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Re: About APU start up / Shut down

Posted: Sat Dec 31, 2016 2:33 am
by Starlionblue
gloom wrote:
The only question remaining is the zoning (I've seen a couple of birds with different temps in front, middle and back zone of the plane), but that's easy trick to do either by multiplying mixers, or adjusting on manifolds, so I can answer myself that :)

Czesc, ;)
Adam


As you say this is a trim air function. Control wise it varies a bit. On the 330 overhead panel we have a knob for the cockpit temp and a knob for the cabin temp. The cabin crew can then set individual zones within +/- 2 degrees from what we've set in the cockpit by using the panel by the L1 door. The 350 also has an additional "Purser Select" setting on the cabin temp knob, allowing the cabin crew to adjust the cabin temp independently.

skywalker92 wrote:

I have another question(sorry for deviating from the topic)
1) How we turned off the APU? I mean the correct sequence.
2) How the APU is fed with fuel and is there a dedicated pump? How the fuel is supplied to the APU for starting it. I have turned on the APU with out turning on any fuel pumps.


On the Airbus there's an APU master pushbutton and a start pushbutton. Press the master pb and then the start one.

On the 330 it is rather complex due to the trim tank. There is a standpipe from the wing tanks to the APU and trim tank. The APU is fed from the trim tank if there is fuel in it by using the aft APU fuel pump, and from the wing tanks otherwise using the forward APU fuel pump. If the standpipe should be empty for some reason (well, not so much empty filled with air) you can't start the APU as the fuel won't be sucked up. There are separate aft and forward transfer tanks for CG trimming with the trim tank.

On the 350 it is way simpler. There's no trim tank so it's just a pump and a standpipe.

Re: About APU start up / Shut down

Posted: Sat Dec 31, 2016 8:30 pm
by skywalker92
The APU is fed from the trim tank if there is fuel in it by using the aft APU fuel pump, and from the wing tanks otherwise using the forward APU fuel pump. If the standpipe should be empty for some reason (well, not so much empty filled with air) you can't start the APU as the fuel won't be sucked up


That means APU has two fuel pumps(forward and aft) one to suck fuel from trim tank and the other one to suck from wing tanks. Am I correct?(if not correct me)
Are they powered up by the master switch?

Thnaks a lot!

Re: About APU start up / Shut down

Posted: Sat Dec 31, 2016 11:37 pm
by skyhawkmatthew
gloom wrote:
The only question remaining is the zoning (I've seen a couple of birds with different temps in front, middle and back zone of the plane), but that's easy trick to do either by multiplying mixers, or adjusting on manifolds, so I can answer myself that :)

Czesc, ;)
Adam


On the 777 it's similar to the A330 - we have a master control in the cockpit, then the cabin crew can vary each of the six cabin zones up to +/- 10°F/6°C from that temperature.

skywalker92 wrote:

I have another question(sorry for deviating from the topic)
1) How we turned off the APU? I mean the correct sequence.
2) How the APU is fed with fuel and is there a dedicated pump? How the fuel is supplied to the APU for starting it. I have turned on the APU with out turning on any fuel pumps.


On the 777, you start the APU simply by momentarily turning the switch to the START position. To shut down, you just move the same switch to OFF - the APU will enter cooldown mode for a couple of minutes before it actually shuts down and the door closes.

If AC power is not available, the APU has a dedicated DC fuel pump in the left main tank. Once there's AC power, the left forward main AC pump will operate even with its switch turned off in order to supply the APU. However, if you want, you can manually turn on a different pump to feed from a different tank instead.

Re: About APU start up / Shut down

Posted: Sun Jan 01, 2017 4:50 am
by Starlionblue
skywalker92 wrote:
The APU is fed from the trim tank if there is fuel in it by using the aft APU fuel pump, and from the wing tanks otherwise using the forward APU fuel pump. If the standpipe should be empty for some reason (well, not so much empty filled with air) you can't start the APU as the fuel won't be sucked up


That means APU has two fuel pumps(forward and aft) one to suck fuel from trim tank and the other one to suck from wing tanks. Am I correct?(if not correct me)
Are they powered up by the master switch?

Thnaks a lot!


That is correct. There are two pumps. I'm guessing the APU master switch activates the Electronic Control Box for the APU, and then the ECB runs the pumps as needed given the status of the fuel system etc. It's all transparent for the pilots. There are no switches in the cockpit like for the engine fuel pumps.

The fuel system on the 330 is annoyingly complex due to the trim tank, or as one ground school instructor put it, "clearly designed by a lunatic."

Image

Re: About APU start up / Shut down

Posted: Sun Jan 01, 2017 5:14 am
by benbeny
Starlionblue wrote:
skywalker92 wrote:
The APU is fed from the trim tank if there is fuel in it by using the aft APU fuel pump, and from the wing tanks otherwise using the forward APU fuel pump. If the standpipe should be empty for some reason (well, not so much empty filled with air) you can't start the APU as the fuel won't be sucked up


That means APU has two fuel pumps(forward and aft) one to suck fuel from trim tank and the other one to suck from wing tanks. Am I correct?(if not correct me)
Are they powered up by the master switch?

Thnaks a lot!


That is correct. There are two pumps. I'm guessing the APU master switch activates the Electronic Control Box for the APU, and then the ECB runs the pumps as needed given the status of the fuel system etc. It's all transparent for the pilots. There are no switches in the cockpit like for the engine fuel pumps.

The fuel system on the 330 is annoyingly complex due to the trim tank, or as one ground school instructor put it, "clearly designed by a lunatic."

Image

What's the purpose of adding heavy trim tanks and associated plumbings?

Re: About APU start up / Shut down

Posted: Sun Jan 01, 2017 5:54 am
by Starlionblue
benbeny wrote:
What's the purpose of adding heavy trim tanks and associated plumbings?


To give check captains a lot of questions they can shoot at you during a line check. :D

In the cruise, fuel is transferred aft, which moves the CG aft. This decreases fuel consumption by a percent or two. As fuel is burned off, small forward transfers are made to keep the CG within target limits. If there is any fuel left in the trim tank 35 minutes before arrival it is all transferred forward. (If we load a lot of fuel there will be some in the trim tank on take-off as well.)

On the 350, cruise trimming is done by extending the flaps a few degrees during the cruise to move the center of pressure, so the trim tank system was done away with. All automatic and way simpler.

Re: About APU start up / Shut down

Posted: Sun Jan 01, 2017 7:29 am
by benbeny
Starlionblue wrote:
To give check captains a lot of questions they can shoot at you during a line check. :D

In the cruise, fuel is transferred aft, which moves the CG aft. This decreases fuel consumption by a percent or two. As fuel is burned off, small forward transfers are made to keep the CG within target limits. If there is any fuel left in the trim tank 35 minutes before arrival it is all transferred forward. (If we load a lot of fuel there will be some in the trim tank on take-off as well.)

On the 350, cruise trimming is done by extending the flaps a few degrees during the cruise to move the center of pressure, so the trim tank system was done away with. All automatic and way simpler.

Hahaha I see :mrgreen:
So let's imagine the valve is stuck, you're flying at about max aft CG, and the trim tank is full, the only way to burn that is via APU :stirthepot:

Re: About APU start up / Shut down

Posted: Sun Jan 01, 2017 8:18 am
by Starlionblue
benbeny wrote:
Starlionblue wrote:
To give check captains a lot of questions they can shoot at you during a line check. :D

In the cruise, fuel is transferred aft, which moves the CG aft. This decreases fuel consumption by a percent or two. As fuel is burned off, small forward transfers are made to keep the CG within target limits. If there is any fuel left in the trim tank 35 minutes before arrival it is all transferred forward. (If we load a lot of fuel there will be some in the trim tank on take-off as well.)

On the 350, cruise trimming is done by extending the flaps a few degrees during the cruise to move the center of pressure, so the trim tank system was done away with. All automatic and way simpler.

Hahaha I see :mrgreen:
So let's imagine the valve is stuck, you're flying at about max aft CG, and the trim tank is full, the only way to burn that is via APU :stirthepot:


There is redundancy in the system, so you'd have to have multiple separate failures for forward transfer to be impossible. In case of a failure of the forward transfer pump, forward transfer is done by gravity through the non return valve. This takes much longer of course so the final forward transfer starts 75 minutes before arrival instead of 35.

Re: About APU start up / Shut down

Posted: Sun Jan 01, 2017 9:00 am
by thepinkmachine
benbeny wrote:
Hahaha I see :mrgreen:
So let's imagine the valve is stuck, you're flying at about max aft CG, and the trim tank is full, the only way to burn that is via APU :stirthepot:


To add to Starlion's answer - even if the fuel in the Trim Tank is completely stuck/unusable, you still have at least 4 hours flying time before you reach the Aft CG limit. In this time you can either continue to your destination, or divert.

Starting the APU won't help, as the fuel to the APU is fed from the trim pipe, downstream the Trim TK Isolation Valve (the one that is stuck in your example)

At 330/340 fuel system is a nightmare to understand. There's so much fuel transfer back and forth happening all the time, its unbelievable. Not only to/from the Trim Tank, but also Outer to Inner Taknks, and Center tank (if you happen to fly the 332). Having said that, it works pretty well and is reliable. The 350 solution is much much simpler though and actually very elegant.

Re: About APU start up / Shut down

Posted: Sun Jan 01, 2017 9:09 am
by Starlionblue
thepinkmachine wrote:
benbeny wrote:
Hahaha I see :mrgreen:
So let's imagine the valve is stuck, you're flying at about max aft CG, and the trim tank is full, the only way to burn that is via APU :stirthepot:


To add to Starlion's answer - even if the fuel in the Trim Tank is completely stuck/unusable, you still have at least 4 hours flying time before you reach the Aft CG limit. In this time you can either continue to your destination, or divert.

Starting the APU won't help, as the fuel to the APU is fed from the trim pipe, downstream the Trim TK Isolation Valve (the one that is stuck in your example)

At 330/340 fuel system is a nightmare to understand. There's so much fuel transfer back and forth happening all the time, its unbelievable. Not only to/from the Trim Tank, but also Outer to Inner Taknks, and Center tank (if you happen to fly the 332). Having said that, it works pretty well and is reliable. The 350 solution is much much simpler though and actually very elegant.


Many other systems are also simplified on the 350. For example the hydraulic systems are identical instead of all different. There is a pump for each system in each engine. Lose an engine and you still have full hydraulics. Dual hydraulic failure you still have the autopilot. On the 330 if you lose an engine you lose one of the systems. Additionally there are a bunch of little quirks like an electric pump running for so and so many seconds to retract the flaps in certain specific failure situations.

Re: About APU start up / Shut down

Posted: Sun Jan 01, 2017 10:51 am
by chimborazo
On the 737 NG:

1.
- Switch off APU bleed air to the pneumatic system
- Switch electrical power to the engines/ ground power as required ( this disconnects the APU from the electrical bus)
- Move the APU switch to the OFF position: it has three positions: OFF; ON & START (spring return from ON)

2. The APU fuel line is from the left wing tank. either the forward or aft electric pump will pressurise the APU feed and is turned on prior to starting the APU. However, the APU has its own pump which will suck fuel from the tank so you can run the APU without the wing tank but as I understand it (this info is all from a few hours in an NG sim) procedure is to start one of the wing pumps- aft as I recall... Further, if the centre tank pump is fuelled and has the left pump running, it generates a higher pressure than the wing tanks (so the centre tank is used first, leaving fuel in the wing tanks for bending relief of the weight in the centre tank until it's used) so the APU would be fed from centre tank. You could also open the cross-.feed from right tank and use those pumps but that's getting a bit unnecessary... :-)

Re: About APU start up / Shut down

Posted: Mon Jan 02, 2017 12:47 am
by skywalker92
skyhawkmatthew wrote:
If AC power is not available, the APU has a dedicated DC fuel pump in the left main tank. Once there's AC power, the left forward main AC pump will operate even with its switch turned off in order to supply the APU. However, if you want, you can manually turn on a different pump to feed from a different tank instead.


Image

Is this true for all aircraft or only for B777? In the image shown above we have FWD APU pump and AFT APU pump powered by aircraft electrical power(if this statement is wrong please correct me). As you said is there a another DC pump located in left wing to feed APU if the aircraft is not powered? and how can left wing pump feed fuel to the APU?

Starlionblue wrote:
That is correct. There are two pumps. I'm guessing the APU master switch activates the Electronic Control Box for the APU, and then the ECB runs the pumps as needed given the status of the fuel system etc. It's all transparent for the pilots. There are no switches in the cockpit like for the engine fuel pumps.


As we all know when feeding fuel to the engine, center tank is emptied first and when looking at the above diagram through which line center tanks feeds fuel to the engines? Is it through refuel and defuel line by the help of inlet valves by transferring center tank fuel to the left and right wing tanks and then feed to the engine?

Starlionblue wrote:
On the 330 if you lose an engine you lose one of the systems


How can that happen? As an example in A330 aircraft lets say we lost engine No1. So we lost green and blue EDP's but we have their electric pumps. I know that electric pumps is not that much capable as an EDP but Cant we use them to pressurize their systems?

So far this topic has been a pool of knowledge for me and a big salute to all who is filling this :thumbsup: :wave:
Thanks a lot

Re: About APU start up / Shut down

Posted: Mon Jan 02, 2017 6:39 am
by richcam427
Starlionblue wrote:
benbeny wrote:
What's the purpose of adding heavy trim tanks and associated plumbings?


To give check captains a lot of questions they can shoot at you during a line check. :D

In the cruise, fuel is transferred aft, which moves the CG aft. This decreases fuel consumption by a percent or two. As fuel is burned off, small forward transfers are made to keep the CG within target limits. If there is any fuel left in the trim tank 35 minutes before arrival it is all transferred forward. (If we load a lot of fuel there will be some in the trim tank on take-off as well.)

On the 350, cruise trimming is done by extending the flaps a few degrees during the cruise to move the center of pressure, so the trim tank system was done away with. All automatic and way simpler.


Interesting to know it's very similar to the MD-11, the difference being that the FSC transfers fuel forward when any main tank falls below 5200 kg, or during descent below FL200 in the scud.

Re: About APU start up / Shut down

Posted: Fri Jan 06, 2017 1:37 pm
by Starlionblue
skywalker92 wrote:
skyhawkmatthew wrote:
If AC power is not available, the APU has a dedicated DC fuel pump in the left main tank. Once there's AC power, the left forward main AC pump will operate even with its switch turned off in order to supply the APU. However, if you want, you can manually turn on a different pump to feed from a different tank instead.


Image

Is this true for all aircraft or only for B777? In the image shown above we have FWD APU pump and AFT APU pump powered by aircraft electrical power(if this statement is wrong please correct me). As you said is there a another DC pump located in left wing to feed APU if the aircraft is not powered? and how can left wing pump feed fuel to the APU?

Starlionblue wrote:
That is correct. There are two pumps. I'm guessing the APU master switch activates the Electronic Control Box for the APU, and then the ECB runs the pumps as needed given the status of the fuel system etc. It's all transparent for the pilots. There are no switches in the cockpit like for the engine fuel pumps.


As we all know when feeding fuel to the engine, center tank is emptied first and when looking at the above diagram through which line center tanks feeds fuel to the engines? Is it through refuel and defuel line by the help of inlet valves by transferring center tank fuel to the left and right wing tanks and then feed to the engine?

Starlionblue wrote:
On the 330 if you lose an engine you lose one of the systems


How can that happen? As an example in A330 aircraft lets say we lost engine No1. So we lost green and blue EDP's but we have their electric pumps. I know that electric pumps is not that much capable as an EDP but Cant we use them to pressurize their systems?

So far this topic has been a pool of knowledge for me and a big salute to all who is filling this :thumbsup: :wave:
Thanks a lot


The 330s I fly have no center tank so I don't know specifically. However center tank feeding tends to be simple. The center fuel tank pumps are more powerful than the wing tank pumps so they simply overwhelm the latter until the center tank is empty.

The Aft APU Pump is powered even if we're down to the RAT. It is only shed when Land Recovery is activated.

330 Hydraulics. There's a blue system pump powered by engine one, a yellow system pump powered by engine two and green system pumps in both engines. So if we lose engine one we lose the blue system, and the same for yellow if we lose engine two, barring windmilling. (If we still have either engine one or two we'll have the green system.) Yes, we have electric pumps, but they only provide about 18% of engine drive pump capacity. In the words of the FCOM, they "should not be used to replace" the engine driven pumps. They're simply not powerful enough to power all the surfaces, the landing gear and so forth.

Re: About APU start up / Shut down

Posted: Thu Jan 12, 2017 12:39 am
by dennis2380
well on the 787 they must start the apu before arriving at the gate, not many airports have the electrical power required to power the a/c. united pilots will not take the a/c with a inop apu.

Re: About APU start up / Shut down

Posted: Sat Jan 14, 2017 10:16 pm
by 77west
dennis2380 wrote:
well on the 787 they must start the apu before arriving at the gate, not many airports have the electrical power required to power the a/c. united pilots will not take the a/c with a inop apu.


Thats not entirely accurate; the 787 has the normal 2x forward 90KVA connections with a third 90KVA point aft. The only difference here compared to, say, a 777, is they recommend having all 3 connected if the APU is inop for engine start.

http://www.boeing.com/assets/pdf/commer ... trical.pdf

EDIT: I can see, however, that if operating into smaller airports they may only have a single 90KVA available, in which case you would need the APU.