ETOPS critical fuel scenario question

[Otterman]

I don’t want to get to technical, but here it is.
The ETOPS Critical Fuel Scenario is based on:
-Simultaneous failure of one engine and the pressurization sytem,
-Immediate descend to and continued cruise at 10,000 MSL (helicopter descend),
-A descend to 1500 feet above the en route aerodrome,
-Holding for 15 minutes,
-An approach followed by a missed approach,
-Execution of an approach and landing.

There are fuel margins on top of this for icing conditions (TAT method), and wind corrections, etc (they play no role in the question).

So here finally the question:
It has always been my understanding that upon arrival (in other words the landing after the second approach), you would still have to have a legal amount of 15 minute fuel. I have had discussion lately with colleagues who’s interpretation of the ETOPS Critical Fuel Scenario will mean that you will flame out during your roll out (pure theory of course), to me this is a substantial difference. My interpretation has been that the 15 minutes holding fuel in the ETOPS critical fuel scenario was based on the consumption of the normal Final Reserve (30 minutes) up to the point where you land with 15 minutes. Any feedback would be welcome.

Greetings O.

ps. my search through pprune and the internet has not provided an answer.

[BizJetJock]

The 15 minutes figure is a reduced final reserve figure, to take into account that you are in an emergency situation and would expect to get priority from ATC.
Of course, the reality is that for most aircraft the critical fuel scenario is not OEI and depressurised. Most jets at 10,000 will burn more fuel with all engines operating than with one failed:confused:

[Otterman]

I fly the left seat of the 777 and I don't want to go to deep into the ETOPS question (DX, DC or 1X scenarios are outside the scope of what I am looking for).

Just to clarify your statement, it is your interpretaton as well; that after the Critical Fuel Scenario you will still have 15 minutes fuel remaining (also classified as holding fuel). So on the runway you still have 15 minutes, iso of a flame out during the roll?

Greetings O.

[Bus14]

In your simplified scenario, if you divert and then hold for 15 minutes before commencing the 2 approaches then, yes, you will flame out shortly after landing.:=

If you divert, immediately commence the 2 approaches and land after the second one, then you will taxy in with the 15 minutes holding fuel still in the tanks.:D

The 15 minutes holding fuel is the final reserve fuel and replaces the 30 minutes reserve fuel in the non-ETOPS rules

[Otterman]

What I wrote in my first post is a copy out of our EEM (Etops Exposition Manual). It is the what the Critical Fuel Scenario is based on. The amount of fuel is dependent on what requires the most fuel, (DX, DC, 1X), but again outside of the scope of the question.

So it isn’t a simplified scenario, it is the minimum requirement on what the fuel scenario is based.

The final reserve fuel is: a legally required; fuel reserve to cover a low fuel emergency. The fuel to fly for 30 minutes at holding speed at 1500 ft. above aerodrome elevation in standard conditions, calculated with the estimated weight on arrival at the alternate or destination. And is indeed non-ETOPS.

But:

Fuel contingency fuel check.
At the point of no diversion (PND) the minimum fuel required in case of contingency, a diversion to an adequate aerodrome is still possible with a minimum fuel endurance of 15 holding minutes upon landing. This is the minimum legal required quantity of fuel on board upon landing.

My interpretation has always been that the critical fuel scenario has to be based on the conditions of my first post. So the 15 minutes holding occurs before commencing your first approach (that is a fact). But my understanding was that you would still have 15 minutes upon landing as it states under fuel contingency fuel check.

With my interpretation there is a correlation between the Final reserve fuel (30 minutes), and the ETOPS critical fuel scenarios (15 + 15 minutes). In the other interpretation there is no correlation, and the ETOPS final reserve is 0 minutes, because the initial 15 minutes holding fuel in NOT a contingency fuel.

That is the essence of the question.

Greetings O.

[chksix]

Not a pilot but very eager ;)

Is ETOPS really counting fuel down to flameout or down to minimum fuel allowed?
By min fuel I mean the level when the Low Fuel alerts come on.

[mutt]

The only people who can answer your question are within your own company.

In our case, the 15 minutes of fuel is based upon holding upon arrival, you can then make the approach followed by a VFR 2nd approach landing with ZERO fuel. We didnt agree with the VFR approach so we added additional fuel to make it a full 2nd approach, but yet again landing with ZERO fuel.

Now before i scare anyone, thats the way the calculation is done. In our case because the destination is beyond the ETOPS area, the fuel remaining is always greater than required.

FAA Regs...

Rgds

Mutt

[AtoBsafely]

chksix,

Not just ETOPS, but all legal fuel requirements are to "flameout" ie based on useable fuel. It would be a very bad day to just lose pressurization and an engine at the most remote point on your flight, let alone then having to go around when you get to your emergency alternate. The rules are made for the worst imaginable case.

The low fuel light is only there to add stress on that day!

[Bus14]

Otterman, when I stated 'simplified scenario', I was merely agreeing with the parameters that you had set in your question. It was not a criticism.

Anyway, I say again:

When considering the ETOPS CFS, you only have 15 minutes final reserve fuel for the ETOPS diversion. However, you have taken account of the possibility of a go around and second approach at the diversion airport.

When considering your destination minimum reserves you have 30 minutes final reserve fuel, but have not taken account of the possibility of a go around and second approach at the diversion airport.

They are both planning considerations but are 2 different cases, the ETOPS CFS is a completely different scenario to the destination case.

The quotes that you have made from your company Ops manual do not contradict themselves. They are considering 2 separate cases.

Pilots may be tempted to add further fuel to the CFS in order to introduce some sort of comfort factor to restore the 15 minutes 'extra' fuel that you thought you had. However, before doing so, consider the chances of a simultaneous double failure, at the CFP, followed by needing to hold despite both you and the destination having had 180/138/120 minutes notice of your arrival, followed by having to throw away the first approach. I'm a cautious pilot, but I'd have given up flying years ago if I really worried about scenarios like that.

I repeat though, nothing I have posted is a criticism of you Otterman. I merely disagree with your understanding of the ETOPS CFS.

[chksix]

Thanks AtoB :ok:

This means I have to edit my xplane fuel planner to reflect this. :}

[Otterman]

Ok, thanks for the answers, the question is circulating within the company. I believe you are right in your assessment that the CFS is based on a flame out on the runway. In my interpretation I had tried to find some relationship between the final reserve and the ETOPS CFS. It has always been clear that they are truly separate things, but it would have been logical to me to have a correlation.

I also agree that in actuality your fuel status is unlikely to get to that point, there are a lot of margins, and conservative assumptions. It was a pure theoretical question, but nonetheless I hope it wasn’t a useless question, and it helps some others also get a greater understanding of this type of operation.

The answer from company will contain, all relevant regulations, and if they provide more clarity I will post them.

Thanks for the input.
Greetings O.

[Blip]

This very question as well as others relating to ETOPS has been on my mind for quite some time. I'm glad to see I'm not the only one with issues with the way ETOPS is practiced at the moment.

Since when did Holding Fuel become Fixed Fuel Reserve??

Who here believes it makes good sense to PLAN to land with only vapors in the tank? Because that is what you are doing if you decide that 15 minutes holding fuel is really 15 minutes Fixed Fuel Reserve. The fact that it is contingency planning and that the chances of the worst case scenario actually occurring is very small is irrelevant.

And why should it come down to a companies interpretation of the ETOPS guidance material? There should not be any interpretation required. The intent should have been made clear from the time ETOPS was conceived.

The UK ETOPS literature can be found here which pretty much agrees with Otterman's original post.
http://www.caa.co.uk/docs/33/CAP513.pdf


Critical Fuel Scenario

The following describes the assumptions to be used in determining the fuel reserve
required to cover the en-route diversion case. The operator should confirm that this
scenario is operationally the most critical, having considered also the possibilities of
no engine failure but total pressurisation failure, and no pressurisation failure but one
engine failure.

i) At that particular critical point, consider simultaneous failure of an engine and
the pressurisation system; (critical point based on time to a suitable alternate
at the one-engine-inoperative cruise speed, for existing conditions, using
forecast winds at the appropriate flight level).

ii) Immediate descent to and continued cruise at 10 000 feet at the relevant one-
engine-inoperative cruise speed (or above 10 000 feet if the aeroplane is
equipped with sufficient supplemental oxygen in accordance with the Air
Navigation Order (1989), Schedule 4, Scale L1).

iii) When approaching the diversion aerodrome, descend to 1500 feet above
destination, hold for 15 minutes, initiate an approach followed by a missed
approach, and then continue to a normal approach and landing.

iv) Unless the operator has an established value for in-service deterioration in
cruise fuel mileage, the fuel calculated under (i) and (ii) above should be
increased by 5 per cent.

My question would be. Is it possible that the authors of the above paragraphs have made the assumption that the aircraft will land with the usual Fixed Fuel Reserve? I mean, why wouldn't they. FFR is a given in every other scenario. Surely if ever you would want the margin for error that the FFR offers is when you're down to one engine and/or the oxygen masks are dangling from the ceiling!

[cjam]

When I first started flying etops i didn't understand it until someone explained that the "critical fuel scenario" is not always the most fuel required for the flight. i wonder if one or two people here were having the same misunderstanding as I had.
eg On a nice day the fuel required from A-B plus 30 min reserve might be 15 tonne,
On same flight fuel to a critical point, eng fail and depressurised flight to either A or B plus 15 mins hold, plus one IFR approach plus a visual circuit to land might be 17 tonne. On this day the CFS is the min deparure fuel.
On crappy day fuel required A-B then divert to C plus fuel for icing at C plus a traffic advisory holding plus 30 min reserve might be 25 tonne, whereas the citical fuel for single eng depressurised flight to C plus 15 holding plus one IF approach plus one visual circuit might be 22 tonne.
The flight planners run separate flight plans for the different scenarios then see which one requires the most fuel. For the crapy day you can see that the CFS fuel is well covered by the fuel required for the flight due weather. ....Man I'm confused again....might lie down.

[Bus14]

I'm not the only one with issues with the way ETOPS is practiced at the moment

This is the way ETOPs (EROPS) has been practised for the last 20 plus years

Since when did Holding Fuel become Fixed Fuel Reserve??

What else would reserve fuel be used for?

Surely if ever you would want the margin for error that the FFR offers is when you're down to one engine and/or the oxygen masks are dangling from the ceiling!

contingency fuel is for margin for error, not FFR

All the rules, whether it be ETOPS, MEL, or whatever, are based on probabilities of an occurence happening versus an accepted risk level.

For the destination case, as I've stated above, there is no margin built in for a go around at the diversion field, but you can use the final reserve fuel to account for a go around, or for some holding, or a bit of both. For the ETOPS case, as I've stated above, the airport has had many hours to prepare for your arrival, as have you (whether you have a rubber jungle or single engine or not) and the CFS has allowed not only for the possibility of a go around, but for 15 minutes holding as well - it is hard to justify routinely carrying extra fuel, particularly as to get yourself in this situation requires a serious double failure, at the most fuel critical point in the flight, on a flight where the CFS required more fuel than the flight plan. The probablity of the occurance does not justify carrying even more fuel, therefore the rules don't require it.

There is a similar logic for MEL issues. For most 2 engined aircraft, having a pack or bleed failure before despatch requires the cruise level to be capped at a lower level than standard. This is because you are now operating a single pack aircraft, and staying lower restores the risk level in the event of the single pack/bleed failing. However, if you despatch with 2 operating packs, and one fails in flight, most regulators do not require you to descend to a lower cruise altitude. The possibility of a dual, independant, failure of both packs/bleeds on the same flight has already been considered and is deemed to be within the accepted risk level.

There are occasions where it is prudent to carry extra fuel for foreseeable contingencies. As far as the ETOPS rules are concerned, I am comfortable that this is not normally one of those occasions. Yes, an approach to a strange airport, on one engine (50% probability, it could have been a bleed problem that forced the diversion), on a second approach, on minimum fuel will have a high 'pucker' factor, but, as a top human factors analyst once said to me - 'you're paid all that money to get it right on the 10 to the minus 9 occasion'.

The bottom line for me is that if I fully understand the philosophy and logic behind a rule, then I am comfortable to follow it and can fully justify the occasions when I carry extra fuel. That, for me, is a professional approach to the task. If there is a rule that I don't fully understand, then I should follow it anyway, but I have lost the comfort level that the understanding of the rule would give me. If my day to day attitude to the job is to interpret the rules in my own way and routinely add 'comfort' rule without justification, then I'm probably in the wrong job.

The last paragraph is not intended to be a criticism of any other posters. It is simply an explanation of how I keep my comfort factor without routinely carrying, and burning, extra expensive fuel to the detriment of my employer's shareholders and the planet. I'm not saying I don't add extra fuel. If I have good reason I carry buckets of the stuff - the ETOPS CFS, however, for me, is not usually one of those occasions.

[Blip]

Bus14 Thanks for your thoughts. I think we might have to agree to disagree.

For me it comes down to this. Fixed Fuel reserve is not the same as Holding Fuel. Holding fuel is extra fuel that allows you to, well, hold, to "hang around" until you have the opportunity to make an approach and landing. It is carried when the need to hang around can be expected due to either the forecast weather, or the forecast traffic delays at a particular airport at a particular time of the day.


The Fixed Fuel Reserve is like the "Reserve Tank" I had on my two stroke motorcycle and the 1962 VW Beetle I used to drive, neither of which had a fuel gauge. When the car engine started spluttering due to fuel exhaustion, I could reach down with my foot and rotate a metal leaver 90 degrees. That allowed it to bypass the standpipe in the fuel tank so that it could utilise the last remaining 5 litres of fuel and hopefully allow me to make it to a petrol station.

(I want to stress here that even though there was no fuel gauge, I kept very good records of fuel uplift vs distance travelled, and I got pretty good at estimating when I needed to fill up again based on the odometer reading. I never planned to use the fuel tank lever, but it was there to save the day when required.)


Now imagine you are flying an aeroplane with the standpipe in the fuel tank. All the flight planning you do both for normal operations and contingency (engine failure, and pressurisation malfunction as required by the regulations) assumes that the last of the fuel below the standpipe level will remain there, always. I repeat we are talking about PLANNING for fuel burn.

You must PLAN for "Normal" fuel requirements...
FLight Fuel allowing for gross weight, enroute winds, chosen altitudes and speed, and an approach allowance, plus;
Holding Fuel for weather, and/or estimated ATC delays, plus;
a Variable Reserve equal to 10% of the Flight Fuel (this is the margin-for-error fuel!)

You must also PLAN for "Contingency" fuel requirements...
You must assume that an engine failure and/or a depressurisation can occur at any time during the flight. You must identify the most critical point on the flight to suffer such a malfunction and then calculate how much fuel it would take to make it to a safe place for landing. Again there are allowances for the gross weight, altitude, enroute winds, and an approach and landing plus;
Holding fuel for weather, and in the case of ETOPS, 15 minutes for "whatever", plus;
a Variable Reserve of 5%;

Now my argument is that all of this fuel is assumed to pass through the fuel tank stand pipe!! (For those who don't know what a standpipe is, it is simply a tube that sits vertically at the bottom of the tank. The top of the tube is some given level above the bottom of the tank from which the fuel is drawn. As the level of fuel reduces to the top of the tube, obviously it is just as though the tank is running out of fuel. But of course it is not empty. By flicking a switch, you can bypass the top of the tube and draw the remaining fuel from the bottom of the tube and hence bottom of the tank.)

If however for a million unforeseen reasons you find that after all this planning for the worst you are still not yet safely on the ground, you do not have to fall out of the sky! Rather than doing that and killing all on board, you have the ability to reach down and select the "Standpipe Bypass" lever which allows you to get access to the last dregs in the fuel tank that gives you just enough time to get the thing on the ground. You get a second chance.

Fixed Fuel Reserve is the equivalent to the standpipe. Basically you pretend it isn't there until the day you would otherwise fall out of the sky for reasons that could NOT be anticipated.

Engine failures and cabin depressurisation CAN and in fact MUST be anticipated.

iii) When approaching the diversion aerodrome, descend to 1500 feet above destination, hold for 15 minutes, initiate an approach followed by a missed approach, and then continue to a normal approach and landing.

To me this is describing in chronological order the anticipated contingency fuel burn requirements. It doesn't mention how much fuel is to be in the tanks at the end of the landing roll, because that is just a given. You are ALWAYS expected to land with a Fixed Fuel Reserve.

I'll put it this way. I am arguing that the paragraph should be amended to read:

iii) When approaching the diversion aerodrome, descend to 1500 feet above destination, hold for 15 minutes, initiate an approach followed by a missed approach, and then continue to a normal approach and landing to arrive with a Fixed Fuel Reserve being the equivalent of 30 minutes holding fuel at 1500 ft.

while you are arguing that it should read:

iii) When approaching the diversion aerodrome, descend to 1500 feet above destination, hold for 15 minutes, initiate an approach followed by a missed approach, and then continue to a normal approach and landing while suffering complete fuel exhaustion and engine flame-out.

Which one sounds reasonable? Which one does not?

[Hardass56]

Folks, I have to agree with Bus14 on this issue.
One has to look at ETOPS as a very unlikely probability of diversion due to the extensive flight planning and aircraft serviceability requirements. The Etops alternates are chosen far more conservatively than destination alternations. Therefore, in the UNLIKELY event of diversion for e.g. illness, cargo fire, the alternate should remain open.
Therefore, because it is extremely unlikely to use the alternate, the fuel reserves are permitted to be reduced below what one would normally consider normal.
U may choose to agree or otherwise, but that is what the regs say.

[BOAC]

Another vote for Bus14. Why would you expect to 'hold' at all? The 'missed approach' fuel is a big safety margin too wrt the weather criteria for ETOPS alternates. If you are to start catering for that 'nasty' failure AT CP AND weather going down/blocked runway etc etc you we might as well not fly! Then of course we COULD lose the second engine on the div.............. Life (and aviation) is all about reasonable risk management.

Even the '30 minutes' reserve we are discussing is a figure 'plucked out of the air' and may not be 'enough' some time with unusable fuel/gauge error etc.


I'm happy!

[FullWings]

Yes, I agree too. The probability of the worst-case scenario is so small it can almost be discounted. If you're really unlucky on the day you could always do the div. at 15,000 or above if you wanted to save fuel; you'll have a small buffer anyway as has been pointed out, the regs. work on an instantaneous descent which won't happen. In my experience on ETOPS routes, it's about once a year that the flight plan includes more fuel for the critical case; rest of the time there is much more than than required so you'd arrive at the diversion with considerable reserves.

30mins fuel seems to be very close to 15mins of fuel plus two approaches (maybe less at some places...) Most are happy planning to arrive at a diversion airfield with the former, so why all the fuss about the latter?

[Bus14]

Which one sounds reasonable? Which one does not?


Blip, I can't fault the apparent common sense and logic of your case. However, the fact remains that, on a commercial jet, guage fuel is useable fuel and the rules are based on probabilities.

Non ETOPS - Final Reserve Fuel is useable fuel and caters for unforeseen events at the diversion airfield ie go around or holding. Under JAA rules, in certain circumstances, we can eat into final reserve fuel at destination if it is likely that a delayed landing at destination would result in landing with more fuel than would result from a diversion

ETOPS - Holding at the diversion and/or a go around has been built into the figures, so there is no Final Reserve Fuel

Perhaps I can risk illustrating my point with a case from a previous life (different rules, but similar probability logic). In good weather, RAF Harrier aircraft do not require a diversion airfield, and do not require a final reserve fuel. All guaged fuel is useable.

Occasionaly one could be in the hover, with a fuel flow of over 200 lbs/min with less than 400 lbs on the guages. Not comfortable, but safe enough (for the next 90 seconds at least!), and, most importantly for this illustration, perfectly legal.

As I stated before, we, as professionals, are paid to get it right on the 10 to the minus 9 day when it all goes wrong. By understanding the rules we can operate both safely and efficiently. From my observation as a senior examiner in an ETOPS airline, there are days when the professionals carry more fuel than the cowboys. Nevertheless, over a year of flying, the professional will usualy, by understanding the rules and applying them correctly, have carried, and burnt, significantly less fuel - safe and efficient.

I have no personal argument with any of the posters on this thread, but I am convinced that my understanding of the ETOPS and non-ETOPS rules is correct.

[BOAC]

Bus - I agree with your last para. We may well have met in your 'fore-life', having been there myself waiting for the 275 light to come on (and secretly hoping it is working:))

I would disagree with your comment on useable fuel, and the 737 has, I believe, a 2 1/2% gauge error for Classics and 1% for NG (not a lot, I grant you at 1200kg, but finite), and an unknown 'unusable' amount depending on attitude (a/c not pilot:8). Hence I never ASSUME I have 30 mins to tanks dry.