The blimps are filled with helium. The helium is maintained under low pressure, so small punctures do not pose serious consequences for the blimp. One inspection element of the blimps is to look into the envelope for pinpoints of light which are indicative of small holes. The Goodyear blimps were non-rigid (meaning their shape is not maintained by a rigid internal structure) dirigibles (directable/steerable airships). Inside their exterior envelope, the Goodyear blimps are fitted with air–filled ballonets. As the blimp ascends or descends, the internal ballonets expand or contract to compensate for density changes and to maintain uniform pressure in the envelope. The latest craft, a Zeppelin NT is a departure from this tradition, as it is a Semi-rigid airship that makes use of a structural truss inside the envelope to provide some of its structural strength. "GZ" stands for Goodyear-Zeppelin, stemming from the partnership Goodyear had with the German company when both were building airships together. However these models came many years after this partnership had dissolved during the start of World War II. The GZ-1 was the USS "Akron" (ZRS-4), the U.S. Navy's fourth rigid airship used for several tests including as a flying "aircraft carrier". According to the Goodyear website, the two active GZ-20 blimps are 192 feet (58 meters) long, 59.5 feet (18 meters) tall, and 50 feet (15 meters) wide. For comparison, the largest airships ever built, the Zeppelin company's "Hindenburg", LZ-129, and the "Graf Zeppelin II", LZ-130, were 804 feet (245 meters) long and 135 feet (41 meters) in diameter. That is, over four times as long and over twice as wide as the current Goodyear blimps. The largest blimp ever made by Goodyear was the U.S. Navy's ZPG-3, at 403 feet (121 meters) in length. Since 1928, Goodyear had traditionally named its blimps after the U.S. winners of the America's Cup yacht race. This naming method is attributed to then-Goodyear CEO P. W. Litchfield, who viewed the airships as "aerial yachts". Although that practice deviated with the introduction of the "Spirit of Akron" in 1987, the tradition ended with the Florida-based "Stars & Stripes" in 2005.