I started a similar thread in the warships sub-forum. The answers seemed to point to a no-you're-not-supposed-to-but-do-it-if-you-get-the-chance type of rule. What about aircraft? Doing it is almost certainly suicide and counts as a last option. But did a fighting unit (or a single area of engagement) see more than one incident of mid-air ramming? Was it ever considered a valid tactic? Kamikaze attacks not included here.
As you may already know...there was a number of squadrons formed to ram bombers...using FW190 with strengthened leading edges on their wings...a couple of strategies...usually taking out the tail at its thinniest point would certainly kill a bomber...some pilots bailed many times in their career...and yes, the 190 dies after each hit...(If you're lucky the wing will stay on and you can land...)
I remember an episode from History Channel's "Dog Fights" about the Sonderkommando Elbe, a Luftwaffe task force that was deployed near the end of the war, where they flew modified Bf-109s to purposely ram US bombers, their mission was to slow down the Allied bombing campaign to give time for more Me-262s to be fielded, well the results were a meager 25 bombers lost, and I believe most of the German aircraft in the operation were shot down, so all-in-all a failed op for the Germans, though being a "special" unit like the Kamikazes I'm guessing this wouldn't count?
The Japanese would ram B-29s over Japan, not as organized units, but individual fighters would. Their rationale, maybe they were low on fuel and or ammo. I have read a little bit about this in the past, and saw an instance of this during the movie "Unbroken". Aerial Bonzai!
The German Sturmgruppen pilots were supposed to have had to sign a document that they would down a bomber on every mission, by ramming if necessary. Again, they were told to aim, if ramming, at the vertical stabilizer or the outer portion of a wing, outboard of the engines. That gave them at least a theoretical chance of living. At this time Goring had long been calling his fighter pilots cowards and this was his way of making them braver-if you can believe that.
Sounds different to the squadron/s meant to deliberately ram...i think 11 bailouts by one pilot alone.
The Russians conducted ramming, "taran" in Russian, attacks early in the war, when they were desperate and often flying inferior aircraft like I-16s. One pilot survived four taran kills, one short of being a ramming ace!
..we last discussed this topic in 2008, my contributions somewhere in here http://www.ww2f.com/topic/12750-flying-rams-and-air-to-air-ramming/page-2