It may sound too simplistic but from a conservative point of view, after touch down the aircraft should be stoppable within the physical proximities of the runway. The current landing distance calculations are based on a touchdown within the first 1500ft but the touchdown zone markings extend till 3000′ on a typical runway.
The pilot is allowed to touchdown latest by 3000′ on a runway length 7800′ and above leaving a distance of 4800′ in a worst case scenario. A risk based safety margin must be adopted by the operators especially considering the long landing distance data.
(1) The braking coefficient of friction obtained from not less than six landing tests conducted in accordance with FA.R 5 25.125 and subject to the additional provisions listed below should be used to establish the ground
(i} The landing should be preceded by a stabilized approach not to exceed an angle of 3° down to the 50-foot height and at a calibrated airspeed not less than 1.4 Vso (stall speed)·
(ii) The average touchdown rate of descent should not exceed 6 feet per second. Longitudinal control and braking application procedures must be such that they can be consistently applied to yield a nose gear touchdown
rate of descent that does not exceed 8 feet per second.
(iii} The runway should be level, smooth, hard-surfaced, and
The parameters affecting the landing distance are published in the Flight Operations Manual. Flight crew should have a good understanding of the sensitivity of the landing distance to these parameters in order to make
sound go-around decisions. The following data shows the effect of relatively minor deviations from a baseline calculation of landing distance for a wet runway. The reference condition is a reasonably attainable performance level following normal operational practices on a nominal wet
runway surface. The reference QRH data on the bar chart below is based on:
1500 foot touchdown
VAPP=VREF+5, 5 knot speed bleed off to touchdown
Sea Level, Standard Day (15 C)
No wind, no slope
Recommended all engine reverse thrust
Braking Action – Good, consistent with FAA wheel braking definition of a wet non-grooved runway.
The vertical line represents the dispatch requirement that is 1.92 times the dry runway capability of the aeroplane.
Each bar as you go down the chart demonstrates the cumulative effect of the operational variation listed. In overrun incidents, you usually see a number of factors that contribute to using up the margin available, especially if the runway has worse wet runway friction capability.
For runways longer than 2400 meters the touchdown zone has a total length of 900 meters and the aiming point markings are located 400 meters beyond the threshold. This means that when touching down at the aiming point the landing distance is increased by 95 meters as compared to a 1000 ft (305 m) reference. When a touchdown would occur at the end of the touchdown zone the landing distance would increase by 595 meters. Runways with a length between 1500 m and 2400 m should have a touchdown zone with a length of 600 m and aiming point markers 300 meters beyond the threshold.
An important thing to consider here is the fact that ICAO Annex 14 prescribes the minimum distance from the threshold and an IFALPA survey indicates that in reality things may be quite different. As an example it was found that for Charles de Gaulle airport all aiming point distances exceed 400 m and the aiming points for runways 27L and 27R actually exceed 500 m.
FAA research has confirmed that actual air distances may exceed the current assumptions used in the manufacturer advisory landing distance tables for dry and wet runways*. The FAA research shows the effect of pilot motivation and glideslope/runway intercept on the actual air distance achieved in service. The figures below present this air distance for two airports: Washington National (6869 ft / 2094 m runway) and London Heathrow (12743 ft / 3884 m runway) with glideslope/runway intercept points of 1000 ft / 305 m and 1157 ft / 353 m respectively. When the data is reviewed it appears that when the runway is shorter and has approach guidance that results in a shorter aiming point the flight crew will put the airplane on the runway sooner (in less distance).
* EASA CS-25 AMC 25.1591 requires an air distance based on a 7 second flare time for contaminated surfaces which results in an allowance of 1300 to 2000 ft.
The landing distance performance calculations are based on a touch down at 1500′ from the threshold and the touchdown zone extends till 3000′. As seen above a touch down at 2500′ with compounding additional factors like tail wind, runway condition etc. the landing distance required is beyond the calculated distance required with margins.
As recommended by FAA SAFO19003 the safety margin of 15% may be insufficient when the runway turns wet. A safety margin of 30-40% could be a more conservative and a safer option. If the runway touchdown zone markings extend till 3000′ then the assumed touchdown point for the purpose of calculations can be assumed at this point for landing distance calculations as compare to the present 1500′.