In October 1961 NORAD mounted its largest-ever air defense test, Exercise Skyshield. In order to be as fully realistic as possible, the Strategic Air Command's Boeing B-47 Stratojet and B-52 Stratofortress units would try to penetrate NORAD's radar and fighter defenses. In addition, No. 27 and No. 83 Squadrons of the Royal Air Force's Bomber Command each sent four Avro Vulcans to participate in Skyshield. No. 27 Squadron's four Vulcans were deployed to Kindley AFB, Bermuda, to penetrate the defense net from the south and No. 83 Squadron's four Vulcans were deployed to RAF Lossiemouth in Scotland to penetrate from the north.
On 14 October Exercise Skyshield began in earnest with the incoming bombers taking off. The northern attack began with the B-47s coming in as low as 500 feet to jam out the ground radars. Behind the Stratojets came the B-52s at 35,000 to 42,000 feet supported by jamming by EB-57 Canberras. And behind the Stratofortresses came the Vulcans at 56,000 feet. The heavy jamming was so effective that only the first Vulcan detected the radar of an F-101 Voodoo. Although NORAD scrambled many fighters, they concentrated on the B-52s and by the time the Vulcans arrived, they lacked the fuel to climb to 56,000 feet. No. 83 Squadron's Vulcans reached their target points all unscathed (which was to land at a base in Newfoundland).
The southern wave followed and used similar tactics only this time the four Vulcans from No. 27 Squadron were on their own. The four Vulcans approached the US Northeast on a broad front with their systems in full ECM and jamming. Fifty miles from the coast as defending F-102 Delta Daggers were scrambling, the southernmost Vulcan turned north and flew behind the other three Vulcans and was effectively masked by the onboard jamming of those three bombers. While the Delta Daggers swarmed the other three Vulcans, the fourth Vulcan penetrated unscathed and undetected to reach its target (which was to land at Plattsburgh AFB in upstate New York).
Source: Vulcan Units of the Cold War by Andrew Brookes, Osprey Combat Aircraft #72. Osprey Publishing, 2009, p21.