Airspeed - GPS v NAV

[Guest allblack]

Hello.Why is the airspeed different when I flick from GPS to NAV when approaching the glideslope?I was barreling in to Zurich, had the selector on GPS, and my airspeed in front of me agreed (as always) with the GS on the GPS screen.So I'm about to hit the intercept, flick from GPS to NAV, and from there on the airspeed is always approx. 20 kts different to both the GPS screen, and what I see if I flick the selector back to GPS.Why?I'm finding that when I'm tracking the ILS, the NAV has to be selected, and I have to use the GPS screen to see what my approach speed is.Or am I just doing this all wrong??????Looking for inspiration.ThanksAllblack

[Guest Stratus_Fractus]

Most likely measuring your groundspeed based on the radio/ILS/VOR signal. Not too accurate, distances are not either, the distance is measured as slant-range.HTH

[Guest allblack]

........so, if this is 'real world', what's the point in it if the info. can be inaccurate?If you're crusing in on the glideslope (in fs2k2), how do you know how fast you're going?

[Guest guyjr]

Uhm, try the airspeed indicator? Indicated airspeed is a lot more important than your speed across the ground. It's the only way to know how close you are to stalling on approach. Obviously it would be bad to have a tailwind pushing you faster across the ground, but as far as keeping the plane aloft, actual speed across the ground is irrelevant.

[Guest Stratus_Fractus]

Allblack: The only point is for smaller airplanes without a GPS to get a rough idea how fast they're going over the ground. But guyjr is right, they will pretty much always use the airspeed indicator.