Constant Descent Angle Approach

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Category:	Controlled Flight Into Terrain
Content source:	SKYbrary
Content control:	EUROCONTROL

Constant Descent Angle Approach

Constant-Angle Non-Precision Approach - CANPA (FSF)

Constant-Descent Final Approach - CDFA (EASA)

Description

Figure 1 illustrates a typical non-precision approach procedure. The path of the aircraft is coloured red. The aircraft approaches the final approach fix (FAF) at the cleared height, then descends until reaching the minimum descent height (MDH); this height is then maintained until either the runway is in sight or the missed approach point is reached. If the runway is not sighted by the missed approach point, a go-around must be flown. The descent to the MDH may be at any convenient rate of descent. More complicated procedures may include check heights at various points during the approach.

The disadvantages of this procedure are that the approach path incudes a number of changes in descent angle, making it difficult to achieve a stabilised approach and smooth transition to visual flight. A landing attempted from the missed approach point would be extremely dangerous due to the steep approach angle.

Many CFIT accidents occur during non-precision approaches.

Figure 2 illustrates the Constant Descent Angle Approach (or Constant Descent Final Approach - CDFA) concept. The path of the aircraft is again coloured red and the standard non-precision approach path is coloured blue for comparison purposes. The aircraft descends from the FAF at a constant rate which, if continued below the MDH, would result in it crossing the runway threshold in a position to land the aircraft. A simple calculation is necessary to determine the rate of descent required. The (constant angle) approach path crosses the MDH at the Visual Descent Point (VDP). If the runway is not in sight when the aircraft reaches the VDP, a go-around is flown.

The main advantage of the constant descent angle approach is that the descent angle is constant throughout, ensuring the best chance of a stabilised approach and smooth transition to visual flight if the runway is seen at the MDH.

With the development of GPS-based navigation, the CDFA concept has been adapted further and this is discussed in the references in 'Further Reading' below. The magazine article briefly describes the concept, which is further developed in FSF ALAR Briefing Note 7.2 - Flying Constant Angle Non-Precision Approaches and further enhanced in the comprehensive Airbus Briefing Note.

Further Reading

For a summary of CDFA see the article in Aero Safety World From Non-precision to Precision-Like Approaches

The basic CDFA Concept is developed in the Flight Safety Foundation ALAR Briefing Note 7.2 — Constant-Angle Nonprecision Approach

The concept is further enhanced and illustrated in the Airbus Flight Operations Briefing Note From Non-Precision to Precision-like Approaches

Copies of the full Flight Safety Foundation ALAR Toolkit on CD may be obtained from the Flight Safety Foundation[1]