Only one FD on visual approach on 320

[speedbird787]

Dear fellow pilots

What are the consequences of only one FD ON. in a visual approach with approach phase activated and speed managed before turning one FD OFF.

[vilas]

Switching off both FDs puts ATHR in speed mode irrespective what the present mode is and which is desired mode for visual approach. If only one FD is off then it will remain in whichever mode the other FD is in. If it was in OP DES then ATHR will remain in THR IDLE. This is a cause of an accident so it is recommended to switch off both and together.

[NorthCountryBoy76]

I believe the thrust will follow the FD which will not be appropriate as you are flying a visual approach, or not what the FD is commanding. Both FD's off will put the thrust into SPEED mode and will command the thrust to give the managed speed (or selected speed).

This is my understanding - which may or may not be completely accurate.

[Amadis of Gaul]

I'm having trouble thinking of ANY circumstance in which flying with only one FD is appropriate (provided they're both operational). All or nothing, I say.

[Microburst2002]

It is all about knowing what u do.

It is not recommended for the above mentioned reason. There is a very real danger in doing it. However, if you know that and you make sure that A/THR remains in SPEED (by selecting V/S before switching off the FD) or better, you just disconnect the A/THR, then it is absolutely safe to fly with one FD only.

It can be useful, the PM would see how the PF is doing. Could be good for practice of hand flying skills.

I am the inventor of a new flight guidance mode, which doesn't have a name yet (we are in the start up stage). It would be like an intermediate layer between FDs ON and FDs OFF flying. The FMA still shows the guidance modes but the bars are not there. They only appear when you deviate from flight parameters beyond a given threshold. Then you follow the bars. This would make actual hands on manual flying practice safer and airlines would be ok with it. It could also work with A/THR, so you would control it manually until it activated in case of excessive deviation.

Am I going to be a millionaire or what?

[FlyingStone]

I'm having trouble thinking of ANY circumstance in which flying with only one FD is appropriate (provided they're both operational). All or nothing, I say.

Agreed, although some people find it too difficult to monitor the other guy's visual/raw data flying without some magenta/green bars. I think it says something of their skills as well. Usually they belong to the "AP until minimums" squad.

[despegue]

Ehm...A VISUAL Approach is done raw data with FD's OFF!:ugh::ugh::hmm:

Or is your airline one of these outfits where one is not allowed to fly anymore and an accident waiting to happen?:=

[172_driver]

One FD on/One off is occasionally used to satisfy FDM requirements, when the airline in question does not allow functional FDs to be switched off


Edit: My old computer does not allow smileys to manifest the ridicolousness of the above. It's true, nonetheless

[Microburst2002]

I was referring to one FD OFF only, for hand flying in an ILS approach, while the other pilot keeps the FD ON. Nothing dangerous if you know what you do, like I said (check speed green, or disconnect A/THR)

In a visual it doesn't make any sense. What guidance would that FD give??

[FlyingStone]

Why do you need F/D ON to monitor flight path progress during other pilot's raw data flying? Have we really come that far that even monitoring flight path requires F/D?

[carrchow]

That's why we have the normal checklist term in the landing checklist says A/THR mode, Check it in SPEED green or manual, just make sure we keep nothing change after we finish the Landing Checklist, that's should be ok.

[vilas]

When a fatal accident has mandated both FDs off just don't be creative. If a captain cannot judge FO'S approach visually he should not be giving the approach to the FO.

[7478ti]

Vilas is correct in asserting that following the OEMs guidance and airline policy is certainly appropriate, and wise, for operating any specific type.

However, in general, and not necessarily related to any specific type, OEM policy, or airline policy, ...there are many circumstances industry wide, where selecting, retaining, or re-selecting "one FD ON" may nonetheless be acceptable, if not even appropriate, and in some circumstances, even necessary. There are specific cases where it is (or was) the policy of an airline, for some aircraft types and circumstances, to select "one FD ON", or to re-select one FD ON. This can be for setting a mode, for protecting mode availability, or for things like presetting the ability to transition to an RNP based missed approach path for a balked landing, in the event of the need for a go-around, or loss of visual reference, or for arming modes or protections, or for retaining some form of FD guidance for a PM, etc.

This issue becomes very difficult for transition between types, or for mixed fleet flying of different types, where habit patterns intervene, and where the philosophy and procedures for FD use may differ, may lead to vulnerabilities, or may outright clash, if not even be mutually exclusive.

So in broad terms, any design where this sensitivity to "both on", "both off", or "one on" becomes an issue, just as Vilas correctly points out, has been known to lead to an accident. So for these designs, is likely that a design improvement would be worth considering, and operating policies and training may need to be reconsidered, to underscore and graphically demonstrate "why" vulnerability exists. Further, such designs may require special care or emphasis during initial or recurrent training, or IOE, to specifically illustrate the risks and vulnerabilities, of not operating according to the OEM's policy or the airline's policy, or per procedures or requirements for that specific type.

Another good example of this principle, of carefully adhering to procedures for a given type, and not assuming that methods from other types will work acceptably, was the previously not emphasized issue of using "rudder checkbacks" and "rudder rolls" the same way in a big jet transport as had been successfully done for decades in tactical fighters, to augment roll control. That pilot information void too, and incorrect assumptions about transfer of procedures from one type to another, once ultimately led to another unexpected fatal difficulty [AA587 - NTSB attributed the accident to the first officer's overuse of rudder controls in response to a wake turbulence event, which caused the vertical stabilizer to fail, and engines to separate from the aircraft.]

[vilas]

In A320 procedure of switching of both FDs does not come from systemic deficiency but rather to take advantage from a smart feature that puts the ATHR in speed mode without any other action on FCU. Heaven is not going to fall if you kept one FD on but you need to ensure proper mode transition by doing other things. It is matter of developing correct and safe operating habits because when under pressure pilots can get confused and make mistakes executing nonstandard procedures. In SFO accident when the crew switched off FDs in B777 the auto throttle went in throttle hold mode which was at idle then and the crew failed to monitor speed. Airbus FD logic in this case is definitely safer and better. FAA has asked Boeing to do something about it. On the other hand the first fatal crash of A320 in 1982 India was similar because the crew switched only one FD OFF and remained in OPDES and imagined they were in speed mode. Had they switched off the other it would have switched to speed mode.

[Chris Scott]

Quote from vilas:
"On the other hand the first fatal crash of A320 in 1982 India was similar because the crew switched only one FD OFF and remained in OPDES and imagined they were in speed mode. Had they switched off the other it would have switched to speed mode."

That's correct, except for the date. The A320 did not enter airline service until 1988. To avoid any confusion, the accident vilas is referring to is the Indian Airlines accident at Bangalore in 1990, as recently mentioned on another thread here (http://www.pprune.org/tech-log/566091-thrust-levers-moving-not.html#post9083108).

Sadly, the experience that we in the launch-customer airlines had previously gained may not have been passed on. A number of incidents involving the speed dropping well below VLS, including some in which alpha-floor had been activated, had led to our SOP of switching both FDs off for visual approaches.

The problem results, as vilas indicates, after a transition from the normal idle-descent phase to a visual profile when one or both FDs are left on in IDLE/OPDES mode and the PF pitches up in manual flight above the FD pitch bar. In those days, unless the crew selected SPD mode, the A/THR would remain in IDLE mode unless and until it was disengaged by the crew or by alpha-floor - regardless of the speed selected by the crew.

Following the accident, a protection was introduced by which the A/THR changes automatically from IDLE to SPD mode if the IAS falls below VLS (vilas may tell us the precise figure, which I don't have to hand).

[ManaAdaSystem]

Have we really come that far that even monitoring flight path requires F/D?

Have we really come that far that a modern jet aircraft becomes unsafe just because you switch off a flight director?

[Chris Scott]

Quote from ManaAdaSystem:
"Have we really come that far that a modern jet aircraft becomes unsafe just because you switch off a flight director?"

In the case we are discussing, it is not the switching off of one FD that is responsible for the problem. It is the PF's failure to comply with the pitch commands of the remaining FD that causes the speed to decay. To put it bluntly, if you tell a non-human system to guide you, and then deliberately ignore its advice, there may be consequences. In the original design, the only safety-net to a pilot's mismanagement and inattention was alpha-floor. After Bangalore, AI introduced a second, higher safety-net.

[olster]

Which is why the A380 only has 1 f/d switch. You have to ensure that the auto thrust is in SPEED mode hence both f/d's have to be switched off. It has nothing to with who is Chuck Yeager and everything to do with system design. Unfortunately, there has been a lack of understanding of this since the early A320 days including at least 1 well documented fatal accident; if the FCOM says f/d's off for visual approach then this is not open to interpretation. If 1 f/d is on then the auto thrust could be in THR IDLE and therein lies potential catastrophe.

[tubby linton]

It is also why we switch them off in response to a TCAS RA, unless you have the TCAS mode embedded in the software.

[olster]

Yes, Tubby and the A380 has a fully automated TCAS mode - sorry to go on about the A380 but the evolution of the Airbus fbw cockpits is precisely due to the ergonomic design issues which have historically caused problems. Having gone through ( endured) a manufacturers A340 course in 93/94 I can confirm that with some exceptions the training did not clarify some of these technical issues with subsequently dire consequences. The disappointing factor is that this question regarding f/d use is still being asked 20 years later. The fundamental flaw was training (or lack of) and it would appear that not much has changed.

cheers

[vilas]

Chris Scott
Sorry typing error. Off course it was 1990. Just to elucidate further they had SPEED, ALT* some where near the minima and here PF asked for 1000ft V/S. Later PF asked for 700ft V/S but PNF got involved in setting up missed approach ALT and an ATC call and by mistake set 700 ft. on ALT SEL and activated OP DES. The PF (under check) switched off his FD without informing the PNF(check captain). So PNF's FD and OP DES remained active. Then onwards PF was followed visual flight path with dropping speed. Even after this mistake everything would have been fine if the other FD was switched off. But the fact remains they did not check the speed till they crashed. The last words were "Hey we are going down" but nothing about speed.