Synopsis - Aircraft Tire Test Evaluation

Statistical Analysis of Aircraft Tire Test Evaluation Program for Heavy/High Performance Aircraft.

Contents:
	Purpose of Research
	Research Objectives
	Research Design
	Major Assumptions
	Conclusions

Executive Summary

Purpose of Research

The results of this statistical research will assist in the decision-making process to determine which aircraft tire manufacturer will be awarded the annual tire contract for a fleet of heavy/high performance aircraft. The test evaluation engineer desires to determine which tire manufacturer offers a tire that yields the lowest dollar-per-landing cost. The engineer also requires statistical analysis for the number of landings until tire wear-out. Statistical analysis of tire performance will allow the test evaluation engineer to evaluate the overall tire performance and cost effectiveness of both brands of aircraft tires.

Research Objectives
There are three main objectives of the statistical research.
1.	Determine if there is a statistically significant difference between the mean (average) number of landings until tire wear-out for both brands of aircraft tires. This determination will be accomplished through hypothesis testing. Establish a 98% confidence interval for the mean number of landings until tire wear-out for both brands of aircraft tires. Summarize the statistical findings.
2.	Determine if there is a statistically significant difference between the mean (average) dollar-per-landing costs of both brands of aircraft tires. This determination will be accomplished through hypothesis testing. Establish a 98% confidence interval for the dollar-per-landing costs. Summarize the statistical findings.
3.	Provide conclusions about the differences between the mean number of landings until tire wear-out and the mean dollar-per-landing costs for the tire manufacturers.

Research Design

A total of 250 tires were used in the test: 125 from each manufacturer. Approximately half of the manufacturers' tires were delivered to two different test sites. The tires were then randomly installed on aircraft on an attrition bases. Aircraft tire position was not a factor of consideration for this statistical research. In addition to other information, the number of full-stops (landings ending in a complete stop) and the number of total landings (full-stops plus "touch-and-go's") were recorded for each tire after installation. The new tire position and/or aircraft number was then recorded if a tire was removed to facilitate maintenance, or for temporary duty (TDY) purposes. A sum total of the number of full-stops and landings incurred by that individual tire was recorded when the tire was removed.

Tires were removed from the test when they wore out; became cut or chunked; or damaged by other means. The total number of landings incurred at the time the tire was dropped from the test was the principal variable in the statistical research.

The dollar-per-landing cost calculations are based on the total number of landings divided into the cost of a single tire. Comparing the performance (number of landings until tire wear-out) and cost effectiveness (dollar-per-landings) should provide the optimal choice of tire brand.

Major Assumptions
There are three main objectives of the statistical research.
1.	The assignment of tires to bases, aircraft, and aircraft tire position was performed on a random basis.
2.	Other factors influencing the test such as; aircraft loading, flying habits of the pilot, crosswinds over the runways, etc., are random occurrences in nature. Over time, the factors affecting the wear of the tires will vary (i.e. different pilots, various landing locations, changes in aircraft loading). Therefore, it is assumed that variance in these factors will approximate a normal probability distribution over time; not significantly influencing the normal probability distribution of the number of landings until tire wear-out for the entire population of aircraft tires.

Conclusions

There is a statistically significant difference between the mean number of landings until tire wear-out for both aircraft tire brands. The one brand of tire demonstrated a greater number of landings until tire wear-out than the other brand. Both the two-sample t-test and analysis of variance (ANOVA) tests support this conclusion.

Hypothesis testing showed that no statistically significant difference exists between the mean dollar-per-landing costs for both aircraft tire brands. The zero value (the mean difference of zero) fell within the 98% confidence interval (-1.41, 1.44). Both the two-sample t-test and analysis of variance (ANOVA) tests support this conclusion.

ANOVA tests were also completed on both tire brands to determine if there is a statistically significant difference in the mean number of landings until tire wear-out based on the aircraft position of the installed tire. The results of the ANOVA test showed that there is no statistically significant difference in tire wear-out based on aircraft position.

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