Theses sounds were coming from the aft part of the forward cargo hold.

Once on the ground, inspection of the forward cargo hold revealed that all container restraining XZ latches were lowered. The hold was loaded with a single container, the noises were therefore attributed to the free movement of that container in the cargo hold.

The inspection, which revealed no damage to the cargo linings, concluded that the XZ latches were not correctly locked before take-off.

The standard A320 Family cargo hold is configured for bulk loading.

Around one third of the fleet is equipped with the optional semi-automatic Cargo Loading System (CLS), which is proposed for all members of the Family except the A318.

The CLS is an electrically powered system that allows ULDs to be carried in the aircraft. The main goal is to reduce manpower and loading/unloading time.

The CLS is composed of several components, which can be grouped according to their intended functions (fig.1):

• Guidance, thanks to entrance guides and YZ guides along each side of the holds.
• Transport and Conveyance, performed by power drive units on ball mats and roller tracks.
• Restraint of ULDs, ensured by XZ latches, installed on the centre roller track.

(fig.1) A320 Family CLS components

The ULDs are maintained in position by XZ latches, which consist of an aluminium frame and a pair of spring-loaded interlocking pawls.

If the outside pawl is lifted up, the inside pawl extends and locks automatically. Conversely, if the inside pawl is depressed into the retracted position, the outside pawl automatically folds down (fig.2).

(fig.2) XZ latch 2842T100 series

Once each ULD is in position, the loading procedures call for these XZ latches to be raised and locked manually (fig.3).

XZ latch in down position

XZ latch in intermediate position
(not yet locked)

XZ latch raised and locked Interface
with ULD/ pallet baseplate

(fig.3) XZ latch manual operation

Each ULD is then restrained by one XZ latch on each side of their baseplate or by one latch and one end-stop for those located at the borders of each holds (fig.4).

(fig.4) A321 FWD cargo hold, ULD, XZ latches and End stops positions

Several possible root causes have been identified, based on the ULD movement events reported by operators.

If the loader does not raise up the latch or does not properly lock it into position, the container/ pallet will not be restrained in the X (flight) direction and will therefore move freely.

Alleviation means:

• Awareness

Loaders should be made aware that instructions call for all latches to be raised and properly locked, independently of whether the positions are empty or not, as specified in the Weight and Balance Manuel (WBM, chapters 1.10.05 and 1.10.06), and Aircraft Maintenance Manual (AMM, chapter 25-52).

• Design improvement

If the outside pawl is lifted up, the inside pawl extends and locks auto­matically.

A weak spring may not extend the inner pawl enough when the outside pawl is lifted up, leaving the latch in an intermediate (unlocked) position. The new design calls for an increased spring force.

This design equally calls for the addition of an easily visible yellow indicator when the latch is in intermediate position, i.e. not raised enough to be in the locked position (fig.5 and 6).

This latch (Part Number 2842T100-3)has been introduced in production from MSN 0573 onwards (Feb 1996) and is proposed through the Illustrated Parts Catalogue (IPC) as alternate.

An inoperative or damaged latch may as well lead to ULD movements.

Alleviation means:

The following A320 Family Maintenance Planning Document (MPD) tasks call for visual and operational checks of the XZ Latches:

• Task 255000-11: “Visual check of the xz latches and door sill latches” Interval: 750 FH
• Task 255000-02: “Operational check and detailed inspection of XZ latches” Interval: 8000 FH

A third task has recently been added (in the July 2013 revision) for a detailed inspection of the latch after removal. The removal of the part allows for an easier inspection, especially regarding the spring condition.

• Task 255000-25: “Remove XZ latches for detailed inspection” Interval 24000 FH

(fig.5) Latch correctly locked: the yellow indicator
is not visible

(fig.6)  Latch in intermediate position, not correctly locked: the yellow indicator is visible

A non-standard or damaged ULD baseplate may also impair the proper functioning of the latch.

Alleviation means:

• ULDs should comply with National Aerospace Standards (NAS) 3610 and IATA specifications during their entire lifetime.
• ULDs should be loaded in accordance with the WBM.
• Airbus encourages airlines to clean their cargo hold to avoid debris that could interfere between the ULD baseplate and the XZ latch, as recommended in Task 12-21-12-100-007-A.

An object falling from an improperly closed container/pallet on the inner pawl could lead to a latch disengagement.

Alleviation means:

To mitigate this risk, a specific loading procedure is recommended when cargo holds are not fully loaded. This procedure, proposed in SIL 25-162, remains identical for all holds and aircraft types. Its application will ensure that the inner pawl of the latch, i.e. the movable pawl, will always be blocked by the adjacent ULD/ pallet
baseplate (fig.7).

(fig.7) Recommended A321 AFT cargo hold loading sequence

The below (fig.8) illustrates, when two containers are loaded in positions 41 and 42, how the inner pawls of the latches are blocked by adjacent containers, making any disengagement impossible.

Latch at frame C56 between ULDs at position 42 and 41

Latch at frame C53 behind ULD at position 41

(fig.8) Latches in locked position after loading

As illustrated above, feedback from airlines led Airbus to propose a design modification of the latches to reduce the number of ULD movements.

Airbus encourages operators to communicate any suspected or confirmed case of ULD movement by means of a specifically designed reporting sheet.

This sheet is available in SIL 25-162 and has been specifically produced to facilitate the search for the root cause of the movement (fig.9).

(fig.9) Airbus ULD Movement Report-Sheet