IRS Alignment time

[Robert3512 21]

I've heard that it shouldn't take more than 10 minutes at the most for the IRS to fully align, but I was at PANC and it took 14 minutes! Is this a geological thing or is PMDG or P3D v4.2 to blame? If it is a geological thing, can someone explain it? If not, how do I fix it? 

[mmerelles 78]

IRS align time on the real life aircraft varies between 5 to 17 minutes based on airplane present latitude. From equator to 78 degree North/South

5 min at the equator (faster earth rotation speed) and you get longer times as you move outer from the equator due to slower rotation speeds

 

Since the earth does not rotate in FSX/P3D, i would speculate PMDG should be using some sort of formula to determine how much it will take on the real thing based on present aircraft latitude

 

 

note: if you do not want to wait for IRS "REALISTIC" times go to the FMC and change the setting. Should be under PMDG -> General -> IRS or so...

 

 

 

[Robert3512 21]

But what's the geological explanation behind this? By the way, the correct setting is PMDG SETTINGS >> OPTIONS >> SIMULATION >> IRS OPTIONS. 

[mmerelles 78]

you are welcome 

 

Sorry i am not at the sim now, not being using p3d for quite some time now so tried to remind the setting location from the top of my head.

Inertial systems are based on gyros by "feeling" earth rotation, because earth rotation produces very tiny movements it takes time until the system can determine its initial reference point.

If you want to know all the details and mathematics behind this, you may google it lot of info available and much better written than my poor english

 

Glad to have helped, i hope. Happy flying.

 

 

Edited March 10, 2018 by mmerelles

[JRBarrett 3,046]

1 hour ago, Robert3512 said:

But what's the geological explanation behind this? By the way, the correct setting is PMDG SETTINGS >> OPTIONS >> SIMULATION >> IRS OPTIONS. 

During the alignment process, the gyros sense the speed of the earth’s rotation from west to east. It uses that value to determine its initial latitude and heading. The speed of rotation is very slow the closer you get to the geographic North Pole. 

Imagine you are 5 feet from the North Pole. How far would the point you are standing move in one hour? If you drew a circle around the pole with a radius of 5 feet, the circumference of the circle world be: 2 x Pi x the radius - which is: 6.28 x 5 or 31.25 feet. If you divide that circumference into 24 equal parts: 31.25 divided by 24 equals 1.3 feet.

In other words, standing 5 feet from the North Pole, your velocity west to east as the earth rotates is a mere 1.3 FEET per hour - i.e. very slow.

At the equator, on the other hand, your velocity would be 1,037 MILES per hour. The earth’s circumference at the equator is approx 24,900 miles, so dividing that by 24 means any given point will travel to the east 1,037 miles in one hour. 

Since the amount of time that it takes for an IRS can “find itself” directly depends on the speed that the earth is rotating at the IRS location, the alignment time gets longer and longer the farther north or south one goes from the equator.

Practically speaking, an IRS system cannot align at all at latitudes much farther north or south than about 70 to 75 degrees.

[downscc 1,327]

Good explanation Jim, I'd like to add that the process Jim is referring to is the bias process that cancels out the motion of the reference frame (earth) when the aircraft is stationary within that reference frame.  This is why the aircraft needs to be stationary.  The IRS is not finding it's location on Earth, only determining how much bias is required to cancel Earth's motion in orbit and through space.  The alignment is complete when you enter the coordinates, and it then proceeds to calculate new position when the aircraft moves within the reference frame.