It's worth taking a step back here, and marveling at the fact that we have such a deep systems simulation that - over a year after release, an oddity appears for the first time, and RSR has to go to BOEING to verify the exact behavior!

 

Not wanting to take us off-topic too far here, but I thought this was a perfect example of what a great 'age' of simming we're in. Just wait until we're all here pondering obscure EFIS messages from our B777 flight decks! ^_^

 

I was just thinking the same thing. I tend to follow the checklist so this is not something I would ever find out except for this post but it does kind of blow me away that the NGX is programmed to this level of detail. Love it!

There is a feature on the 737 NG where the Generator Control Breaker (GCB) closes automatically .

The Generator Control Unit (GCU) sends a close signal to the GCB when these condition occurs;

 

- fire switch is in the normal position

- BTB is open.

- Airplane is in the air mode.

- both GCB's are open on transition to the air mode and the APU is the only power source on the airplane.

- the Auxilary Power Breaker (APB) opens.

 

This occurs only once in flight, This feature gets a reset on transition to the GND mode.

This a quote from the AMM

This means that the automatic transfer only arms if the aircraft is with APU feeding the plane (both busses) at lift off (with idg outputs of a good quality) and it will work when the APU will be shut down.

If the test is made after a normal take off (idgs online) the automatic transfer will not work as the "liftoff" condition is not present.

This as it is written in the manuals.

It's worth taking a step back here, and marveling at the fact that we have such a deep systems simulation that - over a year after release, an oddity appears for the first time, and RSR has to go to BOEING to verify the exact behavior!

 

Not wanting to take us off-topic too far here, but I thought this was a perfect example of what a great 'age' of simming we're in. Just wait until we're all here pondering obscure EFIS messages from our B777 flight decks! ^_^

Not really, because the way the system is currently programmed is not how the aircraft is supposed to operate. The NGX connects the engine GCBs at liftoff and disconnects the APU generator. From what Andrea is saying this isn't supposed to happen unless the APB trips off for some reason.

 

I tested this on my NGX and the engine GCBs closed after takeoff even though the APU was still running.

Kevin, you're right, but I suppose that Mark was talking about the support team avaiable to check, over an year after the release, and eventually fix errors that are not so important for the product. This demonstrates, one more time, that the team wants to stay at the higher level they can!

Well done team!

Kevin, you're right, but I suppose that Mark was talking about the support team avaiable to check, over an year after the release, and eventually fix errors that are not so important for the product. This demonstrates, one more time, that the team wants to stay at the higher level they can!

Well done team!

I agree it's good that PMDG are still concerned to get it right wherever possible. Though I certainly hope they would still be supporting their most recently released product. But once SP2 is released I expect further such changes will be unlikely.

 

This minor issue shows up the fact that design specifications aren't always kept up to date as the design develops. So they aren't always reliable documents.

I'm fairly certain :blush: that if the IDG's are off line (two blue GEN OFF BUS lights) after takeoff, they will not connect automatically to the buses unless Boeing has modified the system in the last 5 years. The APU should continue to power both buses after takeoff until each IDG is manually connected to it's bus.

A big clue that something's not right (besides not looking up at the overhead panel), is the call from the

flight attendant saying there's no galley power (load shedding) and you look up on the overhead panel and see the galley switch is already on! Whoops!

Another early clue if you pick up on it as the airplane lifts off, you might hear some suttle electrical contactors switching behind you in the E&E compartment, but it's not automatically connecting the IDG's to the buses. That has to manually be done.

I'm fairly certain :blush: that if the IDG's are off line (two blue GEN OFF BUS lights) after takeoff, they will not connect automatically to the buses unless Boeing has modified the system in the last 5 years. The APU should continue to power both buses after takeoff until each IDG is manually connected to it's bus.

A big clue that something's not right (besides not looking up at the overhead panel), is the call from the

flight attendant saying there's no galley power (load shedding) and you look up on the overhead panel and see the galley switch is already on! Whoops!

Another early clue if you pick up on it as the airplane lifts off, you might hear some suttle electrical contactors switching behind you in the E&E compartment, but it's not automatically connecting the IDG's to the buses. That has to manually be done.

Hi John

 

There is automatic closing of both GCB's that connects the IDG,s to the busses due to when after takeoff the APU generator is still the only power source on the airplane, So when you shutdown the APU in air mode without connecting the IDG,s to busses, there is a one time automatic power transfer from APU generator to the IDG's, This automatic transfer system exist's since the beginning of the 737 next generation. The 737 present generation didn't had this automatic transfer system because the 737 PG did not had the CDS system installed.

 

The 737 NG have the CDS system installed, one of the problems of the CDS system when there is a power interrupt of more than few seconds the displays can go dark and it can take up to 3 minutes for the indications on the displays can reappear, you don,t want to have dark displays during takeoff.

That's why Boeing developed this automatic power transfer system for the 737 NG in a event the flight crew forget to transfer power from APU generator to IDG's after takeoff and they shutdown the APU.

Thanks Mark. I agree. I didn't read the entire post the first time. :blush:

I see now that he had turned off the APU.

It is interesting to note that the FCOM describes the system differently than the engineering documents. The FCOM appears to indicate that the airplane will stay on APU power until the APU generator is lost (through failure or shutdown) but our model does not currently account for this since we are transferring power at rotation.

The FCOM is correct. The system design takes a least risk approach which makes perefect sense to me. Automatically transferring the power source to the IDGs on rotation would add unnecessary risk and distraction at a critical point in the flight. I would not want the distraction of the change over with screens flickering (my primary airspeed reference), lights, blinking on and off at such a ciritical stage of flight for such a minor oversight.

 

There is no hurry, the system is good to 41,000' with the APU powering the transfer busses. Best to keep the pilots in the game by letting scan flows and airmanship deal with the error when height, speed and configuration are in the crew's favour

I tried this last night in the sim and I believe the functionality is working as it should be. I intentionally did not connect the generators during my after start flows and left the APU running and connected. I taxied and took off, and...nothing happened, just as it should have. The APU remained the power source as evidenced by the AC/DC Metering Panel. I then switched the APU off and again, seemingly nothing happened as all of my power remained. I again checked the Metering Panel and although I hadn't manually switched anything, the Engine Generators were then supplying full pwer.

 

So clearly, the system functioned as it was supposed to in my case. The engine generators were automatically connected only when the APU was shut down AFTER take off. I can only assume that if I had shut down the APU while still on the ground, I would have reverted to battery power.

 

Back to the O/P's issue...I have had instances where during taxi/takeoff roll, my aircraft will "bounce" or "jump" briefly. I think this is an FSX bug (also FS9, FS8, FS7, etc...) but I wonder if maybe that happened and caused the Air/Ground sensor to momentarily trip and therefore deactivate the automatic switching. Since the aircraft briefly becomes "airborne" during these jumps and the NG system is set up to arm only once, I could certainly see this being the reason why the O/P reverted to BAT power when switching the APU off in-flight.

I tested again, the ngx automatically switchs to the engine generators at lift off, so, probably we will see it fixed in sp2...