Irs

[Tan]

Hi Folks

I'm trying to answer this question for another;

"It takes about ten minutes to do a full align so why does the IRS only tell you that the time remaining is >7 minutes rather than telling you there are 8, 9, or 10 minutes to go? Makes no difference to the operation but I wonder if anybody knows the reasoning behind it."

[it will do the trip]

heard recently thru general discussion, with no proof to back up the answer that the first 3 mins are bite tests then 7 mins to align. Sounded good to me at the time.

[Tan]

Thanks I like your take on it better then mine. I thought the length of time that it takes for the IRS's to align, among other things, depends upon your location in relation to the poles. The logarithmic functions taking longer to compute or create a pseudo position for a reference point at the higher latitudes or something like that. Therefore the min time to align would be 7 minutes with the max time dependent on your location..

[QAVION]

If you're referring to the display, say, on the Navigation Display of a 747-400, the 10 minutes is simply a fixed time period from when the IRS selectors are moved to NAV (or ALIGN). There are no complex nav calculations being carried out in relation to the timer. Indeed, when you are operating from high latitudes, you have to force the IRS's to stay in ALIGN mode for at least 17 minutes (by selecting ALIGN and not NAV until the 17 minutes has elapsed). It would be necessary to measure this time period by something other than the IRS timer.

Re the start of timing period... I have seen operational and training notes to the contrary of what I have said (e.g the timing period begins after the PPOS has been entered), but after many observations of the countdown on the real aircraft, what I have said is true.

Sorry, I don't have any info on why the countown display shows 7+ and not 10.

Regards.
Q.

[mono]

I think you guys are all getting in too deep.

In my view (and I'm willing to be proved wrong) it's just like transponder, an octal display and saves a little space on the PCB.