they wouldn´t give me a new engine, because it looked ok to them).


User currently offlineSkydrolboy From , joined Dec 1969, posts, RR:
Reply 18, posted (10 years 9 months 4 days 22 hours ago) and read 7891 times:

Has any work been done in the aft cargo pit lately??
About 6 months ago we had the HMG installed along with all the associated wiring. After the work was done the APU would run fine until the aft cargo pit was loaded and then the APU would shut itself down. We couldnt figure out what the problem was because by the time the a/c was handed over to us to fix it the pit was empty, so we walked up and down the pit till we found the magic spot where if you stood still the APU would shutdown. It turns out that the wiring for the APU was displaced and being pinched in between the floorboard and beltframe, and would short to ground when weight was on the floorboard.

I know its a shot in the dark for you but it might be worthwhile to check the records if anything has been in the pit before the problem started.

User currently offlineAA777-200 From , joined Dec 1969, posts, RR:
Reply 19, posted (10 years 9 months 4 days 17 hours ago) and read 7848 times:

Seems to me that the Gonculator may be broke. Usually when something breaks on an American Eagle aircraft...we just have to replace the Gonculator....or either get the roll of 500mph tape out. That usually fixes it everytime!  Big grin  Acting devilish

User currently offlineMD11Engineer From , joined Dec 1969, posts, RR:
Reply 20, posted (10 years 9 months 4 days 14 hours ago) and read 7836 times:


What the f*ck.....?  Confused


User currently offlineUAL Bagsmasher From , joined Dec 1969, posts, RR:
Reply 21, posted (10 years 9 months 4 days 10 hours ago) and read 7813 times:

Thanks for the info guys. I'll let you know what happens next week when the aircraft is back in town.

AA777-200- We tried ordering a Goncolator, but it was lost in transit in Toledo. That's the last time we ever use PPS to ship a part Angry

User currently offlineCdfMxTech From , joined Dec 1969, posts, RR:
Reply 22, posted (10 years 9 months 4 days 1 hour ago) and read 7767 times:

Don't like to "desktop T/S" without knowing full history, etc, but I like these APU problems...so I'll chime in.

The 331-200 is notorious for the Reverse Flow shutdowns. The Surge Control valve filter will not fix that particular problem. A clogged SCV filter keep the SCV open all of the time. The filter is downstream of the Compressor Discharge Line into the SCV. This air is the muscle air which controls the normally OPEN valve to the closed position. So it being clogged wouldn't allow the muscle air to close the valve.

The Flow Sensing Modules fail, as do the LCIT sensors. IGV problems (not actuator..see below) maybe. But the Flow Sensing modules should be replaced as a unit (This is direct from Honeywell), and some airlines replace them as seperate components.

I don't know if I'd replace the IGV Actuator for that. If the IGV actuator wasn't in it's correct position, the APU ECU should flag that (Check ECU BITE for miniflags). The IGV actuator reports it's position to the APU ECU. The APU ECU uses this position to "assume" the position of the Inlet Guide Vanes. However if there were damaged or missing IGVs, the ECU would not know it. Thats where the Flow Sensing Module comes in. A properly calibrated FSM should be able to compensate for even that situation.

BTW, if that LCIT sensor was to fail foir shorts or opens, the box should catch it (and flag it) and use a pre-programmed value. Obviously a sensor out-of-calibration could cause it, but they say they have changed it. I'd liek to believe that mayeb the LCIT sensor is doing it's job and find out why the APU is reverse flowing (He says he felt the tell-tale shake).

I believe the new APU ECUs (-20 and up) have been updated not to be so sensitive during these conditions but that Surge Control system is a very sensitive system. Every component has to work in perfect harmony, or else everything can be thrown off.

Good luck, the 2 hardest problems to fix on that APU is Slow Starts and Reverse Flows.

User currently offlineUAL Bagsmasher From , joined Dec 1969, posts, RR:
Reply 23, posted (10 years 9 months 2 hours ago) and read 7695 times:

Was out with the aircraft in question today. We fired up the APU. With both packs on, they would repeatedly cycle themselves on and off every minute or so. The duct pressure would drop to about 10 psi when this was happening. The packs are fine when running off engine bleed air. In the rear of the cabin when this cycling was occuring, you could hear a loud banging along w/ the aircraft shaking. We turned the packs off so they wouldn't break anything else. That's where the APU stands. Some new info though. The APU on the aircrtaft was removed earlier this year for overhaul due to some other problems. When the loaner APU was installed, all the problems with the APU and packs disappeared. Now with the original APU back, these new problems have surfaced.

User currently offlineAvioniker From , joined Dec 1969, posts, RR:
Reply 24, posted (10 years 9 months ago) and read 7680 times:

The duct pressure only dropping 10psi when the packs are cycled is a hint.
Sounds like the Load Compressor IGV's are staying wide open.
Time to wring out the wiring and pay close attention to polarity. Use a long wire not the aircraft as a test lead extender.

25 Buzz : Hi Bagsmasher, Buzz here... UAL / PDXMM. Sounds like that APU is doing a compressor stall... "reverse flow". Maybe it's a high-time APU - lots of tip
26 UAL Bagsmasher : The duct pressure was dropping down TO 10psi, not dropping just 10 psi. Anyway, it's looking like an APU swap is in order.
27 Post contains images A/c train : APU's been inop for a while now, you cant keep paying for those air starts away from base !? ! On the history you've given, the APU should have been
28 A/c train : Still a bit confused with the way this storys been told, as VC-10 says, when or why would you ever shut down both packs at once ? and it then goes on
29 Jjbiv : Congrats on finally getting an A&P job, UAL! joe
30 CdfMxTech : A/C Train You should be able to shut down both packs at the same time. The problems with the B757 APUs has led to the workaround procedure which is to
31 HAWK21M : Is there any Manufacturers instructions regarding the same. regds MEL
32 Boeing767mech : Who's overhauling your APU's, we are having our APU's overhauled by a company in SAT and their work sucks. We installed a overhauled APU in a 75 fire
33 TristarSteve : Reading through this thread it amazes me how much work is carried out on APUs on the line. I worked at GF main base for 11 years in BAH. We had a poli
34 Boeing767mech : The idea from Corparate American is that it is cheaper to troubleshoot and repair it on a 10day placard then ship it out, it take a act of god to get
35 HAWK21M : What was the conclusion reported from the Overhaul shop on Investigation its cause. Do you have a pic of this. regds MEL
36 Boeing767mech : Poor workmanship. The rear bearing seal was install incorrectly, That's what my company gets for outsourcing work that was done at our overhaul base.
37 TristarSteve : No sorry it was 25 years ago. But the APU sat in its engine stand, and we had a small trolley the size of a nitrogen cart. This had a fuel tank and a
38 Post contains images HAWK21M : Any compensation.Also was the same Agency carrying out the Ovrhaul & issued the findings as the one the APU was outsourced to. Looks like some one ha
39 TristarSteve : No MEL, read back to my previous posts. When we had an u/s APU, we changed it and did the fault finding on the stand to keep the aircraft flying with
40 HAWK21M : Ok. Was there AC Grd carts located at your Gulf stations at that time. regds MEL
41 TristarSteve : Yes, but they are never as efficient as an APU when you have 35degC and 90pc humidity., they just can't remove enough moisture. Also our B737 were do
42 HAWK21M : Then that makes sense. 35degC seems cool for the region. regds MEL
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