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Chapter 6 - Federal Aviation Regulations: How FAR Can We Go? 6-47

visible to ATC surveillance systems). In addition to keeping it turned on, you must also operate it in Mode C (the alti-
tude mode) if you are so equipped.
You are probably wondering, “What do I do if my transponder, or Mode C, fails in flight or fails at some intermedi-
ate stop on a cross country flight?” Well, fear not. ATC can waive the transponder requirements, regardless of the air-
space in which you’re flying. ATC can honor this request at any time. Simply call the Center or approach controller
with your request. If, however, you don’t have a transponder, ATC wants at least a one hour notice before they
approve a flight in an area requiring a transponder. In other words, if you want to depart the primary airport in Class
C airspace, you should call the tower at least one hour before departure and request a waiver if your aircraft is not
transponder equipped.

FAR 91.225—Automatic Dependent Surveillance-Broadcast (ADS-B) Out equipment

Mode C or S Transponder and Equipment Meeting
The Requirements of ES/ADS-B and TIS-B

Required In
ADS-B (OUT)
Class E Yellow, Blue & Magenta Areas Class E

No ADS-B (OUT)
Mode C Veil Mode C Veil
Required in Hatched Area

Fig. 66

ADS-B or automatic dependent surveillance broadcast is cutting edge technology that is already present in the cock-
pit of many airplanes. It’s a revolutionary new way of identifying and controlling air traffic. The concept is simple.
Here’s how it works. (Here the term ADS-B implies both “in” and “out” capability. I’ll discuss these shortly.)
Unlike radar, which bounces radio waves from ground-based antennas off airborne targets and then captures the
reflected signals for interpretation, ADS-B uses conventional Global Navigation Satellite System (GNSS) technology
and a relatively simple broadcast communications link as its fundamental components (Figure 65).
The regulations now require all aircraft (with the exception of those air-
craft not originally certified with an electrical system) to have ADS-B HOW ADS-B WORKS
“out” when operating in the following areas (Figure 66).
•Class B, and C airspace (operations in Class A airspace require a
Mode C or S transponder meeting the requirements of Extended
Squitter ADS-B and TIS-B);
•Class E airspace at or above 10,000 feet MSL, excluding airspace
at and below 2,500 feet AGL;
•Within 30 nautical miles of a Class B primary airport (within the
“Mode C” ring found at some Class B airports);
•Above the ceiling and within the lateral boundaries of Class
B or Class C airspace up to 10,000 feet MSL;
•Class E airspace over the Gulf of Mexico, at and above 3,000
feet MSL, within 12 nm of the U.S. coast (not shown in Figure 66).
If for some reason, your ADS-B “out” unit stops working while
enroute, you can request a deviation from these ADS-B require-
ments at any time to proceed to your destination airport, includ-
ing any intermediate stops, or to proceed to a place where suit-
able repairs can be made (vs. unsuitable repairs). However, if
you don’t have an ADS-B unit or your unit is inoperative, then
your request must be made at least one hour before the pro-
posed operation. These are the same deviations permitted with
Mode C and Transponders failures. (We’ll cover more on ADS-B in
Chapter-9 and Chapter-17.) Fig. 65