B777 - FMA Modes: THR vs SPD
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In a “normal approach” with low airspeed, your AOA is too high and you must reduce it. Period
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@Vessbot: a very detailed and thoughtful analysis but in day-to-day ops the task is either to follow a defined flight path (pitch for path, thrust for speed), or otherwise pitch for speed and thrust for vertical performance.
V/S and FPA modes can be considered as defined flight paths.
Most of us senior aviators find it incredible that none of the three pilots on the flight deck of Asiana 214 noticed the decreasing airspeed until too late.
V/S and FPA modes can be considered as defined flight paths.
Most of us senior aviators find it incredible that none of the three pilots on the flight deck of Asiana 214 noticed the decreasing airspeed until too late.
The failure was at an earlier point in the brain/control loop. By pulling on the stick with XX pounds, he was commanding the airspeed to decrease by that much! Just “monitoring” implies that they failed to notice some external event that might or might not have come, and they had to be on their guard to catch it in case it did. If they lost airspeed due to a sudden downdraft or tailwind shear, that’s monitoring. If they lost fuel due to a leak, that’s monitoring. If the FA called asking to turn the temperature down because the thermostat went crazy, that’s monitoring. If someone called them on 121.5, that’s monitoring.
If they lost YY knots because they were pulling on the stick with XX pounds, that’s not just monitoring. AOA determines airspeed, and they didn’t merely fail to monitor, but rather they caused that airspeed decrease by pulling on the stick and increasing the AOA.
Someone with an understanding of System 2 sees that clearly; while according to System 1, pitch control is merely for flight path and the airspeed loss was a separate incidental event that they would have caught had they just been monitoring better, like the fuel loss or the temperature in the cabin. No, the connection is causal.
And even in regular everyday flight in the middle of the envelope while following a defined fight path, AOA (even though primarily being used for tracking that path) still determines airspeed just the same. And that fact must be embraced like lives depend on it, because they do.
System 1, while simple and easy for an introductory student to see the direct effect of the controls, is not sufficient to truly understand flight.
Last edited by Vessbot; 1st Apr 2020 at 01:39.
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If airspeed is to be increased for a constant flight path, AOA must be reduced. If the latter does not happen, the former will not.
The alternative statement to yours is that if a constant flight path is to be maintained with increasing airspeed, AoA must be reduced accordingly - which I’d suggest is what we see day-to-day when following any defined path (e.g. accelerating in level flight).
in my younger days I used to fly something that went down final on the back side of the drag curve, using AoA as a primary reference. If for some reason you got increasing AoA/bleeding speed close to the ground, reducing AoA as an initial reaction wouldn’t help much - you’d probably just hit the ground with a higher rate of descent. The only effective response was an immediate increase in thrust, which came naturally as it was the normal technique used anyway. Same goes for flying an airliner. If you feel the arse falling out of it at 100’, are you going to lower the nose, or give it a handful of thrust?
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I’m not sure if it’s what you mean to say, Vessbot, but it sort of seems you’re implying that AoA must first be reduced before speed can increase, whereas I’d argue that one goes hand-in hand with the other. (Of course, if you’re far enough on the back side of the drag curve that thrust is no longer adequate, then the only way that airspeed will increase is to get the AoA off first - but that’s not the case with a normal approach.)
The alternative statement to yours is that if a constant flight path is to be maintained with increasing airspeed, AoA must be reduced accordingly - which I’d suggest is what we see day-to-day when following any defined path (e.g. accelerating in level flight).
in my younger days I used to fly something that went down final on the back side of the drag curve, using AoA as a primary reference. If for some reason you got increasing AoA/bleeding speed close to the ground, reducing AoA as an initial reaction wouldn’t help much - you’d probably just hit the ground with a higher rate of descent. The only effective response was an immediate increase in thrust, which came naturally as it was the normal technique used anyway. Same goes for flying an airliner. If you feel the arse falling out of it at 100’, are you going to lower the nose, or give it a handful of thrust?
The alternative statement to yours is that if a constant flight path is to be maintained with increasing airspeed, AoA must be reduced accordingly - which I’d suggest is what we see day-to-day when following any defined path (e.g. accelerating in level flight).
in my younger days I used to fly something that went down final on the back side of the drag curve, using AoA as a primary reference. If for some reason you got increasing AoA/bleeding speed close to the ground, reducing AoA as an initial reaction wouldn’t help much - you’d probably just hit the ground with a higher rate of descent. The only effective response was an immediate increase in thrust, which came naturally as it was the normal technique used anyway. Same goes for flying an airliner. If you feel the arse falling out of it at 100’, are you going to lower the nose, or give it a handful of thrust?
To expand on this (not for you, you seem to be fully familiar especially given your background; but for the thread generally, as I didn’t have much time last post): it comes from the lift equation, which says that:
Lift = CL times V^2 times a few constants that we can ignore.
Since CL is a function of AOA, you can essentially take “CL” to mean “AOA” as a factor that varies under your control of the elevator. So now we’re left with essentially Lift = AOA times V^2.
For a constant flight path, weight is constant and therefore lift is constant. Since lift is constant, and equals two things multiplied by each other, those two things are inversely related. In other words, airspeed and AOA are inversely related; and if one goes up then the other goes down, and vice versa. Every value of AOA, after flight is stabilized, will yield one and only one airspeed (for a given configuration, weight, altitude, etc.)
Like this representative airplane here:
And yes my “arse falling out” response is thrust, which is part of what I’ve been trying to say the whole time. The real question is are you gonna give it a handful of thrust or raise the nose, which is the reaction of the so-called “straight forward” pitch-for-flight-path thought. (If we’re making a forced choice of thrust or pitch... which is not a forced choice in reality)
Last edited by Vessbot; 1st Apr 2020 at 00:52.
Originally Posted by Vessbot
The real question is are you gonna give it a handful of thrust or raise the nose, which is the reaction of the so-called “straight forward” pitch-for-flight-path thought.
I see it often. At 200ft, the @rse falls out of it because of a bit of a hole. On goes the thrust (overriding the ATS), we get lower and lower below the slope going faster and faster, the thrust now kicks in and we're now going too fast and land long. Simple fix: raise the nose then regain the slope! If the ATS doesn't hold the speed, put on some thrust yourself.
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If you like (and you'd have to do the same with system 2) you can put on full thrust. As soon as you approach the normal slope, stuff the stick forward to maintain it. Or would you (system 2) just pull the thrust off a bit after it had kicked in and just hope you end up going where you want to go?
I see it often. At 200ft, the @rse falls out of it because of a bit of a hole. On goes the thrust (overriding the ATS), we get lower and lower below the slope going faster and faster, the thrust now kicks in and we're now going too fast and land long. Simple fix: raise the nose then regain the slope! If the ATS doesn't hold the speed, put on some thrust yourself.
Last edited by Vessbot; 2nd Apr 2020 at 17:35.
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The cause of the Asiana prang was inadequate control of airspeed rather than inadequate monitoring.
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To control speed you have to look at your speed and thrust.They never looked at speed which reached 31kt below Vapp with thrust stuck at idle all along. SFO is a bad example to discuss normal approach. Normal approach within +_10kts or half a dot below or above is straight forward. You pitch for slope and as thrust comes up you will change AoA to maintain it. In airbus FBW the TW couple is not going to take you to GS.
(Yes if you want to also maintain a straight vertical path then you have to consider thrust, and the smoothness and timing of AOA vs. thrust changes, but that is a separate matter.)
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Day in day out people are flying pitch for GS and thrust for speed. If you raise the pitch off course the speed will decrease so thrust will be increased simultaneously unless your speed is high and you want to loose it. SFO they were on verge of stall so let's not talk about it.
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Day in day out people are flying pitch for GS and thrust for speed. If you raise the pitch off course the speed will decrease so thrust will be increased simultaneously unless your speed is high and you want to loose it. SFO they were on verge of stall so let's not talk about it.
First is the pilots’ intent of what the pitch is to achieve, which is something occurring in their heads. Yes day in and day out they are pitching for path, I don’t doubt that.
But, second, is the physical fact that AOA determines airspeed and pulling on the elevator controls airspeed downward. This is true 100% of the time, whether in a minor GS deviation in a normal approach, or what happened at SFO. I stress again that there is no difference in concept, only a difference in degree which is irrelevant. So the SFO event must be talked about, because it is a function of the same laws of flight dynamics, long before they approached a stall.
On a normal approach half a dot low, if the pilot pulls the elevator back a tiny bit to raise the nose a fraction of a degree, his intent is to adjust the path, which will happen successfully; but, simultaneously, the increased AOA is commanding an airspeed reduction. Whether he intends it or not, whether he’s aware of it or not, it is happening. If everything goes normally, he will increase thrust commensurately with the shallower path and return the elevator to the original position, commanding the wing to the original AOA, and the airspeed to the original value. Hunky dory.
But if he doesn’t come in with thrust and continues to pull the elevator back commanding a higher AOA and a lower airspeed, even by 5 knots, that is the SFO sequence. There is no dividing line. Where would such a line be, between 5 knots and 31? 10? 15? 16? No, The SFO sequence was a manifestation of the exact same flight mechanic as normal flight. Every time you pull on the elevator, no matter how slightly and how day-to-day, you are commanding an airspeed reduction and potentially entering that same loop. And to disregard the relevance of the fact that you’re doing that (regardless of that you’re primarily using AOA for something else at the same time), is to make the same mistake that the Asiana pilot did.
So what are you suggesting, Vessbot? Half a dot low, push the power up? Wait a few seconds to see if it was the correct amount? Meanwhile, you've just landed short. In an Airbus (as Vilas pointed out) nothing happens because there is no pitch-power couple.
I wonder if the FO was doing that in this incident?
https://www.flightglobal.com/safety/...137667.article
I agree with Vilas; AOA has nothing to do with this discussion which is now going off on a AOA tangent. Speed is what we see in the cockpit. You pull the nose up a bit, you might have to increase thrust. Might. You pull it up a lot, you will have to. But you don't teach people to fly by using the power to stay on the GS. Maybe in a 737, but not in anything else.
I wonder if the FO was doing that in this incident?
https://www.flightglobal.com/safety/...137667.article
As the freighter approached Runway 05L, it appeared that the altitude was higher than the approach glide path. While the altitude got corrected immediately, at around 137 feet the aircraft again began to fall below the glide path.
Investigations revealed that the co-pilot had failed to maintain the normal speed and rate of descent by utilising pitch control and thrust. He failed to determine his mistake in time, which led to the hard landing before the touchdown point.
Investigations revealed that the co-pilot had failed to maintain the normal speed and rate of descent by utilising pitch control and thrust. He failed to determine his mistake in time, which led to the hard landing before the touchdown point.
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This:
This “yes” is to you:
Well the discussion is about airspeed, and since AOA determines airspeed, it has everything to do with it.
Again, you’re conflating A. whether an elevator input is made with the intention of an airspeed control, with B. whether it physically constitutes an airspeed control. They're two separate questions, and the answer to A is “probably not” while the answer to B is “yes.” And while most elevator inputs will continue to be made with the intention of controlling path and not airspeed (A), my aim is to make pilots aware that B is simultaneously occurring in the background.
B is part of the bare essentials of the reality of flight. And when stress, surprise, and task saturation cut away everything extraneous in the Asiana pilot’s mind, the bare essentials he was left with in his mind, were a mismatch to the bare essentials of the reality of flight, resulting in a fatal crash. I want to add B to the bare essentials in pilots’ minds so this isn’t repeated.
Originally Posted by Vessbot
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.
Originally Posted by Vessbot
I think this is a clear demonstration of the simultaneous inputs being the most proactive and neat: [...] If the initial deviation is glideslope only, then you need a simultaenous thrust change, attitude change, and trim for the TPC.
Originally Posted by Vessbot
We’re on the same page then. I agree with you a hundred percent that [...] a low and on-speed is fixed with simultaneous pitch and thrust;
Originally Posted by Vessbot
Yes, if you’’re doing everything right.
Originally Posted by Capn Bloggs
you raise the nose so you don't hit the ground. You simultaneously put the thrust up (if you need to) to to stop the speed decreasing.
Originally Posted by Capn Bloggs
I agree with Vilas; AOA has nothing to do with this discussion which is now going off on a AOA tangent.
Again, you’re conflating A. whether an elevator input is made with the intention of an airspeed control, with B. whether it physically constitutes an airspeed control. They're two separate questions, and the answer to A is “probably not” while the answer to B is “yes.” And while most elevator inputs will continue to be made with the intention of controlling path and not airspeed (A), my aim is to make pilots aware that B is simultaneously occurring in the background.
B is part of the bare essentials of the reality of flight. And when stress, surprise, and task saturation cut away everything extraneous in the Asiana pilot’s mind, the bare essentials he was left with in his mind, were a mismatch to the bare essentials of the reality of flight, resulting in a fatal crash. I want to add B to the bare essentials in pilots’ minds so this isn’t repeated.
Last edited by Vessbot; 3rd Apr 2020 at 02:10.
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B is part of the bare essentials of the reality of flight. And when stress, surprise, and task saturation cut away everything extraneous in the Asiana pilot’s mind, the bare essentials he was left with in his mind, were a mismatch to the bare essentials of the reality of flight, resulting in a fatal crash. I want to add B to the bare essentials in pilots’ minds so this isn’t repeated.
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I'm not satisfied with that. Monitoring something as if the factors that go into it are a surprise, is not enough when we have an understanding of their cause (especially when that cause stems in our hands). The AOA-airspeed relationship may not be 2+2, but it's not particle physics either. (Actually, it's y=1/x)
Why do you say to leave Asiana alone? Is there some difference between it and a more moderate airspeed loss that makes it inapplicable?
No doubt had someone seen the airspeed indicator they would have noticed the problem. I agree. But I disagree that that's "all they needed to do." They also needed, at a more basic level, to not cause that airspeed loss to begin with... Which entails an understanding, that they lacked, that aft stick force does that.
And I'm trying to understand why you're resisting that. Back to my crashing a car straight into a wall analogy, you're saying that "all they needed to do" was to watch more carefully how close they're getting to the wall, and make a callout; and they'd eventually retire happy without knowing about how mashing the gas pedal accelerates them toward the wall. And that such knowledge would not have saved them. Why? On the contrary, that's exactly what would have saved them, at a stage prior to monitoring. Monitoring is supposed to add an extra layer of safety, not serve as a cover to remove the basic mechanism underneath.
You called it "poor ... skill," but that doesn't explain anything, it just puts a label on what's poor. And I'm explaining what actually constitutes that poor skill, (so that people can actually do something about it instead of merely tsk-tsk'ing over its poorness) but you're saying no.
You scoff at counting nanoseconds like I'm counting the angels on the head of a pin... no, I'm describing the extreme basics, or at least what should be.
I'm not satisfied with that. Monitoring something as if the factors that go into it are a surprise, is not enough when we have an understanding of their cause (especially when that cause stems in our hands). The AOA-airspeed relationship may not be 2+2, but it's not particle physics either. (Actually, it's y=1/x)
I will say again leave Asiana alone. All they needed to do was scan their speed regularly which is a must in every approach, every type of aircraft, with ATHR or manual thrust. If there was a call SPEED when the the speed was Vapp-5 they would have added thrust and landed properly and eventually retired happily without knowing anything about AoA. It was poor knowledge of AFS and Skill. If you think your AoA theory would have saved them sorry I can't agree. We do things simultaneously. Let's not count nano seconds.
Why do you say to leave Asiana alone? Is there some difference between it and a more moderate airspeed loss that makes it inapplicable?
No doubt had someone seen the airspeed indicator they would have noticed the problem. I agree. But I disagree that that's "all they needed to do." They also needed, at a more basic level, to not cause that airspeed loss to begin with... Which entails an understanding, that they lacked, that aft stick force does that.
And I'm trying to understand why you're resisting that. Back to my crashing a car straight into a wall analogy, you're saying that "all they needed to do" was to watch more carefully how close they're getting to the wall, and make a callout; and they'd eventually retire happy without knowing about how mashing the gas pedal accelerates them toward the wall. And that such knowledge would not have saved them. Why? On the contrary, that's exactly what would have saved them, at a stage prior to monitoring. Monitoring is supposed to add an extra layer of safety, not serve as a cover to remove the basic mechanism underneath.
You called it "poor ... skill," but that doesn't explain anything, it just puts a label on what's poor. And I'm explaining what actually constitutes that poor skill, (so that people can actually do something about it instead of merely tsk-tsk'ing over its poorness) but you're saying no.
You scoff at counting nanoseconds like I'm counting the angels on the head of a pin... no, I'm describing the extreme basics, or at least what should be.
