Why was this changed vs the NACA looking duct on the NG? When in flight, surely the new design is a bigger fuel penalty given that it looks like a speedbrake when it's open? Is the APU so seldom used in flight that it doesn't matter? Weight savings? To match the 787 which has the same thing?

Why would anyone use the APU inflight during normal ops?

Acey wrote:

Is the APU so seldom used in flight that it doesn't matter?

Acey, I have not flown any aircraft where the APU is used inflight during normal operations. From my experience, the APU is only used inflight under abnormal/emergency conditions. I have dispatched with an engine driven generator inop--under MEL--requiring the APU to be used for the entire flight, and in those cases, additional fuel was uploaded to account for APU fuel burn; not necessarily due to drag.

When I went through Airbus A320 initial qualification training, an instructor told us that Airbus originally designed the APU on the A320 series such that it could be used continuously (i.e., inflight).

However, this is not the procedure at my company.

During normal operations, we shutdown the APU immediately after engine start, and after landing, we start the APU so as to have it fully operational just prior to arrival at the gate/parking stand.

e38

paullam wrote:

Why would anyone use the APU inflight during normal ops?

My question was about why the 737 MAX inlet door design was changed.

e38 wrote:

When I went through Airbus A320 initial qualification training

Any insight as to why the MAX APU inlet door was changed?

Acey wrote:

paullam wrote:

Why would anyone use the APU inflight during normal ops?

My question was about why the 737 MAX inlet door design was changed.

e38 wrote:

When I went through Airbus A320 initial qualification training

Any insight as to why the MAX APU inlet door was changed?

Perhaps it is due to a more powerful APU requiring larger airflows.

Acey wrote:

paullam wrote:

Why would anyone use the APU inflight during normal ops?

My question was about why the 737 MAX inlet door design was changed.

e38 wrote:

When I went through Airbus A320 initial qualification training

Any insight as to why the MAX APU inlet door was changed?

The door is open 45* in ground mode, and 17* when in the air. A MAX can be dispatched with door in-op for a 1% fuel burn penalty at 17*.

I suspect (but don't know) the change is related to weight savings and improved reliability, especially with high winds on the ground.

paullam wrote:

Why would anyone use the APU inflight during normal ops?

ETOPS requirement possibly?

AirKevin wrote:

paullam wrote:

Why would anyone use the APU inflight during normal ops?

ETOPS requirement possibly?

Precisely. You won't use it in normal ops, but it needs to be available for ETOPS/EDTO.

Or it could be that the APU is required at the outport.

Starlionblue wrote:

AirKevin wrote:

paullam wrote:

Why would anyone use the APU inflight during normal ops?

ETOPS requirement possibly?

Precisely. You won't use it in normal ops, but it needs to be available for ETOPS/EDTO.

Or it could be that the APU is required at the outport.

Yes, correct, exactly.

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

Usually our packs are sufficient to cool the airplane in anything but the most blistering heat, I've only used the APU once or twice to provide additional airflow in the Carribbean. 99.9% of the time we crank the APU 10 minutes prior to push, start the engines, and it goes off until the next start. Even with a long taxi time (ORD, JFK, or delays) we usually turn it off and just do a cross-bleed start.

bluecrew wrote:

Starlionblue wrote:

AirKevin wrote:

ETOPS requirement possibly?

Precisely. You won't use it in normal ops, but it needs to be available for ETOPS/EDTO.

Or it could be that the APU is required at the outport.

Yes, correct, exactly.

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

Usually our packs are sufficient to cool the airplane in anything but the most blistering heat, I've only used the APU once or twice to provide additional airflow in the Carribbean. 99.9% of the time we crank the APU 10 minutes prior to push, start the engines, and it goes off until the next start. Even with a long taxi time (ORD, JFK, or delays) we usually turn it off and just do a cross-bleed start.

Er what ?


You use the packs without running the APU and they’re enough to cool the aircraft ?


Can you explain how you do that ?!

bluecrew wrote:

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

What aircraft has an APU that produces hydraulic pressure?


For the original question I guess they changed the design to reduce drag and increase fuel efficiency. As noted above the APU is barely ever used inflight, so why would you want that door always creating drag (even when closed its not flush with the fuselage) with that spike sticking out of it (whatever it's called. I'm not familiar with the 737)

paullam wrote:

Why would anyone use the APU inflight during normal ops?

We run it for the entire ETOPS portion of a ETOPS flight + 15 minutes before entering the segment. It’s used as a ‘ready to go’ backup source.

Horstroad wrote:

bluecrew wrote:

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

What aircraft has an APU that produces hydraulic pressure?


For the original question I guess they changed the design to reduce drag and increase fuel efficiency. As noted above the APU is barely ever used inflight, so why would you want that door always creating drag (even when closed its not flush with the fuselage) with that spike sticking out of it (whatever it's called. I'm not familiar with the 737)

Electrical power for the electric hydraulic pump if there's an electrical failure.

Horstroad wrote:

spike sticking out of it (whatever it's called. I'm not familiar with the 737)

The colloquial name is "bicycle seat", the function of which I believe, is to act as a vortex generator to encourage air to go into the inlet if the APU is operated in flight.

Regards, JetMech

The in flight open position is only 17 degrees vs the 45 degrees you see on the ground when watching our planes from the terminal.

Ordinarily the apu is used only in flight as directed by mel or Qrh. However, we also run it continuously on departure during etops flights, it is running as a third electrical source until exiting etops airspace.

The inlet design was taken from the 787 design. Airflow around the new tail one design.

Max Q wrote:

bluecrew wrote:

Starlionblue wrote:

Precisely. You won't use it in normal ops, but it needs to be available for ETOPS/EDTO.

Or it could be that the APU is required at the outport.

Yes, correct, exactly.

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

Usually our packs are sufficient to cool the airplane in anything but the most blistering heat, I've only used the APU once or twice to provide additional airflow in the Carribbean. 99.9% of the time we crank the APU 10 minutes prior to push, start the engines, and it goes off until the next start. Even with a long taxi time (ORD, JFK, or delays) we usually turn it off and just do a cross-bleed start.

Er what ?


You use the packs without running the APU and they’re enough to cool the aircraft ?


Can you explain how you do that ?!

With the engines running... you know... bleed air feeds the packs and the packs cool the plane?
No need on this plane to run the APU to provide additional bleed air to push more air through the packs, and cool the aircraft more quickly.

bluecrew wrote:

Max Q wrote:

bluecrew wrote:

Yes, correct, exactly.

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

Usually our packs are sufficient to cool the airplane in anything but the most blistering heat, I've only used the APU once or twice to provide additional airflow in the Carribbean. 99.9% of the time we crank the APU 10 minutes prior to push, start the engines, and it goes off until the next start. Even with a long taxi time (ORD, JFK, or delays) we usually turn it off and just do a cross-bleed start.

Er what ?


You use the packs without running the APU and they’re enough to cool the aircraft ?


Can you explain how you do that ?!

With the engines running... you know... bleed air feeds the packs and the packs cool the plane?
No need on this plane to run the APU to provide additional bleed air to push more air through the packs, and cool the aircraft more quickly.

I think he was asking because you mentioned not turning the APU on until 10 minutes prior to push, so what are you using to cool the plane before you start the APU. I'm guessing ground air of some sort.

AirKevin wrote:

bluecrew wrote:

Max Q wrote:

Er what ?


You use the packs without running the APU and they’re enough to cool the aircraft ?


Can you explain how you do that ?!

With the engines running... you know... bleed air feeds the packs and the packs cool the plane?
No need on this plane to run the APU to provide additional bleed air to push more air through the packs, and cool the aircraft more quickly.

I think he was asking because you mentioned not turning the APU on until 10 minutes prior to push, so what are you using to cool the plane before you start the APU. I'm guessing ground air of some sort.

Low Pressure Ground air is used usually.

APU in flight can also be used if you need to a APU to packs takeoffs if additional performance from the engines are needed.

The entire aft tailcone was redesigned. The -NG uses an eductor to let ambient air into the APU compartment. That air is then pulled through the oil cooler from suction of the exhaust. The MAX eliminated the eductor design, so a new inlet had to be created that diverts air for the actual APU inlet and for the APU compartment. The APU on the -NG and MAX are the same, except for the mounts. There's a good explanation here: http://www.b737.org.uk/737maxdiffs.htm

bluecrew wrote:

Starlionblue wrote:

AirKevin wrote:

ETOPS requirement possibly?

Precisely. You won't use it in normal ops, but it needs to be available for ETOPS/EDTO.

Or it could be that the APU is required at the outport.

Yes, correct, exactly.

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

Usually our packs are sufficient to cool the airplane in anything but the most blistering heat, I've only used the APU once or twice to provide additional airflow in the Carribbean. 99.9% of the time we crank the APU 10 minutes prior to push, start the engines, and it goes off until the next start. Even with a long taxi time (ORD, JFK, or delays) we usually turn it off and just do a cross-bleed start.

Low pressure ground air doesn't always do it for us. It really depends on the port. Oftentimes we have to use the APU just to get the temp down to acceptable levels for boarding.

Starlionblue wrote:

bluecrew wrote:

Starlionblue wrote:

Precisely. You won't use it in normal ops, but it needs to be available for ETOPS/EDTO.

Or it could be that the APU is required at the outport.

Yes, correct, exactly.

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

Usually our packs are sufficient to cool the airplane in anything but the most blistering heat, I've only used the APU once or twice to provide additional airflow in the Carribbean. 99.9% of the time we crank the APU 10 minutes prior to push, start the engines, and it goes off until the next start. Even with a long taxi time (ORD, JFK, or delays) we usually turn it off and just do a cross-bleed start.

Low pressure ground air doesn't always do it for us. It really depends on the port. Oftentimes we have to use the APU just to get the temp down to acceptable levels for boarding.

Which aircraft?

bigb wrote:

Starlionblue wrote:

bluecrew wrote:

Yes, correct, exactly.

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

Usually our packs are sufficient to cool the airplane in anything but the most blistering heat, I've only used the APU once or twice to provide additional airflow in the Carribbean. 99.9% of the time we crank the APU 10 minutes prior to push, start the engines, and it goes off until the next start. Even with a long taxi time (ORD, JFK, or delays) we usually turn it off and just do a cross-bleed start.

Low pressure ground air doesn't always do it for us. It really depends on the port. Oftentimes we have to use the APU just to get the temp down to acceptable levels for boarding.

Which aircraft?

A330 and A350. Some ports in the tropic get rather hot, and in some places the air isn't quite up to the job, especially if the aircraft has been sitting for a while.

Starlionblue wrote:

bigb wrote:

Starlionblue wrote:

Low pressure ground air doesn't always do it for us. It really depends on the port. Oftentimes we have to use the APU just to get the temp down to acceptable levels for boarding.

Which aircraft?

A330 and A350. Some ports in the tropic get rather hot, and in some places the air isn't quite up to the job, especially if the aircraft has been sitting for a while.

That’s interesting but doesn’t surprise me. I’ve noticed that it all depends on if the Ground Air Unit/GPU that dependent as well. A good unit will cool avaircraft down but cooling a widebody aircraft with LP air only doesn’t work well.

bigb wrote:

Starlionblue wrote:

bigb wrote:

Which aircraft?

A330 and A350. Some ports in the tropic get rather hot, and in some places the air isn't quite up to the job, especially if the aircraft has been sitting for a while.

That’s interesting but doesn’t surprise me. I’ve noticed that it all depends on if the Ground Air Unit/GPU that dependent as well. A good unit will cool avaircraft down but cooling a widebody aircraft with LP air only doesn’t work well.

Even for my tiny jet, the LP air frequently won't do it in certain stations or if they have a terrible air cart. Oftentimes they'll hook it up and it will blow ambient air into the cabin, heating the cabin up really quickly. Very common in the Carribbean where the equipment isn't the best. That's usually the scenario where we're on APU air the whole time at the gate.

bluecrew wrote:

Max Q wrote:

bluecrew wrote:

Yes, correct, exactly.

In my aircraft you never operate the APU inflight unless you need it for non-normals, relight, hydraulic pressure, electric power, etc.

Usually our packs are sufficient to cool the airplane in anything but the most blistering heat, I've only used the APU once or twice to provide additional airflow in the Carribbean. 99.9% of the time we crank the APU 10 minutes prior to push, start the engines, and it goes off until the next start. Even with a long taxi time (ORD, JFK, or delays) we usually turn it off and just do a cross-bleed start.

Er what ?


You use the packs without running the APU and they’re enough to cool the aircraft ?


Can you explain how you do that ?!

With the engines running... you know... bleed air feeds the packs and the packs cool the plane?
No need on this plane to run the APU to provide additional bleed air to push more air through the packs, and cool the aircraft more quickly.

I don’t know any aircraft that allows you to use APU bleed air and engine bleed air to the packs simultaneously

Which type is this ?

Acey wrote:

Why was this changed vs the NACA looking duct on the NG? When in flight, surely the new design is a bigger fuel penalty given that it looks like a speedbrake when it's open? Is the APU so seldom used in flight that it doesn't matter? Weight savings? To match the 787 which has the same thing?

You are forgetting about the Vortex Generator on the NG. That was always there sticking out in the slipstream. The MAX APU Inlet Door design saves fuel in normal ops better than the NG does.

Max Q wrote:

I don’t know any aircraft that allows you to use APU bleed air and engine bleed air to the packs simultaneously

Which type is this ?

You can’t use the air simultaneously, but the sources can be available simultaneously. At idle, most engines barely produce enough air to keep the packs running with low demand. If you asking the packs for a bunch of cold air, the APU is a much better choice, and the aircraft “knows” it. The engine PRSOV’s (or equivalent) will close, allowing the APU bleed valve to open and supply the packs.

Once the demand is gone, or the engines are accelerated off idle, the engine air quickly overcomes the APU air and takes over.

fr8mech wrote:

Max Q wrote:

I don’t know any aircraft that allows you to use APU bleed air and engine bleed air to the packs simultaneously

Which type is this ?

You can’t use the air simultaneously, but the sources can be available simultaneously. At idle, most engines barely produce enough air to keep the packs running with low demand. If you asking the packs for a bunch of cold air, the APU is a much better choice, and the aircraft “knows” it. The engine PRSOV’s (or equivalent) will close, allowing the APU bleed valve to open and supply the packs.

Once the demand is gone, or the engines are accelerated off idle, the engine air quickly overcomes the APU air and takes over.

Yes, this is what I was referencing. By "additional airflow" I meant more, colder air. Depends on the airplane and outside conditions, but sometimes your engines at low power will produce hilariously insufficient airflow, or the packs will be blowing hot air into the cabin, until you get up and going. APU usually provides slightly more comfortable conditions, keeps the cabin below 90 degrees.

We used it all the time in Asia. Turns out it translates to the electric jet if it's super hot and humid, and the sun is beating down on you (SJU and KIN come to mind).

bluecrew wrote:

fr8mech wrote:

Max Q wrote:

I don’t know any aircraft that allows you to use APU bleed air and engine bleed air to the packs simultaneously

Which type is this ?

You can’t use the air simultaneously, but the sources can be available simultaneously. At idle, most engines barely produce enough air to keep the packs running with low demand. If you asking the packs for a bunch of cold air, the APU is a much better choice, and the aircraft “knows” it. The engine PRSOV’s (or equivalent) will close, allowing the APU bleed valve to open and supply the packs.

Once the demand is gone, or the engines are accelerated off idle, the engine air quickly overcomes the APU air and takes over.

Yes, this is what I was referencing. By "additional airflow" I meant more, colder air. Depends on the airplane and outside conditions, but sometimes your engines at low power will produce hilariously insufficient airflow, or the packs will be blowing hot air into the cabin, until you get up and going. APU usually provides slightly more comfortable conditions, keeps the cabin below 90 degrees.

We used it all the time in Asia. Turns out it translates to the electric jet if it's super hot and humid, and the sun is beating down on you (SJU and KIN come to mind).

While two sources of air can be AVAILABLE simultaneously they can’t be used at the same time


You can’t ‘augment’ engine bleed air for pack operation with APU bleed air


It’s one or the other, you implied otherwise