TT Talk - Unplanned uncoupling


Safety of trucks on the highway is a paramount concern for the transport industry, impacting not only the lives of drivers but also all road users. It is imperative that industry stakeholders prioritise and uphold stringent safety measures. This article delves into one aspect truck safety: unplanned uncoupling.

An unplanned uncoupling incident refers to the sudden and unanticipated separation of a tractor unit from its attached trailer unit. This event might occur during transit on a public road or, more commonly, in a transport yard following completion of the coupling procedure. Such incidents have various causes, including mechanical failures, human error or adverse road conditions.

Such incidents can have serious consequences, posing immediate danger to both the driver of the vehicle and others nearby. The vehicle is likely to be damaged, as well as the infrastructure of the transport facility or the roadway. In addition, there may be cargo damage. Moreover, these incidents impact operations, causing delays, schedule disruptions and costly downtime for trucking companies.

It is imperative that industry stakeholders prioritise and uphold stringent safety measures

How incidents occur

In order to connect a tractor unit to a trailer, the driver aligns the trailer's kingpin with the tractor's fifth wheel. The kingpin is a cylindrical steel pin located on the underside of the front of the trailer, while the fifth wheel is a horseshoe-shaped coupling device mounted on the rear of the tractor. Ordinarily, when the kingpin is properly engaged with the fifth wheel, only severe stress will cause the two to separate. However, several factors may affect the coupling of the two units.

During coupling, the jaw (a component of the fifth wheel) should grip the kingpin, but improperly maintained equipment may make it difficult for it to grip fully. In particular, if the grease used to coat the fifth wheel is too thick, it may cause the jaw to engage with the kingpin partially rather than fully. Thick grease may also prevent the driver from seeing that the fifth wheel is improperly engaged. If the jaw is partially engaged, then vibration from the road, harsh braking or shifting cargo may cause the fifth wheel to disengage from the kingpin.

Additionally, drivers and operators should be aware that faults in the kingpin and related components may cause failure. The kingpin is connected to the container table that sits underneath the trailer. If this connection fails, perhaps due to improper welding, then the kingpin itself may fail. This happens only rarely but the consequences can be catastrophic.

A related issue arises from the design of the trailer braking systems that use air rather than hydraulic fluid, engaging when air is emptied from the system. This design means that brake failure due to mechanical malfunction is very rare. However, it introduces a risk to the person connecting the tractor unit to the trailer. If the trailer parking brake is not activated, then the trailer brakes may unexpectedly release when the airline is connected to the trailer during coupling and the trailer may roll. If it rolls towards the tractor unit, it may trap the individual performing the coupling between the tractor and the trailer, leading to serious injury or death.

If the trailer parking brake is not activated, then the trailer brakes may unexpectedly release...and the trailer may roll

Planned preventative maintenance and pre-use checks 

Rigorous pre-use inspections mitigate against unplanned uncoupling, requiring drivers to inspect the tractor-trailer combination meticulously in advance. Proper training and adherence to this procedure ensures that any mechanical issues can be identified and addressed. Such thorough pre-use checks, including for use in the yard, need also to be recorded and available to demonstrate remedial actions have been taken.

Similarly, a planned preventative maintenance schedule ensures that all components, including the fifth wheel and brake systems, are in optimal working condition and within the manufacturer's operating tolerances, and that there has not been a build-up of grease that might prevent effective coupling.

Although pre-use checks are important and will frequently highlight maintenance issues, given the technical nature of certain faults, there is no substitute for regular and rigorous inspection by a trained vehicle engineer. Most jurisdictions require that heavy goods vehicles undergo regular preventative maintenance and operators should be aware of the requirements in particular regions.

There is no substitute for regular and rigorous inspection by a trained vehicle engineer

Further, effective information management will maximise the opportunity for fleet managers to make smart procurement choices and provide feedback to vehicle manufacturers, as well as enabling safety to be strengthened, such as monitoring for common faults and defects, and taking anticipatory actions to prevent future failures.

How procedures can help

Proper driver procedures are central to safety, providing opportunity for drivers not only to identify defects, but also ensure that the tractor and trailer are coupled safely and correctly. Precise steps need to be established that include aligning the kingpin with the fifth wheel, lowering the landing gear, engaging the locking mechanism, and ensuring proper safety chain attachment. Throughout these actions, drivers should ensure that parking brakes of both tractor and trailer are engaged whenever the vehicle is not being driven. After the fifth wheel and the kingpin are engaged, but before the airline is connected, the driver should perform a test of the couple by selecting a low gear and, with the trailer brake still engaged, pulling slowly forward.

Training for such procedures needs to be provided for all involved to reduce the risk of human error. Particular attention is recommended for shunters or yard workers responsible for moving trailers about within a terminal or transport yard, who may couple and uncouple dozens of times each day and may not be fully qualified heavy goods vehicle drivers.

Importantly, as new safety technologies are integrated, such as automatic locking systems and kingpin sensors, particular care is required. Be alert that new equipment or retrofitted technology may result in fleets with differing safety features or protocols. This may be confusing; training regimes should take into account all vehicles used within the fleet, including vehicles hired or leased temporarily.

The importance of safety culture

A cultural emphasis on safety reinforces the importance of following established procedures at all times and empowers the workforce to take decisions that promote the safety of themselves and others.

The transient nature of the driving workforce often makes this challenging; ensure that all drivers, including temporary drivers and sub-contractors, are held to the same high standard and that this standard is enforced through rigorous training and assessment, and evidenced through commitment from management.

Further reading

Coupling & uncoupling - Workplace transport (


If you would like further information, or have any comments, please email us, or take this opportunity to forward to any others who you may feel would be interested.

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Josh Finch

TT Club