A320 Single Engine flying - beta target indication
 

Hi all,

I've noticed (when practising engine failure at V1, no relight, then land single engine) that the blue Beta Target indicator only appears on takeoff and climb, at some point during the flight and on the approach there is no blue beta target indication. I have read that the indication occurs when there is a certain difference in power settings between left and right. Is this a sim anomoly and should the blue indication be displayed all the time there is an engine out and one engine has say 50% more power?

It's in the FCOM. One engine above 80%N1, 35% N1 differential between the two engines (CFM56) and some flap down and you'll have the B target. It indicates minimum drag, not zero side-slip.

On the approach, configured for landing (flap full), your power setting will generally be around 70% N1 (aircraft gross weight + 10% is a good rule of thumb) so there will be a side-slip indicator on the PFD, not a B target. If you go around, on selecting TOGA, you'll have a B target.

It's not a simulator anomaly.

Will that do?

As said by wingswinger: beta target centered=optimum side slip=min drag

Point worth mentioning is that when doing a T/O with Tmaxflex, N1 might well be below 80% for take-off.

PPRuNe TechLog at its best, thank you OP DES.

FCTM OP-020

My question is what creates the rolling moment due to the side slip angle (beta)? Is it because of the 'dihedral effect' or because of the neutral lateral stability?

That's. a really good point OP DES. I had never considered that!
I don't think I've ever seen an N1 that low on T/O but I accept that it is a possibility.
I'll add it to my list of good reasons to select TOGA if an engine fails on TO.
Thanks for that.

The Beta target (blue) is only available in a take-off flap configuration i.e. 1, 2 or 3. It is not available in Config 0 or in Config FULL. It will therefore never be present during an approach (in Config FULL) - irrespective of the power setting on the live engine.

If I understand it corectly, the airbus FBW family have neutral APPARENT lateral stability, because the flight computers ensure the aircraft does not generate a roll rate unless the pilot tells it to by moving the stick left or right.

However, the airframe DOES have inherent lateral stability, due to wing sweep and dihedral among other factors. Therefore any sideslip induced by asymmetric thrust will cause a laterally stable response by the airframe, which the FBW computers have to cancel out using aileron and spoiler inputs. These cause additional drag, so you need to find the sideslip angle that gives the lowest combination of rudder drag and aileron/spoiler drag while maintaining a steady heading sideslip sufficient to counter the asymmetric thrust. This gives you the highest possible L/D ratio and thus the highest possible climb rate. This happy balance of control inputs is what the beta target is helping you to achieve.

As I see it, the key to a successful and not-too-scary OEI climb out, once speed is pegged to the optimum climb speed for the configuration, is to avoid unecessary sideslip as much as possible.

Regarding the flex issue, not entirely sure selecting TOGA rapidly is a good idea, it depends on the airspeed you have at the time and how good your climb rate is. The last thing you want to do in a low-speed OEI situation is rapidly exacerbate the thrust asymmetry and sideslip (which increases drag hugely) if you are only a few kts above the minimum control speed. IMHO any thrust increase on the remaining engine should be smooth and carefully controlled with matching rudder input so as to minimise sideslip excursions, which, if mismanaged, would certainly hurt your rate of climb and could even cause a speed reduction in the worst case.

Having said that, if selecting TOGA causes the beta taget to appear, then at least the aircraft will be helping you manage the sideslip. I can see Airbus's thnking on this, and it seems sensible. What is the usual SOP regarding TOGA use in this situation?

john smith & Weekendflyer
Although OEI performance is certified with Flex it is a good idea to select TOGA once you have centered Beta. In A320 with computers trying to resist the displacement it does not cause any prblems. A320 can be trimmed and flown hands and feet off. Flying the entire procedure in flex will take longer time to reach acceleration altitude and also to accelerate to green dot. Since the profile is related to terrain clearence it is better to be done with sooner than later. Also if you delay too much and select TOGA after SRS is changed to V/S0 the FMGS will go into GA mode.

John smith
Airbus training is not at all against TOGA. The first thing after gear up is"consider TOGA" (PRO-ABN-10 p1/14). Unlike your Airline there are others who advice to take TOGA as soon as engine fails however I am against that. You are not flogging the engine by selecting TOGA. The engine can withstand what is certified.

Guys, this discussion re TOGA or not after engine failure is a perennial one. Already some great points have been made in both directions.
As Vilas said, "consider" is the mantra. But so many times in the sim I see pilots forgetting to "consider" TOGA and so my personal SOP after trimming is:
AP1/2, HDG Pull, TOGA Thrust - I can always consider not selecting TOGA if I have a good reason. But why would you not want to double your rate of climb?

I hope you would all agree that pilots tend to rotate carefully and more slowly after an engine failure to avoid over-rotating and losing speed. So consider a bad day: wet runway, perf limited by a close in obstacle which you are going to clear by 15' (Okay, that's net). Because of the EF you have rotated slowly and so are already behind the expected performance profile. Additionally, you are now flying fast on V2 imp and so are experiencing more drag than expected. All in all, you are in bad shape. TOGA will help here!!

These are only my thoughts and I certainly have no intention of starting a tiddling competition over this but you are welcome to comment.

Hi MCDHU,

Didn´t remember this thread.
Agree on everything.
N1 can definitely be lower than 80% on the A319´s with FLX. This will only occur at very high Flex temps though. Associated performance margins (spread between OAT and Tflex) will be such that beta-target and (conventional) sideslip indication both suffice for flying the a/c inside the envelope. So I would not select TOGA for the sole purpose of getting the blue target back!

Demand of thrust is highest during T/O
Engines run at high thrust level during T/O and airspeed is low.
If an engine fails at this time, other engine will cause excessive yaw due to assymetrical thrust. Yawing causes sideslip and increase in drag. Yaw damper will react to sideslip, but not enough. At Low airspeed, control surfaces are not very effective.
Pilot is required to give rudder input to stop the sideslip. Giving rudder input for zero sideslip will require too much rudder deflection.
It may be the case that even full rudder deflection is not enough to ceter the sideslip indicator or rise in drag is too much for available thrust to handle. Acceleration and climb performance will hamper.
Airbus found a 'sideslip compromise' which overall reduces the drag by limiting rudder deflection. This is indicated by blue beta target. This improves acceleration and climb.
This is the problem of low airspeed when large rudder deflection is required and drag increases drastically. On acceleration, this problem ends, hence when flaps are retracted normal sideslip indicator resumes.

Yawing causes sideslip

Are you sure of that ? Most analyses would have it that, once the initial problem is under control (considered to occur instinctively for the trained pilot) and the aircraft is flying straight, it is the pilot rudder input causing the sideslip .. hence the need for some bank to reduce the sideslip in near Vmc situations.

Pilot is required to give rudder input to stop the sideslip.

Perhaps the rudder input is to control the asymmetric thrust yawing moment ? At low speed, where the concern is greatest, the relevant pilot input is to bank into the operating engine(s) to reduce sideslip... noting that some systems have a problem with small angles of bank so the sideslip has to be accommodated due to other considerations.

Giving rudder input for zero sideslip will require too much rudder deflection.

Perhaps you can explain just how rudder input can result in zero sideslip ? .. presuming wings level.

Airbus found a 'sideslip compromise' which overall reduces the drag by limiting rudder deflection.

I have no specific knowledge of how the indicator functions. However, I suggest that the idea is to reduce sideslip to a value somewhere near zero.

On acceleration, this problem ends, hence when flaps are retracted normal sideslip indicator resumes.

Or, perhaps, sideslip related drag is critical at low speed but, to a large extent, can be ignored at higher speed to reduce pilot manipulative workload ?

A question, though, from a non-AB pilot - do you conduct the initial climb out wings level or with some bank into the operating engine(s) ?

The rudder of course always controls yaw, but our control of yaw has 2 separate jobs -- of controlling heading, and controlling sideslip. If the thrust is symmetrical, they are tantamount to the same thing, and we don't really think about it.

But with asymmetrical thrust, it starts to matter. Let's say we lose the right engine. If we do nothing about it, it wants to yaw right due to the thrust from the left side. So we naturally stop the right yaw with left rudder. The clockwise yaw torque from the thrust, is balanced by the counterclockwise yaw torque from the rudder. Therefore heading is steady.

But the rudder, in addition to producing a counterclockwise torque, is also producing a force to the right - with nothing to counter it. Therefore, we will drift to the right. Heading and direction of flight no longer match, which is a slip. That is why zero heading change no longer equals zero slip, as it does in normal flight. To fix this, we counter that rightward force with a leftward force: a horizontal component of the lift vector, obtained by banking slightly left. Now the left and right forces match, and nothing is pulling us sideways. Ergo, heading and flight patch match, and slip is zero. (But the ball is to the left).

But here's a crucial side effect - and the answer to John T's question. There's another contributor to the yaw torque on the plane, besides the rudder and the asymmetrical thrust. It's any sideways air hitting the back of the plane, i.e. the slip itself. It too must be countered with rudder to maintain a heading. Recall that in symmetrical-powered flight, it takes a constant rudder input to maintain a slip. Without it, the sideways-hitting air will turn the tail into the "new" direction and the slip will disappear.

Well, in our right-engine-out scenario, if we go back to the no-correction situation, the air will come from the right and yaw (and boat-turn) the plane to the right. So we need left rudder to prevent that. If we eliminate the slip by banking slightly left, this will disappear! If we bank more than the required amount to the left, we'll then slip the other way and will need right rudder. <-- But, the rudder inputs laid out in this paragraph are only for the slip; they're in addition to the rudder input for the asymmetric thrust. The actual rudder input is the sum of both. In case you're starting to get lost in the weeds (I am, in writing this) let's try a table format, with sample bank angles and made up "units" of rudder travel.

I had to do weird things, and it still won't show up quite right.

Bottom line is, that the more you bank to the left, the less left rudder you need. You can get so much effective left yaw torque by banking, (at the cost of increasing drag) that you can artificially lower the Vmc. This is why the certification limit for Vmc is a maximum of 5 degrees of bank. Any more yaw authority must be obtained by rudder.

Indeed, that's all fine and, largely, what I was suggesting in the earlier post.

So we need left rudder to prevent that ..

.. however, if one is at, or very near, Vmc, one doesn't have any left rudder remaining to play with .. else Vmc would have been lower (assuming we don't have to contend with rudder stall for instance). Indeed, if there is no bank, rudder becomes more limited and Vmc, necessarily, would be expected to increase. So what does AB suggest in that (near Vmc) situation ? Surely not to relax rudder ?

What one does at higher speeds, where rudder is not limiting, can be tweaked as might be desired. Wings level and a small loss in climb is a lot easier than trying to fly a couple of degrees wing down to get that last bit of climb. As to the final adjustment for drag, AB would have more than enough models and FT data to provide crew guidance via whatever beta indications.

For the AB, do you fly out OEI wings level or with favourable bank ?

Hi John,
For minimum drag (no aileron or roll spoiler deflection) at VMCA, then you would need 5 degrees bank.
The problem with AB normal Law is that you can't "feel" when you have any aileron deflection and the only clue is the position of the Beta Target.

In the sim, with V2>VMCA (often by 10 kts or more), less accurate flying can result in level wings climb out with no clue to the amount of aileron deflection applied. SOPs require us to engage the AP early on to reduce the work load and rudder deflection is then controlled automatically. The aircraft will gradually centre the Beta Target resulting a few degrees of bank, often masked by heading changes.

Fast magazine issue 20
"If an engine fails, the triangle at the top of the PFD horizon will divide (see Figure 2). The lower resulting trapezoid changes to blue and will move out in a similar sense to a conventional slip ball, indicating pilot rudder demand in exactly the same way. However, the function is significantly different from the ball in that the centering of the trapezoid (Beta Target) will provide maximum performance with minimum drag.

If no rudder action is taken to centre the Beta Target, like a conventional aircraft, roll will occur towards the dead engine. However, unlike a conventional aircraft, with stick free (no sidestick roll input), the flight control laws will detect the roll and apply aileron and spoiler to stop the roll. The rate of roll will depend on the severity of the thrust loss. In the worst case, the roll will stabilise between 7-9 degrees bank angle, leading to a slow heading drift of about 0.5 degree per second, without any sidestick roll input or rudder input."

Thanks for that .. I'll wade through the details over a coffee or two later this evening.

However, the question still remains re bank. Your suggestion that

For minimum drag (no aileron or roll spoiler deflection) at VMCA, then you would need 5 degrees bank

doesn't make much sense to me. If you are after the standard bank angle, then that will require an aileron input.

I might be just a bit slow but I can't infer from your post if you target some bank or just go with wings level regardless of speed schedule ? I have some difficulty with any suggestion that wings level is the way to go when one is back near Vmc ... a 10kt margin can get swallowed up by the effect of no bank pretty quickly.

While this will vary with Type, some of the 4-engined machines can see a 40kt plus increase in Vmc if the bank is allowed to go the wrong way .. Real world Vmc is very sensitive to bank angle.