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.
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.
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.
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.
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.
I couldn't say how common it is, but military aircraft definitely fly in American airspace without transponders. A few months ago I hear/saw some Osprey fly overhead. They were not visible on ADS-B Exchange (which does not block military transponders).
What if it accidentally collided with a civil airliner?
Not much. I can't think of anything more recent than Hughes Airwest 706 (and that predates mode s transponders by a few decades), and all that came out of that was a promise to communicate with civilian ATC.
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.
> far easier to install one than to wrangle an exception
You are talking about an agency that builds and operates planes that nobody else knows about. They can have any "exception" they want by just shrugging and saying the aircraft in question does not even exist.
All military aircraft have transponders because the air bases have ATC as well and (at least in the US) are near commercial airports they have to work with to avoid collisions during takeoff and landing.
Civilian planes fly with transponders - something that you can receive with a $20 bit of hardware. Military aircraft do not turn their transponders on unless they want the world to know where they are.
You only need a transponder when flying an aircraft in controlled airspaces (Class A, B, C). You can fly without a transponder in a lot of places as long as you are in VFR conditions and not within 30 miles of most airports. It's just not a good idea. This is why gliders and paramotors can fly without a transponder in VFR conditions.
This is just basic safety for vehicles that can't see each other traveling at 100-1000 miles per hour in conflicting space.
Although it's a little concerning that ADS-B makes it so much easier to identify WHO is flying the planes.
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.
>Why hasn't it been made mandatory that all aircrafts should have a standardized transponder ?
In some parts of the world, it may be. For example, I don't know if Europe allows for aircraft without transponders. Even in the U.S. you must have a transponder to enter certain types of airspace (e.g. the airspace around major airports). As far as I know, no matter where you are in the world, you must have a transponder to fly IFR.
Installing a transponder is a non-trivial expense, especially in older aircraft (which are the aircraft most likely to not have transponders). For a lot of small aircraft in the U.S. this would represent an unnecessary burden on the owners. For example, crop dusters: they typically fly around low and slow in areas with very little traffic, under day VFR conditions, so they have no need to interact with ATC and therefore no real use for a transponder.
>It scares me a bit that, at the end of the day, we rely on pilots avoiding collisions by sight.
We don't rely solely on this: we have transponders (with TCAS in some cases), ATC radar, and (in some cases) airborne radar. All of these tools help us to avoid collisions. Unfortunately, none of them are 100% effective, and in most of the situations where they all fail, the good old Mark I Eyeball usually saves the day. See-and-avoid isn't perfect, either (if it was, we'd never have midairs), but it is still the most effective tool available for avoiding an impending collision.
Eh. Military aircraft would also have a transponder, they just wouldn't necessarily have active broadcasts.
Civilian aircraft do broadcast actively (ADS-B). But they also respond to secondary radar for Mode A/C, which are basically cases of IFF Mode III (okay, maybe not exact term, but the idea applies.) So it's still a challenge-response/IFF, just in this case always responding.
Military aircraft use different modes and presumably don't respond unless interrogated with an appropriate challenge, but the principles are the same.
Have they said whether the transponder was active on the jet? They usually have the ADSB beacon live when flying near airports, but if it was turned off it would be a bit trickier to find. Also, if the retroreflector wasn't extended it might be hard for active radar to track it as well. Worst case scenario would be if it was in "full stealth" mode -- that would be the situation where we discover that NORAD air defense systems can't detect our own jets!
You can actually track a large number of military flights on websites such as https://www.adsbexchange.com/ a large amount of the time they fly with there transponder on because they don’t want to hit other aircraft.
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.
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