Before any flight, the pilot should determine the weight and balance
condition of the aircraft. In the early days of flying, aircraft were loaded
by guess and intuition. On occasion, the results were grim. Through trial
and error the early pilots learned about weight and balance. Today there
is no excuse for following this method. Simple and orderly procedures based
on sound principles have been devised by aircraft manufacturers for the
determination of loading conditions. The pilot, however, must use these
procedures and exercise good judgment. In many modern aircraft, it is not
possible to fill all seats, baggage compartments, and fuel tanks and still
remain within the approved weight and balance limits. If the maximum passenger
load is carried, the pilot must often reduce the fuel load or reduce the
|USEFUL LOAD CHECK
A simple and fundamental weight check should always be made by general aviation pilots before flight. This check should determine if the useful load is exceeded. The check may be a mental calculation if the pilot is familiar with the aircraft's limits and knows that unusually heavy loads are not aboard. But when all seats are being occupied, fuel tanks are full, and some baggage is aboard, the pilot should do some careful calculations.
The pilot needs to know the useful load limit of the particular aircraft. This information may be found in the latest weight and balance report, in a log book, or on a major repair and alteration form, located in the aircraft. If useful load is not stated directly, simply subtract empty weight from maximum takeoff weight. Be especially weight conscious of aircraft which have a limited useful load because they are the ones which cause weight and balance troubles.
IT IS THE RESPONSIBILITY OF THE OWNER AND PILOT TO ENSURE THAT THE AIRPLANE
IS PROPERLY LOADED. THE DATA ABOVE INDICATES THE EMPTY WEIGHT, C.G., AND
USEFUL LOAD WHEN THE AIRPLANE WAS RELEASED FROM THE FACTORY. REFER TO THE
LATEST WEIGHT AND BALANCE RECORD WHEN ALTERATIONS HAVE BEEN MADE.
The check is simple enough - just be sure to include all the load items
included in the useful load - then check the total against the limit. The
calculations might look like this:
The calculations indicate that the useful load is not exceeded and the flight can take place.
Now suppose that Mr. Jones, in the example, is replaced by a new instructor who weighs 210 lb. A useful load check will show that the aircraft is too heavy. The pilot in our example must reduce the load to the specified useful load limit. There is no alternative in this small aircraft but to reduce the fuel load, even if all the baggage has been removed.
Pilots should be aware of, and on the alert for, unusual loadings. They should remember that the manufacturer's initial weight and balance calculations and some examples in the owner's manual make the assumption that the pilot and passengers weigh a standard 170 lb. each. Heavyweight passengers can overload a small aircraft seriously. A student and instructor may easily weigh 220 lb. each in winter clothing; this represents a potential overload of 100 lb. The baggage compartment is another place where pilot vigilance should be directed - the maximum compartment load placard must be obeyed. Frequently, a restriction is placed on rear seat occupancy with the maximum baggage aboard.
WEIGHT AND BALANCE RESTRICTIONS
Be sure to follow your aircraft's weight and balance restrictions. The
loading conditions and empty weight of your particular aircraft (fig.
31) may differ from those in the owner's manual due to modifications
or equipment changes. Sample loading problems in the owner's manual are
intended for guidance only; each aircraft must be treated separately for
weight and balance. The pilot should understand that although the aircraft
is certified for a specified maximum gross weight, it will not safely take
off with this load under all conditions. Conditions which affect takeoff
and climb performance such as high elevations, high temperatures, and high
humidity (high density altitudes), may require operation at reduced weight.
Other factors to consider are runway length, runway surface, runway slope,
surface wind, and the presence of obstacles. Pilot experience and proficiency
should always be considered - if in doubt, reduce the load.
|Some small aircraft are designed so that it is not possible to load them in a condition which will place the c.g. outside the fore or aft limits if standard load schedules are observed. These aircraft have the seats, fuel, and baggage accommodations located very near the c.g. limits. They also have special empty weight c.g. limits listed in their specifications. Loads can be added to or removed from any location within the c.g. range with complete freedom from concern about c.g. movement. Such action cannot cause the c.g. to move beyond the c.g. limits of these aircraft (see fig. 32), but maximum weight limits can still be exceeded.|
Most aircraft, however, can be loaded in a manner which will place the
c.g. beyond limits. Even though the useful load is not exceeded, an out-of-balance
condition is serious from a stability and control standpoint. The pilot
can quickly determine if the load is within limits, if the aircraft is
simple enough to make use of a loading schedule. This schedule may be found
in the weight and balance report, the aircraft log book, the owner's manual,
or may be posted in the form of a placard. A typical placard may appear
similar to the one shown in figure 33.
The loading schedule should be treated as a suggested loading plan only. The pilot should make a check by means of weight and balance calculations to see if limitations are not being exceeded. The assumption in the use of the loading schedule is that each passenger weighs approximately the standard weight of 170 lb. It is obvious that passenger weights could vary widely from the assumed standard.
AIRPLANE FLIGHT MANUAL
Each airplane of over 6,000 lbs. maximum weight is furnished with an airplane flight manual. An airplane of less than 6,000 lbs. may have information furnished in the form of placards, markings, or manuals. When an airplane flight manual is furnished, the following is included:
a. Limitations and data:
(1) The maximum weight.
(2) The empty weight and c.g. location.
(3) The useful load.
(4) The composition of the useful load, including the total weight of fuel and oil with full tanks.
b. Load distribution:
The established c.g. limits are furnished in the airplane flight manual. If the available loading space is adequately placarded or arranged so that no reasonable distribution of the useful load will result in a c.g. outside of the stated limits, the airplane flight manual may not include any information other than the statement of c.g. limits. In other cases, the manual includes enough information to indicate loading combinations that will keep the c.g. within established limits.
LIGHT SINGLE ENGINE AIRCRAFT LOADING PROBLEMS
|Aircraft manufacturers use one of several available systems
to provide the aircraft loading information. The following weight and balance
problems will show how the pilot can determine if the maximum weight limit
is exceeded or the c.g. is located beyond limits.
Assume you are a pilot planning a flight in a light single engine, four place aircraft. Your load consists of yourself, one front seat passenger and two rear seat passengers, full fuel and oil, and 60 lb. of baggage (fig. 34). Here is how the critical weight and balance problems are solved for this case by two different methods (examples 23 and 24).
Solution by index table:
1. From the manual or weight and balance report, determine the empty
weight and empty weight c.g. (arm) of the aircraft.
2. Determine the arms for all useful load items.
3. Determine the maximum weight and c.g. range. (For this case - Max. TOGW = 2,400 lb., c.g. range - Sta. 35.6 to 45.8.)
4. Calculate the actual weights for the useful load items.
5. Construct a table as follows, and enter the appropriate values. Multiply each individual weight and arm to obtain moments.
NOTE - Observe that the oil tank for this aircraft is located forward of the datum. Care must be taken to subtract the negative oil moment when totaling the moment column.
6. Adding the weights produces a total of 2,276 lb., and adding the moments produces a total of 101,178 lb-in. The c.g. is calculated by dividing the total moment by the total weight:
------- = 44.5 in aft of datum
7. The total weight of 2,276 lb. does not exceed the maximum weight
of 2,400 lb., and the computed c.g. of 44.5 falls within the allowable
c.g. range of 35.6 to 45.8 in. aft of datum.
|Weight and balance computations are greatly simplified by two graphic aids - the loading graph and the center of gravity moment envelope. The loading graph (fig. 35) is typical of those found in general aviation aircraft owner's manuals. This graph, in effect, multiplies weight by arm giving moment, then divides the moment by a reduction factor, giving an index number. Weight values appear along the left side of the graph. The moment/1,000 or index numbers are along the bottom. In this example, each line representing a load item is labeled. To determine the moment of any load item, find the weight along the left margin, then project a line right to a point of intersection with the appropriate load item line. For example, the index number of a pilot weighing 170 lb. is 6.1.|
|The c.g. moment envelope (fig. 36) allows the pilot to
bypass the computation of a c.g. number. It gives an acceptable range of
index numbers for any aircraft weight from minimum to maximum.
If the lines from total weight and total moment intersect within the envelope, the aircraft is within weight and balance limits. In solving the sample problem, follow this procedure:
1. Determine the aircraft empty weight and the empty weight index from the weight and balance report.
2. Construct a table such as the one
3. From the loading graph, fig. 35, determine the index number (moment/1,000) of each useful load weight item and enter it in the table.
4. Add the weight and moment columns and write in the totals.
5. Refer to the c.g. moment envelope, fig. 36, and find the point of intersection of a line projected right from total weight (2,276 lbs.) and of a line projected up from total moment/1,000 (101.2).
6. The point of intersection falls within the envelope, therefore, the
weight and c.g. are within limits.
LIGHT TWIN ENGINE AIRCRAFT
Modern light twin engine aircraft are larger than most single engine aircraft; accordingly, their useful load is almost always greater than that found in the smaller aircraft. In these aircraft, it is possible to have many different loading combinations. Their large baggage compartments may be full or empty, and there may be wide variations in the number of seats being occupied. These variations are to be expected and are normal for the types of operations for which the aircraft are used. However, the c.g. is bound to range backward and forward as the loads are varied; therefore, weight and balance control is essential.
If a variety of loads can be placed aboard an aircraft in a number of locations, the pilot must be especially aware of duties regarding weight and balance control. Pilots should use a reliable weight and balance system, preferably the type recommended by the manufacturer, to assure that the weight and balance is within limits for each flight. They should insist that passengers are assigned to the correct seat from a weight distribution standpoint. They should also be sure that passenger baggage or miscellaneous cargo is properly loaded.
The weight and balance systems used on light twin engine aircraft are essentially the same as those used for single engine aircraft. Weights, arms, and moments are the basic factors, and the final c.g. computation must fall within the allowable c.g. limits. Many twin engine aircraft make use of the loading graph and moment envelope system (figs. 35 and 36). Other models make use of index tables similar to those explained earlier for single engine aircraft (fig. 24).
Some light twin engine aircraft have weight and balance control systems which make use of a special weight and balance plotter. The typical plotter is made of plastic material similar to an aeronautical computer. It consists of several movable parts which can be adjusted over a plotting board on which is printed a c.g. envelope. The reverse side of the typical plotter contains general loading recommendations for the particular aircraft. The recommendations may suggest that occupants be loaded progressively from front to rear. In other words, the forward and center seats should be occupied before passengers are assigned to the rear seats. A pencil line plot can be made directly on the envelope imprinted on the working side of the plotting board. This plot can be erased and recalculated anew for each flight. The plotter is to be used only for the aircraft for which it was designed. This weight and balance control system is very similar to one used on air carrier aircraft as illustrated by figure 57.
A typical weight and balance plotter should contain this reminder: "It is the responsibility of the owner and pilot to ascertain that the aircraft always remains within the allowable weight versus c.g. envelope while in flight." This note should serve as a precaution to the pilot to be sure to check the weight and balance condition before takeoff and to be sure that any shift in passenger seating locations does not adversely affect the location of the aircraft center of gravity.
HIGH DENSITY SEATING AIRCRAFT
Many light twin engine aircraft are being used for transportation of passengers, cargo, or mail in the form of commuter or air taxi service to supplement the scheduled and unscheduled air carriers. An increasing number of twin engine aircraft are being used to carry mail on a scheduled basis. Many commuter and air taxi operators carry passengers to and from small cities to make connections with trunk carriers at airports in large cities. The aircraft used for this purpose are in some cases fitted with a large number of seats in relation to fuselage size and are called high density seating aircraft. The aircraft may contain seats for eight to 15 passengers and some of the larger types may seat over 25 passengers. The loading problems are relatively more complex than for aircraft which carry only six passengers. The complexity of the loading situation approaches that encountered in air carrier operations. Weight and balance limits for high density seating aircraft must be respected. The passenger, cargo, or mail load on high density seating aircraft may vary considerably from flight to flight. Some trips may be made with a full load and others with a minimum load. Some of the high density seating aircraft have special weight and balance problems because they have been modified and modernized from older aircraft which originally did not have a great number of seats. Some of these modified aircraft are very sensitive as far as loading toward the rear limit is concerned. The recommended weight and balance checking procedures for modified aircraft must be carefully followed and operators should be sure to make a thorough analysis of weight and balance records to assure currency.
An operator's manual, when required for high density seating aircraft, should contain procedures for assuring compliance with weight and balance limits, including periodic reweighing of the aircraft. The weight and balance procedures contained in the manual should:
1. Be based on sound principles, using standardized terminology, and be compatible with the type(s) of aircraft operated.
2. When followed, assure that the aircraft is properly loaded and will not exceed authorized weight and balance limitations during operation.
3. Provide for blocking off seats or compartments when necessary to remain within c.g. limits. Effective means should be provided to assure that those seats and compartments are not occupied during operations specified.
4. Provide crewmembers, cargo handlers, and other personnel concerned
complete information regarding distribution of passengers, fuel, and other
items, and should give complete information regarding the distribution
and security of cargo to prevent the shifting of weight in flight.
5. Provide other information relative to maximum weights, capacities, and other pertinent limitations affecting the weight and balance of the aircraft.
Pilots of these high density seating aircraft must be aware of the effect of passenger and cargo location on c.g., and they must have a personal knowledge of the means of correcting an out-of-limits condition. They often have no one to assist them with loading problems; they act as pilot, dispatcher, and loading agent in many cases.
In commuter or air taxi operations, pilots are confronted with the problem of frequent trips with varying loads. They need to have a positive, accurate, and fast way to compute the weight and balance. They must have reliable empty weight and c.g. information readily available for use. This information must be updated to account for all the modifications performed on the aircraft.
A load manifest, when required for air taxi or commercial operations, should contain the following information concerning the aircraft loading at takeoff time:
1. The weight of the aircraft, fuel and oil, cargo (including mail and
baggage), and passengers;
2. The maximum allowable weight for that flight;
3. The total weight computed under approved procedures;
4. Evidence that the aircraft is loaded according to an approved schedule that insures that the c.g. is within approved limits.
The execution of a load manifest is always a highly recommended procedure from the standpoint of making a uniform preflight check of the weight and balance condition. A typical load manifest may be a simple form similar to that illustrated in figure 37. This form provides a record of passengers and of all useful loads for the particular flight. The major advantage of such a form is that the pilot has a standardized means of calculating and recording the weight and balance condition of the aircraft for each flight. If care is taken to carry forward or make proper record of the empty weight and c.g., the pilot can be spared the task of a search through aircraft records for this vital information. The form may also be used as a record of passenger identification as may be needed for administrative purposes.
One typical weight and balance control system for high density seating aircraft is based upon the utilization of useful load index tables and a total weight index limit envelope or table. With these tables (figs. 38, 39, 40, 41), it is possible to determine if weight and balance is within limits even in a situation where the passenger, cargo, or fuel loads change fairly rapidly. The tables can be read for intermediate weights by interpolation of values. To simplify and speed up calculations, use the nearest listed weight, but be conservative when checking against particular limits. The system is generally similar to those discussed on smaller aircraft. The pilot adds the weight and moments (index) of the empty aircraft and the useful load items. Then, checks are made against the published limits in this case the index limit envelope or table. Care must be taken to use the empty weight and moments or index from the latest weight and balance report. The pilot must be sure to use the same reduction factor for all moments in the calculations. Sufficient accuracy is obtained by rounding off index numbers to the nearest tenth.
Assume you are a pilot planning a flight in an air taxi aircraft. Your load consists of yourself, your copilot, 11 passengers, 300 pounds of fuel in each of the main tanks, 120 pounds of fuel in each of the auxiliary tanks, 75 pounds of oil, 100 pounds of baggage in the nose compartment, and 200 pounds of baggage in the rear compartment. The sample manifest form in figure 37 has been completed to show the useful load and empty weight items. Use the loading tables and total weight index limit table shown in figures 38, 39, 40, 41 to determine if the aircraft is properly loaded for takeoff. The intersection of the dotted weight and moment/100 lines in figure 41 shows that limits are not being exceeded.
TWIN ENGINE CARGO AIRCRAFT
Small twin engine aircraft can be used effectively for carrying cargo into airports where operations would not be practical with transport aircraft. Cargo which is particularly suitable for twin engine aircraft are high value items or items which must reach local destinations quickly. The scheduled transportation of mail to small cities and towns is an example of the type service these aircraft provide.
Light twin engine aircraft can be designed for more effective cargo operations if some special loading and handling features are employed. Large size cargo doors are a great help when bulky packages are to be loaded. Without the use of large doors, the cargo space may be restricted because big packages cannot be maneuvered through the passenger type doors. Provisions for securing the cargo to the aircraft structure are also needed. Normally, tiedown rings are attached to the floor and to structural members of the side walls for this purpose.
Many of the high density twin engine aircraft can be quickly converted from passenger to cargo use by removing the seats from the main cabin area (fig. 42). In some cases, cargo is carried in the passenger seats and secured by the regular seat belt. It is also possible that only one or two passenger seats will be removed, resulting in a mixture of cargo and passengers in the main cabin. Measures must be taken in this case to protect the passengers from possible cargo movement.
The following are recommended for the loading of cargo in other than approved cargo compartments or bins:
a. If passengers are carried, the cargo must be carried forward of the foremost passenger.
b. The cargo should be properly secured by a safety belt or other tiedown device to prevent it from becoming a hazard by shifting.
c. The cargo must not impose any load on seats or the floor structure that exceeds the load limitations for these components.
d. The location of the cargo must not restrict access to or use of any required emergency or regular exit by any passenger, or access to an emergency exit by a pilot if a regular exit is not accessible to the pilot.
e. The location of the cargo must not obscure any passenger's view of any required sign, unless an auxiliary sign or other approved means for proper instruction or notification is provided.
Cabin cargo is in danger of shifting if the deck angle (floor attitude) is not level as during the rotation and initial climb at takeoff. Unrestrained cargo will shift rearward in this event and cause a tail-heavy condition which may lead to a dangerous takeoff stall. Cabin cargo is also subjected to inertia forces resulting from turbulence, acceleration, deceleration, vibration, and hard landings. These inertia forces act more strongly in some directions than in others and tend to shift the cargo unless it is properly restrained. A forward force is the one most likely to act on cargo. This force may result from a sudden application of brakes, landing on a soft sod runway, or a crash landing. That is why cargo should be located forward of all passengers in a mixed-load configuration. Cargo must also be secured from moving aft, from side to side (laterally), or up and down (vertically).
Cargo may be secured by means of tiedown devices such as straps, ropes, or nets. These devices, when properly used, will restrain the cargo from moving in any direction. Tiedown fittings should be adequate in number and strength to restrain a cargo of any allowable weight and size. Floor structure, particularly where the tiedown fittings are anchored, must be strong enough to resist any anticipated load without distortion. Cargo floor loading limits are usually expressed as maximum weight in pounds per square foot. If a cargo item is loaded in a seat, the pilot would be wise to limit its weight to that of an average passenger. A tiedown or safety belt should restrain its movement in the seat (fig. 43). A single cargo item on the cabin floor should be secured in a manner similar to that shown in figure 44. Its center of gravity may be determined by the method shown in figure 45. The following general precautions should be observed when actually loading the cargo:
1. In a tailwheel aircraft, cylindrical items on their sides should
be chocked until lashed down.
2. Liquid containers should be placed with their outlets at the top.
3. Lightweight items should be stacked on heavier items, or stacked separately.
4. Shoring or planking must be used when the contact area is likely to exceed the floor strength limitations.
Many cargo loads carried in air taxi aircraft will consist of a variety of boxes, crates, sacks, drums, etc. This type of composite cargo may be secured with the type devices shown in figures 46 and 47. Sufficient restraint should be used to prevent shifting because of high deck angle or inertia forces. In arranging composite loads, cargo items should not be arranged so the load is top-heavy. If possible, the height of the load should not exceed its length. Particular care should be taken to secure this type load against slipping out from under the tiedown device. If the individual items of this type cargo are comparatively light, a net type tiedown device is adequate. Heavy items will require ropes or straps.
Cargo should be placed as near to the c.g. of the airplane as possible, roughly at the 30% chord point; but limitations of particular areas should be observed to prevent overloading the structure. Care must also be taken not to block access to an exit in the rear of the cabin or to cut off an aisle needed for inflight inspection of the main cabin cargo.
This problem is an example of the use of a manifest form to determine the weight and balance condition of a cargo flight. The airplane is the same one used in example 25 with the seats removed from the main cabin. Notice that the empty weight and moments have been changed due to the removal of the seats. This change must be carefully noted according to the manufacturer's recommendations. The sample manifest form in figure 48 has been completed to show the useful load and empty weight items.
Using the loading tables and total weight index table shown in figures 38, 39, 40, 41 to determine the loading condition of the aircraft, you should obtain a moment of 10038.7 index units. It is apparent when the index limit table is checked that the cargo is loaded too far to the rear. The aircraft is not safe or legal to fly in this loaded condition. The maximum index limit (rear c.g. limit) has been exceeded by 20.7 index units (2070.0 lb-in). If the cargo in compartment E consists of cartons, each weighing 20 pounds, how many cartons must be moved to compartment A to bring the index within the maximum limit?
The baggage or cargo table can be used to help determine how much cargo must be shifted. At least two methods are available:
1. Select the cargo weight which would make a difference of at least 20.7 index units when compartments A and E are compared. (40 lb. = 64.0 - 32.0 = 32.0 index units)
2. Determine the difference in arms between compartments A and E (Sta. 160 - Sta. 80 = 80 inch). Divide the excessive moments by this arm (2070.0 / 80 = 25.9 lb.).
By use of either method we can see that the movement of 40 lb. (two each 20 lb cartons) would be required to reduce the index by at least 20.7. A new passenger and cargo manifest should now be executed to prove that the c.g. is within limits with the proposed new load distribution. Of course, it would be possible to shift a greater number of cartons than the minimum to be on the safe side. In any case, care must be taken to remain within the compartment maximum weight limit, the floor loading limit, and the minimum and maximum index limits.
HELICOPTER WEIGHT AND BALANCE
The weight and balance principles and procedures which have been described in connection with airplanes apply generally to helicopters. Each model helicopter is certificated for a specific maximum gross weight. However, it is not safe to operate at this maximum weight under all conditions. Combinations of high altitude, high temperature, and high humidity determine the density altitude at a particular location. This, in turn, critically affects the hovering, takeoff, climb, autorotation, and landing performance of a helicopter. Additional factors to be considered are wind, obstacles, type of surface, and space available for takeoff and landing. Just because a helicopter can take off with a heavy load does not mean that flight with that load will be safe. A heavily loaded helicopter has less ability to withstand shocks and additional airloads caused by turbulence. The greater the weight, the less the margin of safety for the supporting structures such as the main rotor, fuselage, and landing gear.
Most helicopters have a much more restricted c.g. range than do airplanes. In some cases this range is less than 3 inches. The exact location and length of the c.g. range is specified for each helicopter and usually extends a short distance fore and aft of the main rotor mast or the centroid of a dual rotor system. Ideally, the helicopter should have such perfect balance that the fuselage remains horizontal while in a hover and the only cyclic adjustment required should be that made necessary by the wind. The fuselage acts as a pendulum suspended from the rotor. Any change in the c.g. changes the angle at which it hangs from this point of support. Many recently designed helicopters have loading compartments and fuel tanks located at or near the balance point. If the helicopter is not loaded properly and the c.g. is not very near the balance point, the fuselage does not hang horizontal in a hover. If the c.g. is too far aft, the nose tilts up and excessive forward cyclic is required to maintain a stationary hover. Conversely, if the c.g. is too far forward, the nose tilts down and excessive aft cyclic is required (fig. 49). In extreme out-of-balance conditions, full fore or aft cyclic may be insufficient to maintain control. Similar lateral balance problems may be encountered if external loads are carried.
Upon delivery by the manufacturer, the empty weight, empty weight c.g., and the useful load are noted on the weight and balance data sheet in the helicopter flight manual. If, after delivery additional fixed equipment is added or removed, or if a major repair or alteration is made which may affect the empty weight, empty weight c.g., or useful load, the weight and balance data must be revised. All weight and balance changes should be entered in the appropriate aircraft record. The helicopter flight manual includes directions for solving loading problems. The procedures are similar to those already described for airplanes. For further information, read the FAA Basic Helicopter Handbook, AC 61-13A.