How long does an IRS take to align?

[greengage22]

This is really addressed specifically at operators of the Boeing 737-400, but I can't imagine that the B747, 757/767, or 777 are very different. Airbus might be.

There is a JAA exam question which asks how long the B737-400 IRS takes to align. I take this to mean from the moment you select ALIGN or NAV.

The candidate is asked to choose from between 9 minutes and 12 minutes.

Now, my view is that it will depend on the latitude because, the higher the latitude, the less Earth Rate there is to detect. However, I would expect that, at ther same latitude, the time taken to align for a particular system would be very consistent day to day.

My question, to all you Boeing operators, is - At temperate latitudes, is it nearer 9 or 12 minutes? I can't find any references.

Any help from the sharp end gratefully received.

[Lynx]

10 Mins for a B757 Honeywell IRS in Temperate Lats.

But really, what a stupid exam question. Who cares if it's 9, 10 or 12 Mins? As long as it's not 45 Mins, when it's aligned its aligned - no one sits there with a stopwatch.
I see the JAA exam questions are no better thought out than the old CAA ones.:mad:

[Cornish Jack]

Generally quoted as 10 minutes for temperate latitudes increasing to 17 to 18 minutes for latitudes 78 and above.

[Sex Kitten]

In real life 11 mins but the status countdown window on the IRS module only shows the count from 7 mins to align, but if you time it, it takes four minutes to change to 6.
So I'd go for 10mins if I was doing the exam.

[ICT_SLB]

Litton LTN-101 FLAGSHIP (CRJ, Airbus) 7 minutes at "normal " temperatures. They were 10 minutes the same as the older LTN-90s. Note that the real arbiter is the temperature of the sensing platform itself. Once had to do a cold temp operational check - took 3/4 hour before the IRU came up.

Latitude has an effect as Cornish Jack rightly said but some IRUs won't even take an initial entry above 70 degrees N (once aligned they will work satisfactorily even flying circles around the pole).

[QAVION]

If you go from OFF to NAV, there is a simple timing circuit which lets the IRS align for set period of 10 minutes. For high latitudes however, you have to go from OFF to ALIGN and wait 17 minutes or more before selecting NAV. This will allow the IRS's to achieve the required accuracy for navigating in, say, Polar regions.

Cheers.
Q.