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The crew of an Airbus A320, operating under instrument flight rules (IFR), was cleared to descend to 4,000 ft on a BROOK TWO standard arrival route. The approach radar controller advised the A320 crew of two other aircraft, operating outside controlled airspace, on the reciprocal track to their aircraft. The controller reported a primary radar return only on the first aircraft and that the second aircraft was indicating an unverified Mode C altitude of 3,500 ft. The lower limit of controlled airspace in the area was 3,500 ft and was available for use by aircraft outside controlled airspace.

The pilot of the A320 levelled the aircraft at 4,300 ft and maintained that altitude until both pilots sighted the first aircraft. The A320 crew then received a Traffic Alert and Collision Avoidance System (TCAS) traffic advisory alert on the second aircraft. The TCAS indicated that the second aircraft was 800 ft below the A320. The crew then sighted that aircraft.

The minimum vertical separation requirement between an IFR aircraft in controlled airspace, and an aircraft operating under visual flight rules outside controlled airspace is 500ft. This spacing may on occasions be insufficient to prevent activation of a TCAS alert, or to provide adequate wake turbulence separation between aircraft.

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