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!

  • 8
    $\begingroup$ Is the premise that "the satellite apparently knew that the transponder was off" even true? How would it know that? $\endgroup$ Mar 31, 2014 at 11:02
  • 2
    $\begingroup$ You are looking at this thing from the hindsight. It's clear what we would need to do when we already know what comes next, but normally we don't know what will happen, and therefore cannot cover every situation. $\endgroup$ Mar 31, 2014 at 13:51
  • 19
    $\begingroup$ Attach a rope to each flight, so when it gets lost, just pull the rope $\endgroup$
    – Firee
    Mar 31, 2014 at 14:11

6 Answers 6


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.

  • 8
    $\begingroup$ There's also no reason to believe such an alarm would have "saved the crew, the passengers, or the airframe" in this situation either - If the aircraft descended to an altitude below radar coverage the operational status of the transponder becomes irrelevant. On the other hand raising an alarm would give the flight crew another problem to deal with if they need to deactivate the transponder in response to an emergency (say it's shorting out and in danger of starting a fire). As you said, the risks simply outweigh the benefits here. $\endgroup$
    – voretaq7
    Mar 31, 2014 at 3:52
  • 9
    $\begingroup$ 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? GOL 1907, where the Embraer pilots inadvertently set transponder to stand-by while the captain (I think) pressed the relevant LSK with his foot(!) while using the foot rest. Also let's not forget that TCAS requires transponder to function. $\endgroup$ Mar 31, 2014 at 6:03
  • 5
    $\begingroup$ Yes, GOL1907. It is probably the only one and it would have helped the most if the ATC radar bitched more loudly about loosing the secondary target instead of just showing the primary data with single symbol change. $\endgroup$
    – Jan Hudec
    Mar 31, 2014 at 6:44
  • 1
    $\begingroup$ @Jan Hudec True and that's the theory of Swiss cheese holes and that one factor by itself is not enough to cause an accident and bla bla bla. The point is to patch as many holes as possibles, and (regarding GOL) both transponder and the RADAR UI are safety holes that should be closed. Always IMHO. $\endgroup$ Mar 31, 2014 at 10:08
  • 1
    $\begingroup$ @SteliosAdamantidis - Yep, GOL1907 is the only one I could think of as well. $\endgroup$
    – Steve V.
    Mar 31, 2014 at 23:28

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?

  • 1
    $\begingroup$ "The airline doesn't generally have a direct line of contact with their aircraft." Can one of the airline pilots here confirm if this is true? I was under the impression that it was normal for airliners to have the ability to talk to the airline dispatch. $\endgroup$
    – reirab
    Mar 31, 2014 at 15:54
  • 1
    $\begingroup$ "Remember, it was nighttime and the aircraft wasn't on radar anyway." My understanding of news reports was that it was tracked on both Malaysian and Thai military radar for quite some time after they turned off the transponder, including the entire time that it was crossing the Malay Peninsula and out into the Straights of Malacca. $\endgroup$
    – reirab
    Mar 31, 2014 at 15:56
  • $\begingroup$ > some jets could have been scrambled > > By whom? Who is paying for that? By the air force of the country that is being over flown by an unidentified aircraft, also unreachable by radio, that's who. I'm surprised that neither Thailand nor Malaysian Air force scrambled some jets. I would think this would be a routine procedure in this type of situation. This could have been another 9/11 type of incident, thankfully, it wasn't. Isn't that what an air force is for? $\endgroup$
    – Gilligan
    Apr 2, 2014 at 0:42
  • $\begingroup$ Granted, it would have been a huge logistical task, particularly on short notice, to follow the airliner until it crashed. Probably involving air to air refueling and the coordination of several nations. Who's paying for the HUGE ongoing search for the downed aircraft? The real-time scramble would no doubt be cheaper. There is a slim possibility some survivors could have been rescued. Virtually no chance of that now. $\endgroup$
    – Gilligan
    Apr 2, 2014 at 0:43
  • $\begingroup$ Again, you're looking at this with the benefit of hindsight. "If only they had done X at time Y, this could have been avoided." Sure, but while it's actually happening it is very difficult to determine the correct course of action. Even for several days afterwards, searchers were looking in a different ocean. At the time, nobody knew that the blob on military radar displays might have been MH370. $\endgroup$ Apr 2, 2014 at 0:49

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.

  • $\begingroup$ then bump up the transponder from the minimum equipment list $\endgroup$ Mar 31, 2014 at 7:41
  • 7
    $\begingroup$ To where? The transponder is already on the MEL. Commercial flights cannot depart without everything on that list installed and working. (hence: "Minimum"). No rules against things breaking mid-flight. Private aircraft don't always need one, and no aircraft needs one if they are not flying through controlled airspace. In that situation, no one would be listening to the transponder anyway. $\endgroup$
    – paul
    Mar 31, 2014 at 10:17
  • $\begingroup$ @paul My understanding is that that's the exact opposite of a Minimum Equipment List: the MEL is precisely those things which can be inoperative (possibly with some modified procedures). On that note, I find it hard to believe that an engine failure doesn't constitute an emergency dictating a landing at the nearest airport that can take the plane. $\endgroup$
    – cpast
    Apr 9, 2014 at 4:11

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.


† 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.


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. ?

  • 2
    $\begingroup$ Soon after ATC noticed that the transponder was off, they tried to raise MH370 on the radio. Failing to contact them, they asked other flights to try to contact the plane. $\endgroup$
    – Phil Perry
    Apr 1, 2014 at 13:12
  • 1
    $\begingroup$ So, someone DID try to contact the flight! I hadn't heard this on the news. And what was ATC's followup? Seemingly nothing, just wait to see if they arrive at the destination? Is there a normal procedure for this type of situation? $\endgroup$
    – Gilligan
    Apr 1, 2014 at 23:44

If pilots are alive and conscious they will generally be sufficiently aware by other clues. Given that ADS-C which MH370 was using is contracted to ATS without a transponder the feedback from ATS would cease.

There are only so many alarms you can bombard a pilot with before it becomes a counter-productive distraction in an emergency.

It is just a presumption that MH370's transponder was turned off since the Flight Aware website notes that MH370's transponder did briefly send signals again about three hours after take-off at 03:48-03:51 MYT.

Because these transponder returns were prompted by MH370 flying through civil radar coverage and being painted by civilian PSR radar then it may be that it flew through a gap in radar coverage and MH370's transponder was not switched off at all.

  • 2
    $\begingroup$ the Flight Aware website notes; any link to substantiate your claims? $\endgroup$
    – Federico
    Jun 19, 2014 at 21:16

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .