During the development of the Antonov An-124 Ruslan, studies were done on how best to transport the large wing panels and wing center section from their place of manufacture in Tashkent to the Antonov final assembly plant in Kiev. Air transport was found to be the most efficient solution and to this end, Antonov modified the An-22 prototype to carry the wing sections externally on the aircraft's back.
Two forward attachment points were mounted on the An-22's wing center section and two rear attachment points were mounted just aft of the wing. The attachment points were then connected to internal bracing inside the cargo compartment that in turn mounted to the cargo deck floor. The whole external structure was then faired. Additional attachment points were mounted just above and aft of the flight deck. To provide additional directional stability, a stock An-26 vertical fin with the rudder locked was fixed to the aft fuselage to supplement the An-22's twin vertical fins.
"Special Cargo No. 1" referred to the An-124 wing center section and "Special Cargo No. 2" referred to the An-124 wing panel assembly. On 15 July 1980 the now-designated An-22PZ took off for the first time from Tashkent with the wing center section attached to its back. Though a brief preliminary flight was made the day prior, this was the first delivery flight to Kiev and soon into the flight significant vibrations and buffeting set in caused by a failure of one of the forward attachment points. Though the flight made it safely to Kiev, it led to a redesign of the fairings used on the cargo itself as well as the attachment points. The opportunity was also taken to add a 1000-liter alcohol deicing system for the cargo.
The first flight with the wing panel assembly took place in February 1982 and by 1983 regular An-22PZ flights were now taking place from Tashkent to assembly plants both in Kiev and Ul'yanovsk. These special flights lasted until 1988 but six additional flights took place between 1987 and 1994 to deliver wing sections for the An-225 Myria.
In 14 years of operation, over 100 flights took place safely.
Source: Antonov's Heavy Transports: The An-22, An-124/An-225, and An-70 by Yelfim Gordon, Dmitriy Komissarov and Sergey Komissarov (Red Star Volume 18). Midland Publishing, 2004, p27-31.