Transponders have to be turned off while you're at the airport and while the plane's being serviced on ground. I think 777 has 2 transponders, so you also need the ability switch from one to the other. Lastly, I believe all electronics on the plane must be on a breaker circuit in case of electrical shorts so that pilots can isolate and turn off faulty circuits.
that you can turn on if you are flying into controlled airspace.
What I think they should have is a new system that pings the location and other data to satellites that can't be turned off and will work anywhere unlike radar transponders.
They have transponders. But military airplanes can and sometimes do turn them off. When airplanes broadcast signals they can be detected. The military sometimes wishes to avoid this.
Planes have transponders to make it easier to find them, though that's mostly for when they are in the air. The issue is that the military likes to turn them off.
It is worth mentioning that transport aircraft carry Mode S [0] transponders which emit a unique ICAO 24 bit address assigned by aircraft registration. This data is available regardless the pilot-entered 4 octal digit Mode A transponder code. The transponder can still be turned off, however.
This contrasts to small aircraft which typically have Mode A/C only (no mode S).
Yeah I've heard stories of some people doing crazy, dangerous, illegal things in small planes and just turning their transponders off.
I was a passenger in a C182 once where the transponder antenna fell off midflight. It was very educational how much trouble ATC had seeing us. Even after telling them exactly where we were, flying the heading they gave us to present the broadside of the a/c to the radar antenna, they basically said "there's something that might be you there, but I can't really tell."
A big jet at FL370 should show up on primary, but I can see it depending on a bunch of factors.
Inside the continental US, if you fly a small not-turbine-engine-powered airplane yourself and only go to rural airports with no tower, it is actually legal to have no transponder (see below, 91.215(b), 91.225(b)).
But, if the transponder is installed, you are legally required to turn it on at all times no matter where you are (see 91.215(b)(3), "all controlled airspace" means everything above 700-1200ft AGL (depending on exactly where)).
So for this to be legal, you can't just turn if off: you have to actually remove it from the airplane. Whether that is legal additionally depends on the airplane.
Yes. I am astonished that you do not already know that.
On 9-11, the combined maximal efforts of civilian and military aviation were unable to locate four airliners because their transponders were turned off. This occurred near the New York and DC metropolitan areas, some of the most carefully controlled air space in the world.
Without a transponder, an aircraft is just a raw radar return among, potentially, hundreds or thousands of other returns from civilian and military aircraft, weather, birds, balloons etc. Add to that glitches, ghosts and anomalies in busy air space and it is not surprising that a large aircraft without a transponder can hide in plain sight.
If you can't trust the pilot then you can't trust the equipment either. Civil aviation requires a lot of trust, so there's no real reason to try to lock out the transponder.
Once the pilot is not trustworthy, you've entered the realm of military defense, and a lot of that is built around the ability to track targets that don't want to be tracked.
My impression was that military aircraft turned off their transponders all the time, especially during sensitive parts of their missions. You don't have to follow any of the OSINT accounts that post flight info to see posts about how some Global Hawk re-emerged on the trackers, after having gone silent some number of hours ago. Sometimes they keep their transponders on, intentionally, to communicate intent and posture to potential adversaries.
Many aircraft, particularly those flying IFR, are required by law in many jurisdictions to broadcast their location to reduce the risk of collisions and facilitate air traffic control. Check aviation transponder interrogation modes and ADS-B in particular
ATC primary radar is pretty weak. Not showing up (particularly after changing flight direction) doesn't mean much. It's very odd (and illegal) for something to be up in flight levels without a transponder though.
Just as a technical point, the transponder would be set to MAS370.
The ACARS would be set to MH370.
It's a minor point but significant in the separation of air traffic control ( transponder ) and airline operations ( ACARS ).
You can think of the flight number MH370 as the interface contract ( 'We will fly you from KL to Beijing' ) and MAS370 as the reification of that. In fact, there could be several physical flights underlying the logical flight number.
Military aircraft have controls on their dashboards for turning individual systems off as required. Turning off ADS-B would be the first thing on the checklist for sensitive flights, and one of the things that are turned on purposefully when entering friendly airspace.
It would surprise me if military transponders do not have the ability to broadcast “I am a Cessna at 3000ft” regardless whether the actual aircraft is a F35 or a Galaxy.
cf the usual copypasta about the SR-71 crew asking ATC for an altitude and speed reading. They use the broadcast radio systems because they want you to know they’re there.
"UPDATE - Monday, March 17, 2014 - 12:15 PM EST
Some have raised the statement that TCAS doesn’t work if the transponder is disabled… this is only partially correct. Other planes TCAS would NOT see MH370 at all. MH370 would not actively query other planes as it’s transponder is off HOWEVER it could still listen to any transponder output from other planes that are actively transmitting. SQ68 would have been actively transmitting while in-range of Subang ATC center.
Even if TCAS on MH370 wasn’t working for some reason, an in-expensive portable ADS-B receiver paired with an iPad and Foreflight app would allow a pilot to receive the ADS-B output being transmitted by SQ68 at that time."
But you're right, ACARS (the system through which the engine information was routed - http://en.wikipedia.org/wiki/Aircraft_Communications_Address...) probably should be updated to add more information...
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