A320 obstacle strategy
 

Good day gents,
I am aware during engine out during cruise (obstacle strategy)
and when ready for descent we must
set speed to greendot and set the driftdown ceiling and pull.
the question is,anybody please enlighten me.
wether we should wait until the speed drop to greendot
or we can descent to driftdown ceiling right away.

thx for any feedback
adwa0001

"Obstacle clearance strategy allows the aircraft to maintain cruising altitude the longest, provides the least possible rate of descent and will result in the highest possible engine out cruising altitude for the conditions. In this procedure, maximum continuous thrust is set and (except in aircraft where AP & AT will maintain the required level of thrust and descent profile) the autothrottle disconnected. The speed target is adjusted to the best engine out speed and altitude is maintained while the speed slowly decays from the all engine cruising speed. When the target speed is achieved, a descent at maximum continuous power/thrust and the target airspeed is initiated. During the descent, speed is adjusted to maintain the best speed for the current altitude and the descent is continued until Drift Down altitude is reached."

Here is some more info

Beacuse you set MCT and then select A/THR off, the thrust is fixed at the high value.

Pulling the ALT selector for OP DES as correctly stated will make the aircraft descend ONLY AFTER the speed drops below the selected target value, and the A/C will dive to maintain it.

OP DES is a speed mode, remember. Technically speaking, if the correct drill sequence is followed, the question makes no sense.

You can pull as soon as you want.

Hi FD,

Not entirely true. Whilst what you say gives an acceptable result in most cases, it could start a shallow descent too early, primarily if an excessive decel rate is present (hot/high/heavy).
The way the AP/FD manages the pitch change in OP CLB/DES (speed on elevator) is a sacrifice/exchange ratio between kinetic vs potential energy.
Likewise if you pull OP CLB in the EFATO case (third segment), before reaching Green Dot, the a/c will start a shallow climb in the level segment: Accelerating and climbing at the same time in accordance with the ‘energy ratio’.

So: pull ALT when reaching Green Dot, not before.

brgds

s

Thanks for all the reply gents, in fcom it only says.
at high altitude and heavy weight,the a/c may go to green dot very fast,thus dont delay descent.
This is my argument to NOT DELAY to descent.
i am aware also by doing this we lose altitude earlier.
Adwa0001

From FCTM : At high flight levels, close to the weight limits, the aircraft speed quickly reduces. Thus, the flight
crew should not delay to descent. The crew must not decelerate below green dot.

When ready for descent, the PF:

- Sets the SPEED and pulls
- Sets the engine out recovery altitude and pulls to engage for OPEN DES


From FCTM As the speed trend arrow reaches Green Dot speed, pull for OPEN CLIMB, set THR MCT when
the LVR MCT message flashes on the FMA (triggered as the speed index reaches green dot)
and resume climb using MCT. If the thrust lever are already in the FLX/MCT detent, move lever
to CL and then back to MCT.

Sonicbum,

You said nothing new here.
Yes the speed at high levels and high weights goes down rapidly, meaning that OP DES should be selected earlier than later. However the aim is fly level for as long as the aircraft performance allows in order to stretch the descent. Hence pulling OP DES indiscriminately for obstacle strategy, regardless of the energy of the a/c, is not the way to go.
Reaching green dot is indeed when the speed trend touches green dot.
Thank you.

I surely don't, the manufacturer does.

Don't want to be pedantic, but it's not. When the speed trend reaches green dot means something, and when the actual (index) speed reaches green dot means something else.
In the case of the EFATO You pull OP CLB when the speed trend reaches green dot and set MCT when the actual speed reaches green dot.

What OP Des said is correct. Idea is to descend as late as possible. Trend arrow is good guide to achieve that.

OPEN DES

Unfortunately, not according to Airbus. Their tutorial for Engine Failure in Cruise, is having set MCT and disconnected the A/THR, set and pulled for GRN DOT, you then set the appropriate Altitude and pull for OP DES. The aircraft maintains level until GRN DOT is reached, thrust is fixed at MCT so no initial descent.

1. A tutorial is not a controlled document, latest one is Dec 2019.

After initial (common) actions:
SPEED Pull & Set GD
if appropriate:
HDG Pull & set
.....
ALT set & pull

In fact the order of actions in the tutorial is different from the FCTM, FCTM is king.



2. The FCTM mentions:

When ready for descent, the PF:
-sets the SPEED and pulls
-sets the engine out recovery altitude and pulls to engage for OPEN DES

I say again:
If you PULL altitude before approaching green dot (spd trend is a good guide) the aircraft may descend earlier than AT green dot. I.e.: level flight is not guaranteed due energy exchange ratio between speed and altitude.
For the SAME reason we don’t PULL ALT in the EFATO profile, before green dot is reached. (Trend touching index, sic!)

cheers

You can "say again" as much as you want, but if the A/THR is set to MCT and disconnected, GRN DOT set and OP DES pulled you will NOT descend early. Your scenario can lead to distraction and speed reduction below GRN DOT, which is NOT safe.

Tutorials are controlled in the ACE.

If You pull to engage OP DES before the the speed reaches green dot the aircraft MAY start a very shallow descent (50-100 ft/min?) which is absolutely no problem and is considered by Airbus in the design of the procedure. On the other hand if You reduce speed below GDOT by pulling ALT "exactly" when Your speed index is at GDOT then you MAY slow down below GDOT which in single engine at high altitude is a big no no no.

When the speed trend reaches green dot vs green dot is reached is DIFFERENT. A speed trend is a TREND : your actual speed will be lower.

Indeed we use the speed trend, just reread my own post..... Both for EFATO and the Eng Fail in CRZ (obstacle) scenarios. Indeed it is not desirable to decelerate below green dot.
The descent may be shallow or not, in fact it depends on FG software. Some older aircraft use a different ratio for speed/altitude trade-off. Another reason to not pull OP DES until the trend vector touches GD.

What we are discussing is pedantic. Tutorials are made for explaining generic drills in type rating. Everyone doesn't have them. So FCOM and FCTM will obviously prevail over them. Tutorial only says Drift Down ceiling Set and pull. It nowhere says the aircraft will decelerate to GD first and then descend. It's not even given in any document. As OP DES says it is linked with energy exchange between speed and Alt. Since the thrust is locked at MCT and descent speed is low the resultant ROD will be low so the energy exchange target will take longer to reach. It may appear that it is waiting for GD but not necessarily so. If it descended 10Sec earlier may not make real difference or if it did a few knots below GD(not desirable) but won't be catastrophic. If FCTM says when ready to descend then you are ready to descend a little before GD.