Descent Speeds.

[cambridge]

A number of UK airlines now seem to be descending at very low speeds. Some also seem to be using similar low speeds at intermediate cruise levels. Is there any recognised technical logic to back up this procedure, or any scientific references? Thanks.

[cortilla]

I don't fly airbus so what happens in their FMC's may be completely different i just don't know.

However in Boeing aircraft you have a cost index (if im preaching to those in the know i do apologise, but not everyone on here knows what a cost index is) which changes the various speeds depending on what you set in the FMC. In the 73 classic you can put any number in from 0 to 200.

The way i understand it 0 is min fuel usage and 200 is max speed and all variations inbetween. As the price of fuel goes up airlines are using lower and lower cost indicies. The lower the cost index the lower the speed in the descent. My airline have been told to use a low cost index and then ammend the descent speed to 280 knots to keep a good speed above fl100. I'm just guessing here but perhaps several uk airlines are telling their pilots to ammend their cost index to a lower setting and not to change the descent speed in the descent. Just a guess.

[411A]

Generally speaking, with jet transport aircraft, the same cruise speed (IAS/MN) should be used for the descent, at idle thrust, decreasing slightly as altitude decreases, for minimum fuel usage.
However, other considerations apply, for example, flight (hours) time.
Quite a large compromise, depending on the specific operator/airframe.

[tubby linton]

There is a new max speed into some airfields in the LTMA of 270kt.There was a discussion about it in the atc section of this board.Low cost index and low weights give some very slow descent speeds in any fms

[Ashling]

Cortilla, thats exactly whats going on.

Seems to me it could cause as much trouble as its worth due to ATC realising which companies fly slowly and which fly relatively fast and prioritising accordingly.

[jpu0509]

More block time:ok:

[Pilot Pete]

Seems to me it could cause as much trouble as its worth due to ATC realising which companies fly slowly and which fly relatively fast and prioritising accordingly. Or, my experience, which is that ATC ask the slow one in front to speed up. Low cost index on our fleets is leading to descent speeds of around 250-255kts. The majority of descents into larger airfields require us to speed up and thus negate any fuel saving from the low cost index!:ugh:

PP

[LYKA]

Pilot Pete:

You have missed the point about CI - It's total cost's not just one component.

[411A]

You have missed the point about CI - It's total cost's not just one component.

Sadly LYKA, many pilots have this misconception.
Some are only proficient in thinking in one direction only (at the moment, the high cost of fuel, and make no mistake this is a large component) however other factors enter into the equation, and are often ignored by those that occupy the pointy end.

[silverhawk]

Fly too slowly and your flight time goes up. You soon reach the point where maintenance costs outweigh the fuel savings thus making the flight more expensive, not cheaper.

Investing in appropriate flight planning software with variable cost index for each sector is the way to go.

[Man Flex 37.5]

At Ezy we are flying cost index 10 giving a very slow descent speed around 250 - 260 depending on cruise altitude. This is all very good, but, with the airbus if you are on profile using managed descent the aircraft will have around 44 - 45 % N1 (the idle factor) to allow for engine anti ice, so we spend longer in the air, longer in the descent with power on!

MF

[OBK!]

Man Flex

Correct, longer in the descent with idle...rather than at cruise with 80-90% N1. You've surely noticed how a slower decent means TOD comes sooner...maybe 5 minutes sooner...saving 5 minutes of cruise power fuel.

silverhawk

For other peoples understanding, there's no such thing as too slowly with cost index. The speed from whatever cost index the company gives you is the most economic, no matter how much longer you spend in the air with the engines running. I imagine you where pointing out that a fixed cost index across all airfields and fleet that an airline operates isn't the correct way to utilise it though.


If all the operators worked out a pretty accurate cost index, we'd all be descending at a slow speed, maybe around 250kts.

[Man Flex 37.5]

There is also an option to change the descent 'Idle' setting, next to the engine performance factor, changing this will steepen or shallow the descent, then we could descend with real idle like open descent.

[silverhawk]

Yes you are correct. It's taken a long time to get that message across to the purse string holders.

Same CI cannot work on there and back sectors unless there is no wind.

Same CI cannot work on a 27 minute sector versus a 3 hour sector.

We are using 15 across every single sector. Refinements can be made.

By the way, 'were' is the word you were looking for. Sorry to sound like an ass, but many viewers are trying to improve their levels of English and I do wonder where all my tax pounds are actually going in the education system. No offence.

[ItsAjob]

On some of the short sectors where we remain in spd, our fmgc has our cruise speed much higher than the descent speed.
Climb at 310, cruise 305 then descend at 270??
For all the calculating it doesn't make much sense when you fly the thing.
Makes me chuckle when I still hear the famous words ` What's IT doing now?`

[flite idol]

"Same CI cannot work on there and back sectors unless there is no wind."

Not so sure about that one, at least on the ship I fly. The FMC takes actual wind into account, speeds up for a head wind, slows down with a tail wind. Thats the concept, a ratio of fixed costs and variable costs ie crew,lease,maintenence etc and fuel. Wind is accounted for and speed adjusted in real time by the FMC.
While it may seem a blunt tool that seems to work on some sectors and not on others, its an across the fleet philosophy. If it was left to the pilots to program what their best theory is then who knows what would happen. Its an average percentile saving that is being sought!

[LYKA]

Man Flex:

Don't go messing around with the idle factors unless you company permits this - it can really produce some very odd results.

RE OPEN DES vs. DES: Airbus has spent considerable amounts of time and money in designing an aircraft as efficient as possible...still find it odd that pilots think that they know better...

[anotherthing]

Tubby Linton

you are partially correct - there is a 3 month trial for sectors outside the LTMA to present traffic to TC controllers at 270Kts - this trial will end soon and the procedure will not be implemented.

It causes too many problems with regards to traffic and can actually cause unnecessary holding if it is used without any thought by ATCOs.

I don't want any pilots here thinking that they are doing ATC a favour by flying 270Kts unless specificaly instructed by ATC when entering the LTMA.

Similarly EZY pilots - it might be an idea to tell ATC that you will be slow in the cruise if what Manflex 37.5 says is correct is his first post - if you don't, you may find yourself being pushed to the back of a stream of aircraft more often, because we often need large speed differentials to stream aircraft into a sequence that prevents or minimises holding... If EZY pilots slow down because of CI, but don't inform ATC you won't be making our jobs any easier and your CI will increase because you will be put to the end of the queue and have to fly for longer than would have been needed!!

[Man Flex 37.5]

Anotherthing

Most of us fly 290 in the descent into London TMA anyway, what you say is understood on the line.

Lykra

Thanks for the lesson mate, I have been flying the bus since 95 so have a vague idea how it works (and no i have never modified the IDLE on the ground, but it is there should a company need it else why would there be an option). It doesn't take Mr Einstein to work out that you are burning more fuel with power on in the descent, if you were to select open descent at the FMGC computed descent point, you would go below the profile more ofter than not.

MF

[LYKA]

MF - It wasn't a personal critism of you or your experiance, so if an apologee is required then please humbly accept it.

Airbus specifically state that the IDLE factors is an operational tool used to adjust the assumptions of the descent path computation and is NOT a tool to save fuel. Improper use might lead to inaccuracy of time and fuel predictions as well as the flight path computation.

Yes you would go below the profile, but in order to re-capture the ideal profile, whilst remaining at idle, one would need to reduce the speed; something managed DES does for you with the use PFD speed brackets/ ATHR combination. Additionally the fuel flow values at idle are not that accurate due to design and sensor location.

LYKA

[Kiltie]

UK 737NG operator - we've recently changed from cost index 35 to cost index 10. This gives a 250kt descent speed from TOC but we are instructed by management to changeover to 270kts which makes it more sensible.

[757jetjockey]

Just for a bit of fun, try putting '999' into your cost index page, and see what you get. I know the boeing book says its fmc ranges from 0 - 200 but trust me '999' will get you home seriously quick!

it basically flys on the barbers pole for the whole flight, from T/O to final approach when you have hopefully, caught up with, slowed down and are about to land the beast