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ATA 49 APU
1 - Training guide APU fadec. 737-3/4/500
2 - APU igniter correct installation. 737-6/7/8/900
3 - Auto cooldown-cycle. 737-6/7/8/900
4 - APU borescope, how to rotate it. 737-6/7/8/900
5 - APU 131-9B fault code reference & troubleshooting flow chart. 737-6/7/8/900
6 - APU 85-129 performance problems due to cracked combustor case. 737-3/4/500
7 - Pressurizing the aircraft with an old APU. 737-3/4/500
8 - Never start an APU when a pitot/static system test is going on! 737-6/7/8/900
9 - APU air inlet plenum panel installation guide. 737-3/4/500
10 - Always check the APU exhaust for FOD. 737-ALL
11 - APU oil cooler contamination (Next Gen 131-9B). 737-6/7/8/900
12 - APU fire warning: sneak circuit inside APU ctrl module. 737-3/4/500
13 - APU generator attachment nuts, different types available. 737-3/4/500
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1-Training guide APU -280
fadec. 737-3/4/500
2 - APU
igniter correct installation.
737-6/7/8/900
A 131-9B APU have been sent to the shop for the repair of the threads in the hole in which the igniter plug is installed. The damage was caused by an improper (crooked) installed igniter plug. Repair costs thousands of dollars. Therefore pay attention that the igniter is installed straight into the hole, you will save lots of money.
3 - Auto
cooldown-cycle.
737-6/7/8/900
As you will know the NG APU has a cooldown cycle: After the APU switch is set to the "OFF" position the APU does not stop directly but it continues to keep running without being used. This ensures that there is less internal wear and that the life span is extended. A cooldown-cycle can only be interrupted by switching the main battery - switch to "OFF" while the APU is still running. The number of interrupted cooldown-cycles are saved in the data memory module (DMM) and can be read out on the CDU (maintenance page). It appears that this number is considerably high, resulting in a shortened life span of the APU. Since a complete shopvisit of an APU costs between 100,000 and 150,000 euro, the life span must be stretched as long as possible. Before the battery switch is set to OFF, it is worth the trip to the tail to verify if the APU is still running (and not to look to the position of the APU switch only).
4 - APU
borescope, how to rotate it.
737-6/7/8/900
The borescope inspection on the 131-9B APU is not scheduled in a maintenance task. Still it can be necessary to carry out a borescope inspection. It has occurred that the compressor and turbine were rotated by turning the turbine through the exhaust hole using a wooden stick. This can cause damage to the turbine (one of the most expensive components of the APU), a reason why this is ABSOLUTELY NOT PERMITTED. The correct way to turning the APU compressor and turbine is stated in AMM task 49-21-00-980 801, and is done by using a wrench which is inserted in the gearbox.
6 - APU 85-129
Performance problems due to cracked combustor case.
737-3/4/500
When you do an APU performance check on a 85-129 APU, or whenever inspecting it for leaks when its running, pay special attention to the combustor case. It can happen that this housing is cracked at the welded area. This causes a big air leak and even the flames are visible through the crack! But this same crack is hardly visible when inspecting it when the APU is not running. See picture below of such an event where you can see the flames in the combustion chamber.
7 - Pressurizing
the aircraft with an old APU.
737-3/4/500
We all experience lot of problems pressurising 737 classic aircraft only with apu, specially old aircrafts with ETC. Here is the solution: ETC limits EGT by closing apu bleed valve which in turn is not able to provide enough pressure to pressurise old leaking fuselage. ETC EGT limit margin can be increased by simulating main engine start mode (MES MODE) remove any start valve connector and move and hold start switch to GRD position. NOTE USE MES MODE ONLY FOR LIMITED PERIOD ENOUGH TO HELP AUTO FLOW VALVE TO CLOSE. MONITOR APU EGT CLOSELY NOT TO EXCEED MAX CONT EGT ! DON'T FORGET TO PUT START VALVE CONNECTOR BACK !
8 - Never
start an APU when a pitot/static system test is
going on! 737-6/7/8/900
Hi, I had a problem that the APU on B737-800 was not starting on ground. The APU initially starts and then does a protective shut-down with a fault light and a msg " NO ACCELERATION SHUT DOWN ". At that time Pitot Static system trouble-shooting was also going on. We never correlated these two events and were in a problem till we actually got the solution. When Pitot Static System check is going on and you happen to simulate a higher altitude e.g. 10000 ft the Electronic control unit of APU takes the altitude information from ADIRU and compares this from inlet pressure sensors of APU. Upon mismatch of both these inputs the APU itself does a protective shut-down. As such it is advised that never start APU when a Pitot Static system test is going on.
9 - APU
air inlet plenum panel installation
guide.
737-3/4/500
APU Air Inlet Plenum Panels Installation.
AMM: 49-15-11-404-014-B00 GTCP36-280 ENGINES
AMM: 49-15-11-404-007 for GTCP85-129 SERIES ENGINES
IPC: 49-00-00-13
To install all panels the sequence and the way to do is a bit duchies.
1. Place the aft bulkhead assembly a few inches aft of STA 1121.
2. Place the floor panel assembly.
3. Place the inboard panel
4. Place the fwd bulkhead panel. (IPC detail A, panel FWD of the Elevator cable tube) Check all flanges to
make sure flanges with holes and flanges with nuts are overlapping in the correct way.
5. Place the outboard panel, with calibrated force.
6. Place the upper panel.
· Start with the inboard aft flange at the outboard upper corner.
· Move the panel a bit aft.
· Rotate the panel. Bent the flange with calibrated force.
· Move the panel aft wards.
· Place, bent the panel to its final position.
7. Check all flanges to make sure flanges with holes and flanges with nuts are overlapping in the correct way.
8. Place aft upper panel.
9. Place fwd upper panel.
10. Move the aft bulkhead fwd with the inboard lower corner first.
11. Install all fasteners. Do not tighten fasteners before all fasteners are installed.
12. Install splitter panels.
Thanks to Martin.
10
- Always check the APU exhaust for FOD. 737-ALL
When you have APU problems or fire/smoke coming out of the exhaust, always check the APU exhaust first, especially when the a/c was parked overnight or for longer periods of time. You'll be surprised what you can find in this area. The pictures show actual debris found in a 737 APU exhaust.....
11 - APU
oil cooler contamination (Next Gen 131-9B).
737-6/7/8/900
During regular inspection (737-700) we found very often dirt accumulation on APU oil cooler, we incorporate regular inspection and cleaning at A-check interval /600FH/.See also FTD 737NG-FTD-49-01015 Thanks to someone from SkyEurope.
12 - APU
fire warning: sneak circuit inside APU ctrl module.
737-3/4/500
With APU Fire handle pulled (P8 panel) and the Master Bright/Dim and Light test switch set to Dim, the fire warning horn sounds in the cockpit. The Engine fire handles and the remote APU fire handle remains stowed. With the Master Bright/Dim and Light test switch set to Bright, the fire annunciation and horn stopped. This complaint is attributed by a "sneak circuit" inside the APU Control Module (M280). The aircraft manufacturer has released a component service bulletin (CSB) 65-52801-49-08 which include a modification from MOD "A" status to MOD "AB" of APU Control Module (M280) p/n: 65-52801-67. Thanks to Jan Hogeveen.
13 - APU
generator attachment nuts, different types available.
737-3/4/500
When you have to replace or install an APU generator on the -280 APU, there may be 2 different types of generator attachment studs mounted on the APU gearbox, a short (22 mm) and a long (32 mm) type. This may require also different nuts to attach the generator on the studs, due to the fact that the high nuts are too high for the short studs. (not enough threads coming through the nuts).
For short studs (22 mm), you can use LOW height nuts p/n NAS1805-6 or BACN11Z6CK or 624AG6HD
For long studs (32 mm), you can use the low nuts but also the HIGH height nuts p/n 624AG5HD or BACN10GW6A
See IPC 24-21-21-02 or CMM 49-10-21
See pictures below of short and long studs and nuts.
