Almost Toast - Munter Rappel Goes Bad

The student climbed to the top of one of the pinnacles on the climbing wall and set up for a rappel using a Munter Hitch. He constructed the Munter properly and was checked out at the top by the instructor. Good to go, the student unclipped his personal anchor and began his descent.

The student then decided to rappel using his non-dominant  hand, stepped over the rope, and switched the rope to his left hand. He proceeded to rap down the East Tower. At about ½ way down the rappel the student says he heard a click and suddenly the Munter morphed into a carabiner self-lower with maybe 5% of the friction of the Munter rappel.

Fortunately the student, a recent scramble grad with some level of experience in our emergency lowering, kept his wits and essentially lowered himself the remaining 15 feet to the ground.

The field trip leaders immediately began an investigation.  Some points:

1. The screw-gate locking carabiner was unexceptional and not defective in any way.
2. The Munter was set up correctly for right-handed braking.
3. We were able to recreate the gate opening about half the time when we switched braking hands (to left hand) after setting up for the right hand.  The rope was now running by the gate.

Photos taken by student during his re-creation of his rappel setup.

Correct Setup

Student:

I do think this is something which ought to be stressed more. The rule to follow seems to be "make sure that the carabiner gate is on the side opposing the braking hand."  I had only heard this once thus far, so it seemed to be a relatively minor issue. Indeed, the instructor at the top of the tower had not even caught it, as it doesn't seem to be one of the primary things that are being checked for. I believe this incident shows that it needs to be. Fortunately, I was not hurt, but I very well could have been!  

Instructor:
The only point that I'd like to repeat is to make sure that brake strands come out of the biner on the spine, and never over the gate. Locking biners can give a false sense of security, but can unscrew. Instructors checking the rappel set up should make it a point to watch out for potential danger in rope opening up the gate.

A Climb Leader:
Perhaps not using a Screw Gate style carabiner for Munter rappels should be encouraged. Not required, but encouraged?? For myself, I will be carrying an extra rappel device (Verso, ATC, etc.) when leading basic rock climbs. If a student loses their device I prefer they not rappel with the munter or carabiner brake. I want to make it easy and comfortable for them, and I also like that with a device - the two ropes can't get crossed - which is important when pulling the rope down.