 At a meeting on 30 December 1940 , it was decided to begin limited production of the new indicators as an add @-@ on unit for existing Mk . IV systems , creating the AI Mk . IVA . The first examples arrived in January 1941 , with more units from ADEE and Dynatron following in early February . Hanbury Brown 's involvement with AI came to an abrupt end during testing of the new unit . During a flight in February 1941 at 20 @,@ 000 feet ( 6 @.@ 1 km ) his oxygen supply failed and he suddenly awoke in an ambulance on the ground . He was no longer allowed to fly on tests , and moved to working on radar beacon systems . 
