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A320: "Thrust Idle"

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A320: "Thrust Idle"

Old 16th May 2013, 02:37
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A320: "Thrust Idle"

Sorry in advance if this material has been previously discussed.

My concern is fuel savings. CFM engines.
When on descent profile, the FMA reads "thrust idle" yet N1 is well above idle, approximately 45% through 30,000'. When I pull for "open descent" N1 drops to about 30%, fuel flow reduces by almost half. So I start figuring it saves more fuel to fly beyond the top of descent by 30 miles or so and open descent in an attempt at constant idle descent as close to final as possible. I've been flying this way for a few weeks now, but there are too many variables for me to know if I'm really saving anything.

When I asked a knowledgeable instructor, he told me our fleet is configured for N1 at 45% when on profile managed descent in case engine and wing anti-ice is selected on the N1 simply drops to real idle and we don't end up above profile. Kinda makes sense, but I'm not entirely convinced. What I suspect is that real thrust idle is actually a negative thrust condition, EPR less than 1, creating some drag like props not feathered. Maybe N1 at 45% is actually what the engineers calculated to be 0 thrust, which renders my current attempts at open descent all the way to finals futile as far as fuel savings is concerned. Any input?
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Old 16th May 2013, 03:00
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Youre not the only one.. After my 10 year career on gliders, the managed descend mode is driving me insane on the 320. Especially if you for some reason, is high, the computer goes in total suicide mode, and vertical speed reaches values of 4000ft/min + which is not really comfortable, yet on the other hand, OP DES with the initial increase in indicated airspeed, due to Mach hold is not so nice either.. I have started to pull OP DES and select a speed target around 20 knots lower than our planned descend speed of 300 knots, and the profile is then constant (The captains that belives in fully managed think im crazy, but yet im one of the few F/O's which can land with the exact planned fuel from our CFP's - where managed descend on the same route, is around 200 kgs higher.)
I doubt that flight idle, is in anyway less economic, as you still generate thrust with the idle power, on ground.
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Old 16th May 2013, 03:16
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I'm not sure if I am entitled to an opinion, after all i only fly a Cessna 150 :P But i understand than the les power you are producing at any given time, the less dinosaurs per second you are burning, right??? So it makes more sense to make a steep approach at idle rather that a shallow one at some power. And also (And this is where i am entering unknown terrain) When you spend more fuel is at low altitude, right? Because of denser air right? Then a quick descent makes also spend you less time on the high fuel consumption altitudes than a shallow one... So from a common sense point of view you should be right, right?

Steeper approach (gliding) = Less fuel consumed BUT you also have to take other stuff into consideration such as traffic sepparation and passenger comfort...

If i stuck my foot in my mouth, please, let me know
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Old 16th May 2013, 05:32
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THR IDLE on the A320 is a lie. In the A330/340 it is rightfully called DSC THR (decent thrust).
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Old 16th May 2013, 05:45
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When you start your descent in managed mode the Auto thrust goes to idle mode but FADEC gives you Modulated Idle till flaps lever is out of zero position. In this mode N1 is regulated by bleed system demand. So if you select Anti Ice the N1 will increase to maintain minimum N1.Also if cabin descent time is longer than a/cdescent time a repressurisation segment is necessary during which the a/cvertical speed is limited to permit cabin pressurization. If you consult Descent table in QRH you will see the required N1from TOD. Since in Open descent you are commencing descent later FAEDEC gives you the minimum idle N1. On ground It is possible to modify idle setting which is done by maintenance.

Last edited by vilas; 16th May 2013 at 05:48.
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Old 16th May 2013, 08:04
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FMC descent

The FMGC calculates the descent path with the ASSUMPTION that anti-ice will be ON.
A you should know, having anti-ice ON increases the demand for bleed air, and in order to be able to deliver that amount, the engine has to run at a slightly elevated level of thrust (that means, a little more than idle).

If, as often happens in real life, the descent is made with anti ice OFF, then idle thrust will not be enough to hold the speed constant, so thrust will be kept slightly above idle by the autothrust. Remember that in descent, pitch mode will keep the aircraft on the descent path.

When you use open descent, pitch mode will steer to maintain speed, while autothrust goes to idle.

The assumption of the FMGC is programmed to keep you from f***ing up your approach: if it was planned with real idle and you needed to employ anti-ice during the descent, you would end up with too much energy at the end of descent and that has proven, on a number of occasions, to be too much to handle for the magenta generation.
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Old 16th May 2013, 08:06
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How much extra fuel are you burning by staying at cruise power for longer?
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Old 16th May 2013, 08:47
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In the A330/340 it is rightfully called DSC THR
a very small point... its actually "THR DSC"
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Old 16th May 2013, 15:19
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On the FMGS status page you'll have an idle factor - that affects the idle thrust set in a managed descent - your company will have decided it.

On top of that, FCOM DSC-70-40-40 P 2/4:

The FADEC has the following three idle modes :

Modulated idle ‐
Is regulated according to :
bleed system demand ‐ Is selected :
In flight, when the flaps are retracted (FLAPS lever at zero position),
On ground, provided reverse is not selected.

Approach idle :
‐ Is regulated according to aircraft altitude, regardless of bleed system demand.
‐ Is selected in flight, when the flaps are extended (FLAPS lever not at zero position)
‐ Allows the engine to accelerate rapidly from idle to go-around thrust

Reverse idle :
‐ Is selected on ground, when the thrust lever is in REV IDLE position.
‐ Is slightly higher than forward idle thrust.
It gives you a buffer for head/tail winds, big temp changes, half asleep, distracted and bored pilots and short cuts.

Consider re-inputting the cost index on the FMGC PERF page every time you pass a constraint to allow the system to recalculate the profile and optimise your descent. Open Descent is rarely going to be more fuel efficient.

Last edited by WhyByFlier; 16th May 2013 at 18:10.
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Old 16th May 2013, 18:15
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Combine information

The above quote about the workings of the engine control computers is nice - it is the process by which your idle will be at slightly higher thrust and RPM's with anti-ice ON than with anti-ice OFF. This quote however, does not explain why the aircraft seems to fall short on energy on most managed descents.

The descent path as stated a few posts above, is calculated by the FMGC, which, for its calculations, assumes the elevated idle thrust will apply for the whole descent. If, in reality, anti-ice is OFF and consequently, the lower level of modulated idle is technically available, the autothrust will have to maintain slightly more thrust to maintain speed while the pitch mode keeps the aircraft glued to the managed descent trajectory.

For fuel economy, a real idle descent would be ideal, all the way from cruise level to the point where your dirty configuration on the final approach slope necessitates thrust to keep speed at the desired approach speed.

Needless to say, real world constraints make it hard to achieve that lofty ideal, but theoretically ... think about the alternatives:
If you descend too soon, you will need level flight in thick air - you're worse off
If you need speedbrakes down low, you have stayed at cruise thrust too long - also a waste.

Descent paths calculated by FMC's (or FMGC's for Airbus folks) are just a means to help you to approximate the ideal situation.
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Old 17th May 2013, 01:07
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In the above descent - what would happen if you Selected 280kts.
Open Descent?

. .and . ../ . but . .

Could you not plug in 280 for the descent speed in the Prog page . ? or would that be the , anyway, the page where you could put in 280 (thereby without having to go to Selected spd yet) (and then go Selected lower down, getting ready for the approach)


Well by doing it this way, you would have a managed profile to keep you on the . . . descent profile but with a MAN input into the FMGS?

Or have I missed the boat again?

Last edited by Natstrackalpha; 17th May 2013 at 01:09.
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Old 17th May 2013, 05:15
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FMGC does not calculate descent with anti ice on, a note in FCOM says a/c will deviate from path with anti ice on. Not surprising because with anti ice on calculation you will end up starting descent early and burning more fuel.In general a slight overshoot with speed brakes consumes less fuel than undershoot and flying level.
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Old 17th May 2013, 09:25
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Thank you, Gentlemen! You're responses are fantastic! Complete with QRH and FCOM references! So what the instructor told me was spot on after all.
In my continued quest for fuel savings I will now experiment in continuing at cruise beyond the TOD but not so far that I'll have too much energy approaching final, pull for real thrust idle until the magenta profile doughnut centers up, hopefully very close to final, and simply push for managed descent so I don't end up level in thick air for long.
But does anyone know that I'm wrong in thinking that real thrust idle may indeed be additional drag, akin to props not feathered? If I discover that that is the case, I will immediately stop attempting to save fuel by using open descent from TOD..
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Old 17th May 2013, 11:04
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In theory, managed descent takes into account all the known variables and constraints and works everything out to get you to the initial approach fix at the appropriate speed and using the minimum fuel (according to your cost index) - but of course you have to have programmed in all the variables first.

As long as you have put in all the descent winds, and realistic speed and height constraints, it should get it right. The managed descent speed is variable between the 'hockey sticks' to allow for differences, such as stronger/weaker headwinds etc. It obviously cannot predict ATC requirements such as headings or speeds. From memory, our A320's usually idle at around 35-37% N1 in descent. Selecting engine anti ice on should increase idle slightly.


Last edited by Uplinker; 17th May 2013 at 11:04.
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Old 17th May 2013, 16:52
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i use managed descents only for stars with multiple constraints and it is much easier for the fmgc to get it right than for me to manually figure out the profile. in such a situation having the extra n1 is OK.

however, landing into a field with no traffic and a straight in ils apporach or something similar, i start my descent on vs, not more than 3 miles beyond the tod arrow. start with a vs of 1500 progressively increasing it in increments of 500 to a vs of about 3000-3500. flying at fl390 at .79 indicated speed is around 245 and our descent speeds lie around 305-315 knots for the company cost index. so taking open descent at tod has the probability of the vs going to 6000 which is really really uncomfortable. so descent on vs till mach changes to speed and then take open descent. at this point my vdev is around +1500 and is very comfortable to to be descending in open descent and configuring and be stabilised by 1000 feet. there are a few occasions where you will have a level segment but taking it on vs helps avoid it. 1 out of 10 times speed brakes are required if atc had asked to reduce speed.

the only catch in this is the cabin v/s which can sometimes go really high and which is a reason why some people prefer managed descent to avoid that.

as for fuel saving as long as you are not having level segments you should be good. incidentally if at cruising level your fuel flow is 1200 kg/hr/eng, at 1500 agl your fuel flow will not be lesser than 1100/kg/hr/eng. thats like a difference of 3-4 kgs a min.

in the end fly the descent that makes you the happiest, and be stabilised by 1000 ft. thats all that matters.
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Old 17th May 2013, 18:44
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Thin air

Speed freak:

If fuel flow at 1500 ft is the same as at cruise level, and your indicated airspeed even is the same, say 250 kts, you are still flying less economical at 1500 ft because your true airspeed down low is about 250 kts whereas at high altitude you would get about 460 kts TAS for your fuel burning effort.
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Old 17th May 2013, 20:52
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How many of the responders here are FOs? I find these differences in descent management style to be quite enlightening. Where I flew recently, the more experienced (older) guys had an awful habit of interfering with the FO's descent management. After a year of flying with a whole variety of captains the only thing I learnt about descent management on the line was to expect the guy on the left to have you do it he's way. Oh and don't learn it as gospel, as the next one will rubbish it and teach you he's way. People you can learn from? Or just dictators with opinions?
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Old 18th May 2013, 04:07
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Oh yes. I knew what I said was somehow not making sense. It would be about 20-25 kg of fuel saving a minute. Thanks.
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Old 19th May 2013, 19:38
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Books, again


With regard to your mention of, FCOM says, Descent is calculated with idle thrust (i.e. anti-ice off), let me give you this quote from FCTM:

The first segment of the descent will always be idle segment until the first altitude constraint is reached.

This would appear to confirm your assertion.
Continue reading however, the next paragraph:

The idle segment assumes a given managed speed flown with idle thrust plus a small amount of thrust. This gives some flexibility to keep the aircraft on the
descent path if engine anti-ice is used or if winds vary. This explains THR DES on the FMA.

Do you now understand my assertion that the descent path is calculated with the assumption that anti-ice is on?
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Old 20th May 2013, 03:17
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I did check the FCTM and it says as you quoted. However what I stated is from FCOM DSC-22_30-70-50 P 5/12. I reproduce the reference below:

The aircraft may deviate from the DES profile while DES mode is engaged if:

‐ Unexpected wind conditions are encountered

‐ Anti-icing is turned on

‐ The lateral flight plan is changed.
So I guess it adds only a small amount as buffer and not the full required thrust. You can make your own opinion.
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