Fuel Flow vs Ground Speed

[5miles]

Question for some B737 jockeys.

Had one coy's B737 westbound across the Bight at F260, GS of 420knots. Other coy's same type (and generation) aircraft, same route at F380, with a GS of 300.

Ride was smooth at both levels with the base of the jetstream at about F290.

That's a 30% difference in GS, and from YPAD to YPPH it was a difference of about 45 minutes in the air. I realise different opertators apply different cost priorities, but how much less would the fuel burn be at the higher level, and what other considerations would the crew give to wearing that headwind?

Blue skies :ok:

[DirectAnywhere]

Can't help you specifically with 737, but for the aeoplane I fly, depending on weight, anything between 40 kts and 80 kts less headwind will allow you to burn the same amount of fuel at the levels you've mentioned. Given that there is probably at least 100 knots less wind at the lower level (taking into account TAS changes between 380 and 260), and assuming that the ride is smooth, I couldn't really see any reason to be at the higher level. ETA would obviously be improved by flying at 260.

In essence, in my aeroplane, you would save both time and fuel at the lower level and I would gladly take 260 unless other operational factors dictated otherwise.

[john_tullamarine]

Principles are similar, regardless of Type ... disregarding turbulence, which might well override fuel considerations ... and the Bight in or near the jetstream can be an extremely uncomfortable ride .....

Most operators do the sums and determine, typically, a schedule of wind gradient tables which provide guidance to the crew regarding whether it is worth or not worth changing cruise level. These normally consider sector length (ie the relative effect of climbs and descents on the cruise deltas) and SGR consequences of different winds and fuel flows at different levels.

Knowing the local wind, and reported winds at other levels, the crew can determine the wind gradient between the levels and, by referring to the guidance material, make a decision as to what makes commercial good sense.

There are other ways to present the data (as in most things) but this is fairly typical.

[18-Wheeler]

I haven't flown a 737, but FWIW in the 747 you can still go nearly as far at low level rather than step-climbing as you normally would.
It's easy, just keep the TAS below 500kts and the fuel burn per air-mile is not too far off that of what you would normally be burning at the higher level.
It's possible to do ten hours no probs at 28,000 .... done it on the Hajj a couple of times. It requires a bit more fuel but not as much as what you think.
For the relatively short distances in Aus, I wouldn't hesitate going from BNE to MEL in the low 20's if the headwind was far lower than at high level.
(ride permitting)

[Capt Fathom]

With the jets I have flown, you need a GS of roughly 80-120kts more at around FL280, to get the same fuel mileage you would get at say FL370 in the headwind. Sounds similar to 5miles senario. Just depends on if you're in a hurry or not. (Or in some cases, the crews' knowledge of the aircraft and its capabilities)

[Next Generation]

As a general Rule of Thumb, I use 6 knots per 1000'

Therefore, in this example, from FL 380 to FL 260, the headwind component would have to be 72 knots (6 x 12) less at FL 260 than it is at FL 380 for it to be considered worthwhile to descend to the lower level.

NG.

[MoFo]

There are other costs involved than just fuel. A late aircraft can have flow on costs to the airline. Engine hours dictate maintenance costs. Delayed passengers can be expensive. Ground staff overtime. Flow on delays meaning the aircraft may push a curfew later in the day. You name it.

One of the airlines mentioned above in its last discussion on the subject to its pilots gave a formula of acceptable extra fuel burn versus minutes saved to regain scheduled arrival time. In other words burn extra fuel if you can save significant time to regain schedule, in accordance with the formula. Don't burn extra fuel to arrive early, but.

Also on days where significant wind GRADIENT occurs you can have both. Get there on time AND save fuel. Other posts above have illustrated this.

Admittedly their fuel hedging at the time was in place and saving them heaps. I wouldn't be surprised if the formula changes in near future based on current fuel hedging figures. However the principal is the same. There's more to costs than just fuel, particulary if you are nicely hedged with your fuel costs.

I'm also amazed at the difference in philosophy between the two airlines.

[Dehavillanddriver]

I suspect that the cost of maintenance between the two airlines is a significant driver - QF would have higher maintenance costs by virtue of its infrastructure.

Additionally, I doubt that the full cost of maintenance is really known within QF - this isn't a dig at QF by the way - but there are many entrenched costs in an "old" business - accountants play games with numbers to justify something else - the TRUE cost is often different from that shown on the books.

As an example just ask any of the guys that used to fly the Electra freighters for Ansett. The Electra operation had huge costs apportioned to it - by accountants trying to make different areas look good - and then someone - who didn't know the background came along and said these aeroplanes have to go they are costing us a fortune - when in reality they were making a fortune.

In a relatively new operation, particularly with a single type it is harder to hide costs - and the real cost is easier to calculate.

Hence maintenance is a different cost driver in VB than QF.

This being the case fuel becomes more important in the grand scheme of things - bingo one goes high and one goes low.

I went SYD to PER in a -800 at 36000 then 38000 only pushing about 60 knots and a QF -400 went at 30000. He left after me but arrived before me - he over took me about 3/4 of the way across, and we were doing M0.785

He must have been going like the clappers and the burn would have been significant - we left and arrived on schedule - so he must have either been late leaving or early arriving or their schedule is too tight - either way they burnt a heap more fuel than I did....

[john_tullamarine]

.. ahhh ... I still have my Wombat tie pin in the drawer somewheres ...

mind you .. the props cost a small fortune and engine shutdowns were not really considered unusual.