Characteristics of Aircraft As They Affect Airports ∗
2. Calculating the allowable maximum payload that may be moved under those specific conditions when the payload is limited by available field lengths
In essence, an aircraft type is required to demonstrate the field length required for the following cases: (a) to complete a takeoff to 35 ft (11 m) altitude with all engines operating, (b) to complete a takeoff to 35 ft (11 m) altitude with an engine failure at a critical point, (c) to stop after aborting a takeoff with an engine failure at the same critical point, and (d) to stop after landing from a height of 50 ft (15 m).
The demonstrations take place under carefully controlled conditions of flying speed, aircraft weight and configuration, and airfield altitude and temperature. Safety margins are then added to these demonstrated distances to allow for variation in pilot perfor-mance, aircraft perforperfor-mance, and environmental conditions in service. The margins are typically 15% in the all-engine-operating takeoff case and 67% in the landing case;
the difference is due mostly to the extra difficulty of controlling and monitoring an approach compared with the relatively fixed and known conditions on takeoff. It is also recognized that the in-service performance of old aircraft, while conforming to adequate maintenance procedures, will be less than the performance of new aircraft under certification demonstrations.
Extra margins are implicit in the procedure just described, insofar as most airports accepting commercial flights do not have obstructions at the ends of their fields and most airfields have either visual or electronic guidance on the approach path. However, experience has shown that margins of this order are necessary if a satisfactorily low rate of hazardous incidents associated with field length is to be maintained. In this way, a required field length is assigned to each certificated aircraft, for every practical combination of variations in weight, altitude, and temperature, and the information is published in the official flight manual as a series of charts. This information is collated by the FAA (2) and the ICAO (3). Figure 3.7 is a modified example taken from the FAA’s runway length requirements for airport design. The example shown indicates the runway lengths required at 1000 ft elevation and STD+ 15C for a B737-900 takeoff with maximum design takeoff weight (174,200 lb) and for takeoff with a restricted takeoff weight of 150,000 lb.∗The FAA provides a companion chart for calculating the landing runway length required.
Published field length requirements can be used for the following purposes:
1. Checking the ability of an aircraft to take a specified payload from, or to land
120
55 3 125 4 5 6 7 8 9
1000 ft
FAR takeoff runway length (1000 m)
10 11 12 13 14 4.5 15
60 65 70 75 80
130 135 140 145 150 1000 Pounds
(1000 kg) Operational takeoff weight
155
Max design takeoff wt 174,200 lb (79,016 kg)
160 165 170 175 4.0
3.5
3.0
2.5
2.0
1.5
1.0
Standard day + 27°F (STD + 15°C)
Tire speed limit
Flaps 5
Flaps 15
Sea level 2000 4000 5000 Airportft
8000 Elevation(m)
(2438)
(510) (1,218) (1,829)
Flaps 25
Figure 3.7 Takeoff runway length chart for Boeing 737-900 (1).
and aircraft weight, the weight being adjustable between useful payload and fuel, as described below.
There are, however, two further variables to be considered in defining the field length requirement. The takeoff distances that must be demonstrated for transport category aircraft are presented in Figure 3.8; the speeds to be controlled during the demonstration are symbolized as follows:
V1 = takeoff decision speed chosen by the aircraft manufacturers: >1.10Vmc, <speed at which brakes overload, <VR, <1.10 Vs
Vmc = minimum control speed: minimum speed at which engine failure can occur and still allow straight flight at this speed in a fully controlled manner
VLOF = liftoff speed: ≥1.1 Vmu(≥1.05Vmuwith one engine out)
Vmu= minimum unstick speed: >minimum speed that allows safe continuation of the takeoff V2 = takeoff safety speed at 35 ft (11 m) ≥ 1.2Vs, ≥1.1Vmc
Figure 3.8 Takeoff field length demonstration requirements for transport category aircraft (2, 3).
Vs = stall speed in takeoff configuration
VR = speed at which nosewheel can be lifted from runway and ≥V1,≥1.05 Vmc
The first variable is V1, which can be chosen by the manufacturer within the limits of controllability, rotation speed, and brake failure. If the engine fails before this speed is reached, the pilot must abort the takeoff; if failure occurs at or above this speed, the pilot must continue the takeoff, despite the loss of power. When only a normal hard runway is available, the minimum engine-out runway requirement is obtained if V1 is chosen so that the distance needed to stop is equal to the distance to reach 35 ft (11 m).
This is called the balanced field length. The field length in this case is determined as the larger of the balanced field length and 115% of the all-engine distance to a height of 35 ft (11 m). This is the only definition applicable to piston-engine aircraft.
Turbojet engines have proved to be so reliable that engine failure on takeoff has become very uncommon. This has allowed the introduction of a second variable, namely, the ability to substitute stopways and clearways for some portions of the hard runway. Stopways and clearways are defined in the U.S. Code of Federal Regulations (CFR), Title 14, Part 1 (4, 5).
A stopway is defined as “an area beyond the runway, not less in width than the width of the runway, centrally located about the extended centerline of the runway, and designated by the airport authorities for use in decelerating the aircraft during an aborted takeoff. To be considered as such, the stopway must be capable of supporting the aircraft without inducing structural damage to it.” A clearway, on the other hand, is defined as follows:
An area beyond the runway not less than 500 feet (150 m) wide, centrally located about the extended centerline of the runway, and under the control of the airport
authorities. The clearway is expressed in terms of a clearway plane, extending from the end of the runway with an upward slope not exceeding 1.25% above which no object nor any portion of the terrain protrudes, except that threshold lights may protrude above the plane if their height above the end of the runway is not greater than 26 inches (66 cm) and if they are located to each side of the runway.
Similar definitions for both the stopway and the clearway are given in Annex 14 (6). A clearway may not be longer than half the difference between 115% of the distance between the liftoff point and the point at which 35 ft (11 m) altitude is reached for a normal all-engine takeoff or longer than half the difference between the liftoff point and the point at which 35 ft (11 m) altitude is reached for an engine-out takeoff.
A stopway may be used as a substitute only for the part of the accelerate-stop distance that is greater than the full-strength runway requirement determined from clearway allowances; that is, the hard runway must extend for the full length of the takeoff run, defined as the point equidistant between the point at which VLOF is reached and the point at which a height of 35 ft (11 m) is attained. The use of stopways and clearways in the declaration of available field lengths is shown in Figure 3.9. Also shown is the way in which the demonstrated performance is converted to the factored performance as scheduled in the aircraft’s flight manual.
The takeoff field lengths scheduled in the flight manual and listed in Table 3.1b must be the greater of the demonstrated engine-out accelerate-stop distance, the demonstrated engine-out distance to 35 ft (11 m) altitude, or 115% of the demonstrated all-engine distance to 35 ft (11 m) altitude. The takeoff decision speed (V1) may be chosen by the manufacturer, within the limits noted in Figure 3.10, but the speed must be used for both the aborted and the continued takeoff.
This flexibility of choice is extended to the pilot who is faced with a particular run-way situation, so that the greater the takeoff distance available, relative to the emergency
*
Figure 3.9 Field length definitions (the asterisk on the horizontal axis of the middle graph indicates engine failure at speed V1).
Figure 3.10 Use of unbalanced field performance.
stop distance available, the lower the pilot will choose his or her V1speed. Similarly, the airport planner can take advantage of these alternatives. It is frequently advanta-geous to use a clearway, because it saves on full-strength runway without penalizing the operation. Then a low decision speed can be chosen to keep stopway requirements to a minimum. Conversely, a high V1 will give an even shorter full-strength runway requirement at the expense of a long stopway in the engine failure case, but the normal takeoff or landing cases may then become critical from the point of view of the length of full-strength runway. These choices are depicted in Figure 3.10. Every airport con-stitutes an individual case for consideration. The runway length requirements depend on the geography and weather at the airport, the possible critical speeds of the aircraft, and the fuel requirements for the critical flight plan.
Field Length Regulations—General Aviation Aircraft∗
Many general aviation aircraft used for executive business, air taxi, and commuter operation are now certified in the United States under Federal Aviation Regulations, Part 25, and so must meet the same field length requirements as those applicable to
†It is the responsibility of individual countries to certify aircraft appearing on their own register. Developed countries, such as the United States, the United Kingdom, France, Germany, and so on, have their own certification procedures. Small countries often avoid the costs of certification by accepting the certification of the FAA or some other authority, such as the CAA in the United Kingdom or the French Ministry of Civil Aviation. This applies both to air transport and general aviation categories of aircraft. The examples given here quote FAA requirements, but these can be considered typical.
aircraft greater than 12,500 lb AUW and/or 30 seats, as described earlier. Other aircraft are certificated under Federal Aviation Regulations, Part 23 (8), which requires only demonstration of all-engine takeoff distance to 50 ft altitude and landing from 50 ft altitude for aircraft weighing between 6000 and 12,500 lb. No specific demonstration is required for aircraft below 6000 lb, but Table 3.1c gives some data relating to normal takeoff and landing distances to 50 ft to assist in runway design.
Reference 2 provides guidance on the recommended runway lengths for turbojet-powered airplanes of 60,000 lb or less maximum certified takeoff weight. In that circular, the FAA presents temperature- and altitude-dependent curves to cover 75 and 100% of the basic turbojet fleet at 60 and 90% load factors. That fleet includes Bombardiers, Sabreliners, Cessna Citations, and Falcons. Load factors greater than 90%
are not considered, because the likelihood of that load occurring on a day when this category of aircraft is not climb limited is very small. For those airports expected to accommodate general aviation aircraft over 60,000 lb, the runway length requirement is calculated on the critical aircraft as it is at a commercial airfield.
Reference 7 gives the runway lengths recommended as a basis for planning the various classes of utility airports as well as the effects of altitude and temperature.
Restrictions on Payload-Range Performance
It is important to realize that the field lengths given in Table 3.1 refer to maximum take-off and landing weights at sea level and 59◦F (15◦C) (ISA). Equation 3.1 (Section 3.2) indicates that lift is proportional to air density. Since air density falls with increase of either altitude or temperature, for operation at maximum takoff or landing weights, higher takeoff and landing speeds must be used, requiring greater field length. If addi-tional field lengths are not possible, landings and takeoffs must be carried out at lower weights to compensate for the lower generated lifts. This is demonstrated in Figure 3.7 and in Table 3.2. The illustrations refer to transport category operation, but the effects on general aviation are equally substantial. The situation is complicated by the effect of
Table 3.2 Increases in Field Length (ft× 1000) due to Changes in Altitude and Temperature
Takeoff Landing
Sea Level 4000 ft Sea Level 4000 ft
Aircraft ISA ISA+ 15◦C ISA ISA+ 15◦C ISA ISA+ 15◦C ISA ISA+ 15◦C
B747-400 3000 3150 4000 4200 2400 2400 2650 2650
B767-300 2600 3400 4400a 4400b 1720 1720 1900 1900
A340-600 3150 3900 3900a 2125 2125 2350 2350
767-200 1720 1860 2350 2620 1450 1450 1630 1630
B737-900 2300 2400 3150 3100a 2100 2100 2300 2300
A321-200 2750 2900 3800 3850a 1625 1625 1830 1830
MD 90-30 2150 2275 2650 2800 1850 1850 2800 2800
De Havilland Dash8 1300 1440 1640 2000 1050 1050 1150 1150
aBreak energy limitation, reduced TOW in some cases.
bTake-off weight limitation.
Source: Manufacturers’ data.
low-density air on engine performance, which tends to produce a greater deterioration in the takeoff case, unless the engine is flat rated (i.e., its output at high air densities is deliberately limited to avoid overloading its components). Therefore, under hot and high conditions, it becomes necessary to use longer field lengths or to reduce weight.
In addition, the hot and high cases frequently make it difficult to meet the requirements for engine-out climb gradients at a given weight, even if the runway is long enough to meet the field length regulations at that weight.
The previous discussion has shown how the payload range can be compromised by runway length and by altitude and temperature. The allowable operation weights, hence the payload-range capability, can also be affected by runway strength limitations.
This is illustrated in Figure 3.11, which indicates the relative effects of strength and length limitations for a given set of operating conditions as well as the reduction in potential profit margin. However, these limitations only apply for more than occasional use above the pavement’s structural limits.
Weight Components
It is not always difficult to reduce weight to meet runway requirements, because air-craft usually are flexible in the makeup of their maximum weight, as indicated by the following definitions:
1. Empty operating weight is a constant weight for a type, made up of all items