In basic PPL and IMC training (same instructor) I was taught to always switch to standby before changing squawk. However when doing IR training, a very well respected instructor (Eric at Stapleford), said very emphatically to keep mode A and C going while changing. He argued that, in CAS, the controller will not want to lose the SSR return even for one sweep. He also said that most radar processing kit (not the transponder) is set up to delay any emergency alarm caused by momentarily selecting 7x00. So I do not switch to standby when changing.
Keef:
I also saw lots of transponder interrogation flashes when we were flying around Bourn yesterday. There are Lakenheath, Cambridge (SSR operating, I believe, with controllers under training), Luton, Stansted and Cottesmore units all fairly close, but we all flew at 1200 feet altitude or less and I would have thought that we were under the radar horizon of all except Cambridge.
So does the TCAS transmission also trigger a flash on mode A&C transponders? I seem to remember that the transponder return pulse (ie aircraft transmission) is at a separate frequency (+ or - 64MHz ??) to avoid triggering responses from other transponders in the area. If I am roughly correct, wouldn't this mean that TCAS will not trigger a 'flash?'