PDA

View Full Version : Do you really have enough fuel? / B737 performance question.


Blip
4th Apr 2004, 05:03
I'm looking for B737 Classic or NG engine out diversion fuel burn figures. By that I mean fuel burn requirements for climb-cruise-descent at long range cruise or even max range cruise speeds from take-off or rejected landing to an alternate aerodrome.

Does Boeing have such figures? Do any operators of B737's have their own figures?

The reason I would like this information is because I believe this information is vital for adequate contingency considerations during flight planning.

Critical Points when considering engine failure or a cabin depressurisation are often identified as points midway between two aerodromes along the planned route. The most critical of these is the last CP between an enroute aerodrome and the destination. The reason this is the most critical of all the CP's is because you reach this point with the least amount of fuel in the tanks.

However if your destination is suffering from fog or other weather which is below landing minima, the most critical point for an engine failure is not enroute between aerodromes, but rather at the ILS minima at the destination!

Think about it. If you're planning a flight to somewhere that is forecast to be suffering fog during your ETA, you will plan to fly to that destination and also plan to divert to an alternate. You will also consider the possibility of suffering an engine failure or depressurised cabin at the most critical point enroute and add any extra fuel required to cover this possibility.

Now the question is this. Where did your flight planning computer consider the most critical point to be? I bet it was somewhere between an aerodrome you pass by enroute and your destination. Then it only considered Drift-Down from cruise to an Engine-Out LRC. But is this really the most critical point along that PLANNED flight path? NO it is not!

The worst place to have an engine failure is during a go-around from the ILS minima.

You knew it was a real possibility that you were going to have to divert otherwise you would not have been carrying diversion fuel. Did you consider suffering an engine failure at that moment of your PLANNED diversion? Probably not.

The question will be asked "Why not??"

Now that you have suffered the engine failure, do you have enough fuel to divert to your nominated alternate airport? "I... I don't know. Maybe."

What about somewhere else? Somewhere closer. Anywhere!! "Err... probably." :uhoh:

It's a bit late now, trying to find a place to divert which will allow you not to run low (if not out) of fuel.

The same should be considered during a low vis take-off where the visibility is above take-off minimas but below landing minimas. You might have nominated an aerodrome (as one you will fly to if you suffer an engine failure after V1) but if this aerodrome is also your destination, do you really know that you will be able to make it there and land with reserves?.

And what if you tail scrape on rotation, hit a bird and crack a windscreen, or simply have a pressurisation problem that requires you to divert (or rather to simply continue as planned) at 10,000 ft? Have your flight planning fuel figures allowed for this possibility?

If you check your regulations, (CAR 234 in the case of Australian aviation), you will surely read that it makes no mention of which flights are exempt from these considerations. All planned flights must consider the possibility of suffering an engine failure or cabin depressurisation at ANY moment during the planned flight.

I have done some number crunching of the performance figures that I do have for the B733/4/8 and have determined that in the enroute cruise, at a given gross weight, when compared to normal two engine LRC:

Engine-out LRC burns 21% more fuel and is 13% slower.
Depressurised (two engine @10,000 ft) burns 49% more fuel and is 20% slower.
Gear down burns 89% more fuel and is 29% slower.

These figures assume that the aircraft is at the Optimum altitude.
The thing to note is that the differences are reduced when the "Normally" operating aircraft is lower than optimum. That is to say when cruising at a low altitude, the percentage difference of the Specific Air Range is much less.

I wonder if this means that the difference is also less during climb. Therefore if a large part of a diversion is taken up by the climb and descent, perhaps the total fuel burn (in the case of a Engine-Out diversion) will not be 21% more than the normal diversion fuel burn but rather some figure less than this, depending on how far away the alternate aerodrome is.)

The way I see it, to overcome this flight planning vs regulations dilemma, add this Engine-Out or Depressurised diversion fuel burn to your orthodox Enroute CP contingency fuel calculations This would allow you to suffer an engine failure or depressurisation at that last enroute (in cruise) CP and continue on to the destination. You would then still arrive with enough fuel to divert to the nominated alternate and land with the required reserves.

Perhaps the contingency diversion ports need not be the nominated alternate aerodrome. Perhaps if it was a lesser "Emergency" aerodrome, an aerodrome closer to the destination could be nominated an therefore the contingency fuel requirements would not be so penalising.

You might say that the normal fuel reserves should take care of these considerations. This might be the case. But this is also just a guess at best.

I hope these ideas stimulate some further thought on the subject.

Regards.

Blip.

:)

john_tullamarine
4th Apr 2004, 05:27
Unless things have changed in Oz (which could well be the case, I guess)

(a) if the departure is below landing minima then you have to have enough fuel for a diversion and prudent performance engineering will address that for an emergency situation arising during the takeoff.

(b) if the destination is below alternate minima then you have to have likewise.

Or have I missed something in your post ?

Vol 3 normally has all the stuff you need to figure the sums although I haven't had an opportunity to run through an NG volume.

Blip
4th Apr 2004, 07:01
The only information our performance manuals have regarding Engine Out performance is in the form of cruise tables where you line up Weight with Altitude to read off IAS, TAS, N1, FF etc. The other is a quick reference chart where you enter with distance to aerodrome and initial drift-down weight, and you read off the Fuel Burn and Time Interval.

There is no Engine Out Climb-Cruise-Descent information.

So John are you saying that you have this kind of information and that your flight planning can and does make engine-out and depress diversion calculations on all flights where your destination requires an alternate?

Thanks.

LEM
4th Apr 2004, 11:24
Agree with Blip, this issue is underrestimated, and will continue to be, until....

BOAC
4th Apr 2004, 11:39
Are we not straying into the land of 'double disasters' here, which have never normally been taken into account in aviation - vis a vis second engine problems following ETOPS first engine shutdown?

john_tullamarine
4th Apr 2004, 12:24
Two points.

(a) the manual data to which Blip refers are, presumably, contained in company performance manuals which will have been produced from the relevant Boeing data ? If so, then the pilot group ought to take the matter up with management if the pilot group believes that the status quo is not quite the way to go.

I have file data for 200/300/400, which may be out of date as I have been away from 737s for a while now.

However the point is .. your flight standards management people will have access to the relevant current data and have only to direct the performance engineering people as to company policy in respect of this failure and that.

I have seen some policies which make me shiver but that is the way it goes provided that the regulatory requirements in the relevant country are met as a minimum standard. I can recall one colleague, who was working for a major airline at the time, telling me that they didn't worry about any takeoff failures other than V1 cases ... certainly not the approach that I would take in performance scheduling work where there are obstacle considerations ... and the failure might well occur before the critical turn ?

(b) BOAC's point is very valid. Certification generally is based on one major problem. Normal practice is to consider subsequent problems which have a reasonably likely probability.

While I appreciate that it may not be easy in a deregulated and weak (or no) union environment sometimes the captain has to earn his money by making prudent risk-based assessments before flight even if that flies in the face of management pressures... easy to say ... yes .. but that is the way things are.

I can recall the odd occasion where well intentioned pilots tried too hard to follow the company preferred options and ended up in uncomfortable positions while they sweated ... watching the fuel gauge tick down towards zero... and the occasional case where the aircraft managed to get in .. and the engines failed on the rollout. I am sure that we all know of such things.

It is not just a matter of adopting an overly conservative policy in the real world .. a good way to go broke quickly. Clearly the two extremes are to have full tanks all the time and patently not enough fuel for the flight. Often a prudent and reasonable fuel load is somewhere in the middle. It must remain a decision process balancing reasonable corporate goals against sensible risk-mitigating flight planning. It is, of course, comforting to meet the regulatory requirements as well.

Granted it can be a tough time for the captain but, if one seeks an always easy life, then perhaps the life of the bank clerk may be the way to go .... and, very occasionally, things go totally topsy turvy once the aircraft is launched and the outcome may not have a rosy ending.

Blip
5th Apr 2004, 10:03
Thanks for the replies.

Sorry BOAC but I think the answer to your question is no.

I haven't mentioned mutiple failures. I don't consider weather at destination below landing minima a "disaster" at all, especially if it was already forecast to be below the minima and you already planned to carry diversion fuel.

It is simply a matter of identifying, before departure, the most critical point to suffer an engine failure or cabin depressurisation and then calculating how much fuel it would take to enable the flight to proceed from that point to an aerodrome that you (or the company) consider usable.

Sure if while enroute the destination weather suddenly and without notice, reduces below Alternate or Landing Minima (or what ever it is that triggers a change in your fuel requirements) and you've passed your PNR or CP then that's when you'd say you're having one of THOSE moments!

All I'm saying is that I reckon in those cases when the destination requires an alternate, the most critical point to suffer an engine failure or cabin pressurisation failure is during a missed approach overhead the destination.

I was hoping someone might have some data that compares two engine diversion fuel burn with engine out diversion fuel burn for the B737. Perhaps B767 or B777 or even twin engine Airbus drivers might have some idea of the comparison.

John I don't think my company (or many others from what I've gleened here) have identified this as an issue. Actually I tell a lie. The company policy is to not pass that enroute CP unless the destination weather comes good. But if that is the case, why bother carrying diversion fuel? Why not take it that extra step and put on the extra fuel and then all eventualities will be covered, the regulations will have been complied with, and everyone is happy.

So yes when I have faced this scenario during the flight planning an have made that decision to put on the extra fuel. But the amount has been nothing more than an educated guess based on the enroute figures I mentioned in my earlier post. I know I could do better if I had the information I am seeking.

Cheers.

john_tullamarine
5th Apr 2004, 13:28
Blip,

I don't disagree with your position. However the data is available. If your company chooses not to publish it in one form or another, why don't you approach your performance people to get access to the data on a one-off basis and then you have the comfort you seek for your own flight planning. Do keep in mind that there is a range of variants for these aircraft and you need to make sure that you refer to the correct set of data.

Certainly, one airline for which I flew provided such data to operating crews ...