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Originally posted by Freedom_for_sum
There is no reason, under normal circumstances, that a pilot would turn off the transponder during flight. Even when they aren't talking to ATC and are flying under VFR (Visual Flight Rules), pilots are required to "squawk" a code of 1200 (standard VFR code) so that ATC can separate IFR (Instrument Flight Rules) traffic from VFR traffic.
Pilots have been known to turn off their transponder, however, when they know they are flying somewhere they shouldn't be. They turn off the transponder so that ATC can't catch them doing something they're not supposed to be doing.
I believe one (or perhaps more) of the hijackers on 911 turned off the transponder to make it more difficult for ATC to track them. But even with the transponder turned off, ATC will usually get a "primary" return on their radar display which is basically a "dot" on the screen. But there won't be any of the information associated with it, such as altitude, that is normally provided by the transponder.
It's been awhile since I read this specific regulation; but I believe transponders are not required above 10,000 feet and while in airspace that's not congested (I'm purposely avoiding using reg-speak). But it's in every pilot's interest to use the transponder because it's a tool that improves safety.
[edit on 20-2-2007 by Freedom_for_sum]
1. 14 CFR Section 91.215(b) provides, in part, that all U.S. registered civil aircraft must be equipped with an operable, coded radar beacon transponder when operating in the altitude stratum listed above [10,000 MSL to 18,000 MSL] . Such transponders shall have a Mode 3/A 4096 code capability, replying to Mode 3/A interrogation with the code specified by ATC, or a Mode S capability, replying to Mode 3/A interrogations with the code specified by ATC. The aircraft must also be equipped with automatic pressure altitude reporting equipment having a Mode C capability that automatically replies to Mode C interrogations by transmitting pressure altitude information in 100-foot increments.