Can I start both engines on Ground Air Cart ONLY?

[Robert McDonald 167]

I can't seem to do it.. I can start ONE engine, but the pressure seems to low to spin up engine #2?  I must be missing something, even after turning the start contact knob to "Flt" for the running engine, the 2nd engine will not go over about 17 N1 (need 25 N1 to engage)

 

If I start the APU, I can fire up both engines. 

I would 'guess' I should be able to start both with ground carts connected?

 

Thanks in advance.

 

[voske 36]

I am not sure whether the ground air source can be used to start engine 2 but, according to FCOM (volume 1, section SP.7.5), engine 2 should be started using the Engine Crossbleed Start procedure.

[Jgid 0]

Do you have bleed air and crossfeed valves set correctly?

[kevinh 409]

You should be able to start engine 2 from ground air. As I understand it it is not recommended because the ground air connection is near engine 2 so disconnecting it while engine 2 is running is dangerous for the ground crew.

 

I've never tried this in the NGX and without knowing how you have the panel set up I can't say why it won't work for you. However selecting FLT on engine 1 has no effect on engine 2 starting. It merely energizes the ignition on engine 1.

 

What is the duct pressure after engine 1 has started and before you attempt the second engine start? Is it the same as it was before engine 1 started? Make sure no pneumatic bleeds are on, for example ensure the packs are off, both engine bleed switches are off, wing and eng anti ice off, etc.

[KriVa 307]

If I remember it correctly (been a while since I've done a crossbleed start), you also need to add a bit of power on the engine that's already running. Most of the time, it will be engine 1, because the connection for the bleed air supply is indeed rather close to the inlet of engine 2.

[machine2035 176]

Yes, as Marc stated above, the laid out procedures for doing a ground-air start are detailed in Supplementary Procedures of the FCOM (PMDG Start Menu -> PMDG NGX -> Operating Manual Vol 1). Page 219 to be exact

[mwilk 1,107]

In real life, our company policy is to start #1 engine at the gate and then disconnect. We will start both at the gate if there is a technical reason for it but we'd prefer not to. It's a safety issue.

[johnsturm 22]

I just tried starting the engines without the use of the APU using the procedures detailed on page 219 of the FCOM volume 1. It worked perfectly. From your post it looks like the key step you missed was advancing the throttle on engine one until you have a duct pressure of at least 30.

 

Engine one is started from the AIR START UNIT using the "Starting with Ground Air Source" procedure.

Disconnect the cart.

Start engine two using the "Engine Crossbleed Start" procedure.

[kevinh 409]

Disconnect the cart.

 

Start engine two using the "Engine Crossbleed Start" procedure.

 

 

The point is you should be able to start both engines from the ground air cart.  The FCOM procedure saying use crossbleed air for the second start is not relevant, it may be normal procedure but that does not mean the second engine start is not possible.

 

I've just had a chance to try this with the NGX and the OP is correct.  In fact you can start engine 2 from ground air if you try that engine first, but then you can't start engine 1. During the first engine start, duct pressure recovers to about 38 psi as you would expect but 15 seconds after starter cutout it drops to 0, so no second start is possible. Ground Air remains connected on the CDU and in the external model. If you shut down the running engine duct pressure from the ground air reappears.

 

Not sure if this is a bug or by design, but I suspect the latter.  Either way it should be changed to reflect reality.  There is no aircraft logic for the ground air pressure to drop like that and so prevent a second start from ground air.  It should not be dependent on whether the other engine is running.  Contriving the model so the user must follow a particular procedure is not good simulation. If PMDG want the user to do this then the ground air should be fully disconnected so it's clear why the air pressure has gone.  But I think it should be up to the user to disconnect ground air and start the second engine after pushback, or alternatively start both engines from the ground cart at the gate if they wish.

[johnsturm 22]

The point is you should be able to start both engines from the ground air cart.  The FCOM procedure saying use crossbleed air for the second start is not relevant, it may be normal procedure but that does not mean the second engine start is not possible.

You may be correct that starting the second engine from the cart is possible. I don't know because I don't have any firsthand experience flying a real 737, do you? If you do then thank you for the input and maybe it is something that can be looked at, if not you are making an assumption that may not be correct.

 

It may be counter intuitive that the available duct pressure from the cart drops when the first engine is started but that does not mean it is not realistic. There may be a very good engineering reason why this happens, maybe someone with a lot more knowledge than me will come a long and explain it.

[scandinavian13 5,192]

 

 

Not sure if this is a bug or by design, but I suspect the latter.  Either way it should be changed to reflect reality.

 

I know the low pressure air system (which I understand is a completely different system, but may have similar design cues) has a way of preventing backflow by closing off the port through a flapper.  I don't have the schematics in front of me and I have no idea what the internals looks like, so this is just baseless conjecture, but still, an idea.

[kevinh 409]

You may be correct that starting the second engine from the cart is possible. I don't know because I don't have any firsthand experience flying a real 737, do you? If you do then thank you for the input and maybe it is something that can be looked at, if not you are making an assumption that may not be correct.

 

It may be counter intuitive that the available duct pressure from the cart drops when the first engine is started but that does not mean it is not realistic. There may be a very good engineering reason why this happens, maybe someone with a lot more knowledge than me will come a long and explain it.

I have a lot of experience in full flight simulator design for airliners including the 737. This means I have had access to aircraft manufacturer's data, and I'm familiar with how the aircraft systems are designed.  I've never seen any reference to this kind of ground air pressure behaviour, and nothing like it has been modelled on a simulator I've worked on.  The Boeing schematics in the FCOM don't make any mention of it.  As far as I'm aware there is no valve in the system to control ground air pressure, not even a non return valve, just the mechanical connection to the pneumatic duct.

 

I'm not ruling out that this is correct simulation, but I very much doubt it. I put in a ticket to PMDG to get them to look into this so if they have data to support it they will presumably let me know.

I know the low pressure air system (which I understand is a completely different system, but may have similar design cues) has a way of preventing backflow by closing off the port through a flapper.  I don't have the schematics in front of me and I have no idea what the internals looks like, so this is just baseless conjecture, but still, an idea.

There is an NRV on the ground conditioned air inlet, but not on the high pressure ground air inlet.

 

http://www.b737.org.uk/pneumatics.htm#Schematic

 

An NRV would be intended to prevent higher pressure from the duct backflowing into the ground source. It can't close off a pressurised source of air unless the pressure in the duct is even higher.  With APU off, the only other source of pressure in the duct is the bleed valve on the running engine (around 30 psi).  Even if you close that valve the ground air pressure still disappears.

[scandinavian13 5,192]

 

 

Even if you close that valve the ground air pressure still disappears.

 

Gotcha - that was going to be my next question.  Interested to hear what you hear from them.

[kevinh 409]

Gotcha - that was going to be my next question.  Interested to hear what you hear from them.

Not expecting an answer any time soon, under the circumstances. I will dredge this thread up if I hear anything.

[Joey_767 0]

Interesting question, allthough I don't see myself using this (theoretical possibility) in the real world because of the placement of the ground air cart and thus the hazard to the ground personell..

 

Start engine 1 using the "Starting with Ground Air Source" SP.7.5 (page 219). After stabilizing engine 1 close the Isolation Valve switch, now look at the duct pressure and try to start engine 2, does it work?

 

The ground air cart is connected on the right side (no 2) side of the Isolation Valve Switch, thus the right side should be isolated from the left side and the ground cart should be able to start engine 2 if I'm right..