For a brief moment, let's just toy with the fantasy of UA93 being shot down.
If a shootdown did occur, we can essentially narrow down the weapon to one of four options.
1. AIM-9 Sidewinder Short-range Heat-seeking Air to Air Missile
2. AIM-120 AMRAAM Medium-range Active-homing Air to Air Missile
3. AIM-7 Sparrow Short-range Active-homing Air to Air Missile
4. M-61 Vulcan 20mm canon
By assessing the most likely outcome of utilising each of these weapons we can conclude whether the crash pattern of UA93 is consistent.
1. AIM-9 Sidewinder
The Sidewinder is a short-range missile in widespread use by the US military, which tracks a target by identifying heat sources such as an engine exhaust. As such, an AIM-9 hit, regardless of the direction it came from, would occur near one of the aircraft's engines. The weapon has an annular-blast fragmentation warhead which is designed to cut through the skin of an aircraft, and would almost certainly result in separation of the engine from the wing. Additionally the detonation could cause rupture of the wing fuel tanks leading to destruction of the wing that was hit (otherwise a second missile would be required to destroy the other engine). A distinct characteristic of a heat-seeking impact would be impact-point debris such as engine and wing parts at a substantial distance from the rest of the wreckage. These would be "upsteam" on the flight path, and would be substantial debris fields in their own right, a long way from the main impact. Further, the loss of an engine would be clearly recorded on the FDR which would notice loss of fuel pressure and readings from one engine. Loss of a wing would result in a severe stall, and the aircraft would fall to the ground in a rapid spin which would be clearly recorded on the FDR.
2. AIM-120
3. AIM-7
Because the characteristics of these two weapons are similar, I have grouped them together. They are both active-homing missiles - the significant differences is that the AIM-7 has shorter range and a larger warhead. Both have a high-explosive warhead, which uses detonation shock to destroy the target. The active-homing missile actively tracks the radar signature of the target aircraft, and as such would be more likely to impact on the body of the aircraft. This would lead to an immediate explosive decompression of the aircraft which would have been recorded on the FDR. Aircraft hull debris at the point of impact would be found a long way upstream of the main crash site. Interior objects in the aircraft would immediately start shedding through the hull breach, leaving a long spread-out trail of small debris from the point of initial impact to the final crash site. This debris field could be expected to be enormous - potentially a hundred square kilometers or more. Depending on the specifics of impact, the aircraft would either continue to fly (requiring additional missile launches and repeats of the above) or would result in disintegration of the hull leading to a mid-air break up, characterised by a widely scattered debris field over an even larger area, particularly identified by the wide scattering of major structural components (i.e. tail, nose and wing section substantial distances apart). Of particular note, a mid-air break up would result in the FDR and CVR terminating in mid-air, not at ground level. A wing impact would result in circumstances much like the AIM-9 impact already mentioned. A specific particular point of impact could be the nose of the aircraft. On the NORAD tapes, staff are heard discussing options for bringing down an airliner (due to its size, not as easy as bringing down a small fighter) and it was suggested that firing into the nose was a good option (presumably to disable the flight deck and flight controls). In this particular instance some very distinct events could occur. Separation/destruction of the nose section would occur, leading to initial impact debris upsteam of the main crash site. This would open the entire fuselage to winds, resulting in rapid break down of the fuselage. Further, as per the case of TWA-800 which lost its nose section, the aircraft would go into a severe and rapid climb until the aircraft stalled and fell to air. Main impact could be expected to be a long way from initial impact point. FDR would terminate prior to ground impact.
4. M-61
The M-61 is a six-barrel rotary 20mm cannon which is used on almost all US military combat aircraft. The method for destroying an airliner with a 20mm cannon can be predicted based on conversations had by pilots of the 121st FS on 9/11 itself. They had only 20mm guns as weaponry - no missiles - and decided the best course of action was to attempt to remove the wing of the aircraft by aiming to shoot along the wing root.
Such an impact would have very distinct characteristics similar to the wing-removal scenario of the AIM-9, with the distinction that the engine would remain intact almost certainly until impact with the ground. Further, the ammunition used by the M-61 is either high explosive or incendiary, and would almost certainly result in detonation of the central fuel tank, which would cause instant explosive decompression of the aircraft, severe hull damage, and potentially cause it to snap in half.
In all cases, a mid-air shoot-down of UA93 would result in the distinct characteristic of leaving at least one piece of substantial structural debris (tail section, wing/s, engine/s or nose section) at a location a long way "upstream" from the final ground impact site. In all cases there would be clear indications on the FDR that an impact had occurred. (Cabin Pressure, Engine Readings, etc). In almost all cases the FDR and CVR would cease recording prior to ground impact.
On 9/11, with regards to UA93, while we can never be absolutely sure what occured inside that aircraft, we can be sure of the following:
1) No debris from UA93 was found upstream of the main impact site. All of the debris was found downstream of the impact site, with all major structural debris contained within the immediately vicinity of impact.
2) The FDR from UA93 recorded no anomalies with any aircraft system in the moments leading up to the crash. Cabin Pressure, Engine Throttle, Engine Vibration, and so forth were all normal.
3) The FDR and CVR for UA93 ended at impact with the ground.
From these three points alone, we can unequivocally conclude that UA93 was not shot down.
