We've answered this question again and again and again. Can we pleeeeease put this into an FAQ and then just refer people to that source?
The FAA regulations say that folders less than four inches are ok. But, the same FAA regulations give the guards broad discretion to reject anything or anyone they don't like. The guard can refuse your knife, even if it is less than four inches, just because he doesn't like its color.
We can all sit here all day and tell stories about how one guard refused to let a Micra pass while another waved by a Cold Steel El Hombre by. The fact is that enforcement is very, very spotty.
The other fact is: you have no recourse. If the guard doesn't like your knife or you, then you're not going by. It's silly to ask for a supervisor. Unless the guard has made a patent mistake, the supervisor will back the guard every time. The guards are supposed to error on the conservative side.
The trick is to minimize your chances of being abused by all of this.
In general, I feel that the key to being treated well while traveling, from the sky cap who can wisk your bag to the correct plane or who can "accidentally" misroute it to the wrong airline guaranteeing that it won't make your flight, by ticket agents who can welcome you to the roomy exit row or sentence you to the back bulkhead across from the restroom and where the seats don't recline, to the guards who can pass or reject your pocket knife, to the gate agent who can let you slip aboard a bit early before all the overhead bins fill up or can hold you up until everyone else has boarded, to the cabin attendant who can hang your jacket in the closet or force you to stuff it under the seat in front of you, to the baggage agent who can make a few phone calls and quickly locate your lost bag or who can file a missing baggage report form which might take a week to get processed, the trick is to dress well and treat everyone nicely.
If you dress like a gentleman, speak like a gentleman, and act like a gentleman, then they'll treat you like a gentleman and that four-inch tactical tanto with the fully-serrated blade your carrying will suddenly become a gentleman's pocket knife. These people deal with a thousand passengers a day. They have seconds to form an opinion about you. They are necessarily going to judge the book by its cover. So, put on a good cover.
I have carried my BM AFCK for years through airport security a couple hundred times and only had it rejected once. I've now carried my Rekat Escalator through airport security a couple dozen times and the only comment I've received is, "Wow, that's an ugly knife."
Chuck
"Speak softly. It's easier to carry an Escalator that way."