System safety approach in manned Spacelab missions for control of hazards by software

In European manned missions, hazard control of safety critical systems are not realized entirely by software. At least one hardware inhibition exists. With the increasing capabilities of microprocessor systems, more and more hazards are controlled by S/W (software) in addition to be required H/W (hardware) inhibit. In previous missions this has been the exception. Wherever fault tolerance had to be assured, a failure of the S/W control meant termination of the mission for the affected facility. From a performance point of view this is a very undesirable solution. A software feature developed to overcome this situation in order to gain flexibility and to increase performance reliability is described. The necessary steps in safety certification are discussed using the D2 facility GFQ (Gradient Furnace with Quenching) as an example. An overwiew of S/W safety in space station applications is presented.