A340 THR DES... What is that??

[Microburst2002]

Not much explained about it in the FCOM.

"Reduced thrust while DES mode is engaged"

Can someone enlighten me about this A/THR mode?

[D-Gold]

This mode is one of the more dificult to understand and explain! The DES mode provides Managed vertical guidance along an FM computed vertical descent profile. The system computes this flight path backwards from the Decel point up to the top of descent (T/D) at the cruise flight level with respect to the speed and altitude constraints. The Decel point is where the guidance begins the deceleration to VAPP, to be reached at 1,000 feet above touch down on the final descent path.
The descent profile takes into account wind data and data from the lateral and vertical flight plans (speed and altitude constraints and SPD LIM), and it is based upon the Managed descent speed profile. It does not take holding patterns into consideration.

The aircraft will not start its descent automatically when reaching the top of descent (T/D). In order to initiate the descent, you set the ATC lower clearance altitude on the FCU, then push the ALT selector knob. The aircraft will then descend immediately:
- If the descent is initiated before the FM computed T/D, the aircraft descends at a constant V/S converging on the descent path from below. The A/THR operational mode will be SPEED/MACH.
- If the descent is initiated after the FM computed T/D, the aircraft descends at idle thrust and attempts to converge on the descent path from above. The A/THR operational mode will be THR IDLE.
- If the descent is initiated at the FM computed T/D, the aircraft is descending on the FM calculated vertical descent profile. The A/THR operational mode will be THR DES.

Hope this answer your question!

[Microburst2002]

It does not! But I hadn't read that.

So what does that A/THR mode do? I mean: THR CLB gives Climb thrust. THR IDLE gives idle thrust. What thrust does THR DES give??

Is it, perhaps, that the profile is calculated using a thrust different to idle, like the 320s? And if so... Why?

[D-Gold]

Well.. DES is a Managed vertical mode. It can only be set when in NAV lateral mode. DES mode is the only time that you will get the Managed Speed target range (normally ± 20 Kts, limited by Vmo – 3 Kts or Mmo – 0.006 or if a descent SPD LIM or a Speed constraint is encountered, the speed is limited to the Constraint Speed + 5 Kts) displayed on the PFD Speed tape.There are three basic cases that can occur when in the DES mode:

On profile:
The aircraft is descending on the FM calculated vertical profile and so VDEV will be about zero.
By setting DES when on profile you command the AP/FD to fly the target SPEED/MACH (Managed Speed target range). The Speed is now controlled by using the elevator to change the pitch attitude by the AP/FD. As the AP/FD is controlling the SPEED/MACH the A/THR cannot – so it must control the THR(ust), which it does and sets THR DES.


Above Profile:
The aircraft is descending but is above the FM calculated vertical profile and so VDEV will be positive.
By setting DES when above profile you command the AP/FD to fly the target SPEED/MACH (Managed Speed target range and it will increase up to the upper limit of the range). The Speed is now controlled by using the elevator to change the pitch attitude by the AP/FD. As the AP/FD is controlling the SPEED/MACH the A/THR cannot – so it must control the THR(ust), which it does and sets THR IDLE in an attempt to intercept the profile from above.

Below Profile:
The aircraft is descending but is below the FM calculated vertical profile and so VDEV will be negative.
By setting DES when below profile you command the AP/FD to fly the trajectory to intercept the profile from below. The Managed Speed target range will be displayed but it will use the set Managed Speed only, not the lower limit of the range. The trajectory (or path) is now controlled by using the elevator to change the pitch attitude to set a V/S of either 1,000 ft/min (Idle segment) or 500 ft/min (Geometric segment). As the AP/FD is controlling the trajectory the A/THR must be in SPEED/MACH mode in an attempt to intercept the profile from below.

Remember GIGO (Garbage In/Garbage Out). The FMGS will only be able to achieve what you input into it. If you put crap in, you can expect a less than optimal performance.

Hope you are a little bit closer now then.. The mode interact with several System wich is helpfull to understand before you jump in to the DES mode!

[99jolegg]

I've not heard of THR DES before, it doesn't feature on the A320.

What's lacking by just having THR CLB, THR IDLE, SPEED/MACH? They're the only applicable modes I can find that the A/THR will give on the FMA for the situation we're talking about here.

If you're on the descent profile and managed, won't the ATHR just maintain SPEED or MACH like it does on the A320?

[Microburst2002]

OK

In the 320, DES mode, on profile, AP/FD pitch will maintain the profile, like the G/S mode maintains the glideslope.

If everything is perfect during the idle segment, A/THR mode will be THR IDLE.

If the airplane tends to get high, speed will increase as a consequence of increasing pitch down to maintain the profile. After reaching the upper limit of the speed range, the airplane will get high unless you intervene.

If the airplane tends to get low, then speed will decrease, up to the lower limit of the speed range, point at which the A/THR will become SPEED.

So in the 340, DES mode, on profile, the AP/FD pitch will maintain target speed, instead of the profile. So what does THR DES do? Does it maintain the profile? If not... Who does?

99Jolegg is right. If in the 320 THR DES is not necessary, why is it necessary in the 340?

[9.G]

M2002 read FCB 11 Idle Factor, it's all in there.:ok:

[I-2021]

Hi MB2002,

The THR DES mode is a feature of the A330/340. Basically THR DES is the sum of THR IDLE added to (sorry I am on a weird keyboard can't find the plus sign:p) the IDLE factor. The IDLE factor is used to adjust the IDLE thrust level and is a percentage of the basic idle thrust. You can insert it and modify it from IDLE/PERF page. It is not to be confused with the PERF factor that is used for fuel predictions to take into consideration the engine wear. If you have a positive IDLE factor the airplane will start the descent earlier and when reaching the profile will go in THR DES. That extra amount of thrust compared to idle thrust will give you some flexibility during descent if conditions change such as wind speed and direction, the need of anti ice, etc.. basically all it does is to make you descend with some more thrust that in idle thrust. If you get high on profile the airplane will accelerate like on the A320 and will reduce the thrust to THR IDLE :)
On the A320 you have the same function but it doesn't show you the THR DES FMA, it's all in THR IDLE.

Cheers !

[PJ2]

Microburst2002;

Some very good explanations here. I'll chuck my 2c in.

THR DES mode is a SPEED/MACH mode. It is like the DES mode in the A320.

The mode has a +/-20kt speed latitude from the calculated target descent speed in order to energy-manage the calculated descent path, essentially to conserve fuel, (same idea as any manual descent we've ever done in any other airplane, by either speeding up a bit or slowing down a bit or if that wasn't enough, by either adding a bit of power if slightly low or using speedbrakes if slightly above what we wanted).

You can imagine the FMGEC-calculated descent path as a geometric, "hard line" in the sky so to speak, and the autoflight/autothrust tries to keep the airplane on that hard vertical path.

The mode is intended to manage aircraft energy and conserve fuel. The FMGEG's calculate the required descent path taking into account all altitude and speed constraints in the programmed arrival route as well as the pilot-entered winds, and sets a descent point.

The speed range with which THR DES manages aircraft energy is plus or minus 20kts from the calculated (target) descent speed. You already know what the airplane does if above or below the calculated path...it speeds up (and the descent rate increases) if above the path or it slows down (and the descent rate reduces) if below the path.

If too high and the speed is at the upper limit, the A/THR mode reverts to THR IDLE, and if still too high the "EXTEND SPD BRK" message is displayed on the top of the PFD. If too low, the pitch is increased slightly to use aircraft energy to reduce the descent rate to stay on the path. If the lower limit is reached, thrust is added because, as stated, the THR DES mode is a SPEED/MACH mode.

PJ2

[Microburst2002]

9G, I2021 thanks, now I get it.

I always suspected that modern MSNs in 320s had something like that when in DES, because with THR IDLE green, thrust was significantly higher than idle and TOD was too far compared to an OP DES TOD.

So that is what the IDLE factor is...

Question solved, thanks everybody!

[SMOC]

It's been years since I flew the bus but doesn't it descend with thrust ON to provide 0 drag ie the A330 (RR) descends with an EPR of 1.0 while if you descend in OP DES you get idle.

So THR DES means thrust with descent while IDLE DES means thrust at idle.

[dream747]

If this helps in any way, the only reference to 'Thr Des' I can find from my books on th A320, is from the FCTM Normal Procedures under descent where it says:

"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."

Never seen it in real flight before though.

[macdo]

If you have a look at FCOM BULLETINS No 11 FCB-FCB11 – IDLE FACTOR, I think this explains the workings of THR DES, as far as I can see it does not have this function on the 320, only the 330/340. If you can't find it in your own FCOMs I'll try to post a link later.

[Trent 972]

When the AP/FD vertical mode is SRS, OP CLB, CLB, OP DES, or DES in idle path, the A/THR is in THRUST mode.
The thrust target depends on the THRUST mode.
When the vertical mode is DES in idle path and the FMS requests a thrust slightly higher than the idle thrust, the thrust mode is THR DES
When the FMS requests idle thrust, the thrust mode is THR IDLE

[macdo]

Yup, this is correct. I was wrong about this being 330 only and the same rule applied in the late model 320 I tried this in last night.
In managed DES the FMA showed THR IDLE, but the N1 was at about 42%. On selecting Open Descent, the FMA stayed at THR IDLE, but, the legend IDLE appeared at the top of the E/WD just above the N1 guages, flashed for about 10secs and became constant along with the N1 falling to about 36%, so properly at Idle. Re-selecting DES and the little bit of thrust came back.

[Microburst2002]

What msn are the 320s in your fleet?

I think this increased idle thing is relatively modern, in the 320s
In the 340s and 330s it seems to be from the first model.