Deriving Benefits from Alternative Aircraft-Taxi Systems (2016)

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11 This appendix provides a copy of the detailed discussion of each of the benefits and issues associated with alternative aircraft-taxiing systems included within the ATAM. It repeats some of the information presented in the main discussion, and some of the information repeats across each of the items that follow, but each discussion item from the ATAM has been included here in its entirety for completeness. Noise Electric onboard aircraft-taxiing systems produce negligible noise and no local emissions. However, there would still be some noise produced by the aircraft’s APU, which would be used to generate the electricity. For an onboard alternative aircraft-taxiing system powered by an APU, Asensio et al. (2007) and Tam et al. (2005) compare APU noise to that of aircraft main engines. This information suggests that the overall noise contribution of APUs is lower than that for an aircraft taxiing using its main engines (estimated to be around 1.5 dB DNL in Page et al., 2009). The frequency (i.e., tonal) characteristics are also different. When an aircraft is taxiing, main engines peak at approximately 125 Hz, and APUs peak at between 250 Hz and 350 Hz. Therefore, compared with main engines during taxiing, the noise from an APU is lower in level, but slightly higher in frequency. Similarly, the noise from external systems, such as a tractor, is likely to be lower than that from aircraft main engines. For an alternative aircraft-taxiing system powered by an additional jet engine, the additional jet engine’s noise would replace that of the main engines during taxiing. Studies undertaken by a number of alternative aircraft-taxiing system developers (electric motor and external systems) into noise levels with and without the alternative aircraft- taxiing system close to taxiways indicates a noise reduction of between 10 dB and 12 dB. Cost for Alternative Aircraft-Taxiing System Vaishnav (2014) estimates that external alternative aircraft-taxiing systems would cost about U.S.$1.5 million for small systems (for use with narrow-bodied aircraft) and U.S.$3 million for a system for use with wide-bodied aircraft. Vaishnav also estimates that onboard systems are likely to cost approximately U.S.$0.26 million for narrow-bodied aircraft and U.S.$1 million for wide-bodied aircraft. One of the onboard system manufacturers will offer a lease arrangement in which the airline pays half of the savings achieved by using the system. Purchasers should consider the fact that external systems can be used for several different aircraft (and can be used for a wider range/number of aircraft than modeled), whereas the onboard systems assessed are installed in one particular aircraft. Any onboard system is also likely to incur aircraft modification costs, whereas external systems will have very low, if any, aircraft modification costs. In addition, airlines should consider the revenue lost during installation for onboard systems. A P P E N D I X A Detailed Description of Information in the ATAM

12 Deriving Benefits from Alternative Aircraft-Taxi Systems Construction Costs Construction costs may be incurred for additional aircraft main engine start-up areas (needed if an airport or airline does not allow main engine start during taxi-out) and vehicle service roads for return of any external system to the gate area. Many larger airports already contain hold or deicing areas near the runway ends and relevant existing service roads, so these airports are unlikely to need additional infrastructure. For those airports that need additional holding areas and roadways, concrete taxiways cost approximately U.S.$200 per square yard (U.S.$240 per square meter) (Pers. comm., 2015). Concrete roadways cost approximately U.S.$75 per square yard (U.S.$90 per square meter) (Pers. comm., 2015). However, costs may vary by region. Asphalt is about 20% cheaper than concrete (Pers. comm., 2015). Other Costs (e.g., fuel, staff, installation downtime) Additional fuel, maintenance, installation downtime (onboard systems only), and staff costs are likely to be related to use by airlines and ground handlers. These costs are likely to be minimal and offset by cost savings. Potential cost savings include the following: • Not needing to use pushback tractors [approximately U.S.$50 per cycle (Morrow et al., 2007)]. • Reduced delays and taxi times (at around U.S.$66 to U.S.$150 per minute per turnaround) due to, for example, eliminating the need to attach or detach aircraft pushback tractors for onboard systems (i.e., nose- or main-wheel electric motors). • Being able to use two gates for passenger loading rather one gate, by having the aircraft taxi in parallel to the two gates (i.e., turning the aircraft around by 90 degrees), subject to the airport/ gate layout. • Reduced fuel use. Other costs for airports are associated with additional taxi-queue management needed for systems with low speeds. For external systems, labor costs may increase since drivers will be needed to operate the equipment for longer than for conventional pushback [at a cost of around U.S.$29.60 per hour (Pers. comm., 2014)]. Safety In the event of any alternative aircraft-taxiing system failure, the aircraft must still be able to taxi using its main engines. None of the systems reviewed prevent the use of main engines for taxiing. There were also safety concerns regarding alternative aircraft-taxiing systems that pilots do not directly control. Other safety issues include concerns regarding the visibility of aircraft pushback tractor equipment on taxiways, the possibility of jackknifing and braking fail- ure (where braking did not use the aircraft’s brakes), and visibility and identification of aircraft with onboard systems to allow for slower speed when crossing runways. Nose Fatigue Issues Webster (2008) highlighted a major concern identified by Virgin Atlantic and Boeing with regard to the stress to the aircraft nose wheel caused by dispatch towing and subsequent reduction in the wheel’s fatigue life. It is assumed that the Virgin Atlantic dispatch trial used a towbarless aircraft pushback tractor (TBLT) based on information in ACRP Research Results Digest 15 (2012). In “Towbarless Towing Vehicle Assessment Criteria,” Boeing (2003) describes the test criteria and procedure for evaluating the impact of TBLT used for pushback and maintenance towing on Boeing aircraft. The loads introduced by the TBLT should not exceed the aircraft nose-wheel

Detailed Description of Information in the ATAM 13 design loads, and tests are required to prove that the tow vehicle and aircraft maintain good stability at all times. While the document is not specifically written for dispatch towing, it can be used to assess the potential for nose-wheel fatigue. Therefore, potential fatigue to the aircraft nose wheel should be considered for any external tractor-type system, depending on how the tractor connects to the nose wheel. Airframe Modifications Required Any onboard system will require aircraft modification. The level of modification needed will depend on the individual aircraft and onboard system but could include larger/new power cables due to increased power needs from the APU, APU modification if the APU is not powerful enough, replacement of the APU, changes to control cables and fuel pipes, and reinforcement of the rear of the aircraft for any additional jet engines. Aircraft Brake Implications Aircraft brakes are attached to the main wheels of the aircraft. Therefore, main-wheel onboard systems may have an impact on aircraft brake cooling or could be affected by brake temperature, depending on the design. APU Modifications Required Any onboard system will require aircraft modification. The level of modification needed will depend on the individual aircraft and onboard system but could include larger/new power cables due to increased power needs from the APU, APU modification if the APU is not powerful enough, replacement of the APU, changes to control cables and fuel pipes, and reinforcement of the rear of the aircraft for any additional jet engines. System Weight For aircraft onboard systems (i.e., nose- or main-wheel electric motors or additional jet engines), Saia (2013) raises concerns about the impact of the additional weight when retrofitted to aircraft. Aircraft carrying extra weight will burn more fuel during cruise, but manufacturers of some onboard systems claim their equipment is weight-neutral because less fuel needs to be carried. However, the main issue with weight gain associated with the installation of an onboard alter- native aircraft-taxiing system is the increased fuel consumption during takeoff, climb, and cruise (assuming the number of passengers and/or cargo tonnage remain the same). Any fuel weight savings associated with taxi-out does not affect weight during takeoff, climb, and cruise because that fuel would have been burned prior to takeoff. Only the fuel saved during taxi-in (which, therefore, would not have been loaded onto the aircraft prior to departure) can be considered to offset the weight gain of an onboard system for takeoff, climb, and cruise. If an aircraft is operating close to its regulated takeoff weight (RTOW), then the impact of the additional weight may require an airline to remove cargo or passengers so the aircraft can operate within its RTOW. Airline operators calculate takeoff weight by taking into account the operational empty weight of the aircraft with the weight of passengers, cargo, and fuel (FAA, 2005). The Joint Aviation Authorities (2007) describes the standard passenger weight for a passenger including 6 kg (13.2 lb) of carry-on (hand) luggage and takes into account passengers carrying infants. For an aircraft

14 Deriving Benefits from Alternative Aircraft-Taxi Systems with over 30 seats, the prescribed standard passenger weight for an average adult is 84 kg (185 lb) on scheduled flights and 76 kg (168 lb) on holiday charter flights. The Joint Aviation Authorities (2007) estimates the standard passenger weight for a child to be 35 kg (77 lb) for charter and schedule flights. The Joint Aviation Authority is a European agency, and the reference is used to illustrate weights only. The FAA similarly prescribes a standard adult passenger weight of 169 lbs, plus 5 lb of clothes (10 lb in winter) and 16 lb for carry-on items. Thus, assuming no reduction in fuel allowance, if an aircraft typically operates at its RTOW, then the increase in weight due to the addition of an onboard system (weighing around 300 to 800 lbs) would be offset by a reduction of two or more passengers. For onboard systems attached to the nose wheel, the additional weight would slightly offset the center of gravity. Similarly, if a jet engine were used to replace an APU at the back of an aircraft, with any necessary strengthening of the airframe around the area, the center of gravity would be offset. However, the actual weight gain of any onboard alternative aircraft-taxiing systems is likely to be relatively small and so would have limited impact on the center of gravity. Center of Gravity Implications For aircraft onboard systems (i.e., nose- or main-wheel electric motors or additional jet engines), Saia (2013) raises concerns about the impact of the additional weight when retrofitted to aircraft. Aircraft carrying extra weight will burn more fuel during cruise, but manufacturers of some onboard systems claim their equipment is weight-neutral because less fuel needs to be carried. However, the main issue with weight gain associated with the installation of an onboard alter- native aircraft-taxiing system is the increased fuel consumption during takeoff, climb, and cruise (assuming the number of passengers and/or cargo tonnage remain the same). Any fuel weight savings associated with taxi-out does not affect weight during takeoff, climb, and cruise because that fuel would have been burned prior to takeoff. Only the fuel saved during taxi-in (which, therefore, would not have been loaded onto the aircraft prior to departure) can be considered to offset the weight gain of an onboard system for takeoff, climb, and cruise. If an aircraft is operating close to its RTOW, then the impact of the additional weight may require an airline to remove cargo or passengers so the aircraft can operate within its RTOW. Airline operators calculate takeoff weight by taking into account the operational empty weight of the aircraft with the weight of passengers, cargo, and fuel (FAA, 2005). The Joint Aviation Authorities (2007) describes the standard passenger weight for a passenger including 6 kg (13.2 lb) of carry-on (hand) luggage and takes into account passengers carrying infants. For an aircraft with over 30 seats, the prescribed standard passenger weight for an average adult is 84 kg (185 lb) on scheduled flights and 76 kg (168 lb) on holiday charter flights. The Joint Aviation Authorities (2007) estimates the standard passenger weight for a child to be 35 kg (77 lb) for charter and schedule flights. The Joint Aviation Authorities is a European agency, and the reference is used to illustrate weights only. The FAA similarly prescribes a standard adult passenger weight of 169 lbs, plus 5 lb of clothes (10 lb in winter) and 16 lb for carry-on items. Thus, assuming no reduction in fuel allowance, if an aircraft typically operates at its RTOW, then the increase in weight due to the addition of an onboard system (weighing around 300 to 800 lbs) would be offset by a reduction of two or more passengers. For onboard systems attached to the nose wheel, the additional weight would slightly offset the center of gravity. Similarly, if a jet engine were used to replace an APU at the back of an aircraft, with any necessary strengthening of the airframe around the area, the center of gravity would be offset. However, the actual weight gain of any onboard alternative aircraft-taxiing systems is likely to be relatively small and so would have limited impact on the center of gravity.

Detailed Description of Information in the ATAM 15 Typical Taxi Speed (i.e., around 20 knots) To achieve reasonable taxiing speeds (and acceleration), aircraft pushback tractors with greater horsepower may be needed. Airbus (2013) states that an alternative aircraft-taxiing system should be able to reach speeds of 20 knots. Similarly, Boeing stresses the need for aircraft to be able to reach reasonable taxiing speeds in a short time period (Paisley, 2015). A system fitted to the aircraft nose wheel (or main wheels) may be limited in speed due to motor size. Slower taxiing speeds at smaller airports could be an airline concern in terms of taxiing delay. At larger airports, with existing delays, slow taxiing speeds may be less of an issue, although all airports are striving to reduce such delays. A general guideline, according to air traffic controllers inter- viewed, is that an aircraft should be able to cross a runway surface and clear the safety area within 40 seconds (Pers. comm., 2015). If this requirement cannot be met for an alternative aircraft- taxiing system, then delays will occur unless main engines are used instead. Pilot Control Any alternative aircraft-taxiing system that is directly pilot-controlled is likely to be considered safer by airlines and pilots than a system where the pilot has to work with a driver of an external system. APU Load When an aircraft is on the ground not connected to gate power, its APU is used to run onboard systems and to start the aircraft main engines. The aircraft onboard systems that are powered by the APU typically include lighting, heating, ventilation, and air conditioning. During conventional taxiing, the APU is often not used, and power is taken from the aircraft’s main engines for the electrical systems. However, for any alternative aircraft-taxiing system, the APU will run whenever the engines are not running to produce power. Aircraft with onboard alternative aircraft-taxiing systems that rely on power from the APU (i.e., nose- or main-wheel electric motors) will need an APU that is able to accommodate these additional power needs. Acceleration (e.g., to allow runway crossing from stop in 40 seconds) To achieve reasonable taxiing speeds (and acceleration), aircraft pushback tractors with greater horsepower may be needed. Airbus (2013) states that an alternative aircraft-taxiing sys- tem should be able to reach speeds of 20 knots. Similarly, Boeing stresses the need for aircraft to be able to reach reasonable taxiing speeds in a short time period (Paisley, 2015). A system fitted to the aircraft nose wheel (or main wheels) may be limited in speed due to motor size. Slower taxiing speeds at smaller airports could be an airline concern in terms of taxiing delay. At larger airports, with existing delays, slow taxiing speeds may be less of an issue, although all airports are striving to reduce such delays. A general guideline, according to air traffic control- lers interviewed, is that an aircraft should be able to cross a runway surface and clear the safety area within 40 seconds (Pers. comm., 2015). If this requirement cannot be met for an alternative aircraft-taxiing system, then delays will occur unless main engines are used instead. Taxi-In and -Out It is less likely that external systems will be used for taxiing in as this would require a number of them to be positioned at runway exits and then be attached to an aircraft before taxiing it to the gate. However, there is no reason that onboard systems (i.e., nose- or main-wheel electric

16 Deriving Benefits from Alternative Aircraft-Taxi Systems motors or additional jet engines) cannot be used for taxiing in as they do not need to be attached to or detached from the aircraft, and there are no associated delays. Attaching and Detaching In most cases, the alternative aircraft-taxiing system negates the need for the use of a pushback tractor, with the possible exception of the replacement of the APU with a certified jet engine. Larger jet engines cannot be started close to the gates due to engine blast. Therefore, a large jet-engine–based system would need to use a conventional pushback tractor before the extra jet engine was started to provide powering for taxiing and onboard systems. For external systems, the attaching and detaching needs to occur in a manner similar to that for conventional push- back, but near the runway. Potential time savings for onboard systems (i.e., nose- or main-wheel electric motors) are estimated at around 1.75 minutes, which is the time needed to attach or detach aircraft pushback tractors for onboard systems. Start-up/Disconnection Area During conventional taxiing operations, the aircraft main engines are started at the gate, where the ground crew is present to observe any issues, and the aircraft taxi serves as the main engine warm-up period. The time needed to warm up most commercial aircraft main engines is con- sidered by Deonandan and Balakrishnan (2010) and Airbus (2013) to be up to approximately 5 minutes. When alternative aircraft-taxiing systems are used, the aircraft main engines are started away from the gate. Virgin Atlantic (2006) found that dispatch taxiing required either aircraft main engines to be started during taxiing or the addition of designated areas for engine start-ups to allow for aircraft main engine warm-up. For external systems, there is the possible need for a designated detaching area near the runway to allow the external system to be removed from the aircraft. The cost for additional start-up/disconnection areas is likely to be similar to the cost for concrete pavements, which are approximately U.S.$200 per square yard (U.S.$240 per square meter) (Pers. comm., 2015). Asphalt is about 20% cheaper than concrete (Pers. comm., 2015), although it is standard practice to use concrete for engine run-up areas for safety reasons. Costs may vary by geographical region. Additional Roadways Once external systems have taxied an aircraft to the runway, they will return to the gate area. Depending on the airport layout, additional roadways may be required to support this. Concrete roadways cost approximately U.S.$75 per square yard (U.S.$90 per square meter) (Pers. comm., 2015). However, costs may vary depending on geographical region. Asphalt is about 20% cheaper than concrete (Pers. comm., 2015). Pushback Tractors In most cases, the alternative aircraft-taxiing system negates the need for the use of a pushback tractor, with the possible exception of the replacement of the APU with a certified jet engine. Larger jet engines cannot be started close to the gates due to engine blast. Therefore, a large jet-engine–based system would need to use a conventional pushback tractor before the extra jet engine was started to provide powering for taxiing and onboard systems. For external systems, the attaching and detaching needs to occur in a manner similar to that for conventional push- back, but near the runway. Potential time savings for onboard systems (i.e., nose- or main-wheel

Detailed Description of Information in the ATAM 17 electric motors) are estimated at around 1.75 minutes, which is the time needed to attach or detach aircraft pushback tractors for onboard systems. Loading of Passengers Additional fuel, maintenance, installation downtime (onboard systems only), and staff costs are likely to be related to use by airlines and ground handlers. These costs are likely to be minimal and offset by other cost savings. Potential cost savings include the following: • Not needing to use pushback tractors [approximately U.S.$50 per cycle (Morrow et al., 2007)]. • Reduced delays and taxi times (at around U.S.$66 to U.S.$150 per minute per turnaround) due to, for example, eliminating the need to attach or detach aircraft pushback tractors for onboard systems (i.e., nose- or main-wheel electric motors). • Being able to use two gates for passenger loading rather than one gate, by having the aircraft taxi in parallel to the two gates (i.e., turning the aircraft around by 90 degrees), subject to the airport/gate layout. • Reduced fuel use. Other costs for airports are associated with additional taxi-queue management needed for systems with low speeds. For external systems, labor costs may increase since drivers will be needed to operate the equipment for longer than for conventional pushback [at a cost of around U.S.$29.60 per hour (Pers. comm., 2014)]. Engine Warm-Up The time needed to warm up the aircraft main engines is considered by Deonandan and Balakrishnan (2010) and Airbus (2013) to be up to approximately 5 minutes. Therefore, the use of alternative aircraft-taxiing systems at small airports with relatively short taxiing times of 5 minutes or less might not be beneficial. Foreign Object Debris (FOD) Damage Issues A reduction in FOD damage is cited by Airbus (2013) as one result of the use of alternative aircraft-taxiing systems. This is because the aircraft main engines are not in use during taxiing, and the likelihood of engines sucking in FOD is reduced.