Why is there no alarm when the transponder is turned off?

Gilligan
  • Why is there no alarm when the transponder is turned off? Gilligan

    When the transponder on Malaysia Airlines Flight 370 went off, there was no ALARM. The satellite kept getting pings, and apparently "knew" that the transponder was off. Would it not be a great idea if the Malaysia Airlines got an alarm indication that the transponder was off on one of their planes? The airline could have radioed the pilot and asked what the trouble was. If the pilots did not respond, some jets could have been scrambled to checkout what is going on. The scrambled jets could have followed the airliner for hours and watched them crash in the ocean, when they ran out of fuel. We would know EXACTLY where the airline crashed!

  • Because it's not worth it.

    Quick, name me another accident where a transponder-off alarm would have been exactly what was necessary to save the crew, the passengers, or the airframe?

    Hint: there aren't many.

    The number of false positives would drastically outweigh the number of actual events where a transponder alarm would have been able to do some good. For example, air traffic control might ask an aircraft to turn off their transponder. Nothing dangerous there, but alarms would be going off back at dispatch. Good idea on paper, but not practical enough to make it worthwhile in the real world.

  • Turning off the transponder, radio, flight recorder, engine, air conditioning, lavatory or any of the other things on the Minimum Equipment List does not usually qualify as an emergency. There are spares which can be switched on as soon as deemed prudent. If the pilots have something to say, they will. If they can't, they probably have more important things to do. Aviate, Navigate, Communicate. In that order.

  • The airline could have radioed the pilot and asked what the trouble was.

    The airline doesn't generally have a direct line of contact with their aircraft. That's the job of air traffic control. It might be possible for the airline to ring up ATC and see whether they can contact the aircraft, but if ACARS is turned off then the only viable option is HF radio which doesn't always work. Some aircraft have a satellite phone in the cockpit, no indication whether MH370 did.

    Anyway, it was the middle of the night. Might be tricky to get somebody at ATC to answer the phone.

    If the pilots did not respond, some jets could have been scrambled

    By whom? Who is paying for that? If jets were scrambled every time an aircraft couldn't be reached over the ocean, there would be a lot of false alarms.

    to checkout what is going on.

    So, you've got jets in the air, now where's the aircraft? Remember, it was nighttime and the aircraft wasn't on radar anyway. If they had turned off the lights, there would be no possible way to find the plane. By the time the jets get to where the flight was last estimated to be, they could have gone in any direction.

    The scrambled jets could have followed the airliner for hours

    What's the range of your jets? Remember they would probably like to get back home too, so cut that in half. Where are they going? Whose airspace? If these are military jets, who clears them to fly into another country's airspace? Would Indonesia mind being overflown by Malaysian fighter jets on short notice in the middle of the night?

  • The question presumes capabilities that don't exist (the ACARS satellite link doesn't "know" about the radar transponder), presumes the transponder was intentionally turned off (likely, but very speculative, we have no evidence), and presumes the airline monitors planes in flight (they do not), while ignoring the fact that Malaysian military radar operators had the transponder-less blip on their display(s), had radios, and were part of the military that would have sent fighters to investigate.

    Because their range is finite and limited, air traffic radar will "lose" transponder signals when a plane flys out of view. Signals stopping is a constant occurrence. If I follow the timeline correctly, the copilot spoke to air traffic control after the transponder signal stopped. Air teaffic control are supposed to monitor the planes they are watching. Why didn't they ask the copilot what was up? When the transponder signal stopped, was the plane at a distance and altitude where disappearing would be expected. ?

  • Why is there no alarm when the transponder is turned off?

    Because airlines trust their pilots and have systems in place to ensure their pilots are trustworthy. It is probably right to concentrate on preventing untrustworthy persons entering the cockpit before worrying about real-time remote monitoring of the actions of untrustworthy persons in the cockpit.

    The satellite kept getting pings, and apparently "knew" that the transponder was off.

    This is not correct. The ACARS system and the transponder are separate systems. The satellite in orbit "knows" nothing, it's just a relay. The satellite's base station receives the data and forwards it to customers whilst monitoring the satellite service/performance. As I understand it, the base station equipment is not expected to peek into their customer's data content and make judgements about it's meaning.

    ACARS uses VHF over land and only uses satellite communications (SATCOM) when out of VHF range. So the satellite systems not receiving data is not unusual and should not trigger an alarm.

    The Inmarsat satellite ground station could not inferred that ACARS had been turned off inappropriately. This is because it doesn't know if the aircraft has landed and systems turned off normally. The Inmarsat company probably don't know every time an aircraft deviates from schedule, e.g. to divert back at the airport to check a fault light. The Inmarsat base station also didn't know the location of the aircraft other than it was somewhere on that side of the planet. The hourly handshake is conducted using the broad regional beam, not one of the narrower beams.

    It is fortunate that Inmarsat keep logs and that the logging system chooses to keep certain data such as signal strength and frequency rather than discarding it. This enabled Inmarsat's engineers to analyse those logs days and weeks after the events. But this analysis is not routine and real-time (and it would probably be a very inappropritae solution to make it so).

    Malaysian Airlines had not subscribed to the more continuous† data reporting systems available through ACARS (and Inmarsat). So the absence of ACARS or other transmission is even more unremarkable from Inmarsat's perspective at the time.

    In the normal course of things it is not the job of Inmarsat to interpret the data being transmitted to it's customers (the airline, plane maker, engine maker etc). It's job is to relay data to it's customers and those customers can then use that data as they need.

    Since those customers had not subscribed for continuous† transmission, no-one could know what was happening in the cockpit.

    Even if the continuous† transmission was subscribed to, there is no guarantee that the state of the transponder would be part of that data, nor that there would be a monitoring system at the airline capable of issuing alerts for unusual settings of the transponder.

    ACARS itself is just a reporting system. I believe the content of the data reported is selected by the airline.


    Notes:

    † By continuous I mean the sort of monitoring used for AF447. As I understand it, this isn't completely continuous, just more frequent reporting and as-needed throughout the flight to report certain events as they arise.

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