Displaced Thresholds.

[Ex Douglas Driver]

My question is "why"? What factors are coming in to play to require that extra runway before the displaced threshold?"

Airmanship (and the law) require that you know the landing distance required vs the landing distance actually available, before committing your tyres to the tarmac.

Landing distance available means the length of runway declared to be available and suitable for the ground run of an aeroplane landing.

For most operations, the Landing Distance Required calculated is factored to give "fat" to the demonstrated distances. This "fat" should be used to give a bit of breathing space during the landing roll, not an attempt to put the wheels just past the weeds.

For info, here are the factors for various aircraft weights here in Aussie (should be ICAO.....!)


...an aircraft must not land unless the landing distance available is equal to or greater than the distance required to bring the aeroplane to a complete stop,..., following an approach to land at a speed not less than 1.3 Vs maintained to within 50 ft of the landing surface. This distance is to be measured from the point where the aeroplane first reaches a height of 50 feet above the landing surface and must be multiplied by the following factors:

RPT SINGLE ENGINE AEROPLANES, CHARTER OPS, AERIAL WORK, & PRIVATE OPS
(a) 1.15 for aeroplanes with MTOW of 2000kg or less;
(b) 1.43 for aeroplanes with MTOW of 4500kg or greater;
(c) for aeroplanes with MTOW between 2000kg and 4500kg, a factor derived by linear interpolation between 1.15 and 1.43 according to the MTOW.

RPT MULTI-ENGINE NOT ABOVE 5700KG
(a) 1.15 for MTOW equal to or less than 2000kg
(b) 1.318 for MTOW equal to 3500kg
(c) for aeroplanes with MTOW greater than 2000kg but less than 3500kg, a factor derived by linear interpolation betwenn 1.15 and 1.318
(d)1.43 for MTOW exceeding 3500kg but not exceeding 5700kg

AEROPLANES ABOVE 5700KG - ALL OPERATIONS (Turbine and Piston)
for jet-engined aeroplanes
(a) 1.67 for RPT when landing on a dry runway, or charter on a wet or dry runway
(b) 1.92 for RPT landing on a wet runway (or the distance given in a wet runway landing chart

For propeller driven aeroplanes landing at a destination aerodrome
(a) 1.43 for a dry runway
(b) 1.67 for a wet runway