Why are these things able to be turned off on commercial aircraft?

Any aerospace engineers here?

They are turned off to prevent ground clutter around the airports.  

bulkhead

The radar automatically ignores anything doing less than 40kts.  That is called coast mode.  The computer keeps the info but the return is eliminated from the screen.  No one has anticipated the unit being turn off.  Just because the unit is off that doesn't mean the radar return is gone, just the info the XPDR provides.  During a hijack the pilot may not be making check ins with ATC either but there is not much you can do with that.  Planerench out.

I figured people would be wondering about that, after it came out in the reports immediately after the terrorist attacks. Keep in mind that just because the transponder is turned off, it doesn't mean the aircraft's radar return automatically disappears from radar screens. If that's all it took, the Air Force wouldn't have had to spend billions on stealth aircraft. All the transponder does is reply with a discrete code (and the plane's altitude, in the case of mode C units) each time its antenna is swept by a ground radar unit's beam. Aircraft on IFR flight plans, as well as those receiving radar traffic separation service, are assigned a code to 'squawk' - a four digit number that's set on the transponder - by the controller who's working that sector. If a pilot has an emergency, he enters 7700 on his transponder. Loss of communication radios warrents a 7600 code. There are codes to enter if you're hijacked - but these bastards would have known all that, and wouldn't have allowed those codes to be used, even if the original pilots were still in control.

I know that the plane still appears on radar, but thanks for the insight.


Crystal clear.

I'm not sure the policy and procedures on a commercial airline concerning their transponders but I can tell you that general aviation aircraft have the same units and when we fly into "controlled" areas (like a major airport), we are assigned a four digit transponder code like 3725. The lingo is called "Squawk".

ATC may say something like this after I report I'm approaching their airspace.

ME: Columbus approach Piper 43 niner seven tango.

ATC: niner seven tango.

ME: niner seven tango, 20 miles east, west bound at 8.5, inbound CMH, request traffic.

ATC: "Niner seven Tango squawk 3725, ident and decend to 7.5, traffic at 8, ten o'clock at 6 miles.

ME: 3725, ident, looking....niner seven tango.

ME: niner seven tango, at seven with traffic.

The transponder is a small in dash unit about the size of a car radio that has four turn type switches and a small window above each switch that displays a single digit from 0 to 9.

This code tells ATC that my plane, Niner Seven Tango is west bound at 8,500 feet and ATC will follow me and I'm under their control while in their airspace. They will tell me to turn to avoid traffic or climb or descend. When I leave their airspace, I'll switch back to a normal 1200 code in uncontrolled airspace. Next to these switches is a single turn type knob that has four positions. Off, Standby, On and Altitude. There's also what is called a Ident button that you can press when requested by ATC that "pings" the system to make sure the right aircraft is sending the signal.

If hi-jacked, the code is set to 7500. If it's an emergency, the code is set to 7700.

Sadly, the unit can simply be switched off and the aircraft still show up on the ATC radar screen but ATC loses altitude and aircraft information. I'm not sure what happens on the ground or in the tower when this happens. One of the commercial guys can explain how TCAS and the "what ifs" from there.

Edit: I can't work and write at the same time[;)]

You don't want, say... 100+ transponders on at the same time on the ground at an airport. I have been in a control tower and was shown what the transponder does to a radar screen(a number pops up). The screen could become so cluttered that you couldn't read it.

That hasn't been a problem for years.

Nice summary!!

Actually transponders use 4 digit squawks with numbers from 0-7.   No 8's or 9's

 A transponder is a device that emits a discreet code that lets ATC identify each aircraft individually while providing ATC service. This code is received by Secondary radar that provides altitude(mode C) relative position on our radar display & airspeed.

 Primary radar which dates back to the beginning is the old styles "sweep" look where the radar is actually just getting a reflection off the aircraft as a return. This is rarely used anymore in ATC except as a backup type situation
ie. if an aircrafts transponder fails.

All commercial aircraft carry 2 transponders & any aircraft flying at 18000ft.
or above are required to utilize a transponder & be on an IFR flight plan.

 Below 18000ft aircraft may fly VFR with a secondary return on their transponder of 1200, the VFR beacon code. If below FL180 & squawking 1200 no ATC service is required unless transitiong busy ATC areas like airport traffic areas(towers/approach controls).
Below FL180 VFR aircraft may also fly with NO transponder.

 I have aircraft loose their transponder on a regular basis & when that happens primary radar identification is difficult.
 A nice defined target becomes a little x or dot & the return can be lost or intermittent at best.  Secondary radar(with a transponder) is what we really rely on 99% of the time with all aircraft.

 So in summary if a Secondary radar return is lost(transponder) it is difficult to track the aircraft with a primary return only & sometimes the target is completely lost & we must rely on pilot postion reports.
 Many areas across the country will refuse ATC service if a plane's transponder is broken. It
significantly increases workload on the controller working a busy sector & is a safety
issue.

 Air Traffic Control Specialist(10yrs)
 Houston TX.

p.s. the military may have better hardware (AWACS etc.)

I guess your question hasn't really received a reply yet has it?

A commercial pilot may have more info. but I know from personal experience sometimes the transponder "freezes" up or fails & the pilot will turn off the transponder to "recycle" the discreet beacon code & it will "fix" itself.

  Kind of like rebooting a computer.

The Xponder has several modes, basically to cut down on nuisance alerts.

Everything in an Aircraft must be able to be turned off.  In case of electrical fire, you eliminate the source, via circut breakers.

What he said.  BTW, that scene in "Con Air", where the guy hot-wires a transponder and shoves it into another airplane is pure Hollywood bs.  What he actually did was stick a bunch of wires into the remote control.  Pointless...

I sure liked the movie though.

Saw it about five times.

Then there is the code I wish controllers had seen on 9-11, and that I hope you never have to see, cnatra.
A fighter on a 'for real' intercept in controlled airspace transmits a code of 4 zeros. Its one of the old codes from the 60s when things like that really happened.

    Actually, you are required to have an operating Mode C transponder anytime you are above 10,000 ft msl AND above 2500 ft agl over the 48 contiguous states and D.C.

-Dave

No eight's or nine's on the transponder! DOH! I should read my post more carefully. Oddly, I know there are 4,096 separate codes and 0000 is reserved military air defense operations. I just typed in the wrong digit.

Just proves the point that I shouldn't type my post, eat my poptart and try to work at the same time. Multi-tasking is not for me! [;)]

Now you know more about aircraft transponders then you really wanted to know eh? lol.

Interesting sidelight:  When transponders first came out in WWII, we didn't want the Germans to know we were using a transponder to enhance the radar returns.  They were called "parrots" for a code name.  Hence the "squawk" term.  To turn it off was to "strangle" the parrot, though I haven't heard that term used in many a year.  When they got the four numbers, sometime in the fifties, the "squawk 1234" request was a logical  extension of the old terms.  

Mode C, or altitude reporting came out when?  In the seventies?

Norm

                
 

Thanks Norm. I learned something new today. I didn't know it was called parrot and I'm a WW2 aviation buff.

Some terms/language I still hear once in a great while over the air is the word "sugar" as in Cessna 5447 "Sugar" instead of the international phonetic "Sierra". (Phonetic alphabet like Alpha, Bravo, Charlie...) I was told that "sugar" was the from the days of WW2 but I never thought to ask anyone.
I'll also hear "No Joy" which means "I don't see the traffic", etc. And "Tally Ho" when you see traffic called out by ATC. I can't recall ever saying Tally Ho...I just say "niner seven tango with traffic" since Tally Ho sounds like I'm ready to get on the tail of the idiot in front of me and shoot him down, lol.

I started flying in 1979 and I'm not sure when Mode C was adopted. I just remember when it was required in my plane and it was an expensive little monster about the size of cell phone but 10 times the cost. Cnatra could give you a better heads up and I bet he's got some great ATC stories.

Actually I a young one, only 10yrs in the FAA but there are a few of the "old salt" pre strike guys around(1981).  
 I'll ask about the Mode C.

 Since I learned on digitized secondary radar I'm a little spoiled. I can't imagine the old days with NO radar & just relying on pilot reporting points at radial/DME fixes for separation. The rare times I have to do it now it's a PITA [:)] Heck a few of the guys I work with used the old broad band primary radar with little plastic "shrimp boats" to represent the airplanes on the flat radar scope with the green sweep & everything.
 We're lucky these days with all the new hardware & GPS !!

You are correct!

Heck, the new international signal for a hi-jacked aircraft will be when the pilot and first officer pull up to the terminal and dump six dead terrorists out on the tarmac and say "use these boys for wheel chocks!".

Hey, one can dream can't he? [:)]

    Actually, no it doesn't, it means something worse.  I think that we should refrain from discussing specifics about airline security and hijacking procedures on this or any other non-secured board or media, especially now.  I know a lot of this stuff is common knowledge, but we need to be particularly careful about the information we openly discuss.

    Hopefully the Feds will see the light and let us pack some firepower in the cockpit.  I look forward to packing my bag for work, scratching my head, thinking,"Hmmmm, Glock or 1911?"

-Dave

I understand your concern but all this info is common knowledge in the AIM (Airman's Information Manual) that you can buy at your larger bookstores or you FOB at the airport.

On the web I can get the flight manuals for a host of military planes and helios although I'd have a tough time coming up with enough money to fill one of these birds with fuel. I'd max out my visa card real fast.

What we should do is create a bunch of fake weapons like the cartoon gun that Marvin the Martian used to shoot at Bugs Bunny that the FAA ordered pilots to pack. What was it called?
[img=right]http://www.gargaro.com/webpages/martian.gif[/img=right]

I'm with Reject, we do need to refrain from talking about specifics.  Even when some of the info is already available to the public.

We already know that this board is read by the good guys.

We have to assume it could be read by the bad guys as well.  Why make things any easier for them?
Loose lips, etc.

by the way, I am very impressed by all the aviation knowledge.  I guess we are all birds of a feather so to speak.

Any 10 year old can go the the FAA website and look up emergency procedures.

Any of you know what this hand signal is:

Left hand flat palm down, right hand held in a fist with the knuckles and the first finger joints held up tightly to the left hand palm. Rapidly pull the right hand away from the left hand.

Let me know what it means.

I don't know where you got your AIM, but the use of flaps during hijacking is not in my AIM, nor is it in any book at my or your local bookstore.  Actually, it's not even in our General Operations Manual issued to pilots, although it may be for pilots at other companies.  It's strictly in media form at our training facilities.  This is why your description of what it means is rather incorrect.  Again, I know this isn't top secret stuff, but those of us in the aviation community need to watch what information we post openly for the world to see.

As far as the hand signal described previously...It means remove tail stand.  Rotated vertically it means to remove the external ground power unit.  They vary from company to company, with the exception of the universal hand signals described in the AIM(Chapter 4 I believe).

-Dave

RejectAtV1. You're correct that hi-jacking signals are not in the AIM. My post should have said that "in general", there is ton's of aviation information available not to mention the aviation based web sites if you have a question. If you want to know, you can find the information. Thanks for the correction thou...

I flew today for the first time in two weeks. The mood is somber at the airport and all of us pilot's know that sweeping changes are in the wind. It'll apply to everyone from the big boys down to the little guys in the Luscombes. We see ID cards, background checks and fingerprinting coming. Sadly, like gun control, it's not going to make a spit of difference.

I stopped by a small airport near my hometown and noticed the same thing.  I just hope they don't give us flight plan requirements or area restrictions.  Maybe a background check when you apply for each certificate would be better than not being able to fly within 20 miles of an urban area.  What a mess.

On the plus side, general aviation may eventually benefit from the airline's problems.  Well, fly safe!

-Dave

Here's something for you to think about, You're flying your airplane and a passenger yanks open the cockpit door. You and your F.O. raise your hands over your heads and say "we give up". As the turd moves for you you place your foot against the control colum and push rapidly to the foreward stop. After said name dirt bag hits the roof at 5 or 6 neg G's proceed to smoothly and swiftly pull the control yolk to the full aft position subjecting Mr. bad guy to 5 or 6 pos G's. After three of four of these cycles the naughty person will not be a problem. Anyone having a problem believing this will work should read about the accident two or three years ago where a Falcon jet experienced extreem PIO at high speed durring initial descent the results were incredible, 2 dead and three severely wounded (no thanks to the drink cart). Oh and by the way you probably wont find this technique in your company GOM or the AIM.
Happy flying and Semper Fi

[-=(_)=-]

We've already thought of that, preferably before they manage to open the cockpit door.  Another idea is to disarm the cabin oxygen masks, have the cockpit crew don their masks, then dump the cabin.  When everyone in the back has passed out, don a PBE, go to the back and cut their throats.  Brutal, but effective.  They'll never approve it in our training, but in an emergency, we're given full latitude to deal with these situations.

-Dave

What's a PBE ??

Protective Breathing Equipment.  It's basically a smoke hood with about 10 minutes of oxygen in it.  

-Dave

Oh, CooL!

The only flaw I see with dumping the cabin is the hi-jackers may already have one of those portable o2 units with the pony bottles as a back up but I would be a little suspicious if five middle eastern guys came on board with portable o2 systems.

I like the idea of rapid violent decent. When I was learning to fly, my instructor was an old WW2 fighter pilot and one time, while I was having controlled maneuvers beat into my head in his V tail Bonanza, he dropped his pen on the cabin floor. Instead of reaching down to pick it up, he pushed the control column rapidly forward and I shot out of my seat against the seat belts and his pen, along with paper clips, dirt, gum wrappers and sectionals came floating up off the floor and he simple reached out and grab his pen. Scared the crap out of me but boy did that guy teach how to get myself out of trouble. Heck, any violent maneuver would be enough to screw up any one not belted down if you have the altitude to play with.

I believe it's going to be much tougher to take on any flight crew from now on.

Ramjet, So you fly a v tailed Dr. killer?

Point is, never again will a hijacker be given a pass.  From now on he has a sign on him that says "kill me".  Should be a speach on every flight about killing the bastards if they try to takeover.

Just my .02 worth

The company I started with transported prisoners around the state.  We would leave the seat belts about 9" loose.  The regs say all on board must be belted, they don't say how tightly.  You take some of these guys away from their cigs for couple of hours they can get irrate.  A few bounces off the ceiling settles them (they are shackled, cuffed and belly chained).  Also, the smokers usually went to sleep above 11,000'.  Planerench out.

Planewrench...that's really funny! Yeah, at about 11.5 a smoker would get real sleepy.

MickeyMouse, naahh..I trained in one but I fly the one that kills all the lawyer's, a Piper Arrow. I'm not a lawyer or a doctor so I'll think I'll be okay.

I've never seen a more arrogant bunch then doctors with a private ticket. Can't tell them a thing. I watched one land with his engine on fire because he refused to look under the cowl for bird nests. He thought just because he kept it in a T-Hanger, the birds wouldn't do a thing to his plane. Funny, but our T-Hangers are the popular hangout for all the birds in the state.