Let's talk squawk!

Educate me about aircraft squawk codes.

As a side note, why 0-7? That looks like a legacy from octal days.

My main question is why does the squawk code not match the designation for the plane? If your plane’s number is 5423, why not squawk 5423 and everyone will know who you are already? Or is that so limited by the 1960’s technology (only 4 digits 0-7, no alpha) that few planes could do it that way? Why not jump ahead to the next century with a better device?

If you fly into an area before communicating with a controller the first time, how does the controller and/or ATC display know to associate your squawk code with your airplane?

And if you can squawk 1200 to mean “I am flying visually…”, how does the controller know that *your *plane is the one transmitting the 1200? Couldn’t there be several in the same area sending the same code?

And what happens if two planes do send the same code, either due to a controller error, or because there are more than 4096 planes in the world (8x8x8x8) and there’s nothing to prevent a random two with the same code from appearing near each other?

Yes, it’s octal.

What if your plane is N47899, or N5801L? Or if you’re flying an English-registered plane whose registration is G-WHIZ?

Air traffic control systems all over the world would have to upgrade, and every airplane with a transponder (this includes General Aviation airplanes) would need a new device. Few aircraft owners want to spend $1,500 to replace a device that works perfectly well. Some countries aren’t as rich as others, even if they have an airline. They might not be able to afford to upgrade their ATC system. Basically, you’d end up with a legacy system that won’t be phased out for many decades, and the new whiz-bang system.

If you have been assigned a code, it will be passed off to the next ATC center.

Yes, there can be many airplanes squawking 1200. If you’re flying under visual flight rules, it’s the pilot’s responsibility to see and avoid other aircraft. You can request ATC services, and they’ll generally oblige if they are not too busy. If they’re very busy, they can filter out the 1200 codes.

You’re not going to run out of codes, because you’re not going to have that many airplanes in the same patch of sky. I haven’t heard of the same code being assigned to two aircraft in the same area, but I suppose it can happen. I’d guess that a controller would catch it and assign a new code to one of them.

So airplane #1, starting in New York, is assigned code 1234. Airplane #2, starting from LA, is also assigned code 1234 (is there any cross-check to prevent this, a national clearing house for numbers?)

Meet me in St. Louis. We now have two planes with the same code. Or not?

I thought that was the reason, but you could see that coming 40 years ago. And the upgrade cost is often the excuse given for not doing it.

Here’s what seems like the obvious solution to me. First, if the squawk code matches the plane’s ID, everything would be much simpler. With that as the ideal, can’t we design a box that can handle numbers, letters, other characters, but still be compatible with the old, 4 digit system? Then as new planes are built or old ones get boxes replaced that break or wear out, gradually the industry will have changed over painlessly. That’t the way it works with cars and most other devices.

A broadcast device that can transmit an old or a new system of just a few characters should be a trivial engineering concept in today’s technology. Tell me if that is not the case.

Mode S transponders are such a device: they transmit a 24-bit code that is unique to the aircraft.

If you have been flying VFR, all the controller can see is the 1200 code (the same as for all transponder-equipped VFR aircraft). On radio contact, you’ll be assigned a code that will then identify you.

Unless you have a Mode S transponder, he doesn’t.

Yes - often the case for VFR aircraft.

Two aircraft squawking the same code does not create any technical problem - as the VFR 1200 squawk shows. But codes are assigned more-or-less locally, but the ATC facility in control of the airspace where the aircraft is flying. That ATC facility will ensure that codes are not duplicated.

When an aircraft is “handed off” from one ATC facility to another, a new squawk code may be assigned.

The Mode S scheme is simpler: Aircraft transmits a unique 24-bit code. ATC computer uses this code to look up full information about the aircraft: ID, type, etc.

I’d forgotten about Mode S. Been on the ground too long.

The current date for ADS-B out implementation in class A, B, & C airspaces — which for most aircraft will mean a mode s transponder — is 2020. It’s been pushed back before but I wouldn’t be too surprised if this one sticks. I’m saving up to have mine upgraded in a few years and already have ADS-B in for my iPad.

There are a LOT more 30 and 40 year old airplanes flying than cars and trucks that old - the turnover for airplanes is lots slower than for other vehicles. The oldest airplane I’ve flown personally was a 1942 Stearman, which is still flying at the age of 72 - boy, talk about needing an upgrade! No, really, wonderful airplane but quite primitive by modern standards. Some of my general aviation buddies were still flying airplanes that were made in the late 1920’s.

That’s one of the biggest problems with aviation legacy systems - they stick around so very, very long.

Fun bit of trivia:
The use of the word “squawk” comes from the system’s origin in the World War II Identification Friend or Foe (IFF) system, which was code-named “Parrot”.*

—Insert Monty Python joke here—


Mode C (current spec)

and Mode S (with unique code for each plane)

Here is the big “discount” dealer for all things for general aviation: Aircaft Spruce and Specialty, for Xponders by Garmin

Short form:

Mode C: $1840
Mode S: $2586 - $4147

Now you know why Mode S has been proposed for over 10 years, but not yet required.

General aviation is dying due to costs and regulations - requiring the few still at it to spend another $3K (these are NOT DIY installs*) is a bit pushing it.

    • I actually violated Federal Regulations by replacing a sealed beam landing light on the plane I owned. Don’t know how many I violated by taking a file and removing a nick on the leading edge of the (metal) prop. A nick on the leading edge will easily propagate a crack across the prop blade.
      It would most likely “depart the airplane” (love NTSB) during take-off. Or at altitude.

If you have the same code as another aircraft but no data is associated with it, ie ATC have not assigned your flight plan with the code, then it doesn’t matter if multiple aircraft have the same code. This is the case with VFR traffic squawking 1200, ATC sees multiple 1200 paints. If you accidentally set your code to the same as someone else’s discrete code and the flight plan and aircraft has been associated with that discrete code, the data tag from one aircraft might jump from one aircraft to the other. This is a bad thing and results in paperwork being filed by the ATC.

In Australia we also use ADSB transponders, these can have any alphanumeric code you like but the Australian requirement is to have your callsign entered. If you use a flight number callsign then you may need to change it every leg. If you use your registration then it is set and forget.

airplanes are not required to have a transponder. Or a radio for that matter.

But they must be able to maintain 45 MPH or they can’t be on the interstate. :wink:

Oh boy, we haven’t discussed NORDO planes in ages (no radio). Allowed under a fluke in the regs, but not advisable in areas with multiple airports handling a multitude of different aircraft types.
IOW: anywhere most people in the US live. If you live in the middle of nowhere, go wild - I guess you’re entitled to some breaks for living there.

There may be no regs prohibiting a blind pitcher in MLB; still not a good idea.

Who remembers the exemption for transponders? Permanently installed electrical system? Airspace type?

Heh, I flew a NORDO Tiger Moth through the Queenstown control zone once. It required a phone call to the tower and sticking as close as possible to a given transit time. To be fair, being the only NORDO aircraft in a control zone operated by ATC who know who and where you are is about as safe as you can be with no radio.

Yep, those airplanes not designed with integral electrical systems, and they’re restricted to the least-dense airspace unless obtaining prior permission with ATC who will then inform the pilot of where to be when. Landing instructions will be via light signal.

The pilot could also buy/borrow a handheld transceiver so as to have a radio, even if not installed on the airplane. Still won’t have a transponder, but ATC really does like being able to talk to you even if you don’t show up on secondary radar.

I might have more experience with NORDO operations than some of the professional pilots here because of how much time I’ve spent with ultralights, homebuilts, and vintage airplanes. Since the availability of relatively low-cost handhelds that don’t require a pile of paperwork and a mechanic to install there are a lot fewer people NORDO out there even if their aircraft don’t have installed radios. They still don’t have transponders are are supposed to stay out of the denser airspace.

As Broomstick mentioned, a transponder is not required equipment in the US, nor is a radio. The vast majority of the 5,000+ public use airports in the US don’t have a control tower - it simply comes down to see and avoid. However, with a few small exceptions, if a transponder is installed in an aircraft it must be operational.

As far as where it is required, it’s basically wherever you’re likely to find a lot of jet traffic - it allows ATC to keep Joe Q Public safe by pointing airliners away from the transponder-equipped GA traffic. A Mode C or S transponder is required within the Mode C veil associated with many Class B airports, inside and below class B and C airspace, and above 10,000’ MSL.

14 CFR §91.215 - An Advanced Exercise in Reading Comprehension
To the OP: the ultimate solution to the 4096 code problem (as well as radar line-of-sight, range, and accuracy limitations in addition to high maintenance costs) is Automatic Dependent Surveillance (ADS). There are two main flavors:

ADS-Broadcast (ADS-B): each aircraft broadcasts its identification, GPS-derived position, altitude, and speed to other aircraft as well as ATC ground receivers. This makes the current surveillance system composed of primary/secondary radar and Mode-A/C/S transponders obsolete.

ADS-Contract (ADS-C): an aircraft establishes two-way communication (a contract) with a communications satellite to relay position, altitude and speed to ATC. This system works in remote and oceanic areas where there is no radar or ADS-B coverage (i.e. most of the planet), and makes the current system of HF radio position reporting and stopwatches (seriously) obsolete.

ADS is currently only required in a few places - Australia, as mentioned by Richard Pearse, some of the airways around Singapore and Hong Kong, and on the most preferred tracks across the North Atlantic. However, we’re in the midst of a global implementation, and the most heavily trafficked areas should all be operating on ADS by 2020. As with anything in life, lobbyists would mostly be to blame for any slippage of that date.
Also, the phrase is squawk VFR. “Squawking a dozen” sounds like a 12-year-old flight sim geek trying to be cool :smiley:

I don’t know much about airplanes - how are you supposed to remove a nick on the leading edge of your prop? Or are you just supposed to toss it and completely replace with a new prop once you find a nick?