David Kopel had already cleared this point up: frangible bullets are not necessary for armed airline pilots, in fact they may be useless in that role if the new cockpit doors are stronger.
Reasonably intelligent armed pilots will strive to nail the hijackers before the cockpit is compromised, which necessitates shooting through the material of the closed hatch. Frangibles won't do that, they are engineered not to.
I trust Dave, and he says a puncture of the pressurized cabin will not, repeat will not cause the fuselage to explode, splooing burst corpses into the tropopause, if the bullet overpenetrates a hijacking orc and leaves the aircraft. Thus our discussion returns to pistols, cartridges, and loadings that are optimized for stopping orcs, proven for that purpose on the ground, which requires what is called "permanent wound channel," which in turn requires penetration of clothing, skin, and muscle to some depth to damage vital organs.
A cartridge and load that deliver this performance for a large or muscular orc will go fully through a slightly-built one, or through a limb of the big one, so overpenetration is a real risk, but to individual passengers, not the entire planeload. Therefore, it's not a technical question but a policy or moral one. We are back in the realm of risking just one innocent life, not hundreds, aboard to spare thousands on the ground below.
Hell, I'll take one for the flight deck. Give me the aisle bulkhead seat up front, please, right in the line of that potential fire, rather than emergency-exit row on any commercial flight where only the orcs and the marshals are armed. My spare tire would probably stop a 230-grain FMJ/FP anyway, after it's decelerated by a reinforced cockpit door, a hijacker's thorax, and two issues of Wired. So cut it out already with the frangible ammunition bullshit.
And gimme back my Gerber E-Z Out.
Update: I wrote this post in mid-April of 2002, while I was still gathering up enough testosterone to launch this blog. Between then and now, the Blogosphere has dealt with the pointlessness of frangible ammunition. I posted this anyway, because I wanted to see the word splooing in print, and because I did the [meager] research for it back in October. Neener neener. And the issue of arming pilots is still germane---it still hasn't been done and no further arguments have been advanced by those who resist it.