The Tsar Bomba was the most powerful nuclear weapon ever created. On 30 October 1961 the Soviet Union experimentally detonated the bomb after dropping it from a plane above Nova Zembla. A second plane was nearby to gather data on the explosion.
According to this article from the bbc:
In order to give the two planes a chance to survive – and this was calculated as no more than a 50% chance – Tsar Bomba was deployed by a giant parachute weighing nearly a tonne.
A similar thing is mentioned on the English article on the Tsar Bomba from wikipedia:
Both aircraft were painted with the special reflective paint to minimize heat damage. Despite this effort, Durnovtsev and his crew were given only a 50% chance of surviving the test.
This 50% chance seems a bit suspicious to me, because it is a conveniently round number, it is quite a low survival rate for an experiment (even for a high profile military experiment during the cold war), and it is not clear to me why it should be so dangerous.
Wikipedia lists the CTBTO website as the source for this claim. While the CTBTO is a respectable organisation, the webpage is not a primary source and does not fully convince me.
There is no mention of the 50% chance on the Russian version of the wikipedia page (using google translate).
This document claims that:
even if the parachute system had failed during the test, the bomber's crew would not have been endangered, as the bomb contained a special mechanism which triggered its detonation only after the plane had reached a safe distance
Which directly contradicts the 50% claim.
Does a primary source exist for the 50% number? Or is there any evidence suggesting that the Soviets either did or did not think beforehand that there was a significant chance of the crew not surviving the experiment?