DC-9 Fuel Efficiency

[rjemery]

Because of the recent ditching in New York's Hudson River, the A320 has been in the news. The WikiPedia site has this comment in its Airbus (http://en.wikipedia.org/wiki/Airbus_A320_family#A320) article:

With jet fuel prices rising dramatically, Northwest Airlines is replacing McDonnell Douglas DC-9 aircraft it has had in service for decades, because the A319 is 27% more fuel efficient than the DC-9.

citing as its source the NY Times article To Save Fuel, Airlines Find No Speck Too Small (http://www.nytimes.com/2008/06/11/business/11air.html?scp=1&sq=airlines%20speck%20too%20small&st=cse) (June 11, 2008).

My dumb questions:


Is it the aerodynamics of the DC-9 itself that is not fuel efficient compared to more modern jet aircraft?
Could not the DC-9 be refitted with newer engines to achieve nearly the same fuel efficiency as with new aircraft?
Or was it simply that NWA's DC-9s were simply too old and at a retirement age?

[BelArgUSA]

Hola rjemery -
xxx
Well, you go compare two airplanes that are quite different.
The NW DC9-30s (and maybe 50s) are 1960-70 era airplanes.
Their engines (JT8D-9 or higher dash number) are .58 SFC for fuel burn.
The A320 family airplanes are 1990 and up-to date technology.
The CFM-56 is an engine with about a .35 SFC efficiency.
Certain that newer DC9s (i.e. MD-87s) would look much better.
xxx
What to replace the old NW DC-9s with...?
Obviously, modern A320 or 737NG airplanes.
I am certain than both are very similar in efficiency/economy.
The bottom line is the financing end.
European carriers definitely favor Airbus products.
And US/Canada operators will probably favor Boeing products.
It is both politics, banks, and high finance.
And put in there the "merge" factor of DL with NW...
xxx
:8
Happy contrails

[beachbumflyer]

Does it make sense to replace a DC-9 that is paid for for a $60M aircraft
that you have to pay, so that you can save 30% on fuel. Doesn't make
too much sense to me. It's like spending $3 to save one.
There has to be more to it, like creative accounting, commissions, etc.
What do you think, guys?

[Rainboe]

Re-engining is a lot more expensive and complicated than you think. It causes major restructural and engineering complications, weight and balance problems and design considerations. It has to be tested in airflow chambers, extensively engineered, flight tested etc. It is VERY expensive. Worth doing for a big program like the KC135, but otherwise, the old airframes don't have the life left in them to make it really cost effective. It really is simpler to start again with the latest passenger-attractive equipment. Those old airframes have far higher maintenance and reliability problems. You're really only putting off having to upgrade at some stage.

I could have been driving round in cheap old bangers for years, maybe even re-engining them. But it is better to get a shiny newer BMW and experience quality, comfort, reliability and more street cred. Better to let my old bangers get scrapped, or shipped to places in the world where people will keep them going, but they'll keep breaking down! Just the same with aeroplanes!
European carriers definitely favor Airbus products.
Not sure I agree! European airfields often look like Boeing Field? Tell that to Ryanair with 170(?) 737-800s. Looks to me like Europe is pretty evenly split, even possibly in Boeing's favour. It all comes down to doing the deal with the man on the day!

[411A]

Then real reason for airlines to obtain new aircraft for their fleets is tax depreciation.
Most new aircraft are depreciated in (generally) fifteen years, some faster, under acccelerated depreciation rules.
Leasing is another option, with many beneficial tax options.

For a large airline to retain old fuel-inefficient aircraft is generally poor economic planning.
Usually...there are a few exceptions.

[galaxy flyer]

411A

That would be true IF there were any profits to apply the depreciation against! :confused:

Regards the DC9, they have, in addition to fuel consumption, just about used up their economic lives. Maintenance is expensive, parts are hard to find, avionics are out-of-date and advanced EFIS FMS systems are almost as hard to change as re-engining. They had their time.

GF

But not the Tri-Star :}

[mathy]

Now you're comparing two different spec sheets. GE and CFM tend to be 1-2% better in cruise than their R-R and PW rivals but this is not dramatic enough sales-speak. The engine burn at sfc 0.35 lb/lb-hr is from a spec sheet for sea level static take-off conditions. The other figure is 0.58lb/lb-hr in cruise mach 0.8 at F350. The original DC-9 engines were much nearer an sfc of 0.8 in cruise. So the money saved is from figures 0.8 bad, 0.58 good.

Please don't fall into the salesman's trap of thinking there are engines that have an sfc of 0.35 in cruise. Apples and Oranges.

There will be a flight level, speed and load condition for any engine airframe combination that produces the best sfc figure. There is an engineering tool called Buckingham's Pi-Theorem which salesmen hijack to bat their eyelids with at prospective customers. I'd call it finding a ruse to put your products in the best possible light at the expense of your opponents and hoping that no-one will notice.

Cheers

[Northbeach]

rjemery,

Your question is not dumb at all; in fact it is a good one. As it has already been pointed out the DC-9 design is forty years old. Therefore it is unable to compete with new engineering, technology and composites. This is especially true in the wing design and the engines.

As we age it gets more expensive to keep us running, so it is with aircraft. At some point it becomes cost prohibitive to maintain an airframe any longer. Our company recently retired their fleet of 737-200 jets for that reason.

It would be possible to put new generation engines on the DC-9 airframe. However the airline would need to pay for the certification process with the governmental regulatory authorities. When it was all done you would have a several thousand-dollar saddle on a hundred dollar pony (substitute your monitory currency of choice). You still have the old wing with its poorer performance, older and heavier airframe with its associated higher maintenance needs. So it turns out that you were right all along the jets had served out their useful lives and needed to be retired.

[er340790]

Good points being made.

Only thing I would add is the inherent 'overengineering' apparent in some older designs like the DC-9. Without CADCAM etc the engineers erred on the cautious side resulting in airframes that could easily last half a century or more. As long as fuel stays cheap they're still viable.

While working throughout in Venezuela in the 90s, the main and regional airports were all full of Avensa and Servivensa DC-9s (many still with their ex-Eastern livery colours still showing through several bad paint jobs). Quite a lot of DC-3s too, but that's another story.

[BelArgUSA]

Hola Mathy - re. the "steady on" BelArgUSA here above, and SFCs...
xxx
I have used engine SFC as instructor when I was in classrooms.
The figures used were provided from reliable source, i.e. engineering.
xxx
Our flight crews were briefed about the SFC of engines.
This as an alternate means of setting takeoff thrust.
Engines normally using EPR can use N1 as approved alternate power setting.
But we also provided the SFC as a NON-approved method of "crosscheck".
I wish the Air Florida 737 crew with iced PT2 had been in my class.
xxx
My 747 crews knew the JT9D-7Q fuel flow of 19,000 lbs for 53,000 lbs thrust.
That JT9D has an SFC of .36 - Just a crosscheck... agreed...?
So it looks that I am not comparing apples and oranges.
I used SFC in the above manner through my career as pilot and instructor.
xxx
As I recall, having flown and taught various airplanes, my notes show -
JT8D (727 or 737) as .58 SFC, no reason the DC9's is any different;
JT3D (707 or DC8) as .53 SFC, their JT4Aa were .75 SFC;
CFM53 (DC8) as .35 SFC, no reason the A320's - 737's CFMs to be different.
JT9D (747) as .36 SFC...
CF6E and RB211 (in 747) are about same .34-35 class SFC.
xxx
Incidentally - if you compare flight total fuel burn comparisons at random.
The SFC factors gives you a very good yardstick.
The 707 with JT3D (.53) used about 2/3 of the fuel of 707 with JT4A (.75)...
So, Mr. Mathy... review your statement about the JT8D...
xxx
The above comes from PanAm, an airline you might not recall, of little value.
And I continued teaching same in a "third world nation" of South America.
In the country you are, you have more accurate/superior expertise.
I wonder how we, were lucky not to crash with our defective knowledge.
xxx
Respectful cheers, too... or "salud"
:*
Happy contrails