Another Airbus question

[Jack1985]

Yes its the Airbus again:O hopefully I'll get a few pilots who fly a member of the A320 family regularly to answer me but I'm wondering do pilots really go by recommend ECON modes for speed following the entry of the Cost Index figure? Particularly in a delay situation would a pilot strictly follow the recommendation or in an effort to pick up time fly as close to the restricted speed as possible?

[A4]

Yes. In terms of cruise this difference, time wise' between M0.78 and M0.80 is very little on 2-3 hour sector. You may save 2-3 minutes but you will burn A LOT more fuel. Additionally, when you arrive at destination you may have to hold (in the air or on the ground) so any time made up is lost.

The place you really make up time if delayed is on the ground. If you really "drive" the turn around you may save possibly 10 minutes - for free.

The other risk of flying round on the barbers pole is that of a possible Overspeed due to turbulence or positive sheer.

Most companies are very focused on fuel at the moment for obvious reasons. Chucking an extra $200 worth out the back to maybe gain 2 minutes just isn't worth it - especially if your airline does >1000 sectors each day. It's why I'm always surprised that a particular very large low cost carrier seems to dash around at 320knots / 0.80-81 much of the time. Companies have SOP's and if they want us to fly ECON I'll do it - after all it just makes my available 900 annual hours get used up quicker :ok:

A4

[Jack1985]

Thanks for the reply! :ok: I ask this because of a recent flight aboard an A320 we cruised at Mach 0.76, but on a recent flight with the same carrier your referring to above who primarily fly B738's we cruised at Mach 0.82 which is just below the restricted speed, is there much difference between those two speeds if both were flying same sectors and departed the same time?

[Dan Winterland]

There are other factors involved. Such as connecting flights on the same airline. A dozen passengers who will need hotel accomodation, feeding and expenses will be far more expensive than an extra ton of fuel. We work hard to ensure the connections are made. For that reason we often bump up the CI when delayed.

[A4]

That's a fair point Dan - but the Euro LCC's don't do connecting flights. They specifically say that if you don't leave at least 2 hours between any connection with the same carrier and you miss your flight - well tough :\

The Airbus will modify its cruise speed depending upon the prevailing winds (don't know about the Boing). So if you've got a 100 knot tailwind the aircraft will reduce to .76 to save fuel and take advantage of the high ground speed. If its a headwind it will increase speed to .79 (max I think) as it's more economical (because you will spend less time in it....). It's also more economical to descend at a slower speed as you will commence descent earlier hence spending less time at cruise power....... We're only talking about perhaps a 40-60kg saving but x 1000/day = $$$$$$$$

Flying at 0.82 on a 2.5 hour sector would probably gain 3-4 minutes but I reckon would burn 300-400kgs of gas (roughly speaking about $300-400 worth). With LCC's working on very small margins it doesn't seem worth it.

A4

[BOAC]

As A4 says, on the shorter routes it is almost impossible to make up a significant delay by travelling at warp speed. As A4 says, there are other better ways to save significant minutes than by burning fuel. Most company's Ops manuals forbid 'high speed' selections by crews unless requested by Ops.

[misd-agin]

Rough estimate is 45 seconds per hour for each .01 Mach or 1.5 minutes per .02 Mach.

As others have mentioned operational decisions (on time, late, connecting passengers, etc) can result in different cruise speeds.

Headwind vs. tailwind also has an impact. Higher than expected headwinds might result in higher CI/cruise Mach to avoid late arrivals and resulting operational difficulties.

Boeing article on Cost Index (CI). First 10 +/- pages cover general concept and cruise applications -
http://www.boeing.com/commercial/aeromagazine/articles/qtr_02_10/pdfs/AERO_FuelConsSeries.pdf

[misd-agin]

Chart (page 9?) has 777 data. Shows MRC of .826 and LRC of .84(1%) less efficient.

1% on a 777 is about 40EU/ $50 of gas savings per hour.

[Craggenmore]

The faster you fly the more hours they can squeeze out of you to reach the 900hr mark..!

[Jack1985]

So does the Airbus detect when it needs to increase speed when flying in a head wind or is that done manually using selected mode? Also in relation to the FD I'm just wondering when you guys are hand flying the approach do you pay much attention to it or do you prefer to (in an visual app situation) go with your own instinct inputs? I would go with the latter I think

[A4]

Yes, the Airbus increases/decreases speed automatically based on encountered cruise wind.

With regard to FD's - yes we follow them on the ILS for example, as they are intrinsically linked to Autothrust. If you wish to fly visually the drill is both FD's off and "bird on". The A/THR can remain engaged if you wish. The "bird" is a symbol on the PFD which shows your climb/descent angle and you track over the ground -very useful.

A4

[exeng]

The faster you fly the more hours they can squeeze out of you to reach the 900hr mark..!

Hours are hours regardless of how fast you fly. Perhaps you meant they can squeeze more miles out of you?


Rehards
Exeng

[max nightstop]

If you are going to programme the FMGS, then let it do its thing the way your employer wants.

If the flight director is on, then follow it, otherwise switch it off! The link between FD and A/T is pretty tenuous, try an engine failure in ALT*, and see what the priorities are!

Flying faster means you can do more sectors, or more days of work...hmmm.

[I-WEBA]

Does airbus change its speed according to Wind inserted on the MCDU bifore departure or according to the real Wind encountered during cruise....?

Is there any table to see how much mach speed changes during cruise to wind ?
A320 cost index around 8, which is descent speed?

[I-2021]

Hi I-WEBA,

Have a look in DSC, 22-20-30-20. There are a few pages that will precisely answer your question.

Cheers.