Aero 07 - ETOPS Maintenance

Extended-range twin-engine operations (ETOPS) have become common practice in commercial aviation over the last 15 years. Maintenance and operational programs for the twinjets used in these operations have received special emphasis, and reliability improvements have been made in certain airplane systems. Many operators are now considering the merits of the ETOPS maintenance program for use with non-ETOPS airplanes.

An increasing number of operators are now providing ETOPS service to their passengers. For example, 76 percent of 767 operators and 42 percent of 757 operators are flying ETOPS routes. Several operators have discovered that the cost of ETOPS maintenance, compared to its benefits, also offers them a significant cost advantage when flying their non-ETOPS routes and when operating their non-ETOPS airplanes.

In 1953, the United States developed regulations that prohibited two-engine airplanes from routes more than 60 min (single-engine flying time) from an adequate airport. These regulations were later formalized in U.S. Federal Aviation Administration (FAA) Federal Aviation Regulation 121.161. The ETOPS program, as outlined in FAA Advisory Circular (AC) 120-42A, allows operators to deviate from this rule under certain conditions. By incorporating specific hardware improvements and establishing specific maintenance and operational procedures, operators can fly extended distances up to 180 min from the alternate airport. These hardware improvements were designed into Boeing 737-600/-700/ -800/-900 and 777 airplanes.

The ETOPS maintenance approach that can be applied to all commercial airplanes includes

Engine health monitoring.
Predeparture service check.
Basic and multiple-system maintenance practices.
Event-oriented reliability program.

1. ENGINE HEALTH MONITORING
ETOPS maintenance procedures were created to ensure the safety and reliability of flights operating at extended distances from alternate airports and to prevent or reduce the probability of a diversion or turnback with one engine out. These maintenance procedures are equally effective for any commercial airplane with any number of engines. Most traditional maintenance programs are based on regularly scheduled preventive maintenance and on the ability to predict or anticipate maintenance problems by studying failure rates, removal rates, and other reliability data. However, the ETOPS philosophy is a real-time approach to maintenance and includes continual monitoring of conditions to identify problems before they threaten airplane operation or safety.

Two items in the ETOPS maintenance program that best illustrate this real-time approach are oil consumption monitoring and engine condition monitoring.

Oil consumption monitoring.
A typical maintenance program requires checking engine oil before every flight (but only once each day on the 737, as approved by the FAA) and the auxiliary power unit (APU) oil less frequently (such as every 100 hr). The quantity of oil added and flight hours for each leg should be noted in the maintenance logbook.

The oil consumption rate, the amount of oil used per hour of operation on the previous flight leg, should be calculated for both engines and the APU during ETOPS before dispatch. The resulting number provides a better indication of oil usage or loss than the quantity of oil added. If the rate is acceptable, the flight can be released; if not, the cause of the increased usage must be addressed before dispatching the airplane on an ETOPS flight. This increase can frequently be caused by an oil leak, which is easy to detect and repair.

The consumption rate data is also logged to track long-term variations in consumption rates (fig. 1). This allows the operator to determine if problems are developing so they can identify and implement solutions before serious engine or APU degradation occurs.

Engine condition monitoring (ECM).
For many years, ECM computer programs have been available for all engines used on Boeing airplanes. The engine manufacturer supplies ECMs to help operators assess the general health of their engines. The programs allow for monitoring of such parameters as N1, N2, exhaust gas temperature, fuel and oil pressures, and vibration (fig. 2). Most operators use an ECM program regardless of whether they fly ETOPS routes. ETOPS operators are required to use ECMs to monitor adverse trends in engine performance and execute maintenance to avoid serious failures. These failures could cause in-flight shutdowns, diversions, or turnbacks. In some cases, oil consumption data and ECM data can be correlated to define certain problems.

2. PREDEPARTURE SERVICE CHECK
FAA AC 120-42A requires certain ETOPS systems to be checked before each flight. Boeing determined that the transit check in the maintenance planning data document was sufficient to meet the AC requirement. This is because certain systems relating to ETOPS were redesigned for greater reliability and dispatch requirements were altered for ETOPS (e.g., minimum equipment list requirements). However, because of the oil consumption monitoring requirements for ETOPS, the APU check interval on the 737, 757, and 767 was changed to the transit check for ETOPS airplanes. The engine oil servicing interval changed only on the 737. These two changes and the calculation of consumption rate are the only changes necessary to the standard transit check to form the ETOPS predeparture service check.

3. BASIC AND MULTIPLE-SYSTEM MAINTENANCE PRACTICES
Two programs -- resolution of discrepancies and avoidance of multiple similar system maintenance -- are outlined in AC 120-42A.

Resolution of discrepancies.
This program requires items that are repaired or replaced to be checked for proper installation and operation before the work is signed off on the maintenance log. This ensures that the item is actually fixed and that no new problems were introduced during maintenance. This maintenance practice is applicable to all airplanes.

Avoidance of multiple similar systems maintenance.
Maintenance practices for the multiple similar systems requirement were designed to eliminate the possibility of introducing problems into both systems of a dual installation (e.g., engines and fuel systems) that could ultimately result in failure of both systems. The basic philosophy is that two similar systems should not be maintained or repaired during the same maintenance visit. Some operators may find this difficult to implement because all maintenance must be done at their home base.

However, methods exist for avoiding the problems that may be introduced by working on two similar systems simultaneously (see "Multi-Engine Maintenance" in Aero no. 5, January 1999). For example, different personnel can perform the required work on the similar systems, or they can ask each other to review the work done on each system. If the systems are checked after performing maintenance according to the resolution of discrepancies program, any problems introduced during maintenance should be identified and corrected before releasing the airplane for flight.

4. EVENT-ORIENTED RELIABILITY PROGRAM
An event-oriented reliability program associated with ETOPS differs from conventional reliability programs, which rely on historical data or alert levels to determine when an item should be investigated for possible corrective action.

In an event-oriented reliability program, each event on an ETOPS-significant system is investigated to determine if a problem could be reduced or eliminated by changing the maintenance program. Examples of events include a failure, removal, or pilot report. Events can also be monitored to detect long-term trends or repeat items. Not all events warrant such detailed investigations; continual monitoring and awareness of problem areas reflects the ETOPS real-time maintenance philosophy.

SUMMARY

Although three- and four-engine Boeing airplanes (as well as some earlier 737s) are not specifically designed or improved for ETOPS, the ETOPS maintenance approach can be applied to those airplanes and offer operators significant improvements in reliability, performance, and dispatch rates. The approach can be applied at minimal cost, which can later be offset by reduced maintenance costs and other costs associated with diversions or turnbacks.

"ETOPS EXPANSION IN THE NORTH PACIFIC MARKET" WAS DISCUSSED IN AERO NO. 4, OCTOBER 1998. -- ED.

OPERATOR EXPERIENCE WITH ETOPS MAINTENANCE

United Airlines (UAL) maintains all its 767 airplane engines and auxiliary power units (APU) in ETOPS configuration, regardless of whether the airplane is used in ETOPS flights. This provides greater maintenance flexibility and reduces the need to carry different engine configurations to supply both ETOPS and non-ETOPS fleets.

Since 1990, UAL has experienced a constant decrease in its in-flight shutdown (IFSD) rate (fig. 1). By 1991, UAL achieved a level of reliability that permitted qualifying its 747-100s for ETOPS if necessary. By applying ETOPS maintenance techniques to both engines and other systems on non-ETOPS airplanes, UAL increased the overall reliability of its 747 fleet and 767-200s (fig. 2).

Trans World Airlines (TWA) recognized the benefits of ETOPS awareness to maintenance early in its experience with the 767. Many aspects of the ETOPS programs were incorporated into its non-ETOPS fleets. Line mechanics working on 747s also received 767 ETOPS training; as a result, TWA saw an improvement in its 747 dispatch rate.

The ETOPS philosophy has now spread fleetwide at TWA and is a contributing factor to the operator�s ability to meet on-time performance goals.

The TWA 767 and 757 airframes and engines are all maintained in ETOPS configuration. As with UAL, TWA sees an advantage in operational flexibility, purchasing spares for a reduced number of configurations, and standardized training and documentation for the ETOPS approach.

The number of other operators who have realized benefits from adopting the ETOPS approach to maintenance continues to grow.

HARRY KINNISON, PH.D.
ETOPS MAINTENANCE PROGRAMS
MAINTENANCE AND GROUND OPERATIONS SYSTEMS
BOEING COMMERCIAL AIRPLANES GROUP