A330 Autothrottle

[EGLM guy]

Hi,
I recently watched a documentary about the fatal Air France flight from Rio to Paris. At one point they mention that one of the reasons that may have lead to a high altitude stall was the fact that the pilots did not immediately respond to a potential loss of airspeed indication due to severe icing on the pitot tubes. This resulted in a number of errors, including both the autopilot and the autothrottles to be disengaged. Supposedly, the pilots should have trimmed the aircraft for a 5º pitch angle and manually set the throttles to 80% to ensure a safe airspeed. Apparently it may have taken the pilots a while to execute this as they were extremely busy troubleshooting a large number of critical errors. The documentary then puts forward the fact that on the A330, when the autothrottles are engaged, the actual physical thrust levers do not move, and one must look at a screen off to the side showing the power settings in order to see what is going on. They say that had the thrust levers moved (as they do in other aircraft) in response to the autothrottle settings, this would have visually cued the pilots to address this particular situation more promptly. My question is actually pretty simple (all the stuff above is just what made me think about it). What happens when you disengage the authrottle at a different setting than the one indicated by the levers? Does the thrust respond immediately to the position of the levers? Is the pilot supposed to readjust the thrust levers close to the setting the thrust is currently at? Why would a modern aircraft like the A330 not have automatically moving levers? Is there any advantage to this? Thanks, and sorry for the length of this post.

[hetfield]

1.What happens when you disengage the authrottle at a different setting than the one indicated by the levers? 2.Does the thrust respond immediately to the position of the levers? 3. Is the pilot supposed to readjust the thrust levers close to the setting the thrust is currently at? 4.Why would a modern aircraft like the A330 not have automatically moving levers? 5. Is there any advantage to this?

On Airbus it's called autothrust, A/THR, anyway

1. Levers are in climb power detent. If you disengage A/THR thrust/rpm will be according lever position e.g. Climb Thrust or thrust lever angle corresponding thrust between idle and CL
2.Yes, regarding spool up time
3.Yes, before A/THR is disengaged
4.Weight (so does Mr. Airbus say)
5. Don't think so

[engfireleft]

In normal flight the thrust levers are set to what is called the climb detent which represents climb thrust. The autothrust system then automatically controls the thrust in a range from idle to climb thrust depending on the circumstances, and no, the actual levers do not move.

If the autothrust is inadvertently disengaged either through a fault or by pushing the AT button on the control panel the system interprets that as an "uncommanded" disconnect and freezes the thrust where it was when the disconnect occurred. A repetitive tone and caution message is displayed until the pilot moves the thrust levers manually, and then the thrust acts conventionally according to where the thrust levers are placed.

If the pilot disconnects the autothrust through the push buttons on the thrust levers the engine thrust goes to where the thrust levers are set, in this case climb thrust, and the thrust is then conventional according to lever placement.

[Cardinal]

What happens when you disengage the authrottle at a different setting than the one indicated by the levers?

engfirelft well describes what happens when the A/THR disconnects abnormally, as was the case on AF447. The recommended means for a crewmember to disconnect the A/THR is via the "instinctive disconnect" button on the side of the thrust levers. The thrust will promptly match the lever position. Incidentally, I hate that Airbus refers to this as instinctive, on my previous two types this is where the go-around button was located. It's the "counter-intuitive disconnect." But I digress.

Does the thrust respond immediately to the position of the levers?

Yes.

Is the pilot supposed to readjust the thrust levers close to the setting the thrust is currently at?

Yes. When the A/THR is engaged, the thrust levers serve in a limiting function. In the climb detent, where they normally belong, climb power is the upper limit of what can be commanded. Bringing the levers back out of the climb detent will limit the A/THR's authority to whatever intermediate setting the levers are at. With the thrust levers at idle, the engine wil be at idle, even if the A/THR still happens to be engaged. Proper technique is to bring the thrust levers back to match the current power setting, then push the button to disconnect. This is readily accomplished, as the thrust lever position is represented on the N1 or EPR gauge with a donut shaped icon. Match the donut to the needle and disconnect.

Why would a modern aircraft like the A330 not have automatically moving levers?

Only God and Toulouse know. It's cheaper, lighter, and mechanically simpler this way. Really moving levers are a throwback to mechanical actuation of the fuel control unit. The FCU is an all digital function now, there are no cables to drive. Like the rest of the digital flight controls architecture, Airbus decided that mechanical feedback was unnecessary. Unfortunately it removes valuable cues to the crew. One can't hardly hear the engines at intermediate power settings, in order to determine what the engines are doing one must look at the engine display, tactile information is unavailable. It prevents elegant incremental power adjustments while the A/THR is engaged. It adds significant complexity to what should be a straight-forward interface with the aircraft. I love the Airbus, yet myself and many of my associates could imagine a more friendly system. This system does work.

[ziggie08]

You've touched on probably the biggest "why Boeing is better than airbus" arguments going around. Any guys with previous boeing experience swear black and blue that moving levers help with situational awareness whereas airbus exclusive pilots seem to have n1 indications embedded in their scans to such an extent that moving levers become unnecessary.

[EGLM guy]

Thanks very much for the very detailed answers. Strangely it seems that although these automated systems were designed to reduce pilot workload, in the end the pilot still needs to make sure he is aware exactly of all the settings he needs to be monitoring and what the computer is doing with them. It appears that when things go (very) wrong, the amount of info suddenly thrown at the pilots is astounding. I'm sure that in the heat of the moment, it would be very easy to miss something that might seem quite basic when you're calmly sitting on the ground, and not about to plunge into the south atlantic with a plane full of pax. I'm just a lowly PPL but my instructor has done a good job of beating this into my head: "Don't forget to fly the plane". With the increased use of automation, is there a tendency for pilots to get sucked into the screens troubleshooting errors and forgetting the basics when things go wrong? Obviously errors need to be resolved but I wonder if this is addressed in training, as I'm sure it can be very tempting to bust out all the checklists and start addressing warnings when a shiny screen is blinking at you before dealing with possibly more basic and critical problems.

[mutley320]

to move or not to move !

I too thought the Boeing moving throttles were a better system than the airbus "detents". However i've been on airbus a good while now and their system, once you get used to it also has its merits.
Reading the Turkish airline 737 accident in AMS, the "moving" throttles didnt do them any favours ! Airbus "Detents" may have helped them that day !
( however i realise there have been incidents where TOGA "detent" has not been fully selected on Airbuses and led too some considerable problems)
Every human/mechanical interface can have its own ergonomic and understanding issues/problems.
I'm sure every operator of each system has their own thoughts on the subject.

[engfireleft]

I have lots of time on both Boeing and Airbus, and I also have no real preference since both have their advantages and disadvantages. I don't find situation awareness with moving levers to be a valid argument for Boeing though. The FMA is what counts, and we should be fully aware of not only what every component of the A/F system is doing at any given time, but what it is going to be doing in the near future.