Measuring Reliability and Availability

A survey of 2,000 business jet operators in the US about the reliability and availability of their aircraft showed their average maintenance dispatch reliability was about 99.6 percent, with relatively little variation from the average regardless of age, usage, or maintenance practices. To achieve this reliability, the operators 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. However, the respondents reported significant variations in the number of down days. For example, the higher the utilization and/or the younger the aircraft, the lower the maintenance days per 100 flight hours.
The importance of reliability in our business is self-evident. Down time translates into a significant cost for operators as fixed expenses accrue whether the aircraft flies or not. Also, charter operators face the additional loss of revenue when the aircraft is down for maintenance. Yet, without hard data, it is difficult for the manufacturers to measure the impact of any changes in maintenance practices. This leads to the conclusion that tracking these vital signs should be important to every operator. Surprisingly, only 36 percent reported that they do. Perhaps not surprising was that operators who measure and track the reliability and availability of their aircraft have higher utilization and lower maintenance days per 100 flight hours than those who don’t!
Setting up a system to measure and track reliability and availability is fairly straightforward. Let’s take a look at the details.

Measuring Dispatch Reliability
Dispatch reliability is expressed as the percentage of flights that depart within a specified time of the scheduled departure time. The first step is to define the margin. The airlines use a 15-minute margin between actual and scheduled departure time for a flight to be considered as having departed "on time." A number of corporate operators report to using between 15 minutes and one hour as their margin. We recommend 30 minutes. This margin is short enough to not affect passenger schedules too significantly and long enough to fix small mechanical problems, assuming spares and maintenance personnel are on-hand.
The second step is to define and categorize the causes of delays. The airlines define at least three categories:
1. Mechanical (including any maintenance problems)
2. ATC/Weather
3. Other (including lack of catering or crew, ramp exit blocked, etc)
Corporate and charter operations should consider adding a fourth, Passenger, because they are often faced with passengers who do not show up on time — except if passengers call ahead to reschedule.
Defining when the clock starts ticking next. The easiest way is to establish a formal scheduled departure time for each flight, and then score as delayed or cancelled if it is has not closed its door and taxied out under its own power within 30 minutes of that time. The only exception is if the passengers call before the scheduled departure time and reschedule the flight.
The last step is to record information such as all trip legs, scheduled and actual departure time, and, if a problem caused a delayed departure, whether the problem resulted in a delay or a cancellation, whether there were passengers on board, and a detailed description of problem. This step is the hardest for many operators, but it is the one that leads to the payoff — hard numbers and reasons that offer the ability to track, analyze, and improve the reliability of the aircraft.

Measuring Maintenance Availability
Measuring maintenance availability takes a similar approach. The first step is to define availability. If all required maintenance is accomplished and the aircraft is airworthy as defined by the regulations, then it is "available" for flight. The aircraft is "not available" for flight if it is undergoing scheduled or unscheduled maintenance, damage repair, application of an AD, etc. Where it can get confusing is if the aircraft is being maintained or modified during times when the user(s) have indicated explicitly that the aircraft is not required for the organization’s travel. It could be argued that as the aircraft was not required for company travel, the maintenance down days should not be scored as "not available." Taking this approach in effect "hides" maintenance days. The aircraft was in maintenance, but it doesn’t show up on the records. Frankly, this detracts from the value of this analysis tool, as it distorts the true number of maintenance down days. We recommend measuring maintenance availability against the 24-hour-per-day, 365-day-per-year calendar as this is the most comprehensive measure.
The second step is to determine the categories of maintenance down days. We suggest: Scheduled and unscheduled maintenance, AD’s, damage repair, and interior/exterior cosmetic maintenance or refurbishment.
Finally, record the information for each aircraft and each day of the year. Information that needs to be recorded is the aircraft tail number, the number of hours that the aircraft was unavailable, the detailed cause, and whether or not this impacted a scheduled passenger flight.
As with the dispatch reliability, the key to success is to record this information consistently and accurately. The payoff is that you will have hard information to analyze the benefits of switching maintenance schedules, installing optional service bulletins that are supposed to improve maintainability, etc. You will also have the information the manufacturers and vendors need to judge the real-world impact of their maintainability and reliability programs and to make real improvements in these areas.