CHAPTER 4 CHAPTER 4 Airport Influences on Aircraft Performance
4.3 Departure Performance
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.
by a 1.25 factor on the demonstrated distance and by the TODR being not greater than the ASDR. These requirements do not always guarantee an acceptable operational solution, however.
Aircraft used for commercial purposes (multiengine) and all aircraft with an MTOW in excess of 12,500 pounds (6,700 kg) are certified in accordance with a multistage certification process, whereby it must demonstrate performance equal to in excess of a minimum gross climb gradient throughout a four-segment climb. The requirements (FAR 25 in the United States) are summarized at Table 4.3.
TABLE 4.3 Gross Climb-Gradient Requirement (%)
The segments begin or terminate at points that are defined by procedures (i.e., speed- related) and aircraft configuration. They are illustrated at Figure 4.6 and can be described as follows:
FIGURE 4.6 Climb-path segments.
First segment. Critical engine inoperative, remaining engine(s) at takeoff thrust, landing gear extended, flaps in takeoff position, and aircraft at the minimum safety speed V2,min.4
Second segment. Same as first segment, with gear retracted, speed increased by 20 percent, and proceeding to an altitude of 400 feet (122 m).
Third segment. Maintaining level altitude, retracting flaps and slats (fully or to appropriate settings), increasing speed by a further 5 percent, and engine power setting reduced to maximum continuous thrust.
Final (fourth) segment. Maintaining configuration at end of third segment, climb to altitude of 1,500 feet (457 m).
The segment capability is measured in flight testing, but in reality a commercial crew will fly to the specific points while at the same time maintaining the most favorable flight conditions to achieve the best possible climb and flying with due regard for the minimum speed conditions. For instance, it would be rare for a crew to maintain a level flight path in the third segment. This criterion is applied in the certification case to replicate a “worst case” scenario. The assurance that is critical in operations is that if the obstacle-limitation surfaces applied to runways are applied with rigor, they should be adequate to ensure that there is acceptable protection.
In normal operations, climb-out technique is frequently also modified by considerations of noise abatement or fuel economy. Efforts to reduce the noise impact of departing aircraft include
• Flying the highest gradient possible (a technique used less as pure jets have given way to high-bypass turbofan aircraft engines)
• Using specific combinations of thrust and heading to avoid noise-sensitive areas
• Using thrust-cutback techniques that will balance the reduced climb rate with a lower perceived noise on the ground and without undue operational penalty
These performance requirements show how the maximum allowable takeoff weight may be limited by field length or by climb performance to meet either weight and temperature (WAT) limits or dominant-obstacle clearances. It is clear that frequently there is a margin available even at maximum structural weight over all these requirements. There are, in fact, other requirements that a dispatcher must check associated with en-route climb performance, landing performance, and limits on tires and brakes, but these are seldom critical. The pilot therefore has the discretion to perform the takeoff with less than takeoff thrust, provided that not less than 90 percent of the available power is selected. The amount of thrust required to stay within field-length and climb requirements normally can be selected accurately for specific airfield conditions by using an onboard computer, with consequent advantages to engine life and fuel consumption. On the other hand, not only must increased tire wear be considered but also the possible overall increase in risk compared with the historical statistical base for accidents per takeoff, whereby those with a significant margin between actual and allowable takeoff weight are predominantly a higher percentage of safe takeoffs.
Because the required takeoff weight is a function of the payload and fuel requirements, ultimately the payload will be tailored to the takeoff weight available by offloading that part
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.