RNAV stands for Area Navigation.
The most common form of RNAV is GPS, but I think youīre asking about whatīs called VORTAC Based Area Navigation, which is basically a computer that allows you to create waypoints or phantom stations by entering a radial and a DME distance and navigate directly to them.
itīs real useful, but now, with GPS, vortac based rnav is becoming less popular. Some old Rnavs wonīt tell you what your ground speed and time to the waypoint is, others will, always read the manual...also, some of them show lateral displacement in nautical miles as opposed to degrees. That way sensitivity stays the same the whole time.
IRS is a set of 3 IRUs (inertial reference units) that use accelerometers, laser 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. When a route is active, say from ESSA to EHAM, the autopilot will fly that route, as long as itīs been activated (LNAV button depressed).
LNAV uses info from the IRS.
VNAV stands for Vertical Navigation. You enter a route, wind conditions, weight, and a cost index, and it comes up with a vertical profile. It tells you the optimum and MAX FL for the conds entered, and a speed (mach number) that is directly related to the cost index. Thereīs a good post about cost index I recommend you read.
Forgot to say, optimum and max FLs will increase during flight as fuel is burned and weight decreases.
VNAV calculates your Top of descent, end of descent and gives you tons of useful info. Also you can program it to reach a certain position at an altitude and speed, and the computer will try to do it. It might ask you to increase drag, if it determines it will overshoot it unless a higher rate of descent is achieved without increasing speed.
Once you start using VNAV LNAV you wonīt want to come back to fossils...I guarantee that.
This is just the gist of it...for detailed info search the net. I found cool stuff about the subject.
FMC, IRS users pls care to comment.