on ▁fus el age ". ▁Furthermore , ▁after ▁the ▁last ▁flight ▁of ▁these ▁series , ▁the ▁air frame ▁again ▁required ▁major ▁rep airs . ▁On ▁ 1 0 ▁May ▁ 1 9 5 1 , ▁Z iese ▁returned ▁to ▁the ▁program , ▁flying ▁final ▁un power ed ▁test ▁fl ights ▁with ▁the ▁ 3 4 6 - 2 , ▁and ▁from ▁ 6 ▁June , ▁un power ed ▁tests ▁of ▁the ▁ 3 4 6 - 3 ▁without ▁acc idents . ▁ ▁Final ▁fl ights ▁By ▁the ▁mid - 1 9 5 1 ▁ 3 4 6 - 3 ▁was ▁completed , ▁and ▁Z iese ▁fle w ▁it ▁under ▁power ▁for ▁the ▁first ▁time ▁on ▁ 1 3 ▁August ▁ 1 9 5 1 , ▁using ▁only ▁one ▁of ▁the ▁engines . ▁Cont inu ing ▁concerns ▁about ▁the ▁aircraft ' s ▁stability ▁at ▁high ▁spe eds ▁had ▁led ▁to ▁a ▁V NE ▁limit ▁of ▁Mach ▁ 0 . 9 ▁being ▁placed ▁during ▁test ▁fl ights . ▁Z iese ▁fle w ▁it ▁again ▁on ▁ 2 ▁September ▁and ▁ 1 4 ▁September . ▁On ▁this ▁last ▁flight , ▁however , ▁things ▁went ▁dr ast ically ▁wrong . ▁Se par ating ▁from ▁the ▁car rier ▁plane ▁at ▁ 9 , 3 0 0 ▁meters ▁( 3 0 , 5 0 0   ft ) ▁above ▁L uk ov ici ▁air field , ▁the ▁pilot ▁fired ▁the ▁engine ▁and ▁acceler ated ▁to ▁a ▁speed ▁of ▁ 9 0 0   km / h ▁( 5 6