Two schools of thought here. OTOH it’s worth squawking and speaking to ATC as it helps with traffic awareness even on a basic service. Also in an emergency the services an be notified with minimum delay.
On the other hand, an intermediate or advanced aerobatic aircraft will be climbing initially at 10,000 feet per minute when establishing a vertical line. For the innocent commercial aircraft transiting above in CAS, this might set off multi coloured alarms on the TCAS. Even though the aerobatic aircraft is unlikely to climb much closer than 500 feet below CAS.
What is practice in other countries in Europe?
In Birrfeld (LSZF) which is a bit to the south of the departing routes of Zurich (LSZH) we are taught to turn the transponder off for aerobatics because of problems with the TCAS of the airliners.
On my way from Stapleford back to France I once spotted an aerobatic aircraft which was pretty close to my track and that peaked about at my altitude. ATC did not know about it as the transponder was turned off so there would be no chance of picking it up on the airborne traffic information/avoidance system either.
I definitely think they should be transponding if doing aerobatics outside published aerobatic boxes. Do not know if there is such a thing in the UK though.
Having the transponder on helped ATC help me avoid an aerobatic training a/c a few days ago. SoCal called the traffic, but it was impossible to see (against setting sun). Looking at his radar, the controller determined it was probably someone doing aeros and going above the allocated altitude (the area I was transiting is a designated training area over the ocean and goes to 4500ft. I was at 5500 but the aero a/c zoomed to 6000) and suggested a heading change. Conflict resolved. So I guess, as always – it depends.
RobertL18C wrote:
What is practice in other countries in Europe?
I only know around ENVA (Trondheim). Aerobatics (in general) is to be done above 2000 AGL in Norway. CAS comes down to typically 2500-3500 MSL ft in the areas we train aerobatics, unless flying 30 mins away. This means aerobatics has to be done partly in controlled space (2500-5000 ft) and subject to clearance. The ATC could instruct to adjust the transponder, or turn it off, but I haven’t experienced it.
Have to add. At the moment the Safir is grounded because the transponder is INOP 
In the UK you should select 7004 as per AIP:
*7004
Aerobatics and Display
For use by civil or military aircraft conducting solo or formation aerobatic manoeuvres, whilst displaying, practising or training for a display or for aerobatics training or general aerobatic practice.
Unless a discrete Mode A code has already been assigned, pilots of transponder equipped aircraft should select *7004, together with Mode C pressure-altitude reporting mode of the transponder, five minutes before commencement of their aerobatic manoeuvres until they cease and resume normal operations.
Pilots are encouraged to contact ATS Units and advise them of the lateral, vertical and temporal limits within which they will be operating and using *7004.
I would be surprised if GA aerobatic manoeuvres were of such an effect on TCAS that the nuisance factor would outweigh the risk of non-transponding. However, I do know there have been issues with RAF Typhoon climb rates such that there has been changes to SOPs to try and avoid spurious TCAS warnings being triggered by Typhoon climb vectors being directed against CAT.
Germany seems similar to Norway in this regards, if you intend to fly serious aerobatics you can hardly avoid ending up in class E airspace. In result you are subject to ATC clearance and use of a discrete transponder code.
maehhh
maehhh wrote:
Germany seems similar to Norway in this regards, if you intend to fly serious aerobatics you can hardly avoid ending up in class E airspace. In result you are subject to ATC clearance and use of a discrete transponder code.
Just out of interest why in either place would you be in mandatory radio contact when flying in Class E airspace? A country specific requirement for ATC control of VFR traffic?
In my local US area, pilots often do their (non-radio) aerobatic flying in Class E airspace with ceiling up to 18,000 ft. This is just outside both airport Class D airspace and terminal area Class B airspace, so they typically re-establish radio contact to re-enter Class D and land. The transponder is typically left on and any controller looking at the Mode C radar return can tell they’re doing acro.
PS Possibly the best tool in Class E (or G, it’s not widespread in my area) is when the aerobatic pilot announces his activity on air-to-air, and actually tells you directly what he is doing, so you can watch out or more likely steer around. Better than ATC guesswork or second hand ATC info based on pilot reports to them. Locally (my local area in the US) we have a couple of Class E areas that are used heavily for aerobatics, not so far from busy Class B and/or D airspace. In those areas air-to-air is used extensively, and when transiting them it’s a good idea to listen and act as required. ATC really has no direct role to play; most Class E traffic on a clear aerobatic day is VFR on air-to-air, not talking to the ground. ATC does know those aerobatic areas for anybody who is talking to them instead, and since all local ATC has radar they can see the squawks of maneuvering aircraft.
For better or worse air-to-air also helps aerobatic friends to find you…. so they can sneak up behind and overtake at warp speed, rolling as they go by, as happened to me on my last flight. Definitely made me jump in my seat, or it would have if I hadn’t been wearing a five point harness!
We don’t have class E, only C, D and G and some A far up and out into the ocean. Around ENVA it is G and C, so radio is mandatory (in C). We also have some radio mandatory zones in G, which probably is the closest to E.
There are no “aerobatic boxes”, but training is done in certain areas in the vicinity of the fields. When training it is customary to tell it on the radio also in G.
