A320 NWO/ B737 Min

[twistedenginestarter]

Airbus and Boeing are going to a lot of trouble to fit new engines that save maybe 5-10% fuel.

Now I know this is rather poor evidence but on the B737 MSFS on a very old laptop I had last night I was testing fuel flow at various speeds. I couldn't get clear figures because the autothrottle was hunting too much but it seemed that knocking 40 kts off your cruise speed gives you something around the same savings.

So why didn't Airbus add a couple of frames in front of the wing root, straighten out the wings a bit, and make them more efficient at slightly lower speeds. I reckon you'd get better savings than adding fancy, new expensive motors, and Boeing wouldn't have to change the undercarriage?

[rudderrudderrat]

Hi twistedenginestarter,
... on the B737 MSFS ... I was testing fuel flow at various speeds....but it seemed that knocking 40 kts off your cruise speed gives you something around the same savings.
Er..... wouldn't flying 40 kts slower take proportionally longer?
If it takes x% longer and your saving is only Y% fuel flow, how would that save anything?

[twistedenginestarter]

I did take that into account.

The normal objection is you get longer sector lengths ie less revenue per hour. That is obviously sensitive to sector length, how much is in the climb, and gate turnaround times.

[DaveReidUK]

If it takes x% longer and your saving is only Y% fuel flow, how would that save anything?

Well it's not quite as straightforward as simply comparing those two percentage values, but you're on the right track.

The main implications of flying slower and taking longer are increased flying hour-related costs (e.g. crew, maintenance) and higher fixed costs per sector (because fewer sectors per year for the same utilisation).

But I agree that those increased costs would likely more than wipe out any resulting fuel savings.

[MarkerInbound]

Welcome to the world of Cost Index.

[twistedenginestarter]

So have you noticed cost indexes being changed over the last few years to fly slower? One of the Scandanavian airlines started flying slower but it doesn't seem to have caught on.

Studies on future aircraft tend to favour lower speeds but they go for more expensive propositions - new truboprops, open rotors etc. Just changing the wing on an existing model looks a much more digestible route.

You might say if this was viable, Boeing would already have thought of it but they didn't support new engines until Airbus forced them to.

[captplaystation]

Yes they have, typical values for B738 are down from 40-35 to 15-10 in most companies.

[twistedenginestarter]

What does that translate to in terms of cruise speed?