Vertime is a system devised to solve the problem of navigation in the vertical plane. By use of existing navigational aids together with only small additions to cockpit instrumentation and controls, a flight path in the incline plane is defined which the pilot can follow from cruise altitude to end of descent point. In addition, provision is made for improving the accuracy of E.T.A.s at the end of descent. This system reduces pilot work-load in the cockpit, ensures an operationally efficient descent from cruise altitude and will be of material assistance to A.T.C.