Most commercial airplane doors cannot open directly outward. They unseal inward and then twist to clear outward. At altitude the internal pressure will push the

Most commercial airplane doors cannot open directly outward. They unseal inward and then twist to clear outward. At altitude the internal pressure will push them directly outward and render them unable to open, even if the locks fail. In this regard they are fail-safe. The DC-10 eschewed this standard in favour of doors that opened directly outward with catastrophic results if the locks failed.