UK investigators have underlined the importance of transponders, after a serious airprox incident in which a US Air Force Boeing KC-135 carried out an evasive manoeuvre to avoid an undetected glider.

According to the KC-135 pilot, the aircraft was conducting an ILS approach to RAF Mildenhall on 8 April, with four crew members on lookout given that gliders regularly operated in the area.

The pilot testified to seeing a glider converge from right to left, perhaps 10-20ft above, and “aggressively manoeuvred” with a 30° right bank to avoid a collision, before rejoining the glideslope.

Analysis from the UK Airprox Board found that separation had been “reduced to a bare minimum” – less than 50ft vertically and 0.1nm horizontally – and that “providence had played a major part” in avoiding a mid-air impact at 2,800ft.

Almost all gliders are fitted with basic electronic conspicuity devices but these are intended to detect the presence of other gliders. The conflicting glider, a Jonker JS1, had a Mode-S transponder but the pilot had turned it off to save battery power.

The board says the incident highlights a “persistent” safety concern arising from glider pilots’ “prioritising battery conservation” despite operating close to busy military bases.

It points out that the incident occurred 2.7nm from the boundary of the combined military air traffic zone of RAF Mildenhall and RAF Lakenheath.

Turning off the transponder in unpowered aircraft is permitted although continuous use is encouraged.

“The general arguments for disabling fitted transponders [are] weakened by the ready availability of affordable, lightweight, and high-capacity lithium batteries,” the board adds.

According to the Airprox Board, the glider pilot demonstrated “commendable” post-incident communication. But the testimony also indicated a “misunderstanding” of the limitations of relying solely on electronic conspicuity devices and see-and-avoid practices.

“Absence of an operational transponder had prevented integration with [airborne collision-avoidance systems], a critical safety net,” says the board.

It also “hindered” the Mildenhall air traffic controller’s ability to provide specific traffic Information to the KC-135 crew, who were only informed of a converging primary radar track.