A precision approach (Cat 1) has a predetermined safety allowance below DH for a GA commenced at or above DH. For Cat 1 this includes altimetry and aircraft height loss. Some crew procedures use a call of “15 (ft) above” so that the PF can respond immediately, with no response or a “GA” call, a GA is commenced at or above DH.
The MOC for a non-precision approach should not be less than 75 m 246 ft or 295 ft no FAF (PANS-OPS), or 250 ft (TERPS) and the obstacle clearance assume no flight below MDA. However, due to the method of procedure construction the obstacle clearance for TERPS is slightly larger than PANS-OPS thus some TERPS based operators have been granted dispensation to use MDA as DH.
In Europe studies have shown that the slightly increased risk of obstacle collision by using PANS-OPS MDA as DH would be offset by the overall increase in safety afforded by flying a continuous descent with GA at MDA. Plans for MDA=DH are being discussed by JAA and ICAO.
See the
draft JAA NPA-OPS 20 relating to the introduction of continuous descent approaches.
“The use of the MDH as a DH is a consequence of the method of not allowing level flight at MDH (or multiple step-downs during non-precision approaches). This method is widely used among major European, commercial operators and more than half of those operators do not apply an add-on to the DH. It was however felt to be prudent to remind the recipients of this NPA that the application of MDH as a DH is a matter, which needs to be evaluated and which may need to be discussed between the operator and his Authority. This matter is currently reviewed by ICAO OCP and OPSP.”