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| Well, first off, transponders are simply a piece of electrical equipment, and they can fail. Or the electrical system powering them can fail. It happened to another 777 just this week. http://www.business-standard.com/article/current-affairs/air-india-...
Second, they have to be able to be controlled by the crew to squawk different codes. Again-pretty plausible that the crew in a smoky cockpit or a decompression emergency could have inadvertently put the transponder in standby while attempting to enter an emergency code.
Transponder codes
Transponder codes are four digit numbers transmitted by the transponder in an aircraft in response to a secondary surveillance radar interrogation signal to assist air traffic controllers in traffic separation. A discrete transponder code (often called a squawk code) is assigned by air traffic controllers to uniquely identify an aircraft. This allows easy identification of aircraft on radar.[6][7]
Squawk codes are four-digit octal numbers; the dials on a transponder read from zero to seven, inclusive. Thus the lowest possible squawk is 0000 and the highest is 7777. Four octal digits can represent up to 4096 different codes, which is why such transponders are often called "4096 code transponders." Care must be taken not to squawk any emergency code during a code change. For example, when changing from 1200 to 6501 (an assigned ATC squawk), one might turn the second wheel to a 5 (thus 1500), and then rotate the first wheel backwards in the sequence 1-0-7-6 to get to 6. This would momentarily have the transponder squawking a hijack code (7500), which might lead to more attention than one desires. Pilots are instructed not to place the transponder in "standby mode" while changing the codes, as it causes the loss of target information on the ATC radar screen, but instead to carefully change codes to avoid inadvertently selecting an emergency code. Additionally, modern digital transponders are operated by buttons to avoid this problem.[6][7]
The use of the word "squawk" comes from the system's origin in the World War II Identification Friend or Foe (IFF) system, which was code-named "Parrot".[9][10]
Code Assignments
Beacon Code Allocated Use
0000
Shall not be used — is a non-discrete mode A code (Europe)[11]
Mode C or other SSR failure (UK)[12]
Should never be assigned (USA)[13]
Military intercept code (USA)[14]
Internal ARTCC subsets assigned by En Route Safety and Operations Support (Discrete codes only except for first primary block to be used as non-discrete if all discrete codes are assigned) (USA)[13]
0021
VFR squawk code for airspace 5000 feet and below — from 15 March 2007 replaced by the international 7000 code for VFR traffic (Germany)[15]
0022
VFR squawk code for airspace (above 5000 feet) — from 15 March 2007 replaced by the international 7000 code for VFR traffic (Germany)[15]
0033
Parachute dropping in progress (UK)[12]
0041-0057
Assigned for VFR traffic under Flight Information Services (BXL FIC) (Belgium)
0100
Flights operating at aerodromes (in lieu of codes 1200, 2000 or 3000 when assigned by ATC or noted in the Enroute Supplement Australia) (Australia)[16]
0100-0400
Allocated to Service Area Operations for assignment for use by Terminal/CERAP/Industry/Unique Purpose/Experimental Activities (USA)[13]
0100-0700
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
0500, 0600, 0700
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
1000
Instrument Flight Rules (IFR) flight below 18,000' when no other code has been assigned (Canada)[6]
Non-discrete mode A code reserved use in Mode S radar/ADS-B environment where the aircraft identification will be used to correlate the flight plan instead of the mode A code (ICAO)[11]
Used exclusively by ADS-B aircraft to inhibit Mode 3A transmit (USA)[13]
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
1100
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
1200
Civil VFR flights in class E or G airspace (Australia )[16]
Visual flight rules (VFR) flight, this is the standard squawk code used in North American airspace when no other has been assigned (Canada and USA)[6][13]
1201
Visual flight rules (VFR) glider operations for gliders not in contact with ATC, through February 2012 (USA)[17]
Assigned via FAR 93.95 for use by VFR aircraft in the immediate vicinity of LAX (USA)[13]
1202
Visual flight rules (VFR) glider operations for gliders not in contact with ATC; effective February 2012 (USA)[13][17]
1203-1272
Discrete 1200 series codes, unless otherwise allocated (for example, 1255), designated for DVFR aircraft and only assigned by FSS (USA)[13]
1255
Aircraft not in contact with an ATC facility while en route to/from or within the designated fire fighting area(s) (USA)[13][18]
1273-1275
Calibration Performance Monitoring Equipment (CPME) “Parrot” transponders (USA)[13]
1276
Air Defense Identification Zone (ADIZ) penetration when unable to establish communication with ATC or aeronautical facility (USA)[13]
1277
VFR aircraft which fly authorized SAR missions for the USAF or USCG while en route to/from or within the designated search area (USA)[13][18]
1300
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
1400
VFR flight above 12,500'ASL when no other code has been assigned (Canada)[6]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
1500
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
1600, 1700
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
2000
Civil IFR flights in Class G airspace (Australia)[16]
Uncontrolled IFR at or above 18,000' (Canada)[6]
The code to be squawked when entering a secondary surveillance radar (SSR) area from a non-SSR area used as Uncontrolled IFR flight squawk code(ICAO countries)[11]
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
2100
Ground testing by aircraft maintenance staff (Australia)[16]
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
2200, 2300, 2400
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
2500, 2600, 2700
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
3000
Civil flights in classes A, C and D airspace, or IFR flights in Class E airspace (Australia)[16]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
3100, 3200, 3300, 3400, 3500, 3600, 3700
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
4000
Civil flights not involved in special operations or SAR, operating in Class G airspace in excess of 15NM offshore (Australia)[16]
Aircraft on a VFR Military Training Route or requiring frequent or rapid changes in altitude (USA)[19]
Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[13]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
4100
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
4200, 4300
Internal ARTCC subsets assigned by En Route Safety and Operations Support (Discrete codes only except for first primary block to be used as non-discrete if all discrete codes are assigned) (USA)[13]
4400-4477
Reserved for use by SR-71, YF-12, U-2 and B-57, pressure suit flights, and aircraft operations above FL600 (USA)[13][19]
4401-4433
Reserved in accordance with FAA Order JO 7110.67 (Fed Law Enforcement) (USA)[13]
4434-4437
Weather reconnaissance, as appropriate (USA)[13]
4440-4441
Operations above FL600 for Lockheed/NASA from Moffett Field (USA)[13]
4442-4446
Operations above FL600 for Lockheed from Air Force Plant 42 (USA)[13]
4447-4452
Operations above FL600 for SR-71/U-2 operations from Edwards AFB (USA)[13]
4453
High balloon operations – National Scientific Balloon Facility, Palestine TX, and other providers, some in international operations (USA)[13]
4454-4465
Air Force operations above FL600 as designated in FAA Order 7610.4 (USA)[13]
4466-4477
Reserved in accordance with FAA Order JO 7110.67 (Fed Law Enforcement) (USA)[13]
4500, 4600, 4700
Internal ARTCC subsets assigned by En Route Safety and Operations Support (Discrete codes only except for first primary block to be used as non-discrete if all discrete codes are assigned) (USA)[13]
5000
Aircraft flying on military operations (Australia)[16]
5000
Reserved for use by NORAD (USA and Canada)[13]
5061-5062, 5100, 5200
Reserved for special use by Potomac TRACON (USA)[13]
5100, 5200, 5300, 5500
Internal ARTCC subsets assigned by En Route Safety and Operations Support (Discrete codes only except for first primary block to be used as non-discrete if all discrete codes are assigned) (USA)[13]
5100-5300
May be used by DOD aircraft beyond radar coverage but inside US controlled airspace with coordination as appropriate with applicable Area Operations Directorate (USA)[13]
5400
Reserved for use by NORAD (USA and Canada)[13]
5600, 5700
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
6000
Military flights in Class G airspace (Australia)[16]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
6100
Reserved for use by NORAD (USA and Canada)[13]
6200, 6300
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
6400
Reserved for use by NORAD(USA and Canada)[13]
6500, 6600, 6700
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
7000
VFR standard squawk code when no other code has been assigned (ICAO)[11]
This code does not imply VFR; 7000 is used as a general conspicuity squawk (UK)[12]
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
7001
Used in some countries to identify VFR traffic (France)
Sudden military climb out from low-level operations (UK)[12]
7004
Aerobatic and display code in some countries (UK)[12]
7100, 7200, 7300, 7400
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
7500
Aircraft hijacking (ICAO, worldwide)[6][13]
7501-7577
Reserved for use by Continental NORAD Region (CONR) (USA)[13]
7600
Radio Failure (Lost Communications) (ICAO, worldwide)[6][13]
7601-7607
Reserved for special use by FAA (USA)[13]
7610-7676
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
7615
Civil flights engaged in littoral surveillance (Australia)[16]
7700
Emergency (ICAO, worldwide)[6][13]
7701-7707
Reserved for special use by FAA (USA)[13]
7710-7776
External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[13]
7777
Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE) (Belgium, Germany, Netherlands, USA)
DOD interceptor aircraft on active air defense missions and operating without ATC clearance in accordance with FAA Order 7610.4 (USA)[13][20]
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Who else thinks that missing Jet was Hijacked/Stolen??
