If requested by ATC, pilots are allowed to turn off the transponder.
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A transponder is not required unless an aircraft is operating: In Class A, Class B, or Class C airspace. Above 10,000 feet Mean Sea Level (MSL), excluding airspace below 2,500 feet Above Ground Level (AGL).
If it's failing and/or on fire, you want the power off. Two: operationally, it is sometimes necessary for ATC to request planes on the ground turn off their transponders to declutter the radar.
A Squawk 7700 indicates an emergency. This can be an emergency of any kind. Pilots may input it into the transponder themselves – or when instructed to do so by ATC. As a result, ground control will know that the aircraft is dealing with a serious issue and needs help.
A Primary Radar System.Typically located on the control tower or other strategic location on the airport, the Primary Radar antenna is able to detect and display aircraft that are not equipped with or have malfunctioning transponders or ADS-B.
Two-way radio communications and operational transponder are normally required for operations within Class C airspace, but operations without radio communications or transponder can be conducted by LOA, facility directive, or special arrangement with Class C airspace controlling facility.
All transponder-equipped aircraft, both VFR and IFR, must have their transponder tested and certified every two years (see FAR 91.413). Aircraft operating under IFR must also have their altimeter(s), encoder and static system tested and certified every two years (see FAR 91.411).
Squawking 7700 in an emergencyThe most well know of these is the code 7700. This is used to indicate an emergency of any kind. A pilot will enter this when in an emergency situation - either instructed by ATC after declaring an emergency or without communication if there is no time.
FlightRadar and similar websites use several sources for aircraft detection, most commonly they use ADS-B. Not all General Aviation aircraft, to which helicopters would count normally, are equipped with transponders that are capable of emitting ADS-B data.
Websites such as ADSBExchange.com, FlightRadar24.com and FlightAware.com aggregate flight data in the United States and abroad using a mix of commercial and citizen-owned sensors that capture the movements of commercial, civilian and military aircraft in real time, 24 hours a day.