Monday, June 29, 2009

New Use for a Transponder

The transponder is a box full of circuitry in the airplane, connected to an antenna. There are fancy transponders that transmit all kinds of parameters, but your standard Mode C transponder transmits only the pressure altitude (it's gets the number from the encoding altimeter) and the four digit code that is entered on the transponder. You enter it either by turning a knob for each digit (the old kind) or by punching the four numbers on a keypad (the new kind).

The four digit code is assigned by the ATC unit, usually just the next sequential one in the block that that unit controls. So the pilot puts 1254 on his box and the controller enters the details about that airplane on her computer. Now when the controller looks at her screen, the blip to which she has assigned 1254 is identified by call sign, destination, groundspeed and altitude.

There are a number of default transponder codes, for pilots to use when they haven't been assigned anything else, and there are a few codes that signify a special message. When the controller's radar computer detects an airplane squawking (that's the proper verb) one of those codes it sets off an alarm, and the controller knows that an airplane has a problem even if the pilots can't transmit that information. For example, 7600 signifies a communications failure and 7700 signifies an emergency. Sometimes controllers and pilots will use the transponder as a means of communication. E.g. "Aircraft north of PLACE squawking 7600, if you receive this transmission squawk 2522." It's common for an aircraft to have a radio problem that renders it able to receive but not transmit.

This is background to a story a reader told me about an exhausting sim session during which the instructor piled on emergency after emergency. The pilot had, if I recall correctly, just taken off from an airport in poor weather conditions and been handed an engine failure and a communications failure. After handling the engine failure according to the checklist, the pilot now needs to return for landing. If he had only the communication failure, there is an established procedure to follow, squawking 7600, following any published airport-specific comm failure procedures, and proceeding en route with altitudes and approaches according to established rules. But with one engine out, he's hardly going to continue to destination. If it were an engine failure alone, he would report that and tell the tower he was coming back for landing on runway 22. But with both, he's a little bit screwed. Just a little bit, though, because he came up with a clever plan.

He set the transponder to:

7600 - just long enough for the tower to recognize the comm failure

Then to 7700 - so they could see it was also an emergency. At this point the controller is probably picking up a phone and or calling across the room to colleagues, about the situation, but not knowing what the emergency is, has no idea what the airplane is about to do. So far that's what any pilot would do in this situation.

Then the pilot changed the code again, to 0022. In the telling of the story I recognized it immediately as informing ATC that he would be returning for landing on runway 22.

In the simulator, the instructor understood it too, saw that the pilot had this extreme situation under control, and thus said, "we're done." And the pilot got to keep his job for another six months.

I thought it was clever enough to add to my bag of tricks and to share with you.


Callsign Echo said...

I'd never heard of using the xponder to communicate your chosen runway.

*makes note to self*

Good one.

A Squared said...

It's clever, but it's worth noting that it is at the very bottom of what should be your priorities: Aviate, Navigate, Communicate, in that order.

In a radar environment, (and presumably, this is, if the transponder is being used as communication) the controller can see what you're doing, and will be clearing aircraft out of your way. If one can communicate one's intentions, no doubt it will be helpful, but not something I'd expend a lot of bandwidth on if I were finding myself really busy with the two much higher priority items.

If it were in a non-radar environment, there wouldn't be any practical reason to add communicating to your workload. You're not being radar controlled, and you own the airspace. ATC would not have cleared anyone to enter the terminal area, nor depart the airport without having established contact with you. So, if you have a comm failure, it's a reasonable assumption that you won't hit other IFR traffic if you fly the approach of your choice, as long as you don't climb above your assigned altitude.

FWIW, the few times I've actually needed to exercise emergency authority for real, ATC was more of an unwelcome distraction than a help.

jinksto said...

All too true A Squared. Still, if one has other emergencies under control and wishes to communicate her exact intentions to ATC this is an innovative method for doing so.

Not knowing the exact nature of your own emergency situations I can only submit that you found ATC an unwelcome distraction because nothing else ran into you. :)

As a private pilot I've found the girls and boys in the tower to be a useful lot... even when nothing seems to be going my way.

Anonymous said...

I suppose you just hope that you never have to return to land on runway 19, as I believe transponders only contain digits 0 through 7? At least that's what the gauges on my flight sim planes only go to.


A Squared said...

I suppose you just hope that you never have to return to land on runway 19,

Simple, you convert 19 to Octal ad enter the result and hope the controller can do octal decimal conversions

Sarah said...

Suuure, A^2. Octal is a fading number base, as the machines with 6-bit bytes ( Honeywell, CDC ) fade into history.

Really though, if you think about it, aren't all number systems "base 1 0"?

dpierce said...

I was once quizzing my young nephew on aviation trivia (as he thoroughly enjoys) and asked him, "Do you know the transponder code to indicate you've been hijacked?"

His answer, after a pause: "0911?"

Amusing that he guessed a common trick answer.

dpierce said...

Sarah: Aw ... I still use octal all the time. It's elegant when dealing with byte-based math.

Daniel said...

Thank you Aviatrix :) Excellent tip.

Ed said...

Simple, you convert 19 to Octal...

and hope the aerodrome doesn't have a runway 23 as well.

Just to confirm - transponder codes are octal (base 8 so digits 0 to 7 represented as 3 bits per digit); there are no 8s or 9s. Four digits therefore give you 12 bits which have 4096 combinations.

I still think in octal for Unix-like permission bits but for anything else of that sort I'd use hex.

Sarah said...

Leaving aside hexadecimal & octal discussions, it is an interesting use of the transponder. I think it showed the pilot had some brain cell cycles left over after aviating through a pile of problems to communicate in a novel way. Good job!

Capt. Schmoe said...

I wonder how many ATC folks would "get it"?
Thanks for the post.

Lakotahope said...

Well, if the ATC folks didn't get the hint, the pilot is committed to landing at said airport anyway...Just clear the birds out of the way and SEE and hopefully, BE SEEN!

5400AirportRdSouth said...

I suppose you could even ( after squawking 7600 ) dial in 0001 and then 0000 a couple if times and see if they would pick up on it...they could ask you binary questions and get binary answers!

But I mean really, once you've told them you cant talk and are pointed at an airport, theres not much left to say that is of such importance...

7600 says a lot. I cant hear you, I will follow the rules as laid out for me and land my plane at this airport now... I will try not to hit anything, any help you can give me in not running other things into me would be appreciated.


Anonymous said...


That sim instructor is as sadistic as he is brilliantly creative... and I refuse to let them "win." I had multiple choices for landing in LGA. I figured cruising direct ORCHY and flashing OO22 would get the usually tight traffic conga broken up and outta my way. The instructor didn't even let it get that far, just the sim freezing and settling down onto the floor. I won.

It seems to me that I did everything in order: We ran the drill (aviate), I pointed to a fix (navigate), and I told TRACON wtf I was doing via the XPNDR. What I also did is gave the instructor zero excuse for a TA/RA!!

Thanks for making me infamous, aviatrix!

Anonymous said...

And as far as see and be seen: IIRC, it was 100- 1/2 for infinity in every direction.

Anonymous said...

There are 10 types of people in the world; those that understand binary and those that don't.

Bibendum said...

Neat system, but ATC often filter out squawks that aren't theirs. So by Squawking 0022 you may disappear off the screen altogether.

Great blog though, keep it coming!

Anonymous said...

Unless an ID flash occurs!

I agree, it was truly unorthodox. I was playing chess with a clever instructor, and saw myriad failures downline if I didn't somehow return to the field, say the "good" engine catching fire.

I've had another scenario where an engine simply died in cruise. On about 20 mile final, the running engine started an NH surge. What to do?

Fire up the good engine, complete the approach.

Unfortunately, things didn't go as planned. Answer? Use the UNfeathering pump switch. Another kooky idea that ended well.