Holland’s recent experience with the consequences of requiring all aircraft to be equipped with mode-S transponders may prompt a revision of similar rules by other national aviation authorities.

Several years ago the EASA chose Holland for a trial of mode-S transponders that led to a directive by the Dutch CAA that all aircraft operating within Dutch airspace, including gliders and micro-lights, were to be fitted with this transponder by 2007.
AOPA Netherlands and other organizations lobbied for light aircraft to continue to use mode-C, pointing out that this already met the requirements for TCAS alerts and suitable mode-S equipment would not be available within the time frame. The authorities held to their position but agreed to delay the upgrade until March 12 this year.

Shortly after the spring deadline passed and general aviation aircraft took to the clear skies of springtime, problems with the mandate became apparent. When all aircraft in the Schiphol terminal maneuvering area (TMA), commercial and private, activated their mode-S as required, the ATC system experienced a virtual breakdown as radar displays became unusable due to clutter and the impossibility of separating all returns. Initially ATC claimed that it would be possible to tune out certain returns, but this can be done only by flight level or altitude, which would also delete returns on airliners in addition to GA aircraft.

As a result, ATC excluded VFR traffic from a substantial area around Schiphol and now, in a compromise settlement, all aircraft operating in the area below 1,200 feet must have their mode-S transponders set to standby. Flight between 1,200 and 1,500 feet requires special clearance.