ATC Q&A

The following ATC questions and answers are from the podcast and website www.atccommunications.com 

This is a very informative web site and gives great insight into communicating with ATC, VFR and IFR.  I am posting some VFR Q & A here.  Check out Jeff Kanarish's web site for more info.

Question: 

To provide traffic advisories and alerts, ATC has to know 3 important pieces of information about your aircraft.

First, ATC needs to know who you are. This is your call sign, of course. Second, an air traffic controller needs to know where you are. Your position includes your aircraft’s location over the ground and your aircraft’s altitude. Third, ATC needs to know where you are going so the controller can project ahead to see if your flight path might conflict with another airplane’s flight path.

Once your aircraft is in radar contact, your aircraft’s identity and location will be updated each time the controller’s radar sweeps across your aircraft and interrogates your transponder. Your flight path is also tracked by radar so ATC can project where you are headed. Mode C on your transponder updates your altitude. When you switch from one controller to the next, your transponder ensures the new controller receives all of the same information provided to the previous controller. Here comes your question: If radar updates your aircraft’s identity, location and flight path, and your transponder’s Mode C reports your altitude to ATC, why does ATC require you check in with each new controller by stating your current altitude?

Answer:

When already established in radar contact with ATC, you do not have to state your current position over the ground when checking in with a new controller. Your current position is continuously displayed and updated by ATC’s radar system as long as you remain in radar contact. You do have to check in with your current altitude even though the new controller has your current altitude displaying on his radar display. Here’s why.

Your transponder’s Mode C system always transmits altitude based on the standard altimeter setting of 29.92 (1013 Millibars in most countries outside of the U.S). Mode C has no idea what the local altimeter setting is where you are flying. Fortunately, when ATC’s radar system receives your transponder’s Mode C altitude readout, ATC’s system compensates for the difference between 29.92 and the current local altimeter setting before it displays your aircraft’s altitude on the controller’s radar screen. If you do not have the current local altimeter dialed into your cockpit’s altimeter, then you may be looking at one altitude readout in your cockpit and the controller may be looking at a different altitude readout for your aircraft on his radar display. Since ATC often separates aircraft using vertical spacing between aircraft, it is critical that both you and the controller agree on your current altitude.

If the altitude confirmation you give on the radio when you check in differs by more than 300 feet from what the controller sees on his radar display, the controller is going to first give you the local altimeter setting. If that does not fix the problem, he will note the error in Mode C reporting and may ask you to switch off Mode C on your transponder.