On the default 737 the airspeed top right of the compass rose is always the same as the GS in the GPS.However, when approaching the ILS, it is necessary to flick the lever from GPS to NAV, and when this is done the airspeed changes to something completely different, and I need to have the GPS open to see what my actual airspeed is.Two questions:1) Why does this happen, and2) What does the new airspeed represent?The difference between the GPS speed and the NAV speed is significant, and I can't see that the 'new' airspeed under NAV relates to anything....I've been wondering this for ages, so looking forward to some enlightenment.ThanksAllblack

Thanks Ryan, that was a great help.But this is where I start getting lost....You (and others) say the relevant speed is the IAS on the 'round dial'. Let's say for arguments sake the approach speed three miles out for a 737 with flaps at 30 is 140kts. I can come in with no wind at 140 showing as the IAS and all's well.But what happens when I'm heading into a 30kt wind? The IAS will show 140, but my ACTUAL speed will be 110kts. Thirty kts below the 'recommended' approach speed. Conversely, the same would happen with a tail-wind, but my approach will be way too fast.Isn't this where you need to look at the GS on the GPS to see the difference between the two, and make sure the IAS is increased or decreased in order to have the approach at 140kts????Assuming my reasoning is correct (huge assumption!!), I don't understand your comment that "the grounspeed is not important at all". Clearly you sound like a man who knows what he's talking about, so maybe you can put me straight.CheersAllblackNZ: 27South Africa 15

HiJust my two cents here. GS IS not important. Not even at approach. The important thing is to have enough air speed for the wings to provide lift. You can land in a 90 knot head wind but you still have to maintain 130 knots IAS to get your wings to induce lift. As a result the approach will take somewhat longer. Your GS will be 40 knots. You sure have a lot of time to align yourself! :DRegards

Ryan, thanks very much.I've copied and pasted into a word document, which I'll read at my leisure.I might come back with a ? or two, and appreciate your time in explaining this.CheersAllblack