KUALA LUMPUR - The Aircraft Communications Addressing and Reporting System (ACARS) cannot be totally disabled from the aircraft as it goes in standby mode much like a telephone on flight mode.
That is why, despite being turned off, satellites could still "ping" the ACARS on Malaysia Airlines flight MH370 after it went off-radar.
Pilots use ACARS on a normal day only to receive short messages and weather reports, or to send short massages.
They can even use a three-step selection process which can alert air traffic controllers of any emergency or hijacking, as long as there is battery life.
According to a commercial airline Boeing 777 pilot who requested anonymity, ACARS is a system that works on Very High Frequency (VHF) and Satellite Communication (SATCOMM).
"It is basically like an SMS, which we use to send data to and receive data from the ground. What happens is that when you power up, ACARS will establish communication with the satellite.
"Pilots can't (totally) disable the communication because every aircraft will have a unique code with the satellite. You use SATCOMM when the VHF communication is out of range.
"So you can turn off the ACARS but not the connection, that is why you have the 'ping' indicating you have a connection and also the aircraft is flying," he said, adding that the network connection will still be present.
He added that it is only when the plane is powered up or flying that the 'ping' is established but it would be at different locations since the aircraft is moving, which would explain the pings or handshakes of MH370.
Another commercial pilot also familiar with the Boeing 777 concurred, saying that ACARS was like any computer, which actually goes on standby mode when you switch it off.
"ACARS is a communication system, not a Global Positioning System (GPS); it does not give positions but only transfer messages.
"It is Mas procedure to switch ACARS, VHF, and High Frequency selection off but this is only for flights to China as the service provider for Mas does not cover China. Some if not all pilots switch them all off for a while and then later switch SATCOMM back on to force the system into SATCOMM mode."
When asked what steps could have been taken in the case of a hijacking or emergency, the pilot said that in the cockpit, one VHF receiver is always in emergency frequency 121.5 for air defence and other aircraft to call.
"I have never approached the ACARS possibility but for the transponder I can't figure out why it was switched off with MH370.
"When in an emergency, the pilot has to change transponder code to either 7700, 7600, 7500 depending whether it's an emergency or hijacking or others."
Meanwhile, former Royal Malaysian Air Force pilot Dr Mohamed Rafick Khan Abdul Rahman, who retired while holding the rank of major, MH370 would probably have been flown higher than 5,000ft while over the peninsula because of the Titiwangsa mountain range.
He said flying nap of the earth, also called terrain masking or treetop flying, was a skill which usually required military training.
Such flying, he said, was usually performed by air force pilots in fighter jets.
"For a jetliner like the Boeing 777 doing terrain masking would require military training as it would be very hard to manoeuvre such a big aircraft. You would usually fly 1,000ft or 2,000ft above the highest points.
"Perhaps over the sea, they could have travelled lower but you would burn more fuel if you did that. In my personal opinion if the plane had crashed, the black box beacon would have sent a signal as the vibration of the G-force (force of gravity, when the crash occurred) would have triggered it," he said.
Rafick Khan also said that the Malaysian military revealing their radar data was something out of the norm as it was showing the world the capability of the armed forces.
"Divulging such information would open a Pandora's box to the ability and vastness of the country's defence system."