An aerospace engineer shows how match can forecast school bus service requirements, and save time and money in the process

Case Study
A school district in north central Indiana covers 160 square miles with 275 active units. Each bus logs an average of 10,732 miles per year. The buses, by law in Indiana, are rotated out of the fleet at 10 years old. The average number of miles the district logs in a year is 275 times 10,732, which equals 2,951,300. The average service life of a school bus in this district is 10 times 10,732, or 107,320 miles.

According to a mechanic in the district, four of the failure modes experienced by the district are wipers, headlamps, fuel pumps and transmissions. A wiper is changed, on average, once every 1,000 miles, as are headlamps. Fuel pumps fail about every 80,000 miles, and transmissions last about 100,000 miles. It takes about an hour to change a headlamp or a wiper, if we round up the service time. It takes about eight hours to change a fuel pump and three days to change out and/or rebuild a transmission.

Based on just this information casually gathered we can make the following calculations:
Major Failures
?w = failure rate for wipers = 1 per every 1000 miles = .001
?h = failure rate for headlamps = 1 per every 1000 miles = .001
?f = failure rate for fuel pumps = 1 per every 80,000 miles = .0000125
?t = failure rate for transmissions = 1 per every 100,000 miles = .00001

Restoration Time
Wipers = 1 hour
Headlamp = 1 hour
Fuel pump = 8 hours
Transmission = 24 hours (8 to remove, 8 to replace and assuming 8 to rebuild)

MTTR = 2340/2022.5 = 1.157 hours required on average to restore a bus to the fleet

MTBF system = 1/nw + nh + nf + nt

MTBF system = 1/(.001 + .001 + .0000125 + .00001) MTBF system = 1/.0020225 = 494.44 The mean time between failures for a bus in this system is 494.44 hours. The mean time to restore a bus to service is 1.157 hours. Mattis is the quality manager at Honeywell Aerospace in South Bend, Ind., and is currently enrolled in a PhD program in technology management at Indiana State University. He is a certified quality manager, quality engineer and quality auditor and a certified Six Sigma Black Belt.

Reprinted from the March 2007 issue of School Transportation News magazine. All rights reserved.