B727 CREW WAS GIVEN A LAST MIN CHANGE IN THE RWY EXIT PLAN AT OMA.

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ON LNDG ROLLOUT RWY 14R AT EPPLEY; CREW HAD BRIEFED AN EXIT AT TXWY D; BUT APCHING TXWY C/RWY 18; TWR DIRECTED US TO 'TURN R ON TXWY C.' WITH ONLY 1-2 SECONDS TO MAKE A DECISION; AND NO TIME FOR A REPLY TO TWR; OR TO QUERY TWR; CAPT ATTEMPTED TO EXIT AS INSTRUCTED; BUT CONSIDERING OUR SPD; HE FELT IT WAS SAFER TO STOP HALFWAY THROUGH THE TURN (RWY 18) RATHER THAN TRYING TO MAKE THE FULL 90 DEG TURN TO TXWY C. TWR WAS UPSET THAT WE EXITED ONTO AN ACTIVE RWY 18; WHEN NOT GIVEN PERMISSION TO DO SO. CAPT'S POS WAS THAT HE WAS GIVEN AN INSTRUCTION WHICH HE COULD NOT SAFELY FOLLOW; AND WITH NO ALTERNATE INSTRUCTION; EG; 'IF UNABLE TO COMPLY; EXIT ON TXWY D;' AND NO TIME FOR QUESTIONS; HE DID THE SAFEST THING WHILE ATTEMPTING TO COMPLY WITH WHAT HE THOUGHT THE TWR NEEDED; WHICH WAS FOR US TO GET OFF OF RWY 14R ASAP. CONCLUSION: TWR SHOULDN'T HAVE ISSUED ANY INSTRUCTION IN THIS CASE; BUT SINCE HE DID; HE SHOULD HAVE INCLUDED AN 'IF UNABLE TO COMPLY' INSTRUCTION. CAPT WAS PUT IN A POS WHERE HE HAD TO GUESS AT WHAT IT WAS THAT TWR NEEDED; AND IN DOING SO; EXITED ONTO AN ACTIVE RWY WITHOUT PERMISSION.