8/18/2011

A320 vs. B737-800 Fuel Burn

There is the never-ending "war of words" between Airbus and Boeing of which aircraft is the more fuel efficient one, and, since the launch of the A320neo, if the A320neo would be more fuel efficient and if yes, by what margin. Further complicating, Boeing meanwhile claims, that it's yet-to-be-named reengined B737 would have the same advantage in terms of fuel efficiency as the current generations of these two narrowbodies have. And as even Boeing does not seem to know how the exact configuration of the reengined B737 looks like, this claim seems (at least) a little bit premature.
As I tried to lay out in an earlier posting, much depends on what you want to compare:  fuel burn per trip of fuel burn per passenger.
The official documents provided by the manufacturers
can provide some answers, as they give us numbers like empty weights, fuel capacities and also range-payload diagrams.
So let's compare the B737-800 and the A320-200 with a similar takeoff weight. I choose the 170 klbf/77.1t MTOW variant of the B737-800 and the 169,756lbf/77t variant of the A320-200 (WV010).
On page 3 and 4 of section 3-2-1 of the Airbus document you can see the payload range diagram of the A320-200 with CFM56-5B and V2500-A5 engines respectively. For a payload of  30,750lbs (205lbs per passenger you get a maximum range of  3,050nm.
Page 15 of the Boeing PDF document shows a range of just below 3,000nm for the B737-800 and the same payload.
To calculate the fuel burned for these two (virtual) missions, we need the "Zero Fuel Weight" (Operational Empty Weight + Payload) and subtract that from the MTOW.
Payload was 30.0750lbf for both aircraft.
Operational Empty Weight (OEW) is
  • 90,027lbf for the A320-200 and
  • 91,300lbf for the B737-800
Yes, contrary to what many people think, the A320-200 - meanwhile - is lighter than the B737-800; although that may be different from operator to operator and on a per-seat basis the B737-800 should be lighter anyway.

Thus the "Zero Fuel Weight" is
  • 120,777lbf for the A320-200
  • 122,050lbf for the B737-800
Fuel hold and fuel burn (minus reserves) thus is
  • 48,079lbf for the A320-200
  • 47,950lbf for the B737-800
Fuel burn per nautical mile flown then is
  • 15.76lbf for the A320-200
  • 15.98lbf for the B737-800
Thus the B737-800 burns 1.4% more to carry the same payload.
What puzzles me a little bit is that Airbus states a cruise speed of Mn0.76, whereas Boeing just states "at LRC". This could be anything between 0.76 and 0.78. If LRC means something higher than the Mn0.76, we would have to correct the range and therefore the fuel burn per nautical mile of the A320-200, so that range will fall and fuel burn would raise.

But - bottom line: for a typical mission of 500-800nm, where cruise speed does not play a big role - neither for trip time nor for fuel burn - the A320-200 and the B737-800 are very close in fuel burn for the same payload.
If one uses the higher passenger capacity of the B737-800, thus transporting more payload, the fuel burn will go up slightly. Fuel burn per trip would be above the A320-200 number then.

Boeing's claim, that the B737-800 would be 8% better can not be backed by this little analysis and was always disputed by Airbus. As Scott Hamilton pointed out recently, airline fleet managers also seem to back Airbus here.
And as the fuel burn improvement for the B737RE is seen by about 10% compared to the 15% of the A320neo, the gap on a per-seat basis between the A320neo and the B737-800RE should narrow, if not close. On a per-trip basis, the A320neo should have an edge anyway. This is particular true for noise sensitive airports where noise fees are factored in the landing charges. The smaller fan diameter of the B737RE will result in higher noise levels than the fuel burn and noise optimized engines on the A320neo.

Update Aug19, 2011:
I would like to comment on some comments made:
Of course the airport planning manuals are not too accurate and I did not intent to claim that I made a detailed and fully accurate analysis. But suggested that both documents have a similar level of accuracy, the conclusions should be not too far off from reality.
The Boeing document is for the non-winglet version of the B737-800, this is right. But for typical (short)missions the effects of winglets on fuel burn are not that big. And with the introduction of the "sharklets" the two aircraft would be on-par again.
The claim that the 737-800 flies missions the A320 cannot (range-wise) is contradicted by flyhigh@tom ina thread over at airliners.net. I cannot comment on that myself... 
Another post from seabosdca states some issues:
1) I already commented on the winglets.
2) I compared a mission with the same paylaod with about the same MTOW to get a feeling about the fuelburn per mile. Fuelburn per mile would not change significantly if I would choose the 79t MTOW version of the 737-800. In fact, it could even go higher as the aircraft has to lift more weight at the beginning. I once made a study and it showed that aircraft as different as the A380-800 and the B787-8 which are both designed for 7000-8000nm have the lowest fuelburn per mile when flown at ranges like 4000nm.
3) The mission I "flew" was the longest mission possible with that particular payload and MTOW, as the payload-range-diagram shows. The reason why I choose same MTOW's I laid out before. Normally, if you are an airline manager, you compare the same mission (same stage length) to compare fuel burn for different aircraft and you get the fuel burn for this particular mission from the aircraft OEM. As I did not have fuel burn numbers for any mission I choose to compare the fuel burn from reading the payload-range diagram. And to have missions with similar ranges I choose similar MTOW's.

9 comments:

  1. The Boeing claim always is "per seat", and the higher seat count of the B737-800 makes it difficult to draw conclusions.

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  2. Fleet managers extrapolate from what their experience is: the A320-200 and the B737-800. If even they back Airbus, it is easily to conclude how airlines will decide on their fleet renewals in the future.

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  3. I've flown both the A320(V2500) and 738W.

    The A320 is heavier (Minimum 500KG) and burns more fuel in the real world.

    The 738 also cruises higher (less fuel burn) and faster than tha A320 on real life missions.

    The 738W has more range with more payload (79t vs. 77t MTOW). My operator uses the 738 on sectors too far for the A320.

    Also the V2500 burns 25% more during taxi.

    Your above numbers don't include winglets either.

    Although, for actual flying, I prefer the A320.

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  4. The publicly available airport manuals are not accurate enough (deliberately so) to make any sense of these comparisons.

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  5. Your numbers may reflect theroy, but not reality. DL and AA fly their B-738s on TRANSCON US everyday, in both directions (non-stop). B6 flying the A-320 TRANCON US during the winter months must make a refueling stop (usually in PHX) before contiuning onto California destinations (west bound). All 3 airlines have their departure airports as BOS, JFK, or IAD. US cannot fly their A-320s westbound non-stop TRANSCON US from PHL or CHL to California without a refueling stop in PHX, either.

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  6. One thing regarding M.76 versus Long Range Cruise (LRC): Theoretically LRC gives best range and therefore beats any other flight Mach number. For an A320 with medium weights M.76 is pretty close to LRC, the B737 usually flies slightly faster as the wing doesn't like higher lift coefficients. So B738 LRC is a tad faster than the A320's LRC. On a 500nm mission (ZFW roughly 58-60t) the A320's LRC is close to M.76 especially if restricted in altitude.

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  7. Can you give me the link to where you got the fuel flows for Boeing and airbus. thank you.

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    Replies
    1. Difference between MTOW and Zero Fuel Weight.

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  8. Hi, I get different numbers while calculating, here you have:
    Trip Fuel + Reserve Fuel:
    B737-800 = 47.950 lb
    A320-200 (WV010) = 48.979 lb

    Fuel burn per nautical mile flown
    B737-800: 15,983 lb/NM (3.000 NM)
    A320-200 (WV010): 16,058 lb/NM (3.050 NM)

    ReplyDelete