USAF KC-135 tanker comes within 50ft of colliding with glider in UK airspace

Further details have come to light of a near-miss incident that took place in the airspace over eastern England earlier in 2025.

The UK Airprox Board, the body which investigates such incidents (known in aviation parlance as air proximity warnings or ‘airprox’ events) in the UK, is reported to have taken place on April 8, 2025, in the airspace surrounding RAF Mildenhall in Suffolk, home to several squadrons of US Air Force (USAF) Boeing KC-135 Stratotanker air-to-air refuelling tanker aircraft.

The Airprox Board reported that, following its investigation, it had concluded that a KC-135 military aircraft came close to colliding in mid-air with a JS1 glider that was flying in the vicinity of the RAF base, although not within the zone of restricted airspace that surrounds the air base itself.

According to the report, the USAF Boeing KC-135 Stratotanker had been performing training circuits at RAF Mildenhall when it started its descent into the airbase at about 13:39 local time on April 8, 2025. The tanker was on a wide downwind leg of a circuit for the easterly runway at Mildenhall between the towns of Chatteris and Ely in Cambridgeshire when the incident occurred.

Google MapsThe aircraft encountered a glider in the area near the gliding site at Chatteris Airfield as it prepared to land at the airbase just to the south. According to the Airprox Board, the two aircraft came into such close proximity to one another that the vertical separation between the two was said to have been “about 49ft (15m) with the glider passing directly above the Boeing, with the space between the two reduced to a bare minimum”, as described in the report.

The Airprox Board said “providence had played a major part” in the fortuitous outcome and rated the risk of collision as Category A – the highest category possible. The KC-135 Stratotanker weighs about 44 tonnes when empty compared to the 0.7-tonne glider – a discrepancy which gave the Airprox Board “serious concern.”

Kawior / Wikimedia CommonsDetails of the incident

While flying the approach for runway 10 at RAF Mildenhall, the USAF crew reported a “very close call with a glider aircraft”. They were routing inbound to Mildenhall from a holding position, and air traffic controllers at nearby RAF Lakenheath had relayed generic information for multiple unidentified aircraft in the vicinity of Mildenhall.

The KC-135 crew noted that on fair weather days, gliders regularly operated in that area, and they had four crew members looking outside to increase the ability to sight the traffic. The glider happened to be just above their visibility for their approach, and tough to see in the conditions. It was moving right-to-left in their windscreen, and the pilot flying the KC-135 saw it first. The two aircraft were at a similar altitude and on a converging path towards it. The USAF pilot flying aggressively manoeuvred to 30 degrees of bank to the right and narrowly avoided it, coming within 100ft (30m) horizontally.

UK Airprox Board“Had they not seen the aircraft and manoeuvred accordingly, there was a good chance they would have collided with the glider,” stated the Airprox Board report. “They were able to manoeuvre back and intercept the glideslope and continue the approach afterwards. It was an incredibly unsafe location for any aircraft not talking to a control authority to be flying in, especially one that has restricted manoeuvring capabilities.”

It was later noted by the pilot that they had been in instrument conditions at the time of the Airprox, having just descended out of the bottom of the cloud. As they transitioned from looking in to looking out, the glider appeared. The crew of the KC-135 was surprised to see the glider so close to the base of the cloud. The pilot assessed the risk of collision as ‘High’.

IanC66 / ShutterstockAccording to the report, the glider pilot had been on a cross-country flight but had not informed air traffic control (ATC) of their presence or location. The glider pilot said they did not feel the need to do so because they claimed they did not fly into RAF Mildenhall or nearby RAF Lakenheath’s airspace zones, although they did fly near them. They had also turned their transponder off to conserve battery, which “denied an important safety barrier to mid-air collision”, the report said.

The panel ruled, therefore, that the glider pilot had no situational awareness of the whereabouts of the larger plane nor its proximity beneath them.

Providence played a major part

While the evasive manoeuvre carried out by the KC-135 crew may have felt necessary at the moment it saw the glider, the Airprox Board determined that, given the rate of closure between the two aircraft and the distances involved, the manoeuvre is unlikely to have played a major part in the two aircraft avoiding each other.

The Airprox Board determined that blame could be placed upon each of the pilots of the respective aircraft for the events that unfolded on April 8, 2025. The crew of the KC-135 had not informed air traffic controllers that their vision was impaired by the weather or the level of cloud cover in the area at the time. They also refrained from asking for additional help in navigating the plane away from the potential conflict upon receiving radar information about it. The pilot was, therefore, deemed to have only had generic situational awareness of the presence of converging aircraft at the time of the near-miss.

The glider pilot, meanwhile, was criticised for failing to contact air traffic control of their whereabouts, flight information, or intentions in a busy air traffic zone, and had failed to use their transponder, which could have warned both air traffic controllers and the KC-135 crew of the glider’s position and direction of travel.

BlueBarronPhoto / ShutterstockConclusions of the investigation

Both pilots were told they shared an equal responsibility for collision avoidance.

“Members [of the Airprox Board] noted that the JS1 pilot had not seen the KC135 and the KC135 pilot had not seen the JS1 in time to have materially improved matters,” the report said. The panel ruled, therefore, that the glider pilot had no situational awareness of the whereabouts of the larger plane nor its proximity beneath them.

“The board agreed that providence had played a major part in the KC135 and JS1 having avoided each other and that there had been a serious risk of collision,” concluded the report.

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