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Aviation Ergonomics: The Ups and Downs of Manual Cart Handling in the Airline Industry

Aircraft Manufacturers Need to Consider Risk of Injury in Manual Material Handling Tasks

Replacing the engine on the plane, working people tap. Concept maintenance of aircraft

Wheels move all types of things in all types of places. There are some unique situations involving manually handling wheeled objects in aviation, both in manufacturing and in the operation of commercial aircraft. There are risks of overexertion injuries resulting from pushing and pulling when the tasks are not well designed, controlled and managed.

Using Liberty Mutual Tables to Evaluate Material Handling Tasks and Reduce Risk of Injury

One example from Malaysia describes an analysis of the risk of overexertion injuries while manually moving carts in an aviation manufacturing facility. The task involved male operators moving carts carrying 500 kg components about 2.1 meters every 45 minutes throughout a 12-hour shift. The Liberty Mutual Manual Handling Tables were used to evaluate the push and pull forces and it was determined that only about 50 percent of male operators could exert the initial and sustained forces required to move the carts. Although some men could move the carts, they are at risk of injury. Liberty Mutual researchers have found that maintaining push and pull forces at or below a level achievable by at least 75 percent of females significantly reduces the risk of injuries.

Conversations with ergonomists and engineers in aircraft manufacturing facilities in the US emphasize the variety of things on wheels that are pushed and pulled. They range from supply carts to scaffolds used to access large aircraft as they are assembled. However, whatever the object moved, the Liberty Mutual Tables are commonly used to evaluate the acceptability of the required forces.

Ramps Used in Aircraft Manufacturing Increase Force Required for Manual Material Handling

One vexing problem associated with the assembly of larger aircraft is rolling carts over small ramps used to protect anacondas, a nickname for bundles of cables and lines carrying power to the aircraft under construction. The ramps that protect the anacondas can be as high as 18 inches (46 cm). Moving carts up these ramps requires extra effort and going down ramps often is complicated by the need to apply brakes to slow down the cart as it descends the ramp.

Flight Attendants at Risk of Injury Pushing Service Carts

Pushing service carts down the aisles on commercial aircraft is also a task that can put operators at risk of injury. A report from Germany notes that flight attendants frequently reported discomfort from pushing service carts during short duration flights. The aircraft are often pitched up or down while the service is taking place, in effect requiring pushing the carts up and down slopes as high as 8 degrees 150 to 250 times per shift. The cart forces were evaluated using the ISO 11228-2 and CEN 1005-3 standards, which recommend force levels that 85 percent of all users can safely exert. The authors determined that the required forces often exceeded those recommended limits and concluded that steeper slopes sometimes increased the force demands to the point where only empty carts could be moved.

Clearly, this is an indication that the manual handling task needs to be optimized in order to keep flight attendants safe. The pitch of the aircraft on these short haul flights is unlikely to be changed and cart weight is also not a changeable consideration. But, there are other options to improve the safety of the task. The design of the cart itself should be the focus. By implementing ergonomic improvements to the cart, the force required to push it would be lowered; two areas to focus on should be handle placement and caster selection.

Airline Industry Should Focus on Ergonomics, During Manufacturing Process and Beyond

Wherever wheels move carts, tools such as the Liberty Mutual Tables or the ISO and CEN standards are useful in evaluating the safety of those individuals who make the wheels turn. Those involved in the design and manufacturing of carts used in the airline industry, whether in the manufacturing process or in-flight service, should focus on ergonomics to decrease the risk of injury in manual material handling tasks that serve to get the planes in the air.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.

Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

Proactive Ergonomics and Cart Handling in the Automotive Industry

Best Practices in Implementing New or Retrofit Industrial Ergonomics Programs in the Automotive Industry

automotive manufacturing industrial ergonomicsThe automotive industry has a long and successful history of employing manual material handling ergonomics programs to reduce the likelihood of musculoskeletal disorders (MSDs) and utilizing proactive ergonomics as a tool for improving both operational efficiencies and product quality.

Early Assessment of Force Requirements

An article in EHS Today, At Ford, Ergonomics Meets Immersive Engineering, describes how Ford has used digital assembly to move ergonomic analysis of tasks far upstream in the design process by building digital models of operators performing assembly tasks with digital components – a virtual operator assembling virtual parts years before a prototype is available!

Combining the digitized operators’ motions with biomechanical models allows Ford ergonomists and engineers to identify and modify high risk operations well before cutting steel to build fixtures and tooling.

So, how did Ford benefit from this process?

  • 80% fewer manufacturing feasibility issues,
  • Very high perception of new product quality from customers – five times the industry average, and
  • A dramatic reduction in injuries, lost time and physical pain, and suffering.

Proactive Ergonomics at Work: Identifying and Retrofitting Existing Designs

As is common practice in the automotive industry, Honda has established internal guidelines for classifying cart push and pull forces. After evaluation of the force required to perform a task, each task receives a color code. Those tasks receiving a red label are high risk and require redesign.

An OSHA Ergonomics Success Story describes how Honda implemented proactive ergonomics. One task required Honda associates to push carts that weighed one thousand pounds or more. The force required to keep the carts in motion was in the green zone (acceptable), but the initial force required to start them moving was deep in the red zone (unacceptable). A team of associates developed an on-demand electric motor assist system to start the carts moving. Once the cart was moving, the motor switched off, which helped to prolong battery life.

What is the ROI on proactive ergonomics programs like this one?

The investment on these systems paid its way and more – a US $1.5 million return over the five-year life of the carts from an initial investment of approximately US $300,000. Honda eliminated any initial cart start forces over 50 pounds-force and the new carts could now carry heavier loads.

Consider Dynamic Braking to Control Stopping of Carts for Improved Ergonomics

Some challenges remain regarding the ergonomics of cart handling. One is stopping carts once they are moving. Sudden stops, such as emergency stops, can place very high loads on the person pushing or pulling the cart.

“Sustained forces aren’t a big issue for us,” said Honda ergonomist José Banaag, “Stopping the cart once it’s moving is a problem.”

Ergonomic Risks of Maneuvering Carts

Operators commonly turn carts; for example, when positioning them on assembly lines. The forces on operators while maneuvering carts is a relatively unexplored area of the ergonomics of manual cart handling that requires more attention.

A study from the Netherlands focused on ergonomic adjustments of catering carts suggests that those forces are equal to the initial force required to start a cart moving. Evaluation tools such as the Liberty Mutual Tables rate acceptable force levels based on the frequency of the higher level of force required to initiate cart movement. Should we be including maneuvering forces in those analyses?

“Maneuvering is a big issue,” says Banaag, “It takes a lot of effort to maneuver carts”.

Ergonomic Testing: Consistent Measurements in Cart Testing & Caster Testing

Brent Bowers, Ergonomics Program Manager at Deere and Company, has responsibility for ergonomics in plants around the world. A big concern is the consistency of the measurement of cart forces.

“We use the ISO 11228-2 measurement protocol to generate the push-pull forces to get more consistency.” said Bowers.

Once they have a representative average force from several trials, they evaluate the acceptability of the required force using the Liberty Mutual Tables.

It is difficult for human operators to push a test cart with the same acceleration for each trial. Consequently, Deere is exploring the development of new techniques to improve the consistency of force measurements. One possible option is an automated system that will consistently assess the forces required to push or pull carts under various loads and with various casters. The system could also be used to assess different cart designs before they are rolled out to the production lines.

Deere is also looking at caster maintenance; for example, over time the convex cross-sectional profile of a caster can wear down, changing the push and pull force requirements. Deere’s innovative idea is to work on the development of a go/no-go caster profile gauge to quickly evaluate when a worn caster should be replaced.

Proactive Ergonomics – Knowledge, Preparation & Design are Key

Proactive assessment of manual cart handling force requirements in the automotive sector has paid off in terms of better cart designs that lower risk of injuries and discomfort, increase operator satisfaction, and gain operational efficiencies.

For a deeper understanding of the ergonomics of safe pushing and pulling and factors to design safer manual material handling tasks, download the Guide to Workplace Ergonomics.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.

Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

Safe Operating Envelope

Just as there are safe operating envelopes for aircraft, there are safe operating envelopes for pushing and pulling carts.

 

“The ‘envelope’ was a flight-test term referring to the limits of a particular aircraft’s performance,
how tight a turn it could make at such-and-such a speed, and so on.” The Right Stuff, by Tom Wolfe

 

flight envelopeTest pilots push themselves and their aircraft to extremes to determine safe operating envelopes for that aircraft. In doing so, they work out the limits of human and mechanical abilities in all the various situations expected while flying the aircraft. A flight test program that only looked at straight and level flight wouldn’t be a good estimate of a safe operating envelope for landing an aircraft. An important component of that testing is understanding the limits of a pilot’s ability to withstand the stresses placed upon him or her while maneuvering the plane; for example, a too high speed or too tight turn may cause the pilot to lose consciousness and crash.

Just as there are safe operating envelopes for aircraft, there are safe operating envelopes for pushing and pulling carts. Just as for aircraft, the strength and endurance capabilities of the human operator play an important role in determining the safe operating envelopes for carts. Operating forces that stress the operator beyond their limits are outside the safe operating envelope. Consequently, measuring the forces exerted by the cart operator and comparing those forces to known operator limits is the basis for determining a safe operating envelope.

Tools such as the Liberty Mutual Tables (Snook tables) describe the limits to forces for starting a cart moving or for keeping it moving. Depending on who is moving the cart (men or women), how far the cart is moved and how frequently the cart is moved, the tables tell us what percentage of people can work safely within that operating envelope.

However, the forces measured on the job and compared to those in the tables must be operating forces used when moving carts with typical floor conditions, slopes, space constraints, temperature, etc. Emergency conditions must be considered too; a heavily-loaded rolling cart has a lot of momentum. Overcoming that momentum while stopping or turning that cart quickly will likely require the operator to exert a much greater force than is needed to start or keep the cart moving.

workplace ergonomics guide darcorDetermining the safe operating envelope for carts cannot be done by limiting operating force assessments to a straight and level flight in optimal conditions, e.g. a brand-new cart rolling on a smooth surface. The essence of developing a complete safe operating envelope for moving carts is evaluating the human performance limits in all the various situations and conditions expected while moving the cart.

Have a safe flight.

For a thorough overview of workplace ergonomics, download the Guide to Workplace Ergonomics.

 

 

 

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.

Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

Maintaining Carts for Optimal Push/Pull Forces

Use of Safety Tables and Maintenance of Carts and Casters to Reduce Injury

Have you heard of the wonderful one-hoss shay,
That was built in such a logical way
It ran a hundred years to a day,
And then, of a sudden, it — ah but stay,
I’ll tell you what happened without delay – Oliver Wendell Holmes

Pushing and pulling heavily-loaded carts by hand is an everyday occurrence in the workplace. There are many parts to a discussion of the best way to systematically manage cart handling, but let’s start first with a discussion of the how the injury risk associated with pushing and pulling carts is assessed. While one might think that the risks are limited to back injuries, there are good reasons to be concerned about shoulder injuries as well. And of course, there are those incidents where a cart ran into or over an unseen individual.

In North America, limiting the risk of injury while handling carts is commonly done by keeping the operating forces at or below a reference force limit. For example, starting a cart moving shouldn’t require a force more than about 220 newtons (or 50 pounds of force).

However, that recommended force changes depending on various conditions:

  • the gender of the person doing the work,
  • the distance that they move the load,
  • whether it’s the force necessary to start the cart moving or just to keep it in motion,
  • how often the loaded cart is moved, etc.

A handy reference tool that takes these various conditions into account when recommending a push or pull force limit can be found in the Snook tables, published by the Liberty Mutual Research Institute for Safety. These tables describe the amount of force workers judged to be the maximum acceptable push or pull forces under a given set of conditions.

Some different tools are used in the European Union to determine acceptable force limits for pushing and pulling loads. One is an international technical standard (International Standards Organization 11228-2: 2007) that gives two different means of calculating acceptable forces for pushing and pulling based on conditions like those used in the Snook tables.

However, of all these tools, only the British Health and Safety Executive’s Risk Assessment of Pushing and Pulling (RAPP) tool calls out the importance of maintaining carts in order to maintain the operating forces at or below the recommended limits.

Unlike the wonderful “one hoss shay” that lasted 100 years to the day, carts wear out. The frames get bent and don’t track quite true. Wheels get cut or go out of round. Bearings wear out. All these change the force required to operate the cart. What was once low risk may no longer be safe. It is not possible to minimize the risk of injury if carts are not well maintained.

A good, systematic approach to managing the safety of pushing and pulling carts must ensure that the operating force is within the recommended limits to begin with and then remains there throughout the cart’s working life. Consequently, a manual cart safety program must do more than an initial test of cart operating forces or documenting a purchasing specification for cart push pull forces. Routinely testing carts to ensure that the operating forces are maintained within safe limits is an essential part of managing cart safety. Not to do so means that an operators’ risk exposure is unknown and uncontrolled.

 

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.

Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

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