10.3 ETOPS En Route Alternates: Adequate vs. Suitable & Critical Fuel

ETOPS Alternates, Arrival Windows, and Critical Fuel

Extended operations require a network of usable alternates and fuel sufficient for the most demanding prescribed diversion. The route's maximum diversion-time boundary, alternate weather window, systems endurance, and fuel requirement are related but separate. A still-air ETOPS circle does not calculate actual diversion fuel, and a single favorable TAF at departure does not establish alternate suitability throughout the flight.

Adequate and suitable airports

An adequate airport satisfies the applicable airport/runway, services, and performance requirements for use in the operation. Part 139 certification applies where required; foreign airports and other approved facilities require their applicable standards. Avoid assuming that every adequate airport worldwide must possess a U.S. Part 139 certificate or the same universal ARFF category.

A selected ETOPS alternate must satisfy § 121.624: enough alternates are listed to keep the airplane within its authorized maximum diversion time; weather meets the ETOPS alternate minima in the issued OpSpecs from the earliest to latest possible landing time; field conditions permit safe landing; and the applicable public-protection requirements are met. “Suitable” commonly describes an adequate airport meeting the additional weather and operational conditions for the proposed use.

Airport suitability includes runway length and condition, aircraft weight and landing performance, approach/navaid availability, wind, operating hours, and emergency services. Verify NOTAMs and service availability across the entire possible use window. An airport may be physically adequate but unsuitable because the forecast, runway closure, or approach outage defeats the plan.

Arrival windows

Calculate the earliest and latest possible landing times using the applicable flight points, approved diversion profiles, winds, and timing assumptions. Every arrival consists of time to the relevant point plus diversion time. Subtracting diversion time from the time at an entry point does not produce a landing time.

For a simplified exercise, the airplane reaches the earliest relevant route point at 0200Z and could land at an alternate 50 minutes later: that candidate arrival is 0250Z. If the latest relevant point is reached at 0410Z with a 70-minute diversion, that candidate is 0520Z. The complete approved analysis determines whether these bound all possible landings. Evaluate the forecast and field reports throughout the resulting window, including overlapping conditional groups.

C055-style destination alternate additions are not a substitute for the carrier's actual ETOPS alternate authorization and table. The common one-facility and two-facility calculations can be used only when the applicable issued authorization permits them, with appropriate approach heights and suitable runways. Do not assume CAT II/III destination capability grants correspondingly low ETOPS planning minima.

Three diversion fuel scenarios

Section 121.646(b)(1)(i) requires the greater fuel quantity for three cases at the most critical point:

1. Rapid decompression, followed by descent to a safe altitude consistent with oxygen requirements.
2. Rapid decompression and simultaneous engine failure, followed by an oxygen-compliant descent and one-engine-inoperative diversion.
3. Engine failure, followed by descent to the one-engine-inoperative cruise altitude and diversion at the approved speed.

Compute all three; do not declare the combined failure the winner without data. An all-engine low-altitude decompression diversion can use more fuel than another case. The most fuel-critical point can differ from an equal-time point (ETP). Terrain, aircraft profile, wind, and airport choice determine the actual result.

Wind, icing, deterioration, and APU

The wind rule in § 121.646(b)(1)(ii) uses a 5 percent wind-speed adjustment to worsen headwind or reduce tailwind when using actual forecasts from an FAA-accepted wind model. Without such a model, carry additional fuel equal to 5 percent of the scenario fuel as the specified fallback. These are different computations; a universal five-percent fuel adder misstates the rule.

Icing requires the greater of the rule's two fuel effects: airframe icing during ten percent of the time icing is forecast, including ice-protection fuel during that period; or engine and applicable wing anti-ice throughout the forecast icing period. An arbitrary two-to-ten-percent drag adder does not replace that comparison. Engine deterioration adds five percent fuel unless the operator has the specified in-service monitoring program. If an APU is required as a power source, include its consumption in the appropriate phases.

Under § 121.646(b)(2), add fuel for 15 minutes holding at 1,500 feet above field elevation, followed by an instrument approach and landing. The text does not universally mandate a missed approach and second landing attempt. Additional margin can be required by the approved operation or chosen for safety, but distinguish it from the base statutory wording.

Worked critical-fuel comparison

All figures below are hypothetical exercise inputs, not performance claims for a named aircraft. Assume wind adjustment, icing, and deterioration requirements are already included in the three scenario burns: 18,500, 20,000, and 19,200 pounds. The governing scenario burn is therefore 20,000 pounds. Holding needs 1,200 pounds, approach/landing 800, and required APU use 200.

With 22,000 pounds projected at the critical point, the plan is short by 200 pounds at that point. Do not simply add 200 pounds at departure and assume the deficit is fixed: carrying the extra fuel can increase burn before reaching the critical point. Recompute uplift, weight, primary flight fuel, and the diversion cases with the approved planning model until all requirements are met.

Before entry and during flight

Section 121.631 requires the applicable check before proceeding beyond the ETOPS entry point. Reassess alternates, field conditions, aircraft systems, and remaining fuel, making needed release changes while still within the authorized diversion time. Enroute weather standards are not identical to the higher predispatch planning table. Maintain communication with the PIC and provide current safety information. An unexpected airport deterioration calls for an operational assessment under the rule; it is not an automatic instruction to turn back regardless of circumstances.

Official References

Source check: 2026-10-12. Engine-out performance § 121.191; ETOPS alternates § 121.624; ETOPS fuel § 121.646; ETOPS maintenance § 121.374. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.