A330 Managed Descent - THR DES vs THR IDLE

[Sidestick_n_Rudder]

Hi!

The VNAV in A330 is quite confusing at times. Most of the time, during managed descent the FMA is "THR DES" and the thrust is slightly above idle.

However, at times you still have THR DES FMA and the E/WD indicates "idle". At other times the FMA changes to THR IDLE...

What's the logic of this?

[Uplinker]

Have a read of:
FCOM DSC 22_30-10
and DCS 22_30-70-50

THR DES is only available in NAV mode and takes account of flight plan, constraints, cabin altitude and selected FCU ALT, to plot a suitable descent profile. The engines may be above idle to achieve this, so EWD highlights to you when the engines are actually at idle.

THR IDLE is just that, minimum thrust. If you are not in NAV but in HDG, and pull for descent, you will only get THR IDLE and you have to fly the descent profile you want via V/S or speed etc.

[vilas]

When the cabin descent time is longer than aircraft descent time the initial rate of aircraft descent needs to be less than obtained at idle thrust till both meet at same point on ground. This is known as repressurization segment. During this phase in A330 FMA shows as THR DES which higher than idle and then it comes back to THR IDLE. Even idle thrust value is subject to bleed demand as anti ice etc. and is called modulated idle. Same thing happens in A320 but FMA shows only THR IDLE.

[Sidestick_n_Rudder]

Thanks guys for answers, but it's not exactly what I have been looking for.

I understand the idea behind THR DES FMA and the FCOM description. What I am getting at is that in MANAGED DECENT mode the FMA is sometimes THR DES, sometimes THR IDLE. Also, with THR DES, the thrust is sometimes idle, sometimes not... Any ideas why/when?

[CaptainMongo]

Profile and corresponding FMA’s in managed descent

Above managed profile
THR IDLE – DES – NAV .

On managed Profile
THR DES – DES – NAV .

Below managed Profile
SPEED/MACH – DES – NAV .
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).

[vilas]

I think you didn't undrestand what I wrote. I told you the reason why it varies.

[Sidestick_n_Rudder]

Hi Vilas,

I think THR DES FMA has nothing to do with repressurization segment. Re-px is rarely required (if ever) and I'd expect SPEED-DES FMA...

I think Capt Mongo's explanation is correct - FMA goes to THR IDLE above the path, otherwise it's THR DES...

Bonus question - can anyone suggest a good, accurate formula for TOD calculation on the 330, taking into account speed, weight and winds?

On the 320 I was usually able to nail it better than the FMGC :) On the 330 it's a bit more tricky, as this thing tends to glide much better...

[hikoushi]

Someone else here will have to find you the references as my A330 manuals are 3000 miles away and last time I flew the aircraft was over a year ago.

How managed descent really works:

In the A/C STATUS page (I think that is the name?) there is a "perf factor and idle factor" field. The idle factor is usually /+1.3 or some such number.

In THR DES from altitude, the plane plans a descent at descent speed, at flight idle PLUS a little bit based on the "idle factor". That is why in managed THR DES you may see .900 EPR / 40% N1 while in OP DES it is .850 / 35%. So based on the descent winds you have entered, it draws a "line" from the T/D point to 10,000 feet or your first altitude constraint, whichever comes first. The "line" is really several FPA segments attempting to give you descent, at the programmed speed, at idle thrust + idle factor. This of course factors in repressurization if required, winds etc.

The airplane gives itself 20 knots on either side of the chosen speed, and will allow the speed to vary with the wind to maintain the planned descent "line". You will see it gently pitch up and down, but the descent angle and power setting remain constant and comfortable.

In the event you get high on profile and the speed reaches the fast end of the "window", power goes to THR IDLE as the plane "dives off" the altitude. In the event you are low on profile, the plane will "stretch the glide" until reaching the low end of the "window". At that point thrust will go to SPEED or MACH, and increase to maintain the bottom of the window.

As you get back on profile and speed, the plane will return the THR DES and all will be happy in the world.

If you have to use a hard-selected speed due to ATC restrictions, sometimes the plane will cycle between THR DES, THR IDLE, and SPEED somewhat rapidly, as it cannot play with the glide to stay on profile. You have "closed its window" so to speak. Sometimes that can be a good time to use V/S for a smoother descent.

Basically THR DES gives the computer flexibility to accommodate changing conditions by both adding or reducing power if the "line" it plans doesn't work as it planned it to!

[SW1]

NOTE:
The precomputed descent path is actually divided in successive descent segments: from TOD to the first
constrained waypoint it is an "idle segment", then geometric segments. The Idle segment assumes a given
managed speed flown with "Idle + Δ" thrust, this Δ thrust above idle gives some flexibility for the FG to keep the A/C
on the descent path in case Eng A/I is used, or winds vary. This explains THR DES on FMA. The Idle factor set on
A/C STATUS page allows to adjust Δ: e.g. if - 2 is set, the Idle segment is steeper