approach procedure must be started, it does not preclude the aircraft from descending below the prescribed DH/DA. Minimum descent altitude (MDA) In a non-precision approach (that is when no electronic glideslope is provided), the minimum descent altitude (MDA) is the lowest altitude, expressed in feet above mean sea level, to which descent is authorized on final approach or during circle-to-land maneuvering in execution of a standard instrument approach procedure. The pilot may descend to the MDA, and may maintain it, but must not descend below it until visual reference is obtained, and must initiate a missed approach, if visual reference has not been obtained, on reaching the missed approach point (MAP). DH/DA, the corresponding parameter for precision approach, differs from MDA in that the missed approach procedure must be initiated immediately on reaching DH/DA, if visual reference has not yet been obtained: but some overshoot below it is permitted while doing so because of the vertical momentum involved in following a precision approach glide-path. If a runway has both non-precision and precision approaches defined, the MDA of the non-precision approach is almost always greater than the DH/DA of the precision approach, because of the lack of vertical guidance on the non-precision approach. The extra height depends on the accuracy of the navaid the approach is based on, with ADF approaches and SRAs tending to have the highest MDAs. Straight-in approach IFR An instrument approach wherein final approach is begun without first having executed a procedure turn, not necessarily completed with a straight-in landing or made to straight-in landing minimums. A direct instrument approach requires no procedure turn or any other course reversal procedures for alignment (usually indicated by "NoPT" on approach plates), as the arrival direction and the final approach course are not too different from each other. The direct approach can be finished with a straight-in landing or circle-to-land procedure. Course reversal procedure Some approach procedures do not permit straight-in approaches unless the pilots are being radar vectored. In these situations, pilots are required to complete a procedure turn (PT) or other course reversal, generally within 10 NM of the PT fix, to establish the aircraft inbound on the intermediate or final approach segment. When conducting any type of approach, if the aircraft is not lined up for a straight-in approach, then a course reversal might be necessary. The idea of a course reversal is to allow sufficiently large