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FD in a stall

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FD in a stall

Old 14th Dec 2015, 06:30
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FD in a stall

The Air France and Air Asia accidents got me thinking about something. What would the FD show while the aircraft descended in a stall?

Will it still be giving commands to capture the FCU altitude? I'm wondering if the PF in each of those accidents, being confused, simply reverted to what they'd been taught so often- "just follow the FD". Would that explain the prolonged nose up inputs?

I wonder how much time each of those pilots had spent flying without the FD? I've seen multiple cases where a student is having trouble flying the aircraft, the instructor will keep pressuring the student to simply follow the FD.

Could these accidents have been cases of pilots blindly trying to follow the FD, and never trying to look behind the bars?
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Old 14th Dec 2015, 07:35
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I believe the MCP control active modes and these are shown on the FD, so they would show previous commands. Like the TCAS as well?
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Old 14th Dec 2015, 10:24
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Hi Check Airman,
The BEA report of AF 447 accident http://www.bea.aero/docspa/2009/f-cp...p090601.en.pdf on page 190 says:

"2.3.2 Operation of the flight directors
When an unreliable airspeed event occurs, the automatic control features (autopilot and autothrust) disconnected automatically. The crew could only then re-engage them by pressing on a dedicated push-button on the FCU. The flight directors behaved differently, insofar as the cross bars disappeared from the PFD, even though the flight directors were still engaged.
....
The credibility of the cross bars is strengthened by their disappearance followed
by their re-appearance: if they appear, it implies that the indications that they
display are valid;

ˆ Since they attract the crew’s attention (green colour and presentation in the
centre of the PFD), the presence of the cross bars could have influenced the
actions of the PF, notably in respect to his reaction to the stall warning;

ˆ It is only possible to be aware of the changes in active modes (when the cross
bars reappear) by reading the FMA, which is probably difficult to do in a high
workload situation induced by piloting or failure management tasks."

When the FDs reappeared automatically, their modes would be HDG and VS. If the aircraft had a rate of climb when the FDs re-engaged, then they would constantly request a pitch up command whilst the aircraft was descending in a stall.
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Old 15th Dec 2015, 14:00
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Only a Wild Guess, but a Fair Question

@Check. You may not recognize it sir, but, I think you have answered your own question. Please see your third paragraph, quietly asking about hand-flying experience. In the SIM or the airplane, I suspect that those unfortunate crews had little or possibly no experience hand-flying their airplanes. "Follow the FD" is not horrible advice, but one must fully understand exactly what inputs the FD is capturing, how it computes instructions for you* and what you should and should not follow.

*Regardless of airplane type, your FD is a small, canned set of brains that can operate far faster than can your bio-brain. That does NOT mean that in extreme circumstances it will come up with a better solution than you can. It simply does its thing much faster. Perhaps a fair answer is, "If the FD's guidance looks reasonable to the experienced pilot. it probably is." If the FD's guidance does not look reasonable to your experienced eye, think for yourself and hand-fly, if you know how to do so. (Sorry, no snark intended, but how many of today's transport pilots have ever hand-flown anything larger than a C172, past gear-up or in the terminal flare, in he SIM or in an airplane. (I'm not certain that the FM SIMS can even do it.) As far as I know, the only live, in-airplane flying of this type is done by Boeing and that other company's engineering test pilots,( those kids CAN hand-fly and do so, regularly). Heavens no! I am not suggesting that you regularly hand-fly at high altitude cruise! I'm simply asking whether you and your colleagues have ever had the experience or been trained to do so. My best guess is no; if/when it may become necessary, you'd have no clue, with Boeing's partial FBW or the other company's full FBW. What do YOU think?
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Old 15th Dec 2015, 14:40
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In the AirAsia report, far too little is said on the FD and other active vertical modes, only that FD were displayed when parameters were inside the flight director envelope.
At least they published more on the FD that they did on the THS ...
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Old 15th Dec 2015, 23:24
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Looking at FD indications at the gate (airspeed zero) on aircraft like the B744, it does seem that directions are based on mode. E.g. In FLCH with an altitude dialled in which is greater than the current altitude, the FD bar will go down (in an attempt to gain airspeed). In V/S mode, I recall that the FD bar went up.

Unfortunately, not all modes can be selected on the ground (e.g. VNAV)... and, of course, I am talking about an older generation (non-FBW) aircraft.
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Old 16th Dec 2015, 04:33
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Thanks guys. As I suspected, the FD was telling them to climb. Hopefully training departments will take note, and actively encourage more raw data flying.
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Old 16th Dec 2015, 05:15
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FFS

FFS guys, what ever ever happened to the basic idea (I learnt it in 1974 FFS) POWER/THRUST PLUS SENSIBLE ATTITUDE EQUALS PERFORMANCE.Why has this basic law of physics been unlearned? I dont get it.Use it and AIR FRANCE/AIR ASIA would not have murdered hundreds of innocent souls FFS.Jesus.
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Old 16th Dec 2015, 05:49
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Check Airman an Goldenrivett
No training department teaches to follow FD when stall warning is blaring. What the training department needs to instil in AB pilots is that in the air unless you were dealing with EGPWS activation or wind shear(which doesn't happen at cruise levels) it is irrational, bizarre and suicidal to pull full back stick and any pitch correction with the stick needs to be cross checked on the PFD.

Last edited by vilas; 16th Dec 2015 at 09:59.
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Old 16th Dec 2015, 09:09
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villas,
see around page 91 http://www.bea.aero/docspa/2009/f-cp...90601e3.en.pdf
The BEA report says repeatedly that the FD initial automatic engagement modes were HDG + VS climb.

"2h 10 min 47
The FD 1 and 2 become available
again (modes HDG/ VS).
The selected heading is 34°.
The vertical speed is 1,500 ft/min.
..."

Last edited by Goldenrivett; 16th Dec 2015 at 10:12. Reason: link to BEA report & removal of text due vilas' reply below
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Old 16th Dec 2015, 09:57
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Golden
You are right about speed protection in descent. I stand corrected. Correcting the original.
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Old 16th Dec 2015, 10:04
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An interesting question from the OP.

As an aside: @ piratepete, I agree with your "FFS" sentiment, but it wasn't murder - that requires malice aforethought or intent to kill.

However, it could perhaps be argued that it was corporate manslaughter, or whatever the term is, because (some of) the pilots involved had obviously not been trained and/or checked thoroughly enough.

And this raises the question: are there really 'pilots' on the line out there who cannot fly without a FD or look through the FD and fly sensibly when/if required? They must wonder what all those funny gauges and scales around the PFD (such as the VS scale and compass) are for..........
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Old 16th Dec 2015, 10:24
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Regardless of airplane type, your FD is a small, canned set of brains that can operate far faster than can your bio-brain. That does NOT mean that in extreme circumstances it will come up with a better solution than you can. It simply does its thing much faster.
Don't buy that explanation.

With enough experience, the flight directors should follow *you*.
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Old 16th Dec 2015, 11:26
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vilas,
No training department teaches to follow FD when stall warning is blaring.
Are you sure they actually heard the stall warning?
See AF 447 CVR Transcript and note that between the following times that the ALT ALERT "C Chord" was sounding continuously.
2:10:20 - 2:10:51 = 31 secs
2:12:57 - 2:13:55 = 58 secs
2:14:02 - 2:14:20 = 18 secs

It appears that for nearly 2 mins of continuous "C Chord", no one bothered to cancel it.
If you look closely at the "ATC, other voices, warnings, remarks and various noises" column, the crew were saturated with continuous audio warnings.
The continuous "C Chord" was only interrupted by "SV Stall Stall", "Dual Input", "cricket", "sink rate", "pull up" etc.

The BEA have asked if the present stall warning noise alone is adequate warning due auditory overload.

See Final AF447 Report Suggests Pilot Slavishly Followed Flight Director Pitch-Up Commands | News: Aviation International News
"Another reason for having ignored the stall alarm could have been a matter of sheer perception, Troadec said. “Audio alarms are no longer heard in some situations,” he admitted. This has prompted the BEA to recommend the addition of a visual stall warning."

I don't think the crew actually "heard" the stall warning. I think they automatically filtered all those "distractions" out. They needed a specific stall warning attention grabber like a vibrator or buzzer felt through the side stick, not a flight deck saturated with noises like an orchestra warming up.
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Old 16th Dec 2015, 13:16
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are there really 'pilots' on the line out there who cannot fly without a FD or look through the FD and fly sensibly when/if required?
You betcha life there are and probably in their thousands. Had a newbie in the 737 simulator with 300 hours total time of which 100 hours in the real 737. He was employed as a first officer in foreign airline

When asked to conduct a take off, circuit and land with FD and AT switched off he became quite agitated and said he had never flown with FD and AT off.

Yet his licence showed he had completed a full command type rating.

Needless to say he was unable to cope with hand flying in even visual conditions without the aid of the FD and AT. Height keeping was +/- 800 ft and airspeed +/- 50 knots. Bank angle at one point was 60 degrees. This was no exaggeration, believe me. These people are out there on line and are the future captains of large jet transports
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Old 16th Dec 2015, 13:39
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So......did you pass him ?
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Old 16th Dec 2015, 14:29
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“… are there really 'pilots' on the line out there who cannot fly without a FD …’ many posts imply that there are, but equally all pilots could encounter mental blocks which limit ‘the hands on’ abilities due to the surprise of a situation.

Slide 2 > in Managing Startle identifies the mental condition of normalcy, where the 'conditioned response' to a surprising event is to revert to normal operations, i.e. it is difficult to remember not to use the FD in a stall.
The complexity of modern operation could sway pilots to seek the ‘comfort blanket’ of the FD for a range of normal situations, not the hazardous surprising events, but the ‘surprises’ of normal flight because these events had not been considered or anticipated. Thus the issue of ‘cannot fly without the FD’ could be rooted much deeper in our industry – normal operations, training, policy, philosophy (pilot as a hazard vs pilot as an asset).

Also see the presentation on AoA research.
Slide 16 shows that pilots seek ‘what to do’ information in critical situations (Rule based thinking, not SOPs).
The FD supplies 'what to do' information and thus reduces workload vice flying raw data, which requires Knowledge-based thinking to generate an understanding of the situation and then the required actions.

Memory fades, but my aircraft did not remove the FD when approaching or at the stall; the autopilot was disengaged at stick shake. But even with an 'erroneous' FD display, stall recovery was not a problem because the stick-push provided the overriding ‘what to do’ action, this is similar to a protected FBW aircraft.
However, in FBW aircraft without protection (no automatic recovery action) then pilots even more so seek ‘what to do’ and if the FD apparently provides that, then it is used, probably without consideration of validity or applicability due to expectation.
The governing activity in the choice of what to do is in the mind – what to do, not how to do it physically.

What would an FD show in a stall, almost certainly a pitch up command; returning to the set altitude, vertical flt path, or vertical speed. This assumes that there are few attitude hold systems or if there are that mode is infrequently used.
An airspeed mode (other than with AT control – generally in the cruise), could indicate a reduction in pitch.
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Old 16th Dec 2015, 18:54
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what ever ever happened to the basic idea (I learnt it in 1974 FFS) POWER/THRUST PLUS SENSIBLE ATTITUDE EQUALS PERFORMANCE.Why has this basic law of physics been unlearned?

the flight directors should follow *you*.


Amen to both. I spent years beating into my students the basic FD philosophy: "if it's correct and helping then follow it, if it's not then reprogram it or turn it off."

No.1. How do you know it's correct? In the days before FD's the same computer was between your ears. You interrogated the 'T' and decided if the a/c was doing what you wanted it to. If it was, you left well alone; if not you adjusted whichever parameter needed adjusting and kept scanning the T. Now, in theory, the FD does this for you. But back to the basics; you need to scan the T (less intensely than before)to confirm the FD is correct & sensible. You must not be an FD slave. The best computer is still between your ears.

No.2. B737NG the FD needs to detect an error between CMD input and actual performance. It will then give guidance, in a direction, to correct the error. It will not give a fixed amount of correction. If you let the error progress the FD deviation will increase. Just look at the guys who rotate too slowly or at too high speed. The FD goes very high >18, way above 15, and see what happens to the speed as they follow it. It then pitches down to 15, or even less. If you are smooth and go 'towards' the FD e.g. 16 it will then descend to meet you. Job done smoothly. Same in LNAV SID's. If the turns are severe & you lead the FD for turns by a gnat's cock it will be smooth and not overshoot at all. Ah, but then you need to scan the MAP; heavens above, there's a MAP?

No3. Same on ILS. Scan the raw data LOC/G.S. and you can smooth out the FD PIO and anticipate what it's going to command you. You can correct the error BEFORE it has occurred.

I did stalls with FD's ON & OFF in ALT HLD. At the stall I froze the sim and asked the students what the FD was showing in guidance? Nothing, because it was centred. Ah, but you are about to stall, so is it useful? What is the ATT? Now repeat with F.D's OFF. Ah Ha!

I've always advocated, but never been given the time, that students should learn to fly the basic a/c. Then add the FD, then add the automatics, then add the non-normals. Nowadays the only non FD flying they get is the tick in the box level turns and the tick in the box raw data ILS. That's it. No wonder they don't know what it should be doing with AFDS in use, and thus they can not monitor it. With A/P engaged the FD's will inevitably be centred, but are they correct? Cadets monitor the A/P via the FD's and if they are centred then everything's alright.....as they stall in ALT HLD. Ah Ha! If you want to monitor A/P performance it would be easier with FD's OFF; then you would have to monitor exactly what the a/c was doing.

The future is not what it was and is getting harder to predict. But will standards improve to what they were? It will be for others to find out and attempt to stop the rot.
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Old 17th Dec 2015, 03:19
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Originally Posted by Goldenrivett View Post
vilas,

Are you sure they actually heard the stall warning?
See AF 447 CVR Transcript and note that between the following times that the ALT ALERT "C Chord" was sounding continuously.
2:10:20 - 2:10:51 = 31 secs
2:12:57 - 2:13:55 = 58 secs
2:14:02 - 2:14:20 = 18 secs

It appears that for nearly 2 mins of continuous "C Chord", no one bothered to cancel it.
If you look closely at the "ATC, other voices, warnings, remarks and various noises" column, the crew were saturated with continuous audio warnings.
The continuous "C Chord" was only interrupted by "SV Stall Stall", "Dual Input", "cricket", "sink rate", "pull up" etc.

The BEA have asked if the present stall warning noise alone is adequate warning due auditory overload.

See Final AF447 Report Suggests Pilot Slavishly Followed Flight Director Pitch-Up Commands | News: Aviation International News
"Another reason for having ignored the stall alarm could have been a matter of sheer perception, Troadec said. “Audio alarms are no longer heard in some situations,” he admitted. This has prompted the BEA to recommend the addition of a visual stall warning."

I don't think the crew actually "heard" the stall warning. I think they automatically filtered all those "distractions" out. They needed a specific stall warning attention grabber like a vibrator or buzzer felt through the side stick, not a flight deck saturated with noises like an orchestra warming up.
Completely agreed. The cockpit is quite ergonomic, but I've long had the opinion that Airbus went a step too far when they removed tactile feedback from the control system. They managed to interconnect the rudders? Would it have been prohibitively difficult to connect the sidesticks?

Interesting note about the altitude alerter. I'd have thought that the stall warning would have taken priority over all other audio alerts. At least they knew they were off their altitude...

Last edited by Check Airman; 17th Dec 2015 at 03:29.
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Old 17th Dec 2015, 03:27
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RAT5,

I see what you're getting at there. People eventually stop monitoring the aircraft, and instead start monitoring the FD, which is invariably centred.

I guess the question becomes, for those of you whose training departments encourage raw data flying (no FD, AP or AT), what can the rest of us do to get other training departments on board with that philosophy?
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