No Stupid Questions

I can only speak for the UK and from a amateur perspective but here's the rough breakdown:

90% of the time it's likely a private pilot that's wandered into a restricted airspace without realising it. Or a faulty radio or navigation equipment or a medical emergency. They're politely escorted out.

9.9999% of the time is an adversarial nation testing the response time of the quick-reaction force defenses. They're politely but forcefully escorted out. Maybe some insults traded over the radio but that'd be about as heated as it gets.

0.0001% they pose a threat and refuse to be escorted out. At that point it's basically the same thing of asking "what would happen if someone climbed the fence to the White House and towards it and when the secret service pointed guns at them didn't stop, would they get shot?"

It's the pilot's call at that point, but if they posed a threat to life then yes they probably would shoot them down.

Edit: there's probably a ridiculous amount of zeros I'd need to add to the last point to indicate how unlikely it is but I can't be arsed to add that many. Basically you might as well round down to 0%

So on 9/11 the fighters scrambled to intercept United 91 had no weapons installed. There were no orders but one of the two pilots sad the plan was to ram the cockpit bringing down the both their plane and the passenger plane.

I'll add that very few fighter pilots will ever scramble a plane. There is a large checklist pilots go through before taking off particularly a fighter. When a fighter is scrambled that's all skipped.

From what I’ve heard from my coworker do actually flew:

Step 1: Attempt radio contact on local frequencies and guard.

Step 2: Try to lead you out of restricted air space

Step 3: Try to REALLY get your attention like doing precision fly so that you are unmistakably aware of them (like a really really close fly by). If this works, try step 2 again

Step 4: Really depends on where you are and/or who you are. If you are willfully violating air space and a civilian, you’ll probably get your certificate revoked and/or face. If you get near something really sensitive and probably are doing so purposefully despite attempts to get your attention, and you’re not over a densely populated area, you could get shot down.

Movies would have people believe that the jets are there to shoot down the errant jet. During the Cold War, this was entirely plausible and did happen. But more commonly, when a fighter jet is sent to intercept an unknown aircraft -- perhaps one that has entered restricted or prohibited airspace -- it may be just to have eyes on the situation.

Airspace is huge. The vastness of the air is like the vastness of the sea. Sometimes it's an advantage because there's fewer things to hit. But on the flip side, if an aircraft needs assistance, there might not be anyone for many miles in any direction. As for what an assisting fighter jet can do, the first is to establish navigational accuracy. History has shown that airplanes can get lost, and sometimes unfortunately end up hitting mountains or running into known obstacles or weather. A second aircraft can confirm the first aircraft's position, since two separate aircraft having navigational problems is exceptionally rare.

The next thing is having eyes on the outside of the aircraft. Things like a damaged engine on a jetliner aren't visible to the pilots, but there's a chance the passengers or cabin crew can look. But damage to a rudder is impossible to see from inside the aircraft; I'm not yet aware of a commercial aircraft equipped with a tail-viewing camera. Checking the condition of the landing gear is also valuable information, if a jetliner has taken damage but still aloft.

Finally, if it should come to it, an assisting aircraft can be the pilot's eyes, if for some reason the pilots can no longer see out their windscreen. At this point, the flight may already be close to the end but it may help avoid additional casualties on the ground. I'm reminded of the flight where volcanic ash sandblasted the windshield, or when a cargo jet had a fire onboard which filled the cockpit with thick smoke.

To be clear, neither incident was aided by fighter jets, but having an external set of eyes to give directions would have made things a little bit easier for the pilots. Other aircraft besides fighter jets can provide assistance, such as any helicopters or private pilots in the area. But of course, fighter jets are on-standby and can get to a scene very fast.

Shoot it down, or just get it's attention. A lot of the time there's a radio fuckup. If some guy cruising in his Cessna sees a fighter pull up along side, he's definitely switching to 121.5.

There was a commercial jet that was non responsive and the fighter could see the pilots passed out.

Otherwise be there to shoot it down quicker if it looks like it might be taking an aggressive action.

Its to scare civillians into compliance. Most of the time, they won't shoot you unless they believe your aircraft is about to strike a building or something like that.

Its also to allow the pilots of the fighter jet to visually see what's happening inside. In Helios Flight 522, the cabin presurization wasn't set properly, so the entire plane lost consciousless. The autopilot flew the flight and went into a holding pattern near the destination, and because the pilots haven't communicated with Air Traffic Control, send they fighter jets to "intercept" it. They didn't do anything besides the Fighter Jet pilot just visually looking in the plane, and see the oxygen masks deployed, and someone (later identified as a flight attendent) with a bottle of oxygen trying to get into the cockpit, but when he got in, the plane ran out of fuel so there was nothing he could do, and steered the plane away from the population center at the final moments.

All of that was witnessed by the Flighter Jets pilots and the testimony helped with the investigation.

Basically, the way I see it, they are there "just in case" something happens and they need to shoot it down.

Its more what they can reasonably do. Keep in mind our commander in chief is ignoring both the congress and the courts right now and if a question like that was asked on lemmy five months ago most poeple would say thats impossible. Even then im 100% sure if they knew what was happening with the 9/11 planes they would have shot them down. I read the 911 report book. Its a hefty read. One of the last real books I read unfortunately. Im like 99% sure some of it talked about this kind of thing although its possible what im thinking of is from another source.

Depends. It's very situational, but the procedure first calls for monitoring and communications (long story short there are ways of communicating between aircraft using maneuvers.)

Beyond that, there will be attempt at guiding the offending aircraft, either to a runway, or to a safer area.

But if it comes to it, the passenger jet can be shot down - A Hijacked passenger liner should be considered a weapon of mass destruction.

I like to think they put a big net around it and carry it home safely

They bark as loudly as possible before shooting because once they shoot it turns into a situation™