Aircraft Radar Explanation

I received a question about radar "pinging" the aircraft and why it wasn't spotted by radar along its (presumably, see below) flight to the northwest. I also received a question via Facebook about aircraft transponders.  I'd like to answer the questions and explain why tracking the hijacked 777 is much more difficult than it might appear:

This is a photo of air traffic control radar as you see it on TV and movies. I have placed an arrow pointing to NWA972 = Northwest Airlines Flight 972. This information is provided to the radar by an aircraft's transponder. When the radar beam hits it (every 12 seconds) it transmits (the jargon is "squawks") the flight number, location above the ground, and altitude. It can also squawk special codes for hijacking and emergencies without using the radio. We know that the transponder was turned off an hour into the hijacked flight.

So, what do aircraft look like without a transponder? Here is a radar image I captured of five aircraft over Kansas yesterday afternoon.
In air traffic control jargon, these are called "primary" echoes because the radar's energy just bounces off the skin of the aircraft. There is no way to know the flight numbers, size of the aircraft (Cessna 172 versus a Boeing 747) or altitude. So, if and when the hijacked 777 takes off again with its transponder off, this is what it will look like. From air traffic control radar, it will be difficult to identify and track.

It s entirely possible that our military and others have good ways of tracking a 777 in the Far East that we don't know about. From ATC radar, it is difficult, at best.


  1. Ok, can't resist idle speculation:

    -The plane flew for seven hours, which is two hours longer than the scheduled flight. This suggests it flew until it ran out of fuel.

    -If it flew over India (especially across India/China or India/Pakistan borders) it would have been detected by the Indian military. Unlikely that India is incompetent enough to miss a jet crossing a powder keg border between two nuclear powers.

    -Transponder off indicates deliberate act. Altitude changes indicate struggle.

    My conclusion: The plane is at the bottom of the South Indian Ocean, and might never be found. The plane was hijacked and the path was diverted, THEN there was a struggle and the pilots perished, but the hijackers didn't know how to fly or were locked out of the cockpit. The plane subsequently cruised until it ran out of fuel.

    It's possible India is withholding information about a hijacked plane transit.

    Do you have weather maps for the Indian Ocean the night of the flight?

  2. Weather was not an issue in the vicinity of the hijacking. I do not have the weather maps for the India-Indian Ocean area.

    Disagree that it "would have been detected" by India. It might have been detected by India. That is the whole point of this post. It is very difficult to make sense of aircraft on radar when their transponders are off.


Post a Comment

Popular posts from this blog

[1:10am Update] Tornado Forecast for Rest of the Night

First Tornado Watch of the Day Issued

Hilary's Forecast Path Shifts West; Updated 9:20am PDT