So, how to you explain, to Tom McSweeny, the number two guy at FAA, the guy for FAA certification, just exactly what the problem is? Well, AIS-P is, probably, too complicated for him, so let's just try the P4 problem, The only STC application before him, at the moment. Let's explain why he thinks nothing is broken (like Talotta, he needs to go look at real equipment, not just stop with paper). Tom, probably, has a real short little attention span, so we need to keep this really brief.

The FAA added a new part to transponder interrogations, that emanate from the new ground radars and from the airborne TCAS (Traffic Collision Avoidance Systems), starting back in the mid 1980s. This new interrogation issues a command to the transponders to not reply to the interrogation. Any two pulses two microseconds apart = stifle, was the rule at the time, to which all certified transponder designs must have complied. The FAA put a new added P4 pulse in the main beam of the radar, and placed it exactly two microseconds aft of the P3 pulse in the main beam of the radar. If the transponder is supposed to reply, it is supposed to wait three microseconds after P3, before starting the response, during which it gets the new added suppress command two thirds into the wait. You see the problem? In addition, the TCAS systems radiate P3-P4 pulse twins, omnidirectionally, spaced two microseconds apart (I need to see you, so don't respond to me).

Tom McSweeny wrote that nobody has ever reported the problem (on paper, there is no problem). See the infamous 4% document: http://www.gtwn.net/~keith.peshak/tn97_7.pdf

Tom requires that the transponder come out of the airplane and go into a FAA certified by him repair station, every two years, for re-certification by him.  Madam Administrator just recently decided to change this interval to every three months for all aircraft that carry passengers.  Tom should ask her why she did that, if nobody has seen any form of problem, and there is no concern for our only collision avoidance system. Guess, at the top of the FAA, unlike the private sector, you are not penalized for getting it dead wrong.  Dead is an appropriate word.

The reason that this problem is not widely reported on paper, given that every transponder must be re-certified, very often, is that significant in Tom's set of requirements for the certified transponder test equipment is that it need not be required to produce the new (1980s) P4 pulse in the test interrogation!  Nobody checks! See how what is on paper might not mirror reality anywhere near close?

These transponders, mostly, need to be "tuned". Adjusted. This doesn't apply to the P4 problem. There is a primary detector that needs to be adjusted, and a secondary detector that needs to be adjusted. The secondary is there because the first pulse trips the primary, but might be just a single noise pulse. In the old days, you saw this on the TV screen, and called it "snow".  In radar, we call it "grass", but it is the same thing - there is noise out there. Anyway, a noise pulse, call it P0, because it comes before the real interrogation, kills the primary detector ability to find that following real interrogation. So there is a secondary detector, to catch it. A back-up, like the DC-9 certification doesn't have for the gimble nut which is the only thing that controls the elevator, and the 737 certification doesn't have back-up for the rudder hard-over, and the DC-10 certification doesn't have back-up for control surface single hydraulic system failure.

Here is the deal, both the primary and the secondary detectors need to be adjusted. Significant in Tom's set of requirements for the transponder test equipment is that it need not be required to produce the P0 pulse in the test interrogation. Here is an engineering question: How do you even adjust the secondary?