IAS is the displayed speed as the one you really want- TAS- you don't have an indicator for. A jet may indicate 290kts IAS, but be flying at 480kts TAS which is its 'correct' speed. Knots are frightfully brilliant for navigation purposes- when you are navigating using dividers and chart, you just measure off your distance on the chart (always in nms), and the speed of the aeroplane, and that's that. Hopefully we will convert Europe first to Imperial units (after all Napoleon did lose didn't he?), then stop everyone using those absurd Statute miles. Eventually we can make it a capital offence to do anything in kilometers- surely a totally useless unit- and then we can start working on getting the world converted to pounds and ounces.