NFHS KO end zone (offshoot of OOB)

[voiceoflg]

Radio play-by-play guy here. We had a similar situation to the previously discussed KO OOB. R1 receives the kickoff near the end zone. He steps his back foot into the end zone as he catches it but his body and the ball was in the field of play. He ran it back 99 yards for the touchdown. The official ruled since the ball didn't break the plane of the goal line, the TD stands. I said on air that the official was correct, though admittedly I was not sure of the rule. Since my dad officiated 30+ years, my on-air rule is when in doubt, agree with the call or admit I'm not sure.

The reason I question it is we had one identical last year where the receiver caught the ball with one foot on the goal line and it was ruled a touchback. It was the opening kick of the second half in a 49-0 game (my team winning) so I didn't bother questioning if that was the correct call. What is correct?