McDonnell  Douglas

 FUEL ECONOMY

e-con-o-my - a......2. thrifty and efficient use of material resources: frugality in expenditures.

Fuel economy is the efficient utilization of fuel to obtain the maximum benefit from each gallon consumed.

Proper fuel management therefore is not fuel conservation at any cost.
Policy should be to operate at minimum cost.
 

LONG RANGE FUEL ECONOMY THINGS TO DO:

ü Observe planned speed profile.
ü Bracket the optimum altitude whenever possible.
ü Don’t descend too early or too late.
 

SOME “BASICS” OF LONG RANGE FUEL ECONOMY

· In terms of fuel burn at a fixed Mach, higher altitude is better. J  However, the same is not always true for fuel economy (most/pound). K
· In terms of speed, minimum cost flight plans include a planned speed which is determined by considering tradeoffs of fuel cost vs. time cost (to be detailed later). Variations from planned speeds may or may not result in using more fuel than planned, but will always result in higher cost than planned. L
· There is a price to pay (in terms of fuel burn) for initiating descent either too early or too late.
FLIGHT OPERATIONS CONSIDERATIONS

· Effect of center of Gravity (CG) on Specific Range.
· Cost of Carrying Surplus Fuel.
· Trimming the Aircraft.
· The buffet Boundary.
 

CENTER OF GRAVITY INFLUENCE ON DRAG RULES OF THUMB

MD-80:   7% shift in CG = 1% change in Specific Range.

Specific Range = Nautical Air Miles per Pound of Fuel (NAM/LBf)

Remember. . . In terms of fuel economy, after C.G. position is always better than forward C.G. position.
 

WHAT PRACTICES CAN BE USED TO INFLUENCE C.G. POSITION

· Passenger seating control.
· Selective baggage handling.
· Aircraft configuration alternatives
 Example:
 Removal of forward airstairs = 1,5% C.G. shift after and 250 lb. weight reduction.
· MD-80 Aux Tank Fuel Management Procedures.
 

THE COST OF CARRYING SURPLUS FUEL

Some operators utilize excess capacity to carry more fuel than required for a flight. Reasons for this include “tankering” of fuel due to relatively high fuel costs at destinations, and conservatisms put into reserve fuel policies.

Both of these are acceptable practices.

But remember . . .

IT COSTS EXTRA FUEL TO CARRY EXTRA FUEL!!
 

TRIMMING THE AIRCRAFT

TRUE FACT: The autopilot will hold an aircraft in straight and level flight with the Control Surfaces mistrimmed. This adds unnecessary drag!
 

TRIMMING TECHNIQUE

· Set thrust symmetrically with autopilot in altitude hold and speed stabilized.
· Disconnect autopilot and observe response. If aircraft does not hold altitude, trim the stabilizer to correct the mistrim.
· Manually fly the aircraft and hold wings level. Center the sideslip indicator (“ball”) using rudder trim.
· Trim the ailerons hands off as required to maintain wings level.
 

THE BUFFET BOUNDARY

Buffet Boundary: The operational envelope, established through flight test, identifying where a consistent variation in load factor of 0,2 g’s peak-to-peak exists.

· Douglas recommends maintaining a margin of 1.3 g’s to buffet onset.
· Some operators elect to reduce this margin by a nominal amount, such as to 1,25 g’s.
· MD-80’s are often prevented from achieving their optimum (most miles/pound) cruise altitude by the 1.3 g boundary.
· Adherence to the 1.3 limit is, in most countries, not required by regulation. However, it is a Douglas recommended limit because, for the purpose of flight planning, it ensures adequate margin from upset in all cruise situations.
 

MAINTENANCE CONSIDERATIONS

· Control surface rigging.
· Aircraft cleanliness.