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ETOPS Critical fuel scenario

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ETOPS Critical fuel scenario

Old 11th Feb 2021, 18:27
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ETOPS Critical fuel scenario

Hello, I haven't flown ETOPS for a while but while I'm awaiting a new course I had a read through the ETOPS section of our OMA.

When determining the critical fuel scenario it is determined by the most critical of the following scenarios:
  • A rapid loss of cabin pressure at the most critical point followed by a descent to a safe altitude as defined by oxygen availability.
  • A rapid loss of cabin pressure and a simultaneous engine failure at the most critical point followed by a descent to a safe altitude as defined by oxygen availability.
  • An engine failure at the most critical point and descent to one-engine-inoperative cruise altitude and diversion at one-engine-inoperative cruise speed.

I can understand and apply this concept but I just can't think (I'm probably missing something obvious) when number 3 would be the most limiting. Surely it will always be number two a simultaneous engine failure and pressurisation failure. Can anyone think of a scenario when 3 would be the most limiting? If not then I wonder why it isn't just 1+2 or even just 2.
Even Boeing (https://www.boeing.com/commercial/ae...icle_02_4.html) say that the decompression scenarios "logically define this reserve [fuel]"

Cheers in advance.
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Old 12th Feb 2021, 04:13
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Hi.
Think terrain.
In some cases the engine failure case (escape route) could be more penalizing than the decompression escape route (especially with lots of oxygen, where vertical profile could keep you quite high for a while)

Having said that, can’t remember the last time when engine fail + decompression was NOT the most critical scenario.
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Old 12th Feb 2021, 20:26
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Thanks OPDES, you're quite right. I've only really done ETOPS across the pond so wasn't thinking about terrain. Thank you for your reply.
In case two it technically doesn't say you have to descend to engine out cruise altitude like case 3 does. It only says "descent to a safe altitude as defined by oxygen availability." . I hadn't notice that before!
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Old 12th Feb 2021, 22:00
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Applecrumble. ‘Across the pond’ I assume refers to the Atlantic Ocean. Not sure if you have ever had to route further north, but might be worth reviewing the MSA over Greenland.
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Old 13th Feb 2021, 12:35
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In Australia each scenario has a different required set of reserve fuel requirements.

If yo uare flying a pressurised piston aircraft, then the engines (as they have a fixed compression ratio) don't have the change in fuel efiiciency with altitude that jet have, and will thus be affected differently in the scenarios you have listed.
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Old 13th Feb 2021, 13:08
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On most twin engined jet aircraft depressurised with both engines wil actually require more fuel than with OEI. One engine working harder is more efficient.

Last edited by BizJetJock; 13th Feb 2021 at 13:10. Reason: Specifying jetengines!
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Old 13th Feb 2021, 13:37
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The EDTO Manual does actually expand a little on case 3:

this condition is rarely limiting because the depressurization scenarios are typically based on a lower diversion flight level and thus carry a higher diversion fuel requirement. Consequently, some operators will not include the engine failure only scenario in their EDTO dispatch fuel calculations

...also:


The depressurization altitude is typically considered to be 3 000 m (10 000 ft); however, a higher altitude may be planned for if the aeroplane is equipped with sufficient supply for the planned diversion time


There is no prohibition on operating higher if oxygen supply permits. The wider commercial fleet cannot sustain it but it is permitted therefore the regs need to account for it. On the assumption that you must carry enough fuel for an extended diversion at 10k anyway, then case 1 vs 2 applies. But as the possibility exists that someone will come along with the capability to provide enough oxygen to sustain flight above driftdown altitude for an extended diversion, case 3 has to exist to head off the possibility of case 2 being used to justify dispatch with a reserve based on a fuel burn that could not be achieved.
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Old 14th Feb 2021, 01:04
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Originally Posted by BizJetJock View Post
On most twin engined jet aircraft depressurised with both engines wil actually require more fuel than with OEI. One engine working harder is more efficient.
However the engine out case comes with a large amount of trim drag, which makes the depressurized engine out case the critical fuel scenario on a typical twin.
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Old 14th Feb 2021, 09:26
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Checkboard,
The massive loss of TAS when a high performance twin piston, eg C414AW, loses an engine and descends from F230-F250 to F120 means that much more fuel is required to cover this case if over remote areas like the sea with no close alternates. We're talking 230Kts TAS to about 175Kts TAS. Similarly with a decompression, you still lose the TAS though not as much coz you can still make 155Kts IAS, say 185Kts TAS.
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Old 14th Feb 2021, 10:17
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On the aircraft I fly, the Specific range at 10,000ft all engines is 0.133 NM/lb, and OEI it's 0.14. That's over 5% better with one shut down. I think the aerodynamics apply across the board.
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Old 14th Feb 2021, 15:09
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it all depends on diversion speed...

I can't think of any scenario, where single engine drift-down is the most penalizing. As far as I know, terrain doesn't need to be taken into account for ETOPS fuel calculation.

However, normally one of other two (all eng decompression, or eng fail+decompresson) will be more penalizing, depending on the diversion speed chosen by the operator. Remember that both eng decompression is planned at [email protected], whereas eng fail+decompression is assumed to be flown at FL100 and Operator chosen OEI speed. The former is always the same, while the latter may vary...

So, if the operator choses a higher speed, 1 eng+deco will require more fuel, whereas if the chosen operator's speed is lower, then 2 eng+deco may be more penalizing
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Old 14th Feb 2021, 20:49
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Originally Posted by BizJetJock View Post
On the aircraft I fly, the Specific range at 10,000ft all engines is 0.133 NM/lb, and OEI it's 0.14. That's over 5% better with one shut down. I think the aerodynamics apply across the board.
I would guess it heavily depends on the configuration. Tail mounted vs wing mounted will give a big difference in asymmetric thrust and trim drag. If your username is an indication of what you fly that might explain.
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Old 15th Feb 2021, 08:54
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  • A rapid loss of cabin pressure at the most critical point followed by a descent to a safe altitude as defined by oxygen availability.
  • A rapid loss of cabin pressure and a simultaneous engine failure at the most critical point followed by a descent to a safe altitude as defined by oxygen availability.
  • An engine failure at the most critical point and descent to one-engine-inoperative cruise altitude and diversion at one-engine-inoperative cruise speed.
So,I think reading the scenarios individually word by word there may be a difference between what the regulations require and how you would actually operate the aircraft.

Clearly in 1 you could actually be at LRC speed and be able to stay quite high depending on the supplementary oxygen you have in addition to the regulatory minimum. I'm always thinking FL140/FL100 but technically could be higher depending on the excess oxygen in addition to the minimum.

In scenario 2 it actually says "followed by a descent to a safe altitude as defined by oxygen availability". So the descent is defined again by oxygen availability with no mention of OEI drift down alt. (Clearly in real life you would have to descent to the most limiting altitude but for the purpose of this regulatory critical fuel it doesn't say that!). So here is a scenario where you may be OEI LRC but held high due to supplementary oxygen similar to above.

In 3 it stipulates descent to OEI drift down at OEI speed.

I can now see in a particular set of circumstances dictated by any terrain escape routes, aircraft type etc that 1+2+3 will all vary and there may be an odd case where 2 is not indeed the highest value.

Thats my take anyway.
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Old 15th Feb 2021, 10:39
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I still struggle with this, did not the regulator simply list the combinations to cover all bases?

Hope I can express myself well:

Discussion on the article is relevant only when the article would apply - i.e. force the dispatcher to load additional fuel. Tankering disregarded, that is never desired. Consequently, when presenting the pilot with diversion strategies, the expert would have needed to say:
- this is your EO diversion and alternate route A-B-C, requiring additional regulatory fuel of XXX.
- this is your EO + DEPRESS diversion alternate route J-K-L, requiring additional fuel of ZZZ but that being less than XXX,
= we're uplifting XXX, the higher of the two, apart from the standard route requirement.

Bollocks. He'd say - in case of a simple EO you will fly the diversion alternate route J-K-L and profile which is designed for the EO+DEPRESS case actually, so that's all simplified for you. The additional fuel requirement is ZZZ, no more required than that.

If someone is suggesting there's a scenario when a certain route good for EO+DX cannot be executed under simple EO - without the help of DX -, let's hear it.

Perhaps an argument can be made that under a simple EO which is far less of an emergency compared to EO+DX, another alternate is preferred that may be further out. Well, no one is going to load greater amount of mandatory fuel for that possibility, plan A is to reach the destination AEO and carry payload instead. Not to mention the obligation to land at the closest (terms of time) adequate airport following a loss of one.

n.b. the posters so far seem to disregard drift down speed and ceilings as opposed to EO LRC ceiling and schedules in their comparisons. Not sure if that is relevant.

There is a theoretical piece missing in that puzzle, not mentioned previously, which I'll keep in a closed hand for a moment awaiting your feedback.
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Old 22nd Feb 2021, 04:06
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ICAO Annex 6 Part 1, Attachment D-1 paras 2.2.2 and 2.3.1 are worthwhile reviewing in this conversation.

There is a difference between the conditions for a 2 engine or a more than 2 engine case. It can often be the situation that the 3/4 engine aircraft depressurized is actually using less fuel than all engine case depressed. In 2 engine case, almost always you are better off on one as far as fuel goes, not so great on your cardiac system though.

I'm only familiar with some NAA regulatory reqts, so season to taste, but of the dozen or so different rules that I have operated and continue to operate ETOPS/EDTO/LROPS, there is a difference per Annex 6. If your NAA has something different, then that is great, they only have to notify ICAO within 6 months of variations to ICAO standards.

My current rides will either end up better off on one engine depressed or will end up with a dive over the side. In either case, fuel is not a big worry at that point.

just checked the OPM on a couple of the different jet types I fly, and both of those show that the SAR improves for the 2 engine jet, with an engine shutdown, at low altitude, which is also what was the situation in the last 4 engine plane I drove, and the last 3 engine jet as well. (results may vary, seek medical advice if... etc)



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