Older aircraft going faster and wasting less fuel?

[Pomo1]

I wonder if anyone can give me some feedback regarding B737- 300.

I fly 737-300 that are now nearly 15 yrs old. They r the most used boeings on Europe. Last week I tried increasing the cruising speed to Mach.767 instead of .75. obvously the optimum altitude for cruise will lower at higher speeds. I tried it on a 4 hr flight and found that i burnt 400kg less fuel than was planned. Can anybody help me come up with some solution? could it be that older aircraft have more drag and need to fly faster?

thanks

[XPMorten]

obvously the optimum altitude for cruise will lower at higher speeds.

I would have thought the opposite ...:confused:

M

[BOAC]

Pomo - there is an 'old wive's tale' about the 737 which says that going at 0.76M reduces the (significant) deck angle enough to produce the same form drag as 'ECON' or 0.74M. Result - less flight time, less fuel.

PS It seems to work! Yes you will cruise 'lower'.

PPS It also helps the c/crew get the trolleys from the back to the front:D

[reynoldsno1]

Didn't a B727 crew try something similar using the leading edge slats and the circuit breaker, and ..........whoops:ooh:

[arrow1]

Pomo - Seems to me that you have saved approx 5% fuel by flying faster that LRC, and at a lower altitude. The figures would go against the 737 FCTM that predicts a higher fuel consumtion by 1-2% for an increase in M No. by 0.01 above LRC (old wives tales notwithstanding). I wonder if the reason could lie in the planned fuel burn or winds.

[Pomo1]

thanks guys...

im quite sure the pllaned fuel burn figures and winds are quite accurate.
Do you think the engines can play an important role?

Older engines can consume more fuel to get to a high cruising level. (more strain) If it cruises at Fl 330 instead of Fl 370 it will waste less fuel getting there due to the optimum altitude being lowered at higher speeds.

[XPMorten]

hmm, I'm still not convinced this is correct.
The below graph is from THIS (http://www.iata.org/NR/ContentConnector/CS2000/Siteinterface/sites/whatwedo/file/Airbus_Fuel_Economy_Material.pdf) document.

http://www.xplanefreeware.net/~morten/jpgs7/mach.jpg

Cheers,

M

[Conan The Barber]

You're not convinced that what is not correct?

Do you fly a 737?

[XPMorten]

This..

... .due to the optimum altitude being lowered at higher speeds.

The above chart to me says the oposite.
Also, the below is from a 734. Higher altitudes
to the right. Note the optimum mach # is
LOWER at lower altitudes!

http://www.xplanefreeware.net/~morten/jpgs7/opt.jpg

no pilot - engineer..

Cheers,

M

[Pomo1]

My point is about optimum altitude for fuel savings not for optimum Mach no.




in other words you are saying that if there are 2 a/c same speed same wind ect.... 1 at Fl370 and 1 at Fl 330, the lower one will be slower than the upper one?

I tend to disagree

[XPMorten]

The table shows an increase FF as well.

My point is just that if you increase speed at a constant level, your deck angle will get smaller - or even negative. This will increase total drag and hit your FF.

If you do the same thing and descent to a lower level,
the scenario gets even worse since you will get
even more lift and induced drag.

If you are talking about total trip fuel, thats offcourse another story.. .

Cheers

M

[Pomo1]

on another note...

once we were to a/c 1 at Fl 370 and 1 at Fl330

we made sure all conditions were the same (same wind Cost index ect) and the lower a/c flew faster than the upper one....

can u explain coz it is gng contrary to what u r saying

[XPMorten]

Guess my english isn't as good as u r s.
Could you please clarify WHAT is contrary, and
give me a few more details about u r 2 acf.
Like which one is flying at or closer to optimum level, weight and speed (M#, KIAS, GS) etc.

With regard to old turbofans, the FF will increase about 3-4 % over time.
The biggest loss is in the High Pressure Turbine.
The tip of the turbine blades will get burned away during time, and you get an
increase in the gap between blades and shroud.
The difference in EGT between a new and old HPT and shroud can be 20 deg C.

M

http://www.xplanefreeware.net/~morten/jpgs7/fuel2.jpg

You see below, the only place where a DECREASE in FL
gives a better optimim level on the A310 is above M .80 . Most likely because transonic drag starts
playing tricks.

http://www.xplanefreeware.net/~morten/jpgs7/fuel3.jpg

[BOAC]

There are/were Boeing tables which gave TAS as well at LRC for the 737 Classic/Ancient at different wts and altitudes. I STILL have mine from 1989, but they are getting very well thumbed (and are still extremely productive!) and I could do with a set for the NG. Thanks to an excellent Captain with whom I flew then, I have two lines across the matrix - best TAS and best fuel flow. Anyone know if they are still around? Couple dwith the other part of his 'training package' I have fuel to height tables/wt which enable accurate assessment of whether to climb in conjunction with the first table.

XP - TAS is higher at lower altitudes for the same Mach (speed of sound reduces) therefore you move faster over the ground (with the same wind, of course). This explains Pomo's observation.

BTW
your deck angle will get smaller reduces form and induced drag?

[XPMorten]

quote:
your deck angle will get smaller.
reduces form and induced drag?

If Boeing had wanted you to cruise at 0 deg deck angle, they
would have designed the wing so that you would do that at optimum altitude.

M

[Milt]

The Cruise Climb

Boffin number crunching and flight testing long ago showed that the max range possible for a turbojet resulted from a 'cruise climb' at best mach, optimum N/root theta and a constant W/delta where delta = atmospheric pressure ratio.

This results in a rate of climb on the cruise for a typical heavy of about 30 ft/min as W slowly reduces.

Perhaps it is about time to introduce the complications of cruise climbs to ATC instead of rigidly adhering to assigned cruise levels now that fuel costs have gone so high.

[BOAC]

If Boeing had wanted you to cruise at 0 deg deck angle, they would have designed the wing so that you would do that at optimum altitude.
- and where would we retract the resulting 16' long main gear?:D

Milt - tends to happen anyway, but in 2000' steps. It would be nice to see our ATC computer systems eventually being able to cope with a cruise climb policy.

[XPMorten]

- and where would we retract the resulting 16' long main gear?


lol..theoretically its just a matter of changing wing incidence like 1-2 deg or adding a tad of lift :P
Your gear will need to be SHORTER ;)

M

[arrow1]

XPMorten,

Pomo1 is right in saying that an increase in speed leads to a lower optimum altitude. The 737 FCM, page 4.6 states "In LRC or pilot selected speed modes, optimum altitude increases as either airplane weight or speed decreases".
The 734 strip that you inserted pertains to LRC control, wherein the effect on increase in alt results in higher LRC speed, and the shaded area is the only one at opt alt. It does not show the effect of increase in speed on opt alt.
If a pilot flies Econ speed, the FMC opt alt reflects min cost, and if a pilot selected speed is flown, then the FMC opt alt is slightly different in that it depicts the alt for min fuel burn.
Having said that, I cannot understand how Pomo1 burnt lesser fuel at a higher Mach No, at a lower alt.