A320 OP DES at low altitude

[Giggey]

Hi All.
As by subject i was once told that using Op Des at low altitude was not a great idea because it could have taken into some unmanaged V/s ratio and maybe activate some gpws warning and to use V/s instead which i have to say i agree pretty much.

I am now trying to find some published reference to this theory but cannot find anything either on fcom 3, qrh nor Fctm.

Any clue if that might be just some company policy or it has an origin from Airbus?

thank you.

cheers

[Olabade]

Also your A/THR mode would be IDLE instead of SPD (SPD with v/s mode), which might not be smart when flying low due to increased spool up time.

rgds. Olabade

[Giggey]

Yep definitely right but i cannot find any officially published documentation about that:ugh::ugh:

ciao.

[aristoclis]

Giggey,

Just out of couriosity, what is your company minimum alt sel to use OPEN DES?
How low is your A320 OP DES at low altitude

The only limitation I am aware of is at FCOM 3.01.22 Page 1:

Use of AP or FD in OPEN DES or DES mode is not permitted in approach, unless the FCU altitude is set to, or above MDA (MDH) or 500 feet (900feet for A321), whichever is the highest.

Also intercepting G/S from above is not recommended in OPEN DES.

[Olabade]

I'd say what is said in the FCOM is valid. I would use it down to initial approach altitude and not below that (or MSA if not going for approach).

rgds. Olabade

[Microburst2002]

I don't think you will find it written.

In final approach (wether NPA or G/S capture from above) it is definitely wrong to use OPEN DES.

But in other stages of the descend there is nothing wrong with the mode you use as long as your parameters are OK. You can have too high a sink rate and idle thrust with V/S green on your FMA, as well as with OPEN DES.

One thing I don't like about the OPEN DES mode is the rate of deceleration after selecting a lower speed. Sometimes it takes ages! It is far better to do it using V/S rather than accepting the result of who knows what algorithms in the computers, whatever it is. It gives more control.

Another problem (potentially disastrous) of this mode at low altitudes is if we fail to switch off one FD when the PF says "FDs OFF" (it can occur and remain unnoticed for several reasons such as distractions, fatigue, etc...). Then we have A/THR active in THR IDLE and not SPEED.

[aristoclis]

I don't think you will find it written.

In final approach (wether NPA or G/S capture from above) it is definitely wrong to use OPEN DES.


As mentioned, it is written in the limitations section of FCOM3. Since it is Airbus (and also company) SOP to set missed approach altitude at GS* or FAF in the FCU selector, OPEN DES is not permitted beyond this point.

Also for the interception of glideslope from above FCTM NO-110 page 3/6 states:
"It is vital to use V/S rather than OP DES to ensure A/THR is in speed mode rather than IDLE mode"

Regards.

[zeus_737]

the lowest u should be able to descend with the open des should be ur IAA( initial approach altitude) or incase of a visual approach it would be ur circuit altitude, assuming these are set in the FCU Alt window.

Now if u were to descend below it, i would have to be only in V/S or FPA, with the ATHR in Speed mode, since pulling the alt knob would lead to open climb.
(All this assuming FDs are on and are being followed)

airbus has set heights of 500 and 900 for A321 with due consideration to the spool up time.
if you were descending with 1200fpm:= which means 20fps, u would loose 160ft at the most while u wait for ur engine to spool up from idle to ga thr.. Worst case scenario...Ofcourse there would be hell to explain to Flight safety but that is besides the point...

hope it helps