Flight Computer of FSX Airbus A321 May Have Suicidal Tendencies

DISCLAIMER:

ALL mentions of the A321 in this article are only pertinent to the virtual A321 that is modelled in Flight Simulator X. Unless I say explicitly nothing in this article refers to any real aircraft or any of its components. Most of what I talk about here is derived from what I have directly experienced in Flight Simulator X, and not from any external sources.

In addition, I am not a real world pilot. The furthest extent of flight training I have was about 30 hours in a Piper Warrior, and that was many years ago and I don't remember much of it. I do not claim accuracy in any of my statements.

Please, feel free to correct me where ever you see fit!

The HAL 9000?

Have you ever felt that the flight computers aboard FSX's A321 just don't let you do what you want? Perhaps sometimes it crashes your plane?!

OK, so the title and opening was a bit sensationalist in order to catch your attention! This article is about my experiences with the FCS (flight control system) on the A321 in Flight Simulator X. This is not a criticism or a value judgement of the A321’s FCS! Obviously it was designed to add safety to the plane and that’s exactly what it does.

By default, all of flight computers on the virtual A321 are switched on. There are 4 buttons that you can toggle on or off, affecting separate parts of the FCS. We are only interested in the middle two buttons. The middle two buttons correspond to the Spoilers/Elevator Controls (SEC). In fact, one of these buttons toggles computer 2’s SEC, and the other one toggles computer 3’s SEC. The SEC on both computers does exactly the same thing, duplicating functionality. Err, so why have the same functionality on two separate computers? I believe the reason for this is redundancy. If one fails the other will still continue working.

The implication of this is that switching off either of the 2 SEC computers will achieve the same effect. If you really want you can go and switch off both. Anyway, what does this SEC button do?

You will find that if you switch off the SEC the plane will start to handle like an F-16. However with the SEC on, the plane’s response to your control inputs are extremely slow as your inputs are regulated by the computer. Most importantly, enabling the SEC also activates the “high angle of attack protection system”, and that’s the primary interest of this article.

As for the other 2 buttons, to be honest I have no idea what they do! Toggling them on and off doesn’t seem to have any effect. If you know please leave a comment!

You have to toggle only one of the two buttons, as shown here

Here is one scenario I encountered while flying the virtual A321. It is concerned with the SEC and the high angle of attack protection, and the aircraft going into over speed:

“The virtual A321 came very close to stalling during climb out - deliberately - and the letters A.FLOOR were lit up on the primary display, telling me that the flight computer (the SEC part) was activating the high angle of attack protection, and that it was asserting control. At that point, the sensible thing to do was to let go of the controls and let the aircraft recover from the stall. The flight computer then advanced the throttles to TOGA power and the nose lowers. I let the flight computer do its thing and then decided to take over a bit later* when aircraft had gained enough airspeed (at about 205 knots).

The problem now was that the airspeed was increasing at an alarming rate! I didn’t want to see the aircraft exceeding its max speed (Vmo) and tearing itself apart. So I immediately throttled down to idle. No response. A.FLOOR was still displayed on the primary display. I pushed the throttle forward and then back again. No response. Apparently I was not allowed to throttle back when A.FLOOR was engaged!

The airspeed was probably passing through about 300 knots at that point. So I pulled back on the joystick to its limit, and the virtual A321 reluctantly responded, raising the nose at a very slow rate. The nose was finally rising above the horizon but by now the aircraft was over speeding dangerously. A few seconds later however, I was able to bring the plane to a safe speed, but not before it almost tore itself apart. However, I’ve had incidents before where the A321 just overstressed itself because I couldn’t slow down in time, because the plane wouldn’t respond to throttle inputs!”

*I admit, I deliberately waited a bit long in before attempting to pull the nose up, to illustrate the point.

The following is a picture story of what happened:

1. I deliberately try to get myself into a hairy situation. This is the point where A.FLOOR engaged. At this point the A/T is being commanded to full power and the nose starts to lower immediately. I’m at about 112 knots and 28 degrees pitch up, banking to the right.

2. Some seconds later, this is what the situation looks like. At this point I pull back fully on the joystick. Airspeed is currently 205 knots. Not too fast.

3. I’m still pulling back fully on the joystick. I am already over speed and at risk of overstressing the plane.

4. At this point I have just recovered with a high pitch up attitude and flying very fast at 330 knots. However, the aircraft doesn’t overstress.

5. Analysis of the flight

After that experiment I continued flying, doing some very high-G manoeuvres for the fun of it. I didn’t notice but A.FLOOR re-engaged at some point in time! Now let’s see what happened:

1. Here A.FLOOR is engaged and the airplane is travelling at 340 knots and a few degrees nose down. I immediately pulled back the throttle, but no response again. I also pull back fully on the joystick and the nose starts to rise … incredibly slowly …

2. … too slowly. The plane becomes overstressed! Not so lucky this time!

Here's another scenario:

“It is a pitch black night as there's no moon. You're lost and disorientated. Suddenly the ground proximity warning begins barking at you. You pull back on the stick, hard. Because the flight computer is on it limits your rate of rotation. You don't pull out in time and collide with the terrain.

Before continuing, note that when the virtual A321 is below 100 feet AGL, SEC no longer has any effect and you have the “F-16 handling”.

To illustrate this scenario, I tried flying a few degrees nose down at a low altitude of a couple of hundred feet AGL - this is during day so I could see what I was doing, but imagine this at night. Once the GPWS warning started sounding I immediately pulled back hard on the joystick. The airplane started to pitch up at a rate so slow the plane would have no doubt hit the water at that rate of rotation.

However when the plane descended below 100ft it rotated violently upwards and suddenly, and the tail struck the water and the engines followed suit. Next, the nose slammed into the water and it crashed. If this was flown with the SEC toggled into the off position, it would have been possible to pull the aircraft out when the GPWS sounded.

What you think of the scenarios (interesting? utterly silly? great? amusing?). Yes, these sort of scenarios are perhaps ridiculous, but the purpose of all this is to push FSX to its limits! Let’s keep going …

Getting the aircraft into the A.FLOOR/high speed situation was one thing, but could I have deactivated A.FLOOR when it was still activated, and throttle back when I needed to?

I heard that in the real A321, retarding thrust (or disengaging auto throttle) should immediately remove A.FLOOR, but I can't be sure.

In FSX I discovered that you can disengage A.FLOOR when it is activated. To do this you must first turn off the SEC by pressing the SEC button(s) on the overhead panel and then manually disengaging the A/T, which was engaged by the flight computer during A.FLOOR. That 2 step procedure seems a bit inconvenient. In FSX it is not possible to directly disconnect A/T when A.FLOOR is in effect.

The Flight Computer & Engine Failure On The A321

I was flying the virtual Boeing 747 the other day and I organized for the No. 4 (the most starboard) engine to fail. Not surprisingly, the plane starts to bank to the right. The correct action was to apply left rudder to produce a twisting force that would counteract the roll to the right. Surprisingly, turning the yoke to the left also completely counteracts this roll in FSX, which I'm not sure would be the case in real life. I tried using only the ailerons to turn the plane at 300 knots and then at 250 knots and maybe even less, and I could counteract the roll in both respects.

After doing this for a while and amusing myself, I wondered how the A321's flight computer would react to a single engine failure. So I got myself into the A321 and took off. Once level, I made sure the SEC computer was enabled, and I made engine No.1 fail. Guess what? The airplane did not begin its bank to the left. It continued flying more or less straight and level. On closer inspection, the aircraft was slowly yawing to the left, but pretty slowly. Then I went to the outside view and zoomed in on the plane's ailerons. I could see that the computer had deflected the ailerons to try and keep the plane straight and level. Cool. But to what extent is this realistic? No idea!

I was still curious. What if A.FLOOR comes into effect when the plane is only flying on 1 engine? So I climbed to 10,000 feet and shut down the No.1 engine (port engine). As expected, I was still flying straight and level due to the cool flight computer. But then, I gave the flight computer a challenge. I raised the nose and reduced power to idle and held it there. Soon, A.FLOOR lit up on the primary display (above the artificial horizon) and I let go of the controls. The flight computer yanked up the power on engine No 2., and because engine No. 1 was not working the plane was producing extremely asymmetrical thrust.

Not surprisingly the asymmetrical thrust caused the plane to begin to bank and yaw to the left at an alarming rate, so I banked to the right. Of course I did try to apply right rudder but the rudder was not controllable with the SEC enabled! Soon the plane was in a bit of a crisis, spiralling towards the sea.

It's easier to describe what happened in pictures:

1. At this point, A.FLOOR is engaged. Remember that the No 1 engine is not working. Right after this screenshot, the airplane starts banking and yawing to the left at an alarming rate. I push the joystick to the right ….

2. A little while later. Airspeed indicator is indicating 0 knots and it seems like we’re totally stalled and have no control.

3. Unfortunately there’s no recovery this time. There’s a reset button though. Moral of the story? Don’t put the plane into a stall if SEC is on and you only have 1 engine available.

FAQs About A.FLOOR

There have people asking why the A321 in FSX goes into A.FLOOR during takeoff. As far as I know, this only happens if you pull back forcefully on the joystick and/or do not apply enough power on takeoff, ultimately resulting in an unacceptably high angle of attack, which in turn forces the computer to kick in.

There also have been questions asked as to why the same thing happens during approach to land. Again, this should only occur if you are flying with a speed and attitude that results in a very high angle of attack, enough for the computer to kick in.

Conclusion

The purpose of this article was to share how I amused myself with the FCS of the Airbus A321 in Flight Simulator X. I have discovered that if you have no experience of how the FCS tends to behave, then you will be baffled and confused by it. So try out some experiments yourself and you may find flying the virtual A321 more fun.

I hope you enjoyed the article! See if you can try out some of these experiments, or make your own.

EDIT: The FCS (flight control system) as written about in the post should probably have been more accurately refered to as the EFCS (electronic flight control system). The EFCS appears to be part of the FCS.