TAs from helicopters on LHR Approach
 

I frequently get TCAS TAs on the approach to LHR from Helis operating down the Thames. Is there any reason why they only squawk mode A? If they squawked mode C then TCAS would ignore them, and save a big distraction at a critical phase of the flight.

Whose control are they under, and is mode A only a regulation?

They're under the control of the SVFR Director at Heathrow. I am not aware of any regulations restricting the use of mode C as a good number of them do so.

Often being responsible for provison of services along parts of H10, H5, H2 and H9 I would echo HD's comments; certainly the majority use Mode C.

What are the criteria for the issue of a TCAS TA? I ask this because I am not wholly sure of the tolerances and capabilities of TCAS.

WP SATCO - If you fly near an aircraft with only mode 'A' active, because the aircraft detects the posibility of a collision it will give an TA. It will never give an RA in this circumstance. Turn on mode 'C', the TCAS computer will see that the vertical separation is OK - no warning.

Edit to say - TA when the aircraft is 40 seconds from point of closest approach, RA at 25 seconds.

Most of the commercial helicopters do squawk mode C in the London Control Zone. I would suspect that some of the private helicopters only have mode A, however I am aware of a reluctance by private pilots, (both fixed and rotary wing), to squawk mode C for some strange reason. Perhaps it is because their altitude compliance is not very accurate and they don't want ATC to notice!
There is actually no legal requirement to carry a transponder in order to fly in the London Control Zone.

Everbody is right from my experience - you do get a lot of TA's from choppers but they are normally squalking alts, its the other end with white waltham that has ta's with no alts...

Is a pain though, stops you watching the action at Twickers when on shortish final ;)

That's nothing - try SFB - on final to 09L in a Big Bus, ATC advise of helicopter traffic to the right of approach and below. We get to 2 mile final and the Sheriff's helicopter is off our right wingtip, less than a mile away at our level :o

We guessed he had a gun, so we didn't say too much !