Consider a scenario where an airplane is operated on an ETOPS 180 route where the ETOPS alternate is open, available and above minima, but there is terrain between the route and the airport which makes it absolutely unreachable. If, in that scenario, an engine failure or decompression alternate is available but more than 180 flying minutes away, and enough fuel is carried to reach it while satisfying the terrain clearance requirements, would that flight be legal (not: practical, safe, desirable - only legal!) according to EASA/FAA regs? In other words, is ETOPS satisfied solely by looking at range circles around available airfields, or are there additional considerations required (not: prudent, desirable - required!) by regulations?

  • $\begingroup$ Maybe I misunderstand, but don't ETOPS regulations apply to flying over water, while terrain between the route and the airport would imply you're not over water, but over land, thereby making ETOPS a moot point? $\endgroup$
    – FreeMan
    Commented Jan 13, 2016 at 17:17
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    $\begingroup$ As far as I can see, ETOPS was originally developed for flying over water, but equally applies to flying over land, as fundamentally there is no difference in flying over water or over a vast desert - neither areas provide suitable diversion options. $\endgroup$ Commented Jan 13, 2016 at 18:40

2 Answers 2


No, ETOPS is not satisfied solely by range circles, but (among other things) by the "time needed to fly to that airport".

See the requirements of 14 CFR 121.633 (emphasis mine):

(a) For ETOPS up to and including 180 minutes, no person may list an airport as an ETOPS Alternate Airport in a dispatch or flight release if the time needed to fly to that airport (at the approved one-engine inoperative cruise speed under standard conditions in still air) would exceed the approved time for the airplane's most limiting ETOPS Significant System (including the airplane's most limiting fire suppression system time for those cargo and baggage compartments required by regulation to have fire-suppression systems) minus 15 minutes.

Paragraph (b), for ETOPS beyond 180 minutes, gives the same "time needed" requirement.

So, if a scenario existed as the question describes where "there is terrain between the route and the airport which makes [the ETOPS alternate] absolutely unreachable", then that flight could not reach that alternate in the required time (since the alternate is not reachable), and the flight could not be dispatched listing that alternate.

  • $\begingroup$ Alright, that makes sense. On the other hand, the "time needed to fly to" that airport is an academic exercise, anyway, isn't it? Or will the circles be adjusted for wind, even though the regs state no wind? What about an event after which the OEI planning speed is not feasible? Or if the speed needs to be brought back for turbulence penetration? Or, similarly to terrain but crucially different, what if enroute weather prevents direct flight to the ETOPS alternate from the route? $\endgroup$ Commented Jan 12, 2016 at 20:59
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    $\begingroup$ @CptReynolds I doubt the FAA would buy the "just an academic exercise" argument. $\endgroup$
    – J W
    Commented Jan 12, 2016 at 21:09
  • $\begingroup$ Well... you're not required to actually fly the diversion at exactly that speed, correct? $\endgroup$ Commented Jan 12, 2016 at 22:04
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    $\begingroup$ @CptReynolds I don't suppose so. These regs are for dispatching the flight. $\endgroup$
    – J W
    Commented Jan 12, 2016 at 22:11
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    $\begingroup$ The regulations are about defining a maximum - at the end of the day they're arbitrary to some extent: what's the real difference between 170 and 190 minutes on one engine? The point is that all aircraft are held to the same standard, which has been decided on as being safe (or rather: reasonably safe). If your last engine gives up at 170 miles or 190 miles, you're still probably ditching/finding somewhere nice and flat to try to land. $\endgroup$
    – Jon Story
    Commented Jan 14, 2016 at 16:24

This isn't ETOPS specific, but is related to high terrain. When overflying terrain that limits your ability to reach your destination or destination alternate you plan additional alternates called "drift-down" alternates. The flight plan is then segmented into drift-down segments that determine which alternate you will land at if needed for that segment of flight.

The drift down information will be part of the dispatch release and assures that from any point on your route with an engine failure you can meet obstacle clearance requirements while drifting down toward a suitable alternate airport.

Routes I flew between the US and Mexico often had drift-down alternates as there were areas on many of our routes that we could neither return to our departure airport or make our destination if we experienced an engine failure. In those cases we'd reference which drift-down segment we were in and head toward the alternate for that segment.

The regulatory basis for the drift-down procedures and alternates is 14 CFR 121.191. The synopsis is that flight with a failed engine must be able to proceed to a destination or alternate from any point along the route while maintaining specific terrain clearance requirements. The "drift-down" alternates must meet the same requirements as normal alternates (weather, suitability) and must be present in the dispatch release.

Combining this with ETOPS, the drift-down alternates would have to be within ETOPS range in addition to the terrain drift-down requirements.

The regs:

§121.191 Airplanes: Turbine engine powered: En route limitations: One engine inoperative.

(a) No person operating a turbine engine powered airplane may take off that airplane at a weight, allowing for normal consumption of fuel and oil, that is greater than that which (under the approved, one engine inoperative, en route net flight path data in the Airplane Flight Manual for that airplane) will allow compliance with paragraph (a) (1) or (2) of this section, based on the ambient temperatures expected en route:

(1) There is a positive slope at an altitude of at least 1,000 feet above all terrain and obstructions within five statute miles on each side of the intended track, and, in addition, if that airplane was certificated after August 29, 1959 (SR 422B) there is a positive slope at 1,500 feet above the airport where the airplane is assumed to land after an engine fails.

(2) The net flight path allows the airplane to continue flight from the cruising altitude to an airport where a landing can be made under §121.197, clearing all terrain and obstructions within five statute miles of the intended track by at least 2,000 feet vertically and with a positive slope at 1,000 feet above the airport where the airplane lands after an engine fails, or, if that airplane was certificated after September 30, 1958 (SR 422A, 422B), with a positive slope at 1,500 feet above the airport where the airplane lands after an engine fails.

(b) For the purposes of paragraph (a)(2) of this section, it is assumed that—

(1) The engine fails at the most critical point en route;

(2) The airplane passes over the critical obstruction, after engine failure at a point that is no closer to the obstruction than the nearest approved radio navigation fix, unless the Administrator authorizes a different procedure based on adequate operational safeguards;

(3) An approved method is used to allow for adverse winds:

(4) Fuel jettisoning will be allowed if the certificate holder shows that the crew is properly instructed, that the training program is adequate, and that all other precautions are taken to insure a safe procedure;

(5) The alternate airport is specified in the dispatch or flight release and meets the prescribed weather minimums; and

(6) The consumption of fuel and oil after engine failure is the same as the consumption that is allowed for in the approved net flight path data in the Airplane Flight Manual.

  • $\begingroup$ With your opening clause, are you stating that your answer is not ETOPS specific, or are you referring to the question? $\endgroup$
    – J W
    Commented Jan 12, 2016 at 19:47
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    $\begingroup$ @JonathanWalters both in a way. He asks about ETOPS over terrain, but the driftdown alternates will be necessary whether or not you are flying under ETOPS rules -- they are necessary if significant terrain is present that you can't clear with a failed engine. $\endgroup$
    – casey
    Commented Jan 12, 2016 at 22:10

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