Surveying the Vital Signs (Part 2)

By Bill de Decker March 2001 Last month, we talked about the basic numbers we got from a survey of 2000 business jet operators in the US about the reliability and availability they are getting with their aircraft. The 233 operators of 353 aircraft responded that their average maintenance dispatch reliability, as measured by flights cancelled for maintenance reasons divided by total flights, was about 99.6%. The operators also reported that each aircraft spent an average of 25.2 days per year down for maintenance. This equates to about 5.75 days per 100 flight hours. This down time translates into a significant cost for the operators because fixed expenses accrue whether the aircraft flies or not. Charter operators face the additional loss of revenue incurred when the aircraft is down for maintenance. This month, we will take a look if there are factors such as aircraft age, annual utilization, maintenance approach and reliability/availability tracking that influence reliability and availability. First let’s look at the impact of annual utilization on availability and reliability as illustrated in the following table: This shows that there is a difference in reliability between aircraft that are flown more than 250 hours per year (99.6% to 99.7%) and those that are flown less than 250 hours per year (99.3%). At first glance this may seem like a very small difference, but in fact the difference is significant – the aircraft that flies less than 250 hours per year experiences a flight cancellation for maintenance reasons every 140 departures. The higher utilization aircraft (over 500 hours per year) will experience a flight cancellation for maintenance reasons only once every 333 departures! The second noteworthy item is the difference in maintenance down days for either unscheduled or all maintenance. The more the aircraft is flown, the less maintenance days per 100 flight hours are required. Part of this is certainly explained by the fact that a number of maintenance actions (particularly airframe annual and heavy maintenance inspections) are dictated by calendar time, not flight hours. This has a significant impact but does not explain the increase in unscheduled maintenance days by a factor of almost 4. One reason is, without a doubt, that high utilization operators are much more conscious of maintenance down days and thus will focus much harder on minimizing these lost days. In any case, both the reliability and the availability numbers indicate there is a lot of truth to the old saying that aircraft are more reliable and require less maintenance when they are flown a lot. The next area we looked at was the impact of age. This is shown in the table on the top of page 64. A look at this table shows that reliability is not very sensitive to age of the aircraft. The total variation is between a dispatch rate of 99.5% (one cancellation for every 200 departures) and 99.7% (one per 333 departures). First, this shows that well-maintained old corporate aircraft are in fact very reliable. This implies that reliability is a primary focus of aircraft maintenance, and the price that is paid is that the maintenance required to achieve this high reliability increases sharply with age. The maintenance days per 100 flight hours have not been adjusted to normalize for the differences in utilization, but the maintenance days for 1 – 2 year old, 16 – 20 year old and 26+ year old aircraft can be compared directly because the average utilization is very close for these three age groups. This shows that maintenance down days per 100 hours increase as follows: In other words, unscheduled maintenance almost doubles when the aircraft is over 26 years old and total maintenance days increase by a factor of 2.6! This is the same trend as shown by the maintenance aging curve our company developed some time ago. This is not unexpected, since the more maintenance is required, the more time (days) are required to accomplish the required maintenance. We also took a look at differences between makes and models to see if there are any that are significantly different. Almost all makes and models in current use were represented in the responses, but, unfortunately, there were not enough responses to draw meaningful conclusions for specific makes and models. However, what is significant is that while there are apparent differences between various makes and models in reliability and availability, these differences are largely within the variations expected due to age and annual utilization. The survey included a number of questions about each operator’s maintenance approach. The first of these questions asked if the operator used a Guaranteed Maintenance Program for the engines, such as provided by engine manufacturers Honeywell and Pratt & Whitney Canada and independent JSSI. The results showed that somewhat over half of the operators subscribe to these programs. The survey data shows that use of these programs has little or no impact on the annual utilization, reliability or availability. This is somewhat surprising since one of the goals of these programs is to increase availability by making sure that loaner engines are available when required. The implication of the data is that loaner engines are readily available for all operators that need them. If that is the case, the primary benefit operators would get from these programs is as a budgeting, spares management and planning tool – a significant benefit, but one that does not change the maintenance that is required and does not impact the numbers in this survey. The second of these questions concerned the use of Continuous Airworthiness programs. These programs are designed to allow operators to divide most inspections into small segments that can be done overnight or over a weekend. The goal is to increase availability of the aircraft without impacting reliability. The survey shows that about 114 operators with 199 aircraft use these programs. And these operators report a higher average utilization, lower maintenance days per 100 flight hours and the same dispatch reliability, as shown above in Figure 1. In short, the numbers support the value of these programs for operators who want to increase utilization. The last question we asked was whether the operators tracked and measured availability and reliability. The responses indicate 85 of the operators (a little over one- third) have a tracking/reporting system in place. Their responses are shown in Figure 2. This shows that operators who measure and track reliability and availability have higher utilization, fewer unscheduled maintenance days per 100 flight hours, fewer total maintenance days per 100 flight hours and somewhat better reliability. Eloquent testimony indeed for measuring and tracking these vital signs of your operation!

Next issue, we will talk about how to set up a measuring and tracking system for these vital measures of your operation’s aviation health and why it is important.