IRS Quick Align

[QAVION]

In certain circles, there seems to be some confusion regarding "quick alignment" of IRS's. Am I correct in thinking there are no shortcuts when it comes to the initial alignment of IRS's ? I.e. around 10 minutes is always required to get the IRS's into full NAV mode (but subsequent movement of the mode selector from NAV to ALIGN to NAV will provide a quick alignment/update suitable for short hops). Seems to operate this way on the aircraft I work on. Are all IRS systems the same in this respect?

Thanks.
Q.

[m&v]

Normally,depending on the'Latitude'it takes about 10-17mins. A 'quick'alignment takes 3mins on a station stop.

[QAVION]

"A 'quick'alignment takes 3mins on a station stop."

Thanks, but... shouldn't it only take a few seconds? (i.e. time enough for you to enter a new position...if you wish to do so).
You can of course put the selector from NAV to ALIGN after arriving at your destination and leave it there as long as you want... After a few minutes, the IRS's will assume that you are making a full alignment.. and start refining their alignment.

The above was not really my concern however. I needed to know if you can switch your IRS's from OFF directly to NAV, then do a quick NAV-ALIGN-NAV movement of the selectors to get the IRS's into full operating NAV mode (in say 30 seconds, rather than 10~17 minutes) and start moving the aircraft immediately(With your destination not being on the other side of a large body of water). I don't believe you can, but I just needed a professional/expert second opinion. Many airline manuals mention this NAV-ALIGN-NAV movement, but I haven't yet come across one that says you have to be in _full_ NAV mode before you do this.

Thanks!
Cheers.
Q.

[HighSpeed]

The reason why it takes so long during the intial IRS alignment is because the IRS has to determine the true north. It does this by sensing the rotation of the earth, thus as you go higher in latitude, it takes longer. If you exceed a certain latitude, you wouldn't be able to align the IRS at all ! The old gyro based INS does pretty must the same but the difference you have to get the gyro up to speed in the first place and it's restricted by it's gimbal travel.

Determining the true north is only half the problem. The other part of the IRS are the accelerometers. They basically worked on the good-ole pendulum principle to determine the rate and direction of travel. These accelerometers are the ones that are giving us most of the problems. Over time, they can give false readings thus causing 'drift'.
During a quick alignment, what we are doing is simply 'reset' the accelerometer drift to zero.

Hope this short explaination helps you understand better.

HS

[HighSpeed]

Q,

To answer your question regarding if you can switch the IRS from OFF to NAV then ALIGN and NAV, the answer is no, as i have explain above. Whenever power is taken off the IRS system, it needs to do a full alignment.

HS

[QAVION]

"Whenever power is taken off the IRS system, it needs to do a full alignment."

Thanks HS. That's exactly what I needed to know :)

And now, perhaps, a slightly harder question....

What part of the IRS gives you ATTitude ONLY mode (Are both accelerometers and gyros still required). I find it interesting to know that you can go from OFF to ATT in flight/motion and still get ATTitude. I've done this in real life, but this feature didn't seem to work in the simulators.

Thanks.
Cheers.
Q.

[HighSpeed]

Hi Q,

When you select ATT, you will 'deactivate' the accelerometer, thus losing your NAV mode. In ATT, the gyros still supply info to your EFIS.

HS

[PaulDeGearup]

Joining in a tad late, as usual, but a quick align on a stop gives you the chance to simply update the PPOS and selecting ALN from NAV and then back will zero the residual GS and drift on the systems.

[QAVION]

"When you select ATT, you will 'deactivate' the accelerometer, thus losing your NAV mode. In ATT, the gyros still supply info to your EFIS."

This is the part I have difficulty with... The manual states that:

"If alignment is lost in flight.... attitude information can be obtained by moving the selector to ATT. The IRU enters align mode for 30 seconds. This relevels the system and provides attitude displays on the PFD. For best accuracy, the aircraft must be in straight and level flight. Some attitude errors may occur during acceleration. After acceleration, errors are slowly removed"

If releveling and accuracy are affected by acceleration, then doesn't this mean that the acceleration is being measured? Doesn't attitude need some kind of earth centre sensing(like your old fashioned spinning gyros)...to know which way is down? Surely the accelerometers in the IRU's would perform this function?

Thanks.
Cheers.
Q.

[Cough]

Talking B737-345 here.

On turnarounds, I place the IRS to align, and enter the Ppos, and leave them there until the checks for the next flight. By doing this, if you look at the STS page on the IRS on the overhead panel, it displays 7 for 3 mins, and then counts down to 0. Now I presume this is it in the process of a full align, but you have the advantage that if the aircraft is to be towed for any reason you can bung it back to NAV so you don't lose your align. Hence you get a full align each time, but with a bit of flexability!

Coug..............h

ps I'm only guessing that its doing that!