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CG TO trim calculation time

Started by ASCTU744, Wed, 15 May 2024 20:39

ASCTU744

Came across this video of a KLM 744 flight, from Schiphol to Sint-Maarten:
https://www.youtube.com/watch?v=9IjfeGAnhc4&t=217s

At 4:10 the captain selects the sensed CG, but it takes quite some time before the trim shows up.

Even the captain is suprised ;D

John H Watson

Was he surprised? He just said he will see that later on. I vaguely recall that the IRUs need to be aligned for the value to appear(?)

Jeroen Hoppenbrouwers

My wild uneducated guess is that the thing needs to be sure that the aircraft is completely stable. So it needs to see the indicated weights on the wheel sets stable, PLUS it needs to know the hull does not move in any axis. For the latter it needs the IRS.


Hoppie

John H Watson

Quote from: Jeroen Hoppenbrouwers on Thu, 16 May 2024 06:05So it needs to see the indicated weights on the wheel sets stable,

Even on aircraft without W&B sensors/computers, the trim will not appear until the IRUs have aligned. However, the aircraft still has a pressure sensor on top of the nose gear strut to confirm ballpark agreement with the computed Greenband range.

B767300

Full alignment or just longtitude and attitude (i.e. 1 min into the alignment). Because in PSX the CG value appears with a small delay prior to full IRU alginment.

Hardy Heinlin

As we know, during alignment the IRS is able to detect unwanted motion. There's even an EICAS advidory message for this case. So it's not quite clear to me why the detection of unwanted motion should be impossible before the alignment is completed ...

Or should the trim check be impossible when the IRS is OFF?

The trim computation delay in PSX is actually a simulation of the computing time of the real FMC; the delay is about 4 sec in the legacy FMC, and 1 sec in the NG FMC. This delay simulation is not related to the IRS alignment time.

ASCTU744

Quote from: Hardy Heinlin on Mon, 20 May 2024 13:30As we know, during alignment the IRS is able to detect unwanted motion. There's even an EICAS advidory message for this case. So it's not quite clear to me why the detection of unwanted motion should be impossible before the alignment is completed ...

Or should the trim check be impossible when the IRS is OFF?

The trim computation delay in PSX is actually a simulation of the computing time of the real FMC; the delay is about 4 sec in the legacy FMC, and 1 sec in the NG FMC. This delay simulation is not related to the IRS alignment time.

I guess that the delay in the video has nothing to do with the IRS, a temporary glitch maybe?