B777 - FMA Modes: THR vs SPD

[rudestuff]

Originally Posted by Check Airman

The age old discussion. You’ll use a combination of pitch and power to get the vertical speed and airspeed that you want.

Exactly this. In a Boeing if I want to descend at 1000fpm I can use V/S (thrust for speed, pitch for r.o.d.) or I can use FLCH or VNAV SPD (Pitch for speed, adjusting thrust for r.o.d.)

In either case the aircraft does the same thing because it's always a combination of pitch and power, the only difference is how you choose to think about it.

[Capn Bloggs]

Originally Posted by rudestuff

In either case the aircraft does the same thing because it's always a combination of pitch and power, the only difference is how you choose to think about it.

Seriously? If you wanted to descend at 1000fpm why you would even think of using FLCH? I know you can, but why?

[rudestuff]

Originally Posted by Capn Bloggs

Seriously? If you wanted to descend at 1000fpm why you would even think of using FLCH? I know you can, but why?

No idea. My point was to show that flight path is defined by pitch AND power.

[maui]

Blogs.

Because it will lock the SPD, which in many areas (not WA) is more important than precise V/S control. Think busy airspace and tight separation.

Cheers Maui

[FlightDetent]

Does this one look like pitch for profile and thurst for speed mis-applied? I am genuinely curious if Q400 does have a relevant pitch-power couple at normal speeds.

Originally Posted by The Colgan report

airplane was at an airspeed of 131 knots. FDR data showed that the control columns moved aft at 2216:27.8 and that the engine power levers were advanced to about 70° (rating detent was 80°) 1 second later.18 The CVR then recorded a sound similar to increased engine power, and FDR data showed that engine power had increased to about 75 percent torque. FDR data also showed that, while engine power was increasing, the airplane pitched up;

Power forward (to add energy enabling certain altitude control) and pitch down (against the flight director) to keep safe flying speed - preferably in the opposite sequence. That is the agreed prevention strategy against re-occurence of that body-count.

I doubt strongly the stall recovery technique for B777 is any different. Sure as hell the A3x0 it is exactly as above. I like to think, within the scope of this thread's discussion, it's better not to unlearn the basics which your passengers might need to survive some rainy night when the pilot will only be reacting instinctively.

The statement "On jet A/C you pitch for profile and thrust for speed (change your habits)" with or without an exclamation mark irks me then.
- It suggests a complete change of perception, removing a vital learned skill. Even for situations where very valid, it sholud only be an overlay finesse how to best manipulate the flight controls.
- It suggests applicability for all jet aircraft but even when valid it does not apply to them all, jets with underslung engines specifically not (unless perfectly path stable which the A320 just isn't).
- It suggests non-applicability for prop aircraft but when valid it does actually apply to them.
- It encourages flight director laziness once a routine builds in.

[AerocatS2A]

You are way overthinking this FlightDetent.

[Vessbot]

This is a complex issue that deserves some thought, but I think the one thing we can all agree on is how far we are from putting the final nail in its coffin

Pitch for flight path and thrust for airspeed is the system that:

• comes intuitively first to mind
• simplest to understand
• most direct in mechanic
• has the quickest reaction (but not always, for thrust for airspeed. This could even act in reverse)

I’ll call this “System 1” from now on for brevity.

Pitch for airspeed and thrust for flight path (System 2, if you will) is the one that is:

• less direct
• takes longer to act (except pitch for airspeed, that is immediate)
• is tougher to understand

All of the above are negatives, but it has one tremendously important thing going for it: it is the more universally applicable one, since it is based on the fundamental flight dynamic relationships that in steady flight A) excess thrust determines climb angle, and B) AOA determines airspeed. These are true regardless of whether you’re talking about prop or jet, straight or swept, clean or draggy, heavy or light wing loading, which way the thrust pitch couple goes, manual or FBW, which control law, or any other distinction.

After all, you can set any thrust setting from idle to full, and with it fly at any airspeed from stall to Vmo/Mmo. Bearing that in mind, I don’t see how one can very confidently say that thrust controls speed . However, if you set one AOA, it will yield one and only one airspeed.

While the above paragraph seems to pose a conundrum for the easy and common sense notions of System 1 (the control inputs don’t do what they’re supposed to!), the astute reader may notice that the conundrum is solved by noting that these System 2 relationships are true in the longer-term, steady-state arena, thus allowing System 1 to act immediately, for quick changes.

(But even in the immediate arena things aren’t so clear-cut for System 1. With a “normal” thrust pitch couple, a thrust increase can yield a quite significant speed decrease, as we’ve explored in so many 737-related discussions.)

Ultimately, both systems must be fully understood, embraced, and used together.

Transitioning from level flight to a climb according to only System 1 (pitching up to go up, then noticing an airspeed loss, and reacting to it with thrust) is sloppy, reactive, and dull-witted. You KNOW the airspeed loss is going to happen, so why not act on it earlier and prevent it by simultaneously increasing thrust? Using only System 2 might be even sloppier. Increasing thrust by itself to inject yourself into the middle of a phugoid (thrust causes a speed increase, which causes a lift increase, which makes you go up, which causes a speed loss, which overshoots and then does everything again but with loss instead of gain) it will take many cycles over many minutes (if at all) to settle down in the climb, and the result would obviously be atrocious.

But a simultaneous combination of thrust increase, along with an attitude increase with the practiced use of trim to lock it into the new attitude (which itself is known from experience) is the way to go. But even this isn’t so clear-cut. Even something as simple as raising the nose, might not involve pulling back on the stick at all, depending on the thrust pitch couple. My current plane has a “reverse” one (thrust gives you nose down, a source of displeasure to me) but back when I was flying a normal one, I took great pleasure in the elegance (which I define as accomplishing what you want with the fewest inputs possible) of starting a climb by increasing thrust slowly (so as to pitch up via the TPC and inject myself into only a mild phugoid), and giving a well-timed blip of nose-down trim. That’s all it took! I started the climb without ever pulling back or trimming nose-up. And if done well, it would go straight to the new attitude and climb angle, at the original airspeed, with no overshoots of anything.

An even more open-and-shut example against only System 2, is being on the glideslope. Even though I’m one of the most outspoken proponents of System 2 awareness and usage, it’s obvious you can’t simply nose down to get speed. (You’ll get speed allright, but...) In this case a System 1-only response is also inappropriate, but more mildly so. If you only increase thrust, it’ll soon lead to a glideslope deviation, which if then corrected, is sloppy and reactive (it’ll then cause a speed deviation, which if then fixed with only thrust puts you back to step 1, in an ongoing cycle of unnecessary deviations and corrections). I think this is a clear demonstration of the simultaneous inputs being the most proactive and neat: If the initial deviation is only a speed one, then the fix is an increase in thrust along with a decrease in AOA, which requires a trim composed of a nose-down component for the AOA, and a component for the thrust pitch couple which could go either way. In my plane, for example, the TPC component dominates. So if I need to speed up a bit, I give it some thrust and some nose-up trim. If the initial deviation is glideslope only, then you need a simultaenous thrust change, attitude change, and trim for the TPC.

Now I’ve gone over (in possibly too much detail) a few different examples in regular flight, but the greast importance of System 2 comes at the edges of the envelope and emergencies. In AF447 or Pinnacle 3701, where there was no excess thrust but they were trying to get altitude, everybody’s fate was sealed by the pilots’ brains being locked into System 1, and treating the stick as the go up/go down control. Whether System 2 was ever even mentioned in their earlier training, (at best it was probably treated as a passing curiosity and never taken seriously) years and years of everyday flight in the middle of the envelope, using the stick as the go up/go down control, wired up their reflexes to only think of it that way. So when the emergencies unfolded and their field of vision shrank to a soda straw, eliminated their capacity of any deliberative thinking, and allowed them to act as only automatons by their wired-in reflexes, there was then no realistic hope of anything happening other than what happened. The going got tough, and they did the only thing they knew in their gut: they wanted to go up so they pulled on the stick.

These accidents will continue to happen, I see no realistic way around it. One way, that’s not realistic, is what I would do if I was emperor of the world: nobody would be allowed to touch a powered plane without some number of hundreds of hours in a glider first. Hopefully then primacy might lock in the correct response to any low airspeed problem, that the immediate way to safety is stick forward and nose down. (The increased thrust to counter the descent is only the cherry on top, that might or might not be available.)

[Capn Bloggs]

Very well written, Vessbot.

[vilas]

but the greast importance of System 2 comes at the edges of the envelope and emergencies. In AF447 or Pinnacle 3701, where there was no excess thrust

These are not examples of the edge of envelope but well beyond it. You don't follow a procedure of stall recovery for normal approach. Whenever one is tracking a flight path it is maintained by pitch and speed managed by thrust. It's straight forward. When thrust is constant whether at idle or climb you are not tracking a flight path but maintaining a certain speed so obviously it has to be maintained by pitch.

[Capn Bloggs]

Vilas, well said.

For those of you advocating/thinking of using System 2/secondary effects of controls for normal operations, have a read through these incidents, both involving use of/demonstrating the trap of using FLCH on an approach:

https://www.atsb.gov.au/publications...r/ao-2011-086/

https://www.atsb.gov.au/publications...r/ao-2007-055/

Maybe they thought they'd get more precise speed control...

[maui]

Bloggs



I do not believe anyone has advocated use of FLCH on approach. Descent yes, approach no. In fact, in my recollection, the only person that has even mentioned it in an approach context, is you.



The Boeing 777 FCTM refers to it thus: (my bolding and underlines)

Non-ILS approaches are normally flown using VNAV, V/S, or FPA pitch modes.

The use of FLCH is not recommended after the FAF. Recommended roll modes

are provided in the applicable FCOM procedure.

The cases you cited are not relevant. Both were related to standards / training, rather than “normal ops”.

BTW you still haven’t answered my earlier question about the existence or otherwise of IAS mode in your 717 AFCS.

Cheers. Maui.

[Vessbot]

Originally Posted by vilas

These are not examples of the edge of envelope but well beyond it. You don't follow a procedure of stall recovery for normal approach. Whenever one is tracking a flight path it is maintained by pitch and speed managed by thrust. It's straight forward. When thrust is constant whether at idle or climb you are not tracking a flight path but maintaining a certain speed so obviously it has to be maintained by pitch.

I wrote some subtle and detailed things in that post, that you seemed to skip right over to argue against something I didn’t say. What I did say about a normal approach is in the paragraph that starts with “An even more...”

But if subtleties aren’t the order of the day, I’ll say something blunt here instead:

In a “stall recovery” your AOA is too high and you must reduce it. Period.

In a “normal approach” with low airspeed, your AOA is too high and you must reduce it. Period.

(Also it is very much not “straight forward” that speed is managed by thrust, whether you’re tracking a flight path or not. If it was, then if your speed was low then you could straight forwardly fix it by simply increasing thrust. And that is simply not the case. I remember vividly one event flying with a student I’d flown with for years, at a new airport in a slightly stressed situation. His airspeed got a little low, I prodded him “watch your airspeed,” and in a startle he simply added some throttle, and we lost more airspeed. That’s when I did a Captain Picard double facepalm and lost some hope that everybody can be really taught to fly.)

[tttoon]

Pitch for speed and thrust for altitude can in certain situations delay the re-stabilization of the aircraft due to an incorrect initial action. Take, for instance, an aircraft that is high and fast on approach. If pitch for speed is used, the aircraft will be pitched up and thrust will be reduced. As the aircraft slows down, it becomes more efficient, and the descent rate will be (much) less than in the other case, delaying the stabilization of the approach. If, on the other hand, the aircraft is pitched down towards the glidepath and the thrust is reduced, the speed will remain higher (but below the applicable placard speed) and as a result the drag will be higher. The descent rate will be higher and a stable approach will be achieved in the shortest possible time.

Pitch for speed etc. is a simplification that may work for small aircraft (and delta wings, but they are a special case), but if you think about controlling the aircraft as a form of energy management (the amount and balance of kinetic and potential energy) one thing that becomes clear is that you will initially always need to pitch towards your desired flightpath, and then adjust thrust for speed. This assumes that you are in control of the aircraft, so not in a stall.

[FullWings]

Vessbot, I like your explanations a lot.

I think this subject causes confusion, as you rightly point out, if someone doesn’t have a complete understanding of static vs. dynamic, primary and secondary effects of controls, flight envelopes, AoA, performance limitations, energy management and path constraints. Quite a list and it helps to be at least slightly mathematically minded.

As someone who started flying in gliders then went into power, when introduced to the “power” method of getting out of a stall I was . Later, we changed to the more sensible option of reducing AoA before doing anything else.

Going back to the original thread title, understanding what each FMA annunciation implies and how the systems interact in various phases of flight is essential knowledge but generally buried deep in multiple sections of the FCOM. Well worth reading and digesting at a time of low workload...

[Vessbot]

Originally Posted by tttoon

you will initially always need to pitch towards your desired flightpath, and then adjust thrust for speed.

You are advocating that if low and correcting up then you should lose speed first, and then increase it back to the target value. Why?!

The necessity of thrust to fix the vertical flight path is primary (in importance, though not necessarily sequence) as, again, excess thrust dictates climb angle.

Had the pilots of Asiana 214 really grokked that, they might have come in with thrust as they sank further and further below the glideslope, instead of leaving it relegated to the back of their minds as some side effect to clean up later once the main problem had been solved (and then, in actuality, forgotten). No, they pulled more and more on the stick (ending up with dozens of pounds of pull force, IIRC) not realizing that by doing so they were commanding a decrease in airspeed. (As, again, AOA dictates airspeed).

Back when I was teaching I remember flying with countless of other people’s students checking them out in new types (along with a few other factors such as that it’s tailwheel, minimal instrumentation, etc., that took them out of their comfort zone and loaded up their task capacity) and with regularity would watch their flight path fall off toward a point short of the runway, and then watch their response of pulling up their nose without power, which after the first few seconds’ initial (and temporary) success of pulling up the flight path, (and thereby tricking them into thinking the job was done) the induced drag at the lower speed would increase and the flight path would consequently fall off even further, then they’d respond with pulling the nose higher, feeding back into the same loop of higher AOA -> lower speed -> more induced drag -> more sinking tendency -> higher AOA. Each of these was a mini Asiana 214 in the making. And each of these could have been avoided with a thorough and early understanding of System 2 and that thrust is the primary control of vertical flight path.

Let’s consider the consequences of a stressed pilot initially forgetting one or the other (of thrust vs. pitch). If they forget thrust, the consequence is described in the above paragraph.

If they forget pitch, the consequence is much less severe. If the plane has a normal TPC, then the low plane would pitch up, beginning the glideslope correction with the same input. May be sloppy, but can be cleaned up later, and not dangerous. If it’s a reversed TPC, then the plane would start pitching down, which would surely catch the pilot’s attention and he’ll then pitch it up. Sloppy and slightly more dangerous than the normal TPC, but far less dangerous than if they forgot thrust. And it’s far less dangerous because total energy is gained which increases the maneuvering ability, thus increasing their available time to react (whether this ability is used toward a re-stabilization, or toward a goaround).

[tttoon]

You are advocating that if low and correcting up then you should lose speed first, and then increase it back to the target value. Why?!

I'm not saying that at all, a pitch up to correct the flight path should be coupled with an increase in thrust to anticipate the increased drag due to the pitch increase if needed. Would you say that if low and fast the correction should be to add thrust and pitch up? That will result in the aircraft getting back on G/P, still fast, and closer to the runway, opening the door for a whole range of other threats. An incorrectly trimmed aircraft will still plow into a field with any amount of excess thrust.

Any pilot should have at least an underlying reflex to increase thrust when they pull back on the tick, and the opposite for the reverse. They should be aware of the energy state of their aircraft and apply the required thrust after making the required pitch change.

To be clear, what I'm describing is the normal way (in my opinion) of controlling the aircraft, and is not applicable for stall recovery, where control is lost and the FCTM recovery should be applied.

Unfortunately, as seen in the Asiana incident and similarly in the botched EK go-around, over-reliance on automatics has removed those reflexes from many pilots. Couple that with a bad understanding of AFDS modes and equally bad monitoring of FMA changes and you have an accident.

[Vessbot]

Tttoon,

We’re on the same page then. I agree with you a hundred percent that a low and fast is fixed by a pitch-up alone, and that a low and on-speed is fixed with simultaneous pitch and thrust; and I agree a thousand percent that everyone should have a reflex to adjust thrust with pitch.

But I’ll add that this reflex is lost not only due to automation overuse, but was never developed in the first place in many people, including all the students I mentioned in my last post, most of whom had only bugsmasher experience. I think two main factors go into that together.

First, there is instinct from non-aviation life, where you point the vehicle (or your human body) in the direction it is to go. It’s common sense. Then when someone starts learning to fly, there is the much more direct-acting and easy-to-understand System 1, which this previous instinct slots right into. If you’re headed a bit short of the runway, it’s extremely easier to make the mental connection from pointing your nose up a bit to the flight path correspondingly going up that same bit, than to abstract heady stuff like total energy rate and excess thrust and AOA and all these things that it takes to really understand System 2.

So the above is instinct, and the first urge of what control to react with. The other aspect is which problem is more apparent. The flight path (associated with pitch in System 1) has a big visual cue, and is much more apparent than airspeed loss, which, for small and moderate amounts, is just a number on a gauge (out of the main vision field) and some small amount of pull force on the stick, which is very easy to miss for an unfamiliar pilot (or a familiar one who’s stressed and death-gripping the stick of his low-stick-force bugsmasher... or even his high-stick-force 777). For a task-saturated pilot, the perception of the flight path falling off takes up all the mental space and doesn’t leave any to share with the perception of the airspeed falling off.

When you combine these two factors, the flight path is seen and takes up all the attention of what needs fixing; and then the common-sense System 1 fix of using pitch is the one that comes in as a reflex, forgetting the thrust. And this is how we enter the loop of all the mini Asiana 214’s from my last post. System 1 edges out System 2 from both ends of the situation: the perceived problem, and the mentally available solution.

System 1 is in no danger of being forgotten. This is why I’m such a huge proponent of System 2 thinking taught from the get-go as primacy: not to overshadow System 1, but rather to bring it (System 2) from itself being overshadowed, up to some level of parity. It is not reflexive, it is not common sense, it takes some brainwork to really understand, and anything with these qualities needs all the help it can get to be recalled and used when necessary. And it’s when it’s the most necessary, that it’s furthest from easy recall due to mental load.

[Vessbot]

And to address your high and fast example from a previous post: Yes you have to pitch down initially. Pitching up and flying even higher above the intended path would be senseless. But the AOA is still increased. It’s just a a slow increase, corresponding with the airspeed bleedoff rate. An immediate increase would result in a balloon, same as a flare with excess speed and a pre-programmed stick pull (that would have been correct for a lower entry speed).

When you’re talking about these tings, you attach labels like “pitch for airspeed” that represent more subtlety about rates, etc., than can really be contained in 3 words. Also that by “pitch” we really mean AOA rather than directly pitch. But the elevator controls AOA and we sometimes use “pitch” as a synonym for “elevator usage” and there you have some confusion.

[maui]

Vessbot.
I agree with all you have said but jeez I didn't realise it was so complicated. I always thought it was point and push or pull as appropriate.

The one thing though that is missing from your analysis, is any mention of the importance of a good scan. Without it, all talked about above, will add to the complexity of the operation. With it, all becomes manageable.
Do like the "death grip" reference, which along with deficient scan, are the two principal causes of manipulative difficulties.

Cheers. Maui

[Vessbot]

Originally Posted by maui

Vessbot.
I agree with all you have said but jeez I didn't realise it was so complicated. I always thought it was point and push or pull as appropriate.

The one thing though that is missing from your analysis, is any mention of the importance of a good scan. Without it, all talked about above, will add to the complexity of the operation. With it, all becomes manageable.
Do like the "death grip" reference, which along with deficient scan, are the two principal causes of manipulative difficulties.

Cheers. Maui

​​​​​​Completely agreed with the good scan. Definitely can't process or act on the relevant information, unless you've perceived it to begin with. Cheers!

​​​​​​