I confirmed the operation of this in the real plane yesterday. With the packs and bleeds on we showed an N1 of 89.2 for both engines. I switched the #2 bleed off and the right N1 changed to 89.8 and the left stayed at 89.2.

I just tried this today. Sitting at the gate, N1 was 91.1 with bleeds ON and packs OFF. I turned OFF a bleed, no change. I turned ON a pack, and the N1 dropped to 90.5.

That's because the NGX is checking whether the pack is running or not. It doesn't look at bleed switches and pack switches like the aircraft. Hopefully they will sort this out next update.

That's because the NGX is checking whether the pack is running or not. It doesn't look at bleed switches and pack switches like the aircraft. Hopefully they will sort this out next update.

I was in a real plane. N1 didn't change when I moved the bleed switch, only when I turned off the pack. IIRC, the ISO VALVE was auto. I didn't do all the switch combinations, but simply turning off the bleed didn't do anything.

 

I'll have to take pics next time, (if I can figure out how to post them).

N1 didn't change when I moved the bleed switch, only when I turned off the pack.

 

That's impossible IMO, because packs are fed by bleed air.

That's impossible IMO, because packs are fed by bleed air.

Yep, that's how I thought it should work, but that's what happened.

 

Let me expand a bit:

 

Sitting at the gate, on ground power. No APU or Engines running.

My Takeoff Performance Report called for 20k Reduced with N1 of 90.5.

The Takeoff page had 91.1 and noting the difference made me think of this thread, so I monkeyed with switches for a few seconds.

 

I only moved two switches while doing this - L Pack and L Bleed.

 

Moving L Bleed didn't change anything.

 

With L Pack Off, I saw 91.1 and with Pack On, 90.5 matching our Takeoff Performance Report.

 

I would think that the engines would say, "I don't care what you're doing with the PACK if I'm not bleeding," but that's not what happened.

 

I'll do it again tonight if I remember.

Yep, that's how I thought it should work, but that's what happened.

 

Let me expand a bit:

 

Sitting at the gate, on ground power. No APU or Engines running.

My Takeoff Performance Report called for 20k Reduced with N1 of 90.5.

The Takeoff page had 91.1 and noting the difference made me think of this thread, so I monkeyed with switches for a few seconds.

 

I only moved two switches while doing this - L Pack and L Bleed.

 

Moving L Bleed didn't change anything.

 

With L Pack Off, I saw 91.1 and with Pack On, 90.5 matching our Takeoff Performance Report.

 

I would think that the engines would say, "I don't care what you're doing with the PACK if I'm not bleeding," but that's not what happened.

 

I'll do it again tonight if I remember.

You tried with isol valv. at close position? maybe when isol valve is at AUTO, and you close left bleed, system automatically opens the isol valve and compensate for lack of pressure?

 

EDIT: no, actually this is nonsense, if you turn off bleed air, the valve is closed and air cannot go there, it can go only farther into engine

 

EDIT2: maybe, when you turn off pack and anti ice, bleedair automatically close valve to prevent over pressurizing of system, regardles of switch position, because, if you turnoff pack and anti ice, there is no way for air to escape and bleed lines would explode?

 

 

EDIT2: maybe, when you turn off pack and anti ice, bleedair automatically close valve to prevent over pressurizing of system, regardles of switch position, because, if you turnoff pack and anti ice, there is no way for air to escape and bleed lines would explode?

I don't think that's an issue. The Engine Bleed Valve is a PRSOV, so it regulates pressure.

Okay try it like this

 

With engines, PACKS and APU running and suppling bleed Air, and ISOL valve in AUTO

 

I get an N1 limit on both engines of 91.6

 

Now go into this configuration.

 

BOTH PACKS OFF

BOTH ENGINE BLEEDS ON

APU BLEED ON

I get an N1 Rating of 92.3 

 

Understandable as theres nothing running off the BLEED air off the engine so you get more thrust hence an increase in N1 LIMIT.

 

Now do this

 

BOTH PACKS AUTO

APU BLEED ON

ENGINE BLEED OFF

 

Notice here this is when things get fishy, with the PACKS running off APU bleed air alone, engine bleed air is off, the N1 Limit reduces to 91.6 (the original value with everything ON). One would think that because the engines are not supplying bleed air to anything at all the N1 Limit would increase from the 91.6.

 

Kick the APU and engine bleed off and leave the packs in auto (they wouldnt be running though obviously) and the N1 limit increases to 92.3.

 

So as above posters said, the PMDG NGX seems to be checking if the PACKS themselves are running and adjusts the N1 Limit accordingly instead of checking if the Engines are supplying bleed air to the systems

I don't think that's an issue. The Engine Bleed Valve is a PRSOV, so it regulates pressure.

I see, I'm not engineer and there is no way I can find technical documentation of 737, just trying to follow common sense logic.  :smile:

 

 

just trying to follow common sense logic.

See, that's your problem right there!

 

In my performance manuals, you get a TOW +900lbs bump for bleeds off but you get a 0.7% increased N1 for packs off.

 

So, you get better performance with bleeds off, as expected. You just don't see it in the N1. You see N1 change based on Pack operation.

I did get the N1 to change with the bleeds off today. I don't know what the difference was.

I did get the N1 to change with the bleeds off today. I don't know what the difference was.

Could have been a faulty switch contact in that aircraft. You can see the state of the switches, as the FMC receives them, on one of the CDU maintenance pages. At least you could on the classic, I assume the NG is similar.

When I did it in the real plane we had all three bleeds on, both packs on and the isolation valve open. We were set for a 22K takeoff. The N1 on both sides was 89.2. When I switched only the right bleed off the right N1changed to 89.8 and the left stayed at 89.2. I switched the bleed on then off three more times and got the same result each time.

I had intended to take some good pics last night with the CDU and the switchology, but we were about 15 behind schedule, so that didn't happen.

Air running through packs could be the culprit, from full power - 0.8%, fixed derate - 0.7%, assumed temp. - 0.6% difference. If air isn't running through packs an increase in N1 limit is displayed moving pack switches; otherwise if air is running the bleed switch change the N1 (maybe also pack if isolation set to close).

Air running through packs could be the culprit, from full power - 0.8%, fixed derate - 0.7%, assumed temp. - 0.6% difference. If air isn't running through packs an increase in N1 limit is displayed moving pack switches; otherwise if air is running the bleed switch change the N1 (maybe also pack if isolation set to close).

 

The FMC only senses switch positions to calculate bleed effect on N1 limits, it doesn't have airflow as an input.