In Part 1 our fuel control, failure mode of flameout was discussed and it was documented that using the Weibull Distribution reduces the number of failure modes by a considerable margin. This equipment condition is a safety-off-light, so finding a solution as quickly as possible became priority # 1. The process started out with 165 failure modes and was reduced to 7, and as stated these 7 failure modes are the start of the analysis process. You may perform RCM (Reliability Centered Maintenance), RCA (Root Cause Analysis), R&M (Reliability and Maintenance), and other processes that lead to a solution or more than one solution in order to improve you product's reliability for the least cost. Since leaks was the primary failure mode, engineering found that the internal Seals were the root cause, so for $15.00 in parts the fuel control started the seal replacement Jan. 2009. Also discussed was the requirement to monitor your Legacy System or Components, this document shows the reduction in leaks, and also showed there were other growing problems (we call these hidden failures because the primary concern was the failure mode of flameouts).
The 2011 article showed that there were 165 possible failure modes for IFA events. Using the Weibull distribution, this was reduced to seven. This allowed the support team to arrive at a solution of changing out the fuel control seals at a cost of about $20 each.
It is evident that the change to the FC has worked. At this time, approximately 63 percent of the FC population, which is 1,650 units in our inventory (US Navy and Marine Corps), has incorporated the change. The model chosen is the Crow-AMSAA, IEC-61164 reliability growth management, specifically with the BETA (slope) being used for comparing baseline (Jan 03 - Dec 08; 72 months before technical change) to post baseline (Jan 09 - Jun 12; 42 months after technical change). In the 2011 Uptime article, it was stated that legacy systems and components must be continuously monitored to detect changes in FMs not only for monitoring the effects of an incorporated change, but to determine if other FMs are "moving to the forefront" in systems reliability and operational availability. When your primary FM effect is reduced, there is a secondary FM that always appears. There is a change in IFAs, as will be seen, and these causes are under investigation, but will probably result in about 80 percent of the internal piece parts changed out*.
The above FMs were combined and resulted in FM 690-Flameouts caused by Fuel Control (FC) internal failure and 177-Fuel Flow Incorrect. Engineering also came to the conclusion that 80 percent of the internal part of the fuel control must be replaced. This study will continue through the end of 2012 and hopefully a First Article Testing (FAT) will be ready in the first half of 2013.
Figure 1: Baseline FM 690 and post change FM690 in a log-log X-Y plot.
Figure 2: The International Electrotechnical Commission(IEC)-61164 reliability growth model; notice the change in BETA, there is a 22.24% change, with less than 65% of the Fuel controls the seals changed out. This is a 22.24% decrease in in-flight aborts.
Figure 3: A log-log X-Y plot with the timeframe divided into four parts; look very closely and you can detect the change in the number of in-flight aborts.
Figure 4: The IEC-61164 model for the data in figure 3 - notice the increase in BETA as marked on the figure.
As previously mentioned, the fuel control flameouts were greatly reduced by our changing of the seals, however, along with the three other FMs, there is another one that is happening now: FM 037-Fluctuates, Oscillates. Flameouts reduce the number of operating engines from two to one, and while the Air Force has 10,000 feet of runway, a Navy aircraft carrier has 200 to 500 feet to stop an aircraft going 230 knots to zero.
It was proven that changing out the fuel control seals reduced our #1 safety issue -- engine flameouts -- and also reduced other failure modes. We also determined that:
It is important to verify that the change to a component is working. If the change is having no impact, or has degraded system or component performance, it is imperative that this is detected very early, if not, then time and money are wasted, and the operational availability continues to degrade, which is not good.
Look for changes in failure modes, but most of all ensure that your safety issues are resolved. Our fuel control flameouts have been greatly reduced, thus addressing our #1 safety issue.
Legacy systems always have a primary and secondary failure mode, so when the primary failure mode is resolved, the secondary failure mode is now the primary.
Legacy systems require constant reliability and performance monitoring.
Figure 5: Removals and a Figure-Of-Merit (FOM) Mean Engine Flight Hours Between Removals (pronounced MEEE FFEERR); the generally accepted MEFHBR for the fuel control was between 825 and 900, here it is barely making the 800 mark.
* The replacement of the internal parts will probably start in mid to late 2013.
Technical Advisors: Dr. Robert B. Abernethy, Mr. James (WES) Fulton, Mr. Paul Barringer
Tyson, Larry. "Improving the Reliability of a Turbofan Jet Engine." Uptime magazine June/July 2011, pages 48-51.
"THE NEW WEIBULL HANDBOOK" by Dr. Robert B. Abernethy;
Weibull Software SUPERSMITH (SS) by Mr. Wes Fulton;
PLAYTIME with SUPERSMITH, by Wes Fulton and various examples by Dr. Bob, Wes and Mr. Paul Barringer;
Barringer, Paul. "Predict Failures: Crow-AMSAA 101 and Weibull 101." www.barringer1.com/pdf/Barringer-Kuwait-1.pdf
International Electrotechnical Commission (IEC) Standards used in Reliability Engineering: www.iec.ch
IEC-61164-Crow/AMSAA (C/A) Reliability Growth
Larry Tyson, retired, has spent 24 years in the U.S. Naval Service. Currently working for government service, his tasks include: support equipment specialist, involved in LIFE-CYCLE-COST ( LCC ), and reliability and maintainability (R&M) for avionics, support equipment, airframes, hydraulics, and power plants, both propeller and jets. Larry is involved in the TURBOFAN Community with a concentration on R&M analysis, RCM, and RCA.
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