Hi. I would like to ask why the 737NGX has an Align Mode on the IRS switch? According to the Oerations Manual V2 Chapter 11.20.5 the selector switches should be moved from OFF to NAV to Align the IRS. It then says ALIGN should be used for fast turn around at airports as it does a quick Alignment.But I would like to also ask,if the IRS as already been setup for its original flight (first flight of the day ) the IRS would stay Aligned through the FMC after completing its first trip,or do pilots in the real world Align the IRS after every trip? Hope that makes sense lol. Tony

I believe that SOP is to do the full alignment when time allows. If there is not enough time then you can do the fast alignment. In any case IRS alignment should be completed prior to every flight.

Quick align does two things. Primarily after a leg is complete it is used to clear out accumulated "lazer Drift". It is also available as an emergency "quick align" in the event that of total alignment loss in flight. You can fly straight and level and do a quick align to regain attitude reference, but you are then left with degraded abilities. You don't have full navigation capabilities but at least you know which end is up.

I have a question. So the quick align method can't be used when IRS is completely misaligned (example when you choose cold and dark). Because I have tried this way and the align button never extinguishes nor does the IRS align.

The full alignment process is used to get all 3 lasers ring gyros in each individual unit aligned with true North, true heading, true vertical and actual latitude. It does this by actually measuring the pull of gravity and the rotation of the Earth (!) as they influence the lazier beams internal to the gyro. Try to visualize spraying a very narrow stream of water from a garden hose. if you move the spray nozzle in your hand the stream of water will follow the nozzle but it will slightly lag behind it, right? And the faster you move the nozzle the faster the water will lag behind. The lazer ring is doing pretty much the same thing, except it is a narrow beam of light instead of water and the lag is much much smaller, but it is still there. The movement during the alignment period is actually caused by the earths rotation! The further from the equator you are, the smaller the latitude circumstance is, and the less movement is detected. Thus, by measuring the movement over a set period of time, the unit can determine exactly what latitude you are sitting on, and exactly which way is straight down. That is why the alignment takes a specific period of time, it is so that these very precise measurements can be made. The quick align does away with these and just measures up and down, that's it.