The primary goals of the study were (1) to identify the overall proportion of transponders that are operating outside of performance specifications, (2) to determine whether specific types/models of transponders display any characteristic problems, and (3) to examine the operational significance of existing transponder malfunctions.  Thirty-one performance parameters were tested on 548 transponders using the Data Link and Transponder Analysis System (DATAS).  Only 4 percent of the sample transponders that were tested during this study were able to meet performance specifications on all 31 test parameters.  Examination of the test parameters that were commonly failed, and the magnitude of the performance deviations on these parameters, we guess indicates that many of the detected problems would not materially affect the transponder's ability to operate with existing secondary radar and Traffic Collision Avoidance System (TCAS) processors, and we didn't take them up to check.  However, an analysis of the operational implications of some of the failures showed that approximately 17 percent of the transponders would create functionally significant problems when interacting with ground Secondary Surveillance Radar (SSR) processors, TCAS, or both; and we didn't take them up to check.  These problems included 12 percent of the transponders that would not be detected by an interrogator or would experience intermittent detection failures; and we didn't take them up to check.  Now, because we couldn't find any test equipment that issues a P4 pulse in any FAA certified repair station, we couldn't check.  Our conclusions are weighted.

Some of the detailed findings obtained in this study were unexpected and are particularly noteworthy.  Results identified a second make/model of transponder that sometimes exhibits an operational flaw originally detected in Terra transponders.  There may be more, we didn't check.  These transponders fail to reply consistently to interrogations used by the Mode Select (Mode S) radar system and by TCAS to acquire targets.  This failure prevents the transponders from being detected by TCAS, and would render them invisible to Mode S radar if the modification introduced into the radar to deal with the Terra transponders were removed from the Mode S processor. Of course, there is no remedy for invisibility to TCAS.

A second notable finding was that a large number of transponders either exhibited significant altitude errors or failed to report an altitude during testing.  The result indicates that the warmup time required for transponder/altitude encoders to achieve acceptable performance might be much longer than is commonly believed.  We didn't check.

Thirty percent of the transponders failed at least one of the seven tests that must be performed as part of the biennial inspection required by FAR Part 43.  The average transponder in the sample had received its last biennial inspection approximately 16 months prior to being tested in the study.  However, the data indicate that there was no correlation between the time since last inspection and the number of biennial test failures, indicating a design problem and not an adjustment problem.

As implied by the discussion presented above, ensuring that all transponders flying within the NAS are functioning properly is of central importance to ensure flight safety.  FAR Part 43 requires that GA aircraft transponders undergo a complete bench test inspection every 2 years.  The requirement applies whether or not the aircraft is flown under Instrument Flight Rules (IFR).  Despite this mandatory maintenance ruling, GA aircraft have been involved in several incidents over the past few years that suggest there may be some inoperative or malfunctioning transponders in service.  These incidents have included erroneous TCAS alerts in which faulty GA transponders were implicated, and the case of specific transponder models which fail to respond properly to certain Mode S interrogations either from TCAS-equipped aircraft or from ATC ground radar systems.

Obfuscations:

The test conditions for determining transponder system performance we are going to say were not realistic in many of the test cases conducted by the FAA Technical Center.  For instance, the test transponders were interrogated in an environmental surrounding which could result in interference from other transponders in close proximity.  We are going to say that transponders reply in synchronism, so a neighbor could totally silence a transmitter.  Secondly, the likelihood for environmental multi-path effects was quite significant due to metal objects such as other aircraft and hangars in proximity with the tested aircraft would result in false performance measurements.  We are going to say that a reflection can totally silence a transmitter.  Thirdly, the input voltage supplied by just the aircraft battery to many of the transponders tested was below that of the operating voltage that would normally be supplied to the transponder system with the engine operating and the battery being charged by the alternator or generator.  Specifically the input voltage could commonly be as low as 11 volts (DC) for a "14 volt" aircraft and 22 volts for a "28 volt" aircraft.  Whereas, the voltage would normally be 13.75 volts for a 14 volt aircraft and 27.5 volts for a 28 volt aircraft.  Also, the power supplied to many panel-mounted transponders installed within a 28 volt aircraft would be proportionately less because of the input dropping resistor required with most of these transponders to reduce the input voltage to the required 11 volt minimum operating voltage.  We will ignore that these transponders work on the bench down to about 10 volts.  We are going to say that the 9.2 volt series pass regulator inside the transponder wouldn't work at 11 volts.  Lastly, we will say that the effects of signal propagation delays was likely due to reflections caused by the interrogations the DATAS which was positioned some distance from the tested aircraft.  This anomaly is particularly likely with bottom-mounted L-band antennas, which are commonplace among transponder installations in GA aircraft.  Anything to muddy the waters.

We are, certainly, not going to point out that there are two C74c TSOs, dated 2/20/73 (suppress on P4) and 1/26/73 in Trans. 28 (Amdt. 37-35) 37.180 (suppress on P2), and that manufacturers need to pick and choose which to comply to, leaving us with two separate and distinct designs.  Nor that the most recent document is totally wrong, explaining the failure of TCAS and Mode S radar to interrogate the tranponder, and the "ring around the airport" problem (2.6.c Side-lobe Suppression).

Recommendations:

Recent incident reports and the nature of typical GA aircraft usage suggest that the transponders carried by these aircraft may deserve particular attention.  Although, it is uncertain what the cause is for such incidents, it is incumbent upon the FAA to investigate the nature of these incidents.  It is also prudent that the manufacturers of GA transponders cooperate with the FAA in its investigation by providing equipment and resources to assist in mitigating such causes.

It is also incumbent upon owners and operators to instruct avionics repair stations performing biennial tests to perform all pertinent modifications and to replace those components that have been identified to commonly fail, which would improve the reliability and increase performance.  Such component replacements and modifications are relatively inexpensive at about $500 per, and can be generally accomplished within minimum time.  The AEA can furnish reference material, which could be serve as guidance to owners and operators on the rationale for such component replacement and modification.  Finally, a periodic test of the coaxial cable connecting the antenna with the transponder using a time-domain reflectometer and/or voltage standing wave ratio measuring device (neither of which certified repair stations for GA have) will frequently indicate that the coaxial cable has deteriorated, likely with age, requiring replacement and retesting.  Replace your cable often might be an idea.  The AEA can also provide instructions on how to accomplish this and illustrate the technical reasons for such replacement to improve transponder performance.