The seven-layer safe data model, based on the Open Systems Interconnection,(OSI), described in the first edition of the Data Safety Guidance document,(Ref 1. Parsons et al 2016), is the basis on which a smart tablet or phone application, (App), which could give safety assurance during maintenance periods is proposed. The design is illustrated by showing how the technique could have prevented the Uberlingen mid-air aircraft collision using such an App. It will be shown how the technique can be used in system design, even when it includes systems from various suppliers and including human operators, to provide improved data safety. After implementation, it can also be used at maintenance planning events to give confidence to the system management organisation that the maintenance period will not affect safety, even when conducted with the system still running, albeit in a reduced mode. In use, the App would require maintenance teams, (prior to taking down the systems to be serviced or modified), to sign in and thus automatically alert systems and operators that the reduced mode of operation will be in effect, highlighting in doing so any fallback procedures that need to be followed during the period of maintenance.