Is it true that if a flight disappears from the website Flightradar it has crashed

How could a flight disappearing from the screen on the Flight Radar app equate to a plane crash. Thats what the news agencies would have us believe. Its not true is it.

No, it isn’t true. It just means that the site isn’t currently getting flight data from the plane which can and does happen for any number of reasons. Even ATC doesn’t receive real-time flight data for all flights, especially those above the open oceans. Flight tracking sites try to minimize this problem by guestimating where the plane is based on the last reports but it can’t smooth out or predict everything. If you watch enough flights, you will see some that suddenly change heading or location abruptly. That doesn’t mean the flight is actually changing its vector that quickly; it is just the app catching up because it received updated data.

How about a cite for news agencies saying this?

Are you sure they said the plane disappeared from the Flightradar website, and not a more generic “disappeared from radar”?
“Disappeared from radar” is usually a simple way of saying all contact has been lost between the plane and control tower. I don’t think that normally happens unless something bad has happened.

And “disappeared from radar” isn’t a bad euphemism, as it’s easier for air traffic control to obtain flight information from the Air Traffic Control Radar Beacon System (transponders) than from the primary radar system people usually think of when they think of radar.

SkyTV news reported the disappearance of the EgyptAir plane last week in this manner. They had the flightradar screen on their VT showing the plane disappear off the screen.

Means nothing. Sky (and other TV channels) are always looking for arresting images, and the little dot disappearing is an arresting image, when we already know the plane is missing/has crashed. But it doesn’t mean that every time a dot disappears a plane has crashed.

To elaborate, primary radar as you say is the traditional, bounce a radio wave off the aircraft body and see how long it takes to get back to you. That tells you the distance, and you know the direction is where your antenna was pointing.

“Secondary radar” is the transponder system - the ground antenna sends an interrogation command, and the transponder on the plane responds. With many transponders on light aircraft, it responds with just a code that the pilot sets at the request of the controller, so the controller can keep track of the blips. Higher-end transponders reply with altitude (primary radar doesn’t tell you altitude) along with the code, and some more info as well.

Secondary radar is easier, since it doesn’t suffer from the distance-power problem of primary radar. The signal you get back with primary is reduced by the fourth power of the distance, so a plane that’s twice as far away, you get 1/16 of the power back. This means that to get long distances, primary radar takes huge amounts of power.

Secondary radar relies on the transmitter in the plane, so the power is inverse distance squared.

Controllers use secondary radar pretty much exclusively. Primary is there in an emergency backup, but the day-to-day traffic control is done with secondary radar.