If an aeroplane were caught in an EMP strike, it could destroy all its guidance, navigation and communication systems making it virtually impossible to fly. An electromagnetic pulse (EMP) is a burst of high-energy photons. A flash of light is also a burst of photons, but visible light photons have much lower frequency and energy than the photons found in an EMP. When you detonate a nuclear bomb in the atmosphere, the high-energy gamma rays ionise the air molecules and essentially cause a massive EMP. The USA has developed specialised EMP weapons designed to create a more focused pulse over a smaller area.
Answered by Luis Villazon