Post by SIR_PAGANINY on Mar 31st, 2009 at 10:46am
IRS (Inertial reference System) is very very important, if you notice you also have 3 autopilot masters...
Each IRS computes its own position independently; consequently they will diverge slightly during the course of the flight. After the alignment process is complete, there is no updating of either IRS positions from any external sources. Therefore it is important to set the IRS position accurately, If an IRS fails, the IRS transfer switch is used to switch all associated systems to the functioning IRS, uses accelerometers, l@ser gyros, etc, to determine the a/cīs position. They crosscheck their results against signals received from VORs and DMEs.
They compare the info they come up with against each other, and if one of them shows conflicting info the other 2 units kick it out.
When no ground radio station signals are received for more than 12 mins (I think) the IRS will announce "IRS NAV ONLY" alerting the crew thatīs the only source of navigation. After, say an 8 hour flight over the ocean, when it first picks up a VOR nav accuracy upgrades, but most of the time you donīt notice the plane modifying its course in response to the upgrade, itīs real accurate.
LNAV stands for Lateral navigation and VNAV for vertical , when a route is active, say from LPPR to EHAM, the autopilot will fly that route, as long as itīs been activated (LNAV button depressed).
LNAV uses info from the IRS.
Hope it helps! ::)