How Does Autopilot Determine Rate of Climb / Decent

[G550flyer 1,014]

2 minutes ago, Bob Scott said:

"Safety-Comfort-Reliability"...SAMFOX, baby!!  😉

Yes Sir!!!😉

[prolixindec 17]

Just to be clear, passengers only sense acceleration, not speed. So the aircraft could climb at 5000 fpm and as long as the rate of climb is steady, the passengers will sit in comfort at 1g and will have no sense whatsoever of the vertical speed. 

It's just like horizontal speed, which is more intuitive (for some reason). Sitting in your seat at 500 kts feels indistinguishable from sitting in your seat at 0 kts on the ground. 

If the vertical speed pointer is flapping about wildly, sure, the passengers will feel that, because of all the acceleration. But if your VS gauge smoothly goes up to 5000, and then smoothly back down again, the passengers will have no idea. 

And yes, big vertical speed values are common in jet aircraft. No cause for alarm.

Edit: While we're on the topic, you can go down at -2500 fpm at 1g, just saying. You'd feel less than 1g if the vertical speed transitions briskly to -2500, but your body wouldn't sense it if the transition is smooth. Once you're established at -2500 fpm, you're back to 1g again and your body would feel like you're not moving at all. You can only sense acceleration, not speed. Put another way, your body senses changes in speed (acceleration) but your body has no way to sense steady-state speed in any direction. If the wings fall off and you start falling, you'd feel that, because falling things accelerate. 

Edited February 24 by prolixindec

[psychedelic_tortilla 170]

If ATC keeps you high and fast (looking at you, Stansted), descent rates of up to -5000 fpm are absolutely nothing out of the ordinary.

As @prolixindec explained, our "motion sensors" in our ears respond to changes in force (well, pressure to be exact), which is mass times acceleration. So if there is no acceleration (i.e. a steady climb/descent rate), we can't perceive speed (other than visually by comparison with a reference, but that isn't given in an aircraft, either).

[Bob Scott 7,721]

12 hours ago, prolixindec said:

Just to be clear, passengers only sense acceleration, not speed. So the aircraft could climb at 5000 fpm and as long as the rate of climb is steady, the passengers will sit in comfort at 1g and will have no sense whatsoever of the vertical speed.

You can only sense acceleration, not speed. Put another way, your body senses changes in speed (acceleration) but your body has no way to sense steady-state speed in any direction.

The 6 degree-of-freedom motion systems in high-end simulators take advantage of the human somatosensory system to, in essence, "trick" the body into an artificial sense of fore/aft acceleration by tilting the motion platform to change the direction of the 1g gravitational vector.  People notice and in fact do sense what their bodies interpret as vertical speed when an aircraft climbs or descends, not because the acceleration changes, but because the direction of that 1g gravitational vector is not straight down through their butts when seated.  Hence the age-old flying term "seat of the pants"...

Many people don't find large deck angles comfortable.  Even if it isn't uncomfortable, to most it still does not feel natural.  Since (usually) a high rate of climb or descent in a fixed-wing aircraft is accompanied by a significant deck angle (pitch), a pilot trying to maximize comfort will try to avoid high deck angles (up or down) when not necessitated by safety or operational considerations--and that means limiting vertical speed in a climb/descent when possible, as well as managing configuration/speed to accomplish the same outcome.

[Luis Hernandez 570]

12 hours ago, prolixindec said:

Just to be clear, passengers only sense acceleration, not speed. So the aircraft could climb at 5000 fpm and as long as the rate of climb is steady

If the pressurization system is good enough and keeps a low cabin altitude V/S. Otherwise, pax will definitely notice it in their ears.

[prolixindec 17]

There are a lot of cues that you can sense that suggest motion: the view out the window, the deck angle, the rotation of the aircraft around its various axes, acceleration in any direction, changes in cabin pressure, the direction of the gravity vector, and wind in your face (if a door panel blows off). All of these are present to some extent, except the wind part--usually.

But there's no way your body can tell you whether you vertical speed is +50 or +5000 fpm.

There isn't any reason to avoid any particular vertical speed for comfort purposes, because you can't sense that. You do want to keep the deck angle in the comfort zone, and you want to avoid abrupt maneuvers, and you want to manage the pressurization, (and keep the door panels on), but a lightly loaded jet can still climb in the 1000s of fpm doing all of that, and it's perfectly fine. 

People shouldn't make a mental connection between the vertical speed and comfort. Instead, associate comfort with things you can sense, like deck angle, smooth movements of the controls, pressurization, and so on.

Edited February 24 by prolixindec

[Bob Scott 7,721]

3 minutes ago, prolixindec said:

There are a lot of cues that you can sense that hint at motion: the view out the window, the deck angle, the rotation of the aircraft around its various axes, acceleration in any direction, changes in cabin pressure, the direction of the gravity vector, and wind in your face (if a door panel blows off). All of these are present to some extent, except the wind part--usually.

But there's no way your body can tell you whether you vertical speed is +50 or +5000 fpm. 

In normal airliner ops, the cabin pressure changes gradually, the rotations and accelerations into and out of climbs are gradual, the deck angle is kept comfortably low, and so on. Sure, you can feel it in the seat of your pants if there's a change, we all know that, but original question was whether a 5000 fpm climb was something to avoid for passenger comfort. The point I'm making is that there isn't any reason to avoid any particular vertical speed for comfort purposes, because you can't sense that. You do want to keep the deck angle in the comfort zone and you want to avoid abrupt maneuvers, but a lightly loaded jet can still climb in the 1000's of fpm doing all of that, and it's perfectly fine. 

So you shouldn't make a mental connection between the vertical speed and comfort. Instead, associate comfort with things you can sense, like deck angle, smooth movements of the controls, pressurization, and so on.

Well, I fundamentally disagree...this is a semantic argument that loses sight of the forest for the trees, and disregards the practical reality of how a pilot actually manages flight parameters. 

There's a clear physical relationship between vertical speed and deck angle when flying normal climb or descent profile airspeeds.  I don't believe there exists a fixed-wing passenger aircraft that can climb at 5000 fpm without the pointy-end aimed significantly skyward--in other words, with a high and uncomfortable deck angle.  When the pilot lowers the nose (IOW reduces the deck angle) the result is a lower vertical speed.  Conversely, when you climb in selected vertical speed mode and reduce the target VSpeed, the result is the nose is lowered--a lower, more comfortable deck angle.  Do you propose some other means of controlling deck angle?

So yes, I advocate avoiding high vertical speeds during climbs and descents where possible in the interest of pax comfort, because there absolutely *is* a connection between vertical speed and deck angle, and by extension pax comfort.

[prolixindec 17]

I'm saying you should manage the passengers' comfort by looking at the attitude indicator, not by looking at the VSI. 

[Bob Scott 7,721]

21 minutes ago, prolixindec said:

I'm saying you should manage the passengers' comfort by looking at the attitude indicator, not by looking at the VSI. 

And I'm saying that managing the VSpeed has the effect of managing the attitude.  The two are not independent of one another. 

I know that if I select a VS of 3000 fpm or less, it will result in an acceptably moderate deck angle for a climb.  And I know that if I were to set an acceptably moderate deck angle (e.g. hand-flying in a FBW acft like a Scarebus), it would result in a VSpeed of less than 3000 fpm.  Either way, if I am climbing at 5000 fpm, it's gonna be accompanied by an excessive deck angle, and it will be out of the comfort zone.  Both high attitude and high VSpeed are valid and complementary indications, and directly controlling either attitude or VSpeed indirectly controls the other.