A Study on Pilots’ Behavior on Decision of Maneuvering Aircraft for Fuel Efficient Flight Operation

It is necessary to understand the situation of aircraft control decision related to fuel efficient operation by pilots. As well known in aviation industry, the flight operation needs to be conducted under strict safety regulations. The safety regulations are usually not compatible with fuel efficient flight operations. However, the airlines are very sensitive in fuel savings in flight operations because the cost of fuel is the one of major cost items in airline operation. Fluctuating fuel price and supply, ever increasing financial competition and emerging worldwide environmental consciousness have forced most airlines to develop some type of fuel management program for their operations. (IATA, 2011).

The response to climate change of airline industry, like the CORSIA has become additional driver for fuel savings in the industry. Min and Kim (2011) emphasizes that we need to introduce aggressive incentive for reducing fuel consumption and GHG reduction. Yoon et al (2019) assert that the total amount of emission all the airlines made in the last three years was 116% more than the emission allowance imposed by the central government resulting in 10.7 billion KRW additional emission expense. They also found that to reduce the emission, airlines are washing engines, using ultra-light ULD and carts in the cabin, increasing the use of flaps and preventing the use of APU.

The pilots in airlines are key personnels in fuel efficient flight operations. However, they are traditionally more aware for safety than efficiency in flight. Therefore, it is necessary certain kind of incentive for pilots to conduct fuel efficient operation. According to the survey through airlines pilot of this research, there are only 4.6% of airlines pilots, who recognize their airlines provide obvious incentive to the pilots who achieves fuel efficient flight operations.

ICAO recognizes the potential of fuel burn reduction in decisions made by flight crew in its Circular 303 AN/176 document. The flight crew can decide whether to opt for safety or efficiency.

Two areas for fuel efficiency improvements that involve flight crew decisions before flight are mentioned in Circular 303 AN/176. One possible decision that flight crew can take part in is the reduction of discretionary fuel.

The second area for potential improvement in efficiency by flight crew is in the decision of flight altitude and speed in accordance with the freezing point of fuel. In the take-off and climb stage, flight crew can decide whether to use full thrust or less thrust to get airborne and climb to cruise altitude. While derated and reduced take-offs can increase fuel consumption, it can significantly reduce engine wear and NOx emissions can also be reduced.

In the landing phase, flight crew can decide whether to reduce speed with reverse thrust and brakes or brakes only. According to Boeing, a 747 could save 65 kg to 70 kg of fuel by stopping using the brakes only.

IATA also provides guidance material to airlines for reducing fuel burn. In it’s Guidance Material and Best Practices for Fuel and Environmental Management, IATA emphasizes the importance of timely, accurate and accessible advanced flight planning data for flight crew. This is especially so for flight crews preparing for flight out of an airport other than the airline’s home base where access to data can be more limited. With less confidence in predicting flight conditions, flight crews can be motivated to make conservative decisions over decisions for improved efficiency.

IATA also notes the importance of minimizing fuel uplift for efficiency, but at the same time, emphasizes safety and risk management. Statistics show that the accurate flight planning systems used today allow contingency fuel to be usually left unused. The use of the flight management system (FMS) for pre-flight fuel calculations can also aid in more accurate prediction of fuel usage. During take-off, IATA recommends the use of reduced thrust and reduced flaps when possible. Reduced thrust take-off does not increase fuel efficiency, but lower engine temperature can significantly prolong the life of the engine. One percent reduction of thrust from full thrust at take off can result in ten percent increase of engine life as the most damage occurs at the highest few degrees. Reduced flap take-offs can result in significant fuel savings and improve second segment climb performance. Retracting flaps at lower altitudes can also reduce fuel burn as drag is reduced.

Constant monitoring and reporting of fuel burn during flight can provide higher levels of risk management and earlier decisions related to planned and estimated arrival times, flight profiles or unforeseen contingencies. Early estimation of arrival times can help in reducing holding time and plan more efficient flight paths and speeds.

During descent, the FMS will also manage an accurate and efficient descent profile unless irregular conditions arise. For accurate calculation of the descent profile by the FMS, wind information for the altitudes in the profile can be inserted. If this is not done, the FMS will assume wind profile based on constant decreasing wind level from winds at cruise level. Descent altitude and speed, or the energy of the aircraft, must be kept ideal to avoid alternating use of speed brakes and thrust.

Where conditions permit, implementation of constant descent operations (CDO) by airports can lead to significant fuel savings. CDO allow aircraft to continuously descend to the final approach fix on idle power and low drag without thrust or brake interruptions. CDO not only allow savings in fuel, but also on flight time and lowers noise levels while reducing workload of controllers and flight crew. Best in class airlines may utilize CDO wherever possible.

When landing, reduced flap landings can decrease fuel burn when the aircraft type and the runway length permit it. Reduced flap landings can reduce fuel burn, decrease noise and chemical emissions and improve safety margins in case a go-around is required.

Engine-out taxi-in procedure as a standard operational procedure (SOP) can reduce engine use and fuel consumption where conditions permit. Well designed engine-out taxi-in SOPs can encourage flight crew to use the process with minimal additional workload. While engineout taxi-in can save fuel and brake wear, ground traffic conditions, taxi-in time, parking locations, local airport and airline regulations must be adequate.

Fuel efficiency schemes however, must be implemented with caution. Although flights generally carry fuel for contingencies, aggressive efficiency measures can lead to adverse effects of causing excessive inefficiencies and sometime lead to serious safety hazards. A report by Spanish Civil Aviation Accident and Incident Investigation Commission regarding an emergency landing due to low fuel on a Ryanair flight, states that Ryanair’s fuel policy was found to be based specifically on minimizing fuel uplift for maximum efficiency. The investigation discovered that Ryanair aircraft frequently land with minimum landing fuel. The report states that if several aircraft with minimum fuel on board are forced to arrive at one airport, this could lead to a chain reaction of several simultaneous emergency landings due to insufficient fuel on board. Emergency landings are granted priority at the cost of delays to aircraft with fuel left over the final reserve level. Long holdings above the alternate airport can force these aircraft to consume additional fuel and possibly dip into final reserve fuel and in turn, declare another emergency (Civil Aviation Accident and Incident Investigation Commission (CIAIAC), 2010).

A structured technique introduced by Thomas Saaty (1970), named the Analytic Hierarchy Process (AHP), is an effective tool to handle complex decisions. it is based on maths and psychology and potentially allows the decision maker to make the best decision by allowing for better priorities. AHP also has a unique usage in group situations for multiple situations. It essentially assists in understanding of the problem and allows the decision makers to find a solution that is best suited for them. The AHP will provide an in-depth and a logical framework for organizing a problem, which allows to quantify its elements and to relate those elements to the end-goal, as well as to evaluate alternative solutions. By reducing complex decisions to a series of pairwise comparisons, and synthesizing the results, AHP can capture both qualitative and quantitative aspects of the decision. Finally the AHP also includes a useful techniques to ensure decision makers’ consistency in assessments to reduce process biases.

We tried AHP analysis with four flight phases and four factors of flight control decision for AHP hierarchy structure. The Fig. 1 is the hierarchial structure of the analysis. The AHP questionnaire tries to find the relative importance of each phase of flight operation, in terms of pilots’ decision power. The second level is to decide the importance of each factor for their flight decision during each of flight phase.

Fig. 1. Structure of control decision for pilots’ flight operation

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The data was collected by the survey through sample pilots. The respondents sampled from the airline pilots in South Korea. Following Table 1. is to show the characteristics of the sample pilots of the survey.

In the AHP analysis, the Consistency Index (CI) of the respondents is considered to be reliable when it is less than 0.10. As shown in Table 2, the level 1. CI value is 0.18, indicating low reliability. Level 2 CI values corresponding to Climb, Cruise, Descent, Approach, Land, and Engine Shut-down of Level 1 were slightly larger, indicating lower statistical effectiveness.

Respondents at Level 1 found that Pre Flight Planning & Briefing was the most important factor for fuel saving during the flight phase. At Level 2, all respondents were found to have an impact on aircraft operation. However, the reliability of the value is too low to insist the results.

We experienced serious difficulties in getting consistent responses from sample pilots from the AHP questions formulated based on the hierarchy of the figure above. Because the questions which ask the importance of each phase or factor seemed vague to respondents. Especially, for the second level, the factor, “Safety” is so predominant that other factors are almost disregarded.

Because the factor of ‘Safety’ is predominant to all other factors, the respondents might ignore other factors in AHP response, and the result of analysis has become inconsistant. So, we revised the hierarchy of the study based on the insight attained through the initial survey and additional discussions with airline pilots and managers in flight operation departments of commercial airlines.

Changes in expressions:

For the second level we used specific flight decisions for each phase of flight operation based on pilots’ interviews conducted for this study.

Fig. 2. Refined structure of control decision for fuel saving

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Survey on 24 national FSC and LCC pilots to discover in which phase efficiency improvements can be made.

Consistency Ratio (CR), representing the consistency for each level was smaller than 0.1 and the results of the AHP was considered acceptable.

Responses considered the cruise phase (0.337) to have highest potential for efficiency improvements. That is because the cruise is the longest flight phase, and there would be more chances to achieve fuel efficiency.

On the other hand, ground operation, with a weight of 0.092, was the phase considered to have the least potential for efficiency improvements. Because the portion of consumed fuel at this phase is very small, they recognize there would be least chance of fuel savings.

In the second level, the ‘Direct Route’ factor in the Cruise Phase was the most important according to responses. In general, pilots may request a direct route after making a detour from bad weather area, but they do not request it to their destination on the way from a planned air route for fuel savings.

The second most important factor is ‘Power Idle Decent’ and the third is ‘Climb Profile by FMCS’. In order to do ‘Power Idle Decent’, it requires descending from the point indicated by the FMCS, but it is often difficult to do due to air traffic conditions. The procedure of ‘Climb Profile by FMCS’ is also not easy to apply due to heavy traffic, airspace separation, and military activities.