Main Air Tank, Yak 50, New, unused
This unused Yak 50 Main air balloon is in hydro until December 2021.
Over the years many Yak 50 owners have needed to come up with creative solutions for failed air tanks, including Halon tanks from Boeing 727, Stainless replacements and so on.
The factory air tanks are made from a low carbon steel.
Over the years it has become clear they have three failure modes. The first is pin-holing, where internal rust spots have drilled through to the outer surface, manifesting themselves as a slow, overnight, otherwise inexplicable loss of tank pressure.
The second failure mode is catastrophic rupture as happened in the well documented G-YAKK incident.
And the third is excessive material strain, discovered during regular Hydrostatic testing, where a test pressure creates excessive expansion of the tank, indicating the material properties are no longer safe for normal duty, and the tank must be condemned.
Corrosion due to moisture in Mild steel tanks is not a foregone conclusion, and the correct maintenance periods and procedure to prevent it are equally well-documented.