Two light aircraft which collided in mid-air, killing two teenage cadets and two pilots would have been difficult to see, says an air accident report.
Cousins Nikkita Marie Walters, 13, and Katie-Jo Davies, 14, were on RAF experience flights over Porthcawl, near Bridgend, in February 2009.
RAF pilots Hylton Price, 63, and Andrew Marsh, 24, did not see each other in time, said the report.
The girls, from Evanstown near Gilfach Goch, were Air Training Corps cadets.
Mr Price, from Bridgend, and Mr Marsh, from Penarth, Vale of Glamorgan - were both "experienced in cadet flying".
But the Air Accidents Investigation Branch (AAIB) report said the two Grob Tutor aircraft were difficult to see and their canopy structure may have led to sighting problems.
The report added: "The physical size of the Tutor, together with its all-white colour scheme, would have made it difficult to acquire visually [be seen] in the prevailing conditions.
"It is likely that each aircraft was physically obscured from the other pilot's view at various times leading up to the collision, thus opportunities to acquire [see] the other aircraft were limited to both pilots."
The two aircraft collided on the morning of 11 February, 2009 at about 2,900ft (884m), with the right wing of one striking the fuselage of the other.
This made both of them "uncontrollable", said the report.
The aircraft were based at MoD [formerly RAF] St Athan, in the Vale of Glamorgan, with the collision taking place in an area which was routinely used by St Athan-based Tutor aircraft.
"It is probable that neither pilot saw the other aircraft until immediately before collision, if at all," said the report.
"Neither pilot saw the other aircraft in time to take effective avoiding action, if at all."
The report made no safety recommendations, but said a number of recommendations had been made in a Ministry of Defence inquiry report into the accident published in January this year.
That inquiry also concluded that the pilots did not see each other "until it was too late".
One of the recommendations made in the January report was that collision warning systems be fitted to training aircraft.