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I am a long-time flight simulator player and also very interested in aviation. In FSX, all aircraft have a GPS/NAV knob which allows the pilot to switch between direct GPS autopilot and VOR navigation Autopilot. However, when I look at the main panel of a real Boeing cockpit, I don't see a GPS knob but I do see NAV V and NAV L buttons.

I know what NAV V and NAV L are (V for vertical autopilot, and L is the same as the NAV mode in FSX that keeps the plane on course), but do real aircraft (specially Boeing) have any function to keep the aircraft automatically on course and direct it toward the destination without using a VOR? Something that works very much like the GPS mode in FSX?

And in FSX I have to manually set the VOR frequency based on the navigation chart, in real life do the pilots have to do the same, or do they have some more advanced features?

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    $\begingroup$ Most real airliners have flight management systems which can be set to automatically direct the autopilot to fly the route programmed into the FMS before departure. $\endgroup$ – reirab Apr 9 '15 at 18:54
  • $\begingroup$ @reirab: So you mean that does exist in real aircrafts? and what is ADF? is it the same as what you explained? $\endgroup$ – Transcendent Apr 9 '15 at 19:15
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    $\begingroup$ An ADF is an Automatic Direction Finder. In its most common form it merely points an arrow on a compass card in the direction of an AM radio station or at a "NDB" (Non-directional Radiobeacon). Now considered primitive even obsolete in today's cockpits, the system of NDB/ADFs was a primary means of radio navigation for many years, and is still in wide use overseas, particularly in third world countries. There are still some approaches in the US that depend on the good old ADF - although a modern GPS can substitue for them. Many use them to listen to ball games on cross country flights. $\endgroup$ – Skip Miller Apr 9 '15 at 19:20
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In the Boeing world, LNAV is Lateral Navigation, and VNAV is Vertical Navigation.

When LNAV is engaged, the autopilot will follow the Flight Management Computer (FMC) guidance laterally, i.e. left & right. When VNAV is engaged, the autopilot will follow the FMC guidance vertically, i.e. climbs & descents.

LNAV is conceptually pretty simple: you have a magenta line on the map, your desired course, and the autopilot will follow it. The points defining your course are database points in the FMC that may but certainly do not have to correspond with VOR's or other conventional NavAids. You can use LNAV to fly VOR-to-VOR, or it can fly point-to-point-to-point equally easily. (You don't have to tune any frequencies in either case.)

VNAV is a little more complicated, because you may only be cleared for some of what the FMC wants to do vertically. Simple example: you're cruising at FL370, planning to start down at a given point so as to cross AAA at FL240, and then continue the descent to cross BBB and 10,000' and 250 knots. ATC clears you to "descend at pilot discretion, maintain FL 290." That means you can start down at whatever point you want, but until you get cleared lower later on, you can't descend all the way to FL240 or 10,000. Yet. Or maybe the controller needs to put you at FL350 right now to avoid a traffic conflict 100 miles ahead, and your clearance is "descend now and maintain FL350, pilot discretion FL290."

It's a lengthy section of Ground School how the pilot sets up the FMC and the mode control panel to accomplish all of this, but hopefully that illustrates some of the complexity of VNAV.

It's a good system; under typical conditions you can go into LNAV and VNAV shortly after takeoff and stay in them both until shortly before landing, and the FMC gives you smooth (mostly) and efficient guidance.

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  • $\begingroup$ Thank you very much indeed, you actually explained it all the best possible way. $\endgroup$ – Transcendent Apr 10 '15 at 3:25
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    $\begingroup$ LNAV and VNAV are not exclusive to Boeing/Airbus/airliners. I've flown G1000-equipped Cessnas and Columbias with the ability to have the AP handle LNAV and VNAV. It's simply FMS terminology. $\endgroup$ – egid Apr 10 '15 at 5:57
  • $\begingroup$ It should be added that the LNAV is a "GPS" mode except the FMC combines INS, GPS and VOR/DME input to determine the current position. A pure VOR/ILS mode is still available (and mainly used for ILS). $\endgroup$ – Jan Hudec Apr 11 '15 at 7:05

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