N1 split in detent, ALT-TO1?

Mitch · 03-18-2008, 04:05 PM

The other day we were taking off and the first officer noted a 0.6% N1 difference between the two engines. Not just the actual N1, but the scheduled N1. Both engines were operating at the exact scheduled/commanded N1 in the detent, but they were 0.6% different from each other. Though not a major difference, and certainly not a safety concern at that level, it did get my curiosity going.

The FADECs matched themselves up with no problem after departure (shortly after setting CLB thrust,) but I had never noticed more than 0.1 or 0.2% difference before and thought the FADECs talked to each other to keep relatively equal thrust between sides.

I'm guessing that the FADECs were just getting slightly different data from somewhere and computing using that. It was windy, but not any more so than other days. There was no icing involved.

What is the maximum difference between sides that the FADECs are kosher with? No limitations come to mind, like I said, just curious.

(Don't have Vol 2 with me to look up everything handled on the inter-nacelle databus, and didn't immediately find anything on asymmetrical thrust margins in Vol 1.)

kidicarus5897 · 03-19-2008, 01:42 PM

Was the ITT a lot higher on the engine with lower N1?

Mitch · 03-20-2008, 10:43 AM

No engine parameters were significantly different and those that were were nowhere near where I would think they would become a factor. (They were all "normal.") As far as the ITT goes, I always thought that, in normal ops, it was the result rather than a contributing factor. Maybe my understanding of it is backwards, but doesn't the FADEC schedule the N1 and then see what the ITT is and only adjust if a limit is exceeded? And then wouldn't we get the ATTCS NO MARGIN message if the FADEC thought the normal ALT-TO1 N1 thrust was too high?

kidicarus5897 · 03-20-2008, 10:00 PM

yea, I was just pulling **** out of my *** trying to sound smart. I figure the fadec's would have a good reason for doing what it was doing. Maybe one fadec was on B and the other was on A providing different data like temp, N2 or flux capacitance. I would have aborted, blown the bottles and evacuated. You can never be too safe

Yariv · 03-25-2008, 12:44 PM

Quote:

Originally Posted by kidicarus5897

yea, I was just pulling **** out of my *** trying to sound smart. I figure .... flux capacitance. I would have aborted, blown the bottles and evacuated. You can never be too safe

LMAO

at the risk of pulling asterisks out of my asterisks, here are a few pages from the mx manual that make this crystal clear. (you'll have to right-click and rotate the view on some of the pages. sorry mac guys, you're SOL, read sideways and get a PC

)

since you didn't have any messages, it seems like the only thing that can be different is the sensed ambient temp from each ADC, which is not shared between engines during takeoff, and the difference in target N1 that resulted was within limits (the limit is not specified.)

Torch · 03-25-2008, 08:14 PM

Quote:

Originally Posted by kidicarus5897

yea, I was just pulling **** out of my *** trying to sound smart.

Yeah, I often do that too, except I just do I do it to shun F/O's. LOL

? · 03-25-2008, 08:15 PM

i think Mitch's man left him..............