What is RNAV? And IRS

[Topper187]

What is Rnav, i.e. Is the just the combination of LNAV and VNAV? or something more.

Whilst you are at it could someone explain the inetial reference system to me. I totally understand the INS(inetial navigation system) so how does it differ from this??

Thanks

[sudden Winds]

RNAV stands for Area Navigation.
The most common form of RNAV is GPS, but I think you´re asking about what´s 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.
it´s real useful, but now, with GPS, vortac based rnav is becoming less popular. Some old Rnavs won´t 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/c´s 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 that´s 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 don´t notice the plane modifying its course in response to the upgrade, it´s 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 it´s 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. There´s 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 won´t 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.

[TR4A]

I believe the big difference btween INS and IRS is IRS uses a Flight Management Computer (FMC). The newer aircraft like the Next Generation B737 (-700, -800, -900) use GPS for updates.

[Intruder]

To expound a bit on Drag Bucket's explanation (which is quite adequate for most purposes):

IMU -- Inertial Measurement Unit
INS -- Inertial navigation System
IRS -- Inertial Reference System

GPS -- Global Positioning System

RNAV -- Area Navigation System

LNAV -- Lateral Navigation
VNAV -- Vertical Navigation

FMS -- Flight Management System

The various Inertial* stuff are various names appended by various vendors to various means to navigate via a "stable internal reference" around the world. The "system" may consist of 1 or more (usually no more than 3) IMUs, which may be the "old" mechanical gyroscopic systems or the "new" RLG (Ring Laser Gyro) systems.

RNAV is any system that allows you to navigate directly from any randomly-selected "waypoint" to another. The "old" RNAV systems used VOR and DME to define the waypoints; the newer INS and newer yet GPS systems use Latitude and Longitude.

The FMS is a system of computers that integrates the position information available from the INS/RNAV/GPS system(s) with the performance information from the aircraft. It provides, among other things, the functions we call VNAV and RNAV that attempt to best integrate the airplane's performance characteristics with where we want to go in 3 dimensions.

All the alphabet soup plays in the equations, but the equations are different for each airplane...

[mrdaz]

Also in answer to your question the differences between IRS & INS is INS being the older system which was used on the Apollo missions did not use VOR or DME signals where as IRS does in conjunction with the FMC.

[Topper187]

All this within 20 Hours. Thank you very much...