Sunday, August 1, 2010

Engines Turn Or Passengers Swim !

Prior to the 1960's, there existed something called the "60 minute rule" for twin-engined aircraft. The rule stated that, in simple terms, the flight path of a twin-engine airplane should not be more than 60 minutes away from a suitable emergency airport. The Federal Aviation Administration (FAA), keeping in mind the limitations of the piston-engine, had in 1953, introduced this rule to enhance safety during long-distance twin-engine flights, as they were considered very risky. The downside to this rule was, for a twin-engine flight to take place between two airports at a considerable distance apart, there had to be sufficient emergency airports at most 60 minutes away from any point along the way. This very often resulted in twin-engined flights having to follow a "staggered" flight path, just to stay within the 60 minute diversion period. Many routes were off-limits to twin-engine operations, simply because there weren't sufficient 60 minute diversion points available enroute, and these routes were called exclusion zones.


By the 1960's, it was evident that jet engines had higher thrust and were more reliable than piston-engines available during that period. In 1964, the 60 minute rule was waived for three-engined jet airplanes. However, the twin-engined jet airplanes were still restriced to the 60 minute diversion period. This led to the development of intercontinental jets like the Boeing 727, Lockheed L-1011 Tristar, and the McDonnell Douglas DC-10.

Boeing 727
Lockheed L-1011 Tristar
McDonnell Douglas DC-10
Hawker Siddely Trident

The 60 minute rule was an FAA mandate intended for twin-engine operations within the United States. Twin-engine operations in other parts of the world, especially in European countries, were subject to the ICAO's "90 minute rule", which required a less stringent 90 minute diversion period. This time relaxation was exploited by Airbus, and the world's first high-bypass turbofan engine widebody airliner, the Airbus A300, entered service in 1974. The A300 could carry just about as many passengers as far as the DC-10, and was 30 percent more fuel efficient than the Tristar. Very soon, the A300 was certified for extended range operations over water, providing for more flexibility in routing (outside the US). In America, airlines like Pan Am and Eastern began inducting A300s into their fleet as a replacement for existing tri-jets, owing to their higher fuel efficiency and better performance. By 1981, Airbus was growing rapidly, having sold over 300 aircraft to more than forty airlines worldwide - and Boeing rolled out the 767.

Airbus A300
Boeing 767 - Note the striking similarity in design to the A300.

In May 1985, for the first time, the FAA extended the 60 minute diversion period to 90 minutes, for TWA's Boeing 767 service between St. Louis and Frankfurt. In fact, a little later, this was further extended to 120 minutes. And so it was, that the first "Extended Range Twin-engine Operational Performance Standards" or ETOPS "rating" was applied to a twin-engine airplane. In this particular case, the rating was called an ETOPS-120 rating, in keeping with the 120 minute diversion period. As airplane engines became more reliable with technological advancement, the ETOPS-180 rating was soon approved, in 1988, by the FAA, subject to the engine meeting very high technical standards and qualifications, and twin-engine airplanes like the Boeing 737, 757, 767, and the Airbus A300 were ETOPS certified. This brought an end to the intercontinental tri-jets, which by now, were relatively more uneconomical to the company.

The Tupolev Tu-154 is one of the few commercial tri-jet airliners still in service.


ETOPS certification consists of an ETOPS "type approval" and an ETOPS "operational certification".

1. ETOPS type approval consists of certifying the engine/airframe combination on the basis of tests conducted to meet such standards as prescribed by the ETOPS requirements. These tests are conducted during the type certification of the airplane, and may involve shutting down an engine and flying the entire diversion time on the remaining engine, very often over the middle of oceans. It must be demonstrated that, during the diversion period, the flight crew is not unduly burdened by excess workload due to the lost engine, and the probability of the remaining engine also failing is extremely remote.

2. ETOPS operational certification refers to the certification of the operator (eg. airline) to conduct ETOPS flights, having satisfied regulatory requirements pertaining to training and qualification of flight crew in ETOPS procedures, as well as experience in conducting ETOPS operations. For example, an airline with extensive experience in long distance operations may be granted immediate ETOPS approval by the regulatory authority, compared to a relatively less experienced airline which may have to be put through a number of certification tests before being granted ETOPS approval.

Under current regulations, the following ETOPS ratings may be awarded:


Approval for ETOPS is granted in cautious increments, to allow airlines to build in-service experience and expertise in operating over extended routes with a particular airframe-engine combination. An airline (to be more specific, one or more aircraft in the fleet) is normally granted ETOPS approval in increments of 75, 120, and 180 minutes. For example, an airline seeking ETOPS-120 approval must first prove that it is capable of successfully operating under ETOPS-75 for a year, before being granted ETOPS-120. One of the important parameters that is considered during ETOPS certification is the in-flight shutdown (IFSD) rate, which represents the number of engine shutdowns per 1000 hours of operation, all engines in service (for a particular engine-airframe combination) put together. For example, one in-flight shutdown in an airline fleet logging 50,000 hours would be represented by an IFSD of 0.02 failures per 1000 hours of operation. To gain ETOPS-180 approval, an air carrier must operate its extended range fleet for at least one year in extended range operations, recording an IFSD of 0.02 per 1000 hours. Any increase in IFSD would be grounds for re-evaluation by the regulator, of the capability of the air carrier to safely conduct ETOPS operations.


In 1988, the FAA ammended the ETOPS regulations to extend the 120 minute diversion period to 180 minutes, subject to stringent techical and operational qualifications. This made 95 percent of the earth's surface available for ETOPS operations. In European countries, the JAA extended the 120 minute diversion to 138 minutes (15 percent more) to take into account the non-availibility of some of the emergency diversion airports during winter/bad weather, thereby allowing airlines to operate across the North Atlantic under less stringent (and less expensive) ETOPS-120/138 rules, instead of ETOPS-180. Similarly, in 2000, the FAA approved some air carriers for ETOPS-180/207 (a 15 percent extension to the 180 minute diversion period) on certain routes during unfavourable weather conditions over the North Pacific, considering the non-availibility of sufficient emergency airports.

For further reading on the FAA's new ETOPS rules (2007) click here.

In 1995, the Boeing 777 was the first airliner to have an ETOPS-180 rating on entry into service, and in 2009, the Airbus A330 became the first airliner approved for ETOPS-240 operations on entry into service, opening up new routes in the South Pacific, South Atlantic, and Southern Indian oceans, to twin-engine operations. Aviation regulatory authorities worldwide are now working towards having common extended-range standards that would also include all three and four engine civil airliners, under a new system that will be called Long Range Operational Performance Standards (LROPS).

Boeing 777
Airbus A330

No comments:

Post a Comment