FMC versus moving map

[hanspetter]

In FS we're spoiled by having access to moving map GPS displays for any aircraft. Even if the panel in question doesn't have a button for GPS one can always put one there or invoke the FS GPS through the instrument view drop-down window. While small aircraft tend to have a GPS these days it seems that airliners use the "less interesting" FMC display that doesn't show any terrain.

For proper navigation you enter legs to waypoints and your final leg will be the four letter airport code of your destination. This provides you with a red (or, green) line to track to your destination. If the AP is used in FMC mode it tracks the course at the altitude you've entered and may even control the speed. I tend to set up the flightplan and track the leg manually, using ALT and heading hold for longer legs.

So, the practical difference between an FMC and a GPS is that the first one shows no terrain. A map is more interesting to look at and you get navigational cues as mountain ranges and coastlines.

Do modern airliners have a colorful GPS display in addition to the FMC? Strictly speaking it may not present additional useful information for standard airline flights while the GPS is invaluable to bush pilots who may be flying low over rugged terrain with no navaids.

As FS flightsimmers a lot of us have been accustomed to using the GPS to view entire continents as you start out and zoom in as you get closer to your destination. I assume that real pilots take a more myopic view and concentrate on trailing the track line whether they use an FMC or a GPS moving map.

[Tom Goodrick]

Let's consider what the differences are between a Flight Management Comuter or "FMC", an autopilot, and a GPS Navigation Computer (which is what the Garmin 530 or 500 in FS really is). In any aircraft that does not have an FMC, the autopilot and the GPS nav computer are separate items though they can be made to operate together. In an aircraft with an FMC, I think the autopilot is integrated into the FMC.

The FMC does everything for you and you just sit back and watch it work. You enter your flight plan. Your flight department has already entered Company preferences as to how that type of aircraft is to be flown. You line up with the runway, turn on the FMC and the aircraft goes. You stand ready to abort if needed. The FMC controls the aircraft - raising the gear, raising the flaps, flying the departure path and profile, getting set on cruise and performing cruise to the point for descent, perforing the descent and getting on the approach. You can can make the landing. The FMC obviously either contains the autopilot or controls it and also controls the autothrottle system. It may show you the route on the moving map. It may use the GPS for direct flight or, more likely, uses the GPS to fly established "jet routes" as noted in the flight plan.

The autopilot, as we are find in FS, can manage the flight after takeoff and before landing with our input to control the gear, flaps and throttles. We can couple it to the GPS with a list of waypoints. It will then fly the waypoints at whatever altitude we set.

The moving map of the GPS shows where we are and shows the path we have entered for the flight. The actual Garmin GPS 530 looks the same but does many more functions. It can switch through the various departures and approaches, it computes and displays ground speed and speed and direction of the wind the aircraft is experiencing at any moment. It will adjust the heading when enroute so that the aircraft compensates exactly for wind drift and flies the path exactly. There is no reason to use manual heading vectors when enroute unless there is a big blue prohibited or restricted area you must fly around. Otherwise, let the coupled autopilot do its superior job. I do use manual vectors to the autopilot to fly the departure and the approach and I switch to manual before entering the final approach.

I do not think you can make an automated takeoff in FS even if you have a box labeled "FMC." I don't know anything about FMC devices in FS and am not interested in finding out. Its no fun if you can't do something to help the aircraft go from point A to point B. But I read pilot reports about flying new airliners and it is mostly about setting up and turning on the FMC.

[hanspetter]

I don't think the FMC and the AP are integrated into one unit, at least not in the sense of making manually controlled AP assisted flight impossible. While I was trying to read up on the use of FMC I noticed that there was a warning not to turn it on until you're airborne. In other words, don't allow the AP / ATC to start climbing before you have left the ground and have attained sufficient speed.

Further, even when you have an FMC you don't have use it. The AP has all the usual modes that can be set manually.

Autopilots are also different in different aircraft. In FS we have an autopilot that will climb / sink the moment you enter an altitude and click "hold" and then keep doing so until the altitude is reached. Other APs lock unto the current altitude the moment you click "alt hold" with no regard to what number you've entered. Then you may switch to vertical speed mode to climb or sink, define the vertical speed and click alt hold again when you have reached your desired altitude.

I don't know whether the FMC will deploy flaps and lower / raise the gear for you. However, flap overspeed will be avoided automatically in some jetliners by automated retraction of the flaps if you exceed the limit / refusing to deploy them. Then, if you've set 3 notches of flaps and lost one due to an excursion into overspeed it will redeploy once you get down to the speed range again.

Those who fly or sim for fun will hardly love an FMC-controlled night flight between continents but I like to understand how it works.