s†z 58
BALANCE COMPUTER BALAÁICE COMPUTER OVERLAY
FWD AFT FIGURE 55. Balance computer and overlay.
location of the c.g. relative to its limits can easily be seen.
A diagram of the aircraft fuselage is normally included on the load adjuster. This diagram is used to help identify various compartment locations.
Balance computer. A slide rule can be circular as well as linear in construction-the aeronautical computer is an example of the circular type. Circular slide rules or computers have been adapted to the solution of weight and balance problems.
Essentially, these balance computers are circular load adjusters, and like the linear type, can be used only for an aircraft of one make, model, and configuration. The identification markings of a balance computer should be carefully checked to insure its correct use.
The circular balance computer consists of a card on which are printed curved scales for load items and a curved c.g. grid. A different card is provided for each configuration aircraft and can be included in the weight and balance section of the aircraft flight manual. A transparent plastic overlay sheet with a simple straight line similar to the cursor on the load adjuster is used on top of the balance computer card (fig. 55).
The procedure for use of the circular balance computer is similar to that used on the slide rule type load adjuster. Starting with basic index units, progressive movements of the overlay are made in clockwise or counterclockwise directions. A pencil mark is made on the overlay before making each adjustment for a load item. After the final fuel item adjustment is accounted for, the line on the plastic overlay indicates the c.g. location where it crosses 59
GRID
TEMPLATE FUEL
PASSENGERS
AR CARGO FWD FIGURE 56. Template and grid.
the gross weight line on the c.g. grid. The c.g. is indicated in % MAC on the grid and the grid itself is a limit envelope (fig. 55).
The design of the circular balance computer takes into account several general assumptions pertaining to the normal operation of the aircraft. Among these is the assumption that passengers will have a preference for empty seats next to the windows and that the fuel will be loaded and consumed according to standard schedules.
If standard practices are followed, the computer allows omission of the intermediate steps of calculating moments and indexes for the load items.
Template and grid. A similar system makes use of a large weight and balance grid upon which the weight items are directly plotted. The grid indicates the weight and balance limits of the aircraft as 60 expressed in % MAC. The grid can be printed on a hard plastic material from which the plot is erased after use or on a paper form which is kept as a record for each flight. The values for the load items are plotted progressively by the use of a plastic template (fig. 56). The final plot of the fuel load terminates at the intersection of the total weight and c.g. in % MAC.
As in all other systems, this system relies upon getting off to a good start with a reliable basic operating weight and basic operating c.g. in % MAC. Since the final plot represents the fuel load, the plotted fuel line can be followed down the chart as•fuel is consumed in flight to get a continuous graphic indication of c.g. location during flight (fig.
57).
Advanced systems. New systems of weight and
BALANCE LIMITATIONS CENTER OF GRAVITY - % MAC 44 40 36 32 28 24 20 16 12 MAXIMUM TAXI WEIGHT 182,000 180,000 MAXIMUM INFLIGHT WEIGHT 180,000 TAXI WT. AND C.G.
170,000 160,000 MAXIMUM LANDING AIRPLANE GROSS WEIGHT - LB. WEIGHT
• FUEL 150,000 AFT C.G. LIMIT TAKEOFF -FORWARD C.G
FLIGHT AND LANDING LIMIT INFLIGHT 140,000
MAXIMUM ZERO
FUEL WEIGHT
ZERO FUEL WT, 130,000
• PASSENGERS REAR
• FORWARD C.G. LIMIT
TAKEOFF AND LANDING
FORWARD 120,000
REAR - CARGO
FORWARD
- BASIC WT. AND C.G.
100,000 44 40 36 32 28 24 20 16 12 FIGURE 57. Balance limitations grid.
WEIGHT AND BALANCE MANUAL 5-10-61 Equipment List N-113.
Balance Chk Description and Identification Quantity Weight Arm Compressor-Air Fuel Starter 50.0 875.6 ABC Co. 4056728 Pressure Regulating Shutoff Valve 1 7.5 835.4 Jones Inc. 1726001-10 Jones Inc. 1726001- 9 (Optional) Boost Pump, Fuel, High Pressure 2 7.8 756.3 Smith Co. HD 309721-1 2 7.8 779.2 2 7.8 801.5 2 7.8 835.7 Boost Pump, Fuel, High Pressure 1 10.2 753.0 Lotco Inc. XX45678-9 FIGURE 58. Large aircraft equipment list.
balance control are being devised and will undoubtedly be adopted in the future by the large aircraft operators. Electronic computers can be used to calculate and print out the weight and balance answers; the computer input possibly being based upon passenger and cargo reservation information.
Other systems have been investigated utilizing devices which measure the weight applied to each landing gear when the airplane is on the ground.
These devices are installed in the landing gear system and produce a numerical read out of gross weight and c.g. position. The read out could be located in a convenient location in the flight com-
LARGE AIRCRAFT WEIGHT AND BALANCE
RECORDS It is apparent when a study is made of the weight and balance control systems introduced in this chapter, that the use of a valid Basic Operating Weight (BOW) is essential for accurate results. Operating personnel must rely upon BOW information in the aircraft records. Maintenance personnel must assure that the recorded BOW is correct.
BOW is defined as the weight of the aircraft as loaded ready for payload and fuel. The BOW, there- 62 fore, includes the empty aircraft with all permanently installed equipment, normal oil and fluids (except fuel), crew and crew baggage, passenger service equipment, emergency equipment, and special equipment.
The aircraft is weighed to establish the empty weight and c.g. before it is put into service. When a record is made of this weighing, an equipment list is established which shows each item of installed equipment included in the empty weight. From then on, the condition for empty weight can be duplicated by assuring that this same equipment is installed An excerpt from the equipment list of an FAA aircraft is shown in figure 58.
When equipment changes take place, an appropriate entry is made on a weight and balance log or the equipment list itself is amended so that the empty weight and c.g. are properly corrected. It is essential that any change in structure or equipment be entered in the records, otherwise all other calculations will be inaccurate. Part of a weight and balance log for an FAA aircraft is shown in figure The use of "empty fleet weights" and "basic operating fleet weights" are explained in FAA Advisory Circular 120-27. When feet weights are used,
ACTUAL WEIGHT AND BALANCE LOG Serial: 18066 Registration: N 113 Balance Index Item and Description Weight Arm Moment CG %MAC Units
SUMMARY Pre-flight Weighing 98,320 847.3 83355696 25•5 +3,783 Additions +978 +795556 Deductions -429 -338838 BASIC WEIGHT - 98,869 (847•7) 83812414 25.4 +3,803 FIGURE 59. Weight and balance log.
weight and balance procedures can be standardized and periodic weighings reduced; these advantages are possible when a large number of similar aircraft are used by a particular operator.
The term "zero fuel weight" indicates the maximum authorized weight of an aircraft without fuelthis weight is the sum of BOW and payload. It should be understood that passengers or cargo are included in zero fuel weight. When the zero fuel weight is high because of a large payload, the fuel load must be proportionately low to prevent exceeding maximum takeoff or landing weights. On the other hand, if a large fuel load is needed for a long-range flight or for low-altitude operation, IFR holding, or traffic delays, a reduction of payload and zero fuel weight may be required.
Average weights of adult passengers are assumed to be 160 pounds in summer (May 1-Oct. 31) and 165 pounds in winter (Nov. 1-Apr. 30). Children under 2 are considered "babes-in-arm" and children 2 through 12 are averaged at 80 pounds each. Passenger carry-on baggage is calculated at 5 pounds each. Most weight and balance control systems assume that passengers will select the window seats first, the aisle seats second, and the center of threeabreast seating last. With these assumptions, passenger index tables are calculated. Allowances are made for those loading situations where passengers happen to group in extreme forward or aft locations.
In flight, movement of passengers or crewmembers is also considered and load adjustments made. Any such allowances have the effect of reducing the load-carrying flexibility of the aircraft.
A typical passenger load index chart is shown in figure 60. It should be noted that this chart assumes that passengers are seated in the normal distribution around the center (centroid) of the forward and aft passenger compartments. A separate table in figure 60 provides information about cargo loads. The assumption is again made that the cargo is loaded evenly around the centroid of the cargo hold.
Fuel is loaded in large aircraft according to a schedule presented in the FAA approved manuals.
Such a procedure distributes the fuel among the fuel tanks in a manner which will cause a minimum of difficulty with weight and balance (fig. 61).
Personnel involved with weight and balance are, therefore, able to determine the fuel moments or index from standard tables or by calculating the moment of the fuel in each tank.
Fuel is used according to procedures given in the flight manual. Besides ensuring structural integ.
rity, an important reason for following a standard sequence of fuel tank selection is to control the c.g.
location. The flight crew has a much less complex 63
PASSENGER LOADING TABLE CARGO LOADING TABLE Number Weight Moment Moment 1000 1000 Pass. Lb. Forward Hold Aft Hola FWD. COMP. CENTROID 486. 3 Weight Lo. Arm 581 1066 Arm 5 850 413 10 1,700 827 15 2,550 1,240 20 3,400 1,653 25 4,250 2,067 29 4,930 2,397 AFT. COMP. CENTROID 928.8 5 850 789 10 1,700 1,579 15 2,550 2,368 20 3,400 3,158 25 4,250 3,947 30 5,100 4,736 58 35 5,950 5,526 40 6,800 6,315 45 7,650 7,105 50 8,500 7,894 54 9,180 8,526
FUEL LOADING TABLE Weight Tank 1 and 3 Tank 2 Weight . (3 Cell) Moment Moment 1000 Arm 1000 Lb.
7,583 817.5 6,949 8,500 8,037 817.2 7,355 9,000 8,492 817.0 7,762 9,500 8,947 816.8 8,168 10,000 9,402 816.6 8,574 10,500 9,857 816.5 9,32 11,000 10,313 816.3 11,500 10,770 816.1 9,793 12,000 18,500 906.8 16,776 815.1 15,079 18,500 19,000 907.8 17,248 815.0 15,485 19,000 19,500 908.9 17,724 814.9 15,891 19,500 20,000 910.1 18,202 814.9 16,298 20,000 20,500 911.7 18,690 814.8 16,703 20,500 21,000 913.4 19,181 814.7 17,109 21,000 21,500 915.5 19,683 814.6 17,514 21,500 FIGURE 60. Loading tables.
STANDARD FUEL LOADING Total Outboard Center Total Outboard Center Fuel Tanks Tank Fuel Tanks Tank Load 163 2 Load 163 2 29,550 9,850 9,850 40,000 11,600 16,800 30,000 10,000 10,000 40,500 17,300 30,600 10,200 10,200 41,000 17,800 31,050 10,350 10,350 41,500 18,300 31,500 10,500 10,500 42,000 18,800 32,100 10,700 10,700 42,500 19,300 32,550 10,850 10,850 43,000 19,800 33,000 11,000 11,000 43,500 20,300 33,600 11,200 11,200 44,000 20,800 34,050 11,350 11,350 44,500 21,300 34,500 11,500 11,500 45,000 21,800 35,000 11,600 11,800 45,500 22,300 35,500 12,300 46,000 22,800 36,000 .. 12,800 46,500 .. 23,300 36,500 13,300 47,000 23,800 37,000 13,800 47,500 24,300 37,500 14,300 48,000 .. 24,800 38,000 14,800 48,500 25,300 38,500 15,300 49,000 25,800 39,000 15,800 49,700 11,600 26,500 39,500 16,300 50,000 11,750 26,500 50,526 12,013 26,500 FIGURE 61. Fuel load distribution table.
job in determining in-flight c.g. travel if standard burn-out procedures are followed.
When the aircraft is loaded ready for takeoff, the total weight of the aircraft is called taxi gross weight or ramp gross weight. Because of high fuel consumption during ground operation, this weight may be 500 to 1,000 pounds more than takeoff weight.
Maximum takeoff weight is the maximum allowable just before brake release. This weight may be limited to a lesser weight for any particular takeoff by many factors related to aircraft performance.
The actual takeoff weight limit is calculated by the use of various performance charts, taking into account such factors as: Runway length, temperature, density altitude, runway slope, and wind conditions.
Maximum landing weight is a structural limit and is the maximum weight authorized at touchdown.
All weight in excess of maximum landing weight must consist of disposable fuel. A fuel dumping system is required on the aircraft if the maximum takeoff weight is more than 105 percent of maximum landing weight.
Conditions may arise during flight, such as engine failure or an aircraft system failure, which may require a landing sooner than originally planned In this event, fuel may have to be dumped to reduce weight to maximum landing weight. If the fuel dumping procedures contained in the aircraft flight manual are followed, c.g. travel will remain within prescribed limits. Calculations of the effects of fuel dumping are similar to those used for fuel burn.
Use of the fuel-load table moments makes the calculations simple. Some manuals will provide a fueltank centroid as a single arm, then the effect of fuel dumping can be calculated by a single remove weight formula such as the one which follows:
EXAMPLE:
STAB TRIM ENTER AT 25% MAC, PROCEED TO DIAGONAL LINE, READ RIGHT OR LEFT, 2 UNITS NOSE UP AIRPLANE NOSE DOWN -1- AIRPLANE NOSE DOWN LIMIT -1-1
PILOT'S TRIM SETTING UNITS - AFT
- MID 4 & FWD.
.......
• 10
AIRPLANE NOSE UP LIMIT'
AIRPLANE
NOSE UP 15 20 25 30
CENTER OF GRAVITY - PERCENT MAC FICURE 62. Stabilizer trim.
Weight before dumping. 160,000 lb. at Ve) ; or on the other hand, the nose may pitch
c.g. before dumping -Sta. 615.5 up uncontrollably depending on which direction the Fuel to dump 40,000 lb. stabilizer is out of trim. Once the aircraft is air- Centroid of fuel tanks -Sta. 750.2 borne, the trim is adjusted by the pilot to a setting Find c.g. after dumping. which will enable him to fly with minimum control Use remove weight formula: pressure. The only acceptable procedure for takeoff Wt. removed New total wt. - Dist. from c.g. to tank Ac.g.
120,000 - 484,7 Ac.g. = 44.9
c.g. after dumping = 615.5-44.9=570.6 in.
STABILIZER TRIM SETTING Before starting a takeoff in a large turbojet aircraft, the stabilizer trim must be placed in a position dictated by the c.g. location. An improperly set stabilizer trim may have such a powerful effect that it cannot be overcome by elevator control. During takeof, it may be difficult to raise the nose (rotate is to set the trim according to c.g. location before the takeoff roll is started.
Some of the c.g. control systems previously explained (see Convair 880M loading. summary, fig.
53) provide a stabilizer setting directly when the total weight c.g. is calculated. Other systems require the use of a simple chart to couvert from c.g. in % MAC to stabilizer setting for a particular takeoff weight. A typical chart from an aircraft flight manual is illustrated in figure 62.
• When the c.g. is determined to be located at a particular station, that location must first be converted to % MAC to use the stabilizer trim chart.
A typical problem follows:
Given: 2. Cargo:
Takeoff c.g.-Sta. 575.0 Fwd. ... Aft ... .. 3,000 .. 900........***** MAC -Sta. 540.0 to Sta. 745.0 3. Passengers:
Stabilizer trim chart (fig. 62) Fwd. 20 ..
Find: Stabilizer trim setting for takeoff Aft 40 (Winter wt. = 165 + Solution: 5 lb. carry-on baggage)
1. Determine c.g. in % MAC: 4. Fuel load:
c.g. % MAC= C.g. -LEMAC Tank 1 .20,000.
Tank 2 . ..20,000.
MAC = 575.0-540.0 5. Average fuel consumption to point of first intended Tank 3 .20,000
205.0 landing-3,600 lb./hr./engine. Three-engine op- = 205 35 = 17.1% MAC fuel management for entire flight. eration for 2:30 hours. Straight tank to engine
2. Determine stabilizer setting to nearest ½ 6. Limitations: a. Max. ramp wt.-165,000 lh.
unit: b. Max. zero fuel wt.-110,000 lb.
Enter chart at bottom on 17.1 % line. c. Max. landing wt.-135,000 lb.
Proceed upward to diagonal line. Read stabilizer setting = 5 nose up. d. c.g. limits-15% to 35% MAC e. MAC-Sta. 750.0 to Sta. 955.0 Find:
Ramp and landing weights and c.g LARGE AIRCRAFT LOADING PROBLEM Investigate if limitations are exceeded.
Stabilizer trim setting.
Given: An air carrier aircraft has the following load condition: Solution:
Weight Index moment/ 1,000 Use loading tables-figure 60.
1. Basic operating weight .85,000 65720.0 Use stabilizer trim setting chart-figure 62.
Complete the loading form below:
Takeoff conditions check Load item Weight Index Basic operating weight Payload:
Cargo:
Fwd.
Aft Passengers Fwd.
Aft Sub total (zero fuel weight) Fuel load:
1 Main 2 Main 3 Main Total (ramp and takeoff wt.)
Ramp c.g.-% MAC Stabilizer trim setting Landing conditions check Load item Weight Index Zero fuel weight Remaining fuel load:
1 Main 2 Main 3 Main Total (landing wt.)
Landing c.g. % MAC Notice that the fuel arms (fig. 60) vary with weight because the aircraft used in the problem is a swept-wing type. The correct procedure is to add a new fuel index to the ZFW index. Subtraction of index units for fuel burned will not properly account for the change in arm.
Using the principles explained in the preceding chapters, you should be able to solve the sample problem. As a spot check, here are several answers:
1. No limits are exceeded.
2. Stabilizer trim for takeoff is one-half nose up.
3. C.G. at landing weight is 22.2% MAC.
•US COVERNMENT PRINTING OFFICE: 1993.719. 204, 80005