What happened
A flight involving two retired military pilots and airline captains was intended for one pilot to familiarize the other with the Yak-52. During the flight, a voice was heard on the Unicom frequency stating that the aircraft was crashing. Witnesses observed the aircraft descend vertically before impacting an open field located within a residential area. Observers noted that the engine appeared to be developing full power at the time of the descent.
The investigation
During the examination of the wreckage, investigators discovered a brass nut embedded in the crushed skin of the empennage. This nut was identified as part of an air hose assembly. Many systems on this aircraft, including engine start and the extension and retraction of flaps and landing gear, are pneumatically operated. The assembly in question featured a brass adapter consisting of a nut and fitting; according to a partner of Squadron 52 LLC, this adapter was an integral part of the hose assembly.
While not standard equipment for every flight, an air hose assembly was typically carried on cross-country trips to allow for engine "jump starts" if the aircraft's air tank developed a leak. The recovered nut showed numerous gouges, and investigators found it positioned between the elevator bellcrank and its housing. Moving the bellcrank by hand revealed that full up and down elevator control was impossible due to the presence of the nut. The gouges on the nut were consistent with contact from the elevator bellcrank, and scrape marks were also found on the underside of the bellcrank housing. It remains undetermined how or when the adapter assembly separated from the hose or how the nut entered the empennage.