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.)