Expert: D. Norkus - 8/10/2005

Question
Could you please provide me with a better insight on the difference between MSL and AGL. In what context are each used and is there a way to remember when to use what?
Also, the same for TAS and GS

Thanks

Answer
Not sure if it's 'better insight' (as both are just standard definitions) but MSL starts at the surface of the ocean while AGL begins at the surface of the land where the  measurment is taken. AGL may start several hundred or even thousand feet above MSL. Airports are always measured in MSL while cloud reports are given AGL.

TAS & GS: TAS is what you read off the surface of your instrument corrected for nonstandard temperature & pressure (fancy a/c will do that for you and display it). TAS could indicated 100 kts and in a no wind situation you will also be moving across the ground at 100kts. Add in a head wind, like 20 kts, and you will still indicate 100kts but you will only be moving 80 across the ground. Kind of like a boat in a river: the current is carrying you downstream at 5 kts even if the motor is taking you upstream at 15kts. Your net gain upstream is only 10kts.

Hope this can help a bit. I am off on a trip now and will be away for two days.

D.