Planes can’t make a large scale EMP attack. The atmosphere absorbs/shields the blast to limit the size of the EMP to less than 20 miles.
Large scale EMP attacks require the nuke be set off high above the earth. The US test used a 1.4 Mt nuke set off at 250 miles up. The EMP damaged electrical equipment 900 miles away in Hawaii.
Well, that’s good news then.
To make a very big immediate effect they’d have to do like “Jericho” (a TV show a few years back; I didn’t see it but read about it) - with simultaneous attacks in places around the country so that the electrical grid was destroyed.