CHAPTER 4 CHAPTER 4 Airport Influences on Aircraft Performance
4.2 Aircraft
This section considers largely the operation of commercial airliners, but the principles are similar for all types of aircraft. An aircraft is a heavier-than-air-machine that depends on the movement of air, either by the engine(s) or through the influence of the airframe’s shape, and most significantly by the wing, to attain and sustain normal flight. An overriding parameter for an aircraft operator is that the aircraft must be able to perform services cost- effectively in order for operators to sustain viable service conditions.
The main consideration is the carriage of a useful load (the payload) over a declared range. The payload can comprise many elements—principally passengers, baggage, cargo or freight, and consumables (i.e., food stuffs and water/fluids for toilets, etc.). All-freight operations are also affected by the same considerations.
The nominal mass of the crew (flight crew and cabin crew) is usually included in the operational empty weight (OEW), which is the basic aircraft (with a nominal cabin configuration) without fuel or payload. Consumables, passengers, and their baggage (and/or, as appropriate, freight or cargo) are classified as payload. While much of these can be weighed prior to a flight, passengers are assessed using nominal mass values. The data in use vary from country to country, but typically 200 pounds (90 kg) is used for a crew
member—this includes an approximate 20-pound (9-kg) baggage allowance—and 220 pounds (100 kg) is used for a passenger, including a typical 40-pound (18-kg) baggage allowance. In the United States and Europe, there are periodic assessments of passenger mass values (around once in 20 to 25 years), and the certifying authority may choose to revise the nominal values that certified operators must use on load sheets. Invariably, it rises over time. For very small aircraft, individual passenger mass values will be used, whereas for large aircraft there are mass categories—adult, child, and infant. While these tend to have only a minor impact on the majority of payload assessments, they can be significant for certain operations.
Many manufacturers report, in addition to OEW, an aircraft-prepared-for-service (APS) mass. This is greater than OEW because it adds (or subtracts—but this is rare) cabin variations and consumable allowances. Certified aircraft APS mass can be expected to include allowances that are specific to the airline and even the particular aircraft’s seating configuration.
There will be a maximum structural payload limit, determined by loading criteria such as floor strength and the maximum allowable payload in sections of the fuselage. These criteria also affect where the aircraft center of gravity (CG) will be, and while CG location is an overwhelmingly important flight safety issue, it is of no direct consequence to an airport operator other than in contributing support pertinent to the loading of an aircraft. The aircraft operator’s flight dispatcher holds the ultimate responsibility for assuring that an aircraft load is of the correct mass and that it is distributed appropriately within the aircraft.
The operator will regard airport ground staff as reliable in terms of their observation of a loading operator, in that they are in a position to sense when anything out of the ordinary and that may have escaped their attention is taking place.
Fuel is loaded at the request of an operator. The operator will have knowledge of the prefueling content of an aircraft’s fuel tanks, as well as the fuel load that is designated for an operation. The actual fuel load for a specific operation is determined at the time of the operation because it will take into account the sector distance, the actual route to be flown, and meteorological conditions. The operator may be able to load an aircraft with extra fuel, say, to conduct an outward and return flight, or to minimize the pickup volume at an airport where fuel is expensive—a practice often referred to as tankering. There may be requirements for the distribution of the fuel mass among the tanks and the sequence in which the tanks are filled, but these issues are either controlled by specialist staff or handled automatically by systems on the aircraft. The most important criteria that will be observed in determining a fuel load are that the requested load causes the aircraft neither to exceed its maximum takeoff weight (MTOW)—with its estimated payload—nor to exceed the declared fuel capacity of the aircraft type. Most fuel delivery is conducted in terms of volume (liters or gallons), but the crucial attribute is the mass of fuel on board. This depends on temperature and will be an issue handled by the airline and the fueling agent.
As indicated earlier, the maximum takeoff weight2 (MTOW) must not be exceeded, although an aircraft may depart the stand with a designated taxiing fuel load that will allow a small (in relative terms) additional increase in weight. Table 4.1 presents overall mass data for the Boeing 777-300ER, and the data are extracted from a company publication
that presents aircraft characteristics for airport planning (ACAP).
TABLE 4.1 Basic Mass Data for the Highest Gross Weight Variant of the Boeing 777- 300ER
It is rare to find that adding OEW, maximum payload, and maximum fuel is a summation that equals MTOW. It is usually considerably greater. Because MTOW becomes the limiting attribute, if the aircraft carries its full payload, it cannot carry its greatest fuel load and thus attain its maximum range. As payload mass is reduced, equivalent fuel mass can be added, maintaining MTOW. Thus, when carrying sufficient fuel to achieve its maximum range, an aircraft can carry only a proportion of its maximum payload. Eventually, no more fuel can be added, and as payload is reduced further, the additional range benefit is relatively small.
This can be plotted as a payload-range diagram, as shown at Figure 4.1. This example is again the Boeing 777-300ER. The plot has load plotted vertically and range plotted horizontally. The overall shape, with a flat maximum payload line, starting from the maximum zero-fuel weight (MZFW) on the vertical axis reaches the maximum takeoff weight (MTOW) at the point where it then begins to slope downward. Along this section, payload and fuel masses are exchanged, and the aircraft is at MTOW. At the next change of slope, the maximum fuel load has been reached and thereafter is a steeper drop to the maximum range with no payload, often referred to as the ferry range.
FIGURE 4.1 Boeing 777-300ER payload-range diagram.
There are many ramifications associated with the point at which payload and range intersect on this plot. The only combinations that are allowable are within the envelope (enclosed by the bold lines and the axes). In general, the further to the right an operation is on this plot, the lower is the aircraft’s operating cost per seat. This is a major issue for operators.
What is important to the airport is to appreciate that any aspect of its runway that limits the aircraft’s takeoff weight will cause the payload-range attainable from the airport to be reduced and any reduction in allowable takeoff weight [sometimes referred to as regulated, or restricted, takeoff weight (RTOW)] will cause the steepest-sloping sector to commence at a shorter range and to maintain a similar gradient as the plotted lines at different aircraft weights shown in Figure 4.1. In fact, these are not straight lines but are slightly concave, but the significance is not of consequence in the applications referred to here. Note too that the takeoff-weight line that corresponds to the MZFW is a higher value: about 30,000 pounds (13,600 kg) higher in the example. This is attributable to allowances for reserves and is a variable that will not necessarily be constant across all operators of the type.
Figure 4.2 shows a diagrammatic payload-range plot and shows the major points that relate to aircraft mass values quoted in Table 4.1. The plot, because it shows the payload- range attributable to lower aircraft mass values, additionally provides an illustration of the effect of RTOW usually caused by limited takeoff distance on the attainable payload-range combinations that can be accommodated.
FIGURE 4.2 Diagrammatic illustration of a typical payload-range diagram.
The Aircraft Characteristics for Airport Planning (ACAP) publications already quoted are produced to a format agreed on among manufacturers. They are specifically for planning and do not substitute for a flight crew operations manual (FCOM). In specific circumstances the FCOM is the best source of actual data.
Airport operations staff should be able to access FCOM-derived data through aircraft operations staff, and it is necessary that they do so if an operation is regarded as critical, for example, when assessing the capability of a specific aircraft type to operate at or close to its performance limitations. (This is usually in planning but is a task deferred to operations staff.) The ACAP is available from most major aircraft manufacturers through their website, or smaller manufacturers usually will provide the information on request.
Companies often do not release performance data other than in response to a request from an airport. All information within these documents is generic in that the information pertains to a model specification that will be modified by the choices made by an operator with respect to fixtures and fittings in the cabin (e.g., number of galleys and toilets and even individual seat specifications), and the user has to appreciate how significant or not this might be. These data can be used by an airport planning team if they are content to refer to a general aircraft type in planning future operations.
In general, if current-day operations are being considered, the airport team is recommended to discuss requirements with the flight operations team of incumbent or target operators. The aircraft manufacturers will provide specific data to an airport when the circumstances are critical to safety and they are the consulted in confidence.
Figures 4.3 and 4.4 show the takeoff plots for an aircraft type and are diagrams again extracted from the example ACAP manual.
FIGURE 4.3 Boeing 777-300ER takeoff runway length chart, standard day.
FIGURE 4.4 Boeing 777-300ER takeoff runway length chart, standard day + 27°F (15°C).
These plots provide an indication on how critical a runway distance value is to an aircraft operator and the significance of the effects air temperature and altitude on aircraft performance.
Entering the plots with the takeoff runway length [ACAP definitions are not specific about this but generally use declared takeoff distance available (TODA)], the user can read
pressure altitude) and then drop a line vertically onto the horizontal scale that will reveal RTOW. Referring back to the payload-range chart, the best payload and range combinations possible can be obtained. Charts often show international standard atmosphere (ISA) performance [with 15°C (27°F) ambient at sea level] and a plot of an ISA + 15°C (or ISA + 27°F). These can be interpolated to roughly to assess likely performance.
They should not be extrapolated into lower or higher temperatures because the relationship of takeoff performance and air temperature is not linear. The general performance relationships to note are that, all other parameters being unchanged,
• As air temperature rises, the attainable takeoff weight decreases.
• As airport elevation increases, the attainable takeoff weight decreases.
Table 4.2 shows a sample of aircraft data indicating the airfield design requirements for each category of operation and gives some pertinent data for a representative set of aircraft. The weights and dimensions are from published data, but care must be taken when using specific values.
Sources: Largely manufacturer’s data.
TABLE 4.2 Aircraft Data
There are several principles involved in matching aircraft to infrastructure. These reflect
• Demonstrated performance of the aircraft
• Application of assessed acceptable probability of any relevant failure
• The safety regulation of operations
Aircraft performance, as demonstrated for aircraft certification, is referred to as the
within which it is considered safe to operate, the gross performance is factored, becoming net performance to take account of in-service variables. The variation is predicated to allow for variations caused by such influences as pilot skill, instrument inaccuracies, weight growth, and engine thrust reduction between overhauls.
Thus, for example, the demonstrated landing distance is factored by 1.67 under some regulations, including those of the United States, to derive the schedule landing-field length, and the gross climb performance is reduced by 0.9 percent in order to derive the performance that can be guaranteed. This information is published in the aircraft flight manual (and in the generic data in ACAP publications).
Regulations require that each new aircraft type demonstrates the distance required to land and take off under closely controlled conditions, with defined limitations on the pilot’s reactions and carefully constructed safeguards to obviate any actions that might be inherently unsafe. Similarly, all other certificatory performance measures must be demonstrated for all applicable configurations of power and geometry, with all engines operating, and with the critical engine inoperative.
These result in the aircraft performance requirements being presented as
• Takeoff run required (TORR)
• Takeoff distance required (TODR)
• Accelerate-stop distance required (ASDR)
• Landing distance required (LDR)
Takeoff run required (TORR) is the net performance-assessed distance that the aircraft might need to travel while still in contact with the ground. This clearly sets a minimum runway length, but on its own this is not adequate and for safe airport operations much more is required.
Takeoff distance required (TODR) is the net performance measurement to reach a screen height, the distance being measured from the point at which the takeoff run commences. The screen heights used in the Federal Aviation Regulations (FAR; United States) and Joint Aviation Regulations (JAR; Europe) certification requirements are similar, but vary among aircraft types and can introduce different circumstances depending on the aircraft’s susceptibility to critical events. For most multiengine commercial jet airliners, a screen height of 35 feet (10 m) is used.
Accelerate-stop distance required (ASDR) is the distance required to accelerate to V13 with all engines at takeoff power, experience an engine failure at V1, and abort the takeoff and bring the airplane to a stop using only braking action without the use of reverse thrust.
This should not exceed the paved runway length available at an airport.
Landing distance required (LDR) is measured from the threshold on the runway in use and includes the distance to the touchdown point and the landing run itself. The approach will be assumed to be conducted at the normal approach speed but that only brakes will be available after touchdown. As noted, the certified net performance typically will be the demonstrated gross performance increased by 1.67. Additional factors will be applied, typically 1.15 times, to account for a wet runway.
So that operators can match field-length requirements with the distances available, the airfield is required to publish for each runway the following so-called declared distances,
which have been established on the basis of paved length, runway category, and local obstacles:
• Takeoff run available (TORA)
• Takeoff distance available (TODA)
• Accelerate-stop distance available (ASDA)
• Landing distance available (LDA)
When the operator has determined that the aircraft scheduled performance (corrected to the appropriate aerodrome elevation, air temperature, runway slope, wind, and runway surface condition at the required takeoff weight) results in a required distance for takeoff run and takeoff distance that are less than the declared distances available, the operation is deemed acceptable.
The operator also must conduct analyses of performance requirements and relate them to the declared distances available at any alternative airfield(s).
Declared distances are promulgated in the nation’s Aeronautical Information Publication (AIP) or through the Electronic Aeronautical Publication (EAP) databases that are more common nowadays (see Chapter 11). They need to be accompanied by the airfield reference temperature, the runway elevation, and the runway slope. It is the responsibility of the airport to notify, by means of Notices to Airmen (NOTAMs), any changes in these data caused by, for example, work in progress or accidents.
Declared distances take into account displaced thresholds, stopways, clearways, and starter strips, as shown in Figure 4.5.
FIGURE 4.5 Runway declared distances.
An excess of TORA and TODA may allow a reduced-thrust takeoff to preserve engine life or an increased speed at the screen to improve climb performance. An excess of LDA may allow flexibility in planning for bad weather at the destination or may allow tanking of fuel. A more detailed discussion of field-length requirements is given in Section 3.2 of Ashford, Mumayiz, and Wright 2011.