A320 Beta target unavailability in Conf full & Clean

[iaf_22]

Hi all,
in case of engine out, we know beta target is available only in conf 1,2,3.
But why? Why does beta target actually reverse to a sideslip indication when going clean ? Same question when going conf full ?? :confused:

Thanks

[vilas]

iaf_22
Basically beta target is to to give you optimum climb performance in take off and GA situation where the thrust on live engine is high and you are dealing with performance issues like gradient etc.. In both these situations your flap setting will be 1, 2 or 3.

[gAMbl3]

After an engine failure the aircraft will try to roll and yaw towards the failed engine side. As per lateral Normal Law if there is no input on the sidestick the roll rate is 0. So to counter the roll rate caused by engine failure, the ailerons and spoilers will deflect. Net effect will be approx 5 deg bank angle, constant sideslip and a slow diverging heading rate.

Deflected ailerons and spoilers will increase the Drag. This increase in drag reduces Climb performance {Climb gradient = (T - D)/W}. So from a performance standpoint it will be better to fly a constant heading with roll surfaces retracted. Beta target indicates the amount of rudder required to fly a computed residual sideslip which when centered will allow the aircraft to fly a constant heading will roll surfaces close to neutral position.

Climb performance is critical during Take off and Go around and so Blue Beta target replaces the yellow sideslip index in Take off and Go around configs i.e. Config 1,2 or 3 {along with any ENG N1 > 80 % / EPR > 1.25 or one Thrust Lever > MCT (≥ FLX if FLX or DERATED TO)and the difference between the ENG N1’s / EPR's exceeds 35 % / 0.25}

[JABBARA]

My thoughts may be a little bit controversial,
First, I consider, Airbus naming the top indexes, when yellow, "Sideslip Index" is not correct. When yellow, they are indication of lateral acceleration, in simple words they indicates the turn rate and bank angle are consistent (coordinated turn) or not. As in conventional ball, if airplane turn rate is more than the bank angle vs. speed combination required (which can be simply-trigonometrically calculated, check google), the lower trapezodial index move out of turn, which is known as "skid". if airplane turn rate is less than the bank angle vs. speed combination required, the lower trapezodial index move inside of turn, which is known as "slip". Normally this kind of coordianation is either naturally accomplished by aerodynamic design of the airplane (with a little bit help of pilot at low speeds) or by the help of yaw damper (one of the multiple functions of yaw amper is Turn ccordination) for most transport category airplane. In fact Boeing names theses indexes as Slip-Skid Indicator.

When one engine fails (assymetric thrust), without any rudder application, it is possible to fly with a constant heading. However this is possible with quite a lot bank to running engine side. In this case, not only a lot deflected ailerons and spoilers, but also a Relative Wind not hitting the nose of the airplane which is known as "Sideslip", creates a lot of drag. The ideal minimum drag with single engine is possible with slight bank on good engine side and adjusting the rudder so that RW hits exactly from nose. So how can a pilot understand Sideslip is "zero", so applied rudder is enough? He/she cannot use Slip-skid indicator, because in zero sideslip case it will deflect to good engine side like conventional ball, the reason airplane is flying straight heading (no turn) but there is a slight bank angle towards good engine side (Uncoordinated turn). The classic system is a yarn located out of Windshield center, like gliders. Since this is not practical for transport category airplane, there should be another method. A 380 has side slip vanes (similiar to AOA probes but mounted vertıcally) located just on top of radome. For 320/330? That is most likely a calculated value by computers (FCC, or FMGC etc) as differentiating the sensed static pressures at either side of airplane due to sideslip. I said "most likely" because if you search Pitot/Static boom manufactures products in Google, you will see, in their explanation they claim their products can detect the "SideSlip" on the airplane. This parameter is also an item in Alpha Call Up menu known as "AOS". That is available in AMM, however I could not see in real airplane (A330) ACMS page, I guess it requires engine failure in real plane, to be calculated and displayed.

So briefly, when Indexes are blue, that is the time we can name them as "Sideslip Index", but technically not before. When they are blue, I consider AOS calculation starts to display on PFD and when centered, even with some bank to good engine side (which is a "must" for minium drag), it represents "zero sideslip" which not necessarily minimum control deflection but optimum deflection for minimum drag.

[C_Star]

@Jabbara
First, I consider, Airbus naming the top indexes, when yellow, "Sideslip Index" is not correct. When yellow, they are indication of lateral acceleration

According to the FCOM, the index shows lateral acceleration on ground and side slip in the air:

Sideslip Index (yellow)
This trapezoidal index moves beneath the roll index. On ground, it represents the lateral acceleration of the aircraft. In flight, it shows sideslip (as computed by the FAC). One centimeter of displacement indicates 0.2 g. The sideslip index is against its stop at 0.3 g.

In case of engine failure at takeoff or go around, the sideslip index changes from yellow to blue.
Note:
The sideslip target is blue, if:

CONF 1, 2, or 3 is selected, and

Any ENG EPR > 1.25 or one Thrust Lever > MCT (≥ FLX if FLX or DERATED TO), and

The difference between the ENG EPR’s exceeds 0.25.
In this case, the sideslip index is called β target.
When this index is centered with the roll index, the sideslip equals the sideslip target for optimum aircraft performance.

The way I read it:
- yellow index: when centered, shows ZERO sideslip
- blue index: when centered, the sideslip is non-zero and is a compromise between sideslip and control deflection for optimum performance (ie. minimum total drag)

P.S. IIRC from aerodynamics classes, you can't have "zero sideslip" AND wings level in asymmetric flight. Something has to counter the rudder force...

[iaf_22]

Before going into jabbara's controversial version, I'll need to understand fully gAMbl3 and vilas version that do concur with Airbus version stated in FCTM > OP >020> Normal Law.

I understand that beta target requires from us to center it with rudder to minimize drag and get max performance in case of high engine thrust.

Does that mean that when we are maintaining level flight before glide interception N-1, clean configuration, Airbus think that rudder deflection is not primordial and thus no beta target necessary?
Same question with engine failure in cruise, while applying drift down procedure : we'll have high engine thrust on one side, performance issues due to mountains , but... no beta target to request any rudder help from us.

Do we assume in those 2 cases that FAC & normal law compensation is simply sufficient ??

[JABBARA]

C+Star

My Critics is not for you but for Airbus.
What I said is "When yellow index it is not a Sideslip indicator, but when blue yes it is"

It is contradicting with Airbus simple explanations.

When Airbus is trying to simplify the subjects, it is ignoring some flight mechanic facts. Maybe practically it is supposed to be so. It is ok. I guess they do not want to go so much details from the perspective of pilot.

If you read Page 68 of A 330 (not A 320)FLIGHT DECK AND SYSTEM BRIEFING (AN AIRBUS DOC, You can download), they explain the functions a little bit clearer.

P.S. IIRC from aerodynamics classes, you can't have "zero sideslip" AND wings level in asymmetric flight. Something has to counter the rudder force...

That is exactly what I want to say, "airplane should bank for zerosideslip". If indexes were behaving like ball, they wouldn`t be centerlized, and pilot couldn`t understand when the side slip is zero. Therefore at given assymetric thrust condition and configuration, it automatically starts to display as a function of sideslip amount.

[JABBARA]

Iaf 22

If I understand you correctly, you are right,

In critical phases where Prasit Drag is high and runing engine is used to its peak values, Sideslip drag is intended to be zero. In other cases like in cruise, assuming you are flying manually, you have two option:

1st OPTION: Maintain wings level, stop heading drift with rudder (apply to good engine side), so you can maintain constant heading. Index (yellow) is definitely aligned with sky pointer. Flying technique is relatively easy, but relatively more drag occurs, becasue sidelip is definitely not zero; RW is hitting from failed engine side.

2nd OPTION: Maintain slightly bank to good engine side, apply relatively less rudder than 1st option to good engine side to maintain constant heading. Index (yellow) definitely slipped slightly to bank side. Flying Technique is relatively difficult, but definitely less drag occurs than 1st option. Sideslip is definitely less than 1st option, but is it zero? Nobody cannot know, because sideslip (AOS) is not displayed to pilot in blue (betatarget) format.

[vilas]

iaf_22
Let me put it together. Beta is designed for TO, GA situations when speeds are low and thrust is TOGA to give you optimum performance during the initial climb out. It basically takes out all the theory and calculations. If you centre beta target and maintain the heading by using lateral control all requirements are met. Now how does beta work in the situations you mentioned? With OEI flying level beta is active till Conf3 and you won't have flaps full till descent. During drift down you have MCT which is much less than TOGA at lower altitudes and your IAS is much higher than take off GA making the controls more effective so spoiler deflections is not an issue which is the main reason for existence of beta. Even after TO or GA after acceleration to GD speed and thrust reduced to MCT beta disappears as it is not required.