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.
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
and multiple-system maintenance practices.
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.
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.
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.
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.
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.
BASIC AND MULTIPLE-SYSTEM MAINTENANCE PRACTICES
Two programs -- resolution of discrepancies and avoidance of multiple
similar system maintenance -- are outlined in AC 120-42A.
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.
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.
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.
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.
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.
EXPANSION IN THE NORTH PACIFIC MARKET" WAS DISCUSSED
NO. 4, OCTOBER 1998. -- ED.
EXPERIENCE WITH ETOPS MAINTENANCE
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
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.
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
philosophy has now spread fleetwide at TWA and is a contributing
factor to the operatorís ability to meet on-time performance
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
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