ETOPS Comes of Age

In recognition of growing interest in twin engine long range operations, particularly in the North Atlantic region, the FAA undertook an effort to thoroughly define and standardise requirements for gaining approval for operations beyond the 60 minute threshold. Draft FAA Advisory Circular 120-42 was released in November 1983, with the final version being approved in June 1985. This advisory circular resulted from the involvement and input of airlines, manufacturers, pilot organisations, regulatory authorities and others. Parallel and follow-on efforts by other regulatory authorities resulted in release of similar guidelines by the UK, Canada and the European Joint Aviation Authorities (JAA), as well as others including Australasia.

The original release of Advisory Circular 120-42 provided for diversion times of up to 120 minutes with an allowable operational extension to 138 minutes. A later revision, 120-42A, released in December of 1988, provided for diversion times of up to 180 minutes from an adequate airport. Diversion time has become a central theme with regard to discussion of the viability of extended range twin engine operations (ETOPS). It first of all determines the routes that are available for flights conducted under ETOPS rules. Further, it is a factor that must be taken into consideration with regard to the safety of such flights.

Those who would argue that increased diversion times have an adverse impact on safety suggest that the resulting increase in exposure to a diversion during the longer ETOPS portion of the flight, and corresponding potential increase in single engine time to an alternate, have significant adverse effect on the overall safety of such flights. It is important to note, however, that increased diversion times open up new route opportunities and inevitably increase the number of ETOPS flights. Some experts argue that increased diversion times have benefit in that they result in more optimal flight planning (i.e., shorter flights) and offer flight crews more options should a diversion become necessary.

It is the author’s opinion that when such operations are properly controlled and conducted, extended range twin engine operations are safe.

Effects of Diversion Times

Under a 60 minute rule for maximum diversion time few route opportunities exist, particularly over the Atlantic and Pacific. Operations from North America to Europe are possible, but severely constrained, forcing any flight far to the north and significantly increasing trip length and time while severely limiting alternates available for diversion. Increasing diversion time to 120 minutes provides benefits in terms of increased route opportunities, as well as in the number of alternates available for diversion, particularly in the North Atlantic region. However, opportunities in some areas, such as the South Pacific, are still significantly limited at 120 minutes diversion time.

Extending diversion time to 180 minutes opens the vast majority of the world’s air routes to ETOPS and maximises the number of available alternates. The benefits of increased diversion time with regard to route planning can be illustrated by comparing the route based on a 60 minute rule with one resulting from a 120 minute rule for a flight from the Seychelles to Singapore. Application of the 60 minute rule results in a far more complex route that is also more than 40% or 1200 nautical miles (2225km) longer than a route derived under a 120 minute rule. Further, the route based on the 60 minute rule shows little advantage in alternate options and may, at times, cause the flight to be conducted in an area where en route weather is worse.

Another useful example is the route based on the great circle track from New York to London. This route illustrates the benefit of alternate options available with increased diversion time, particularly when the diversion time is increased to 180 minutes. Under the 120 minute rule only one alternate is available at just past the midpoint of the trip, while under the 180 minute rule, eight alternates are available. One must also remember that the 120 minute diversion time would probably not result in the trip being cancelled if the one alternate is unavailable to a 120 minute operator. The operator would alter the route of flight enough to remain within 120 minutes of another alternate and still at times have few other options for an alternate. In contrast, a 180 minute operator could remain on the simpler great circle track and have the same alternate options available to the 120 minute operator, plus many others.

The benefits of increased diversion time are real and of advantage to both the airline and the pilots, and many airlines continue to seek increased diversion times whether or not they have near term plans for new routes that require 180 minutes.

ETOPS Data

Since approval of the first airframe and engine combination (767/Pratt & Whitney PW4000) in May 1985 using the then draft advisory circular, the FAA has approved over a dozen airframe and engine combinations of the 737, 757 and 767 for 120 minutes. Airbus too has 120 minute approval for its A300, A310 and A320. All airframe/engine combinations of the 757 and 767 have now been approved for 180 minutes as has the long haul products (A300-600 and A310) of Airbus Industrie. As of early 1994 there were more than 75 operators of Boeing 757s and 767s alone conducting ETOPS under the 120- and/or 180 minute rules. The number of airlines conducting such operations with both Boeing and Airbus twinjets is expected to continue increasing at a rapid rate.

At early 1994, Boeing 757 and 767 operators were conducting more than 10,000 flights per month under ETOPS rules. This ETOPS fleet had accumulated since 1985 some 380,000 flights and 15 million flight hours. The 767 dominates these statistics, accounting for nearly 9000 of the 10,000 flights made every month by Boeing twinjets.

Current Boeing 767 ETOPS are also extensive in terms of the regions of the world in which they are conducted. Operations are commonplace in both the Atlantic and Pacific regions. ETOPS operations of the 757, while less extensive than the 767, are still significant, particularly in the Atlantic region. Airbus A300 and A310 twinjets are also used extensively on the North Atlantic; in fact, more than three quarters of all trans-Atlantic passengers now complete their journey in an ETOPS certificated twinjet. Today, the number of US airline flights made by Boeing and Airbus twins on routes to Europe exceed the collective total flight frequency for all three and four engine airliners operated by US airlines.

To achieve dominance in the highly competitive North Atlantic environment in just a few years is testimony to both the viability and acceptance of ETOPS. While economics plays a major role in the viability of airline operations, no carrier can succeed in such an environment unless acceptance, particularly by passengers, is high. Acceptance by passengers or pilots would not have occurred if ETOPS had resulted in significant problems.

In many respects, the success of such operations is a result of the exhaustive preparation and planning that went into the initiation of ETOPS. Airlines, pilot groups, manufacturers, regulatory authorities and others had significant influence on the policies and practices that were put into place and contributed to enhancements in aircraft design. The 767, for example, was required to undergo several design changes to comply with ETOPS specific design criteria. In addition to a number of changes intended to increase engine reliability to meet inflight shutdown criteria, design improvements were made in the areas of equipment cooling, cargo fire suppression, electrical power and auxiliary power unit (APU) reliability. Of interest is that the reliability of the engines themselves is only one factor in the overall ETOPS equation as the proven reliability of the electrical and hydraulic systems, etc. are just as crucial in developing a viable ETOPS package. Fortunately, with the possible exception of changes required from improved engine reliability, none of the design changes have thus far proven necessary to preserve an adequate level of ETOPS safety.

Comprehensive data is accumulated and maintained with regard to all known ETOPS “events”. An ETOPS event is considered to have occurred whenever a situation, system failure or an anomaly causes the flight crew to consider turning back or diverting to an alternate – even if the crew decide eventually to continue to the intended destination.

Examination of data associated with Boeing 767 flights operated under ETOPS rules show that 99.8% of all flights successfully flew to their destination. Only two flights per thousand turned back or diverted to an alternate airport. Since the inception of 767 ETOPS in 1985 through March 1993, the 767 has been involved in 745 ETOPS events. In 602 cases, the crew made the decision to turn back or divert to an alternate. Similarly, 99.8% of all Airbus A310 flights successfully reached their destination, with a near identical rate of turn backs and diversions.

Many of the flights turned back (especially when the event occurred prior to the ETOPS portion of the flight) even though continuing to the destination would have had no significant adverse effect on safety. Often the motive for turning back appears to have been the desire to have a problem dealt with at a home maintenance base rather than a foreign location. Close examination of the data reveals that of the events resulting in a turn back or diversion, only 30 of them (5%) occurred during the ETOPS portion of the flight, while 85% occurred prior to this phase of flight.

Examination of the 745 events reveals that nearly 80% were caused by problems that were not propulsion related. Further, of the 21% caused by propulsion related problems, only one third resulted from inflight shutdowns, and of these only a very small percentage occurred during the ETOPS cruise portion of the flight. Inflight shutdowns, while a major topic of discussion and focus during examination of ETOPS safety, actually occur at a very low rate. The modern turbofan engines of today have inflight shutdown rates that are more than 10 times better than those of the piston engines of the 1950s that were the basis for the original 60 minute rule. Further, the next generation of turbofans are anticipated to enter revenue service with a shutdown rate equivalent to that of the mature turbofans currently used in ETOPS. Indeed, some of today’s turbofans both entered revenue service and subsequently maintained an inflight shutdown rate consistent with 120 minute ETOPS approval.

Virtually all of today’s turbofan engines enjoy very low shutdown rates. Further, cruise shutdowns – those occurring during the cruise phase of flight – occur at extremely low rates. This means that the ETOPS portion of the flight, which is inevitably within the cruise phase, presents little incremental risk. In reviewing all 60 inflight shutdowns that have occurred during eight years of ETOPS flights through March 1993, it is apparent that only 13 shutdowns occurred during the ETOPS portion of the flight. Of these, four occurred in 1985 during the first five months of ETOPS.

In assessing ETOPS safety, the total safety record of modern twins must be viewed in comparison with the hull loss accident rates of three and four engine aeroplanes that have long been accepted in Atlantic and Pacific operations. When looking at engines as a factor in the primary cause of hull loss accidents, it is useful to note that among Western built jetliners there has been only one such accident in the cruise phase of flight, and it did not involve a twin engine aeroplane. In fact, when considering hull loss accidents for all reasons as a function of phase of flight, it can be concluded that exposure to risk during the ETOPS cruise phase of flight is relatively insignificant when compared to other phases of flight.

When accident rates for modern twins are examined together with the extremely low shutdown rates for their engines, it can only be concluded that ETOPS flights are, in total, at least as safe as flights on existing three and four engine aeroplanes. In fact, when inflight shutdown rates reach values as low as those for the ETOPS fleet, the probability of total thrust loss on twin engine aeroplanes presents no greater risk than that resulting from the increased risk of uncontained engine failure on three and four engine aeroplanes.

In assessing all factors associated with safety for a given journey, one should also consider the fact that most accidents arise from factors other than aeroplane (e.g., engine) failure and are much more affected by flight cycles (takeoffs and landings – where accidents are most likely to occur) than by flight hours. The number of flight legs associated with a trip far overshadows safety related benefits or risks associated with the number of engines. As the operating economics of twin engine aeroplanes often result in an increase in point-to-point travel opportunities without necessity for a connection or intermediate stop, a safety advantage may result.

With regard to monitoring the degree of success, ETOPS is unique. The requirements of Advisory Circular 120-42A make it necessary to collect extensive data which is used to justify not only airframe and engine type design approval, but also to support both initial and continued operational approval for airlines. Additionally, the advisory circular requires that the FAA Propulsion System Reliability Assessment Board (PSRAB) perform initial and continuing assessments of airframe/engine suitability and requirements for ETOPS. While propulsion related systems are the principal focus of the PSRAB, the board is provided, and indeed examines, data for all events which may have an effect on the safe continuation of flight.

In no other area is safety of flight operations more rigorously monitored than with ETOPS. The mere suggestion of a safety related concern results in immediate and concentrated action to positively resolve the issue. The strength of ETOPS arises from the fact that all relevant segments of industry and government have worked together to assure its continued success. When one examines the effort directed toward making ETOPS a success and reviews the data which allows us to understand the degree to which we have been successful, one can only conclude that the record so far has been excellent. ETOPS continues to improve and is now considered a routine, safe and highly successful means of conducting long haul flight operations.

Capt Ekstrand is the Director of Flight Training & Industry Regulatory Affairs in the Customer Services Division of Boeing. Article courtesy of ICAO.