10.1 One/Two-Engine-Inoperative En Route Net Profiles & Driftdown

Engine-Inoperative Enroute Performance and Driftdown

A flight that can cruise over mountains with all engines operating may lack sufficient performance after an engine failure. Dispatch planning must examine the approved net enroute flight path, the temperature and winds expected, and the route to a suitable landing airport. The one-engine rule in § 121.191 and the applicable two-engine rule in § 121.193 are different from an ordinary all-engine airway MEA check.

Gross, net, and aircraft-specific data

Gross performance describes the demonstrated aircraft performance basis. Net performance includes the prescribed reduction for planning safety margin. The dispatcher uses approved AFM net data with the correct weight, engine state, airspeed schedule, atmosphere, and configuration. Do not combine a gross ceiling with a net terrain-clearance requirement and call the calculation conservative.

For modern turbine transports, § 121.191 provides two compliance alternatives. Both start with a takeoff weight that, after normal fuel and oil consumption, satisfies the approved one-engine-inoperative enroute data. The operator must use expected ambient temperature. A standard-day diagram alone is insufficient when the forecast is substantially warmer than standard.

Alternative 1: positive-slope route evaluation

Under § 121.191(a)(1), the approved net path must have a positive slope at least 1,000 feet above all terrain and obstructions within five statute miles on each side of the intended track. For airplanes certificated after August 29, 1959, there is also a positive-slope requirement at 1,500 feet above the airport at which landing after an engine failure is assumed.

This branch does not add a universal 2,000-foot terrain-clearance requirement from the other branch. The test is positive net climb capability at the specified terrain-relative altitude, together with the applicable airport condition. Showing that a driftdown path clears a ridge by 2,000 feet is not the same calculation as demonstrating positive slope at 1,000 feet above every relevant obstacle.

Alternative 2: net path to a landing airport

Under § 121.191(a)(2), the net path must permit continuation from cruise to an airport meeting § 121.197, clearing terrain and obstructions within five statute miles of track by at least 2,000 feet vertically. The airport positive-slope requirement is 1,000 feet, or 1,500 feet for the certification dates specified in the regulation. It is not 2,000 feet above the alternate elevation.

Paragraph (b) adds assumptions: failure at the most critical point; the prescribed relationship to the critical obstruction and an approved navigation fix unless another safeguarded procedure is authorized; an approved adverse-wind method; permitted fuel-jettison conditions where applicable; a specified alternate meeting required weather; and fuel/oil consumption consistent with the approved net data.

Driftdown procedure and route geometry

Driftdown is the descent needed when the remaining engine capability cannot sustain the original cruise altitude. The crew follows the aircraft's approved engine-out speed and thrust schedule, commonly using maximum continuous thrust with a performance-efficient speed. Do not equate every approved driftdown speed exactly with maximum L/D or prescribe a universal FL220 endpoint. Weight, atmosphere, engine state, and aircraft design determine the profile and stabilized altitude.

A driftdown decision point distinguishes the appropriate escape route after failure at different positions. If returning clears a western ridge but continuing clears an eastern ridge, the dispatch analysis must establish where each option remains valid. The critical point need not be the midpoint or the equal-time point between airports. A route's highest ridge also need not be its most restrictive obstacle after considering descent distance and winds.

Worked terrain comparison

Use a hypothetical airport at 4,000 feet MSL and a modern aircraft subject to the 1,500-foot airport condition. A positive-slope airport check is then at 5,500 feet MSL. If a ridge is at 12,000 feet MSL, the alternative-2 path must be at least 14,000 feet MSL at the relevant crossing. These are two separate comparisons with different reference surfaces.

Five statute miles equal about 4.34 nautical miles. Evaluate the entire prescribed corridor on each side of the route, including a ridge offset from the centerline. A planner using a five-NM corridor may be conservative in width, but a planner checking only the centerline can miss a legally relevant obstruction. Keep chart units explicit and use the approved route-analysis method.

Two-engine-inoperative requirements

For aircraft subject to § 121.193(c), the route must either remain within 90 minutes at all-engines-operating cruise power of an airport meeting § 121.197, or satisfy the prescribed two-engine-inoperative net path. That path clears the five-statute-mile corridor by 2,000 feet, has positive slope at 1,500 feet above the landing airport, and includes the required arrival and 15-minute cruise fuel conditions. Earlier certification-date branches have different wording and must be read separately.

The 90-minute performance-rule alternative is not the twin-engine ETOPS 60-minute definition. Do not use one threshold to answer the other question. A twin's ETOPS route and critical fuel assessment also do not automatically prove terrain clearance after failure.

Combine failure scenarios responsibly

An engine-out plan assumes a particular cabin-pressure state. A simultaneous depressurization can require a lower altitude or an oxygen-compliant escape profile over high terrain. Evaluate it with approved oxygen and ETOPS fuel data where applicable. Do not promise that the aircraft can immediately descend to 10,000 feet when mountains obstruct that level.

Before release, crosscheck temperature, wind, ice/anti-ice penalties, weight at critical points, airport weather and performance, and the recorded escape route. During flight, updated weather or aircraft status can invalidate the planned assumptions. Provide timely information to the PIC and reassess the route; the dispatcher supplies operational support while the crew executes the aircraft-specific abnormal procedure.

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.