737-800 aux gearbox
Thread Starter
Join Date: Mar 2005
Location: My house
Posts: 1,339
Likes: 0
Received 0 Likes
on
0 Posts
737-800 acc gearbox
Hello all,
I have a rather interesting question that was raised by one of the instructors today during a sim session.
With an accessory gearbox failure and subsequent N2 seizure, would the engine driven hydraulic pump show a low pressure indication? With the engine separation there is a low pressure alert and master caution. However with our sim's N2 seizure there is no low pressure alert.
As the EDP is driven by the N2 rotor, surely the pump would stop rotating as well?
Thanks
edited aux GB to Acc GB
I have a rather interesting question that was raised by one of the instructors today during a sim session.
With an accessory gearbox failure and subsequent N2 seizure, would the engine driven hydraulic pump show a low pressure indication? With the engine separation there is a low pressure alert and master caution. However with our sim's N2 seizure there is no low pressure alert.
As the EDP is driven by the N2 rotor, surely the pump would stop rotating as well?
Thanks
edited aux GB to Acc GB
Last edited by nick14; 26th May 2012 at 23:36.
Join Date: Feb 2005
Location: Past the rabbit proof fence
Posts: 242
Likes: 0
Received 0 Likes
on
0 Posts
Pretty extreme senario - but yes, if the N2 does stop there would be no EDP output and the low pressure light would be on. There is a check valve to prevent system pressure 'backdriving' the EDP transducer. 
Join Date: Jun 2006
Location: Australia
Posts: 1,186
Likes: 0
Received 0 Likes
on
0 Posts
With an auxiliary gearbox failure and subsequent N2 seizure
When an Accessory gear box failure is actuated in the simulator, the engine simply flames out and there is no vibration. The N2 indicator immediately indicates zero rotation. But the compressor still rotates. It is not severe damage by definition. Presumably if the same defect happens in the real engine the symptoms are the same. Just because the N2 indicator goes off the clock to zero does not mean the engine is grinding itself to death.
Last edited by Tee Emm; 26th May 2012 at 12:24.
Join Date: Apr 2010
Location: Home soon
Posts: 0
Likes: 0
Received 0 Likes
on
0 Posts
Why would an Accessory gear box failure cause an N2 seizure in the first place?
The AGB drives the Engine Hydraulic Pump, so no pressure.
Why N2 would indicate 0 immediately?I am not 100 perc sure but the N2 sensor is connected to the front of the AGB,so if the AGB failed,the sensor would not get any info about the N2 rotors speed.
Last edited by de facto; 26th May 2012 at 13:58.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
N2 display??
Working from memory here -
The 738 (and other NG series) have the CFM56-7 donk w/FADEC. The N2 sensor is on the mainshaft in the #3 bearing area, and thus should still function in case of AGB or quill (radial) shaft failure.
And of course the HP fuel pump stops turning (thus flameout), and lube pump likewise (zero oil pressure).
But the little control PMG that powers the FADEC also stops, and thus I believe there will be no "processed" N2 signal available to the cockpit,,,
UNLESS the cockpit receives its N2 DIRECTLY from the mainshaft sensor - which should still work OK and show the windmilling N2.
So the answer is: I DON'T KNOW. N2 display might or might not be available!
The 738 (and other NG series) have the CFM56-7 donk w/FADEC. The N2 sensor is on the mainshaft in the #3 bearing area, and thus should still function in case of AGB or quill (radial) shaft failure.
And of course the HP fuel pump stops turning (thus flameout), and lube pump likewise (zero oil pressure).
But the little control PMG that powers the FADEC also stops, and thus I believe there will be no "processed" N2 signal available to the cockpit,,,
UNLESS the cockpit receives its N2 DIRECTLY from the mainshaft sensor - which should still work OK and show the windmilling N2.
So the answer is: I DON'T KNOW. N2 display might or might not be available!
Thread Starter
Join Date: Mar 2005
Location: My house
Posts: 1,339
Likes: 0
Received 0 Likes
on
0 Posts
Apologies, yes I did mean accessory gear box, long day.
This question has indeed thrown up more than I had anticipated. In my recent LPC I was given this failure, which was expected to be treated as severe damage, action the memory items and not attempt a restart. We did however treat it as an engine failure, which was mentioned in the debrief.
As a previous poster has suggested, should this not be treated as severe damage? If it is treated as an engine failure our company SOP's dictate an attempt at a restart, which in this instance would be futile.
Interesting thread!!
This question has indeed thrown up more than I had anticipated. In my recent LPC I was given this failure, which was expected to be treated as severe damage, action the memory items and not attempt a restart. We did however treat it as an engine failure, which was mentioned in the debrief.
As a previous poster has suggested, should this not be treated as severe damage? If it is treated as an engine failure our company SOP's dictate an attempt at a restart, which in this instance would be futile.
Interesting thread!!
Join Date: Apr 2010
Location: Home soon
Posts: 0
Likes: 0
Received 0 Likes
on
0 Posts
As a previous poster has suggested, should this not be treated as severe damage? If it is treated as an engine failure our company SOP's dictate an attempt at a restart, which in this instance would be futile.
Therefore if your AGB fails,the engine will flame out and the N2 indication will show zero.
As you are told to treat it as a severe damage(as N2 shows 0),do so and no restart.
If you tried to do so ,you wouldnt get any N2 increase anyway as the sarter uses the AGB to drive the N2 shaft.
Last edited by de facto; 27th May 2012 at 05:11.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
If the N2 sensor were located on the AGB, and it's the quill shaft that failed, you WOULD get an N2 indication when engaging the starter. Think about it; starter drives AGB, AGB turns the N2 gage.
But that N2 is a false indication, because the AGB is uncoupled from the core.
(I'm speaking from first-hand experience here...)
But that N2 is a false indication, because the AGB is uncoupled from the core.
(I'm speaking from first-hand experience here...)
Join Date: Feb 2005
Location: Past the rabbit proof fence
Posts: 242
Likes: 0
Received 0 Likes
on
0 Posts
The AGB sends torque from the N2 rotor through spur gears to turn the engine and airplane accessories.
But that N2 is a false indication, because the AGB is uncoupled from the core.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
How is it "uncoupled" from the core - if the accessory drive seized in flt I recon the metal would be flying from the accessory drive, there would be too much momentum.
Join Date: Apr 2010
Location: Home soon
Posts: 0
Likes: 0
Received 0 Likes
on
0 Posts
I predicated my proposition on a quill shaft failure, not a AGB seizure. I cannot recall a single outright AGB failure
Last edited by de facto; 28th May 2012 at 13:53.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
Back to the sim scenario:
Yes, the AGB stops turning rather quickly. It's driving pumps and accessories that take a lot of horsepower, and without a driving torque from the N2 rotor, everything associated with that donk will quickly fall to zero (except N1, which is windmilling). This will be true regardless of whether there's a bona fide internal seizure or simply a loss of rotation a la quill shaft failure.
Meanwhile, nick14, find a good technical description of the CFM56-7 control system, and verify whether the cockpit gets its N2 intelligence from the AGB, or else directly from the main shaft next to the #3 bearing. Let us know, because it affects the fidelity of the simulator logic. (If I were still operating in the real world, I'd look it up myself, but as a retiree access is more difficult...)
Yes, the AGB stops turning rather quickly. It's driving pumps and accessories that take a lot of horsepower, and without a driving torque from the N2 rotor, everything associated with that donk will quickly fall to zero (except N1, which is windmilling). This will be true regardless of whether there's a bona fide internal seizure or simply a loss of rotation a la quill shaft failure.
Meanwhile, nick14, find a good technical description of the CFM56-7 control system, and verify whether the cockpit gets its N2 intelligence from the AGB, or else directly from the main shaft next to the #3 bearing. Let us know, because it affects the fidelity of the simulator logic. (If I were still operating in the real world, I'd look it up myself, but as a retiree access is more difficult...)
Last edited by barit1; 28th May 2012 at 19:04.
Join Date: Sep 2009
Location: germany
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
AGB fail/driveshaft fail
Seems that the AGB itself can fail as well, not only in simulated conditions:
http://airinsight.com/2014/08/26/exa.../#.VLvm5MY-BE4
fly fast
http://airinsight.com/2014/08/26/exa.../#.VLvm5MY-BE4
fly fast
