The unusually cold weather across the Northeast this week was the culprit for the lightning strikes on the planes landing at the Philadelphia International Airport. Why? The freezing level was only about 3,000 feet above the ground, which is a level that is more common in March. The air aloft where thunderstorms grow was very cold with temperatures lower than 10 degrees up through 15,000 feet. That means any thunderstorms that develop will immediately have ice crystals and hail that lead to the charge separation that produces lightning. Now, add an airplane with a metal casing flying through a highly charged cloud and bang, a lightning strike is produced. In Thursday evening's flight, the storms moved into the approach path to Philadelphia International Airport; two airplanes most likely encountered the highly charged clouds and probably generated the strikes. Lightning strike data in that area did not show any cloud to ground strikes, which leads to the theory that the planes themselves generated the strikes. In addition, the bases of the thunderstorms were high enough that cloud-to-ground lightning strikes were harder to come by due the insulation affect of the air between the ground and cloud base. The airplanes being closer to the clouds became the closer target for the lightning.

Although the lightning strikes were loud, bright and nerve-racking for passengers, the planes were safe from them.

The preliminary FAA report on the 3 incidents as reported by WPVI-TV in Philadelphia states...

AirTran Airways Flight 628, Boeing 717, landed on runway 27R at 6:22
p.m.

after reporting it had been struck by lightning. aircraft declared emergency, landed safely.

Republic Airlines Flight 3407, an Embraer 170, reported being struck by
lightning over NE Philadelphia at 6:15 p.m., landed runway 27L at 6:20 p.m.

Did not declare an emergency.

America West Flight 1036, a Boeing 757, reorted being struck by lightning three minutes (6:18 p.m.) after RPA Flt 3407 was hit. Aircraft declared emergency, landed on runway 27L at 6:34 p.m.

FAA inspectors will examine each aircraft today to determine if there was any damage.

Republic Airlines crew reported no damage last night. America West reported some, and AirTran crew reported the aircraft was struck "right on the nose." It is estimated that every commercial airplane is hit by lightning at least once per year. With tight schedules and the sheer number of planes in the air, it is inevitable that a commercial airplane will eventually be hit by lightning. The fact is that in most cases the airplane will actually trigger lightning when flying through a heavily charged cloud. The aluminum casing of the airplane combined with the friction produced as the plane flies through the charged cloud will result in a lightning strike that originates at the plane and extends away from the plane. It's the same principle of a person walking across a carpet and causing a static charge shock when the person comes in contact with a metal object or another person.

The airplane itself is engineered to be protected by lightning strikes despite all the electronics and computer systems. The last known commercial crash due to a lightning strike occurred in 1967. Since then, millions of airplanes have been hit by lightning or generated lightning with little incident.

Story by AccuWeather.com Senior Meteorologist Henry Margusity