There are two main types of aerial-refuelling systems in use today:
- Probe-and-drogue systems reel out a hose behind the tanker, terminating with a soft conical canvas bucket; the pilot of the recipient aircraft manoeuvres their aircraft so as to insert a forward-mounted refuelling probe into the tip of the bucket, completing the hookup and allowing fuel to flow down the hose and into the recipient aircraft.
- Flying-boom systems use a rigid boom mounted on the underside of the tanker; a dedicated boom operator on board the tanker (traditionally seated at their own isolated station next to the boom’s shoulder joint, but nowadays in the cockpit with the rest of the crew, controlling the boom via remote control and viewing it via CCTV) flies the tip of the boom into a receptacle on the recipient aircraft using flight control surfaces mounted on the boom’s wrist, whereupon the boom engages a valve inside the receptacle, completing the hookup and allowing fuel to flow down the hose and into the recipient aircraft.
In either case, if the recipient pilot isn’t careful (for flying-boom tankers, the boom operator is another potential culprit), the recipient aircraft can collide with the refuelling equipment; with a probe-and-drogue tanker, this is a (relatively!) benign occurrence, since the soft canvas drogue, attached to its long, flexible hose, can usually just bounce harmlessly off the aircraft (although damage, occasionally severe, can still occur to the recipient aircraft on rare occasions).
However, with a flying-boom tanker, the hard, rigid structure of the refuelling boom (coupled with the fact that, unlike with the probe-and-drogue system, both the recipient and the boom are actively manoeuvring [the recipient to match the motions of the tanker and correct for the turbulence created by said tanker, the boom to engage with the fuelling receptacle], considerably increasing the potential closing speed between the two should the recipient correct back towards the tanker at the same time as the boom operator adjusts the boom’s trajectory back towards the recipient) makes things considerably worse. As the boom is solidly-built and not designed to break away upon impact, the impact forces generated by a collision between the recipient and the boom are transmitted in full force to a fairly small area of each aircraft (the boom’s shoulder joint in the tanker’s case, and wherever the tip of the boom happens to hit on the recipient), which will likely destroy the boom and cause (at the very least) severe structural damage to the tanker’s lower aft fuselage, and, on the other end, can (depending on the point of impact) easily be enough to (say) tear through even a large, heavy, stoutly-built aircraft’s upper fuselage and then snap one of its wings right off the fuselage; should any fire or explosion resulting from the impact travel up the boom, the tanker is probably doomed as well.
Most things with a fairly high risk of being hit by aircraft (approach lights and ILS antennae, for instance) are made to be frangible (harmlessly disintegrating upon impact), to minimise the potential damage caused to an aircraft that happens to hit them; likewise, most things that stick out the bottom of an aircraft (such as engines and landing gear) are made to break away cleanly if excessive force is applied in the wrong direction, rather than risking damage to things like fuel tanks. The same principles could conceivably be applied to refuelling booms, which, like approach lights or ILS antennae, can easily be struck by an aircraft if one of the pilots involved does something even slightly wrong (and which one would logically expect to actually be much more frequent victims of errant aircraft than approach lights or ILS antennae, given the relative extreme smallness of the margins of error involved), and which, like engines or landing gear, could potentially cause severe damage to their attachment areas by transmitting excessive impact forces (and which one would logically expect to be much more likely to be subjected to excessive impact forces than engines or landing gear, given that landing accidents are rare, whereas aerial refuelling is literally an everyday occurrence), such that, if a recipient aircraft came in a bit too fast, or a boom operator swung the boom a bit too hard, and a boom-recipient collision resulted, the boom itself would give way before either the tanker or the receiver suffered severe structural damage.
Designing a boom to break away in a collision would, admittedly, make the detached boom a piece of potentially-hazardous flying debris, which could strike the recipient aircraft, or be sucked into an engine, and, thereby, cause additional damage; however, this would very likely be preferable to the major structural damage, or complete destruction, incurred by a recipient aircraft involved in a significant collision with a non-breakaway boom. Alternatively, the boom could be made collapsible, such that, if subjected to excessive axial compression loads from a collision, it would telescope into itself, or possibly be pushed into the tanker’s aft fuselage (this would require that space be provided for the boom to collapse into the fuselage without damaging anything; it should not be particularly hard to find space for this, however, given that even tanker aircraft have to leave large empty spaces within the fuselage in order to be light enough to take off), rather than standing firm and causing severe damage to one or both aircraft.
So why are frangible/breakaway/collapsible refuelling booms not (to the best of my knowledge) a thing?