Thrust Idle on A320?

[FLEXPWR]

Any guys or girls who can give me some info on the following:

During descent, the A320 can have various modes for the A/THR, Mach, Speed, THR IDLE..

Most of the time, when THR IDLE is displayed on FMA, the "IDLE" message is also flashing on the E/WD and N1 pointers move to around 35%, but on some occasions, it is not, and the thrust set by the A/THR is far from idle. In fact, it can be anything I have seen up to 50-52% N1. Nothing to do with a geometric profile, and on top of it, my last observation was during a tailwind descent. After a while (last observed when passing FL210), the engine did throttle back to "real" idle, and the FMA annunciatiors did not change a bit. Note: no anti-ice operation.

I have looked in FCOMs, but it only mentions that when FMA is showing THR IDLE, the engines are commanded at idle power. It happens only on rare occasions, I have asked a couple of trainers and no one seems to come with an answer, although they have seen it before.

Any thoughts?

Thanks

Flex

[John001]

Hi,

I've seen this also. Good question. I wonder if it might have anything to do with avoiding certain NI speed regimes (eg flutter avoidance) by the FADEC?

Just a guess, I look forward to hearing from the more learned contributors! :-)
J

[FLEXPWR]

I did not think about that, indeed could be the case.

Any reference to a N1 range that shall be avoided? (within the normal operating range, i.e. 30 to 100%)

I forgot to mention it's on a V2500.

Flex

[John001]

Hi FLXPWR,

I found a reference in FCOM 1.70.20 pg 4 which refers to an N1/thrust lever angle 'keep out zone' however this, it says, relates to "on ground" operation;

" ......The EEC protects against fan flutter. In doing so the EEC prevents the engine from being stabilized between an approximate range of 60 % to 74 % N1 (depending on the OAT)..."

So this would seem to offer little in the way of explanation as to why the A/THR
does as you and I have observed in flight. :-)

[rudderrudderrat]

Hi,

It's a mystery because there is no useful information in FCOM.

I think "IDLE" during a managed DES is above flight idle - so we could use Engine Anti-ice without spoiling the profile.

I've noticed that "Open DES" sometimes selects a lower flight idle setting.

[RA44471]

Noticed that at 319/320
In my opinion THIS BIG COMPUTER keepeing cabin rate around default 350 ft/m

[OPEN DES]

In my opinion THIS BIG COMPUTER keepeing cabin rate around default 350 ft/m

You're confused here with the 'Repressurization segment' which occurs at higher altitude at very low weight. In QRH 4.12 (ALL ENGINES DESCENT) you can see that N1 is higher than idle to cater for this segment.

Basically there are three type of segments calculated by the FMGS for the descent profile:
-repressurization (very rare)
-geometric (in between two altitude constraints)
-idle

THR IDLE is normally displayed on the FMA when on the 'idle segment' under forecast conditions.
The 'steepness' of the idle descent is a function of the IDLE FACTOR on the A/C STATUS page.
Especially at higher altitude the 'calculated idle profile' requires more than idle thrust because it is conservative to allow for anti-ice and/or shortcuts. Especially on the A319 this is noticeable.
My thoughts are that Airbus intends to have a uniform type of descent planning/calculated idle segment for the whole A320 family. The A319 is typically lighter than the rest but does fly very similar Managed DES speeds meaning a worse L/D ratio than the rest of the family flying the respective speeds. (i.e. flying further away from green dot than the rest). Hence requiring some additional thrust when on the 'idle segment'.

Sorry for the lack of references, there aren't any. Hope it helps.

S.

[Phil Squares]

THR IDLE---The auto thrust is armed; with the most forward thrust lever above the IDLE detent and anywhere up to and including the CLB detent. (MCTsingle engine) Airspeed (Managed or Selected) will be maintained by
varying pitch.

IDLE---The auto thrust is armed, with the most forward thrust lever forward in the CLB detent. Idle power has been commanded.

[RA44471]

In OPEN DES mode with no wind A319/320 always desends below managed profile/or flying with THR IDLE at FMA but above idle at E/WD in DES
I've monitored cabin rate in both conditions.

321 OPEN DES and DES look similar. And cab rate is close to 350ft/m

[RA44471]

sorry for addition.
If you're flying the iron, it will falls with :eek: 10000ft/m or more. You'll need thrust to bring it to normal descend path wiht convinient cabin rate.

[FLEXPWR]

Still a mystery for most of us then...

I thought of the cabin pressurization, but as mentionned, any lag would make the computer to adjust the profile with a repressurization segment.

It seems that the FCOM description is far from complete...but that's old news! :}

Is it possible that the N1 above idle has anything to do with the Handling of Bleed from the compressor, e.g. to reduce the chance of flow separation or something, as air density is much less at higher levels? Just a guess.

Flex

[TyroPicard]

FADEC is using Modulated Idle, which varies with bleed demand and ambient conditions.... that's as deep as my knowledge goes.

[OPEN DES]

FADEC is using Modulated Idle

True. But we're talking about the FMGS commanded THR IDLE which is no engine idle but some sort of assumed idle for the descent profile. (Idle does not flash on EWD when FMA reads THR IDLE).

My bet is on uniformity of descent profile calculations:

e.g. given weight (no exact numbers, just for illustration)
A319 green dot 210
A320 green dot 220
A321 green dot 230

Managed descent speed for all 3 a/c: .77/290

At this fixed speed schedule the A321 has the best relative L/D ratio because it is flying closer to Vmin drag=G.D.. Hence has the shallowest descent profile. This A321 profile adopted for all 3. This means that the A319 and A320 require thrust to stay on this profile.

This also explains that DES and OP DES result in more or less the same profile on the A321 while on the A319 and A320 the a/c will descent below the managed profile in OP DES.

Just my thoughts.

Anyone?

[Cough]

My experience is a heavy 321 will have a higher DES speed (and climb...) than a 319. So most of the time, for a fixed ci DES, the target speed for all types will be a similar increment from green dot.

Today's 321 gave 291, yesterdays 319 gave 267 as DES speeds, both relatively heavy for their configs.

I agree, modulated idle in DES mode is higher than open DES, just when the speed runs away it should wind itself back to normal flight idle.

[riedle74]

2.1. Thrust Setting
The standard engine rating for descent is “Flight Idle Thrust”. For fly-by-wire
aircraft, the thrust throttle position doesn’t change when autothrust is engaged. The
throttles remain on the “CL” (climb) gate for the entire flight (Figure H6). The enginemonitoring
computer, or FADEC (Full Authority Digital Engine Control), adjusts the
thrust level to the required value.
In case of an altitude constraint or a repressurization segment (see ”Cabin
Descent”), the aircraft’s vertical speed may have to be limited during descent. This is
achieved at a thrust called “Adapted Thrust”. The adapted thrust may vary between
flight idle thrust and maximum cruise thrust. It is delivered by the engines, when
autothrust is engaged, as soon as the aircraft descent speed plus one of the two
descent parameters (gradient or rate) have to be maintained at fixed values.


2.4. Cabin Descent
The cabin pressure rate is optimized during descent, so that it reaches the
landing field pressure + 0.1 psi just prior to landing.
Depending on the initial cabin and destination airport altitudes, the FMGS
calculates the necessary cabin descent time. This time is obtained from the selected
cabin rate of descent, defaulted to –350 feet per minute in the FMGS, but which can
be modified up to a maximum of –750 feet per minute.
IN FLIGHT PERFORMANCE
DESCENT
3.05.30 P 2
SEQ 110 REV 31
DESCENT - M.78/300KT/250KT
IDLE THRUST ISA
NORMAL AIR CONDITIONING CG=33.0% MAXIMUM CABIN RATE OF DESCENT 350FT/MIN
ANTI-ICING OFF
WEIGHT
(1000KG) 45 65
FL
TIME FUEL DIST. N1 TIME FUEL DIST. N1 IAS
(MIN) (KG) (NM) (MIN) (KG) (NM) (KT)
390 16.1 181 102 73.0 17.0 125 103 IDLE 241
370 14.6 152 90 74.0 16.2 121 98 IDLE 252
350 12.9 115 77 75.9 15.6 118 93 IDLE 264
330 11.7 90 69 IDLE 14.9 114 88 IDLE 277
310 11.2 87 65 IDLE 14.3 110 83 IDLE 289
290 10.8 84 61 IDLE 13.7 107 79 IDLE 300
270 10.2 81 57 IDLE 12.9 103 73 IDLE 300
250 9.6 77 53 IDLE 12.1 98 67 IDLE 300
240 9.3 75 50 IDLE 11.7 95 64 IDLE 300
220 8.6 71 46 IDLE 10.9 90 59 IDLE 300
200 8.0 67 42 IDLE 10.1 84 53 IDLE 300
180 7.4 63 38 IDLE 9.3 79 48 IDLE 300
160 6.8 58 34 IDLE 8.5 73 43 IDLE 300
140 6.1 53 30 IDLE 7.6 66 37 IDLE 300
120 5.5 49 26 IDLE 6.8 60 32 IDLE 300
100 4.8 44 22 IDLE 5.9 53 27 IDLE 300
50 1.8 17 8 IDLE 2.2 21 9 IDLE 250
15 .0 0 0 IDLE .0 0 0 IDLE 250
CORRECTIONS LOW AIR
CONDITIONING
ENGINE
ANTI ICE ON
TOTAL
ANTI ICE ON
PER 1° ABOVE ISA
TIME − + 10 % + 10 % −
FUEL − 2 % + 60 % + 75 % + 0.60 %
DISTANCE − + 10 % + 10 % + 0.60 %
R

2.4. Cabin Descent
The cabin pressure rate is optimized during descent, so that it reaches the
landing field pressure + 0.1 psi just prior to landing.
Depending on the initial cabin and destination airport altitudes, the FMGS
calculates the necessary cabin descent time. This time is obtained from the selected
cabin rate of descent, defaulted to –350 feet per minute in the FMGS, but which can
be modified up to a maximum of –750 feet per minute.


As soon as the cabin descent time is longer than the aircraft descent time, a
repressurization segment is necessary, during which the aircraft vertical speed is
limited to permit cabin repressurization (Figure H9).
Figure H9: Cabin Repressurization Segment
The above A320 descent table (Figure H8) shows that to descend from FL390
at a weight of 45 tons, the N1 parameter must be maintained at 73%, from the start of
the descent, in order to limit aircraft vertical speed.
Note that, in some particular cases (landing at high altitude airports) , the cabin
pressure at cruise level is higher than the pressure at the landing airport. Therefore,
the cabin pressure has to decrease during descent, which means that the cabin’s
vertical speed is positive while the aircraft’s vertical speed is negative.

[Bula]

"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."

Funnily enough this is the concept of THR DES on the A330/340. This mode does not exist on the A320 but essentially does the same thing.

However, have you ever noticed with THR IDLE on the FMA, and the thrust slightly above IDLE on the E/WD, the aircraft tends to accelerate to the +20kt tolerance and only then trust is reduced, and thrust IDLE on the E/WD.

Maybe your forecast to fly into an increasing headwind on descent so it makes an additional allowance. I don't think it's that clever but you never know.

[Microburst2002]

I can't find the other thread about this, but here is a sum up.

A320 also has a THR DES mode, so to speak, but it is transparent to the pilots.

The IDLE FACTOR in the FMGS determines this higher idle when using DES mode. It is intended to give an allowance for anti ice or give some "flexibility" that is, to give you a few more miles from TOD just in case.

I hate it, because it makes my own computations and FMGS's differ.

Note: the repressurization segment also involves a higher idle but this is to reduce v/s and time to descend so that the airplane can lower the cabin altitude without exceeding the preset cabin v/s (350 fpm default)

Where is this written? I can't remember. It is somewhere in the FCOM"4" and I think there is also an FCOM bulletinq about it

[riedle74]

http://www.captainpilot.com/files/AIRBUS%20PERFORMANCE/GettingToGripsACPerformance.pdf