Maintainability

In the airline industry, maintainability has evolved a high profile. The maintenance costs that result from poor maintainability are increasingly less acceptable to major airline customers than was the case in years past. The airline customer is now demanding very high levels of maintainability in their aircraft. Maintainability is a quality, and it results from system design. Whether the system requires minimal maintenance or is maintained at great cost to the owner is determined during system design. One of the purposes of this section is to demonstrate that maintainability requirements are to be viewed as design constraints on the system and that those constraints must be addressed early in the design process.

Maintainability is basically the idea that a product can be maintained in operational condition. How much effort and how many dollars have to be expended for maintenance is a measure of the product’s maintainability. We can all think of examples of cars we have owned that required different degrees of maintenance. There are perhaps some models we would never buy again just because the cost of maintaining them was too high—their maintainability was so poor. Likewise with the airline operator: If the cost of ownership is too high, the airplane will not be purchased. Enormous changes have occurred in the airline industry in recent years. One result is the current intense focus on maintenance—getting costs down to manageable levels, eliminating unnecessary maintenance, improving maintainability in design, etc.

An airplane flying to four or five other locations before returning to home base could require some maintenance at each of these locations. That meant more spares, more inventory, and more mechanics. The scheduling of flights also became extremely tight, which meant that delays in dispatching the next flight because of maintenance problems became very costly in lost revenues. Another consequence of hub and spoke scheduling is that maintenance access to the airplane became very limited. In the point-to-point scenario, maintenance access between flights could be measured in hours. Now access is limited to fractions of an hour.

With this increase in maintenance costs and constraints, airlines have been very interested in increasing the level of maintainability of the airplane and each of its parts.

An airline’s maintenance program is developed into three areas:

1. 1. Scheduled maintenance
2. 2. Unscheduled maintenance
3. 3. Fixed interval maintenance
4. 1. Scheduled maintenance: Scheduled maintenance is made up of tasks done at regular intervals, called checks. Checks are a packaging of a number of tasks done at regularly scheduled intervals. The basic checks are:
   1. a. Overnight
   2. b. A check
   3. c. C check
   4. 1.a. An Overnight check includes fluid checks (oil, hydraulics, etc.), cockpit maintenance and cabin replacements (blankets, pillows, magazines, cleanup) and the clearing of logbook items. The pilot’s logbook consists of items the pilot wrote down during the day’s last flight when he observed a failure of some kind in the airplane’s systems. Overnights take place when the aircraft is parked about 8 hours. The aircraft is not taken out of service for an overnight check.
   5. 1.b. After Overnights, the next scheduled maintenance is called the A Check. A Checks consist primarily of preventive maintenance tasks and inspections for hidden failures. There are large numbers of potential system failures that are not made known to cabin or flight crew. These have to be looked for and repaired where a failure is found.
   6. 1.c. The next scheduled maintenance is the C Check. Usually 10 A Checks are done between each C Check. A C Check is done at approximately 4800 flight hours or 15 months, whichever comes first. This is a 3- to 5-day out-of-service hold and is the heaviest check done on the airplanes.
      One attractive feature of scheduled maintenance under the FAA rules is that the maintenance tasks may be “escalated” to higher, i.e., longer, intervals between checks. Escalation is permitted when experience has demonstrated a higher than expected reliability. This allows an airline to tailor its maintenance program around operations.

1. 2. Unscheduled maintenance. A significant percentage of total maintenance done on an aircraft is unscheduled. By far the most important occasion for unscheduled maintenance is the turnaround between flights. This is called “turn unscheduled maintenance.” Turnaround or turn refers to the time period between the end of one flight and the beginning of the next. The typical time frame for a turn can vary between 20 minutes for the 100-seat airplanes, to 45 minutes for the 100 + seat airplanes. The actual time for the turn is largely determined by the time needed for refueling. Usually by the time fuel is on board, passengers, pilot, and crew are ready to go. An average figure of 30 minutes is often used as a baseline to measure delays in departure. “Delay” then means any time beyond the baseline 30 minutes plus 15 minutes. This is the basis for dispatch reliability (or dispatchability) goals.
   During a flight the pilot makes notes in his logbook of faults or failures that have been reported to the cockpit electronics from the airplane’s various systems and equipment. It is an FAA regulation that all such logbook items must be “cleared” before the airplane is dispatched on its next flight.
   Clearing logbook items can pose a considerable problem for the airline. Passengers are waiting and delays beyond a few minutes or outright flight cancellation will drive business away from the airline rather rapidly.
   The average 30-minute time frame for a turn is not enough time to repair anything except the most obvious “quick fixes.” Consequently, the solution resorted to most to keep the airplane moving is the MEL, which is an FAA-blessed and airline-engineering produced document that lists what aircraft equipment can be inoperative (and under what conditions) and still fly the airplane safely. The repair of an item on the MEL can be deferred for 24 hours or up to 10 days. And if it cannot be deferred, then the airplane has to be parked until it can be fixed. If the equipment is not listed in the MEL, it must be operative at all times. It is, therefore, of great importance to the airline operator to have as much equipment as possible on the MEL.
2. 3. Fixed interval maintenance: Fixed interval maintenance is a kind of scheduled maintenance but unlike the regular A and C Checks these intervals are usually out of sync with the A and C Checks. The intervals are also FAA mandated. Another disadvantage is that the interval cannot be escalated as the A and C Checks can. Airlines look upon fixed interval maintenance with considerable disfavor, as it disrupts operations. The more equipment that falls into this category, the more often the airplane has to be taken out of service and the greater the potential loss of revenue.

Equipment and systems subjected to fixed interval maintenance are the result of an FAA-mandated Certification Maintenance Requirement (CMR). A CMR is a required periodic task established during the design certification of the aircraft as an operating limitation on it.

CMRs usually result from a formal, numerical analysis conducted to show compliance with catastrophic and hazardous failure conditions as defined by FAR Part 25.1309.

CMRs may also result from properly justified engineering judgment. A CMR is intended to detect safety-significant latent failures that would, in combination with one or more other specific failures or events, result in hazardous or catastrophic failure conditions. A CMR task then is designed to verify that a certain failure has or has not occurred. It essentially “restarts the failure clock to zero” for latent failures by verifying that the item has not failed or causes it to be repaired if it has failed.

Generally, spares are not part of scheduled maintenance, so only unscheduled failures affect their total cost. When a pilot logbook item has to be cleared before takeoff and the system is not on the MEL, replacing the faulty LRU from spare inventory is the only solution.

The following items drive the number of spares and/or the cost of a single spare:

• • Minimum Equipment List (MEL)
• • Mean Time Between Failures (MTBF)
• • Mean Time Between Unscheduled Removal (MTBUR)
• • Mean Time to Repair (MTTR)
• • Noninterchangeability
• • Sole-sourcing
• • Number of route line stations (stops)
• • Transit time for a spare, including customs delay

Despite the complexities of spares provisioning, it is clear that a large percentage of the cost drivers lie within the sphere of influence of the systems engineer/manager. Getting it on the MEL, raising the MTBF and the MTBUR, providing for interchangeability, reducing repair time, listing more than one source are all factors over which the designer has some measure of control.

All airplane systems must be analyzed to determine the ability to dispatch with equipment inoperative. Systems should be designed to maximize the number of pieces of such equipment.

Here are some of the design factors that affect whether or not the equipment is on the MEL:

1. 1. Oversight: did the equipment have the ability to dispatch inoperative but it failed to be included on the MEL? This is just an incomplete job.
2. 2. Redundancy: is there built-in duplication, or multiple or alternative paths? Other ways to perform the same function are often necessary for inclusion on the MEL.
3. 3. Simplicity: is the system so complex that a dispatch inoperative analysis is difficult, if not impossible? Simple systems are easier to defer than complex ones.

Airline customers are demanding higher maintainability of their aircraft. To remain competitive airplane OEMs must respond quickly and positively. To sell an aircraft, the cost of spares required must be kept as low as possible: 20% of purchase price is far too high.

The systems engineer needs to focus on these factors during the initial design phase. Scheduled maintenance is driven primarily by the hidden failure. The high number of CMRs are normally driven by design oversight and incomplete, inaccurate, or done-after-the-fact fault tree analyses. Many of the maintenance cost drivers lie within the sphere of influence of the systems engineer.