Recently, there have been a lot of published comments about flight delays and the passengers’ reactions to them. Those getting the best coverage  have been related to weather. Actually, if it were not for the design actions to improve reliability and airline logistics planning to carry out the hours of required maintenance in a non-intrusive manner, there would be a lot more delays, and passenger complaints.

When anyone goes to the airport to board a flight they expect to find the airplane at the other end of the loading ramp ready to go.  It is usual for an arriving passenger to be able to look out the waiting area window and see the cockpit crew busy at their preflight tasks in the cockpit, and various and sundry personnel busily moving around under the airplane. It is assumed that the airplane will be ready to take off to your destination as soon as everyone is on board. In almost every case this will happen as planned because there was much care in the design, manufacture, and maintenance of this airplane to assure that it is ready to fly. I will call the probability that the airplane will take off as planned and complete the flight to its destination Mission Effectiveness.

This Technical Note discusses the Reliability, Maintainability and Logistics Engineering activities that go into an airplane’s development cycle, and in the airline’s operations planning  to assure a high level of Mission Effectiveness in airline type operations.
Mission Effectiveness is the term used for the probability that an airplane planned to be used for a particular flight (mission) from point A to point B will depart on time and continue to function properly until it arrives at the desired destination.
An airplane is composed of thousands of parts that must function in harmony for successful operation.  The probability that these parts will perform their desired function is called Reliability. When a part does not function properly and must be replaced, the replacement activity is called Maintenance. The design discipline that assures that replacement actions for damaged and failed parts is simplified is called Maintainability.  The tasks associated with adding fuel, oil, food, etc. are known as Servicing. The supply of parts and test equipment necessary to make repairs, and the locating of personnel with the required skills can be grouped into the category of Logistics Support. Passenger loading activities and baggage handling are important to on-time departure, but are not discussed here.

RELIABILITY

Reliability can be defined as the probability that the system will function satisfactorily when called upon. The "system" may be an automobile, an airplane, a rocket, etc. made up of many parts intended to function together.  System reliability (Rs) is measured as the product of the reliabilities of the individual parts (Rs  =  R1 x R2 x R3 x……. x Rn). For example, if a system is composed of  ten parts, each having a reliability of 0.999, the system reliability would be  0.990. Modern transport aircraft achieve a mechanical dispatch reliability of approximately 0.98. Thus, for an aircraft with  thousands of parts to have a reasonable probability of mission success, the individual part reliability must be in the range of 0.999997. System and part reliability is often measured in terms of Mean Time Between Failure  (MTBF), or Between Maintenance (MTBM). MTBF is  computed by dividing the sum of all respective operating times to failure  by the total number of failures.

MTBF = Sum of Operating Times Between Failures / Total Number of Failures

MTBM = Sum of Operating Times Between Maintenance Actions / Total Number of Maintenance Actions

The typical reliability concerns for an airplane might be related to probability of no failure that would delay takeoff, or the probability of completing a flight mission. without a critical failure that would necessitate an unscheduled landing. Another operating concern is arrival at an airport with no failure that would limit the ability for a timely  dispatch on a subsequent flight. A part failure in flight seldom creates a safety situation necessitating an unscheduled landing. However, engine failure is a consideration in over-water flights and current twin-engine jet transports are limited to routes where they are always within an acceptable distance (currently180 minutes flying time) of a satisfactory airfield. The probability of completing a flight without a failure critical for a subsequent dispatch  is a concern for airline operations into those  airports where it is not feasible to station maintenance personnel or stock spare parts.  A flight critical failure at an airport where there is no maintenance capability would result in a flight delay while maintenance (or a substitute aircraft) is acquired, or "ferrying" the aircraft to a  properly equipped maintenance facility.

Reliability is  achieved by two methods; by selecting parts that have  design features that assure a long life and ability to function under high stress, and/or by arranging parts in multiples (redundancy or back-up) such that failure of one of the parts can be tolerated for that mission. Redundancy is usually applied in electronic design applications.  Back-up would apply in such applications as flight instruments, radios, lights, etc. Landing gears and their tires represent parts that have no back-up and are flight critical. Engines are critical, however operating rules take into account an engine failure on take-off (or in flight) and flight restrictions assure that multi-engine aircraft can continue take-off or flight after the loss of power on one of the engines.

Maintainability

Thousands of man hours are expended in maintaining a fleet of airplanes. The airplane’s structure, all of the mechanical parts, and many of the electrical parts are maintained on a scheduled basis. This is called scheduled maintenance or routine maintenance. When parts fail and must be replaced on an as-required basis, this is called corrective maintenance. Much effort is expended during an airplane’s design to simplify maintenance. Design considerations include ease of access, standardized tools, positive fault isolation, and design of large parts (e.g., flight control actuators, engines, etc.) so that the higher failure rate components can be replaced without removing the major part. Simplified testing and remove-and-replace tasks help assure low maintenance costs and are even more important in assuring minimum downtime (schedule delay) during corrective maintenance.

Maintainability performance is defined as "The measure of the ability of an item  to be retained in or restored to a specified condition when maintenance is performed by personnel having specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair."  Maintainability performance can be measured in several ways; Mean Time to Repair (MTTR), Maintenance Man Hours per Operating Hour (MMH/OH), Maintenance Down Time per operating interval (MDT), and Probability of Repair (PR). Availability (A) is another performance measure. The equations for each of these are:

MTTR = Total Average Maintenance Time / Total Average Maintenance Actions
MMH/OH = Total Maintenance Man Hours / Total Operating Hours
MDT = Sum of (Maintenance Actions x  each action’s down time)
PR = Relationship of the distribution of all repair times for an item to the
                Time available before a mission delay will occur.
       A = MTBF / (MTBF + MTTR)

The author has advocated the term, Operational Availability (Ao) to introduce the Maintainability performance parameters into readiness predictions.

 Ao = Ro + (1 – Ro) x M              where

 M =  Pr  x  Pp  x  Pd  x  Ps           and

 Ro      Probability that equipment is in an unfailed state
 M       (Maintainability factor) Probability that failed equipment can be repaired
in the time available.
Pr       Probability that the design permits a repair/replace time less than the time available.
 Pp      Probability that the required number and skills of personnel are available.
 Pd      Probability that the failure is detectable/detected in time to permit correction
 Ps       Probability that supply support is in place (proper spares and tools vailable)

This equation illustrates the complex considerations of design (Ro, Pr and Pd) and logistics support aspects (Pp and Ps).

Figure 1 illustrates a logic model for simulating the considerations for Operational Availability.

Figure 2  shows a plot of frequency distribution for maintenance times, assuming that the times are log-normally distributed. This might correspond to the historical data for 100 remove and replace tasks for a single part or for a random 100 tasks for the total airplane. The area under the curve to the left of the value for time available (percentage of total time) represents the probability that the repair can be performed within the time available (Pr). This probability can be read directly if the maintenance data are plotted on log-probability paper, Figure 3.

Figure 4  is a table that shows the number of parts that must be kept on hand to assure that  sufficient spare parts are available, for several probability levels (Ps). Part cost considerations enter into the risk assessment evaluation in determining a desirable stock level.

Logistic Support

Logistic Support  includes all aspects of selecting spare parts,  equipment, and trained maintenance personnel, and locating  these in proper quantities to best support the planned operation of the airplane(s).  This becomes a very complex task. Predicted life or failure rates (MTBF and MTBM), along with allowances for prediction accuracy are the basis for estimating quantities.  If the part is to be stocked at various locations, the scheduled maintenance at that location and flight frequency into the station are considered in selecting quantities. Aircraft flying time, scheduled maintenance locations, planned methods for correcting emergency repairs are all part of the analysis.