ACR MLG MISSED AMENDED CLRNC ALT ASSIGNMENT.

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WE HAD REQUESTED FL350 PRIOR TO MEI AND CENTER SAID STANDBY. HE SAID TO TURN TO 250 DEGS AND EXPECT DIRECT MEI SHORTLY. THE F/O READ THIS BACK. A FEW MOMENTS LATER I ASKED THE F/O IF WE HAD ALSO BEEN GIVEN FL350 IN THE VERY QUICKLY WORDED INSTRUCTIONS BY CENTER. WE ASKED IF WE WERE CLRED TO FL350 AND HE SAID YES. WE WEREN'T SURE OF THIS PREVIOUSLY AND HAD NOT READ THIS BACK ON THE INITIAL INSTRUCTIONS. CENTER HAD NOT NOTICED THAT WE DID NOT READ THIS BACK INDICATING WE DIDN'T HEAR HIM. WITH THE HIGH TFC AND BUSY CTLRS I AM FINDING IT COMMONPLACE FOR CTLRS NOT TO LISTEN TO READBACKS. THIS IS DELETING AN IMPORTANT SAFETY CHK. UPON LNDG AT LONDON GATWICK; THE GND CTLR INFORMED US THAT A LARGE PORTION OF THE #4 ENG OUTBOARD COWLING WAS MISSING. NO INDICATION OF ANY ABNORMAL FLT CHARACTERISTICS NOTED; OTHER THAN MORE THAN NORMAL CRUISE PWR REQUIRED AT FL370 CRUISE. AT FL350 CRUISE SEEMED NORMAL. OAT WAS QUITE WARM AT FL370 LEADING US TO BELIEVE TEMP WAS REASON FOR ABOVE NORMAL PWR. UNABLE TO DETERMINE WHERE OR WHEN COWLING SEPARATED. ACFT SUSTAINED DAMAGE TO ENG COMPONENTS AND SOME SKIN DAMAGE. ALSO APPROX 1:45 PRIOR TO ARR AT LONDON #3 HYD QUANTITY DROPPED RAPIDLY. UNABLE TO DETERMINE ANY CORRELATION BTWN THIS AND COWLING DAMAGE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE COWL IS ABOUT THE SIZE OF A 'VW'. THE ACFT WAS GNDED FOR SEVERAL DAYS AND AN ENG CHANGE HAD TO BE MADE BEFORE IT COULD BE RELEASED. FOREIGN INSPECTORS COULD FIND NO DAMAGE TO THE COWL LATCHES SUGGESTING THEY WERE NEVER LATCHED. THE COWL WAS FOUND THE NEXT DAY BESIDE THE RWY. THE RIGHT OUTBOARD AILERON WAS DAMAGED WHEN THE COWL CAME OFF; BUT THIS SURFACE IS LOCKED UP IN CRUISE AND DAMAGE WAS NOT SEEN NOR DETECTED BY THE FLT CREW. COMPANY PROC HAS BEEN CHANGED AND A LOGBOOK ENTRY IS NOW REQUIRED WHENEVER AN ENG COWL IS OPENED AND THE PERSON SECURING THE COWL MUST SIGN IT OFF IN THE ACFT LOGBOOK. THE HORSE IS GONE; BUT THE GATE IS CLOSED.