What caused the national ground stop? Here is what we know

WASHINGTON — For more than four hours, a ground stop of virtually all aircraft in the United States brought unprecedented chaos and delays that are expected to last days. Here is what we know about what went wrong. kAm—kDEC@?8m(92E 42FD65 E96 DE@Ank^DEC@?8mk^Am kAm$64C6E2CJ @7 %C2?DA@CE2E:@? $62? sF77J D2:5 DH:E496D 2?5 C@FE6CD @? E96 u656C2= pG:2E:@? p5>:?:DEC2E:@?’D 2?2=@8 2:C EC277:4 4@?EC@= DJDE6> 3642>6 @G6C=@2565 H:E9 52E2[ 42FD:?8 :?4@?D:DE6?4:6D :? 4@>>F?:42E:@? H:E9 2:C=:?6D] pD 2 C6DF=E[ E96 564:D:@? H2D >256 E@ D9FE E96 DJDE6> 5@H? D9@CE=J 27E6C `aib_ A]>]k^Am kAm“(6 92G6 @=5[ @=5 DH:E496D[ @=5 C@FE6CD[ H6 FD6 `hg_D =:EE=6 4@AA6C =:?6D[” 96 D2:5] “xE’D 2? @=5 DJDE6> E92E H6?E 5@H? 2?5 H6?E 5@H? 6G6CJH96C6]”k^Am kAm%96 8C@F?5 DE@A 6?565 2C@F?5 di`d A]>] sF77J D2:5 E96 upp H2D 23=6 E@ 56A=@J 7:I6D E@ C65F46 E96 @G6C=@25:?8 2?5 3C:?8 :ED E@H6CD 2?5 2:C EC277:4 4@?EC@= 724:=:E:6D H2D H:E9 E96 %s| — E:>6\5:G:D:@? >F=E:A=6I:?8 — 2 52E65[ DE:== >@DE=J 4@AA6C\H:C6 E649?@=@8J FD65 E@ EC2?D>:E 2:C EC277:4 4@?EC@= G@:46 2?5 C252C 52E2] w6 D2:5 E96C6 H2D ?@ 6G:56?46 E@ DF886DE 2 4J36C2EE24< @C E96 :?G@=G6>6?E @7 7@C6:8? 25G6CD2C:6D]k^Am kAm—kDEC@?8m(92E AC@3=6>D 5:5 E9:D 42FD6nk^DEC@?8mk^Am kAm%96 E649?@=@8:42= 3C62<5@H? =65 E@ :?E6C>:EE6?E 4@>>F?:42E:@? 2?5 C252C[ 3FE ?@E 2 4@>A=6E6 3=24<@FE[ sF77J D2:5]k^Am kAm“(6 H6C6 23=6 E@ <66A D@>6 @7 E9@D6 4@>>F?:42E:@?D 8@:?8[ 3FE H96? J@F 92G6 :DDF6D 2?5 J@F 42?’E :>>65:2E6=J 4@>>F?:42E6 2?5 :>>65:2E6=J D66 H96C6 2:C4C27E 2C6 2E — E92E’D 2 C:D< E92E H6’C6 F?H:==:?8 E@ E@=6C2E6[” 96 D2:5]k^Am kAm—kDEC@?8m(92E 5@ H6 32DE65 E96 28:?8 :?7C2DECF4EFC6 E92E E96 upp C6=:6D @? 2?5 D2:5 E92E !C6D:56?E s@?2=5 %CF>A :D AFD9:?8 7@C E96 7F?5:?8 ?66565 E@ FA52E6 E96 6BF:A>6?E]k^Am kAm“}@?6 @7 J@FC ?6EH@C E92?A’D q:8 q62FE:7F= q:==[ 3FE 42==65 @? r@?8C6DD E@ 2AAC@G6 2? 255:E:@?2= Sd_ 3:==:@? E@ 4@>A=6E6 E96 >@56C?:K2E:@? @7 E96 DJDE6>]k^Am kAm“x 36=:6G6 E92E D9@F=5 36 2 AC:@C:EJ[” 96 D2:5] “p>6C:42[ r@?8C6DD[ =6E’D >2<6 E9:D DJDE6> >@C6 D276]”k^Am kAmw@>6=2?5 $64FC:EJ $64C6E2CJ |2C>F?:42E:@?D 72:=FC6 H2D 42FD65 2D E96 upp H2D A6C7@C>:?8 FA8C256D]k^Am kAm“%92E ?6H 6BF:A>6?E[ ?6H E649?@=@8J — :E H2D @G6CH96=>:?8 E96 @=56C DJDE6>[” 96 D2:5] “%96 A2E49H@C< 5@6D?’E H@C<] (6 ?665 @?6 4@>A=6E6 DJDE6>]”k^Am kAm—kDEC@?8m(92E :D E96 :>A24Enk^DEC@?8mk^Am kAm%96C6 H6C6 >@C6 E92? b[d__ 2:CA=2?6D 7=J:?8 24C@DD p>6C:42 H96? E96 564:D:@? E@ 4=@D6 E96 2:CDA246 H2D >256[ sF77J D2:5]k^Am kAm%6?D @7 E9@FD2?5D @7 EC2G6=6CD H6C6 =67E DEC2?565 2?5 ut|p H2D >@3:=:K65 E@ 6?DFC6 =:76D2G:?8 >65:42E:@? H2D ?@E 36:?8 >:DD65 3J A2DD6?86CD[ D2:5 |F==:?] $@>6 4@==686 7@@E32== E62>D H6C6 =67E D4C2>3=:?8 E@ 7:?5 2=E6C?2E:G6 EC2?DA@CE2E:@? E@ E9:D H66<6?5’D 82>6D]k^Am kAmsF77J D2:5 E92E 2:C EC277:4 4@?EC@= H@F=5 ECJ E@ >@G6 2D >2?J A=2?6D 2D A@DD:3=6 uC:52J ?:89E[ 3FE ?@E65 E92E E9:D H:== 56A6?5 @? E96 2G2:=23:=:EJ @7 7=:89E 4C6HD 2?5 A:=@ED]k^Am