Cargo Loading and Floor Limits

On a passenger aircraft the holds carry the passengers' checked baggage, mail and paying freight; on a freighter they carry everything. Loading them is more than finding room. The load in each compartment enters the load sheet at its own arm, so it sets the centre of gravity (CG), and it presses on a floor that was designed for a limited load per square metre, per metre of length and per compartment.

The mass and balance rules therefore ask for three things of every hold load: that it be placed as the load sheet says, that it stay within the structural limits of the floor and the compartment, and that it be restrained so that it cannot move in flight. The exam tests the arithmetic of floor limits; the line pilot mostly meets the result on the load sheet and in the loading instruction.

Freight and cargo holds

In mass and balance terms, freight is commercial cargo carried for revenue, distinct from passengers' baggage, which the rules define as personal belongings. Freight is normally weighed, because its mass varies too much for standard values; baggage may be counted at standard masses on larger aeroplanes. Freight, baggage, mail and passengers together form the traffic load.

The lower-deck holds of an airliner have fire-resistant sidewalls and ceilings, are usually pressurised and heated, carry fire detection and protection, and have published maximum floor loading and running load limits. On passenger aircraft they are normally Class C compartments: a separate smoke or fire detection system warning the flight deck, a built-in extinguishing system controlled from the flight deck, and means to keep smoke and extinguishing agent out of occupied areas. Class E compartments are found only on freighters (see cargo and lavatory fire protection).

The A320 is a typical narrow-body example. It has three lower-deck compartments, forward, aft and bulk, all pressurised and temperature-controlled. The forward hold accepts containers or loose cargo, and the bulk compartment at the rear takes loose items only.

The EASA rules require loading to be done under the supervision of qualified personnel and to be consistent with the data used for the mass and CG calculation. It must also respect:

• the dimensions of each compartment;
• the maximum load per running metre of the floor;
• the maximum mass in each cargo compartment.

Bulk cargo

Bulk cargo is loaded loose, piece by piece, rather than in a container or on a pallet. A bulk cargo hold is a compartment, or part of one, set aside for it. On many airliners it is the area at the aft end of the rear hold, separated from any containers ahead of it by a restraining net attached to the floor, ceiling and sidewalls. Late baggage, mail and items of awkward shape that do not fit a container are typical bulk loads.

The A320's bulk compartment has its own door, a manually operated plug-type door that opens inwards and upwards and, unlike the forward and aft cargo doors, can also be opened from inside. Loose loads must be stowed so that they cannot shift: moving freight can damage the aircraft and its load and, if enough of it moves, shift the CG.

Unit load devices

A unit load device (ULD) is a container or a pallet with a net on which baggage and freight are built into a single unit away from the aircraft, then loaded, locked in place and unloaded as a whole. Each ULD is weighed, and its mass and position are recorded for the load sheet. ULDs cut turn-round times, protect the load and make the hold contents easier to account for.

Containers are shaped to fit the lower deck. The best-known is the LD3, a container for the lower holds of wide-body aircraft with one angled lower corner that follows the curve of the fuselage; the A320's forward hold accepts a version designated LD3-45W. Pallets carry larger items under a net. Locks in the hold floor engage the base of each container or pallet and hold it in position.

ULDs are moved in the hold by power drive units (PDUs), powered rollers mounted in the floor and controlled from a joystick at the door. They move containers sideways across the doorway and then forward or aft along the compartment, and the forward and aft holds typically have separate systems.

Floor loading limits

A floor can fail in two ways, and there is a floor loading limit for each:

• The running load, or linear load, is the mass per unit length along the fuselage, in kg per metre, kg per inch or lb per foot. It protects the underfloor structure, the beams and frames that carry the floor.
• The area load, or floor loading intensity, is the mass per unit area, in kg/m² or lb/ft². It protects the floor panels themselves.

Both must be met, and neither replaces the compartment mass limit. An item can satisfy the area limit and fail the running limit, or the other way round.

Running load

The running load is the item's mass divided by the length it occupies along the fuselage, fore and aft. The least running load is obtained with the item's longest dimension laid fore and aft.

Take a 900 kg crate measuring 3 ft × 4 ft × 12 ft, with a running load limit of 20 kg per inch:

When a load exceeds the running limit, the simplest cure is often to turn it through 90° so that its longer side lies along the fuselage. A 500 kg item bearing on 10 in of length gives 50 kg/in against a 45 kg/in limit; removing 5 kg would not help, since 495 kg on 10 in is still 49.5 kg/in, but turning it so that it bears on a longer side would.

Concentrated and distributed loads

The general floor limit is published as a uniformly distributed load (UDL), an allowable mass per square metre or per square foot that the floor can take when the load is spread evenly over it. A concentrated load is one whose intensity on its small base exceeds that UDL limit, such as a heavy machine on feet or a crate on narrow runners. It may still be carried, but only if approved load spreaders bring the effective intensity down within the limit.

The area load is the mass divided by the area in contact with the floor. A 480 kg machine standing on a base of 0.6 m × 0.4 m = 0.24 m² presses on the floor at 480 ÷ 0.24 = 2,000 kg/m². If the floor limit is 1,000 kg/m², it cannot go straight onto the floor.

Load spreaders

A load spreader is a rigid base placed under a concentrated load to spread its mass over a larger area of floor; the standard spreader is timber planking about 2 inches thick. Continuing the example, a spreader of 1.0 m × 0.6 m = 0.6 m² with a mass of 20 kg gives (480 + 20) ÷ 0.6 = 833 kg/m², within the 1,000 kg/m² limit.

Three points are easily missed:

• The spreader's own mass is part of the load, in the area calculation, the compartment total and the load sheet.
• The spreader reduces the area load, and its length along the fuselage can help the running load, but both limits must be checked again with it in place.
• Spreaders also protect the floor panels from indentation and damage by hard or sharp bases. Items with a smooth base that already meet the UDL limit do not need one.

Compartment limits and restraint

Every hold has a cargo compartment mass limit, a maximum mass for the compartment as a whole. On the A320 family the certified limits are:

The A321neo ACF and the A321XLR have a bulk compartment limit of 800 kg instead. A total below the aircraft's maximum masses does not make an overloaded compartment acceptable.

The load must also be restrained. ULDs are held by floor locks; bulk loads are held by nets, including the cargo restraining net that separates the bulk area from containers; heavy individual items are tied down to the floor fittings. Restraint protects the structure and other cargo and stops the CG moving in flight. Ballast loaded to correct the CG counts against the same compartment limits and must be secured in the same way.

The CG must remain within limits on the ground as well, during loading and unloading. Holds ahead of and behind the main wheels are therefore loaded and unloaded in a planned order, and on some types a tail stand is fitted to stop the aeroplane sitting back on its tail (see centre of gravity).

Cargo in the passenger cabin has its own rules. Under 14 CFR 121.285 it must be in an approved cargo bin or, if loose, properly secured behind a bulkhead or divider, within the seat and floor limits, not above seated occupants and clear of exits, the aisle and required signs. Dangerous goods in any compartment are subject to their own acceptance, loading and segregation rules and are notified to the commander in writing before departure (see dangerous goods).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.