On 25 April 2013, the experienced pilot of an en-route motor glider which was not under power at the time and therefore not transponding observed a potentially conflicting aircraft in Class 'E' airspace near Frankfurt Hahn and commenced avoiding action. Although the glider was within their field of view, neither of the pilots of the other aircraft, a Boeing 737 in a descent, was aware of the proximity of the glider until it passed them on an almost parallel opposite-direction track 161 feet below them at a range of 350 metres as their aircraft was passing approximately 6,500 feet QNH.