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Drum-Buffer-Rope Streamlines and Simplifies Aircraft Maintenance

  • Satya S. Chakravorty
July/August 2017

Less is more” is a phrase that indicates a focus on simplicity and those bare essentials that can deliver a big impact. Well-known examples of this philosophy include Abraham Lincoln’s Gettysburg Address, which had only 272 words yet a brilliant message that became the ethos of democracy; Gustave Eiffel’s iconic iron lattice tower; and the minimalist masterpieces of Pablo Picasso. In fact, many of the finest art critics in the world believe that the more stuff there is in something, the busier and worse off it is.

The Warner Robins Air Logistics Complex (WR-ALC) recently adopted a less-is-more approach to performance improvement. This organization provides depot maintenance, engineering support and software development for major weapon systems. The facility’s team supports peacetime maintenance requirements, wartime emergency demands, aircraft battle damage repair and ready maintenance of critical items.

In order to enhance maintenance services for the C-5, C-17 and C-130 cargo planes as well as the F-15, a fighter aircraft, a tactical procedure drum-buffer-rope (DBR) initiative was employed. The APICS Dictionary explains that DBR is a “theory of constraints (TOC) method for scheduling and managing operations that have an internal constraint or capacity-constrained resource.”

Epitomizing the less-is-more concept, TOC is known for its focus on the vital few instead of the trivial many. It is based on a five-step process:

  1. Identify the constraint.
  2. Decide how to exploit the constraint.
  3. Subordinate everything to the constraint.
  4. Elevate the constraint.
  5. If the constraint is broken in any of the previous steps, return to step 1 and repeat the process.

“Even a very complex system comprising thousands of people and pieces of equipment can have, at any given time, only a very, very small number of variables,” the APICS Dictionary notes. “Perhaps only one, known as a constraint … [limits] the ability to generate more of the system’s goal.” Once that constraint is identified, actions can be taken to exploit the constrained resources. It is here that DBR scheduling is applied.

The system

Typically, maintenance activities at WR-ALC involve

  • de-painting
  • disassembly and inspection
  • repair or modification
  • the build up
  • a functional test
  • painting.

Cargo aircraft de-painting and painting activities are performed through a shared resource of five facilities for C-5, C-17 and C-130 aircraft maintenance operations. This system is managed by the support squadron. There is some restriction involved, as not all de-painting and painting activities can be performed in all facilities. Therefore, aircraft maintenance generally is performed in designated maintenance hangers. While there was considerable variation in workload from one month to another, the de-painting and painting processes were persistent constraints.

Warner Robins Air Logistics Complex F-15 plane maintenance

Specifically, the required workload was 15 aircraft per month, but the system’s throughput was only 14 aircraft per month. In addition, traditional performance metrics emphasized high resource utilization. To comply with this objective, the previous leaders pulled more aircraft into empty facilities or scheduled work outside the processing capability, causing high variation or oscillation. As work in process (WIP) increased, the system became cluttered, resources — especially the workforce — were stretched thin, and throughput suffered even more.

The insufficient workforce was divided into three shifts, but this put fewer people on an aircraft and ended up delaying things even more. Additionally, working with three different shifts mandated excessive handoffs from one group to another — and these rarely went smoothly. Many times, one shift spent as much as 50 percent of its time correcting problems created or overlooked by the previous shift. Consequently, de-painting and painting tasks were fraught with expensive rework.

DBR scheduling

Decision makers chose to use DBR scheduling to concentrate WR-ALC resources, reduce the number of shifts and introduce intermediate checkpoints to catch quality issues. With DBR, the first step is to develop a master schedule for the constraint, which becomes the drum that beats a tempo for the entire system. Because the constraint dictates throughput, it is imperative that the constrained resource never be idle as a result of being starved of input. To prevent such idling, a buffer is used to protect against system variability.

The size of a buffer is defined as the estimated time necessary for the constrained resource to process all the work inside the buffer. It is a function of both system variability and the amount of protective capacity available at other points in the process. In addition, to avoid uncontrolled growth of WIP, work is released to the system at the pace of the constrained resource — like a rope from the constraint. In this manner, as soon as the constraint resource completes its work, the next task is released from the buffer for processing at the constraint.

The support squadron’s initiative would be a significant change for WR-ALC, requiring modifications in almost every aspect of the de-painting and painting activities. To help with the transition, squadron leadership established a multiskilled steering team composed of process engineers, master schedulers, managers, a Labor Relations Office (LRO) representative, and union stewards from the de-painting and painting areas.

With input from the workforce, steering team members began by developing a detailed transitional plan. It consisted of setting up the drum, creating the buffer and establishing the rope. The drum was determined by considering available manpower. The steering team estimated that the system enabled work on four aircraft — two for de-painting and two for painting. These people also noted that there is variability in processing times because of the different aircraft sizes. Thus, the entire workforce would not be busy 100 percent of the time. However, the goal was for personnel to be able to respond the moment work is ready to be done.

It was decided that the number of workers per shift would be increased from 12 to 21. The steering team was aware that supervisors likely would find daily distribution of work to be challenging as a result of this change. Therefore, the team met with all stakeholders to familiarize them with the new plan. They also explained that workers might be needed to assist others in order to speed up the completion of a process. In addition, a process engineer must always be present to collect pertinent data, such as the actual number of people working and how many were dedicated to an aircraft. Finally, supervisors would be on the floor 100 percent of the time, and quality and safety personnel would be present as much as possible. 

Next, team members created the buffer, which involves all aircrafts that reach complete status. At that time, they are included in the buffer, which generally follows a first-in-first-out priority rule. If, on occasion, two aircraft arrive at the buffer at the same time, the one with the earlier due date receives higher priority.

Lastly, the rope was established for the release of aircraft into the system. As one aircraft completes de-painting or painting, it leaves the system, and another aircraft enters. At no point in time does the system contain more than four aircraft.

Less is more

The WR-ALC steering team completed the DBR implementation in six months. Noteworthy results of this newfound simplicity include a 20 percent reduction in WIP inventory, 45 percent less overtime, 54 percent more throughput and 82 percent quality improvements. Plus, after the initial rollout, system throughput increased from 14 to 16 aircraft per month; and, since then, it has reached 21 aircraft per month.

There also are some interesting and noteworthy intangible benefits. For instance, as low-to-medium-skilled painters now work closely with highly skilled painters, this interaction has given rise to enhanced learning of de-painting and painting skills. This is improving both speed and quality. Second, more manpower per aircraft has allocated more painters on difficult-to-reach areas, such as the belly of an aircraft, which is serving to alleviate worker fatigue. Third, a significant reduction in multitasking is bringing about greater speed and a meaningful reduction in flow days.

Overall, the maintenance system at WR-ALC has become much less cluttered, resources are more concentrated, facilities are being fully utilized, time and energy are saved, and throughput has been maximized. As the organization continues its DBR journey, leaders are confident that processes will only continue to improve.

Satya S. Chakravorty, PhD, CFPIM, Jonah, LSSMBB, is the Caraustar Professor of Operations Management at Kennesaw State University. He may be contacted at schakrav@kennesaw.edu.

To comment on this article, send a message to feedback@apics.org.

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