Jet engines are fragile and a bird-strike can, and does, take out many aircraft every year (http://www.airsafe.com/birds/birdrisk.htm, http://www.airsafe.org/birds/BirdstrikeRates.pdf, for info on bird-strikes).
As a mishap investigator in the USAF, I worked one bird-strike incident where the jet was lost but the crew ejected. I also experienced a bird-strike and lost the engine.
Thing is, a bird-strike hitting the fan blades causes much damage and basically ‘unbalances” (if not destroy) the blades, and when those blades spin at over 22,000 RPM, the effect on the engine is like a shaggy dog shaking off water drops. . .pieces fly everywhere.
Which brings me to a question: If the winds at LHR were calm or even 10 kts tailwind, why would the pilot elect to fly over downtown London when the engine could very well spit parts (if not completely come apart), thereby spewing FOD all over the city and people below? Why not land at LGW and avoid over-flying a population center?
Anyway, jet recovered and no harm. . .other than to the bird(s).
See the Yukla 27 tragedy.
http://aviation-safety.net/database/record.php?id=19950922-0
Canada Geese (weighing 5-14 lbs) showed terrorists what can happen if two engines on a large four-engine aircraft are destroyed at VR speed at the end of the RKZ. The aircraft continues briefly into the air but fails to gain altitude on the remaining two engines and then crashes:
From FlightSafety.org:
“Boeing E-3B Sentry 77-0354 was military Boeing 707-derivative, a.o. equipped and AWACS system. As the plane rotated for lift-off numerous geese were ingested in the no. 1 and 2 engines resulting in a catastrophic no. 2 engine failure and a stalling no. 1 engine. The crew initiated a slow climbing turn to the left and began to dump fuel. The aircraft attained a maximum altitude of 250 feet before it started to descend. The plane impacted a hilly, wooded area less than a mile from the runway, broke up, exploded and burned.”
Note that no blame was assessed to the Yukla 27 E-3B pilots for this crash. They calculated their V1 speed for this flight according to regulations. Above V1 the aircraft could have proceeded to VR and taken off with loss of power if only one engine failed, but there was solution available for what to do if two engines failed. If terrorists can knock out two engines on a multi-engine heavy aircraft, the pilots of that aircraft will not have a “plan B.”