Saturday, November 3, 2012

Availability Makes a Difference … (Part II)

The Modular Airborne Fire Fighting System II (MAFFS II) being installed in a C-130 at Peterson Air Force Base, CO in 2009.[xxii]  (Photo courtesy of 302 AW, Air Force Reserve Command)
The MAFFS II as installed in a C-130 cargo bay, Peterson Air Force Base, CO in 2012.[xxiii]  (Photo courtesy of 302 AW, Air Force Reserve Command)
On 23 June 2012, a wildfire was quickly growing in the vicinity of Colorado Springs, CO.  Ultimately known as the Waldo Canyon fire, this fire lead to the U.S. Forest Service’s (USFS) 24 June 2012 decision to activate its Modular Airborne Fire Fighting System II (MAFFS II) for the 2012 wildfire season.[i] “On June 25 … MAFFS-equipped C-130s … received their first launch orders for 2012 and began to fly fire suppression missions on what would become the costliest wildland fire in Colorado’s history.”[ii] By 17 September 2012, when the MAFFS was deactivated for 2012, eight MAFFS equipped C-130s had “released almost 2.5 million gallons of fire retardant during 1,011 [aerial] drops on fires in 10 states.”[iii]
So, what is the MAFFS system? 
“MAFFS is a joint Defense Department and U.S. Forest Service program designed to provide additional aerial firefighting resources when commercial and private air tankers are no longer able to meet the Forest Service's needs ….
As a self-contained aerial firefighting system owned by the U.S. Forest Service, MAFFS can discharge 3,000 gallons of water or fire retardant in less than five seconds, covering an area a quarter of a mile long by 100 feet wide. Once the load is discharged, it can be refilled in less than 12 minutes.”[iv]
There are three Air National Guard wings and one Air Force Reserve Command wing that are MAFFS-qualified and provide the C-130 aircraft when the USFS activates MAFFS.[v] 
Why are these aerial firefighting systems so critical? According to the USFS’s January 2012 Large Airtanker Modernization Strategy, “Airtankers are used to deliver fire retardant to wildfires, thereby reducing fire intensity and rate of spread until ground personnel can reach the fire. Airtankers play a key role in successful initial attack, which is one of the most difficult and critical components of wildfire management.”[vi] The USFS report goes on to state that up to 70,000 communities across the U.S. are at risk of wildfires, and the annual costs of suppressing and recovering from wildfires “amounts to billions of dollars each year.”[vii]
Like any complex mechanical system, maintaining the MAFFS units and keeping them available requires skilled technicians, proper tools and equipment, and a ready source of spare parts. A 7 July 2012 article in The Seattle Times notes that the “Forest Service has stockpiled enough major parts, can source many smaller parts, and can mend the biggest parts no longer being made to keep the system running”[viii] according to Scott Fisher, MAFFS coordinator for the Forest Service. The article continues, quoting Mr. Fisher, "The system was built for at least 20 years … I would not be surprised to see this thing fly for a full 30 years."[ix] 
Of course, to make that projection a reality is where applying readiness-based sparing (RBS) and its availability-based sparing recommendations would be invaluable. For example, RBS can determine the proper range and depth of spare parts to keep the MAFFS units fully operational and available to continue their wildfire suppression mission for another 20 to 30 years.

System availability revisited

As noted in a previous posting on this blog, the availability of a fielded system in normal operations is dependent upon its supporting maintenance infrastructure. There are two basic categories of maintenance. The first category, corrective (or unscheduled) maintenance, generates independent part demands which must be forecasted. The second maintenance category, preventive (or scheduled) maintenance, generates dependent parts demands whose timing and quantities are derived from the maintenance schedule. Modern RBS applications can accommodate both of these maintenance types when sizing supply support.  Let’s now take a closer look at some well-known availability relationships.

Inherent availability

When it is important to examine the reliability-maintainability trade-offs for a system’s design, then inherent availability is an appropriate system performance measure.[x] Inherent availability [Ai] is calculated as (EQ. 1)  [xi] 

where the mean time between failures (MTBF) is (EQ. 2) [xii]

and mean corrective maintenance time  (also called Mean Time to Repair) is “For a sample of repair actions, a composite value representing the arithmetic average of the maintenance cycle times for the individual actions.”[xiii]

Operational availability

Unfortunately for operators and logisticians, the systems they support don’t operate in such an ideal environment and operational availability [Ao] becomes the more appropriate measure (EQ. 3) [xiv]


where MTBM is the mean time between maintenance. According to the DoD Reliability, Availability and Maintainability (RAM) Guide, MTBM considers “all maintenance actions, including repairing design/manufacturing failures and maintenance-induced failures, performing preventive maintenance, and other actions (e.g., remove an item to facilitate other maintenance).” [xv]
Mean maintenance downtime (MDT) measures “the total elapsed time required (when the system is not operational) to repair and restore a system to full operating status, and/or to retain a system in that condition. MDT includes mean active maintenance time (M bar), logistics delay time (LDT), and administrative delay time (ADT).[xvi]  In this definition, M bar is the actual hands-on maintenance time [xvii]; LDT is the time maintenance is delayed due to the lack of a spare part, maintenance facilities or equipment; or transportation [xviii]; and ADT is the time maintenance is delayed due to personnel availability, workload scheduling, etc.[xix] It is important to note that implicit to these availability calculations is the assumption that they represent the long-run average performance for a system.

Availability applications

The above measures describe availability from a traditional logistics viewpoint. RBS practitioners have adapted these definitions to support their day-to-day management of reparable item inventory systems, viewing availability as the product of maintenance and supply availability. In later posts we’ll define supply availability from the RBS perspective more precisely, but for the time being a reasonable working definition is “What is the probability that any randomly selected end-item (e.g., an aircraft) in a fleet is not down due to the lack of a spare part.” 
Stockage policy has a dramatic effect upon supply performance, and is how RBS influences the systemwide availability performance measure. Accurately estimating this performance is critical, especially when you are sparing for a small fleet of high demand assets such as the MAFFS II units and their C-130 aircraft. If the fleet has insufficient availability, then missions may be scrubbed and lives and property (in the case of wildfire fighting) will be placed in jeopardy.


On 1 July 2012, MAFFS 7, the call sign for a MAFFS II equipped C-130 from the 145th Airlift Wing at Charlotte-Douglass Airport, NC, crashed while performing its fourth airdrop of the day over a South Dakota wildfire.[xx] [xxi] Four North Carolina Air National Guard crewmembers were killed – Lt Col Paul K. Mikeal, Maj Joseph M. McCormick, Maj Ryan S. David, and SMSgt Robert S. Cannon and two crewmembers were seriously injured (names not released). 
For more information about MAFFS 7, there is a very touching memorial page to this brave crew at . Our hearts and prayers go out to the members, families and friends of this crew.

[i] Skarban, Ann. “MAFFS 2012,” Citizen Airman, Vol. 64, No. 5: 14-17 (October 2012).  Downloaded from on 31 Oct 2012.
[ii] Skarban (p. 14).
[iii] Air Force News Service (AFNS), “Forest Service deactivates C-130 firefighting operations,” 17 Sep 2012.  Downloaded from on 20 Sep 2012.
[iv] AFNS, 17 Sep 2012.
[v]  U.S. Air Force, “Fact Sheet: Modular Airborne Fire Fighting System,” (undated) downloaded from on 13 July 2012.
[vi] USDA Forest Service.  Large Airtanker Modernization Strategy.  17 Jan 2012. (p. 2)  Downloaded from on 26 Aug 2012.
[vii] USDA Forest Service (p. 3)
[viii] Gruver, Mead, “Some experts worry about a lack of parts for C-130 wildfire sprayers,” The Seattle Times, 7 July 2012.  Downloaded from on 5 Sep 2012.
[ix] Gruver, 2012.
[x] Ebeling, Charles E. An Introduction to Reliability and Maintainability Engineering. Long Grove, IL: Waveland Press, Inc., 1997. (p. 255)
[xi] Blanchard, Benjamin S.  Logistics Engineering and Management (Second Edition).  Englewood Cliffs, NJ: Prentice-Hall, Inc., 1981. (p. 66)
[xii] Blanchard (p. 400)
[xiii] Department of Defense. DOD Guide for Achieving Reliability, Availability, and Maintainability. Office of the Under Secretary of Defense (Acquisition, Technology and Logistics).  Washington: 3 August 2005. (p. 3-8)
[xiv] Blanchard (p. 67)
[xv] DoD RAM Guide (p. 3-11)
[xvi] Blanchard (p. 46)
[xvii] Blanchard (p. 45)
[xviii] Blanchard (p. 46)
[xix] Blanchard (p. 46)
[xx] Church, Aaron M., ed. “Four Airmen Die Fighting Western Wildfires,” ­Air Force Magazine, Vol. 95, No. 8: 10 (August 2012).
[xxi] Schogol, Jeff, “Legacies remain for airmen killed in C-130 crash,” Air Force Times, 16 Jul 2012, p. 16.
[xxii] Air Force Reserve Command, downloaded from  on 12 Sep 2012. 
[xxiii] Air Force Reserve Command, downloaded from  on 12 Sep 2012.