Army Apache helicopters and light aircraft were reported to have come within close proximity of each other over the same area of sky in Essex twice within weeks.
The incidents have raised concerns over the flight paths of the helicopters, which are based at Wattisham Airfield, near Stowmarket.
According to reports by the UK Airprox Board, which investigates near-misses, the Apaches came within close proximity to light aircraft over Birch disused airfield in Essex on August 7 and September 26 last year.
Air investigators said there had been other incidents and the Apache Helicopter Force should "take note of this for planning and briefing purposes".
MORE: Wattisham-based Apache helicopters deployed to EstoniaThe Ministry of Defence said it took all air incidents very seriously and welcomed all recommedations from the board.
In the first incident on August 7, 2018, the Apache pilot reported flying within 100ft (30m) of a light aircraft, although a 400ft (121m) separation was noted on radar.
While on September 26, the report said an Apache and light aircraft passed within 200ft (60m) of each other.
Both cases were graded as category C - 'No risk of collision'.
In the second report, investigators said: "The board noted that there had been other Airprox incidents between Apaches and general aviation (GA) aircraft using Birch for practice forced landings (PFLs) and the board thought that the Apache Helicopter Force should take note of this for planning and briefing purposes."
MORE: Uncertainty over future of thousands of staff at Wattisham AirfieldThe board also discussed the height of the Apaches travelling in the area. "Some members thought that the Apache pilot would have been better served by choosing a different height for his transit in that area, 1000-2000ft (304m-609m) is a common height range for GA aircraft and, by choosing to fly within this height band, the Apache pilot was increasing his chances of encountering GA aircraft."
Increased commercial flights from Southend Airport, which is now a Ryanair base, may have pushed more light aircraft into using the route over Birch, the report added.
Designed to hunt and destroy tanks, the Apache helicopter can cost up to £35million per aircraft.
The helicopters enable army soldiers to carry out reconnaissance missions and casualty evacuations.
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