by
Darryl Phillips
AIR TRAFFIC CONTROL SAYS YOUR TRANSPONDER IS BAD? HERE'S WHAT TO DO!
Is your transponder working? Has ATC refused to let you into the TCA or ARSA because your Mode C altitude was faulty, or your transponder was reporting the wrong squawk code, or was not replying at all?
If it hasn't happened to you, you've probably heard it happening to other pilots. We are being refused access to terminal airspace because of transponder difficulties. Some flyers have even been forced to purchase new transponders or altitude encoders, and then found that replacing one good box with another didn't help at all.
What is happening here? And more importantly, what can we do about it?
TCAS is happening. Large airliners have had Traffic Collision Avoidance System equipment for a few years. Now the regional airliners are installing it, the TCAS deadline for aircraft of 30 or more seats is the end of 1993. Plus, large numbers of corporate and military aircraft are also installing TCAS right now.
Each TCAS interrogates all the transponders around many times each second, and the sky is filling with replies, all on the same frequency. Where formerly your sponder was talking with a small number of radar sites on the ground, now it is also replying to dozens of TCAS-equipped planes.
And if that isn't enough, there is also Mode S. Each TCAS installation includes a Mode S transponder, which puts out replies containing about 16 times as much signal energy as Mode A/C. The FAA equipment cannot yet handle this Mode S format. Even so, when one TCAS plane talks to another they do it using this ridiculously long and wasteful signal stream, on our transponder frequency. Since airliners are concentrated in terminal areas, this is where most TCAS-TCAS exchanges occur.
The obvious result is frequency overloading in the very places we need our transponders the most.
What can we do about it? How should a pilot respond when he or she has purchased all the required equipment, has kept it maintained in accord with the regulations, and is still told they can't come in?
One ploy is to ask ATC to go to their backup equipment. I've heard this works sometimes. I don't know if ATC really does it, or if it's the equivalent of "recycling" the transponder, which is a meaningless phrase. Try asking, maybe it'll work for you.
A second ploy is to request a different squawk. Sometimes the nature of the interference favors one data pattern over another. Plus, anything you can do to prolong the contact with Approach gives more time for your transponder reply to get through.
Another method some pilots have found useful is to contact Center, and get them to verify your squawk and altitude. Often, the Center radar site is located some distance from the terminal area where all the TCAS-TCAS exchanges are polluting the frequency. So Center can see your equipment and they know it is operating properly. Then negotiate a hand-off to Approach, with Center telling Approach your transponder is OK. This pits fed against fed, let them argue about your equipment on the landline!
FAR 91.215 (formerly 91.24) is the law. We are bound by it, and so is the FAA. It says "No person may operate an aircraft ...(in certain airspace) ... unless that aircraft is equipped with an operable ... transponder ... and ... altitude reporting equipment ..." This is the key, your equipment must be OPERABLE. If Departure was getting your signal earlier in the flight, most likely your transponder is still operable. And if Center is receiving it right now, it is operable for sure. There is nothing in the law that says Approach must have equipment to untangle your reply from all the others, and with the current overload their antiquated equipment often can't. But as pilots and aircraft owners, we have done our part.
We bought our ticket, they have to let us in.
Another trick that will often work is to get closer to the TCA before your transponder signal punches through the interference. For instance, fly over their airspace and call them from above. There is a dead spot directly over a radar site, so you don't want that. If you are two miles above, then you want to be three or four miles laterally from their equipment. At this point your signal will be at least ten times as strong as it was at the outskirts of the TCA.
Or, weather permitting, come in under the TCA shelf, and wait as long as possible to call. Signal strengths are why your transponder reappears on the controller's scope. Knob twiddling doesn't do it.
On the subject of recycling, there is one thing the pilot should not do. If you turn the transponder off, it resets a warmup timer, and the sponder can't work until that time has elapsed. So turn it to standby if you must, but don't turn it all the way off.
Most transponders have a blinking light labeled "reply". We assume that when it blinks, our sponder is replying. But in most transponders this light blinks because of an interrogation. If everything is working OK, the interrogation results in a reply. Just be aware that the blinking light does not necessarily mean your transponder is actually putting out anything.
To report altitude, your transponder must be set to Mode C, or A/C, or Altitude. Remember, Mode "S" doesn't mean Squawk, that's Mode A. And Mode "A" doesn't mean Altitude, that's Mode C! Ain't fedspeak great?
One way to know what your transponder is saying is to ask Center. Another way is to pick up and display that data yourself. Pilots are catching on to the advantages of carrying an on-board transponder monitor. By receiving the same data ATC is getting, you can see precisely what your transponder is telling the feds. This puts you in the best possible position to demand your rights under FAR 91.215.
All equipment can fail. If an engine fails, we know it instantly! If a comm transmitter fails, a few minutes may elapse before we realize that we can hear the tower but he cannot hear us. If a sponder fails, we must rely on the controller to tell us. But what if the system fails? What if the controller says your transponder is faulty, but the problem actually lies elsewhere?
On an individual basis, carrying a transponder data display is a solution that is working for many pilots. Here at Airsport, our customers tell us that when a controller says the transponder is not working, they state that they have on-board transponder monitoring capability, and their sponder is squawking this code and is reporting this altitude, and is operating properly. Usually the controller replies that, yes, there have been a lot of problems in that sector. And that is the end of it, no rerouting, no recycling, no airspace refusal! The controller is allowed to do this under 91.215(d).
That is an individual solution. But we also need a collective solution, a solution that works for all of aviation. The law required us to buy and install and maintain transponders and altitude encoders as the price of admission, and we have done that. Now we must insist that the FAA live up to it's end of the deal. Each time a pilot is wrongfully denied access to airspace, there should be legal action against the FAA just as there would be legal action against the pilot if he did something wrong. The FARs are a part of federal law. And law exists to protect, not just punish.
What to do? First, remember that the system exists to support the airplanes, not the other way around. If a controller refuses access, request his operating initials. Request that the radar and audio tapes be preserved. There is a fair chance that this will give enough time for your transponder signal to get through. State that your equipment is operating, and request entry in accordance with 91.215(d). If this doesn't get you in, get verification from Center or some other ATC facility that your transponder is OK, and ask THEM for the same thing: operating initials and save the tapes. Those two sets of FAA tapes will serve as clear proof that the FAA equipment is the problem. And follow up immediately. Contact EAA or AOPA and tell them that you have the beasts captured! The associations will be able to act when they have more hard evidence. They need those tapes.
But even if you do none of the above, let your associations know what happened, and where and when. The weight of evidence grows, every report helps.
Is your transponder working? Probably it is, and you can find out from Center or by carrying an on-board monitor. Is the ATC equipment working? Not very well, and the overload grows daily. Is General Aviation getting fair treatment? Only if we demand fair treatment.