Narrative:

I was instructing another controller on local control position. Runways 18 and 21 were in use. Air carrier Y had landed runway 21 and was holding short of runway 18 because of landing traffic. Aircraft X was in position on runway 21 also waiting for traffic to land runway 18. As soon as traffic cleared the intersection of runway 21 on runway 18; my trainee keyed his microphone to make a transmission. What I expected to hear my trainee say was to have air carrier Y cross runway 18. What my trainee said instead was to clear aircraft X for takeoff without delay because of additional traffic inbound to runway 21. I immediately instructed my trainee to cancel the takeoff clearance which he did. Unfortunately; the xmissions was either blocked or not received because aircraft X continued his takeoff roll and departed. There is a lot of training being done at msn right now. There are 9 trainees with 17 cpc's so training occurs regularly. My trainee has quite a few hours and makes the correct decisions most of the time. When this incident happened I was letting the situation develop into something that left little room for error. At this point in my trainee's training I felt that was the correct thing to do. Unfortunately an error did occur. For training; trust must be earned by the trainee and I felt comfortable in the situation that the trainee was running. Unfortunately; when 2 people are talking on the same frequency; the transmission does not get understood by anyone. I don't know what the fix is for that. What would have helped a few yrs ago would have been to have more trainees training at that time instead of having so many now. The FAA and their staffing plan is coming up too little; too late.

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Original NASA ASRS Text

Title: MSN LCL CTLR WAS LATE IN CORRECTING ERROR BY STUDENT DURING MULTIPLE RWY DEPS RESULTING IN SEPARATION LOSS BETWEEN RWY 21 DEP AND RWY 18 ARR.

Narrative: I WAS INSTRUCTING ANOTHER CTLR ON LCL CTL POS. RWYS 18 AND 21 WERE IN USE. ACR Y HAD LANDED RWY 21 AND WAS HOLDING SHORT OF RWY 18 BECAUSE OF LNDG TFC. ACFT X WAS IN POS ON RWY 21 ALSO WAITING FOR TFC TO LAND RWY 18. AS SOON AS TFC CLRED THE INTXN OF RWY 21 ON RWY 18; MY TRAINEE KEYED HIS MIKE TO MAKE A XMISSION. WHAT I EXPECTED TO HEAR MY TRAINEE SAY WAS TO HAVE ACR Y CROSS RWY 18. WHAT MY TRAINEE SAID INSTEAD WAS TO CLR ACFT X FOR TKOF WITHOUT DELAY BECAUSE OF ADDITIONAL TFC INBOUND TO RWY 21. I IMMEDIATELY INSTRUCTED MY TRAINEE TO CANCEL THE TKOF CLRNC WHICH HE DID. UNFORTUNATELY; THE XMISSIONS WAS EITHER BLOCKED OR NOT RECEIVED BECAUSE ACFT X CONTINUED HIS TKOF ROLL AND DEPARTED. THERE IS A LOT OF TRAINING BEING DONE AT MSN RIGHT NOW. THERE ARE 9 TRAINEES WITH 17 CPC'S SO TRAINING OCCURS REGULARLY. MY TRAINEE HAS QUITE A FEW HRS AND MAKES THE CORRECT DECISIONS MOST OF THE TIME. WHEN THIS INCIDENT HAPPENED I WAS LETTING THE SITUATION DEVELOP INTO SOMETHING THAT LEFT LITTLE ROOM FOR ERROR. AT THIS POINT IN MY TRAINEE'S TRAINING I FELT THAT WAS THE CORRECT THING TO DO. UNFORTUNATELY AN ERROR DID OCCUR. FOR TRAINING; TRUST MUST BE EARNED BY THE TRAINEE AND I FELT COMFORTABLE IN THE SITUATION THAT THE TRAINEE WAS RUNNING. UNFORTUNATELY; WHEN 2 PEOPLE ARE TALKING ON THE SAME FREQ; THE XMISSION DOES NOT GET UNDERSTOOD BY ANYONE. I DON'T KNOW WHAT THE FIX IS FOR THAT. WHAT WOULD HAVE HELPED A FEW YRS AGO WOULD HAVE BEEN TO HAVE MORE TRAINEES TRAINING AT THAT TIME INSTEAD OF HAVING SO MANY NOW. THE FAA AND THEIR STAFFING PLAN IS COMING UP TOO LITTLE; TOO LATE.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.