AC 91-23A
PILOT'S WEIGHT
AND BALANCE
HANDBOOK U.S. DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
PILOT'S
WEIGHT AND BALANCE
HANDBOOK Revised 1977 U.S. DEPARTMENT OF TRANSPORTATION Federal Aviation Administration Flight Standards Service For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402
CONTENTS Page
FOREWORD
CONTENTS ILLUSTRATIONS • Vii CHAPTER 1. WEIGHT AND BALANCE CONTROL Effects of Weight ..
Weight Changes ..
Balance, Stability, and Center of Gravity Effects of Adverse Balance Shifting of Loose Cargo .
Management of Weight and Balance Control CHAPTER 2. TERMS AND DEFINITIONS CHAPTER 3. EMPTY WEIGHT CENTER OF GRAVITY Weighing Equipment Weighing Procedure Finding Center of Gravity Percent of Mean Aerodynamic Chord (MAC) Aircraft Modifications Weight and Balance Report CHAPTER 4. INDEX AND GRAPHIC LIMITS Index Numbers Center of Gravity Variable Limit Graph Index Envelope CHAPTER 5. CHANGE OF WEIGHT Weight Shifting .
Weight Addition or Removal CHAPTER 6. CONTROL OF LOADING-GENERAL AVIATION Useful Load Check ....
Weight and Balance Restrictions Airplane Flight Manual...
Light Single-Engine Aircraft Loading Problems Light Twin-Engine Aircraft ....
High-Density-Seating Aircraft .
Twin-Engine Cargo Aircraft ... 46 Helicopter Weight and Balance 50 CHAPTER 7. CONTROL OF LOADING-LARGE AIRCRAFT Weight and Balance Procedures Center of Gravity Travel During Flight Records ...........
Weight and Balance Systems ...
ILLUSTRATIONS Figure Page
1. Lift and weight ..
2. Overweight causes longer takeoff run 2
3. Lateral or longitudinal unbalance 3
4. Forward c.g. critical on landing . 4
5. Aft c.g. critical in a stall4
6. If tailheavy, move passengers to front seats 4
7. Effect of fuel in swept-wing aircraft .
8. Definitions 7
9. Electronic weighing kit
10. Measurement of reaction points . 12
11. Weighing aircraft on platform scales 12
12. Empty weight c.g. formulas14
13. Empty weight and empty weight c.g 15
14. Computer solution-empty weight c.g. 16
15. Percent of mean aerodynamic chord 16
16. Computer solution-c.g. in percent MAC 18
17. Computer solution-converting percent MAC to inches 18
18. Large airplane weight and balance calculation diagram 19
19. FAA aircraft type certificate data sheet excerpts 20
20. Typical weight and balance data 21
21. Typical equipment list 22
22. Weight and balance report . 23
23. Oil index table 26
24. Fuel index table 26
25. C.G. variable limit graph 27
26. Index envelope 28
27. Weight shifting diagram 31
28. Computer solution-shifting weight
29. Computer solution-limits in percent MAC
30. Empty weight + useful load = takeoff weight
31. Weight and balance data
32. Changing weight between c.g. limits 37
33. Loading schedule placard37
34. General aviation aircraft-weight and balance diagram
35. Loading graph 39
36. C.G. moment envelope 40
37. Passenger manifest 42
38. Occupants loading moments ... 43
39. Baggage and cargo loading moments
40. Fuel and oil loading moments ..
41. Total weight index limits
42. Cabin tiedown diagram vii
43. Securing cargo in seat
44. Securing cargo to floor
45. Determining c.g. by means of a roller
46. Securing composite cargo with straps
17. Securing composite cargo with net .
48. Cargo manifest
49. Effect of c.g. on helicopter attitude in hover
50. Crewmember index table
51. Flight loading manifest form
52. Loading summary chart-DC-6B
53. Loading summary chart-CV-880M
54. Load adjuster
55. Balance computer and overlay
56. Template and grid.
57. Balance limitations grid 58. Large aircraft equipment list 999909
59. Weight and balance log.
60. Loading tables ..
61. Fuel load distribution table ...
62. Stabilizer trim viii
Chapter 1
WEIGHT AND BALANCE CONTROL Weight is a measure of the attractive force of the earth's gravity upon a material body. It is an indication of the mass or heaviness of the body. Weight is also one of the greatest enemies of the flyer. It is a factor which must be respected if flight is to be conducted safely.
The force of gravity acting on the mass of the aircraft continuously attempts to pull it down from flight. The force of lift which is generated by the airfoils of the aircraft is the only force available to counteract weight and keep the aircraft in flight.
However, the airfoils can produce only a limited amount of lift for use in resisting gravity; therefore, any increase in aircraft weight is to be avoided if possible. The total lift of the aircraft depends on the design of the airfoils, the speed and angle of attack of the airfoils as they move through the air, and the density of the air through which the airfoils are moving. If the generated lift does not equal aircraft weight, level flight cannot be maintained and the aircraft must descend.
EFFECTS OF WEIGHT Any object aboard the aircraft which increases the total weight significantly is an undesirable object as far as flight is concerned. However, aviators must accept a compromise and load some heavy objects in LIFT WEIGHT FIcURE 1. Lift and weight.
the fuselage or wings to make flight possible. Fuel is an example of a heavy but necessary item. It is always easier to fly when the aircraft is light and more difficult and dangerous when the aircraft is heavy. Therefore, it has always been a primary rule of flight to make the machine as light as possible without sacrificing strength or safety and to include only those loads essential for the particular flight.
The total weight of a vehicle changes as the contents (passengers, fuel, or cargo) are varied. If care is not taken, the vehicle can be weighted down with objects to a point where it can no longer function efficiently as a mover of loads. The operator of the vehicle and especially the pilot of an aircraft should always be aware of the consequences of overloading.
An overloaded boat might sink, a truck or automobile might not be able to climb a hill, and an aircraft may not be able to leave the ground. Each vehicle has its limits, beyond which excessive weight leads to inferior operation and possible disaster. Of all common vehicles, the aircraft is most susceptible to trouble if weight considerations are disregarded;
its limits are most easily exceeded. Furthermore, when the aircraft has weight problems, the initial indication of poor performance will be during takeoff; an unfortunate place for the vehicle and the pilot to be in trouble.
Excessive weight (fig. 2) reduces the flying ability of an airplane in almost every respect. The most important performance deficiencies of the overweight airplane are:
Higher takeoff speed.
Longer takeoff run.
Reduced rate and angle of climb.
Lower maximum altitude.
Shorter range.
Reduced cruising speed.
Reduced maneuverability.
Higher stalling speed.
FIGURE 2. Overweight causes longer takeoff run.
2 Higher landing speed.
Longer landing roll.
The pilot must appreciate the effect of excessive weight on the performance of the aircraft. Every preflight check should include a study of performance charts to see if the aircraft weight may contribute to hazardous flight conditions. Most pilots have been trained to recognize and avoid such aircraft performance-reducing factors as: High-density altitude, frost on the wings, low engine power, and severe or uncoordinated maneuvers. Excessive weight reduces the safety margins available to the pilot when these conditions are encountered. The pilot must also consider the consequences of an overweight aircraft if emergency conditions arise. If an engine fails on takeoff or ice forms at low altitude, it is usually too late to reduce the aircraft's weight to help keep the machine in the air.
WEIGHT CHANGES The weight of the aircraft can be changed easily by varying the payload (passengers, baggage, and cargo). But, if weight has to be decreased by reducing the payload, the flight will be less profitable.
Weight can also be changed by altering the fuel load. Gasoline or jet fuel has considerable weight- 30 gallons may weigh more than a paying passenger.
But, if weight is lowered by reducing fuel, the range of the aircraft is shortened. Fuel burn is normally the only weight change that takes place during fight. As fuel is used, the aircraft becomes lighter and performance is improved; this is one of the few good things about the consumption of the fuel Changes of fixed equipment also have a major effect upon the weight of the aircraft. Many aircraft are overloaded to a dangerous degree by the installation of extra radios or instruments. Repairs or modifications usually add to the weight of the aircraft; it is a rare exception when a structural or equipment change results in a reduction of weight.
As with people, when an aircraft ages, it just naturally puts on weight. The total effect of this growth is referred to as "Service Weight Pickup."
Most service weight pickup is the known weight of actual parts installed in repair, overhaul, and modification. These parts should have been weighed or the weight calculated when they were installed. In addition, an unknown weight pickup results from the collection of trash and hardware, moisture absorption of soundproofing, and the accumulation of dirt and grease. This pickup can only be determined by the accurate weighing of the aircraft as a unit.
BALANCE, STABILITY, AND
CENTER OF GRAVITY Balance refers to the location of the c.g. (center of gravity) of an aircraft. It is of primary impor tance to aircraft stability and safety in flight. Pilots should never fly an aircraft if they are not personally satisfied with its loading and the resulting weight and balance condition. The c.g. is the point about which an aircraft would balance if it were possible to support the aircraft at that point. It is the mass center of the aircraft, or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated. The c.g. must be within specific limits for safe flight.
The prime concern of aircraft balancing is longitudinal balance, or the fore and aft location of the
c.g. along the longitudinal axis. Location of the c.g.
with reference to the lateral axis, however, is also important. The design of the aircraft is such that lateral symmetry is assumed to exist as far as weight is concerned. In other words, for each item of weight existing to the left of the fuselage centerline, there is generally an equal weight existing at a corresponding location on the right. This lateral mass symmetry, however, may be upset by unbalanced lateral loading. The position of the lateral c.g. is not computed, but the operating crew must be aware that adverse effects will certainly arise as a result of a laterally unbalanced condition. Lateral unbalance will occur if the fuel load is mismanaged by supplying the engine (s) unevenly from tanks on one side of the aircraft (fig. 3). The airplane pilot can correct the resulting wing-heavy condition by the use of aileron tab adjustment or by holding a constant lateral control pressure. However, this puts the aircraft controls in an out-of-streamline condition and results in a lowered operating efficiency Since lateral balance is relatively easy to control and longitudinal balance is most critical, further reference to c.g. in this handbook will mean longitudinal location of mass balance.
The c.g. is not necessarily a fixed point; its location depends on the distribution of items loaded in the aircraft. As variable load items are shifted or expended, there is a resultant shift in c.g. location.
The pilot should realize that if the mass center of an aircraft is displaced too far forward on the longitudinal axis, a nose-heavy condition will result.
Conversely, if the mass center is displaced too far aft on the longitudinal axis, a tail-heavy condition will result (fig. 3). It is possible that an unfavorable location of the c.g. could produce such an unstable condition that the pilot could lose control of the aircraft. In any event, flying an aircraft which is out of balance, either in a tail-heavy or a nose-heavy direction, may produce increased pilot fatigue with obvious effects on the safety and efficiency of fight.
The pilot's natural correction for longitudinal unbalance is a change of trim to remove the excessive control pressure. However, excessive trim has the effect of reducing primary control travel in the direction the trim is applied.
EFFECTS OF ADVERSE BALANCE Adverse and abnormal balance conditions affect the flying ability of an airplane with respect to the same flight characteristics as those mentioned for an excess weight condition (p. 2). In addition, there EMPTY — - FULL
LATERAL UNBALANCE WILL CAUSE WING HEAVINESS EXCESS
BAGGAGE
LONGITUDINAL UNBALANCE WILL CAUSE NOSE OR TAIL HEAVINESS FIcURE 3. Lateral or longitudinal unbalance.
are two essential airplane attributes which may be seriously reduced by improper balance; these are stability and control. Loading in a nose-heavy direction causes problems in controlling and raising the nose, especially during takeoff and landing. Loading in a tail-heavy direction has a most serious effect upon longitudinal stability even to the extent of reducing the airplane's ability to recover from stalls and spins.
Limits for the location of the aircraft's c.g. are established by the manufacturer. These are the fore and aft limits beyond which the c.g. should not be located for flight. The limits are published for each aircraft in the FAA Aircraft Type Certificate Data Sheets or Specifications. If, after loading, the
c.g. does not fall within the allowable limits, it will be necessary to shift loads before flight is attempted The forward c.g. limit is often established at a location determined by the landing characteristics of the aircraft. It may be possible to maintain stable and safe cruising flight with the c.g. ahead of the prescribed forward limit, but since landing is one of the most critical phases of flight, the forward c.g.
limit is placed at a relatively rear position to avoid damage to the aircraft structure when landing (fig.
4).
FIGURE 4. Forward c.g. critical on landing.
A restricted forward c.g. limit is also specified to assure that sufficient elevator deflection is available at minimum airspeed. When structural limitations or large stick forces do not limit the forward c.g. position, it is located at the position where full-up elevator is required to obtain a high angle of attack for landing.
The aft c.g. limit is the most rearward position at which the c.g. can be located for the most critical maneuver or operation. As the c.g. moves aft, a less stable condition occurs, which decreases the ability of the aircraft to right itself after maneuvering or after disturbances by gusts (fig. 5).
FICtRE 5. Aft c.g. critical in a stall.
For some aircraft, the c.g. limits, both fore and aft, may be specified to vary as gross weight changes. They may also be shifted for certain operationai procedures, such as acrobatic flight, retraction of the landing gear, or the installation of special loads and devices that change the flight characteristics.
The actual location of the c.g. can be altered by many variable factors--usually under control of the pilot. Placement of baggage and cargo items can both determine c.g. and be used to control c.g. In addition, the assignment of seats to particular passengers can be used as a means of obtaining the most favorable balance (fig. 6). If the aircraft is tailheavy, it is only logical "horse sense" to place a heavy passenger in a front seat.
The loading and selective use of fuel from various tank locations can have a decided effect on aircraft
ALL PASSENGERS MOVE TO THE FRONT OF THE AIR-BUS, PLEASE!!
FIGURE 6. If tailheavy, more passengers to front seats.