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TMCNet:  How to disable communications [New Straits Time (Malaysia)]

[March 16, 2014]

How to disable communications [New Straits Time (Malaysia)]

(New Straits Time (Malaysia) Via Acquire Media NewsEdge) KUALA LUMPUR: JUST how easy is it to disable communications systems in an aircraft? There are three in an aircraft - Very High Frequency (VHF), High Frequency (HF) and a Satellite Communication Group (SATCOM), which includes the transponder and Aircraft Communications Addressing and Reporting System (ACARS).


FAS Udara Flying Club president Captain H.S. Dharan, who has been flying for more than 10 years and has spent 24 years in the aviation industry, said the transponder provided and indicated the position and altitude of the aircraft.

He said ACARS was a digital datalink system for transmission of short, simple messages between aircraft and ground stations via radio or satellite.

Dharan said all of this was performed automatically by the ACARS management unit and other associated avionics systems, without flight crew intervention.

As part of the growth of ACARS functionality, the ACARS management units also interfaces directly with a control display unit located in the cockpit.

"All the above must be switched on and off manually by the technical crew, who, in MH370's case, is the pilot," Dharan told the New Sunday Times yesterday.

He said the power supply for all these components could also be disabled permanently by disconnecting the circuit breakers in the cockpit or in the avionics bay below the cockpit.

"The avionics bay is a space inside the structure of the airplane where electronic devices, such as receivers of communications radios, navigation systems, radar equipment, transponders and other electronics, are installed.

"It is in a secured location within the aircraft with restricted access. On most aircraft, it is below the cockpit." On how long it would take for these electric components to go offline should it be turned off manually, he said the effect was immediate.

"The tools used to be switches but the new generation of aircraft need only the touch of a button," said Dharan.

Aircraft maintenance engineer Sri Ramani Kugathasan said it was not accurate to call the transponder a communications device as it only reports the aircraft status, such as its position, speed, heading and altitude, back to air traffic control.

"This is done automatically by a computer installed in the aircraft and doesn't require any sort of pilot input. The only input they do is keying in the four-digit 'squawk' code given to them by air traffic controllers (ATC).

"The pilots can also use the transponder to report a state of emergency back to the ATC if needed by changing the squawk codes to an approved international standard, which is 7500 for a hijacking, 7600 for radio failure and 7700 for an emergency.

"The transponder can be switched on or off just by using a knob in the cockpit." Kugathasan said the transponder did not automatically transmit information to the ATC. Rather, it transmits when "interrogated" by ground radar. Hence, the name "transponder", which is an amalgamation of "transmitter-responder".

Among the information obtained by the ATC is the aircraft's call sign, altitude, speed, heading and coordinates of its current position.

Kugathasan said most modern jetliners had two systems for redundancy, or failsafe, meaning, if one system failed, they could manually switch to a second system.

"About shutting down automatically, I believe this only happens when both systems have failed, failure of the transmission/ receiving antenna or a technical glitch. It is rare that both systems would shut down.

"However, it is possible to shut down the system, via a knob on the control panel, deliberately." (c) 2014 ProQuest Information and Learning Company; All Rights Reserved.

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