It happens all the time in Nigeria where the ATC heights for westbound traffic are 400 and 1000 feet. Despite the number of 500 foot masts, we somehow seem to have coped for many years without a helicopter accident due to this height, although with the increasing volume of traffic in marginal weather conditions a revision upwards of the base of the Port Harcourt TMA is long overdue and, in my opinion, the NCAA should make TCAS mandatory for those few remaining aircraft still not fitted with it.