PPRuNe Forums - Approach angle vs. landing distance

In theory, a steep approach will provide a shorter landing distance, but this is not necessarily the same as a duck under the glideslope manoeuvre. Both aspects depend on defining and fixing some of the variables. e.g. landing distance required is measured from the threshold – from threshold crossing height (normally at 50 ft).

For a duck under approach, with the same aircraft, landing speed, flare technique, flare height, etc, the aircraft arrives at the ‘threshold height’ before the actual threshold (defining the landing distance required), this enables a touch down near the end of the runway. This apparently reduces the landing distance – feel good factor, but in certification terms, there is no reduction. However, there is the increased risk associated with destabilising the approach and eating into the obstacle clearance margins, which is a less safe operation.

The touch down geometry resulting from a duck under normally gives a flatter arrival that for some aircraft / pilots appears to make landing easier. However, whilst the flatter arrival is perceived to give a more accurate touchdown the spread of error in touchdown position increases at shallow angles as described by BOAC. Similar arguments can be made for airspeed and the dissipation of energy but these involve more variables.

For a true steep approach (>4.5 deg) where the glideslope angle is maintained until flare, and again with the constants as above, the landing distance is slightly shorter due to the GS origin (aiming point) being closer to the runway end and there will be less spread in touch down position due to any error in speed / technique.

The certification of the BAe146 / Avro RJ aircraft on steep approaches resulted in a reduction in the actual landing distance required (AFM performance amendment). I recall that the benefit ranges some 80 -130 meters depending on the aircraft variant. These values resulted from the combination of geometry, aircraft capability, and flight test; the spread of values were due to test technique, and that the larger aircraft have a restricted GS angle and wind limits.

The geometric effect in part follows the description above, but the dominant term was the reduction in threshold crossing height from 50 to 35 ft, an even closer aiming point than for just the GS increase.
The aircraft capability also relates to the reduced threshold height, the 146 wing lift and pitch control characteristics enabled the flare to be retained at 35 ft, avoiding any longer distance associated with an early flare. Alternatively, the Saab 2000 can fly a steep approach but due to its steep-approach flare characteristics (circa 60 ft) no landing distance benefit is allowed and there may even be a penalty.
The other advantage that the 146 has is that during the certification of steep approaches (first jet ever?) many of the landings were measured for performance calibration. This followed from BAe’s use of the ’steep approach’ method of assessing normal landing performance during certification. Thus, there was considerable evidence as to the aircraft’s actual performance opposed to its projected performance. I also recall that the CAA did an in-service check of 146 landing operations at LCY in a similar way as they did for the DHC-7, a report is published somewhere.

IMHO approaches should be flown in accordance with manufactures advice, within the certification rules and assumptions, and with the consistency and stability promoted by safety initiatives, all of which add up to the safest operation - do not duck under. See also the discussion in thread ‘VRef speeds and half the HW comp.’