As an example, when setting my MCP speed at 210k, and establishing 210k as the speed, I show a ground speed of 225k in still air at 59 degrees F. This is at an altitude of 4,000'. Is it normal for there to be a 15k difference between IAS and GS at such a relatively low altitude?Second question: Do the speed limits (flap extension, 250k below 10,000', etc.) refer to IAS or GS? Obviously, for instance, there is quite a difference between the two speeds at 10,000'- I'd be going way over 250k GS at 10,000' if I refer to the IAS as target speed.Thanks for clarification.BTW, I'm using the POSKY 767-300ER with Lee's merge.

That sounds about right for a true airspeed vs. indicated airspeed. See http://www.csgnetwork.com/tasinfocalc.html for a calculator. I think you actually want to use pressure altitude (or density altitude?) because those are affected by temperature (and humidity). Of course, GS will also be seriously affected by winds.That speed restriction is definitely 250kts indicated. Same for the 200kts under class B shelf, etc. They couldn't really do the restriction on ground speed because not everyone necessarily knows that, and if there were a strong tailwind your airspeed may be insufficient for a given ground speed.Likewise, if ATC tells you to fly a heading, they mean a heading and not a ground track.Lee Hetherington (KBED)PS: Even putting around in a lowly C172 at 105kts at 4,500 feet, you're really going 130mph as the crow flies (ignoring winds)!

Lee,Thanks for the clarification, for the link to the calculator, and for all the work you've done to make PIC767 so much better than it already is!Chuck in KCAE (Columbia, SC)

As Lee has pointed out below, the difference sounds about right. You can use a fancy forumla or a flight computer ("whiz wheel") to accurately calcuate the difference. However, as a quick in the cockpit rough gouge, I was taught to use 4 x alt in 1000s) for the difference between IAS and TAS (back in the days when I flew aircraft that didn't have a TAS readout). Obviously if the atmosphere is far from standard then the forumla will be a little off. So for your example, at 4000 ft the TAS / IAS delta would be 4 x 4 or 16 knots. Pretty close to what you were seeing. This formula starts to break down at the higher alitudes (above 20000ft) but is useful down low. At higher altitudes a useful rule of thumb is Mach No. x 6 for a rough gouge on TAS.Since IAS is directly proportional to the airload experienced by the aircraft (similar to the force felt by sticking your hand out the window of your car as you are driving) you will see it used for all ancillary and other limits (gear, flaps, Vne etc). GS is only of interest for navigation purposes and perhaps performance purposes on an approach (min groundspeed techniques come to mind that some use for windshear mitigation). Similary, TAS is also mostly of interest for navigation purposes (although it will impact turn rates, etc, rolling inertia etc).All that to say is that most pilots think and use IAS (or Mach No at high alitudes) as a reference for most of their flying (an AOA gauge is better but not commonly used).Kevin in CYOW