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737-8XX APU

Old 15th Oct 2012, 14:00
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737-8XX APU

Greetings to all!

Just a quick question, Boeing 737-8xx 's APU (Garret or Sundtrand) panel, you have only the EGT monitoring. IS there any way to check the RPM? as to know the APU load?.

when you start the engines by using the APU bleed, the duct pressure shows 19psi (because the APU is idle!) but when the ground switch moves to GRD position of the engine, the Duct pressure rise to about 35-40psi as the APU is now loaded. So I was wondering is there any way to check the RPM of the APU while starting the engine?. What would be the starting sequence for APU bleed induced engine start?

Thanks

(for some this may sounds like kindergarten! if so pls ignore! )

Code0

Last edited by code0; 15th Oct 2012 at 14:15.
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Old 15th Oct 2012, 14:13
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The APU is a constant-speed turbine engine, which means that it's RPM is constant. This is achieved by the fuel control unit, which schedules the amount of fuel required to keep the engine turning at constant speed, therefore the only true way to see the engine load is through EGT: if you increase the load on the APU, more fuel will be required to turn it at constant speed - more fuel, the higher the EGT as in any turbine engine. One positive thing of APU being constant-speed engine is that you don't need a CSU between the engine and the electric generator, as the APU itself is turning at a constant speed.

I stand to be corrected though.
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Old 15th Oct 2012, 14:25
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Thanks Stone! but dont you think quit a hand full of airliners do have the option of monitoring the RPM including 757s and 320s with an APU page!

I understand that the APU is a constant speed engine but if I may say the "idle" of the APU is at about 95% isn't that wrong? and if you demand for any electrical or in my case pneumatic pressure the APU is getting loaded hence the RPM moves to 100% But as you said if the Pneumatic is demanded the EGT will go high but why the 737s do not have the option other than in a BITE test page (RPM is shown in the BITE test page - but this is not monitored in Normal Procedures : Engine startups?)

If a moment you can think that the NGs fixed with Sundstand APU's are FADEC ~ ECB controlled hence lets say there is no very much of a requirement for a RPM indication, but how about in classics and originals? do they have ?

Why doesn't the APU RPM is not a requirement ?

Sorry! one question trigger thousands!

Thanks for ur reply!

Cheers

Code0
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Old 15th Oct 2012, 14:45
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Try going on the Maint Page on the FMC and do a APU Bite test. This will give you all the information you need i.e Oil qty, inlet door open/closed etc.

I havent been on the page in a while so not sure if it will give the RPMs.
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Old 15th Oct 2012, 14:53
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Hi 900ER yeah thanks! in fact the BITE page does! it does give the RPM and other few parameter such as the position of IGV, etc! But generally do you keep the BITE page open on the FMC every time you powering up the engines? and is that the way to monitor the RPM on NGs?
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Old 15th Oct 2012, 15:23
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But generally do you keep the BITE page open on the FMC every time you powering up the engines? and is that the way to monitor the RPM on NGs
When we start engines, we monitor the engines
The APU has auto shutoff .
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Old 15th Oct 2012, 15:36
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sorry my mistake! powering the APU! not engines!

When starting the APU no RPM monitoring?

or when the APU is demanded ELEC or BLEED is there any way to monitor the APU RPM ?

thanks

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Old 15th Oct 2012, 17:24
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Go to MAINT > ACMS > ALPHA > ALPHA CODE (not sure of title, it's the LSK 1R)
Use the codes N1A for N1, FFA for Fuel Flow and EGTA for the EGT.
I use it to monitor the APU inflight starts during ETOPS flights.

Hope this helps
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Old 15th Oct 2012, 21:26
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Just a quick question, Boeing 737-8xx 's APU (Garret or Sundtrand) panel, you have only the EGT monitoring. IS there any way to check the RPM? as to know the APU load?
Without going into the CDU, no. You can get a idea of the RPM by watching the APU generator freq's.

when you start the engines by using the APU bleed, the duct pressure shows 19psi (because the APU is idle!) but when the ground switch moves to GRD position of the engine, the Duct pressure rise to about 35-40psi as the APU is now loaded.
Theoretically the APU doesn't ever idle, it runs at a constant speed whether unloaded or loaded. You could have a full generator load on the APU and you will still have the 19psi with APU bleed air selected but no bleed air demand. This APU has a separate load compressor with IGV's that control the pressure. So when the eng. start switch is moved to ground, the APU ECU commands the IGV's to open to allow the load compressor to increase air pressure/volume.

What would be the starting sequence for APU bleed induced engine start?
When start sw. is moved to ground, the start valve opens, IGV's open, air is supplied to the starter, starter turns the N2 rotor, at 56% N2 the start switch returns to off/auto, start valve closes, starter disconnects (through a sprag clutch) IGV's close, duct pressure returns to unloaded pressure.

But generally do you keep the BITE page open on the FMC every time you powering up the engines? and is that the way to monitor the RPM on NGs?
No and (APU RPM) yes.

When starting the APU no RPM monitoring?
Not normally. I only watch RPM when troubleshooting.

If a moment you can think that the NGs fixed with Sundstand APU's are FADEC ~ ECB controlled hence lets say there is no very much of a requirement for a RPM indication, but how about in classics and originals? do they have ?
No RPM indication on the jurassic and classic. There is test equipment we can use to monitor RPM, but normally any adjustments made to the RPM is done to attain the desired generator frequency, so we don't usually care what the actual RPM is.
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Old 15th Oct 2012, 21:49
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APU RPM = APU Gen freq. divided by 4.
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Old 16th Oct 2012, 01:51
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Code 0 , you can watch more of the apu's parameters via the input monitoring page via the maint page of the CDU. Dont go messing with maint stuff without speaking to you engineer first.
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Old 16th Oct 2012, 05:06
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Thanks for all replies! made it clear

And thanks Yeelep for the points and thanks Amp lol yeah no hands on for experiments! but I did try this on simulator! Thanks Safe for the equation!

Yeah now it seems like the monitoring of the RPM of the APU is not a requirement. So now it triggers that in a APU overspeed (lets say during startup) if the APU has to be shutdown, you only rely on the EGT gauge (?) Those classics and originals how do you know your APU has overspeeded? again monitoring EGT? (in fact they have a red marking!)

I think my question now points to manufacture in fact this is now confirmed a product feature than anything else. I dont know about other Boeings but 757 do have APU RPM monitoring during startups (and continuous)

I am sure 737 MAX will have the APU RPM monitoring if not that at least Southwest can demand that

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Old 16th Oct 2012, 09:15
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Originally Posted by code0
Yeah now it seems like the monitoring of the RPM of the APU is not a requirement. So now it triggers that in a APU overspeed (lets say during startup) if the APU has to be shutdown, you only rely on the EGT gauge (?) Those classics and originals how do you know your APU has overspeeded? again monitoring EGT? (in fact they have a red marking!)
High EGT is only indication of high load (or maybe FCU problems during start), not overspeed. On the classic, you have a very simple indication that the APU has overspeeded - the OVERSPEED light on the APU panel illumintaes and the APU shuts itself down automatically.

I don't see a point in having tons of parameters instantaneously available to the crew, since the APU is very simple and fully automatically controlled turbine engine. It can detect and shut down for many types of failures (low oil pressure, overspeed, high oil temperature, fire, ...) and it can also prevent excessive EGT by modulating or fully closing bleed valve.

We could have same set of indications for an APU that we have for an engine, but then we could have some more indications for some other systems, panel space would be a problem and we could reintroduce flight engineer - just to monitor something automatics have been doing quite OK for the last 30 years.

By the way, does APU RPM monitoring on more "sophisticated" types than 737 works when doing battery electrical power-up (e.g. no ground power)?
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Old 16th Oct 2012, 09:46
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So now it triggers that in a APU overspeed (lets say during startup) if the APU has to be shutdown, you only rely on the EGT gauge (?)
For all practical purposes the NG APU monitors itself and automatically initiates a protective shutdown when it goes outside of normal parameters. The following are monitored and can cause a protective shutdown: APU fuel valve, DC powerloss, ECU, Fire, Inlet door, Inlet overheat, Loss of EGT, Loss of speed, No acceleration, No APU rotation, No flame, Oil filter, Oil temperature, Overtemperature, Reverse flow, Sensor failure, Underspeed, Overspeed low oil pressure, fuel solenoid valve fails open.

Those classics and originals how do you know your APU has overspeeded? again monitoring EGT? (in fact they have a red marking!)
Also has automatic overspeed protection, triggered at 110% by a centrifugal switch. EGT won't tell you anything about the speed of the APU.

Last edited by Yeelep; 16th Oct 2012 at 10:14.
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