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I am curious how the ETOPS minutes categories are arrived at. i.e. Say, ETOPS 330 vs 370.

What extra abilities would the 370 type have demonstrated that the 330 didn't have? Was it just that the test flight on single engine was longer? Or something else in the design limits the type to a 330 minute rating while on the drawing board? Is it the MTBF of a component? Or is the 330 minute time back calculated based on actual empirical testing and the associated failures?

On ground and maintainance procedures is there any substantial difference when dealing with a ETOPS 330 aircraft vs and ETOPS 370 aircraft?

Could there be (say) an ETOPS 430 in the future?

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    $\begingroup$ Some elements of the answer: At least the cargo fire suppression time and the in-flight shutdown rate (IFSD) have to be better. ETOPS time (now renamed EDTO by ICAO) isn't limited, though there is no point to go further than what is necessary to cover the south pole area between Latin America and Australia. See Module 4 of this series of ICAO documents. $\endgroup$ – mins Jun 12 '17 at 6:34
  • $\begingroup$ Thanks @mins. I'd love to find out if anyone knows which components of say an aircraft type had to be changed for the in-flight shutdown rate when ETOPS 370 was obtained instead of ETOPS 330. $\endgroup$ – curious_cat Jun 13 '17 at 3:59
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If there is a formula currently used to generate an ETOPS time, no regulatory agency has made it public that I can tell. ETOPS originally grew out of a "60-minute rule" that was the result of earlier distance-based rules (see this report for a thorough history). My guess is that 60 minutes was a case of "well, that sounds good" rather than the result of any rigorous analysis. From there, it's mostly been manufacturers slowly pushing and getting approved for longer and longer extensions (usually just a percentage increase that results in round-ish numbers—again, this is policymaking, not engineering).

However, take a look at the formulas on pages 9–11 of this report. I hesitate to declare them the way to generate an ETOPS number because the report is thirty years old and a lot of it is critical of the methodology, so the regulators may have changed their approach between then and now. In fact, the report does not explicitly state that the formulas within were in use at the time, either. Nevertheless, if there is a formula in modern use, it likely uses a similar approach of quantifying the probabilities of failure and using a factor to convert them into a do-not-exceed diversion time.

The difference between certification at 330 minutes (777) and 370 minutes (A350) may be due to FAA vs. EASA methodologies and/or decreases in inherent failure probabilities as technology matures. With that, I doubt there is much difference in maintenance of a 777 vs. an A350 (at least as far as ETOPS goes—obviously their construction necessitates very different maintenance programs), as that 40-minute increase for the A350 was approved without any in-service maintenance data. It may be different for the 787, which was not initially certified at 330 minutes.

Going forward, I have no doubt we'll see ETOPS 430 and above. Airbus already toyed with 420 minutes for the A350, and perhaps we'll see that after some time in service. At 420 minutes, there are only a few non-compliant routes; I expect that, eventually, any new twin will be able to fly unrestricted between any two points within its endurance.

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  • $\begingroup$ Great answer! I didn't even know there were single engine designs in scheduled operations. Which ones can you elaborate? $\endgroup$ – curious_cat Jul 18 '17 at 6:01
  • $\begingroup$ @Peter Schilling. There will never be single engined aircraft carrying 10s or 100s of passengers. It got nothing to do with power and efficiency, and every thing to do with reliability. Think what happens when an engine failure occurs. While IFSDs are rare on a per plane basis, there is at least on every day. See Aviation Herald: avherald.com $\endgroup$ – Penguin Oct 18 '18 at 11:13
  • $\begingroup$ Not sure how I missed these comments almost a year ago, and also not sure why I went on a tangent about single-engine designs; I've removed that part of the answer. $\endgroup$ – Peter Schilling Sep 21 at 17:00
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The answer ETOPS minutes categorised arrived from the Engine failure diversion range and the time is calculated based on OEI (One ENgine Inoperative) max range distance or other speed selected by applicant. I do agree with previous answer there is no point to go to 300+ minutes. see image. enter image description here The north and south pole concern is more critical and additionally there is Polar certification where the operator have to prepare the polar survival suit for crew member and additionally passenger arrangement incase they divert to this remote airport in advance (such as food, clothing, accommodation, etc)

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