system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can aircraft dissapear from radars?

We are getting a lot of duplicates of this question in the past hour so I'm putting up a sticky to consolidate all the questions and explanations here.



For those out of the loop, this is regarding the missing EgyptAir flight today.
gpt
Because we don't use "Primary Radar" all the time.

Primary radar sends out a signal, bounces off an object and comes back to the sensors displaying a location. 

Why don't we use this for everything? Because it requires insane amounts of power, has a somewhat limited range, and picks up LOTS OF STUFF. Meaning the return is can be confusing with irrelevant information.

Planes use secondary radar which is basically a transponder which tells the radar where the plane is, without the radar having to send out an initial signal. If that transponder stops working (plane crash, or someone simply turns it off), the plane "disappears" because it's not telling you where it is anymore.

However, there's no good reason (aside from cost) that planes can't report their GPS co-ordinates somehow so we have a specific location all the time - although someone with more knowledge could probably explain this better.