Before continuing further I would like to present the summary of documents which Safetypee has provided, in reference to using the safety factor for landing performance.
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FAA Advisory Circular (AC No: 91-79 ) on Runway Overrun Protection.
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- The unfactored landing distances in the manufacturer-supplied AFM reflect performance in a flight test environment that is not representative of normal flight operations. The operating regulations require the AFM landing distances to be factored when showing compliance with the predeparture landing distance requirements. These factors are intended to account for pilot technique, atmospheric
and runway conditions and other items to ensure that the filight is not dispatched where it will be unable to land.
- As part of the operator’s Safety Management System (SMS) and SOP, the FAA recommends using either factored landing distances or adding a safety margin to the unfactored landing distances when assessing the required landing distance at the time of arrival. This landing safety margin should not be confused with the regulatory predeparture runway requirements.
- Operators should use the appendices, which contain detailed information about all of these hazards, to develop their SOPs. The 15 percent safety margin additive recommended by the FAA is intended only to account for slight variations in achieved performance.
- The unfactored certified landing distance may be different fom the actual landing distance because not all factors affecting landing distance are required to be accounted for by certification regulations.
- A survey of numerous operators’ Flight Operations or General Operating Manuals by the FAA’s Landing Performance Team indicated that approximately 50 percent of the operators surveyed did not have adequate policies in place for assessing whether sufficient landing distance exists at the time of arrival at the destination airport. Not all operators performed landing distance assessments at the time of arrival nor did all of the operators who performed assessments account for contaminated runway surface conditions or reduced braking action reports, nor did they apply a consistent safety margin to the expected actual landing distance. Operator specific SOPs should be developed for the assessment of landing performance, including the application of a safety margin, to ensure a consistent evaluation of airport conditions at the time of arrival.
- Unfactored landing distances determined in compliance with certification regulations and published in the FAA-approved AFM do not reflect operational landing distances. Landing distances determined during certification tests are aimed at demonstrating the shortest landing distances for a given airplane weight with a test pilot at the controls and are established with full awareness that operational rules for normal operations require the addition of factors to determine minimum operational field lengths.
- Flight test and data analysis techniques for determining landing distances can result in the use of high touchdown sink rates (as high as 8 feet per second) and approach angles of 3.5 degrees to minimize the airborne portion of the landing distance. Maximum manual braking, initiated as soon as possible after landing, is used in order to minimize the braking portion of the landing distance. Therefore, the landing distances determined under §§ 23.75 and 25.125 are much shorter than the landing distances achieved in normal operations.
- Aircraft Landing Weight. Calculate the anticipated landing weight by starting with the aircraft gross takeoff weight at departure and subtracting the anticipated normal consumption of fuel and oil required to arrive at the destination airport. If en-route fuel burn is less than planned, then the aircraft will arrive at the destination at a weight heavier than planned. Therefore, a recalculation of the aircraft’s landing performance must be done prior to attempting a landing maneuver. The pilot may control the landing weight by adjusting fuel burn en route or decreasing fuel load or payload prior to departure.
- Touchdown Point. Extended flare and runway slope are two factors which affect pilot control of the touchdown point. Turbine airplanes should be flown onto the runway being held off the surface as speed dissipates. A firm landing is both normal and desirable. An approach flown using a 3.5 degree glidepath with a touchdown at 8 feet/sec. rate of descent witouchdown down at the target touchdown point, which for certification purposes is approximately 1,000 feet beyond the runway threshold. The typical operational touchdown point
in the first third of the runway, and it may be farther down the runway than the 1,`000 fpoint. This additional distance should be accounted for in the landing distance assessment at the Air Safety Foundation Approach-and-Landing Accident Reduction (ALAR) Briefing Note 8.3, each 1 percent of runway down slope increases the landing distance by 10 percent.
- SOPs should clearly outline procedures to accommodate changes from the original plan. Preplan contingencies. Include the safety margin (factor) required by company SOPs or the appropriate regulation in all landing distance calculations. SOPs should address a process for conducting a landing distance assessment
- Develop procedures to ensure that a full stop landing, with a reasonable safety margin beyond the actual landing distance, can be made on the runway to be used.
- In the absence of other specific guidance, and to ensure that an acceptable landing distance safety margin exists at the time of arrival, the FAA recommends, as a best operating practice, that a 15 percent safety margin be applied to the actual airplane landing distance at the time of arrival. The 15 percent safety margin accounts for actual performance considering the meteorological and runway surface conditions, airplane configuration and weight, and the utilization of ground deceleration devices.
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U.K. Aeronautical Information Circular AIC 14/2006 (Pink 91) on Landing Performance of Large Transport Airplanes
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4.3.1 Another common factor in landing overrun incidents is a deep touchdown, which is most usually caused by excessive height at
the threshold. An additional 200 ft of runway is needed for every 10 ft of excess height at the threshold.
4.3.2 An extended flare, concluding an otherwise accurate approach, might result in a smoother touchdown, but will use up valuable
runway in the process. Aeroplanes can decelerate far more quickly on the ground than by floating along just above it. This merely
reduces the available runway length in which the stopping procedures can have effect.
4.3.3 An additional factor to consider is that for a given threshold height and glideslope angle, a downhill runway will tend to yield an
increased distance from threshold to touchdown compared with a level runway, if allowance is not made in the flare manoeuvre.
Runways with a significant amount of downslope at the beginning of the landing run are most likely to cause problems in this area, and
those in the UK are now highlighted in the AIP.
4.3.4 The conditions that create optical illusions which give misleading indications of height relative to the glidepath, created by, for
example, sloped runways or by particularly narrow or wide runways are well known and should be anticipated when such
characteristics are encountered. To help compensate in conditions where the visual perception of the flight path during a visual
approach may be affected, all available runway approach aids should be utilised.
Inflight re-check of Landing Performance
As stated earlier, an aeroplane's suitability to land at the intended destination is predicted upon forecasts at the time of despatch.
order to ensure that the assumptions made at despatch remain valid, JAR-OPS 1.400 prescribes an inflight re-check requirement,
requires the commander to satisfy himself prior to commencing an approach to land, that a safe approach and landing can be
made, taking into account the actual state of the aerodrome. This re-check is particularly important given that JAR-OPS 1 permits
despatch on the basis of dry landing distances if the landing runway is forecast to be dry at the estimated time of landing.
Any significant departure from the flight plan, such as an unscheduled diversion, would clearly justify a review of the landing
performance. If such a diversion was as a result of an emergency aeroplane condition which necessitated a prompt landing, then
clearly this would be justification for contemplating a landing on a runway which, although nearby, could not fully accommodate the
increase in the factored LDR due to the failure condition. Such a decision requires a sound knowledge of the principles involved to
make an assessment of the conflicting considerations.