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konde_landi
27th Sep 2012, 11:57
Here is the scenario:
- You are in flight, everything is normal. You are cruising leveled at FL390 and the MAX FL is FL390. You shut an engine off using the master switch and just watch what happens without taking any actions. For both the A320 and the A330 the speed will start to drop as the aircraft tries to keep leveled flight.


My question is: will alpha floor activate for the A330? How about for the A320?

Microburst2002
27th Sep 2012, 15:58
before that, AP will trip. Then I don't know what comes first, in that scenario, if high aoa protection or alpha floor.

konde_landi
27th Sep 2012, 17:23
Yeah. For the A330 here is the sequence:

1-) Aircraft start decelerating.
2-) At Alpha Prot autopilot disengages.
3-) Aircraft start descending between Alpha Prot and Alpha Max.
4-) As you did not touch the thrust levers, the operating engine thrust is limited by the position of the lever in the CLB detent.
5-) Alpha floor is not activated, as it is inhibited above M 0.53 for the A330.

I repeat my question:
How about for the A320? In the same scenario, will alpha floor be activated?

konde_landi
27th Sep 2012, 17:29
Microburst,

But you said "before that the AP will trip". Before what? Before alpha floor activation? Then you must be talking about the A320, since for the A330 alpha floor does not activate under those circunstances.

And that is my question: will alpha floor activate in this case for the A320? If it does activate that is indeed on great operational difference between the two acft.

HazelNuts39
27th Sep 2012, 19:15
3-) Aircraft start descending between Alpha Prot and Alpha Max.
4-) ...
5-) Alpha floor is not activated, as it is inhibited above M 0.53 for the A330.
Re (3): Would it not maintain Alpha Prot?

EDIT:: At Alpha Prot the flight control law changes from an Nz command law to an alpha command law. The commanded alpha is Alpha Prot for sidestick neutral, and Alpha Max for sidestick fully back. Alpha floor is not activated because the alpha threshold for its activation is not reached. At some point in the descent to the single-engine ceiling with AoA=Alpha Prot the speed may drop below M 0.53 .

konde_landi
27th Sep 2012, 20:15
HazelNuts39,

Totally agree. You went deeper on the explanation and it was perfect. That's exactly how it works for the A330. At sometime the speed is gonna drop below M 0.53 and alpha floor will be activated. Alpha floor however will not be triggered AT cruise flight level, it will be triggered at some point during descent.

Will the same happen for the A320? The "M0.53 rule" does not appear on the manual for the A320. Knowing that, will alpha floor be activated AT cruise level for the A320?

HazelNuts39
27th Sep 2012, 21:27
At sometime the speed is gonna drop below M 0.53 and alpha floor will be activated.With the sidestick remaining in neutral the airplane stays at Alpha Prot and alpha floor will not be activated.

konde_landi
27th Sep 2012, 22:26
HazelNuts39,

I got that. The aircraft will enter alpha floor only if you give a further pitch up input AND you are below M.053 on the A330, which is gonna happen at somepoint during the single engine descent at alpha prot (in the proposed scenario).

ON THE A320, if I shut down an engine during cruise at FL390, don't take any actions, wait for the aircraft to reduce to alpha prot, and give a further pitch up input to the sidestick, will alpha floor activate at FL390?

nitpicker330
28th Sep 2012, 01:06
Yes Alpha floor should only activate if you are below .53 Mach, AOA protections are active AND Sidestick deflection is more than 14 deg back.

From the A330 FCOM 1

Normal Law Pitch protection:-

HIGH ANGLE OF ATTACK PROTECTION

Under normal law, when angle of attack becomes greater than Alpha prot, the system switches the elevator control from normal mode to a protection mode in which the angle of attack is proportional to the sidestick deflection. That is, in the Alpha prot range, from Alpha to Alpha max, the side stick commands Alpha directly. However the angle of attack will not exceed Alpha max, even if the pilot gently pulls the sidestick all the way back. If the pilot releases the sidestick the angle of attack returns to Alpha prot and stays there.

In addition to that it goes on to say:-


Alpha FLOOR is activated through autothrust system when :
• Alpha is greater than a threshold depending on the aircraft configuration, the ground speed variation, and the difference between ground speed and air speed or,
• Sidestick deflection is above 14 deg and:
— pitch altitude is greater than 25 deg,
or AOA protection is active

Alpha FLOOR inhibition: (Refer to 1.22.30).

nitpicker330
28th Sep 2012, 01:18
The ALPHA FLOOR protection is triggered when the FMGECs receive a signal elaborated by the PRIMs. This signal is sent when the aircraft angle of attack is above a predetermined threshold function of the aircraft configuration. The A/THR is automatically activated and commands TOGA thrust regardless of thrust lever positions. This protection is available from lift off to 100 feet RA in approach. Following indications are then provided : — A-FLOOR on the FMA and on the EWD as long as Alpha floor conditions are met — TOGA LK on the FMA when the aircraft leaves the Alpha floor conditions. TOGA thrust is then frozen A FLOOR and TOGA LK are displayed in green and surrounded by an amber flashing box. In order to cancel the ALPHA FLOOR or TOGA LK thrust, disconnect the A/THR.


Alpha floor is inhibited:-
— in case of engine failure with flaps extended
— in case of engine failure with derated TO selected
— in case of a single outer engine failure, with the flaps extended. (A340)
— in case of a single outer engine failure, with derated TO selected. (A340)
— in case of a dual engine failure on the same wing. (A340)
— below 100 feet at landing
— above M.53

Alpha Floor protection is lost in case of A/THR

nitpicker330
28th Sep 2012, 01:28
So, Engine failure in cruise and your speed drops to Alpha Prot:-

1/ the A/P disconnects
2/ the Pitch reduces to keep Alpha Prot AOA
AND Alpha floor will activate if you are pulling back more than 14 deg, UNLESS inhibited.

Microburst2002
28th Sep 2012, 05:36
In the 320, I guess it depends on the deceleration rate.

If it was very fast it could hit the alpha floor threshold, whereever it is... If it is not fast, high aoa protection will keep the airplane at alfa prot, so no alpha floor activation would occur

nitpicker330
28th Sep 2012, 08:52
I'm sure it would be the same on the 320.

HazelNuts39
28th Sep 2012, 09:21
In the 320, I guess it depends on the deceleration rate. If it was very fast it could hit the alpha floor threshold, whereever it is...In the conditions stated in the OP, the A330 decelerates at about 0.04 Mach per minute.

konde_landi
28th Sep 2012, 12:56
On the A320, alpha floor is not inhibited above M0.53. In this case, we could have alpha floor activation at FL390 (example). That is my understanding of the whole thing.

I done it on the simulator for the A330 and alpha floor does not activate whatsoever until the aircraft is at an altitute that allows flight at M0.53.

I'm sure the rate of deceleration does not play a role here, since the alpha floor trigger is AOA, and not a "projected time to reach alpha max based on current deceleration rate assuming you are gonna give further pitch up input when you reach it".

I found nothing so far written on the manuals that convinces me that alpha floor will not activate at cruise FL for the A320. If someone experienced this situation or could try it on the sim it would be great!

Microburst2002
28th Sep 2012, 15:17
What does trigger alpha floor, anyway??

Deceleration rate matters, because by the time high aoa protection is able to arrest the loss of speed, you will reach alpha prot or not depending on how quick the protection was

HazelNuts39
28th Sep 2012, 17:56
Deceleration rate matters, because by the time high aoa protection is able to arrest the loss of speed, you will reach alpha protHigh AoA protection does not arrest the loss of speed, it does arrest the increase of AoA, which in the proposed scenario is relatively slow: from approx. 3 degrees in cruise to alpha prot of about 6 degrees in about four minutes.

CONF iture
28th Sep 2012, 19:34
4-) As you did not touch the thrust levers, the operating engine thrust is limited by the position of the lever in the CLB detent.
As you put it here, we can assume A/THR is ON. This was not specified in the initial scenario.
Point 4 is of no importance as there is no 'thrust limitation' at FL390 since climb thrust is nothing short of MCT or TOGA. Nothing to gain to have alpha floor anounciated on the FMA as thrust on the remaining engine is already at its max possible output.
As mentioned earlier, the important point is the alpha prot activation preventing a further AoA increase.

It is still a good scenario to think about.

mcdhu
28th Sep 2012, 22:21
What actually happens in the 320 sim is that the speed slowly comes back to V alpha prot, the autopilot disengages, tha ac then pitches down to regain/maintain V alpha prot and it drifts down at that. While flashing LVR MCT at you!
mcdhu

Microburst2002
29th Sep 2012, 05:07
Exactly

High aoa protection pitches the airplane down to prevent aoa from exceeding alpha prot (sidestick neutral)

If the deceleration rate is very high, the aoa will reach a higher value, maybe avtivating alpha floor. If it is slow, the protection doesn't let the aoa to trigger alpha floor.

With an engine in MCT, deceleration rate should be relatively slow, even at max alt. If we did the experiment of retarding both thrust levers maybe alpha floor would be triggered

HazelNuts39
29th Sep 2012, 08:28
High aoa protection pitches the airplane down to prevent aoa from exceeding alpha prot (sidestick neutral)Not exactly.

During the deceleration to V-alphaprot the airplane is pitching up to maintain lift at decreasing airspeed. The elevator moves NU, the THS follows. That process is arrested at V-alphaprot. The autotrim stops trimming NU, the elevator stops moving NU, the AoA is at Alpha-Prot and stays there, the FPA is level at that point.

As the airspeed continues to reduce below V-alphaprot, the lift reduces as airspeed squared. When lift it is less than weight the airplane starts to descend. It then pitches down to maintain a constant AoA relative to the increasingly downward flight path angle. At some point the trend of decreasing airspeed is reversed, it increases beyond V-alphaprot, lift becomes greater than weight and the flight path starts levelling off. Somewhat later the airspeed trend reverses again and the airspeed reduces through V-alphaprot. That cycle is repeated until the pilot intervenes.

Microburst2002
29th Sep 2012, 13:44
Well, yes, I just made a sumary. Eventually the airplane potches down and will tend to maintain alpha prot.

But if deceleration rate is very high, the aoa can exceed alpha prot. Somewhere in between that and alpha max, flight augmentation will trigger alpha floor. It can happen, but only if high aoa protection is unable to arrest the aoa increase before.

HazelNuts39
29th Sep 2012, 15:25
Or an altitude is reached where available single-engine thrust is sufficient to maintain V-alphaprot and zero VSI.You think so? Would the phugoid motion not continue, altitude cycling about the ceiling and airspeed cycling about Valphaprot?

Konrad_Huebstedt
29th Sep 2012, 18:58
Beside the facts of circumstances which led an Airbus to enter this regime, its not unimportant to know how to leave this flight regime.
Sadly Iberia learnt this lesson on Feb. 7th 2001 in Bilbao on Flight IB 1456.

To state the official Accident Report from the CIAIAC:

"...De-activation of the AOA protection To de-active the AOA protection the sidestick has to be pushed either to 8° nose down for at least 0.2 seconds or to 0.5° nose down for more than 0.5 seconds with α < αmax.
As a consequence of this logic in the activation of the elevator flight control, imple- mented on the ELAC L80 standard computers, it can be stated that: While in flight, if the angle of attack is high, or its value is increasing fast, and at that specific moment the sidestick is pulled fully backwards, the system will detect a very high anticipated angle of attack and, if it is higher than αprot, will activate the AOA protection. Once this protection has been activated, the system will preclude the aircraft from exceeding the maximum angle of attack but, if it also detects that airspeed is diminishing, it will lim- it even further the possible angle of attack, pitching the aircraft nose down to dampen an eventual phugoid movement..."

Microburst2002
30th Sep 2012, 06:47
All of which was and is "transparent to the pilot"