We Survey the Vital Signs

We Survey the Vital Signs...

Reliability and availability are major factors in aircraft economics

Bill deDeckerBy Bill deDecker

February 2001


Very little data for GA
Unfortunately, there is very little data on how general aviation operators maintain their aircraft. Individual aircraft and engine manufacturers, as well as operators track data that is important to them, but almost no statistical data is available for the operators as a group. To start getting some answers, we worked with a graduate student at Embry-Riddle Aeronautical University to obtain some basic data by means of a survey. The goal of the survey was to find out from a representative cross section of operators how they rate the reliability of their aircraft (as measured by cancelled flights) and how they rate the availability of their aircraft (as measured by days used for scheduled and unscheduled maintenance). In addition, we asked how they maintained their aircraft, their annual utilization and the make/model and age of the aircraft to see if there are any strong correlations between these factors and the basic reliability and availability data. Lastly we asked if the operators have a formal system of tracking and reporting reliability and availability.
Number of operators 233 Multiple aircraft operators 73 Largest fleet size 6 (4 0perators) Aircraft operated 353 Average age of fleet aircraft 12.35 Years Average flight hours per year 438 Hours Average hours per flight 1.38 Hours Figure 1

The survey was sent to about 2,000 operators of corporate jet aircraft and we received 233 responses. This 11.5 percent response rate is typical for surveys in this industry and the results are fascinating. First, let’s look at a profile of the operators that responded (Figure 1).
The aircraft operated covered almost all business jets and range from 30+ year-old Sabre 40’s, Learjet 25’s and Falcon 20’s to some brand new Gulfstream V’s, Challenger 604’s and Learjet 31’s. In short, this sample represents a good cross section of the business aviation community.

A big problem in determining availability and reliability is defining the measurement terms. In the airline industry, there are a number of standard measurements for these Dispatch
Reliability - Mx 99.6 % Maintenance days
per aircraft per year 25.2 Maintenance days
per aircraft
per 100 flight hours 5.75 Figure 2

items, but that is not the case in general aviation. To address this problem, we asked about reliability in terms of "Cancelled Flights Due to Maintenance Problems" in the last 12 months and we asked about availability in terms of "days of down time" in the last 12 months due to scheduled and unscheduled maintenance. We then analyzed the reliability data by dividing the number of cancelled flight by the total number of reported flights to yield a "dispatch reliability" percentage. The availability was analyzed both in terms of total number of maintenance days per year and in terms of maintenance days per 100 flight hours. The latter measure was used to allow direct comparison between aircraft with different annual utilizations. Based on these admittedly broad terms, the operators reported the data in Figure 2.

Deciphering the results
What this means is that about one flight in 200 is cancelled for maintenance reasons. Given an average flight length of 1.38 hours and average annual utilization of 438 hours per year, another way to express this statistic is that the average corporate aircraft experiences between one and two cancellation for maintenance reasons per year. It is also interesting to note that this reliability percentage (99.6 percent) is very similar to the reliability figure the airlines target for their aircraft.
The results for maintenance days per year or per 100 flight hours reveal that it takes a lot of maintenance down time to operate these aircraft. The maintenance itself costs a lot of money, but the maintenance down time is also expensive in another way — fixed costs, such as insurance, salaries, overhead, finance/lease payments, depreciation, hangar rent, etc. are incurred regardless of whether the aircraft is flying or down for maintenance. At the same time, the operator is not getting the benefit of why the aircraft was acquired (nobody buys an aircraft so that it can be in the hangar!). This applies to any aircraft operator and can involve substantial sums. Figure 3 shows the cost per day (based on a 365-day year) and per year for these fixed costs, based on the fixed cost shown in our Aircraft Cost Evaluator using market depreciation.
For charter operators, there is an additional cost – the revenue lost during the down time. This is equally substantial, as shown in Figure 4. The numbers in this table are based on two charter hours per day at a typical rate for each class aircraft.

Maintenance down time can be managed
In short, the cost of maintenance down days is very substantial whether for a corporate operator or for a charter operator. Airlines have been aware of these costs for a long time, particularly the cost of lost revenue. For that reason, they measure availability carefully, make extraordinary efforts to keep the aircraft in service, and work with the airframe and engine manufacturers to minimize the number of days aircraft are out of service for maintenance.
Our conclusion is that this data shows that corporate aircraft have excellent reliability but that putting a sharp focus on availability will pay handsome dividends. Specifically, it is worth the effort for operators to start tracking both reliability and availability and discuss with the manufacturers possible ways to maintain high reliability while decreasing the maintenance down days.

Next issue, we will take a look at factors such as aircraft age, annual utilization, maintenance approach and reliability-availability tracking that influence reliability and availability.

Fixed Cost Incurred During Maintenance Days
Cost per Day
Cost per Year
(25.2 Maintenance Days/Yr)
$1,500
$37,800
$2,750
$69,300
$4,100
$103,320
$5,150
$129,780
$6,650
$167,580
Figure 3
Charter Revenue Lost During Maintenance Days
Revenue Loss per Day
Loss per Year
(25.2 Maintenance Days/Yr)
Figure 4

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