The launch proceeded according to plan until T+288.6 seconds at an altitude of 145 kilometres (90 mi), when the second and third stages of the booster began separation. Only three of the six locks holding the stages together released and the third stage's engine ignited with the second stage still attached below it. The third stage's thrust broke the remaining locks, throwing the second stage free but putting the booster under unanticipated strain that caused it to deviate from the proper trajectory. At T+295 seconds, the deviation was detected by the Soyuz's guidance system, which activated an automatic abort program. This separated the Soyuz spacecraft from the third stage booster and then separated the orbital and service modules of the Soyuz from the re-entry capsule.

At the time when the safety system initiated separation the spacecraft was already pointed downward toward Earth, which accelerated its descent significantly. Instead of the expected acceleration in such an emergency situation of 15 g (147 m/s²), the cosmonauts experienced up to 21.3 g (209 m/s²). Despite very high overloading, the capsule's parachutes opened properly and slowed the craft to a successful landing after a flight of only 21 minutes.

The capsule landed southwest of Gorno-Altaisk at a point 829 kilometres (515 mi) north of the Chinese border. The capsule landed on a snow-covered slope and began rolling downhill towards a 152 m (499 ft) sheer drop before it was stopped by the parachutes' becoming snagged on vegetation.

Having landed in chest-deep powder snow and a local temperature of - 7 °C (19 °F), the cosmonauts donned their cold-weather survival clothing. Lazarev, fearing they had landed in China, burned papers relating to a military experiment he had been scheduled to perform in space. Soon, the crew was in radio contact with a rescue team in an approaching helicopter, who confirmed their landing point was in the Soviet Union, near the town of Aleysk. The deep snow, the high altitude, and the terrain meant the rescuers had great difficulty in making contact with the cosmonauts. It was the next day before they were safely air-lifted out. The crew were returned to Star City; the capsule was recovered some time later.

Initial Soviet reports stated the men had suffered no ill effects from their flight. Vladimir Shatalov, the Director of Cosmonaut Training, reported they were fit to fly another mission. However, subsequent reports claimed that Lazarev was injured by the high acceleration of re-entry. Makarov took part in Soyuz 26, Soyuz 27and Soyuz-T3 flights.

In Brezhnev's time it was rare to disclose anything about Soviet failures, and so the first (Soviet) publication about the realities of the flight was not made until 1983 in the Army newspaper "Red Banner".[ The Americans were informed on April 7 after the crew had been rescued. However, as the failure occurred during preparations for the joint Apollo-Soviet Test Project, the Soviets were 'requested' to provide the Americans with a more detailed report. (There was even a U. S. Congressional Inquiry regarding this failure and several others.) It was revealed that the booster used had been an older model than the one planned for use in the joint mission that July. In the report the Soviets made to the Americans, the abort was referred to as "the April 5th anomaly" and as this was the only term the Soviets ever used for the incident, it became the 'official' designation for years afterwards. The mission is referred to in the literature as Soyuz 18-1 or Soyuz 18a, since the following Soyuz mission in May 1975 received the name Soyuz 18 (the Soviets only gave numbers to successful launches).

The Soyuz 18a flight has been the only case of a manned booster accident at high altitude. The exact landing site of the capsule had been a subject of debate amongst space historians in subsequent years. A Russian source quoted by James Oberghas stated that the landing occurred in Mongolia.

Soyuz 18a (Russian: Союз 18a, Union 18a) (also called Soyuz 18-1 and the April 5 Anomaly) was a manned Soyuz spacecraft launched by the Soviet Union in 1975, intended to dock with the orbiting Salyut 4, Space Station, but which failed to achieve orbit due to a serious malfunction during launch. The crew consisted of commander Vasili Lazarev, an Air Force major, and flight engineer Makarov a civilian.

The accident was disclosed by the normally secretive Soviets as it occurred during preparations for their joint Apollo-Soyuz Test ProjectProject with the United Stateswhich flew three months later. The crew, who feared initially they had landed in China, were successfully recovered.

Soyuz 18a was intended to be the second mission to take Cosmanauts to the Soviet Salyut 4 space station for a 60-day mission. Both cosmonauts were on their second mission and had flown their first mission together, Soyuz 12 in September 1973 to test a new type of Soyuz spacecraft after the fatal accident of Soyuz 11.