The purpose of this Flight Operations section is to define the optimum flight profile in terms of fuel used and to present the penalties encountered when deviations are made from the optimum conditions. The information contained in this section is basic to conduct of a trade-off study to arrive at a minimum cost operation, or to enable management to quantify fuel cost if a different operating formula should be adopted. This section, therefore, is written not as an operating manual, but as a guide for management to establish operating procedures.

Out on the line, air traffic control, weather, or other operational considerations may force a deviation from optimum performance. Knowledge of the “less than optimum” effects on fuel consumption will provide the flight crew with means to select the next most fuel-economic set of conditions. Fuel savings during ground operation can also be realized by procedural efficiencies.

GENERAL CONSIDERATIONS.
EXCESS WEIGHT. Substantial fuel savings are realized through careful planning by minimizing tankering reserve fuel or reducing basic aircraft weight. A flight dispatched with 5.000 pounds of surplus fuel (or excess weight) flying at FL350, long range cruise, will consume 4,2% (210 pounds) of the surplus fuel for a 300 nm flight, or 9,2% (462 pounds) of the surplus fuel for an 800 nm flight.

CENTER OF GRAVITY. In cruise, the normal CG movement has minimal effect on fuel flow. However, the effect between the forward and aft limits can be as much as 5,6% for the 300 nm mission and 7,3% for the 800 nm mission.
At an aft CG, the amount of aerodynamic force required on the horizontal stabilizer to maintain proper longitudinal trim is reduced. Consequently, the required lift provided by the wing is reduced and the airplane flies at a lower angle of attack. Accordingly, the stabilizer setting will be near zero and drag at a minimum.

Aircraft loaded at full forward vs full aft CG limits results in following increase in fuel cost.:
 

300 nm flt $21.17/flt or $63.510/year/aircraft
800 nm flt $64.83/flt or $77.796/year/aircraft

AERODYNAMIC CLEANNESS. A preflight of the airplane should obviously include close scrutiny to confirm that all of the access panels, blow-out doors, service panel doors, slat seals, etc. are in place and correctly faired.

FLIGHT PLANNING. Modern airborne/ground navigation systems allow greater flexibility in route selection during preflight planning and while en route. When it is feasible to do so, an effort to shorten the route distance may result in minutes saved and dollars earned. The value of a minute is surprisingly high:

One minute of cruise flight time saved per flight saves:

300 nm - $7,23/flight or $21.690/year/aircraft.
800 nm - $7,40/flight or $8.877/year/aircraft.

USE OF AUTOMATIC FLIGHT CONTROL SYSTEMS. The professional pilot sincerely believes he is able to coax a little more performance from the airplane than the autopilot can. This may be true over the short term (short first sector) but human fatigue usually degrades performance more than automatic pilot tolerances over the long haul. For example. during a 20 minute climb to cruise altitude, the autopilot can maintain the desired airspeed within + 5 knots or + 0,015 Mach. While crew attention is temporarily diverted by ATC, departure procedures, or other cockpit duties, the automatic systems are continuously at work monitoring and correcting for changing conditions. Although deviations of + knots during climb will not result in significant fuel consumed increments, the goal of consistent optimum performance argues for maximum use of the automatic flight control system. Specific penalties will be discussed, where appropriate, by phase of flight.

GROUND OPERATION. APU OPERATION. The APU consumes 270-300 pounds per hour during ground operation. For flight crew consideration, the following provides the dual benefits of fuel savings and longer APU life: delaying start of the APU prior to departure when the airplane is powered by a ground power unit and shutting the APU down as soon as possible after engine start when no longer needed.

Each extra minute of APU operation each flight at 300 pounds per hour F/F costs:
1.200 flights - $448/year
3.000 flights - $1.119/year.

ENGINE START. The clearance to start engines is generally governed by traffic conditions. However, crew awareness of potential fuel savings may minimize early start times and long engine idle times through  close communication with ATC and consideration of existing traffic conditions. A typical fuel flow during ground idle for two engines is 46 pounds per minute.

Each extra minute of running two engines at ground idle each flight costs:
1.200 flights - $4.120.
3.000 flights - $10.299.

TAKEOFF. GENERAL. During takeoff, the aircraft should be “cleaned up” as soon as possible consistent with other operational factors. The drag reduction by the early retraction of flaps, slats and the landing gear will increase fuel economy.

DERATED TAKEOFF. Even though use of derated thrust results in small fuel penalties, long term benefits may be realized by the increase in engine life and the resulting lower rate at which specific fuel consumption deteriorates.

PENALTIES FOR NOT USING OPTIMUM SPEED. The penalty for flying faster than the theorical optimum is:

 
Mach nº lb/nm % Change Cruise fuel  $ cost/acft/yr Increased $ cost/acft/yr
   300nm 800nm 300nm 800nm
0,75 12,90 0 54.873 572.991 0 0
0,76 13,16 2,0 55.979 584.539 1.106 11.548
0,79 14,28 10,7 60.743 634.287 5.870 61.296
0,80 15,38 19,2 65.422 683.147 10.549 110.156

 The optimum cruise speed for maximum range is at the peak of a typical speed versus specific range plot. Operating on the peak of the curve, however is not practical because constant thrust adjustments are necessary due to reduced speed stability at the peak of the curve. If atmospheric disturbances should occur at this speed, the airplane would be difficult to keep in equilibrium. Therefore, to achieve an optimum balance between fuel mileage and speed/thrust ability, a speed slightly greater than the optimum range called long range cruise has been adopted by the industry.

CRUISE ALTITUDE COMPENSATION FOR HEADWINDS AND TAILWINDS. When operationally possible, the cruise altitude should be modified to achieve fuel savings as a function of wind component. Cruising at M.76 at 35000 feet is used as an example:
 
ALTITUDE BELOW OPTIMUM WIND INCREMENT REQUIRED
1.000 3   knots
2.000 7   knots
4.000 20 knots
8.000 47 knots

 For instance, flying the aircraft 4.000 feet below the optimum altitude requires a 20 knot increase in tailwind (or a decrease in headwind ) to provide the same performance. A wind advantage greater than 20 knots would provide fuel savings at this lower altitude.
These fuel savings become measurable when the wind advantage is 20 knots or greater. A 20 knot wind advantage under the cruise conditions stated over a one hour cruise distance will equate to $21 savings in fuel.
Attempting to fly too high above the optimum cruise altitude in order to take advantage of winds is not practical because the airplane will be exceeding aircraft pressurization limits.

THRUST SETTING TECHNIQUE. Improper level off and thrust setting in cruise will result in excessive fuel consumption. As the cruise altitude is attained, it is important that the aircraft speed not be permitted to drop below the target cruise speed. Regaining the target speed requires considerable thrust increase to overcome the drag resulting from the higher angle of attack at the lower speed and thus higher fuel flow.

As a general rule, accelerate to the cruise speed while maintaining climb thrust, then set the EPR values obtained from the cruise thrust tables when at or above the cruise speed. The speed should be allowed to stabilize (but not drop below target cruise speed) and incremental thrust adjustments made to arrive at the final thrust setting for the desired cruise speed. Assuming stable air conditions, subsequent thrust adjustments will be required approximately every 20 minutes to maintain the target speed.

The situation to avoid is a cyclic decrease, then increase (or increase then decrease) of speed which occurs when the initial and subsequent thrust settings are incorrect. Speed excursions that fall below optimum result in rapidly increasing drag and a requirement for a large thrust increase to correct. Speed excursions that drift above optimum will result in increased fuel consumption as previously discussed under penalties for not cruising at optimum speed.

MISTRIM. Estimates run as high as 2.200 USD per year in wasted fuel for less than a half degree of aileron mistrim alone. The cost of one degree rudder mistrim is 10.200 USD per year. A mistrimmed rudder can cause a mistrimmed aileron and viceversa compounding the problem.

The following technique should be used for trimming the aircraft in cruise: ensure that thrust is set symmetrically with the autopilot in altitude hold and the speed stabilized and disconnect the autopilot and observe aircraft response. If the aircraft should slowly ascend or descent, trim the stabilizer to correct the mistrim. Next fly the aircraft manually wings level and observe the position of the sideslip indicator (ball). center the “ball” if it should be displaced, with the rudder trim while continuing to hold the aircraft wings level. After this is accomplished, trim the ailerons hands off as required to maintain a wings level attitude (zero bank angle). Check the correctness of the trim by observing that bank angle or heading changes do not occur. Repeat the procedure to refine the trim point if necessary.

HOLDING. Holding maneuvers should be conducted at the highest possible altitude in the clean configuration with 1,5 Vs minimum speed for reduced fuel consumption. This configuration provides approximately a 11 percent decrease in fuel consumption as compared to the slats extended configuration. This is equivalent to a 430 to 520 pound fuel savings per hour.

DESCENT. The most important factor in the reduction of descent fuel is the use of idle thrust at a fixed speed. In comparison, the use of power to maintain a fixed descent rate at a fixed speed is much more costly in fuel consumption.

A fuel penalty is incurred for early initiation of the descent. For example: should the descent be started too soon so that the distance had to be recovered at 10.000 feet and 250 kias, the fuel penalty for a 10 nautical mile early descent would be 80 pounds.

The same “fuel conservation” awareness and application of fuel economy measures taken during ground and inflight operations should be extended to the approach and landing. The approach intercept maneuver and point at which the landing configuration is established should be optimized. When it is possible, the approach intercept distances should be minimized and high drag configurations delayed as long as operational feasible.

USE OF AUTOMATICS. The automatic pilot and throttles should be used as much as possible. Performance and operation of the automatic systems over a period of time may contribute to fuel savings.

SUMMARY. Awareness of fuel conservation principles and diligent application of fuel conservation procedures and techniques are the keys to reducing the fuel used. each effort toward this goal by the flight crews, no matter how small, will pay off in dollars saved and vital resources conserved.

In the total operational picture, flying at optimum fuel flight profiles may not be practical as other economic considerations may be overriding. These savings are not necessarily cumulative but do indicate their relative importance as well as their potential effect on fuel economy.

COST OF ELECTRICITY FOR GROUND SUPPORT. Most of fuel required for aircraft ground activities is used for supporting aircraft preflight, postflight, loading, servicing, cleaning and repairing as well as for lighting, heating and cooling. Most of the energy consumed is in the form of electricity, except when pneumatic power is required for cabin air conditioning (heating or cooling).

Electricity may be purchased from a utility company or generated by ground power units (GPUs), aircraft engine generators or aircraft auxiliary power units. The cost differences between these sources are significant and costs are often inversely proportional to convenience.
Use of the aircraft engine or the APU is often an extremely costly convenience. For example, when operated solely to generate electricity, the APU runs less than 3 minutes on a gallon of fuel, whereas some GPUs will run for more than one-half hour on the same amount.
It should also be noted that if the APU is not being used for pneumatic power, the cost for electrical power is quite high.

POTENTIAL SAVINGS. The Douglas Aircraft Company conducted a study to determine potential savings through improved ground support power management. This survey revealed that aircraft were electrified on the ground approximately 12 hours each day. Monitoring of aircraft at specific ramp and maintenance facility locations showed several instances where power could have been turned off between scheduled servicing and maintenance operations.

Conclusions based upon the survey were:
· Opportunities for implementing savings vary with operators, schedules, type of ground facilities available, geographical locations, weather and other factors.
· Savings of approximately 20% can be achieved by most operators through training and motivation.
· Savings of an additional 10% can be achieved by some operators through revision of ground servicing and maintenance schedules (compression).
· Savings of up to 25% can be achieved by some operators through capital expenditure for more efficient GPUs and installation of commercial power outlets at ramp, dock and hangar locations.