Summary
- ETOPS and EDTO standards have evolved based on twin-engine aircraft propulsion reliability.
- Losing an engine in flight means twin-engine planes lose 50% of total power compared to four-engine counterparts.
- Takeoff performance must account for losing an engine immediately post-lift-off to ensure safe climbing and obstacle avoidance.
With the decline in demand for quad-jets, efficient twin-engine aircraft continue to lead the widebody aircraft market. While two engines produce sufficient power to perform a safe flight, the aircraft must be able to operate safely if one engine goes inoperative. That means each engine (when intact) must be able to produce plenty of power during critical phases of flight.
Losing an engine mid-flight means losing 50% of total power for twin-engine aircraft, compared to a 25% loss for a four-engined counterpart. The Extended-range Twin-engine Operational Performance Standards (ETOPS) certification determines how far the aircraft is certified to fly with one inoperable engine.
Extended Diversion Time Operations (EDTO) standards
In 2012, the International Civil Aviation Organization (ICAO) introduced the Extended Diversion Time Operations (EDTO) regime in place of ETOPS. The new standards are based on best practices and lessons learned over the years from ETOPS. The ICAO ensures that all operators achieve the same level of safety during long-haul operations.
Photo: Air France
EDTO Standards
- Allow longer EDTO operations for twins, based on propulsion reliability and overall operational safety of current ETOPS Twins.
- The related criteria have been evolved from the existing ETOPS standards.
- Following similar precautions to EDTO operations for aircraft with more than two engines with few additional operational requirements.
- There are no additional maintenance or certification requirements for EDTO operations of aircraft with more than two engines.
While the EDTO standards have been widely accepted, most aviation regulatory authorities, including the Federal Aviation Administration (FAA) still use the term ETOPS to comply with ICAO certification and regulations.
Single-engine performance requirements
- Validation or acceptance of the EDTO certification.
- Conformity of the aircraft, including APU and engines, to the applicable EDTO configuration requirements.
- An established system to maintain and dispatch an EDTO aircraft.
- Demonstration of the maintenance program, including checks, servicing, and reliability programs.
- Demonstration of the operational limitations.
- Approval of the operator based on routes, desired diversion time, fleet, area of operations, etc.
Takeoff is one of the most critical phases of flight due to the various worst-case scenarios involved in the takeoff performance parameter. One such scenario is the inoperability of an engine directly after lift-off. Such a scenario occurs when the aircraft has surpassed V1 (decision to continue flight) and reached V2 (safety speed), at which it must climb.
Photo: Terry K | Shutterstock
The required engine performance must be such that the aircraft avoids an obstacle or terrain a certain distance and height from the liftoff point. For example, the thrust needed on the remaining (functional) General Electric GE90 engine fitted on a Boeing 777-300ER is approximately 110,000 Ibf.
A narrowbody Boeing 737 MAX 8 requires the remaining CFM International LEAP-1B engine to produce around 24,000 Ibf of thrust during takeoff. With both engines having a comparable bypass ratio of 9:1, the engine core must drive the fan to produce 90% of the required takeoff thrust. The remaining 10% of thrust comes from the core, producing shaft power for the fan.
Photo: Carlos Yudica | Shutterstock
The performance impact of losing an engine during the cruise is much lower. However, the remaining (functional) engine must generate all required thrust while overcoming the drag incurred on the inoperable engine. A typical Boeing 737 MAX cruising at Mach 0.78 creates a total drag (parasite and lift-induced drag) of 8,000 If. The thrust produced by the functional engine must be greater than the drag incurred to keep the aircraft airborne and moving forward.
ETOPS certification testing
Engines go through vigorous testing to receive ETOPS certification. From varying conditions to long operating cycles, the engines must operate within a minimum failure rate, showcasing in-flight reliability. Speaking about the requirements for ETOPS certification for LEAP, François Baroin, the Executive Vice President of CFM, commented,
“The testing required for this approval is in some of the most grueling conditions to which an engine would ever be subjected. To start, the engine is deliberately unbalanced to a level that no airline would ever be allowed to operate for even one hour. Then, in this unbalanced state with very high vibrations, it is run for 3,000 consecutive flight cycles (a simulated takeoff and landing sequence).”
“We ran this engine in a way that we will never see in commercial service. Once the testing is complete, the engine is entirely disassembled to the piece part level and laid out on tables for the regulatory agencies to inspect. The state of the parts upon inspection was incredible; they looked practically new.” – François Baroin, EVP, CFM.
Among numerous performance parameters of a jet engine, single-engine ETOPS compliance is one of the significant milestones for engine manufacturers.