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737 JT8 Vs. CFM56 Fuel Economy  
User currently offlineMiles_mechanic From Canada, joined Sep 2001, 137 posts, RR: 0
Posted (12 years 2 months 1 week 2 days 18 hours ago) and read 10328 times:

I live in Canada, and our one airline Westjet, flies a combination of 737-200 and brand new 737-700, I was reading there latest company report and they were saying that they are trying to phase out the 200 series aircraft as soon as possible, because the fuel savings would more then offset the cost of getting the new aircraft. I have been to the Boeing website and can get range and max fuel loads and size differences, but how much more fuel efficient are the CFM56 over the JT8D? I remember reading on the Boeing website about the CFM56 is 25% better fuel efficient then the old JT3D on the 707. And I would be curious in the actual fuel flow per hour and if you could specify either per engine or total, as sometimes it gets confusing to know when people mention fuel flows on here if that is total or per engine.
Thanks for any help with this question.

21 replies: All unread, jump to last
User currently offlineAAR90 From United States of America, joined Jan 2000, 3530 posts, RR: 44
Reply 1, posted (12 years 2 months 1 week 2 days 11 hours ago) and read 10300 times:

I know nothing about the earlier model 737s, but the CFM56 powered -800 is a real fuel "sipper." At max weight takeoff, fuel flow during cruise will be about 3,000 lbs/hr/eng [6,000 lbs/hr/total]. At average North American flight weights that equates to about 2,500-2,750 lbs/hr/eng cruise fuel flow. The highest I've ever seen was 3,500 lbs/hr/eng at max gross weight at max speed [31,000' altitude for best true air speed].

*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineMandala499 From Indonesia, joined Aug 2001, 7354 posts, RR: 78
Reply 2, posted (12 years 2 months 1 week 2 days 9 hours ago) and read 10285 times:

I think the 200s consume about 3 tons a block hour... but it only carries 100 -130 seats where the 733 burns similar amounts with 10 - 20 more seats, and -400 with 20 - 30 more seats depending on the configuration.

732 prices are about US$2m to $5m, and pre 1990 733s can go in the market for about US$6 - 10m a piece... in places where fuel is expensive, dump your 732s...


When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineB747Skipper From , joined Dec 1969, posts, RR:
Reply 3, posted (12 years 2 months 1 week 2 days 9 hours ago) and read 10283 times:

Something to compare the two engines is - their own TSFC...
Thrust specific fuel consumption...
Suppose a 737 needs 10,000 lbs of thrust in a given cruise at a given moment.
CFM-56, the TSFC is about .34 (I may be about 1 or 2 % off on that number)
JT8D, it is .58...
That means, CFM will have 3,400 lbs of FF to get 10,000 lbs of thrust.
The JT8D will have 5,800 lbs of FF to get same 10,000 lbs of thrust.
Assume of course that both planes have EQUAL drag (weight) etc...
You can also use that TSFC using metric if you wish... kilos of fuel and thrust.
Happy contrails  Smile
(s) Skipper

User currently offlineBuzz From United States of America, joined Nov 1999, 697 posts, RR: 20
Reply 4, posted (12 years 2 months 1 week 1 day 13 hours ago) and read 10201 times:

Hi Miles Mechanic, Buzz here. I'm trying to recall what the numbers were the last time i ran a JT8D engine (we had a 727 stuck at Calgary for a couple days in '00 and i saw a lot of Westjet Guppies taxiing by)
I seem to recall it was about 3500 lbs per hour - per engine - at cruise. I haven't ridden 737-300 pit for a while and can't recall what those numbers were, less than 2,000 per hour. At idle i recall about 1100 lbs/ hr. on the JT8D engine. On the 737-300 (CFM56) it idles around 800 lbs / hr. The A320's we have give similar numbers.
When we take a 737-300 out for a taxi (to the far side of PDX) and high power run (acceleration check) we normally only use 1200 lbs of fuel. But i ask for 18,000+ lbs for the weight, i've made skid marks when there wasn't enough weight on the wheels. I just tell people i've got "valuable off-road experience in a 737 that few can match".
In the tri-jet after a high power run we'd come back at least 2000 lbs lighter than when we left. All that fuel went out the exhaust nozzles.
g'day from the Pacific Northwest
Buzz Fuselsausage: Line Mechanic by night, DC-3 Crew Chief by Choice, taildragger pilot for fun.

User currently offlineMiles_mechanic From Canada, joined Sep 2001, 137 posts, RR: 0
Reply 5, posted (12 years 2 months 1 week 1 day ago) and read 10137 times:

Thanks guys for all your help, I was curious about the difference in specs between the engines. These engines actually seem to be more fuel efficient then what I usually work on, when you consider the size difference of the aircraft. I work on Jetstream 31 and Beech 1900 aircraft. So thanks for the help, makes the numbers in the report make a bit more sense when you can see the differences between the 2 aircraft as far as fuel used, and estimate that over a whole year. Keep up the good work of keeping us informed and the good discussion that goes on in here.

User currently offlineTimz From United States of America, joined Sep 1999, 7031 posts, RR: 6
Reply 6, posted (12 years 2 months 1 week 22 hours ago) and read 10123 times:

Jane's used to give SFCs for some engines. At 35000 ft, maximum (?) cruise thrust, both JT3D and JT8D were somewhere around 0.81-0.83 lb of fuel per pound of thrust. As I recall the newer CFM56s were getting down toward 0.6.

In other words, a CFM56 is only something like 50% more efficient than a JT3C/JT4A turbojet. Zat sound right?

User currently offlineAcidradio From United States of America, joined Mar 2001, 1879 posts, RR: 9
Reply 7, posted (12 years 2 months 1 week 15 hours ago) and read 10102 times:

Questions that might be related-

Since the CFM56 is bigger around than the JT8D, I take it that would add onto drag a bit, right? But since the engine is significantly more efficient in itself, that added drag is negligible. Oh and how much do the engines weigh? With that big fan, I'd think the CFM56 is heavier. But then again it is a newer age piece of machinery which is more than likely made of lighter materials which would offset things.

Also, since there are going to be more electronic controls (lighter weight, solid state) in a CFM56 powered aircraft (hopefully) as opposed to manual controls, would this be accounted for as well?

Ich haben zwei Platzspielen und ein Microphone
User currently offlineMiles_mechanic From Canada, joined Sep 2001, 137 posts, RR: 0
Reply 8, posted (12 years 2 months 6 days 19 hours ago) and read 10055 times:

Hey Acidradio

well those are a couple of good questions you raised, but the added drag and weight penalty, wouldn't really affect the performance of the engine itself, it would affect the performance of the aircraft, as it would either slow the plane down or require a higher power setting to achieve the same performance. But as far as the engine fuel burn and other performance it would be the same as sitting in a test stand, it wouldn't notice the added weight or drag there. So that is where as a couple of the guys pointed out the specific fuel burn per pound of thrust produced is the best indication of the efficiency of an engine when comparing even a CF6 to a CFM56, as it will all be relative.
If I am wrong with that statement then I know those of you who have more experience then I will correct me, and I will learn as I always do from all of you out there.

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 9, posted (12 years 2 months 5 days 20 hours ago) and read 9999 times:

I dont understand one thing there...
In my previous posting I inform of the respective TSFC of JT8 and CFM...
Is this experimental calculus to some of you...?
And then continues like... "well... this burn this in cruise and that... burns that..." - I give you the most accurate base for comparison, totally disregarded, some of you still dont get it... Some of you want to be pilots or aeronautical engineers in the future...?
Well, then, read ( and UNDERSTAND) my postings...
(s) Skipper  Sad

User currently offlineTimz From United States of America, joined Sep 1999, 7031 posts, RR: 6
Reply 10, posted (12 years 2 months 2 days 2 hours ago) and read 9892 times:

For what it's worth, here's what Jane's says:

Later editions don't list sfcs. In the 1993-94 the CFM entry shows

Under "Specific Fuel Consumption (cruise, as above):"

(among others) "CFM56-5A1, -5A3, -5B1, -5B2 16.87 mg/Ns (0.596 lb/h/lb)"

The "cruise, as above" apparently means "Cruise, installed, 10,670 m (35,000 ft), Mach 0.8, ISA". This is/was the engine for the A320.

I take "mg/Ns" to mean milligrams per Newton-second, which would mean we needed to multiply that figure by 0.03530394 to get pounds per hour per pound. So the two figures are consistent.

In the 1978-79 Jane's, Pratt and Whitney shows

Under "Specific Fuel Consumption"

"Max cruise rating, as above:"

(among others) "JT8D-1, -1A 22.24 mg/Ns (0.785 lb/hr/lb st)"

(and the higher-thrust versions increase sfc up to)

"JT8D-17, -17R 23.37 mg/Ns (0.825 lb/hr/lb st)"

Here the "as above" means "Max cruise thrust (10,665 m; 35,000 ft at Mach 0.8):"

Maybe I didn't make clear that I have no idea what the sfc is for the CFM56/JT8D or any other engine-- I was just quoting Jane's.

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 11, posted (12 years 2 months 2 days 1 hour ago) and read 9878 times:

Dear Timz -
My own table of TSFCs are quite different in numbers -
I realize you quote Jane's, good reference, but I have Boeing books that give me quite different numbers, i.e. for various engines -
JT3D-7 = .53
JT8D-17 = .58
JT9D-7Q = .37
We need a Sherlock Holmes again... Get your magnifying glasses...
And your pipes...
(s) Skipper

User currently offline777236ER From , joined Dec 1969, posts, RR:
Reply 12, posted (12 years 2 months 2 days ago) and read 9873 times:

TSFCs (in lmb/hr/lb or kg/hr/N) vary with altitude and speed. TSFCs at cruise will vary significantly from TSFCs that engineers use. Engineers want to know what TSFCs are when the engine is on the ground in a wind tunnel. TSFCs for altitude use a few dubious assumptions you might want to watch out for.

If the Boeing figures are for "cruise", you might want to check your units. Pound mass/hour/pound is the standard imperial unit (yuk yuk yuk).

User currently offlineTimz From United States of America, joined Sep 1999, 7031 posts, RR: 6
Reply 13, posted (12 years 2 months 1 day 23 hours ago) and read 9867 times:

I forgot to check Jane's figures for SFC at sea-level, maximum takeoff thrust. Those are of course much lower than SFC at 35,000 ft cruise. I figured the cruise SFC was what the original questioner was after.

I see no problem with the units; should I look harder?

User currently offlineMiles_mechanic From Canada, joined Sep 2001, 137 posts, RR: 0
Reply 14, posted (12 years 2 months 1 day 22 hours ago) and read 9859 times:

Hi everyone, well I want to thank all of you for helping me out with all these numbers.

as far as what I was looking for, I was just looking for a clear comparison of the two engines, probably cruise is a good idea since that is where the aircraft spend most of there time, I just didn't know how much more fuel efficienct the new high bypass turbofan of the CFM56 was over the old low bypass turbofan of the JT8D. Just was hard to see how fuel economy would justify replacing aircraft at such a investment required. But I guess when you factor in scheduled maintenance that along with the fuel savings might make sense, along with the added seat availability also.

So thanks again for all the help everyone. So basically if I do the math quickly, you are looking at about 30% improvement in fuel economy, that is very good.

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 15, posted (12 years 2 months 1 day 22 hours ago) and read 9856 times:

The way I use TSFC in everyday flying, is to compare the fuel flow with the thrust of the engines. Since most of you are from USA, I will work with lbs, but this works with metric (kilos) as well...
Example - The JT9D-7Q (my 747's engines) are 53,000 lbs thrust (max takeoff power)... the TSFC of that engine is .37 -
Putting all that on a slide rule, to get 53,000 lbs of thrust, with TSFC of .37, it takes 19,600 lbs of fuel flow on that engine to have "takeoff power"...
On every takeoff I have that figure in my head. If EPR failed, if N1 failed, the FF gage would still tell me that I get my takeoff power... For those of you who are metric, you can work this with kg of thrust and kg of FF as well, the TSFC remains the same, .37 as well in metric...
(s) Skipper  Smile

User currently offlineExpratt From United States of America, joined Mar 2000, 311 posts, RR: 0
Reply 16, posted (12 years 2 months 1 day 8 hours ago) and read 9818 times:

In your experience, how many other pilots or flight engineers would use TSFC as a back up to determine engine performance. The JT9D-7Qs on your airplane use EPR [engine pressure ratio] as the primary indicator to determine power output. For the 9s, EPR is core exhaust flow divided by total fan inlet flow. But as EPR changes, so do the other parameters like N1 and N2 rpm, fuel flow, and even EGT. It is my experience that most crews do not understand the direct relationship between EPR and N1, N2, EGT, and fuel flow. While I would be a little uncomfortable using N2, EGT, or fuel flow as a alternate to EPR, N1 would be acceptable and in fact, N1 is the primary indicator of power on a GE engine. I have talked to several crews about engine problems where the only thing they looked at was EPR and were seemingly clueless about what the other engine instruments were showing. I understand the crew's primary objective is to fly the plane and not gather data for me. But if I am to correct the problem, I need more to go on than just the engine doesn't work. I am not picking on pilots per se because I have encountered numerous test cell crews that also didn't understand the relationship between EPR and the various engine performance parameters and were needlessly rejecting engines instead of fixing the (what should have been) obvious test cell problem. I commend you for having the knowledge to understand what your engines are doing and how to tell when they may or may not be acting up. Flying is a tough business and there is a lot for you as the pilot to know and to be asked to make decisions on in a split second, but flying is also a lot more than just aiming the nose of the airplane down the runway.

User currently offlineAaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2
Reply 17, posted (12 years 2 months 23 hours ago) and read 9776 times:

I think the good skipper got you on that one...

>>>While I would be a little uncomfortable using N2, EGT, or fuel flow as a alternate to EPR, N1 would be acceptable [expratt]

but he said...
>>> If EPR failed, if N1 failed, the FF gage would still tell me [b747skipper]

Your good point stands however...

not trying to split hairs...

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 18, posted (12 years 2 months 12 hours ago) and read 9739 times:

Good posting, Expratt  Big grin
You see, pilots have a problem with me... I am a line pilot, an instructor and the flight crew training manager... and I have instructed in the airline and military environment since the mid 1970s...
In the classroom, I have had to answer questions I did not know anything about - and educated myself a lot... further, all pilots anf flight engineers know about FF and thrust relation...
If the Air Florida pilots had known about FF, they would not had been fooled by their frozen PT2 oribe in the sad Washington accident with the 737... If I would have been on that flight, that airplane would have flown...
Yes, PanAm crews had no clue about FF, in the Air Force, my trainees slept in the classrooms... but here they listen in Argentina, and are outstanding guys who are trained to my tough standards - after which we enjoy a beer together and laugh about our flying careers...
There is only one limit to knowledge, the one you want to set as your own standards. I want "my troops" to have the highest, and I will make any effort to help then reach it.

User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 19, posted (12 years 2 months 9 hours ago) and read 9723 times:

Another pilot trick trivia with FF...
You are in a 4 engine aircraft, i.e. 747...
You fly an ILS, 4 engines, flaps 25...
Fuel flow is 2,500 kg per engine - total FF 10,000 kg...
On 3 engine ILS (1 engine out) - same 25 flaps setting -
Fly the approach with same total FF (10,000) = 3,300 kg FF each engine...
QED... good stuff to know about FF and what it is...
Happy contrails  Smile
(s) Skipper

User currently offlineExpratt From United States of America, joined Mar 2000, 311 posts, RR: 0
Reply 20, posted (12 years 1 month 4 weeks 1 day 17 hours ago) and read 9678 times:

Being an engine person, I have seen many, many engines behaving in strange ways over the years. Assuming the engine is healthy, fuel flow could be used to approximate engine power. However, if the engine is deteriorated or sick, fuel flow and EGT could give a false indication of a higher power setting, hence my discomfort. When I was teaching engine test, I tried to get the test stand operators to think of the engine as an entire system and that almost all of the performance parameters were directly related to each other. As EPR rise; so do RPMs, EGT, fuel flow, internal pressures. Inlet pressure is the one that goes down as power increases because of the Bernoulli effect of increasing air flow through the inlet. I cannot even count the number of times I have seen stand crews reject an engine for low EPR when all of the other parameters were high. When questioned how could all the parameters be high except for EPR and what did it mean, the usual response was "I dunno." I have seen crews write up one engine as having malfunctioned when in fact it was the opposite engine that had the problem. They just did not recognize what was normal and what was abnormal. Not trying to put words in Skippers mouth, but I think he was trying to look upon the engine indications as a system and that an engine's performance indicators have to be in agreement with themselves and also to its wingmates. The Air Florida accident (Palm 90) at DCA is a classic example where sole reliance on EPR led a crew to a crash. The FO was questioning the power settings, but he deferred to the Captain's incorrect assumption that the RPMs were down because of the cold air. RPMs for a given thrust setting will increase or decrease for warmer or colder temperatures, respectively. But not to the extent that was seen on Palm 90. If they had recognized that all of the other engine indications were abnormally low, they could have rejected the takeoff and averted the crash. The other thing that I have always been mystified by with the Palm 90 crash was why they didn't push the throttles to the stops when they were going down. I have seen many reports where an operator had to change all of the engines on an airplane because they were overtemped and oversped when the crew firewalled the engines usually because of a windshear encounter, although there were a couple of runway incursions there too. But back to the original point, in the absence of EPR, N1 is the next best thing. If no EPR or N1, fuel flow could work, but be careful as it can give a false indication.

User currently offlineAaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2
Reply 21, posted (12 years 1 month 4 weeks 21 hours ago) and read 9649 times:

no arguments from me

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