Retrograde Transit Normalization Study (RETRNS)--a preliminary investigation into variance in retrograde processing.(Candid Voices)(Report)

Introduction

The speed with which carcass-constrained retrograde assets flow through the retrograde cycle (from base supply to the depot) matters. It matters because the Air Force's procurement and planning processes use the depot repair cycle time (DRCT) as a variable in worldwide buy and repair requirements. Our primary interest lies within a component of this large pipeline called reparable intransit (RIT). (1)

In 2006, Air Force Materiel Command, Supply and Engineering Requirements Division (AFMC/A4Y) ran a simulation using the Aircraft Availability Model (Logistics Management Institute) and the September 2005 D200A annual year (AY) data set to test the consequences of reducing reparable intransit time by 10 to 30 percent. At a 30 percent reduction in processing time, buy requirements would decline by $12.5M and repair requirements would decrease by $4.8M for a total savings of $17.3M. (2)

In February 2007, AFMC/A4YR ran this simulation on the September 2006 D200A annual year (AY). The simulation showed that a 30 percent reduction in retrograde cycle time would result in a reduced buy requirement of approximately $32M and a reduced repair requirement of about $11M. These estimates must be tempered by the fact that in an actual production run additional D200A business rules would come into play as well as a post-D200A process that completes the budget estimate?

Whether faster shipping times would lead to cost savings has recently been called into question. (4) The essence of the argument is that while carcass-constrained assets should be expedited, other assets should be moved more slowly to save on transportation costs. Supporting this perspective is the fact that the Air Force employs a repair on demand (ROD) system for reparables--assets not in demand end up being stored at a depot until a specific repair request is made. In short, the point is made that it makes little sense to ship assets express (given the attendant costs)just to have them sit on a shelf waiting to be needed. The authors used a Monte Carlo simulation to demonstrate that at the 99.99 percent confidence level adding a couple days to shipping time does not increase back orders.

In the 2006 AFMC/A4YR simulation cited previously, 7,138 assets were implicated in the projected savings. At the time the simulation was performed, all of these assets were carcass constrained by definition because they were in a buy position. (5)

While it is true that D200A does not include depot storage time as a variable in DRCT, this fact does not invalidate the projected savings due to increased retrograde pipeline velocity. In effect, DRCT encapsulates storage time, but since the DRCT pipeline clock stops after the receipt of the asset at the depot and does not start again until a repair order is placed--storage time has no impact on its calculations. Hence, storage time is not relevant for the calculations. Another way of looking at it is that there is storage time associated with an asset only if the asset is not carcass constrained. Therefore, whether the ensconced model always reflects the reality of asset movement and repair demands is beside the point, because it is only changes in model parameters that affect buy and repair requirements. Whether a more suitable and responsive model can be developed is beyond the scope of this study but has been recently addressed by the RAND Corporation. (6) Perceived limitations in the current D200A model are discussed in that report.