boxsmasher
Topic Author
Posts: 51
Joined: Wed Aug 14, 2002 3:03 pm

747 APU in-flight

Sat Nov 03, 2018 9:24 pm

What was Boeing's reasoning behind not letting the 747 start the APU infight? I know you can start it on the ground and use it for bleed air only up to 15,000 feet but once airborne it cannot be started. There has been at least one instance of a 4 engine flame out due to volcanic ash. With APU powering electrics and bleed air, they would have been in much better shape with functioning electrics and hydraulics. Very curious to see if there is a reason other than $.
 
stratclub
Posts: 439
Joined: Fri Jan 05, 2018 10:38 pm

Re: 747 APU in-flight

Sun Nov 04, 2018 9:14 pm

Probably because there is no need for it. With 4 engines providing electrical and bleed air and the possibility of having all 4 engines flaming out being a 1 in billions chance, an airborne APU would be redundancy of an extremely absurd level. Not dollars really. The mods required to make the 747's APU operational in flight would be insignificant.
 
GalaxyFlyer
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Joined: Fri Jan 01, 2016 4:44 am

Re: 747 APU in-flight

Sun Nov 04, 2018 9:38 pm

It’s happened at least twice! BA over Indonesia and KLM near Mt. Redoubt in AK. The C-5 had both a RAT and two in-flight APUs

GF
 
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Florianopolis
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Re: 747 APU in-flight

Mon Nov 05, 2018 2:07 am

I thought it was because four windmilling engines will provide enough hydraulics to keep flying, and the batteries are big enough to run electrics until you 1) restart an engine, or 2) reach the ground.

The -8 added a RAT (for hydraulics only, I think) because windmilling its engines wouldn't keep the hydraulics going.

Here are some old forum topics:
www.airliners.net/forum/viewtopic.php?t=759841
www.airliners.net/forum/viewtopic.php?t=761799
 
stratclub
Posts: 439
Joined: Fri Jan 05, 2018 10:38 pm

Re: 747 APU in-flight

Mon Nov 05, 2018 6:44 am

GalaxyFlyer wrote:
It’s happened at least twice! BA over Indonesia and KLM near Mt. Redoubt in AK. The C-5 had both a RAT and two in-flight APUs

GF

True. It must be why it was thought the 747-8 needed to have a RAT (Ram Air Turbine) as backup. Still, 2 similar volcanic ash incidents in almost 50 years certainly is an anomaly.
 
stratclub
Posts: 439
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Re: 747 APU in-flight

Mon Nov 05, 2018 7:22 am

Florianopolis wrote:
I thought it was because four windmilling engines will provide enough hydraulics to keep flying, and the batteries are big enough to run electrics until you 1) restart an engine, or 2) reach the ground.

The -8 added a RAT (for hydraulics only, I think) because windmilling its engines wouldn't keep the hydraulics going.

Here are some old forum topics:
viewtopic.php?t=759841
viewtopic.php?t=761799

The battery bus will not run hydraulics or fuel boost pumps. You would have whatever hydraulics that windmilling engines provide and of course your standby instruments and systems indication and control from the battery bus. And like you said, the -8 does have a RAT.

A little slide show on the 747-400 electrical systems: https://www.slideshare.net/theoryce/b74 ... ical-power
 
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Starlionblue
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Re: 747 APU in-flight

Mon Nov 05, 2018 7:48 am

stratclub wrote:
GalaxyFlyer wrote:
It’s happened at least twice! BA over Indonesia and KLM near Mt. Redoubt in AK. The C-5 had both a RAT and two in-flight APUs

GF

True. It must be why it was thought the 747-8 needed to have a RAT (Ram Air Turbine) as backup. Still, 2 similar volcanic ash incidents in almost 50 years certainly is an anomaly.


I have a vague memory of reading that the -8 got a RAT because due to redesigned flight controls, windmilling was not enough in case of all engine flame out.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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747classic
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Re: 747 APU in-flight

Mon Nov 05, 2018 10:15 am

Starlionblue wrote:

I have a vague memory of reading that the -8 got a RAT because due to redesigned flight controls, windmilling was not enough in case of all engine flame out.


The GEnx-2B engines, installed at the 747-8 series, are not providing enough hydraulic power when windmilling, so a RAT, powering hyd syst #3, had to be installed to satisfy the regulations in case of an all engines out scenario.

Regarding the APU on the 747.
In flight operation is not necessary to comply with all regulations
At the 747 classic it was from the production start possible to have two APU options :
- operation allowed untill 15.000ft after APU starting at the ground.
- start and operation possible in the air (max 15.000 ft.)
Only APU pneumatic air could be used during APU flight operations. It was not possible to power the electrical system via de APU generator(s).
In practice this feature was only used during T/O and initial climb to provide max thrust (no bleed delivery) out of the initially underpowered early 747's
The APU delivered air for airconditioning pack #2 with both isolation vlv's closed and pack vlv's #1 and #3 closed.

After a few years of operation (after the fuel crises in the seventies !) a kit was developed to alter the APU air inlet door, to reduce the drag from the original inlet door,(even in closed position this door created a lot of drag.)
After this service bulletin APU in flight use was prohibited, indicated by a decal above the APU panel.
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
744lover
Posts: 188
Joined: Mon Nov 06, 2000 5:29 am

Re: 747 APU in-flight

Mon Nov 05, 2018 5:18 pm

747classic wrote:
In practice this feature was only used during T/O and initial climb to provide max thrust (no bleed delivery) out of the initially underpowered early 747's
The APU delivered air for airconditioning pack #2 with both isolation vlv's closed and pack vlv's #1 and #3 closed.



Wasn't this procedure what, in the end, cause the Lufthansa D-ABYB crash in NBO in the early 70s? With the slats being pneumatically operated, once the X-feed bleed valves were closed, the slats closed shut once the aircraft rotated.

I'm just curious how this was remedied in later versions of the 747.


Thanks,
744lover
 
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747classic
Posts: 2417
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Re: 747 APU in-flight

Mon Nov 05, 2018 5:38 pm

No, the crash of D-ABYD was initialy caused by the original bypass function of the engine bleed air valve during engine starting
It was possible to start the engines with the bleed air vlv switches closed. By checklist you had to open the engine bleed air valves after starting (before taxi checklist)
This was omitted (and not checked !) and conseq the bleed system was not pressurized and the LE flaps were not extended during T/O, causing the crash of D-ABYB at NBO..
One of the lessons of this crash : The engine bleed air function has been changed : only engine start possible with bleed air sw. in the open position.
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
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7BOEING7
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Re: 747 APU in-flight

Mon Nov 05, 2018 5:46 pm

747classic wrote:
Starlionblue wrote:

I have a vague memory of reading that the -8 got a RAT because due to redesigned flight controls, windmilling was not enough in case of all engine flame out.


The GEnx-2B engines, installed at the 747-8 series, are not providing enough hydraulic power when windmilling, so a RAT, powering hyd syst #3, had to be installed to satisfy the regulations in case of an all engines out scenario.

Regarding the APU on the 747.
In flight operation is not necessary to comply with all regulations
At the 747 classic it was from the production start possible to have two APU options :
- operation allowed untill 15.000ft after APU starting at the ground.
- start and operation possible in the air (max 15.000 ft.)
Only APU pneumatic air could be used during APU flight operations. It was not possible to power the electrical system via de APU generator(s).
In practice this feature was only used during T/O and initial climb to provide max thrust (no bleed delivery) out of the initially underpowered early 747's
The APU delivered air for airconditioning pack #2 with both isolation vlv's closed and pack vlv's #1 and #3 closed.

After a few years of operation (after the fuel crises in the seventies !) a kit was developed to alter the APU air inlet door, to reduce the drag from the original inlet door,(even in closed position this door created a lot of drag.)
After this service bulletin APU in flight use was prohibited, indicated by a decal above the APU panel.


To be clear, APU to pack takeoff is still a procedure on the 744 -- can't speak to the 748.
 
GalaxyFlyer
Posts: 1504
Joined: Fri Jan 01, 2016 4:44 am

Re: 747 APU in-flight

Mon Nov 05, 2018 8:49 pm

747classic wrote:
No, the crash of D-ABYD was initialy caused by the original bypass function of the engine bleed air valve during engine starting
It was possible to start the engines with the bleed air vlv switches closed. By checklist you had to open the engine bleed air valves after starting (before taxi checklist)
This was omitted (and not checked !) and conseq the bleed system was not pressurized and the LE flaps were not extended during T/O, causing the crash of D-ABYB at NBO..
One of the lessons of this crash : The engine bleed air function has been changed : only engine start possible with bleed air sw. in the open position.


Wouldn’t this be a failure of both the F/E (BLD VLVs closed) and the crew (failure to correctly check the slat/flap configuration) and the T/O WARN system?

GF
 
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747classic
Posts: 2417
Joined: Sat Aug 15, 2009 9:13 am

Re: 747 APU in-flight

Tue Nov 06, 2018 6:41 am

GalaxyFlyer wrote:
747classic wrote:
No, the crash of D-ABYD was initialy caused by the original bypass function of the engine bleed air valve during engine starting
It was possible to start the engines with the bleed air vlv switches closed. By checklist you had to open the engine bleed air valves after starting (before taxi checklist)
This was omitted (and not checked !) and conseq the bleed system was not pressurized and the LE flaps were not extended during T/O, causing the crash of D-ABYB at NBO..
One of the lessons of this crash : The engine bleed air function has been changed : only engine start possible with bleed air sw. in the open position.


Wouldn’t this be a failure of both the F/E (BLD VLVs closed) and the crew (failure to correctly check the slat/flap configuration) and the T/O WARN system?

GF


My answer was in response to 744 lover's question about the bleed isolation vlv's switching.
The initial engine bleed vlv logic was prown for a failure and that is the reason for the modification following this crash..
But you are correct : If the crew did the correct checklist reading and checking of the T/O configuration the crash of D-ABYD would have been avoided.
Operating a twin over the ocean, you're always one engine failure from a total emergency.

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