CHAPTER 5 CHAPTER 5
5.3 Operating Constraints Visibility
Air traffic moves under either visual flight rules (VFRs) or instrument flight rules (IFRs) depending on weather conditions and prevailing traffic densities. VFR operations are possible where weather conditions are good enough for the aircraft to operate by the pilot’s visual reference to the ground and to other aircraft. Operational runways are classified according to the weather conditions in which they can operate. The worse the condition in which a runway is to operate, the greater is the amount of visual and instrument navigational equipment that must be provided. Runways can be classified according to their ability to accept aircraft at different degrees of visibility (ICAO 2010).
Noninstrument runway. This is a runway intended for the operation of aircraft using visual approach procedures only.
Instrument-approach runway. This is a runway served by visual aids and a nonvisual aid providing at least directional guidance for a straight-in approach.
Precision-approach runway, Category I. This is an instrument runway served by an instrument landing system (ILS) and visual aids, intended for use in operations down to a decision height of 200 feet (60 m) and visibility of no less than 2,600 feet (800 m) or a runway visual range (RVR) of 1,800 feet (550 m).
Precision-approach runway, Category II. This is an instrument runway served by ILS and visual aids, intended for use in operations down to a decision height of 100 feet (30 m) and an RVR of 1,000 feet (300 m).
Precision-approach runway, Category III. This is an instrument runway served by ILS to and along the runway with further subcategories.
Category IIIA. This is intended for operations down to an RVR of 575 feet (175 m) and zero decision height using visual aids during the final phase of landing.
Category IIIB. This is intended for operations down to an RVR of 160 feet (50 m) and zero decision height using visual aids for taxiing.
Category IIIC. This is intended for operations without reliance on visual reference for landing or taxiing.
Runway visual range (RVR) is defined as the distance over which the pilot of an aircraft on the centerline of the runway can see the runway surface markings or the lights delineating the runway or its centerline. This range is now frequently determined automatically by RVR sensors, such as those shown in Figure 5.1, which are set just off the runway shoulders. Decision height is defined as the minimum height at which the pilot will make the decision either to land or to abort the attempt to land.
FIGURE 5.1 Runway visual range (RVR) measurement equipment types.
Low visibility and ceilings can, in addition to presenting safety challenges, result in reduced airfield capacity. Figure 5.2 shows the sort of record that should be available to help determine the economic viability of high-category operations—“Potential regularity”
refers to the percentage of flights that could operate on a runway under various approach categories. Simply recording the number of hours that the RVR is below the limit for Category I is not particularly helpful. At airports where low RVRs occur at night or in the very early morning when there is little traffic, the number of hours of poor visibility overestimates the level of traffic disruption poor visibility would cause. At other airports, however, severe and prolonged morning mist or haze could affect peak-hour operations, and without Category II or III capability, the development of the airport might be made difficult.
FIGURE 5.2 Impact of reduced visibility on potential regularity at London Heathrow. (BAA.) Note that each major category requires significant improvements in runway visual aids, secondary power backup specifications, aircraft equipment, pilot training, and airport procedures. In other words, moving from one category to another represents a significant investment that affects various elements of the airport system. The investment decision will
be based on financial analysis of the number of weather days per year that would close the airport if precision-approach operations were not possible.
The figure shows graphically the results of a computation done by the BAA on the effect of reduced visibility in terms of potential regularity (i.e., operational impact). It can be seen that the proportion of operations requiring Category II and III operations is less than 2 percent. Category IIIc conditions affect less than 0.05 percent of operations. Nevertheless, as far back as the 1980s, the principal operator at Heathrow, British Airways (BA), decided that a “blind landing” capability was economically justifiable because the airline finds itself able to operate when its competitors are grounded. Completely automatic “hands off”
landings of BA aircraft at London Heathrow are now routine, as are operations of many other carriers.
Crosswind Effects
Regulating bodies such as ICAO and the FAA require that an airport has sufficient runways, both in number and orientation, to permit use by the aircraft for which it is designed, with a usability factor of at least 95 percent with reference to wind conditions.
Modern heavy transport aircraft are able to operate in crosswind components of up to 30 knots without too much difficulty, but for operational purposes runway layouts are designed more conservatively. Annex 14 requires an orientation of runways that permits operations at least 95 percent of the time with crosswind components of 20 knots (37 km/h) for Category A and B runways, 15 knots (27.8 km/h) for Category C runways, and 10 knots (18 km/h) for Category D and E runways (ICAO 2010). FAA regulations differ slightly (FAA 1989). Runways must be oriented so that aircraft can be landed at least 95 percent of the time with crosswind components not exceeding 15 miles per hour (24 km/h) for all but utility airports and 11.5 miles per hour (15.5 km/h) for utility airports.
The usability factor should be based on reliable wind-distribution statistics collected over as long a period as possible, preferably not less than 10 consecutive years. As aircraft have become heavier, the provision of crosswind runways has become less important at large hubs, where there is a generally prevailing direction of wind. However, crosswind runways are still operated at many airports when winds vary strongly from the prevailing direction or where light aircraft are operated.
The usability of a runway or a combination of runways is most easily determined by the use of a wind rose,2 which is compiled from a tabular record of the percentage incidence of wind by direction and strength, as shown in Table 5.2. For clarity of presentation, the table shows a record of the percentage of time the wind falls within certain speed ranges (in knots), with the direction recorded to the nearest of 36 compass directions.
Source: FAA (1989).
TABLE 5.2 Wind Table: Wind Direction and Number of Recorded Hours over a 10-Year Period
A wind rose is drawn to scale with rings at 0 to 10, 11 to 16, 17 to 21, 22 to 27, and 28 and over knots, as shown in Figure 5.3. The percentage of time that a crosswind component occurs in excess of 13 knots3 can be determined using the following example with runway 10–28 (oriented 105 to 285 degrees). (For the purposes of this example, it is assumed that true north and magnetic north are identical; in practice, runway bearings are magnetic, and wind data are referred to true north. Therefore, runway bearings must be corrected prior to plotting.) The direction of the main runway 10–28 is plotted through the center of the rose, and 13-knot crosswind-component lines are plotted to scale parallel on either side of this centerline. The sum of percentages of wind components falling outside the parallel component lines is the total amount of time that there is a crosswind component in excess of 13 knots. Table 5.3 indicates that this occurs for a total of 2.72
percent of the time for this particular runway direction. Therefore, this runway would conform to FAA standards if proposed as the only runway of a U.S. airport. The reader is invited to check that it also would meet ICAO standards. Note that estimates of part areas of the rose had to be made in compiling Table 5.3.
FIGURE 5.3 Wind rose. (FAA.)
Source: FAA (1989).
TABLE 5.3 Wind Table: Estimated Area Not Included as Being Below 13-Knot (24-km/h) Crosswind Component
BirdStrike Control
Since the beginning of aviation, birds have been recognized as a hazard to aviation. In the early days, damages tended to be minor, such as cracked windshields, dented wing edges, and minor fuselage damage. However, fatal accidents owing to bird strikes occurred as far back as 1912, when Cal Rogers, the first man to fly coast to coast across the United States, was subsequently killed in a bird strike. As aircraft have become faster, birds have become less able to maneuver out of the way, and the relative speed at impact has increased. Damage also increased when turbine-engine aircraft were introduced. Ingestion of birds into the engine can cause a blocking or distortion of airflow into the engine, severe damage to the compressor or turbine, and an uncontrollable loss of power. On January 15, 2009, USAirways Flight 1549 lost thrust in both A-320 engines after striking a flock of Canada geese shortly after takeoff from New York’s LaGuardia airport. In what has come to be called the “Miracle on the Hudson,” the crew successfully landed the aircraft in the Hudson with no loss of life.
Loss of life through bird strike may be unusual, but airport operators must be aware that the potential for a disaster can exist in the vicinity of an airport where aircraft are operating at the low altitudes at which they are likely to come in contact with birds.
International and national aviation regulating bodies therefore have prepared advisory documents that guide airport operators in methods of reducing the risk of bird hazards through programs of birdstrike control (ICAO 2011).
ICAO recommends that the airport operators assign bird-control duties to a wildlife
officer, preferably someone with training in biology or related sciences. As a first step, the wildlife officer should commission a study of the bird species that present a hazard at the airport, including size and weight, flying and flocking patterns, density of population, and nesting locations relative to the approaches. From this information and from a history of reports of bird strikes and evidence of bird impacts (i.e., bird remains found during daily inspections), the degree of risk presented by bird hazard can be assessed.
Any bird, if present in sufficient quantities, can present a hazard to aviation at an airport. However, because different birds exhibit remarkably different behavior patterns, only a few are likely to create hazards. Past accidents involving large passenger aircraft on the U.S. East Coast indicate the particular hazard associated with gulls and Canada geese.
Birds present on the airport are there because the facility provides a desirable environment for such natural requirements as food, shelter, safety, nesting, rest, and passage for migratory routes. Successful birdstrike control largely depends not on driving birds off but on creating an environment on the airport and in its immediate vicinity that is not attractive to birds in the first place.
Typical countermeasures will
1. Control garbage, especially the location of garbage dumps near the airport.
Garbage-disposal dumps should not be located within 8 miles (13 km) of an airport.
2. Control of other food sources, such as insects, earthworms, and small
mammals, by a variety of measures, such as poisons, insecticides, cultivation, and hunting to ensure that the open space at and near the airport does not encourage a food supply likely to be attractive to the troublesome species.
3. Eliminate as much as possible the occurrence of surface water that can form suitable habitat for water birds. Control measures include filling or draining areas or netting open-water areas.
4. Where possible, control farming in close proximity to operations. If open areas held by the airport for future expansion are leased as farmland, a ban on cereal cropping, for example, should be written into the leases.
5. Promote vegetation that discourages the presence of birds and avoid
vegetation, such as trees, hedgerows, and berry-bearing shrubbery, that attracts birds.
6. Ensure that buildings in the airport area do not provide suitable nesting places for birds such as swallows, starlings, and sparrows that have become used to living in human-made environments.
Even where habitat-control measures have been taken to discourage some birds from being attracted to an airport, other birds might appear in significant numbers. It might become necessary to disperse and drive off birds using more active measures. These measures should not be used instead of habitat control because, given an attractive environment, when one bird is driven off, another is likely to take its place. Dispersing and expulsion techniques include
• Pyrotechnic devices (e.g., firecrackers, rockets, flares, shell crackers, live ammunition, gas cannons)
• Recorded distress calls
• Dead or model birds
• Model aircraft and kites
• Light and sounds of a disturbing nature
• Trapping
• Falcons
• Narcotics and poisons
If the presence of birds is a serious problem that threatens to disrupt the safe operation of the airport, the operator has no choice but to initiate a control program that will reduce the hazard to an acceptable level.
However, effective action cannot always be taken by the airport operator alone.
Indeed, an effective control program must include all airside tenants and possibly property owners adjacent to the airport because these areas could be a source of bird or animal activity hazardous to air traffic. ICAO recommends that the larger community be organized to form a wildlife hazard committee that jointly considers and approves measures to reduce bird and other wildlife risk to an acceptable level.