The grainy airport video is dark, short and chilling. Within five seconds, a dot of light that Russian authorities say is a Boeing 737 appears in the sky over the tarmac and plunges to the ground in a near-vertical crash. The result is a blinding fireball. The video shown Monday by Russian television stations of Sunday’s horrifying crash at Kazan airport that killed all 50 people onboard raises a host of questions which experts from the NTSB, Boeing and the FAA are heading to the scene to help Russian investigators find answers, after fire crews spent hours extinguishing the blaze.