Blip
4th Apr 2004, 04:03
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.
:)
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.
:)