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taguilo
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757/767 APU Switch Start Question

Sat Mar 11, 2006 8:37 am

Hi,

Just an specific question:

When starting a 757/767 APU, after rotating the switch to START and releasing it,

1) Does it return back to ON immediately, starting the double flash of RUN light immediately as well, or

2) Does it hold on START position for a short time (1-3 secs) and then returns back to ON only when the test phase (RUN light flashing) is over? or

3) Switch actually needs to be hold on START for 1-3 secs to enable the start sequence?

Thank you

Tom
 
aogdesk
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RE: 757/767 APU Switch Start Question

Sat Mar 11, 2006 8:47 am

After rotating the switch to start, it releases immediately back to run, and you'll see the double flash of the run light.

Thats if I remember correctly.......god, I see these things every day too......
 
AvionicMech
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RE: 757/767 APU Switch Start Question

Sat Mar 11, 2006 8:47 am

Number One is correct, once the start signal has been sent to the APU ECU(I can't think of its correct name at the moment) the start sequence will begin. There is no solenoid that holds the switch in start like with the engines when they are put to GND.

You beat me to it Mr Desk.  Smile

[Edited 2006-03-11 00:49:43]
 
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fr8mech
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RE: 757/767 APU Switch Start Question

Sat Mar 11, 2006 10:32 am

Our procedure is to hold the switch in place for at least at 1 sec. and allowit to return to the run position. The only reason for this is to be sure that we have initiated the start sequence.
When seconds count, the police are minutes away, or may not come at all.
It’s hard to win an argument with a smart person, but it’s damn near impossible to win an argument with a stupid person. ~B. Murray
Ego Bibere Capulus, Ut Aliis Sit Vivere
 
cdfmxtech
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RE: 757/767 APU Switch Start Question

Sat Mar 11, 2006 2:05 pm

Quoting Taguilo (Thread starter):
When starting a 757/767 APU, after rotating the switch to START and releasing it,

1) Does it return back to ON immediately, starting the double flash of RUN light immediately as well, or


2) Does it hold on START position for a short time (1-3 secs) and then returns back to ON only when the test phase (RUN light flashing) is over? or

3) Switch actually needs to be hold on START for 1-3 secs to enable the start sequence?

In all the years that I've started this APU, I have NEVER EVER had to hold the switch for 1-3 seconds. I always go startaight thru, and back to ON. That being said, the Maintenance manual says to go to ON, wait 1 second, then to START. You are then supposed to manually move the switch back to ON (or make sure it doesn't spring back to ON on it's own). The switch will spring back to on as soon as it is let go, but it is not recommended to allow it do do so. There is a possiblity to damage the contacts.
That's why the 1-3 second thing came from. There were some issues with intermittent contacts in the control panel. So a note came out to sort of "slow down" the entire process. .
 
taguilo
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RE: 757/767 APU Switch Start Question

Sat Mar 11, 2006 10:27 pm

Thank you guys for the great answers!

Now I'd bother you with another group of specific questions.

1) Manual says BATT switch ON is needed to start the APU on ground. Likewise, if BATT is turned OFF while APU is running on the ground, the APU starts its shuts down process (wait for cooling time and/or shut down immediately); however turning BATT OFF in flight does not "affect APU operation" (literally quoted). This means APU will not shut down here, and my doubt is, would it be possible to start the APU in flight with BATT switch OFF ? - Besides, when the APU auto shuts down, does it switch remain in the ON position (FAULT light ON after shutdown) and must be turned to OFF manually to cancel the Fault light and reset the circuity?

2) When APU shutdown is commanded (switch to OFF), while it remains ON waiting to cool down (RUN light still ON), shutdown is automatically cancelled if switch is rotated to START and released. Now, as long as the shutdown process begins (RUN light OFF), is it possible to start the APU (switch to START) before it completes the process (0 RPM)? If so, what would happen to EGT, would rise to near limits? (I guess 760 ºC or so).


3) Which would be a normal range for stabilized APU EGT values :
a) with no electric and pneumatic loads
b) with electric load only
c) with electric and pneumatic loads
d) at engine startup


Again, thank you very much!

Regards,
Tom
 
AJ
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RE: 757/767 APU Switch Start Question

Sun Mar 12, 2006 12:51 pm

Another quick note on the APU slector. When rotated from OFF to ON the APU door is cycled open. An advantage of this is that the APU start sequence is half the time with the door already open. For long and distracting taxi ins I often open the door clear of the runway. This means that if the APU is forgotton prior to parking the embarassing wait for it to start is halved!
 
Buzz
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RE: 757/767 APU Switch Start Question

Mon Mar 13, 2006 7:40 pm

Hi Tom, Buzz here. It's been a while since I did an APU health check, but the basic numbers you requested are...
Idle, no bleed and generator off: Approx 360 c.
Generator on: about 7 degrees higher .... 365 or so
Bleed air on, but not being used (pack valves off) 370
Both packs on: about 450 c
Main Engine Star mode: about 505 c
And limiting EGT is just shy of 800c. Often with a tired APU battery we might see about 765 degrees at 35% RPM while trying to start the APU.

g'nite
 
taguilo
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RE: 757/767 APU Switch Start Question

Tue Mar 14, 2006 1:24 am

Quoting Buzz (Reply 7):
Hi Tom, Buzz here. It's been a while since I did an APU health check, but the basic numbers you requested are...
Idle, no bleed and generator off: Approx 360 c.
Generator on: about 7 degrees higher .... 365 or so
Bleed air on, but not being used (pack valves off) 370
Both packs on: about 450 c
Main Engine Star mode: about 505 c
And limiting EGT is just shy of 800c. Often with a tired APU battery we might see about 765 degrees at 35% RPM while trying to start the APU.

Thank you Buzz! This is more than useful info! I understand those temp peaks (ie from 360 to 505) happen in less than 10 secs, maybe 5 ...?

BTW, do you know something in reference for the first two questions?  Smile

Regards
Tom
 
Buzz
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RE: 757/767 APU Switch Start Question

Tue Mar 14, 2006 10:37 pm

Hi Again Tom, Buzz here. I'm working on a couple of 757's tonight. So I experimented a bit.
This airplane has a marginal APU start switch, I had to hold it until 50% to keep the APU started. I'm surprised none of the flight crew has griped it. Every time I let go of the switch the APU would spool down. After 50% the starter cuts out and the APU accelerates.

To answer your first question, with the APU running I put the airplane in Air mode by pulling the air-gnd system breakers, and landing gear position breakers over head. Then I selected the battery switch to Off. The APU kept running. When I pushed in the breakers the APU shut down.

The APU start switch remained in the "On" position and after several seconds the Fault light flashed once as the fuel valve cycled closed.

I didn't try working on question #2, restart after shutdown. I wanted to get back to the Ready Room and sign off the gripes, the night is almost over and people want to fly these airplanes. But when you shut down the APU it won't re-start unless the RPM is below... either 5 or 7%. EGT is not normally a problem with a good battery (and a good turbine wheel). There's enough airflow to keep the EGT reasonable. Tonight after the 3rd start i saw 760 degrees briefly, outside air temperature is about 35 degrees.

g'nite
Buzz Fuselsausage: Line Mechanic by night, DC-3 Crew Chief by choice, Taildragger pilot for fun, Beliver in Christ for Life.
 
taguilo
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RE: 757/767 APU Switch Start Question

Wed Mar 15, 2006 4:42 am

Hey Buzz many thanks for all this info!

Quoting Buzz (Reply 9):
To answer your first question, with the APU running I put the airplane in Air mode by pulling the air-gnd system breakers, and landing gear position breakers over head. Then I selected the battery switch to Off. The APU kept running. When I pushed in the breakers the APU shut down.

Did you ever try to start the APU in Air mode, with Batt switch OFF?

Quoting Buzz (Reply 9):
The APU start switch remained in the "On" position and after several seconds the Fault light flashed once as the fuel valve cycled closed.

Shouldn't the Fault light remain ON in this case, as last fuel valve position (closed) disagrees with switch position (ON) ?

And another good one  Smile :

Do you know what happens if you try to start the APU with no fuel at all in the left wing tank? Does the ECU prevents the starter to run when no fuel is sensed, or maybe it accelerates to a "fuel on" speed and then spools down ?
Is there a minimum fuel for the APU to start?

Thank you for your kind help

Tom
 
Buzz
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RE: 757/767 APU Switch Start Question

Wed Mar 15, 2006 4:39 pm

Hi again Tom, Buzz here. Often at night we have external power On, and the battery Off (so the battery won't run down if external power drops off at night). The APU logic is such so that it won't crank if the battery is selected off.

The fault light blinks for a moment, it's not a continuous "disagree" light. Just they way it's wired.

No fuel? Every once in a while we totally empty a fuel tank for a fuel quantity calibration. The APU won't light off for a while... sometimes it takes 3 or 4 attempts to get it running. What happens is that the APU cranks, senses there's no 50 degree temperature rise, then cancels the start. A normal APU start causes about a 250 degree rise, so if the fire is lit the logic is satisfied.

I forget what the minimum fuel is, when the APU won't pick up any more fuel.
g'nite
 
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HAWK21M
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RE: 757/767 APU Switch Start Question

Sat Dec 30, 2006 7:37 pm

Quoting Buzz (Reply 11):
Every once in a while we totally empty a fuel tank for a fuel quantity calibration

Whats the Scheduled Frequency for this.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
Valcory
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RE: 757/767 APU Switch Start Question

Sat Dec 30, 2006 8:14 pm

I always leave the apu switch in the on position for a few seconds(give the intake door some time to open up) then i go to start and release
 
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HAWK21M
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RE: 757/767 APU Switch Start Question

Sat Dec 30, 2006 11:41 pm

Quoting Valcory (Reply 13):
I always leave the apu switch in the on position for a few seconds(give the intake door some time to open up) then i go to start and release

Always Advisable as sometimes the momentry contact in Start posn might not be made.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
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fr8mech
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RE: 757/767 APU Switch Start Question

Sun Dec 31, 2006 1:04 am

Quoting Valcory (Reply 13):
I always leave the apu switch in the on position for a few seconds(give the intake door some time to open up) then i go to start and release

The procedure, from our AMM, is to move passed the 'On' position to the 'Start' position and hold for 2 seconds, then allow the switch to return to 'On'.

As I recall, the door does not open in the on position, but I'd have to check the AMM about that.
When seconds count, the police are minutes away, or may not come at all.
It’s hard to win an argument with a smart person, but it’s damn near impossible to win an argument with a stupid person. ~B. Murray
Ego Bibere Capulus, Ut Aliis Sit Vivere
 
Valcory
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RE: 757/767 APU Switch Start Question

Thu Jan 04, 2007 5:16 am

Quoting Fr8Mech (Reply 15):
As I recall, the door does not open in the on position, but I'd have to check the AMM about that.

Please do check.Lets see replacing the intake door actuator,part of the ops check is to put the apu switch in the on position to make sure the intake door open.
 
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fr8mech
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RE: 757/767 APU Switch Start Question

Fri Jan 05, 2007 12:26 pm

Quoting Valcory (Reply 16):
Please do check.Lets see replacing the intake door actuator,part of the ops check is to put the apu switch in the on position to make sure the intake door open.

You are correct and I am once again humbled because I used my memeory instead of the book.
When seconds count, the police are minutes away, or may not come at all.
It’s hard to win an argument with a smart person, but it’s damn near impossible to win an argument with a stupid person. ~B. Murray
Ego Bibere Capulus, Ut Aliis Sit Vivere
 
Valcory
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Joined: Fri Dec 22, 2006 6:56 pm

RE: 757/767 APU Switch Start Question

Fri Jan 05, 2007 2:29 pm

Fr8Mech We are only human i am guilty of going my memory too
 
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HAWK21M
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RE: 757/767 APU Switch Start Question

Fri Jan 05, 2007 10:39 pm

Quoting Fr8Mech (Reply 17):
because I used my memeory instead of the book.

Its fine as long as it applies only to the Forum & not at work  Smile
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
Valcory
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RE: 757/767 APU Switch Start Question

Sat Jan 06, 2007 12:02 am

Happy six year airliners.net birthday Mel
 
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HAWK21M
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RE: 757/767 APU Switch Start Question

Sat Jan 06, 2007 11:27 pm

Quoting Valcory (Reply 20):
Happy six year airliners.net birthday Mel

Thanks.I remembered when I noticed the cake  Smile

Anyone faced a problem with the Springloading to On posn on the Switch ever.We did.Had to be manually moved from start to ON.
regds
MEL
I may not win often, but I damn well never lose!!! ;)

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