Can I start both engines on Ground Air Cart ONLY?

[davierosoft 13]

At fl300 you have enough altitude and probably speed to try a windmilling restart, as per manuals.

The windmilling restart does not need air supply from pneumatic duct. engine rotation is done by the airflow. Faster you go, faster N1 and N2 will be. You need to have at least 25%N2 if I remember, however, manual will give you correct speeds and values.

Ops, I didn't read the entire post... disregard my post

!

You don't need to turn off bleeds on the engines if they are off, but this is not the problem.

The problem is that both pneumatic and electrical can be used together up to 10000fts.

You can use pneumatic (WITH NO ELECTRIC GENERATOR CONNECTED) up to 17000fts.

So basically, start the APU and use its bleed to start the engine wich can be started with only battery (standby bus powered by battery). If you want to use APU to start the engines you can do with pneumatic only. 

[Adam T Lutley 12]

Is it possible for the APU to supply duct pressure in flight?

 

I only ask this because the other day I was flying over Edwards doing systems failures testing and decided to shut down both engines at FL300. After setting both fuel control levers to cutoff and returning the throttles to idle, I started the APU and selected it on bus for electrical power. I turned off anti ice, set isolation valve to open, turned off both engine bleeds, turned off both packs and set APU bleed on. There was 0 psi duct pressure with the APU running. I wasn't sure if this was due to my altitude so I continued down to 15,000 before I was forced to perform windmill starts on both engines due to still having 0 psi duct pressure. Am I setting bleed air wrong or missing something?

The APU on the 737 NG can be used for electrical power at 32,000 ft and below, pneumatic power at 17,000 ft and below, and electrical and pneumatic at 10,000 ft and below. Now, with regards to the previous posts regarding dual engine starts, using an air cart. Don't see any reason why it wouldn't be "theoretically possible" but why would you? I've never seen it done in real world operations. With respect to the 737 family, performing a #2 eng start is ,with a ground cart, potentially hazardous in several ways due to its proximity to the ground cart and the flexible pneumatic line. Cross bleed starting the #2 eng from the #1 involves opening the isolation valve, advancing power on the #1 eng to achieve at least 30 psi of duct pressure, and performing a normal start on the #2 eng. Happy flying!. Regards

[rcpistonhead 47]

 

 

You don't need to turn off bleeds on the engines if they are off, but this is not the problem.

The problem is that both pneumatic and electrical can be used together up to 10000fts.

You can use pneumatic (WITH NO ELECTRIC GENERATOR CONNECTED) up to 17000fts.

 

 

The APU on the 737 NG can be used for electrical power at 32,000 ft and below, pneumatic power at 17,000 ft and below, and electrical and pneumatic at 10,000 ft and below.

 

Thanks for the response, this is the information I was seeking. I was unaware of the APU's altitude restrictions regarding electrical and pneumatic supply. Thank you guys!

[davierosoft 13]

Normally I am one who asks for 100% of system functionality, as in real life. So, theorically and technically speaking the dual engine start on ground cart is something that could be done in real life (also if with some risks).

But, in this case we are not speaking of an onboard system, and PMDG coded a fully automated logic for air cart, we have only the connect-disconnect manual inputs.

As PMDG decided to do in this way, it is correct that it stops to supply pressure at the end of the first engine start. This is also because in this way you can check duct pressure for crossengine start before disconnecting the cart. To me is pretty logical and simple to use. Adding only the support for the second engine start is useless.

Adding the support for it will probably require to make a submenu in the CDU or a popup menu for ground comunication (it could be cool to have a crew-ground comunication menu) with all the necessary commands for the various operation of the cart. To me it is good as it is, pretty real and working.

Thanks for the response, this is the information I was seeking. I was unaware of the APU's altitude restrictions regarding electrical and pneumatic supply. Thank you guys!

You're welcome!

[Adam T Lutley 12]

Thanks for the response, this is the information I was seeking. I was unaware of the APU's altitude restrictions regarding electrical and pneumatic supply. Thank you guys!

Bear in mind that these are the APU altitude operating limits on the actual aircraft. They may or may not reflect how it modeled in the NGX. Regards.

[rcpistonhead 47]

 

 

Bear in mind that these are the APU altitude operating limits on the actual aircraft. They may or may not reflect how it modeled in the NGX. Regards.

Thank you for pointing this out Adam. I will do another test flight tonight and check to see if the APU's operational limits are modeled correctly. I will assume that they are since it's PMDG but just in case it isn't modeled correctly, perhaps they could include this fix in the upcoming service pack. Thanks again!

[rcpistonhead 47]

 

 

Normally I am one who asks for 100% of system functionality, as in real life

 

Bear in mind that these are the APU altitude operating limits on the actual aircraft. They may or may not reflect how it modeled in the NGX. Regards.

I did some testing with the APU operational limits over the weekend and found that the APU will start and provide electrical power far above FL320.

 

The rest of the modeling seems to be accurate. Electrical power supply only above 17,000. Either pneumatic OR electrical between 17,000 and 10,000. And both electrical and pneumatic below 10,000 all work properly.

 

I wonder if it possible to see this fix in the up coming service pack? (APU electrical power inoperative above FL320)

[davierosoft 13]

What is your APU load? APU will supply 90KVA up to 32000fts but is limited to 66KVA till 41000fts.

So it can be used most of the time.

[Adam T Lutley 12]

I did some testing with the APU operational limits over the weekend and found that the APU will start and provide electrical power far above FL320.

 

The rest of the modeling seems to be accurate. Electrical power supply only above 17,000. Either pneumatic OR electrical between 17,000 and 10,000. And both electrical and pneumatic below 10,000 all work properly.

 

I wonder if it possible to see this fix in the up coming service pack? (APU electrical power inoperative above FL320)

I believe the APU can be started all the way up to the service ceiling of the aircraft which is around 41,000 ft. Didn't mention that in the original post. Not sure why the 32,000 ft is the listed restriction for using the generator but can't think of a reason why one couldn't use it. Regards

[Fabo 213]

Maybe it cannot deliver the power needed to get a "rating" for it up to ceiling?

 

In any case it is often turned on for ETOPS crossings so that you have a standby source ready at a moments notice. In any case, if one of your engines fail, you will probably drift down to under 320 quite soon.

[rcpistonhead 47]

 

 

What is your APU load? APU will supply 90KVA up to 32000fts but is limited to 66KVA till 41000fts.

So it can be used most of the time.

I can't remember what the exact electrical load was. However, I was not aware of the load capability / altitude relationship. I will do another test with this and check.

 

 

 

I believe the APU can be started all the way up to the service ceiling of the aircraft which is around 41,000 ft. Didn't mention that in the original post. Not sure why the 32,000 ft is the listed restriction for using the generator but can't think of a reason why one couldn't use it

This was my original thought as well. It does make sense however that the electric load would be limited above FL320; since the air is thinner the APU can only handle so much load. Either way as I mentioned above I will do another test flight all the way up to FL410 checking the electrical load with the APU generator.

[nsksleeper 0]

hello folks,

 

i'm lucky to work at an airport as a pushbackdriver, so i have some experience.

First of all, the panel for pneumatic panel is right on the fuselage about few feets next to the engine,

 

and you have a safety distance to the engine about 23 feet.

 

So that's the only way it works

 

you cannot start Nr. 2 engine, because it would suck your equipment.

 

and pilots are only allowed to start the sec. one after PB is completed, because they have to rise the power on running engine

[davierosoft 13]

I can't remember what the exact electrical load was. However, I was not aware of the load capability / altitude relationship. I will do another test with this and check.

 

 

 

This was my original thought as well. It does make sense however that the electric load would be limited above FL320; since the air is thinner the APU can only handle so much load. Either way as I mentioned above I will do another test flight all the way up to FL410 checking the electrical load with the APU generator.

I'm pretty sure the load on the APU was lower than 66KVA because pmdg used the real manual to simulate the other things, and doing this, probably they simulated also the load limit with altitude above fl320

You can only see current draw from the APU, the power is a different thing and must be calculated.

[nsksleeper 0]

update from my side: safty distance to engines 15 ft

[kevinh 409]

Normally I am one who asks for 100% of system functionality, as in real life. So, theorically and technically speaking the dual engine start on ground cart is something that could be done in real life (also if with some risks).

But, in this case we are not speaking of an onboard system, and PMDG coded a fully automated logic for air cart, we have only the connect-disconnect manual inputs.

As PMDG decided to do in this way, it is correct that it stops to supply pressure at the end of the first engine start. This is also because in this way you can check duct pressure for crossengine start before disconnecting the cart. To me is pretty logical and simple to use. Adding only the support for the second engine start is useless.

Adding the support for it will probably require to make a submenu in the CDU or a popup menu for ground comunication (it could be cool to have a crew-ground comunication menu) with all the necessary commands for the various operation of the cart. To me it is good as it is, pretty real and working.

Andrea,

 

Once again, I must remind you: PMDG did not intend to code it like this, it's an error, a bug. They are going to correct it, or at least look at the cause, in SP2.  If they had meant to do this the ground cart would be disconnected along with the air pressure.  But the ground air remains connected and if the first engine is shutdown the air pressure returns.  Also a start with engine 2 first is possible which is just as dangerous for the ground crew.  If your idea was correct any ground air start from engine 2 should be inhibited too.

 

As for it requiring some new dialogue and ground crew interaction to implement removing the cart after the first engine start, why exactly?  Is there any such interaction simulated with connection of external power or air at present?  No, so all you will have to do is remove the ground air via the CDU and start the second engine using crossbleed after pushback.

 

It is bad simulation to program a simulator to reflect operating procedure rather than correct logic and physics.  Likewise if they had programmed the starts so they wouldn't work if the fuel pumps were off (as is often the case in addons).  Procedure says turn the pumps on before starting the engines, but if you don't the engines will start anyway.