Expert: D. Norkus - 3/6/2007

Question
Hi Dottie,
When you are approaching class B,C, or D airspace and you are required to contact approach control "within 20nm", does this mean that you call them 20nm's before entering that airspace?  Or is that how far out from the airport that the airspace begins?  
Also, if you are navigating using VOR/DME without GPS, and there is not a VOR station on the airport, how would you know when you are within a certain mileage of the airport?  
And lastly, if the outer ring in class C airspace indicates 50/22, as long as I remained below 2200ft I would not have to contact ATC, correct?
Thanks, Dan

Answer
Dan

When approaching controlled airspace in which you need a clearance to enter, you should call them up at least 20 miles from the perimeter of that airspace. This gives the controller advance notice of what you want so they can work you into their traffic flow. If they cannot, that is when they tell you to remain outside the airspace.

You can determine the distance, or about where you should make your ATC request, by using your handy flight plotter. Line the end up on the edge on the B/C/D airspace and then see where the 20 mi mark falls (be sure to use sectional scale!). By dead reckoning, you can find this point on a chart, no VOR or GPS needed. If you do happen to find a VOR or two nearby, even if one is not located on field, then see what radial seems to fall across the airport and again use your plotter to estimate distances. You can then double check your dead reckoning or even triangulate a fix if you have two navaids.

In your example you said "50/22" on the class C marker, meaning 5,000 ft above the horizontal line and 2,200 underneath. Not knowing what airport to which you are referring, I cannot say for sure you wouldn't have to call for clearance. Other controlling airspace may lay under or around that if you are in a congested metropolitan area but generally speaking, (or if you are sure there is no other overlapping airspace) then yes as long as you were to remain above 5,000 or below 2,200 you would be clear to pass under.

Dottie