CHAPTER 4 CHAPTER 4 Airport Influences on Aircraft Performance
4.4 Approach and Landing Performance
of the payload which will produce the least revenue. The primary fuel requirement may be reduced by fuel-management techniques, but a considerable proportion of the fuel uplift is to allow reserves for en-route winds, holding, and diversion to alternative landing fields.
Thus, on shorthaul flights, the reserves can exceed the primary fuel requirements, and this leads to an increase in the takeoff-weight requirement, which is particularly significant when it is realized that in an extreme diversion case, a long-haul jet might burn a quantity of fuel equal to a quarter of the reserve fuel simply to carry the reserves. It is open to question whether aircraft really need to carry the reserves traditionally required, with modern improvements in fuel flow-management, navigational accuracy, and weather forecasting.
On shorthaul flights, with these improvements and with excellent destination weather at departure time, the need to carry reserves for an alternative destination is particularly questionable if the destination has two independent runways. Operators can minimize fuel use and/or maximize payload by filing for closer destinations and then refiling in flight for the original destination (technically this can be referred to as an en-route diversion), but as air-traffic-management (ATM) stringencies increase, this is becoming a less usable option.
In summary, departure performance is dominated by the allowable takeoff weight, which is determined as the lowest of
• Maximum structural takeoff weight
• Climb performance limited by the WAT curve
• Takeoff field lengths: TORA, TODA, and ASDA
• Obstacle clearance
• En-route climb requirements
• Maximum structural landing weight
• Landing-field length, WAT, and diversion requirements
• Tire and brake limits
Any resulting margins between these limits and the required takeoff weight then may be used for tankering, to ease other limits, or to alleviate economic or environmental considerations.
propeller aircraft with large flaps. At 90 knots ground speed, the rate of descent is around 1,000 to 1,100 feet/minute.
Aircraft aim for the touchdown point, and this will be 1,000 feet (330 m) or so beyond the threshold marks on a typical commercial operations runway. Aircraft are expected to be around 50 feet (15 m) above ground at the threshold on a 3-degree approach and to flare, losing speed and reducing rate of descent, to land on or beyond the aiming point. After touchdown, the aircraft will be decelerated with brakes and any other mechanical means (reverse thrust on a jet or reverse pitch or flight idle on a propeller). There is considerable scope for variations in the performance of the landing, and these can include meteorological and runway surface conditions, including runway slope, that can lead to the distance from the landing reference—the threshold—to the end of the landing roll being very different on the same runway and with the same aircraft type. It is for this reason that the LDR requirement used in a flight manual has some onerous factoring applied to calculated and demonstrated distances.
Most regulations allow the manufacturer a choice of demonstrating the landing performance. One option is to use the most long-standing method, landing on a dry, hard runway with conservative assumptions as to height at the threshold, a large factor of safety, and no credit for reverse thrust. The second option is to demonstrate landing on a wet, hard surface from a lower height and higher speed at threshold and using all forms of retardation for which a practical procedure has been evolved. The latter case uses a much smaller factor of safety.
Specific flight plan calculations must take account of forecast runway conditions and wind, with the limiting field length being the lower of the no-wind and forecast-wind cases.
The diversion airport also must be taken into account, but in this case the wet-runway factor is allowed to be 0.95. Although the regulations are easy to state, it should not be inferred that the operation is similarly easy to carry out. There are all the problems of accurate alignment and speed control on the approach; adjustment of speed and heading for crosswinds, gusts, and wind shear; and maintenance of direction on the runway, as well as the primary problem of arresting the descent rate without inducing an extended float.
The aircraft design is severely tested in this phase of the flight, yet the pilot seldom can compromise in favor of sparing the structure. Indeed, the latest shorthaul aircraft specifications are very concerned that brake cooling should not affect turnaround times, that crosswinds should not affect regularity, and that the autopilot should be able to cope with nonlinear and decelerating approaches.
Performance and handling on the approach are just as important as the ground phase of landing in producing a safe completion of a flight. The most vital consideration is accurate achievement of the correct conditions at the threshold (i.e., height, speed, descent rate, track, and power). In order to attain these conditions consistently, the ground aids must be satisfactory, and the aircraft must have adequate performance on the approach to correct discrepancies in the flight path and to respond to emergencies.
The high vertical momentum of modern jets combined with wind gradients, gusts, and wind shear make it essential to provide slope guidance in the form of visual approach slope indicators (VASIs) or the most common precision approach path indicators (PAPIs). A full-
precision approach, of course, is the best aid to accurate flying. In the limit, the approach can be flown completely automatically, the various categories being tabulated as in Table 4.5. The original purpose in developing automatic landings was to increase regularity and save the costs of diversions.
*No decision height applicable.
TABLE 4.5 Decision Heights (DHs) and Runway Visual Ranges (RVRs) for Precision- Approach Runways
One of the most popular aircraft in production is the Boeing 737-800, which has good operating economics, but it is a design that illustrates the ultimate mass that current technology can accommodate on a single-axle main gear that will fit in the space available on this particular airframe. Therefore, it is an unusual aircraft in that during shorthaul applications it can require a longer LDA than TORA. This example is quoted as a reminder that generalizations can be dangerous and that generally accepting that aircraft need a greater takeoff distance than landing distance is not always true. Bear in mind, too, that Boeing has remedied the susceptibility of the type by offering advancements that have reduced the LDR of appropriately equipped models of the aircraft marque. Figure 4.7 shows the ACAP declaration (for airport design) of the Boeing 737-800 landing-distance requirement. Detail on the plot shows the influence of airport elevation and the allowance for a wet runway. The latter is guidance, and flight crews will have to make judgments on surface conditions. They depend on air traffic services to report relevant information, such as measured braking efficiency or the depth of water on a runway.
FIGURE 4.7 Boeing 737-800 landing-distance performance.