ETOPS in Practice: Alternates, Depressurization and the ETOPS Sector

ETOPS Twin Engine Operations in Practice

Most explanations of ETOPS stop at the definition: rules that let twin-engine airliners fly routes far from the nearest airport. This article follows a single ETOPS twin engine operations flight from the planning desk to the oceanic crossing and shows what dispatchers and crews actually check: which airports count as alternates, which failures are planned for, how the fuel is calculated and what happens when conditions change en route. For the background and history, start with how ETOPS works.

Why Twins Need Special Rules

Under the basic rules, a two-engine airliner must stay within a set flying time of an adequate airport, calculated at one-engine-inoperative cruise speed in still air; for most operators this threshold is 60 minutes. Many of the most efficient routes, across oceans, polar regions or deserts, take the airplane further than that. ETOPS approval allows the operator to go beyond the threshold, up to an approved maximum diversion time such as 120 or 180 minutes, and on some modern types much longer, provided the airplane, the engines and the airline’s procedures meet additional standards.

Key Terms Used in ETOPS Planning

Step 1: Choosing the En-Route Alternates

The dispatcher begins with a list of adequate airports along the route and checks their forecasts for a window around the time the flight could need them. Because the crew might arrive with an engine out or other failures, planning minima for ETOPS alternates are set higher than the normal landing minima, typically with extra margin above the published approach minima. The selected alternates must cover the whole route so that no point lies further from one of them than the approved diversion time. Remote islands, military airfields with civil access and polar airports often play this role.

Step 2: Drawing the Diversion Circles

On a planning chart, each alternate is surrounded by a circle whose radius equals the approved diversion time flown at the one-engine-inoperative speed. The route must stay inside the union of these circles. Between each pair of alternates the planning system calculates an equal time point; at that point the decision to divert to one or the other is equally quick, and it is usually where the diversion fuel requirement is highest.

Step 3: The Three Failure Cases

ETOPS fuel planning asks what would happen if a serious failure occurred at the worst possible place. Three cases are normally evaluated:

1. Engine failure. The airplane drifts down to a lower altitude where one engine can sustain flight and cruises to the alternate. See driftdown in twin-engine aircraft.
2. Loss of cabin pressure. Both engines work, but the crew must descend quickly to an altitude where passengers can breathe without oxygen masks. At low altitude a jet burns much more fuel per mile, so this case often drives the fuel figure.
3. Engine failure with loss of pressure. The combination of a low altitude and one engine, usually the most fuel-hungry of all.

Whichever of these needs the most fuel from the equal time point becomes the critical fuel scenario.

Step 4: Building the Critical Fuel Figure

The critical fuel calculation adds up everything the airplane would need after a failure at the critical point:

• fuel for the descent and the diversion cruise at the planned speed and altitude;
• a contingency allowance for forecast errors;
• holding fuel near the alternate, followed by an instrument approach, a missed approach and a second approach and landing;
• extra fuel when icing is forecast, because engine and wing anti-ice increase consumption and ice adds drag;
• APU fuel if the APU is required as an electrical or air source;
• penalties for any inoperative equipment allowed by the minimum equipment list.

The flight is then fuelled for the higher of two figures: the normal fuel plan to the destination, or the amount that guarantees the critical scenario can be completed. On many flights the normal plan already covers it; on long, low-weight sectors the ETOPS figure can add fuel.

Step 5: Before the ETOPS Entry Point

Conditions change between dispatch and the ocean crossing. Before the airplane passes the entry point, the crew and dispatch review:

• updated weather and status for each en-route alternate;
• the airplane’s technical state, including any defects that would reduce its ETOPS capability;
• fuel on board against the plan.

If an alternate has dropped below minima, they can select another one, change the route or, if nothing works, return before committing to the ETOPS segment.

Step 6: Flying the ETOPS Segment

In the ETOPS segment the crew flies normally but keeps the diversion picture current: which alternate is closest in time, where the equal time points are, and how fuel compares with the plan. Fuel balance between the tanks is watched closely. If air traffic control offers a reroute, the crew checks that the new track stays within the approved area before accepting it.

Step 7: When Something Goes Wrong

If an engine fails or the cabin loses pressure, the crew follows the aircraft manufacturer’s procedures and turns toward the most suitable alternate; the planning work means that fuel and an airport are already available. If an alternate becomes unusable while in the segment, another suitable airport is designated. Whatever the plan says, a time-critical emergency such as an uncontrollable fire still calls for landing at the nearest airport where a safe landing is possible.

ETOPS Checklist for Students and Simulator Pilots

• Identify the approved maximum diversion time for the airplane and operator.
• List adequate airports along the route and check forecasts against ETOPS planning minima.
• Confirm that every point of the route lies within the diversion circles.
• Find the equal time points and calculate the critical fuel case.
• Re-check alternates, weather and the airplane’s status before the entry point.
• Track the nearest suitable alternate throughout the segment.

Key Takeaways

• ETOPS turns a regulatory limit into a planned safety case: approved alternates, approved diversion time and fuel for the worst case.
• Depressurization, not only engine failure, often drives the fuel requirement because the diversion is flown low.
• The decision point that matters most comes before the entry point, when alternates and weather are reviewed.
• ICAO now uses the broader term EDTO for the same concept; see extended diversion time operations.

For airline maintenance and reliability requirements behind ETOPS approval, read how airlines maintain ETOPS certification, and for how far a modern twin can fly on one engine, see one-engine range of a twin jet.