The Battle Of The Overwater Twins

The major national variation is that of the United States. There the FAA (Federal Aviation Administration) has adopted a limit of 60 minutes at one engine inoperative cruising speed. This is approximately twice as restrictive as the ICAO Standard because the distance covered in 90 minutes at ICAO’s all engine operating speed would take nearer to 120 minutes at a one-engine inoperative speed. A sweetener for US operators is that the FAA Administrator may vary this limit at his discretion. The landmark decision in this regard was approval in the late 1970s for Eastern Airlines to extend the limit to 75 minutes (at single engine cruising speed) for its A300 operation from New York to Puerto Rico.

It seems opportune at this stage to consider the need for these restrictions. Let us first take the case of engine failure. In the case of a single-engine failure in a twin-engined aircraft of this type, power on the operative engine would be increased slightly, the aircraft would normally descend to a lower altitude and fly at a lower speed to the nearest suitable airport for a landing. The regulatory authority must be sure that the probability of this first engine failure being followed by a second, independent failure before a safe landing can be effected is so remote as to be extremely unlikely to occur at any time during the life of the fleet of aeroplanes being considered. Take heart that since 1946, when IATA (International Air Transport Association) records were established, there has not been any case of independent failure of both engines on a twin-engined aircraft in cruise while on scheduled airline service.

Systems reliability is as important as engine reliability. Modern aircraft of this type are dependent on uninterrupted supplies of electrical and hydraulic power for the operation of flight controls and for navigation and communications. Thus independent sources of power for these functions must be supplied to cover the diversion requirements, even if a single engine failure has rendered the generator and hydraulic pump associated with that engine unusable. Flight instrument reliability and . redundancy must be of a high order for long flights over water or inhospitable terrain as total failure of these systems would render the aircraft uncontrollable on a dark and storm night. The reliability of cargo fire warning and extinguishing systems plus the ability to contain a cargo fire within a particular cargo hold for an extended period is of primary concern. Consider also the reliability of the aircraft pressurisation system; aircraft range is considerably reduced at the lower altitudes necessary after pressurisation failure. While some of these concerns are common to aircraft with more than two engines the larger aircraft tend to have more available systems redundancy.

Both the airlines and the aircraft manufacturers offer a great deal of information to support their contention that these restrictions should be relaxed if not completely removed. Preceeding generations of twinjet aircraft have proved extremely safe and reliable. Several airlines have operated narrow-bodied twinjets on extended-range operations in our own region, notably Air Nauru with Boeing 737s and Air Pacific with 8737s and BAC111s, with no obvious problems. Eastern’s A300 operations to the Caribbean and Hapag Lloyd’s Trans Atlantic cargo flights, also with an A300, have proved uneventful.

Proponents of ETOPS make the point that demonstrated engine reliability for the models of big fan engine in question is at least seven times as good as that for the piston-engined types considered when the present rules were formulated over thirty years ago. As these rules provided an acceptable mathematical probability of safety with the higher known failure rate it could be argued that the 90 minutes limitation could be extended. It is also argued that any engine failure in cruising flightis an extremely rare occurrence as the engine is operating under steady conditions of temperature and pressure at a small fraction of its rated takeoff power.

Boeing and Airbus, as the major manufacturers poised to take advantage of any relaxation of the restrictions, have made significant changes to their products to ensure that systems reliability improves to match the engine development. To take the Boeing 767-200ER as an example, the following changes have been made from the short to medium range base version of the aircraft.

1. A fourth generator, hydraulically powered, can provide electrical power if the two engine-driven generators and he auxiliary power unit generator should become unavailable.

2. Fire suppression in the lower cargo holds will provide a very high probability of fire suppression for at least two hours.

3. A more sophisticated system of instrument cooling provides

Similar improvements are offered by Airbus as options on their A310-300.

ICAO’s response to these developments have been sensibly conservative. While agreement between the states has not yet been achieved, it seems that a threshold will be set (somewhere between the FAA’s 60 minutes at one engine out speed and 90 minutes at all engines cruise speed) below which twin-engined aircraft may operate away from a diversion airfield without any special requirements. For any operator wishing to operate beyond that threshold, it will be necessary for both the operator and the aircraft to be approved for that particular operation by the state of registration of the aircraft. In giving this approval the state should consider airframe, engine and systems reliability, suitability of en-route and off track alternate airfields from the point of view of runway lengths, radio aids availability and weather, and the capability of the airline to carry out such a potentially demanding operating If these conditions can be met approval may be given for spec1f1c routes, operators and aircraft. FAA, for instance, has let it be known that it would consider operation out to 120 minutes if it considers that its conditions are satisfied.

So where does this leave us? The international regulatory process is by nature slow-moving. I expect that when a threshold standard is set, national legislation will quickly follow from those states whose carriers wish to make use of these types of aircraft on ETOPS. There is a major argument against the proposal from the various pilots’ unions, provided that the conditions above are properly applied.

Assuming an appropriate level of self-discipline from the regulatory authorities of the states involved, I would expect ETOPS to become normal part of the international airline scene.

When that happens, I would expect the acronym ETOPS to lapse into well-deserved obscurity.