FW: Analysis from a JHU CS Prof

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1

Quite more interesting is why nobody noticed that 4 airliners where

hijacked

almost the same time.

Not surprising. Aircraft are "flight followed" by a series of control
centers across the nation, each responsible for a given chunk of
airspace. Something happening in an area controlled by Center "A", for
example, wouldn't be passed on to Center "B" (which has it's own problems
to work) unless it impacted Center "B". Furthermore, unless someone TELLS
Center they're being hijacked, there's no way for a controller - looking at

a blip - to know what's up. And any controller can tell you that pilots do

Somehow the people who did this managed to turn off the transponders on
these planes. Normally a plane flying in controlled airspace squawks a
unique id and altitude which is decoded by their radar and associated with
each blip. Sometimes low cost homebuilts/ultralights fly with no
transponder, but Boeings <sarcasm>usually</sarcasm> do. If you set a
transponder to 7500, it means you're being hijacked.

BTW if you see your friend Jack at the airport, be sure to say, "What's up,
Jack!" instead of "Hi Jack!"

So how do you deal with this? Blowing up a whole country? I wonder if the
US should adopt a 'fire w/ fire' approach and invest in intelligence, covert
ops and assassinations. It would seem that it is open season on terrorism
by every democratic nation, I expect to see very conspicuous Samuel Jackson
style ass whoopins on whiny extremist groups to satiate America's anger.
Terrorize the terrorists.

Oh yeah, obviously Echelon should probably have MacOS loaded on it.

-b

2

I despise posting off-topic, but I want to say two things...

1. If a transponder is turned off, it doesn't mean that you don't show up
on radar--a blip appears on the radar screen as long as you're high enough
to be detected. If however you fly low enough, you can fly below the
radar's detection capability. I don't offhand recall what height that
is--it's been years since I was active as a pilot and prospective Air
Traffic Controller.

2. What's the point of having transponder codes for hijacking if they're so
well published everyone is aware of them? The purpose of the codes was so
that the pilot could communicate this information without the hijacker
becoming aware of what was happening. I have always REALLY DISLIKED the now
common practice of advertising this information. You're taking away one of
the pilot's best tools...

  -- Leigh Anne

3

Also sprach Leigh Anne Chisholm

I despise posting off-topic, but I want to say two things...

1. If a transponder is turned off, it doesn't mean that you don't show
up on radar--a blip appears on the radar screen as long as you're high
enough to be detected. If however you fly low enough, you can fly
below the radar's detection capability. I don't offhand recall what
height that is--it's been years since I was active as a pilot and
prospective Air Traffic Controller.

That would depend on the distance from the transmitter...the farther
from the transmitter, the higher you could fly and not be detected.

Another issue is the efficiency of the transmitter and receiver at
picking up the reflected energy. From what I understand, air traffic
control radars are, in this day and age, really quite pathetic as true
radars. They largely depend on transponder responses to be able to pick
up aircraft, as even fairly large aircraft, at some distance from the
transmitter, may only give a secondary (transponder) return, and not a
primary (reflected energy) return.

4

It is my understanding that the planes were _always_ on Radar, but that
without the transponder, the altitude of the planes was unknown.

DJ

5

Then you would probably consider it a good thing that the code he gave is
not, in fact, the transponder code for hijacking. And no, I'm not going to
detail what the correct one is. Go ask a pilot (such as the one living in
my house), if you want to know. Pretty much all of them know it; even those
who don't fly planes one could reasonably hijack.

6

Some obvious things to do:

  1) Turn off altitude reporting -- most of the transponders I've
  used have 3 settings (off, on, and on with altitude reporting)
  2) Then sqwak VFR.
  3) Turn the transponder off
  4) Pull the breaker. (All flight avionics are on resettable
  breakers, accessible to the flight crew. There is good
  reason for this.)

  I wouldn't find it exactly surprising that any of the transponders
had been switched off. It only takes a moment.

  --msa

7

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It's a moot point anyway, it's pretty clear the hijackers had flight
training of some sort AND were well organized, and would be familiar
with the steps that the pilots would take.

Regards,
Matt

- --
Matt Levine
@Home: matt@deliver3.com
@Work: matt@easynews.com
ICQ : 17080004
PGP : http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x6C0D04CF

- -----Original Message-----

8

Since most commercial aircraft have multiple transponders, it should be
assumed that if an aircraft stops squacking, something is BAD
WRONG. Beyond this, the ability of the flight crew (any of them) to push
a "panic" button that would cause the transponder to squack a
predetermined distress code may me a good measure to implement. This
"Panic" button should NOT provide any visible confirmation in the
flight-deck instrumentation but instead, the ATC should in some cryptic
form acknowledge the fact that the aircraft is in distress via radio
communications. (perhaps a minor heading correction that is not needed,
etc.)

9

Leigh Anne Chisholm wrote:

I despise posting off-topic, but I want to say two things...

1. If a transponder is turned off, it doesn't mean that you don't show up
on radar--a blip appears on the radar screen as long as you're high enough
to be detected. If however you fly low enough, you can fly below the
radar's detection capability. I don't offhand recall what height that
is--it's been years since I was active as a pilot and prospective Air
Traffic Controller.

That depends. The FAA has begun removing "Primary Target" service from
some
of it's control center, citing cost-savings, etc. Some ARTCC and TRACON
facilities (Air Route Traffic Control Center and Terminal Radar Approach
Control)
have primary-target capabilities on their scopes, some do not. Some of
the
recording systems record the primary targets, some do not.

As to the height at which you are undetectable, there are so many
determining
factors, that it's on a location by location basis. Factors include
height
of the antenna, terrain, distance from the antenna, number of radar
sites
covering the area (yes, there is an overlap in most of the US these
days),
etc. For example, I know bay can see me on the ground at PAO if I leave
my transponder on. However, Oakland Center can't see me in some parts
of Northern CA if I drop below 12,000 feet MSL (~4000 AGL in some of
those areas).

2. What's the point of having transponder codes for hijacking if they're so
well published everyone is aware of them? The purpose of the codes was so
that the pilot could communicate this information without the hijacker
becoming aware of what was happening. I have always REALLY DISLIKED the now
common practice of advertising this information. You're taking away one of
the pilot's best tools...

Yes and no. There was a time when this was so poorly publicized that
many of
my fellow pilots didn't know that there was a hijack code. The fact is,
this
has always been published in the AIM, so anyone who cared could easily
pick
up a copy of the AIM (Airmans Information Manual) at their local pilot
shop
and find out. I don't think the knowledge of the code necessarily
prevents
the pilot from using it. Any hijacker that's going to know about the
code
from any publicity it may have will already know enough to turn the
transponder off anyway for other reasons.

  -- Leigh Anne

Owen DeLong
PP ASEL, Instrument Airplane