Quick Flight Director question

[A320 Tony]

I know there is a very simple answer here but I'm struggling to find it...

Why is it that when you go visual to land on a precision approach (ILS), you leave the FD's on all the way to touchdown, but on a non-precision you are required to turn them off and take the bird? Surely if you are visual the events that transpired to get you there are irrelevant? What is the difference here? If you're visual you're visual! This is based on the A320 family.

Please can somebody point out the obvious to me!

Thanks

[Meikleour]

Non Precision Approaches only use a pseudo glideslope if it is coded in the FM database or may just be flown using V/S or FPA modes on the F/D therefore when visual it would be possible to be following inappropriate F/D steering commands depending on how the approach had been flown.

On the Airbus, switching off the F/Ds forces the ATHR into speed mode which is "failsafe" Try researching the "Bangalore A320" crash.

[Meikleour]

A320 Tony: When one asks politely on this forum for advice and this is given equally politely then you do not even acknowledge this ............

Some people might regards this as quite rude!!!!

[A320 Tony]

Meikleour - No offence intended and thank you for your detailed response. As you can see I'm not a frequent visitor to the site & have been rather busy lately. I have also been mulling your answer over to get my head around it and come up with an 'intelligent' response (struggling with that!).

In my experience every approach ends up in speed mode (with athr engaged), it's part of our landing checklist. I see your point about the pseudo G/S compared to the actual ILS G/S. The FD's could be following a slightly incorrect vertical profile if in v/s or fpa so you wouldn't want to be distracted by the FD's even if visual. There is always a natural tendency to follow them. I also wonder if the go around has any part to play? On some aircraft there is no SRS engagement below MDA during NPA's, so you wouldn't want to be following the FD's in that instance. I just wish the manuals gave more clear explanations of 'why' things are done in a certain way and what would happen if you left the FD's on until touchdown?

Thanks again

[Jetdriver_JAT]

General advice: If you don't follow flight directors, switch them off.

Lets say you keep flight directors and descend below your target altitude in the FCU.
A/THR and F/D will then command climb power while you're descending because of ALT*.
Hello overspeed of flaps and gear (tried it with a captain who didn't believe in disconnecting F/Ds on a visual approach) - only instant idle on the thrust levers saved us from the alarm clock and a meeting with the chief pilot.

[A320 Tony]

Thank you Jetdriver. So would that situation still occur if you disconnect the AP at say 700', athr in speed mode, with the missed app altitude set (eg 3000' in FCU)? Would the FD's suddenly command a climb if you drop below MDA? Or want to level out perhaps? I don't quite understand what would happen if you have manual control of the aircraft, flying visually on to a rway with them turned on. Why not also switch them off on an ILS approach once visual?

I feel like I'm missing something startlingly obvious here!! Perhaps this is something I need to see in the sim!

[Meikleour]

Tony: All NPAs have to be flown manually after MDA (by definition) On the Airbus the A/P will disengage itself at MDA-50ft anyway.

On an ILS the flightpath control is via the elevator tracking the GS with the speed being maintained by the ATHR. After MDA if the F/Ds were continued to be used they would be following a command which may or may not be appropriate to fly the approach. Hence they are switched off.

If you are still having trouble with this concept consider this scenario:

Visual after MDA, F/Ds still on , A/P off and OPEN DESCENT pulled on the FCU.
If you continue to fly the PAPIS - what speed will the aircraft get down to?

I am deliberately not telling you the answer to get you to think it through yourself!!

[AirRabbit]

… So would that situation still occur if you disconnect the AP at say 700', athr in speed mode, with the missed app altitude set (eg 3000' in FCU)? Would the FD's suddenly command a climb if you drop below MDA? Or want to level out perhaps? I don't quite understand what would happen if you have manual control of the aircraft, flying visually on to a rway with them turned on. Why not also switch them off on an ILS approach once visual?
I feel like I'm missing something startlingly obvious here!! Perhaps this is something I need to see in the sim!

With ABSOLUTELY no intent to criticize or ostracize anyone, I simply have to ask ... am I the only one here who sees this series of questions as a continuously developing condition that only SOME are willing to openly acknowledge – which is the confusion about the differentiation between the following:
(1) the pilot flying the airplane;
(2) the pilot using automatically computed information that is displayed in a visual format that may or may not be followed by the pilot, as the pilot elects, through the use of the airplane’s flight controls;
(3) the pilot retaining responsibility of flying the airplane although using the automatic systems to manipulate the actual flight controls;
(4) the pilot abdicating direct responsibility of flying the airplane to the automatic systems, essentially accepting whatever the airplane’s systems decide; and
(5) at some point (which may or may not be accurately communicated or logically understood) where the automatic systems assume control of the airplane despite what the pilot does with the flight controls provided to the pilot (and either not knowing or not caring what, if any, point this change of control occurs).

… and the significance of the inability of at least some pilots to recognize and understand these differences?

[ahramin]

They have named this automation confusion AirRabbit.

Just went through this the other day. 10 miles back, airport in sight, autopilot off, "both f/ds off". Of course we left the ILS raw data on, and I turned the FPV on as well. Then the comments start:

"I'm going to leave my flight director on" (Scared to fly without it. No understanding of autothrust behavior with pnf FD left on).

"Why are we in track fpa? That's only for non precision approaches." (No understanding of track/fpa mode. No understanding of what the FPV really is for).

These guys are fine as long as the autopilot is flying the ILS or fully managed NPA, and they have the required incantations in both SOPs memorized, but they don't understand what the airplane is doing. Take them outside of those two situations and they are totally lost. As for your point number 4, the mental gymnastics these guys are capable of to justify blindly following the plane into the ground rivals the intelligent design crowd. During the same day as the above, after having 3-4 red lights on the PAPI for the last 500' while following the brick all the way to touchdown on an NPA (in temperatures lower than -20°C):

"I don't know why the PAPIs were red the whole way, but I don't know what eye to wheel height they are good for anyway".

To provide more information on your question Tony, the F/D will provide good guidance all the way through touchdown and rollout on an ILS, but will not do so on an NPA. So you CAN leave the F/D on during an ILS, but you are not REQUIRED to. Feel free to turn them off. On an NPA though, the F/D will not provide good guidance below the MDA and therefore MUST be turned off.

So in both cases, the FDs can be turned off. Once they are off, it would make sense to turn the TRK/FPA mode on, as this is the only way to get your FPV on, which is a handy thing to have.

[ahramin]

And what would these conditions be? Do you have an example of an RNP-AR approach with minimums above 300' that allow use of autopilot to 250'?

[BARKINGMAD]

"All NPAs have to be flown manually after MDA (by definition)"

Is that a HARD FACT, applicable to all aircraft?

If so, why did my 737-800 FCOM (yes, I read it a few times de facto!) quote a "minimum autopilot disconnect height of 158 ft"?

Stuck in my brain because I found it difficult to perceive when I was exactly at that particular height, so rapid do the digits in the radalt turn over!

[PEI_3721]

Some AFMs show the autopilot minimum use height diagrammatically and this might relate to specific operations, e.g. AWO > Cat 1
Flight tests to determine the MUH are usually made from an ILS approach; thus the MUH assumes an obstacle clearance zone proportional to the ILS (1:29 slope?) Unless stated in the AFM, the AP does not have to disengaged at MDA, but pilots must be aware of the possibility of reduced clearance from obstacles / ground in the event of failure. In addition NPA could involve descent rates higher than those for an ILS (or assumed by certification), then the obstacle clearance may be even less; thus an early disengagement (before MUH) would be prudent.
For precision like approaches, as the MDA is approached the decision is either to GA (use AP as required), or land – disengage AP. If it is judged necessary to continue with the AP, then perhaps the land decision was marginal and a GA should be flown.

For the FD, leaving the display on or turning it off may not matter. It should not be used to aid visual flight after a land decision because the guidance for a NPA might be invalid – not to a GS origin. However, leaving the FD on may make it more accessible in the event of a GA from low altitude.

[Meikleour]

Barkingmad: Smash Bugger

Thank you for your inputs. As is often the case on PPRUNE this is spiralling into more and more arcane postings.

The original poster asked a very simple question relating to Airbus procedures.
I attempted to give a concise, simple reply. I am deeply sorry if my precis has failed to fully cover all the variations that exist between different operators and equipment fits. The general principle holds as to the reasons for the procedure nevertheless.

SB - perhaps you have only seen the Airbus with full twin GPS enabled capabilities? I had the good fortune/misfortune to fly the Airbus models from their inception and with an overlay NDB/VOR approach then the A/P does indeed disconnect at MDA-50

My reading of the OP was a question of why in manual flight does the F/D go off for an NPA and stay on for an ILS. My initial reply still answers that!

[Jwscud]

Barking Mad - my 737 FCOM 1 says this:

Airplanes operating under JAA Rules: The autopilot must be disengaged before the airplane descends more than 50 feet below the minimum descent altitude (MDA) unless it is coupled to an ILS glide slope and localizer or in the go-around mode.

For JAA I imagine read EASA these days.

[Meikleour]

Smash Bugger:

I note that you mention the "No Technical Objection" caveat that I guess your airline uses to allow the use of F/Ds below MDA on NPA approaches however I have had a chance to review my own airline's manuals and, guess what, we do not have this facility!! Perhaps many other operators are the same?

As I said before, I chose to answer a simple question in GENERAL terms and you jump in here and criticise me in very SPECIFIC terms for not being 100% accurate!

Please explain to me how your posting has helped the OP in his understanding of Airbus procedures?

[Piltdown Man]

A rather well run simulator session many years ago showed me that whenever you have the FD and an autothrottle on, you must follow the FD. Not following the FD may result in inappropriate A/T commands. The A/T will only perform as expected if it's in the correct mode for the task in hand. If you are not going to follow it, dump it. Why? On a non-precision approach, NAV or HDG with V/S or FPA work to give you the desired track, speed and V/S. When you disconnect the A/P, what mode will the A/T be in?

[flyboyike]

With ABSOLUTELY no intent to criticize or ostracize anyone, I simply have to ask ... am I the only one here who sees this series of questions as a continuously developing condition that only SOME are willing to openly acknowledge – which is the confusion about the differentiation between the following:
(1) the pilot flying the airplane;
(2) the pilot using automatically computed information that is displayed in a visual format that may or may not be followed by the pilot, as the pilot elects, through the use of the airplane’s flight controls;
(3) the pilot retaining responsibility of flying the airplane although using the automatic systems to manipulate the actual flight controls;
(4) the pilot abdicating direct responsibility of flying the airplane to the automatic systems, essentially accepting whatever the airplane’s systems decide; and
(5) at some point (which may or may not be accurately communicated or logically understood) where the automatic systems assume control of the airplane despite what the pilot does with the flight controls provided to the pilot (and either not knowing or not caring what, if any, point this change of control occurs).

… and the significance of the inability of at least some pilots to recognize and understand these differences?


It's nothing that sinister, AirRabbit. The forum is simply being flooded by Microsoft pilots, who've never actually flown anything. At least some of them are making the effort of framing their questions so that they seem legit, but I'm not buying it.