DC-8 Fuel Burn

[christopher1955]

Hi. Although I read everything here I seldom post anything, just sit on the sidelines and learn. However I would like to ask why the DC-8 40-series aircraft have so much greater fuel burn than the Boeing 707-120?

I do all my fightplanning on paper and track fuel usage and overall have found that the early Douglas's burn about 50% more fuel than do the Boeings. Comparing 14 DC-8 flights to 17 Boeing flights, all 2000nm plus non-stop, the DC-8's consumed some 54.3% more fuel in cruise. I found this somewhat surprising since they filled similar commercial niches.

While this might be an operator issue I tend to think not since both are operated in accordance with the handling notes provided by HJG and given the sample size and lack of significant variation it is unlikely related to FS9 weather either. However I could be wrong.

This is not a big deal nor am I complaining. Editing the fuel_flow_scaler in the aircraft.cfg file is not on the table since I know from experiance that doing so might invoke the Laws of Unintended Consequences and I would also not presume to know more than the FDE author. The aircraft are the v5 core files with the latest panels all installed as per instructions.

I am very greatful for all the hard work done by the HJG Team but remain curious as to why the DC-8's are such kerosine hogs compared to the Boeings.

Thanks in advance.

Chris

[aerofoto - HJG Admin]

The short and simple answer to this is ....

The DC8 FDE (CFG file in particular) has not yet undergone the same degree of "fine-tuning" over time as has the equivalent B707/720 data .... and which "may" therefore account for any discrepancies in relation to fuel burn figures between both aircraft types.

George CARTY has sent me (a while ago) FDE revisions to be applied to all our DC8's .... BUT .... due to the imposition of other recently released projects here at HJG, as well as others about to be released too, I've not been able to test these thoroughly yet and in advance of their being released here. I'll be working on this though .... as soon as I'm able to do so.

Again .... I'm only assuming my explanation is indeed the reason for any currently observed fuel burn discrepancies between both RR CONWAY powered B707-420 and DC8-40 type aircraft.

I'll hazard a guess here and say ....

I think the fuel consumption figures for both of these aircraft types should be roughly "similar" .... with maybe 1 of these 2 aircraft having a very slight "edge" (possibly the B707-420 in this case) over the other and due to the natural drag characteristics of both of these aircraft types when compared.

There "WILL" be a general DC8 FDE update .... "LATER ON".

Whether or not these makes any major difference at all is sometrhing we'll just have to wait'n'see.

We can usually get "VERY NEAR" the realworld fuel burn figures for any particular aircraft/engine type .... BUT .... FS itself "WILL" always have the "FINAL SAY".

Mark C
BOG/CO

[christopher1955]

Thanks for the quick response. Figured it was probably something like that and the models are still great fun to fly.

Chris

[Herman]

Hello Chris;

In addition to Mark's response, I noticed that you are comparing fuel burns of the 707-120 to that of the DC-8-40. As Mark correctly noted the comparison should be made using the 707-420 or 707-320. I can't recall exactly, but I think the DC-8-40 series and 707-320/420 were fairly similar in size and weights, where as the 707-120 series was quite a bit smaller and lighter, which I would think would result in a lower fuel burn even though the engines were probably a bit less efficient.

Also,( I think you probably took that into account) even though your stage lengths were about th same there are a few other factors that effect fuel consumption, which I think you are probably aware of.

Cheers; Herman

[christopher1955]

Thanks for the input Herman, while I had noted the big difference in MTOW (some 15 tons) I figured that they were economic rivals since most of my DC-8 flights were from the 1960 Trans-Canada Airlines timetable but the Boeing flights were largely from the American Airlines and Pan Am 1959 schedules. But on reflection I suppose it might be more accurate to say that the counterpart for the DC-8-40 would have been the B707-320 and not the B707-120. I have little stick time on the 320, it coming along a bit later than the FS9 period am most interested in.

It was the vastly different economic potential between the two that prompted the original question (even with aviation kerosine dirt cheap) rather than simulator performance but if the models were not serving in rival roles then any comparisons are apples to oranges.

Thanks again; Chris

[aerofoto - HJG Admin]

Actually .... Herman is quite right !

In my haste to reply yesterday I missed "CHRISTOPHER1955's" very important reference to the "B707-120" .... too many interruptions going on around me !

The early P&W turbojet engines which powered B707-120, -138, -220, and -320 .... along with DC8-10, -20, and -30 too, were extremely "THIRSTY" indeed.

The RR CONWAY engines which powered both B707-420 and DC8-40 represented the first of the next generation engines and were a little more fuel efficient .... but .... not a hell of lot more-so really.

B707-320B/C and DC8-50/61, -62/63 .... and finally B707-700 and DC8-71/72/71 each represent progressively greater advances, in power, fuel efficiency, and evironmental friendliness (smoke and noise) as technology advanced.

There rest is as I mentioned yesterday .... in relation to the DC8 FDE not yet having benefitted from the same degree of fine-tuning as that of all our B707/720 series aircraft .

Mark C
BOG/CO

[Herman]

Hello again,Chris;

Regarding your fuel burn comparisons, I am just a bit curious as to what some of your other flight perameters were, keeping in mind the weight difference between the 707-120 versus the larger DC-8-40 and that your overall distances were pretty well the same.

A few of the major things that would affect the overall fuel consumption would be;

1. Your cruising altitude
2. Your cruising speed....remember that the DC-8's cruise a bit lower then the 707's
3. Wind speed and direction at cruise altitude

There are,of course a few other perameters that affect F.C. but I think the above are the major one's.

How did these compare with your different aircraft and flights?

In the meantime,I am looking through some of my books to see if I can find some actual numbers.

Herman

[christopher1955]

Hi Herman
Most of my flight plans and notes are written on scrap paper and go into the recycling regularly however I do have some post flight reports that are generated by FS Passengers (FsP) that detail weights speeds, altitudes and fuel usage.

Realizing that two flights don't make a trend, here is the data from two typical trans Atlantic flights:

HJG Pan Am B707-120v6 New York JFK -> Paris Orly 3149 nm
TOW 254,457 lbs with max fuel load
Time in air = 7:27 Cruise time 6:10
Max alt = FL330
Avg cruise speed = M.75
Fuel used = 85,282 lbs
Cruise fuel used = 10,277 lbs/hr
Avg fuel burn = 11,447 lbs/hr

HJG TCA DC8-40v5 Montreal CYUL -> Prestwick EGPF 2593 nm
TOW 266,526 lbs with max fuel load
Time in air = 6:12 Cruise time 5:23
Max alt = FL370
Avg cruise speed = M.75
Fuel used = 104,710 lbs
Cruise fuel used = 15,157 lbs/hr
Avg fuel burn = 16,889 lbs/hr

Note distance is point to point, the actual routes were longer in distance but it seems that FsP calculates its statistics based on point to point distances rather than distance flown hence the rather low cruise mach number.

Note that with the high weights it was necessary to step climb to final cruise altitude. Sorry cannot recall winds or weather or initial cruise level but used real wx with winds aloft ON and FS METAR.

It was the significant difference in fuel burn and the corresponding greater profitability of the Boeing in FsP despite the fewer numbers of seats compared to the Douglas. The only reason I made the original post is because this didn't seem right and the FsP environment levels the field for comparisons even if the actual data is historically invalid.

However as was pointed out the 707-120 and larger DC8-40 were not really rivals so all of the above may well be statistically meaningless.

Chris

[Herman]

Hi Chris;

I've decided to do two flights using the DC-8-40 and 707-420 both from Montreal to Shannon under the same overall flight conditions.

I'll let you know the fuel burn results probably by tomorrow.

Herman

[Herman]

Hi Chris;

Here are my fuel burn numbers; DC-8-40 versus 707-420

Toronto to LA. I used standard FS fair weather so that conditions would be the same for both flights.ie. wind direction and speed & temps. Wind speed were a constant 25kts. from the west.

I step climbed from FL290 to FL350 at 1hr. intervals for both flights.

Total distance was 1886NM
GTOW for both was about 260,000lbs.
Max. TOW for the DC-8-40=315,000lbs.
Max TOW for the 707-420=316,000lbs.

The DC-8-40 cruising at it's normal M0.80 consumed 67,000lbs.
Fight time (lift-off to TD) 4hrs.12min. An average of 15,765\hr.

The 707-420 cruising at it's normal M0.82 consumed 55,000lbs.
Flight time was 3hrs.56min. An average of 13,750\hr.

The DC-8-40 there fore consumed 12,000 lbs. more which is about 18%, if my math is correct.

18% I suppose, is a considerable difference, even for the cheap fuel days.

Unfortunately I have no other data to compare, so I do not know if there really was such a large difference between the two aircraft.

Cheers; Herman

[christopher1955]

Thanks Herman. Nice to know it isn't just my imagination and, given the small sample size, looks to be pretty consistant across the rival models. Knowing that the HJG Team has a lot of really good stuff in the pipe, fixing the DC-8 fuel consumption is probably for the far future.

I will continue to enjoy them until they are fixed and if no fix is forthcoming I will enjoy flying them just the same.

Best Regards: Chris

[aerofoto - HJG Admin]

We'll certainly be releasing "SOMETHING" in the form of both FDE and a minor gauge update for the DC8's .... and the CV990's too .... in due course.

As mentioned near the start of this thread George has sent me FDE revision for all these DC8's.

At this stage I'm not sure how these changes might impact DC8 fuel burn indications and engine spooling rates, but, I expect they will likely result in observations similar to those which we currently see in our B707/720 performances.

In respect of the said "minor gauge update" ....

This amendment will apparently correct/improves the metric observations (fuel burn rates) for those whom prefer to run FS with the METRIC/KG (only) option selected. Those whom prefer to run FS with the US/LBS system selected will not benefit any. This particular amendment will apply to both the DC8's and CV880, and CV990 aircraft panels.

Mark C
BOG/CO

[Herman]

Hi Chris & Mark;

DiDo; I will continue to enjoy flying them, regardless. Besides, who can say that the 707 fuel consumption rates are completely accurate?

Herman