Determining CG Changes Caused by Modifying the Cargo (Part Two)

Determining the Maximum Amount of Payload That Can Be Carried

The primary function of a transport or cargo aircraft is to carry payload, which is the portion of the useful load, passengers, or cargo that produces revenue. To determine the maximum amount of payload that can be carried, both the maximum limits for the aircraft and the trip limits imposed by the particular trip must be considered. In each of the following steps, the trip limit must be less than the maximum limit. If it is not, the maximum limit must be used.

 

These are the specifications for the aircraft in this example

Basic operating weight (BOW) 100,500 lb
Maximum zero fuel weight 138,000 lb
Maximum landing weight 142,000 lb
Maximum takeoff weight 184,200 lb
Fuel tank load 54,000 lb
Estimated fuel burn en route 40,000 lb
  1. Compute the maximum takeoff weight for this trip. This is the maximum landing weight plus the trip fuel. [Figure 9-30]

    Figure 9-30. Finding the maximum takeoff weight.

    Figure 9-30. Finding the maximum takeoff weight.

  2. The trip limit is lower than the maximum takeoff weight, so it is used to determine the zero fuel weight. [Figure 9-31]

    Figure 9-31. Determining zero fuel weight with lower trip limits.

    Figure 9-31. Determining zero fuel weight with lower trip limits.

  3. The trip limit is again lower than the maximum takeoff weight, so use it to compute the maximum payload for this trip. [Figure 9-32]

    Figure 9-32. Finding maximum payload with lower trip limits.

    Figure 9-32. Finding maximum payload with lower trip limits.

Under these conditions, 27,500 pounds of payload may be carried.

 

Determining the Landing Weight

It is important to know the landing weight of the aircraft in order to set up the landing parameters and to be certain the aircraft is able to land safely at the intended destination.

In this example of a four-engine turboprop airplane, determine the airplane weight at the end of 4.0 hours of cruise under these conditions:

Takeoff weight 140,000 lb
Pressure altitude during cruise 16,000 ft
Ambient temperature during cruise –32 °C
Fuel burned during descent and landing 1,350 lb
Figure 9-33. Standard atmosphere table.

Figure 9-33. Standard atmosphere table.

Figure 9-34. Gross weight table.

Figure 9-34. Gross weight table. [click image to enlarge]

Refer to the U.S. Standard Atmosphere Table in Figure 9-33 and the gross weight table in Figure 9-34 when completing the following steps:

  1. Use the U.S. Standard Atmosphere Table to determine the standard temperature for 16,000 feet (–16.7 °C).
  2. The ambient temperature is –32 °C, which is a deviation from standard of 15.3 °C. (–32° – (–16.7°) = –15.3°). It is below standard.
  3. In the gross weight table, follow the vertical line representing 140,000 pounds gross weight upward until it intersects the diagonal line for 16,000 feet pressure altitude.
  4. From this intersection, draw a horizontal line to the left to the temperature deviation index (0 °C deviation).
  5. Draw a diagonal line parallel to the dashed lines for Below Standard from the intersection of the horizontal line and the Temperature Deviation Index.
  6. Draw a vertical line upward from the 15.3 °C Temperature Deviation From Standard.
  7. Draw a horizontal line to the left from the intersection of the Below Standard diagonal and the 15.3 °C temperature deviation vertical line. This line crosses the fuel flow–100 pounds per hour per engine index at 11.35 and indicates that each of the four engines burns 1,135 (100 × 11.35) pounds of fuel per hour. The total fuel burn for the 4-hour cruise is shown in Figure 9-35.
Figure 9-35. Determining the total fuel burn for a 4-hour cruise.

Figure 9-35. Determining the total fuel burn for a 4-hour cruise.

The airplane gross weight was 140,000 pounds at takeoff with 18,160 pounds of fuel burned during cruise and 1,350 pounds burned during the approach and landing phase. This leaves a landing weight of 140,000 – (18,160 + 1,350) = 120,490 pounds.

 

Determining Fuel Dump Time in Minutes

Most large aircraft are approved for a greater weight for takeoff than for landing. To make it possible for them to return to landing soon after takeoff, a fuel jettison system is sometimes installed. It is important in an emergency situation that the flightcrew be able to dump enough fuel to lower the weight to its allowed landing weight. This is done by timing the dumping process.

In this example, the aircraft has two engines operating and these specifications apply

Cruise weight 171,000 lb
Maximum landing weight 142,500 lb
Time from start of dump to landing 19 minutes

Average fuel flow during

Dumping and descent 3,170 lb/hr/eng
Fuel dump rate 2,300 lb/minute

To calculate the fuel dump time in minutes:

  1. Determine the amount of weight the aircraft must lose to reach the maximum allowable landing weight. [Figure 9-36]

    Figure 9-36. Determining the amount of weight the aircraft must lose to reach the maximum allowable landing weight.

    Figure 9-36. Determining the amount of weight the aircraft must lose to reach the maximum allowable landing weight.

  2. Determine the amount of fuel burned from the beginning of the dump to touchdown. [Figure 9-37] For both engines, this is 52.83 × 2 = 105.66 lb/minute. The engines burn 105.66 lbs of fuel per min for 19 minutes (the duration of the dump), which calculates to 2007.54 pounds of fuel burned between the beginning of the dump and touchdown.

    Figure 9-37. Determining the amount of fuel burned from the beginning of the dump to touchdown.

    Figure 9-37. Determining the amount of fuel burned from the beginning of the dump to touchdown.

  3. Determine the amount of fuel needed to dump by subtracting the amount of fuel burned during the dump from the required weight reduction. [Figure 9-38]

    Figure 9-38. Determining the amount of fuel needed to dump.

    Figure 9-38. Determining the amount of fuel needed to dump.

  4. Determine the time needed to dump this amount of fuel by dividing the number of pounds of fuel to dump by the dump rate. [Figure 9-39]

    Figure 9-39. Determine the time needed to dump fuel.

    Figure 9-39. Determine the time needed to dump fuel.

 

Weight and Balance of Commuter Category Airplanes

The Beech 1900 is a typical commuter category airplane that can be configuredto carry passengers or cargo. Figure 9-40 shows the loading data of this type of airplane in the passenger configuration.

Figure 9-40. Loading data for passenger configuration.

Figure 9-40. Loading data for passenger configuration. [click image to enlarge]

Determining the Loaded Weight and CG

As this airplane is prepared for flight,a manifest is prepared.[Figure 9-41]

Figure 9-41. Determining the loaded weight and CG of a Beech 1900 in the passenger configuration.

Figure 9-41. Determining the loaded weight and CG of a Beech 1900 in the passenger configuration. [click image to enlarge]

  1. The crew weight and the weight of each passenger is entered into the manifest. The moment/100 for each occupant is determined by multiplying the weight by the arm and dividing by 100. This data is available in the AFM and is shown in the Weight and Moments— Occupants table. [Figure 9-42]

    Figure 9-42. Weight and moments—occupants.

    Figure 9-42. Weight and moments—occupants.

  2. The weight of the baggage in each compartment used is entered with its moment/100. This is determined in the Weights and Moments—Baggage table. [Figure 9-43]

    Figure 9-43. Weight and moments—baggage.

    Figure 9-43. Weight and moments—baggage.

  3. Determine the weight of the fuel. Jet A fuel has a nominal specific gravity at +15 °C of 0.812 and weighs 6.8 pounds per gallon, but at +25 °C, according to the Density Variation of Aviation Fuel Chart [Figure 9-44], it weighs 6.75 lb/gal. Using this chart, determine the weights and moment/100 for 390 gallons of Jet A fuel by interpolating between those for 6.7 lb/gal and 6.8 lb/gal. The 390 gallons of fuel at this temperature weighs 2,633 pounds, and its moment index is 7,866 lb-in/100.
    Figure 9-44. Density variation of aviation fuel.

    Figure 9-44. Density variation of aviation fuel. [click image to enlarge]

  4. Add all of the weights and all of the moment indexes. Divide the total moment index by the total weight, and multiply this by the reduction factor of 100. The total weight is 14,729 pounds; the total moment index is 43,139 lb-in/100. The CG is located at fuselage station 292.9. [Figure 9-45]
    Figure 9-45. Weights and moments—usable fuel.

    Figure 9-45. Weights and moments—usable fuel. [click image to enlarge]

  5. Check to determine that the CG is within limits for this weight. Refer to the Weight and Balance Diagram. [Figure 9-46] Draw a horizontal line across the envelope at 14,729 pounds of weight and a vertical line from the CG of 292.9 inches aft of the datum. These lines cross inside the envelope, verifying the CG is within limits for this weight.
    Figure 9-46. Weight and balance diagram.

    Figure 9-46. Weight and balance diagram. [click image to enlarge]