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VPA481

IRS off then back while in flight

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Hi all, Technical question. Last night, I was cruising in the NGX, but due to a wrong movement, the electrical switches turned off. I immediatly turned them back to on. One of the consequences is that the IRS looked inactive, no more heading and IRS alert lit. I turned the IRS switches to OFF then back to NAV: in due time, the heading was back to normal, BUT, no more route displayed (VNAV and LNAV not available as consequence).Tried on the align mode, no more effect. I've looked for a solution, but did not find any. So, what would have been the process ? Suggestions are welcome.


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For any fault indication the QRH has the answer. In this case, section 11.9. It describes a method for restoring the IRS in flight.


Graeme Brown

 

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Well, it looks like even if the IRS is back, it says

Note: Autopilot(s) cannot be engaged.
. So manual flying and diversion to alternate Airport ...

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Switching off/lose both IRS while in flight keeps you awake on the night flights LOL.gif I have tried to only run on ATT position and the NGX is really easy to trim for level flight and you can just make your destination airport the alternate one :) BrgdsPatrik Soderberg

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For the IRS to align it needs to where it IS, not where it WAS. Hence, you can not be moving.


Kevin M. Manley

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I think if your AC power was just cut for a while, you should have waited for about a minute or so for the GPSes to come back online. Then your data will be restored. Apparently they need some warm up time after being cycled off. Your IRS will be covered by dc power so its still there. However, once you break the IRS using ALIGN or OFF, your never ever get them back in flight. (only on ground). The little NAV stuff is gone even if your GPS turn on after 1 minute or so. (NOTE!!, from my observation, may not correspond to real life). At the same time your FPS (with this current build), should drop heavily (although unnoticable in straight level flight), while waiting for your gps to come back. Ryan mention the FPS drop will be fixed. Marcus Jian

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Hi all, Technical question. Last night, I was cruising in the NGX, but due to a wrong movement, the electrical switches turned off. I immediatly turned them back to on. One of the consequences is that the IRS looked inactive, no more heading and IRS alert lit. I turned the IRS switches to OFF then back to NAV: in due time, the heading was back to normal, BUT, no more route displayed (VNAV and LNAV not available as consequence).Tried on the align mode, no more effect. I've looked for a solution, but did not find any. So, what would have been the process ? Suggestions are welcome.
What is the doctor's motto? First, do no harm? In the cockpit, sometimes things may scare you and you jump up from your seat. When you land, make sure you land on your hands. ;) The IRS were probably fine and you didn't need to switch them off. The right can run for 5 minutes "ON DC" power and the left will continue to run until the battery starts to die. If you followed the "Loss of Both Engine Driven Generators" checklist, it would lead you to your solution.

Matt Cee

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I made that mistake on a night flight. I was suddenly completely hopeless and wished I haden't gotten bored and started flipping switches! I had to resort to getting help from the fs default atc something I thought I would never do. Straight%20Face.gif

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You cannot realign the IRS in flight. There is a reversion mode - move the selector to "ATT" and will allow the attitude component of the IRS to realign, giving you attitude information on the PFD. In addition, you need to enter the current heading into the FMC.


David Zhong

 

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