In the following document re A320 notes for pilots, the author writes on page 58:

http://www.airbusdriver.net/airbusnotes.pdf

"If in Open climb or descent and you allow the speed to hit max or min the autothrust will go to SPEED mode and attempt to regain the selected speed while the flight director bars will be removed! Turn OFF FD when hand flying!"

Can someone please explain this?...I can't quite understand why it is recommended not to follow flight director cues when hand flying,,,seems a relatively intuitive, natural and innocuous thing to do...

Is this in any way a nuisance for A320 series pilots? Is this also recommended for other Airbus aircraft? Is it also present on the 737NG's?


Faro

He's not saying not to follow flight director cues, As long as you follow the FD cues you'll be fine.

He's mentioning the case where you're hand flying with autothrust ON and you decide for whatever reason not to follow the FD cues. In that case it's better to turn BOTH FD off. Otherwise you'll end up in the case of expecting the autothrust to do something and it doesn't.

An example your climbing in managed speed, hand flying after take off. You're approaching a level off and the plane is climbing at 3000 fpm. You decide that you're going to reduce the rate of climb so as not to cause spurious Tcas alerts with other aircraft, but you haven't gotten ALT* yet so as you push the side stick forward to reduce the rate of climb, the autothrust remains fixed and your airspeed increases past the managed speed bug.

The first time this happens to you you're wondering why the autothrust isn't reducing the thrust to maintain the managed speed. So as you're rocketing past 300kts. You grab the thrust levers in frustration to flight idle and you're cursing the STUPID airbus...

NOPE not STUPID airbus, the airbus is doing what the FMA is indicating. The FD cue is still pointing way above telling you to climb to maintain that high rate of climb.

The other guy is snickering and after the whole episode is done, the other guy whose seen it before, will tell you that as soon as you decide not to follow the FD, turn both FD off.

If you just turn yours off, the autothrust will follow the other FD and the same result happens when you decide not to follow the FD cues.

Woodreau wrote:

You grab the thrust levers in frustration to flight idle and you're cursing the STUPID airbus...

NOPE not STUPID airbus, the airbus is doing what the FMA is indicating. The FD cue is still pointing way above telling you to climb to maintain that high rate of climb.

The other guy is snickering and after the whole episode is done, the other guy whose seen it before, will tell you that as soon as you decide not to follow the FD, turn both FD off.

If you just turn yours off, the autothrust will follow the other FD and the same result happens when you decide not to follow the FD cues.

Well put. It's a very clever airplane and a lot confusion comes from not properly understanding automation modes. "Mode Awareness" is a must.

As my first Airbus sim instructor would frequently bellow at me when I made some rookie mistake: "Read the f***ing modes! What you need to know is right there in front of you!".

Sooo the autothrust follows what the FD wants the aircraft to do and not what the aircraft is actually doing? This sounds strange to me. Or did I understand this wrong?

The way I understand it (I'm a non current PPL so no so experienced in the workings of the bus) is that if you have asked the plane to give you 1500fpm climb and the autothrust + autopilot is giving you this then you manually turn off the autopilot and 'force' a less steep trajectory then the autothrust thinks it needs to provide more thrust to still maintain 1500fpm? Is that correct or am I totally off?

Fred

The key is understanding that speed can be controlled in two basic ways. Either in SPEED mode, using variable thrust to achieve a certain speed. Or in THR IDLE/THR CLB using a constant thrust, with the Trimmable Horizontal Stabilizer changing pitch angle to achieve a certain speed. (Of course the latter doesn't work in level flight.)

To use an analogy: If the autothrust was the throttle hand of a Cessna 172 pilot, SPEED would be the pilot controlling speed by moving the throttle knob to change engine RPM. THR IDLE/THR CLB would be the pilot leaving the throttle knob at idle or full respectively and using pitch commands to control speed (in a descent or a climb).

flipdewaf wrote:

The way I understand it (I'm a non current PPL so no so experienced in the workings of the bus) is that if you have asked the plane to give you 1500fpm climb and the autothrust + autopilot is giving you this then you manually turn off the autopilot and 'force' a less steep trajectory then the autothrust thinks it needs to provide more thrust to still maintain 1500fpm? Is that correct or am I totally off?

Fred

Not quite. THR CLB is a constant thrust mode. The thrust will remain constant. But since you are in a less steep climb you will now accelerate past your speed target.

Horstroad wrote:

Sooo the autothrust follows what the FD wants the aircraft to do and not what the aircraft is actually doing? This sounds strange to me. Or did I understand this wrong?

The autothrust is following the FD because in the example above case you're in a managed mode. If you want the FD to stop managing the speed, either turn the FD off or pull the speed knob to select the speed.


Going back to Woodreau's example.

Say you're hand flying, and you're climbing with autothrust in THR CLB, meaning constant thrust, with speed controlled by the stabilizer changing the pitch angle. Since you're hand flying you're controlling the pitch directly. Nose higher means higher rate of climb and lower speed, and vice versa.

You want to decrease the rate of climb. You push the nose down a fair bit and you're now deviating from the flight director. Autothrust is still in THR CLB, meaning the thrust doesn't change. You're now accelerating past your speed bug because you didn't tell the autothrust to stop holding a constant thrust.

That's why you should turn the FDs off if you deviate from the FD. If you turn them off the autothrust mode will revert to SPEED and the thrust will vary to maintain your selected (or managed) speed regardless of variations in climb rate.


Side note: This is why we turn off the FDs for a TCAS RA. That way we get SPEED mode guaranteed. It's no good raising the nose for a traffic avoidance climb if the autothrust doesn't increase thrust as well.

Starlionblue wrote:

Thanks, I know this is slightly off topic but how is constant thrust controlled? Is there a constant flow of fuel or is there some sort of force measure within the engine or is it like a predictive algorithm that gives you near as possible thrust so that you can't even notice any changes?

Fred

flipdewaf wrote:

Starlionblue wrote:

Thanks, I know this is slightly off topic but how is constant thrust controlled? Is there a constant flow of fuel or is there some sort of force measure within the engine or is it like a predictive algorithm that gives you near as possible thrust so that you can't even notice any changes?

Fred

FADEC is a form of PFM technology.

Of course the thrust isn't really constant. It will vary with altitude if nothing else. It might be more accurate to say that THR CLB is a a nominal "highest normal" thrust (but still below MCT), adjusted for environmental conditions, and THR IDLE is a flight idle value adjusted for environmental conditions.

Starlionblue wrote:

flipdewaf wrote:

Starlionblue wrote:

Thanks, I know this is slightly off topic but how is constant thrust controlled? Is there a constant flow of fuel or is there some sort of force measure within the engine or is it like a predictive algorithm that gives you near as possible thrust so that you can't even notice any changes?

Fred

FADEC is a form of PFM technology.

Of course the thrust isn't really constant. It will vary with altitude if nothing else. It might be more accurate to say that THR CLB is a a nominal "highest normal" thrust (but still below MCT), adjusted for environmental conditions, and THR IDLE is a flight idle value adjusted for environmental conditions.

Thanks!

Plus I learned a new acronym, PFM will be used at work very often.

Fred

Starlionblue wrote:

Not quite. THR CLB is a constant thrust mode. The thrust will remain constant. But since you are in a less steep climb you will now accelerate past your speed target.

So what we are saying, in Boeing speak, is essentially turn the FD off if you have it in N1 + FLCH mode and you want to handfly vertical profile other than what FD commands?

Would it be not more prudent to just go to speed mode?

Woodreau wrote:

He's not saying not to follow flight director cues, As long as you follow the FD cues you'll be fine.

He's mentioning the case where you're hand flying with autothrust ON and you decide for whatever reason not to follow the FD cues. In that case it's better to turn BOTH FD off. Otherwise you'll end up in the case of expecting the autothrust to do something and it doesn't.

An example your climbing in managed speed, hand flying after take off. You're approaching a level off and the plane is climbing at 3000 fpm. You decide that you're going to reduce the rate of climb so as not to cause spurious Tcas alerts with other aircraft, but you haven't gotten ALT* yet so as you push the side stick forward to reduce the rate of climb, the autothrust remains fixed and your airspeed increases past the managed speed bug.

The first time this happens to you you're wondering why the autothrust isn't reducing the thrust to maintain the managed speed. So as you're rocketing past 300kts. You grab the thrust levers in frustration to flight idle and you're cursing the STUPID airbus...

NOPE not STUPID airbus, the airbus is doing what the FMA is indicating. The FD cue is still pointing way above telling you to climb to maintain that high rate of climb.

The other guy is snickering and after the whole episode is done, the other guy whose seen it before, will tell you that as soon as you decide not to follow the FD, turn both FD off.

If you just turn yours off, the autothrust will follow the other FD and the same result happens when you decide not to follow the FD cues.

This same thing would happen in a Boeing airplane if climbing in VNAV SPD or FLCH while hand flying. Those are speed through elevators modes. The F/D sets pitch to maintain the selected speed. If you reduce the pitch, the airplane is going to accelerate past the selected speed. In the case of VNAV SPD, the autothrottle is in the THR REF or N1/EPR mode (depending on model) which sets a constant climb thrust.

Fabo wrote:

Starlionblue wrote:

Not quite. THR CLB is a constant thrust mode. The thrust will remain constant. But since you are in a less steep climb you will now accelerate past your speed target.

So what we are saying, in Boeing speak, is essentially turn the FD off if you have it in N1 + FLCH mode and you want to handfly vertical profile other than what FD commands?

Would it be not more prudent to just go to speed mode?

I don't know the Boeing well but that sounds right.

You want the autothrust in SPEED mode in any case. Given the example at hand, you can do things in in two ways and there are arguments for both.
- Pull the speed knob to change autothrust mode to (selected) SPEED and dial in your desired speed. Then dial your desired V/S and pull. This changes the vertical mode to V/S. Then follow FD guidance. Advantage: FD guidance all the way through including altitude capture.
- Turn the FDs off. Autothrust automatically reverts to SPEED and you fly a "hand-on-stick profile". Advantage: Not having to fiddle with V/S.

Fabo wrote:

Starlionblue wrote:

Not quite. THR CLB is a constant thrust mode. The thrust will remain constant. But since you are in a less steep climb you will now accelerate past your speed target.

So what we are saying, in Boeing speak, is essentially turn the FD off if you have it in N1 + FLCH mode and you want to handfly vertical profile other than what FD commands?

Would it be not more prudent to just go to speed mode?

I think I understand now...so the author of the opening quote is exaggerating a little when he writes "Turn OFF FD when hand flying!"...as Fabo says it should be:

"Turn off the FD's when hand flying a vertical profile other than that which the present, thrust-based autopilot mode is following"


Faro

A dumb side-question but one which nags me sometimes...does turning off the FD always turn off the associated autopilot? Any exceptions to this?


Faro

Faro wrote:

A dumb side-question but one which nags me sometimes...does turning off the FD always turn off the associated autopilot? Any exceptions to this?


Faro

On the 'bus, if you turn off the FDs with an AP on, the AP remains engaged. You just don't get the FD crosshair display in the PFD.

Faro wrote:

Fabo wrote:

Starlionblue wrote:

Not quite. THR CLB is a constant thrust mode. The thrust will remain constant. But since you are in a less steep climb you will now accelerate past your speed target.

So what we are saying, in Boeing speak, is essentially turn the FD off if you have it in N1 + FLCH mode and you want to handfly vertical profile other than what FD commands?

Would it be not more prudent to just go to speed mode?

I think I understand now...so the author of the opening quote is exaggerating a little when he writes "Turn OFF FD when hand flying!"...as Fabo says it should be:

"Turn off the FD's when hand flying a vertical profile other than that which the present, thrust-based autopilot mode is following"


Faro

Indeed. A more accurate statement would have been: "Turn off the FDs when hand flying if you're not going to follow FD guidance." You're perfectly fine hand flying with the FDs on. You just have to make sure the FD commands, as given by the FM and/or FCU, give you the path you want to fly manually.

Starlionblue wrote:

Faro wrote:

A dumb side-question but one which nags me sometimes...does turning off the FD always turn off the associated autopilot? Any exceptions to this?


Faro

On the 'bus, if you turn off the FDs with an AP on, the AP remains engaged. You just don't get the FD crosshair display in the PFD.


[quote="Faro"]

Same as Boeing. You can turn off the FD on either Primary Flight Display and it will not affect the Autopilot. It will remain engaged.

The 757 and 767 had an old option that does not allow the FD to use the same channel Flight Control Computer (Autopilot Computer) that is in command of the airplane. In other words, if you select the Left Autopilot and the FD is also using the left FCC, the FD will disappear. The by-product of this is that when the Autopilot goes Triple-Channel at 1500 feet during an Autoland, you'll lose both FDs (since all three FCCs are being used for autopilot and you can't have an FD sourced off an FCC that is giving autopilot commands). I think this option is rare now, and not applicable to other models like the 777 and 787.

Starlionblue wrote:

On the 'bus, if you turn off the FDs with an AP on, the AP remains engaged. You just don't get the FD crosshair display in the PFD.

But then if I understand correctly one would have to turn off both the FD and AP if one wanted to hand fly a different profile than what the AP is set up for...is this correct?


Faro

Faro wrote:

Starlionblue wrote:

On the 'bus, if you turn off the FDs with an AP on, the AP remains engaged. You just don't get the FD crosshair display in the PFD.

But then if I understand correctly one would have to turn off both the FD and AP if one wanted to hand fly a different profile than what the AP is set up for...is this correct?


Faro

That is correct.

If you use the FDs, make sure the profile you've set up in the FM and/or FCU (glareshield) is the one you want to fly. And in any case you can't handfly without turning the AP off.

Example of when we always turn the FMs off: Final stages of an RNAV approach. Since there is no defined vertical path to follow like on an ILS, the FD doesn't have "real" any vertical guidance to give you. Another example is a visual approach.

Contrast with the 350, where you keep the FD on during an RNAV approach because the FD can in fact give "real" guidance all the way to the runway.. The FM on the 350 computes lateral and vertical guidance on RNAV and VOR approaches. The new modes are known as F-LOC and F-G/S and are indicated by double diamonds instead of the single ones you get for ILS. Basically it looks like you are flying an ILS but you're actually flying an FM computed path with no reference to ground installations.

You can see it on page 4 in this preso. http://www.ae-expo.eu/wp-content/uploads/2016/04/Frederic-Belloir-Airbus-xLS-concept.pdf

Starlionblue wrote:

Faro wrote:

Starlionblue wrote:

On the 'bus, if you turn off the FDs with an AP on, the AP remains engaged. You just don't get the FD crosshair display in the PFD.

But then if I understand correctly one would have to turn off both the FD and AP if one wanted to hand fly a different profile than what the AP is set up for...is this correct?


Faro

That is correct.

If you use the FDs, make sure the profile you've set up in the FM and/or FCU (glareshield) is the one you want to fly. And in any case you can't handfly without turning the AP off.

Example of when we always turn the FMs off: Final stages of an RNAV approach. Since there is no defined vertical path to follow like on an ILS, the FD doesn't have "real" any vertical guidance to give you. Another example is a visual approach.

Contrast with the 350, where you keep the FD on during an RNAV approach because the FD can in fact give "real" guidance all the way to the runway.. The FM on the 350 computes lateral and vertical guidance on RNAV and VOR approaches. The new modes are known as F-LOC and F-G/S and are indicated by double diamonds instead of the single ones you get for ILS. Basically it looks like you are flying an ILS but you're actually flying an FM computed path with no reference to ground installations.

You can see it on page 4 in this preso. http://www.ae-expo.eu/wp-content/uploads/2016/04/Frederic-Belloir-Airbus-xLS-concept.pdf

What you are describing sounds like an IAN approach on the Boeing models that have it. The FMC builds an ILS look alike profile. Instead of LOC and G/S, the modes are annunciated as FAC and G/P for Final Approach Course and Glide Path. The FDs or AP will fly the lateral and vertical guidance just like an ILS approach, except the AP must be disconnected by 100 feet.