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CHAPTER 7

CARGO RESTRAINT

INTRODUCTION

This chapter provides guidelines for restraining cargo on military and civilian aircraft. Cargo is restrained (tied down) in an aircraft so that it remains stationary in the cargo compartment when the aircraft is subjected to rough air, vibration, acceleration, deceleration, and rough landings. The greatest force exerted on the cargo is usually the forward movement encountered when the aircraft slows rapidly on landing. When the pilot applies the aircraft brakes on landing, the cargo tends to keep moving at a higher speed. The cargo is also restrained in proportion to its weight, so that it will not shift when the aircraft turns, takes off, lands, or encounters other forces while flying. Tie-down equipment is aboard the aircraft to be used as restraints.

PRINCIPLES OF CARGO RESTRAINT

Cargo loaded in an aircraft is restrained so that it will not shift during any condition the aircraft experiences in flight. Basic principles of restraint apply to tying down cargo. Although the details vary for different kinds of cargo, the basic principles of restraint do not change. The basic principles follow:

  • Tie down cargo to prevent movement in all directions.
  • Install tie-down devices to provide adequate restraint without overstressing the tie-down fitting or damaging the cargo.
  • Ensure the tie-down leads directly from the tie-down fitting on the aircraft floor to the load being restrained.
  • Attach tie-down devices in symmetrical pairs. Unsymmetrical tie-downs cause uneven load distribution and could result in tie-down failure (Figure 7-l).
  • Ensure tie-down pairs in a given direction are equal in type and length. (Any material subjected to a tension load stretches to a given percentage of its length. Therefore, the greater the length, the greater the potential amount of stretch. If two tie-downs of the same type and capacity restrain a load in a given direction and one tie is longer than the other, the longer tie has a greater stretch potential. The shorter tie assumes the majority of any load that may develop. If as a result the shorter tie is overstressed and fails, the longer tie would be subjected to the full load and it too would probably fail.)
  • Use the nylon CGU-1/B strap on cargo that may be damaged by chains, such as fragile/crushable cargo or baggage.

RESTRAINT CRITERIA

Restraint criteria for aircraft cargo are based on the weight of the cargo and the forces imposed on it due to changes in motion (changing direction, slowing down, or speeding up). The force increases as the rate of change in motion increases.

*The primary restraint criterion is the minimum amount of restraint needed to keep cargo from moving in a specific direction. A numerical factor (g factor) called restraint safety factor or load factor has been determined for cargo aircraft. This figure determines the number of tie-down devices to use.

*Imagine a passenger traveling in a car at 50 mph. The driver jams on the brakes for a sudden stop. What happens to the passenger when the brakes are applied? The same thing happens to the cargo in an aircraft. A sudden change in direction or speed of the aircraft moves the cargo in the same manner. The change in motion is called the outside force. The amount of outside force to which a unit of cargo may be subjected is called the load or g factor. Multiplying the weight of a unit of cargo by the g factor results in the amount of required restraint for that unit of cargo:

*Weight x G Factor = Required Restraint

*For example, a unit of cargo weighing 5,000 pounds is to be restrained from moving forward. The forward g factor for the aircraft is 3. Use the formula to determine the total load to be restrained: cargo weight (5,000 pounds) times g factor (3) equals the weight to be restrained against forward movement (15,000 pounds).

DIRECTION OF RESTRAINT

The direction in which the cargo would move if it were not restrained identifies the restraint criteria applied to the cargo to prevent its movement. Forward restraint keeps cargo from moving forward in the aircraft; aft restraint, from moving backward; lateral restraint, from moving to either side; and vertical restraint, from moving up off the aircraft floor. The aircraft floor is downward restraint. Use the restraint criteria in Table 7-1.

TIE-DOWN DEVICES

Three tie-down devices are used to secure cargo in the aircraft:

  • The MB-1 tie-down device has a 10,000-pound rated capacity.
  • The MB-2 tie-down device has a 25,000-pound rated capacity.
  • The CGU-1/B tie-down device has a 5,000-pound rated capacity. This device is commonly called a 5,000-pound strap.

The MB-1 and MB-2 devices (Figure 7-2) are similar in looks, use, and the way they operate. The only significant difference is their load capacities and size.

The CGU-1/B tie-down device (Figure 7-3) is a 20-foot nylon web strap with two metal hooks attached. One hook is stationary at one end of the strap, while the other hook has a ratchet device and can be moved over the length of the strap. The ratchet tightens the device when it is being used.

Similar methods are used to restrain all types of cargo. The details for restraining each cargo item vary with its bulk, weight, configuration, and location in the aircraft and whether it is equipped with tie-down provisions. These variations make restraining each piece of cargo a separate problem.

Turbulence and other violent motions expose airlifted vehicles to extreme gravitational forces that may compress pneumatic tires and thus ease the tension on tie-down chains. When the motion suddenly stops and the aircraft quickly climbs, the chain snaps taut and imposes abnormal loads on aircraft and cargo tie-down fittings and tie-down devices. This same reaction may occur when the vehicle's springs flex under the same conditions. Special-purpose vehicles equipped with large soft tires, such as rough terrain forklifts, can encounter these stress conditions in the aircraft.

Some vehicles are made so that each major component part must be tied down. An example is the truck-mounted crane. The crane is mounted on the truck chassis by a large-diameter kingpin. Because vertical acceleration might disengage the kingpin, both the truck chassis and the crane must be tied down.

The basic rules for applying tie-down devices follow:

  • Use standard tie-down devices that are provided aboard the aircraft.
  • Know the capabilities of each tie-down device used.
  • Use an even number of chains symmetrically in pairs.
  • Know the restraint criteria in each direction (forward, aft, lateral, and vertical), then compute the restraint required.
  • Maintain equal tensions throughout the tie-down arrangement when attaching the tie-down devices to cargo and tie-down fittings.
  • Install tie-down devices at a 30-degree angle from the cargo compartment floor and 30 degrees from the longitudinal axis whenever possible.
  • Ensure the right capacity tie-down device is used with the tie-down fitting. All aircraft floor tie-down fittings are not the same capacity. Avoid placing a 25,000-pound-capacity tie-down device on a 10,000-pound-capacity tie-down fitting. Otherwise, it provides only 10,000 pounds of restraint.
  • Turn the rings in the floor tie-down fittings so that the tension is applied to the top of the ring, not the sides.
  • Consider the capacity of the tie-down device and the strength of the attaching points when attaching chains to vehicles. Attach tie-down devices to lifting shackles or other points that were designed for this purpose first. If additional restraint is required, attach devices to available strong structural points, such as tow hooks, bumper supports, axles, or frame members. (Do not place chains against brake lines, hydraulic lines, fuel lines, tires, or electrical wiring.) Do not attach tie-downs to steering mechanisms, tie rods, driveshafts, grills, or fender and body braces (Figure 7-4).
  • Do not attach more than 50 percent of required tie-down devices in a given direction to vehicle axles.
  • When chains cross over one another, make sure that they pull in a straight line and not against one another.
  • When forming chain loops around axles and bumpers, do not depend on friction or tension to prevent the chain from sliding laterally. Place the chain loop against some solid part, such a differential housing or bumper bracket.
  • When using CGU-1B tie-down devices to tie down cargo, do not use nylon devices over sharp edges.
  • Attach the tie-down devices to the aircraft floor and the chain to the cargo item.

TIE-DOWN DEVICE STRENGTH

Every tie-down device is rated to withstand a given force. The tie-down devices restrain up to their rated capacity only when applied so that the force exerted is parallel to or straight onto the device. When the tie-down device is applied like this, all of its rated capacity is available to prevent the cargo from moving in the direction of the plane angle.

To determine the number of tie-down devices required to properly secure any given item of cargo, know how each of the following influences cargo tie-down:

  • Weight of cargo.
  • Restraint factor for each direction (g force).
  • Floor and plane angles of devices when attached.
  • Rated strength of tie-down devices to be used.
  • Strength of tie-down fittings on aircraft floor and cargo item.

*To find the force that must be restrained, use the first two factors: weight of the cargo and the restraint factor. Written in formula form, it is--

G x W = F

Where:

Restraint factor = G

Weight of the cargo = W

Force to be restrained = F

REQUIRED NUMBER OF TIE-DOWN DEVICES

The two methods for determining the number of tie-down devices needed to secure a load in an aircraft are the percentage restraint chart and the percent of angle of tie. The percentage restraint chart is a quick method for advance estimating of the number of tie-down devices required. This method is not as precise as the percent of angle of tie. The percent of angle of tie is the most precise method to find the exact restraint achieved. It uses several formulas that require knowing the exact angle of tie and cannot be used for advance planning.

Percentage Restraint Chart

The percent of angle of tie can be determined by using the percentage restraint chart (Figure 7-5). Floor angle degrees are in the top horizontal row, and plane angle degrees are in the left vertical row. To find the percent of effectiveness of a 30/30-degree angle, first read the floor angle across the top of the chart (30 degrees). The figure directly below the floor angle degree is the percentage of rated strength for vertical (up) restraint. Next, read the plane angle down the left side of the chart (30 degrees). Next to the plane angle are LON (longitudinal) and LAT (lateral), the directions in which the tie-down will be effective. Read across the table until the 30-degree plane angle line intersects the 30-degree floor angle column.

The numbers at this intersection represent the restraint provided by a restraint device applied at a 30/30-degree angle. These numbers express a percentage of the maximum rated strength of a tie-down device. A device rated at 10,000 pounds would provide 7,490 pounds of longitudinal restraint, 4,330 pounds of lateral restraint, and 5,000 pounds of vertical restraint.

The formulas will help determine how many of each type of tie-down device should be used for each piece of cargo. Do not mix the types of devices. If the formulas say to use MB-2 devices, do not substitute a lower-rated device or the restraint will be insufficient.

Remember also to use the devices in pairs. If the answer is not an exact even number, always round up to the next highest even number when using chains. For example, if the figures came out to 2.2 devices, apply 4.

*If a cargo item weighs 8,000 pounds, its restraint must withstand a 3 G. forward force using MB-1 tie-down devices attached on 30-degree floor and plane angles. Attached at these angles, the effective holding strength of the MB-1 is 74.9 percent of its rated strength of 10,000 pounds.

The formula for determining the required number of devices is--

 

Restraint Factor x Weight of Cargo
Strength of Device x Percent of
Angle of Tie-Down
 

=

Force to be Restrained
Effective Holding
Strength of Device

 

=

Total Number of Devices Required, or
 

G x W
R  x P

  =  

F
S

  =   N    

Substituting numbers, the calculation is--

       3G x 8,000 pounds
10,000 pounds x 75 percent

  =  

24,000
7,500

  = 3.2 or 4 devices needed

Multiplying the rated strength of the device by the percent of angles at which it is attached equals the effective holding strength of the tie-down device. Written as a formula, it is--

R x P = S

Where:

Rated strength = R

Percent of angle of tie-down = P

Effective strength of each device = S

Combine the results of the first two formulas to find how many devices to use for each piece of cargo:

F
S

 =  N

Where:

Force to be restrained = F

Effective strength of each device = S

Number of devices required = N

The product of this formula gives only the number of devices required for one direction of restraint. Use the same process for aft, lateral, and vertical restraint as well. Displaying all the calculations in table form makes the total amount of calculation easier (Figure 7-6). The example in Figure 7-6 represents a 45/45-degree angle.

To use the chart, follow the order of calculations as they are listed along the top. Blocks 1 and 2 (Direction of Restraint and Restraint Factor) always contain the information shown in Figure 7-6. The restraint factors for Air Force aircraft have been determined through scientific analysis and cannot be changed by the unit. The unit must supply the information in the rest of the blocks.

Block 3:

Cargo weight is the total weight of the vehicle or cargo. It includes any cargo in the vehicles, vehicle fuel, shoring, and any other additions. It does not include the weight of the driver or crew of any vehicle.

Block 4:

Force to be restrained is the answer when Block 2 is multiplied by Block 3.

Block 5:

Effective strength of device is found using the restraint percentage chart (Figure 7-5). Determine the angle of tie to be used, read right for plane angle and down for floor angle, and find the percent of effectiveness for the device at this angle. Multiply the percent of effectiveness by the rated strength to get the effective strength of the device. Enter the result in Block 5.

Block 6:

Devices needed is the answer when Block 4 is divided by Block 5. If the answer is a fraction, always round up to the next highest even number. (This number is always even because the tie-down devices are used in pairs.)

Block 7:

Restraint achieved is the answer when Block 5 is multiplied by Block 6. This is a cross-check to ensure that enough restraint is being used. If the figure in Block 7 is lower than that in Block 4, more devices must be added.

NOTE: The devices used versus the devices needed could be different from the figure in Block 6. When tie-down devices are applied at an angle, they will provide restraint in more than one direction. The same chains used for fore and aft restraint will also provide vertical and lateral restraint. Only if the fore and aft restraint is insufficient for vertical and lateral restraint will more devices have to be added.

Percent of Angle of Tie

It is not always possible to apply tie-down devices at a known or desired angle because of cargo configuration or interference by other cargo. After tie-down devices have been applied to a cargo item, their effective restraining strength is found by measuring the lengths of the chains. The percent of angle of tie is used to determine if enough restraint has been applied to a piece of cargo after it is loaded and restrained in the aircraft.

Tie-downs attached to a load usually provide restraint in three measurable directions: on a vertical plane, a lateral plane, and a longitudinal plane. The vertical angle is the angle between the chain from the attachment point or tie-down fitting (Figure 7-7, A and B) and the aircraft floor. The lateral plane angle (Figure 7-7, A and C) is the angle between the chain and a line which runs across the cargo compartment through the attachment point. The longitudinal plane angle (Figure 7-7, A and D) is the angle between the chain and a line which runs fore and aft in the cargo compartment through the attachment point.

NOTE: For ease of illustration, Figure 7-7 shows only one tie-down device. However, tie-down devices must be attached in pairs, with each device having the same angles. Attaching a pair of tie-down devices to the opposite ends of the cargo item will provide restraint against movement in all directions.

When a tie-down device is attached at an angle, its effectiveness is reduced; the greater the angle, the greater the reduction.

EXAMPLE 1:

Use a 25,000-pound-capacity MB-2 tie-down device applied to a cargo item. Figure 7-7 shows a method to determine effective restraint for cargo tie-down. As shown, determine tie-down ratios by dividing tie-down chain length into the effective length for the direction in which restraint is required. Then multiply this ratio by the strength of the tie-down chain or attachment point, whichever is less, to find the restraint received from the tie-down pattern used.

  1. Measure the length of the tie-down chain (A) from the tie-down fitting to the attachment point on the cargo: 50 inches.

  2. Measure the effective vertical length (B) from the attachment point on the cargo to a point directly beneath it on the cargo floor: 25 inches.

  3. Divide the tie-down chain length (A) into the vertical effective length (B) to determine the ratio:

    25
    50

      =   0.50 ratio

  4. Multiply this ratio by the rated restraint (25,000 pounds) of the tie-down chain:

    25,000 x 0.50 =

    12,500 pounds of effective vertical restraint received from the chain

  5. To determine the effective forward or aft restraint, obtain a forward or aft effective length (C) by measuring from a point directly beneath the attachment point on the cargo along a longitudinal axis to a point lateral to the tie-down fitting being used: 37 inches.

  6. Divide the tie-down chain length (A) into the forward or aft effective length (D) to determine the ratio:

    37
    50

      =   0.74 ratio

  7. Multiply this ratio by the rated restraint (25,000 pounds) of the tie-down chain:

    25,000 x 0.74 =

    18,500 pounds of effective forward or aft restraint received from chain

  8. To determine the effective lateral restraint, obtain a lateral effective length (E) by measuring from a point directly beneath the attachment point on the cargo to the row of tie-down fittings being used: 22 inches.

  9. Divide the tie-down chain length (1) into the lateral effective length (8) to determine the ratio:

    22
    50

      =   0.44 ratio

  10. Multiply this ratio by the rated restraint (25,000 pounds) of the tie-down chain:

    25,000 x 0.44 =

    11,000 pounds of effective lateral restraint received from chain

EXAMPLE 2:

If the tie-down device in Figure 7-7 were an MB-1 rated at a maximum capacity of 10,000 pounds, it would provide 50 percent or 5,000 pounds of its vertical, 74 percent or 7,400 pounds of its longitudinal, and 44 percent or 4,400 pounds of its lateral restraint capacity.

NOTE: If the tie-down chain is attached to a pallet ring, the rated restraint would be 7,500 pounds. The ratio would be multiplied by 7,500 to determine the effective restraint received.

PALLETS

The Air Force 463L cargo-handling system is designed to increase loading and unloading efficiency and to reduce operating costs. Small items of cargo are difficult to properly restrain. Cardboard CONEX inserts or standard wood pallets may be used to consolidate small items into a larger unit. CONEX inserts and wood pallets with cargo will then be placed on Air Force 463L cargo pallets and secured with Air Force cargo restraining nets.

The three nets of the 463L pallet will restrain up to 10,000 pounds of general cargo on any single pallet without having to use any other tie-down devices. The 463L pallet may be used as a mobility platform for other than general cargo weighing more than 10,000 pounds. Palletized loads over 10,000 pounds cannot be restrained with nets and must be secured with chains and devices to the aircraft floor, the pallet rings, or restraint rail tie-down rings.

NOTE: The Air Force loadmaster (or boom operator on the KC-10) has final authority in determining adequate cargo restraint.



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