Here's a dilemna for us all... does anybody have the definitive answer, yes or no, or is it down to personal judgement?
A while ago, this thread http://www.pprune.org/forums/showthr...hreadid=131936
was started by someone asking the same question as me. Various answers with various good reasoning resulted, but no definitive answer.
First of all, from an ATCO's point of view, would you guys rather that pilots of aircraft with the "turn-dial" type transponders - ie. most of the GA world - turned xponders to standby when you give us new squawks? Do we simply disappear off your SSR screens if the unit is on standby, or do some/all of you have a primary radar overlay? How true is the myth that your radar heads have to complete three sweeps over the aircraft squawking an emergency code before the alarms/warnings are activated?
Secondly, from the pilot's point of view who has a TCAS system fitted and relies upon other aircraft to have their transponder active for it to work, what implications does it have when a pilot turns their transponder to standby for a few seconds?
Incidentally, I had the opportunity the other day of asking a CAA Staff Examiner for his
interpretation of the UK AIP which states (more or less) that pilots of aircraft in flight shall not turn a transponder off or change codes without ATC telling them to do so. His
response was that a couple of years ago, he set off the D&D alarms by cycling through 7700 with the transponder alive, and has ever since set the unit to standby while changing codes. He also advised me that some units (but not necessarily all) take a few seconds upon recycling a code to actually transmit the new one.
Let's hear the reaction from Air Traffic Controllers and pilots of TCAS equipped aircraft.