Historically, aircraft with three (trijets) and four (quadjets) engines held a significant advantage over twin-engine jets for long-haul, transoceanic flights.
This superiority was primarily rooted in mechanical redundancy and a stringent regulatory restriction: the “60-minute rule.”
Before the mid-1980s, jet engines were less reliable, and aviation regulators, notably the FAA, stipulated that a twin-jet’s flight path could not take it more than 60 minutes’ flying time from a suitable diversion airport while operating on a single engine.
This restriction severely limited twin-jets to inefficient, circuitous paths, making them unsuitable for the most direct, great-circle routes over oceans, remote landmasses, or polar regions.
Multi-jet aircraft, such as the Boeing 747 and the McDonnell Douglas DC-10, were effectively exempt. If one engine failed, they still had ample power remaining to continue safely to a distant airport.
The market was revolutionised by massive advancements in jet engine technology.
Modern engines became exponentially more powerful, fuel-efficient, and, most crucially, reliable.
The new era of reliability paved the way for ETOPS (Extended-range Twin-engine Operational Performance Standards).
ETOPS created a regulatory framework to certify specific twin-engine aircraft and operators to fly routes with diversion airports further than the restrictive 60-minute limit.
Certifications like ETOPS-180 (allowing the aircraft to be up to 180 minutes away from an alternate airport on one engine) became the standard for transoceanic travel.
The final evolution in this safety philosophy is EDTO (Extended Diversion Time Operations). In 2017, ICAO replaced ETOPS with this broader term.
The key difference is that EDTO applies to all turbine-engine aircraft with two or more engines, reflecting a shift in the regulatory mindset.
EDTO takes precedence because it formalizes the recognition that flying far beyond a suitable airport requires assurance about the total system reliability of the aircraft, not just its engines.
Under EDTO standards, a quad-jet like the Airbus A380 may need regulatory approval for an extended flight if a critical non-engine system—such as the cargo fire suppression or a major electrical component—fails.
By focusing on the entire airframe system (hydraulics, electrics, fire suppression, etc.) and applying rigorous standards to all planes, EDTO ensures consistent safety margins.
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where we convert standards in ICAO Doc 10085 Extended Diversion Time Operations (EDTO) Manual, EASA AMC 20-6 Rev 2 Appendix 8 and UK CAA AMC 20-6.