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Is there a ceiling to the amount of force exerted on a plane’s control surfaces or does every human have a limited range of what situations they could wrestle a

Is there a ceiling to the amount of force exerted on a plane’s control surfaces or does every human have a limited range of what situations they could wrestle a plane out of? I’ve got to imagine that put into a fast enough dive, nobody’s pulling up?

WalterBright 1y ago
The "feel forces" exerted by the pilot are taken into account for the maximum ability to control the airplane. A Boeing airliner has a travel limiter on the control surfaces that is automatically adjusted based on the speed. Exceeding that travel puts enough torque on the airplane to literally tear the structure apart. There was an Airbus incident years ago where the pilot stamped on the rudder pedals to violently move the rudder full travel lock to lock, and it tore the rudder vertical stabilizer off. There are also situations where no amount of control input is going to wrest control back from an uncontrolled dive. In other words, designing the "feel forces" and travel has a large number of variables to it, and is not simple at all to get right. For a dive fast enough, you run into an effect called "separation". This is where the air flowing over the wing no longer converges at the trailing edge, but separates. This separation can engulf the elevators such that even at full travel, the elevators are operating in dead air and from then on, you're just along for the ride into a crash. This is a problem for many, many aircraft designs. You always gotta watch your airspeed. This happened to a 727 once, where the autopilot went berserk and sent the airplane in a dive so bad that separation happened. The quick thinking pilot decided to lower the landing gear in a desperate attempt to slow the airplane down. He succeeded, although the slipstream tore the doors off and bent the gear backwards, and saved everyone. It also happened to a 707, where the pilots weren't paying attention and the airplane slipped into a dive. By the pilots realized what had happened, they no longer had pitch control. One of them desperately began cranking the trim wheel by hand, and they managed to pull it up just before it hit the ocean. Yeager's X1 rocket aircraft had this problem, as did the F-86 jet fighter. For another fun anecdote, the F-80 Shooting Star had a problem in that its engine was too powerful. If you oversped the airplane, it would go unstable and pitch up, tearing the wings off. That meant the pilot always had to have one eye on the airspeed (yes, my dad was an F-80 pilot, and flew them in combat). One day, a pilot had a Mig on his tail he couldn't shake. He thought, I'm dead anyway, let's see if I can pitch up and lose the Mig. So he firewalled the throttle, and sure enough the F-80 went through a violent maneuver and miraculously the wings stayed on and he shook the Mig off his tail. When he landed, the wings were bent up and the airplane was scrapped. Dive bombers used "dive brakes" to keep from overspeeding the airplane. My dad's favorite method for attacking an anti-aircraft gun emplacement was to dive straight down on it. AA gunners did not want to fire vertically (think about it). But it required my dad to have one eye on the target, one eye on the airspeed, and his third eye on the altitude.
- PNewling 1y ago
  These are all incredible stories. I appreciate you sharing them with us. Could you link-to/tell-us more about the 727 affair? I've always had a fascination with autopilot (and autoland) systems, so I would love to read/hear more about this incident.
  - WalterBright 1y ago
    That's all I know about it. I was told the tale by my lead engineer at Boeing, Erwin Schweitzer. One of the great things about working with such engineers is all the amazing stories. You won't find much information on this incident or the "mach tuck" one because they didn't crash. But you might try googling "727 autopilot landing gear dive".
  - WalterBright 1y ago
    > These are all incredible stories. Thank you. I enjoy relating them. There are other airplane junkies who come to our monthly D Coffee Haus meeting, and we often wind up talking shop about airplane engineering.
- nayuki 1y ago
  > This separation can engulf the elevators such that even at full travel, the elevators are operating in dead air and from then on Does this refer to the deep stall that affects T-tail aircraft ( https://en.wikipedia.org/wiki/Stall_(fluid_dynamics)#Deep_stall https://en.wikipedia.org/wiki/Stall_(fluid_dynamics)#Deep_st... ), or is it a phenomenon that applies to nearly all fixed-wing subsonic aircraft?
  - WalterBright 1y ago
    Great question! I don't know the answer. The separation affects the ailerons as well, so you lose roll control.
  - potato3732842 1y ago
    Comparable physics, different route to the same effect. It has to do with air compression, speed of sound, etc, shielding the control surfaces and the shape of that effect around the plane as you increase speed. IDK if it has a name. Any youtube explanation of supersonic flight should cover it. Most higher speed subsonic aircraft recycle the design principals from supersonic aircraft to alleviate the problem. You could on paper dive a Learjet or 747 or other "designed for the higher end of subsonic" aircraft to Mach 2 and retain control authority if you wanted and if the airframe could take it.
unsnap_biceps 1y ago
Mentour Pilot did a video on the second MCAS crash where they basically crashed due to being unable to pull up. MCAS changed the trim to a level where the pilots were pulling with a recorded 180 lbs pull weight but still couldn't hold the nose up https://www.youtube.com/watch?v=DcB31RgeL50 https://www.youtube.com/watch?v=DcB31RgeL50 is the full video, well worth watching IMHO, but the talk about the fatal trim is at 51:00 to 53:00
- WalterBright 1y ago
  The thumb switches would have pulled the nose up. But the pilots didn't read the instructions, and turned off the stab trim system with the airplane nose down. They also had the engines at full throttle, and the increased speed made it impossible to hand crank the stabilizer. The instructions were: 1. restore normal trim with the thumb switches 2. turn off the trim system
  - throwaway5752 1y ago
    Surely you know this, but MCAS was a badly designed system that failed in a fatal manner in the event of malfunctioning non-redundant sensor (10s of failure events per year). Boeing knew this, but allowed it kill everyone aboard two flights. Documentation in the instructions or not it had no business being on a commercial aircraft, ever.
    - signatoremo 1y ago
      It still exists today after modification that requires input from both AOA sensors. Why do you think it’s badly designed? The implementation was badly done for sure.
      - filleduchaos 1y ago
        Design and implementation are not completely distinct things, though. If e.g. a software company's backups were implemented solely as daily dumps to the same SSD the database was running on, and then they implemented a more sane backup solution after the practically-inevitable data loss, would we just go "well the implementation was badly done" or would people not call it out as bad systems design? Some designs are so poorly thought through (and/or conceived for the wrong reasons to begin with, like MCAS was) that they cannot help but yield poor implementations. (Also, input from one or two sensors was far from the only problem with MCAS' implementation. The fact that it could activate multiple times and that it could trim far harder and faster than human pilots could manually trim were equally contributors to making the saga a tragedy).
Merad 1y ago
Generally speaking these days I think that small planes with manual controls (i.e. your private pilot's Cessna) just aren't capable of going that fast. Most of the planes that will be able to go that fast in a dive will have hydraulically boosted controls if not full fly by wire systems. However this was a real problem during WW2, and high speed dives killed more than a few pilots. By 1942-43 most fighter planes were able to dive fast enough to get into trouble (500+ mph/800 kph) but weren't designed for it at all. The aerodynamics of supersonic flight just weren't understood at the time. In some cases the design of the plane made it essentially impossible to pull out once you went beyond a certain speed, all you could do was attempt to slow down enough to regain control. The P-38 Lightning was notorious for this. I don't have any specific sources handy but I recall reading accounts of things like pilots bracing their feet against the control panel trying to pull the control stick with all their strength, taking extreme measures like lowering the landing gear to slow down (risking it being ripped off and maybe tearing the whole plane apart), or only surviving due to sheer luck - sometimes the thicker air at lower altitudes would slow them down enough before it was too late.
dlcarrier 1y ago
A 737 can be flown by hand, without hydraulics or electricity, despite weighing over 50x more than a Cessna 172, or other small manually controlled airplanes. Airplanes' control surfaces have have smaller control surfaces inside them called tabs. Just as a small movement in a control surface is amplified into the larger movement of the entire airplane, a control tab's even smaller movement is amplified into the movement of the rest of the control surface, which in turn is amplified into the movement of the entire airplane. Here's a good descriptions of different types of control tabs: https://www.boldmethod.com/learn-to-fly/systems/4-types-of-trim-tabs/ https://www.boldmethod.com/learn-to-fly/systems/4-types-of-t... Many small airplanes, and pretty much every large airplane, effectively use tabs as a gross adjustment and the yoke/rudder as a fine adjustment, so under normal operation there shouldn't be large forces on the yoke or rudder. The only time it takes a lot of force to control an airplane is when the tab can't be easily controlled. Flying even a small Cessna 152 out of trim can take tens of pounds of force and be impossible to sustain for long periods of time. I once tried flying a 152 from one airport to another, only a few minutes away, without adjusting the elevator trim, and my arms were sore the next day. Going into too fast of a dive has two much larger dangers, one is that the aircraft can break up from the aerodynamic forces, and the other is that the air becomes turbulent near the trailing of of a control surface, making it no longer able to control the aircraft. When the later happens, the controls don't take much force to move, but they also don't do much.

WalterBright 1y ago

The "feel forces" exerted by the pilot are taken into account for the maximum ability to control the airplane. A Boeing airliner has a travel limiter on the control surfaces that is automatically adjusted based on the speed. Exceeding that travel puts enough torque on the airplane to literally tear the structure apart. There was an Airbus incident years ago where the pilot stamped on the rudder pedals to violently move the rudder full travel lock to lock, and it tore the rudder vertical stabilizer off. There are also situations where no amount of control input is going to wrest control back from an uncontrolled dive. In other words, designing the "feel forces" and travel has a large number of variables to it, and is not simple at all to get right. For a dive fast enough, you run into an effect called "separation". This is where the air flowing over the wing no longer converges at the trailing edge, but separates. This separation can engulf the elevators such that even at full travel, the elevators are operating in dead air and from then on, you're just along for the ride into a crash. This is a problem for many, many aircraft designs. You always gotta watch your airspeed. This happened to a 727 once, where the autopilot went berserk and sent the airplane in a dive so bad that separation happened. The quick thinking pilot decided to lower the landing gear in a desperate attempt to slow the airplane down. He succeeded, although the slipstream tore the doors off and bent the gear backwards, and saved everyone. It also happened to a 707, where the pilots weren't paying attention and the airplane slipped into a dive. By the pilots realized what had happened, they no longer had pitch control. One of them desperately began cranking the trim wheel by hand, and they managed to pull it up just before it hit the ocean. Yeager's X1 rocket aircraft had this problem, as did the F-86 jet fighter. For another fun anecdote, the F-80 Shooting Star had a problem in that its engine was too powerful. If you oversped the airplane, it would go unstable and pitch up, tearing the wings off. That meant the pilot always had to have one eye on the airspeed (yes, my dad was an F-80 pilot, and flew them in combat). One day, a pilot had a Mig on his tail he couldn't shake. He thought, I'm dead anyway, let's see if I can pitch up and lose the Mig. So he firewalled the throttle, and sure enough the F-80 went through a violent maneuver and miraculously the wings stayed on and he shook the Mig off his tail. When he landed, the wings were bent up and the airplane was scrapped. Dive bombers used "dive brakes" to keep from overspeeding the airplane. My dad's favorite method for attacking an anti-aircraft gun emplacement was to dive straight down on it. AA gunners did not want to fire vertically (think about it). But it required my dad to have one eye on the target, one eye on the airspeed, and his third eye on the altitude.

PNewling 1y ago
These are all incredible stories. I appreciate you sharing them with us. Could you link-to/tell-us more about the 727 affair? I've always had a fascination with autopilot (and autoland) systems, so I would love to read/hear more about this incident.
- WalterBright 1y ago
  That's all I know about it. I was told the tale by my lead engineer at Boeing, Erwin Schweitzer. One of the great things about working with such engineers is all the amazing stories. You won't find much information on this incident or the "mach tuck" one because they didn't crash. But you might try googling "727 autopilot landing gear dive".
- WalterBright 1y ago
  > These are all incredible stories. Thank you. I enjoy relating them. There are other airplane junkies who come to our monthly D Coffee Haus meeting, and we often wind up talking shop about airplane engineering.
nayuki 1y ago
> This separation can engulf the elevators such that even at full travel, the elevators are operating in dead air and from then on Does this refer to the deep stall that affects T-tail aircraft ( https://en.wikipedia.org/wiki/Stall_(fluid_dynamics)#Deep_stall https://en.wikipedia.org/wiki/Stall_(fluid_dynamics)#Deep_st... ), or is it a phenomenon that applies to nearly all fixed-wing subsonic aircraft?
- WalterBright 1y ago
  Great question! I don't know the answer. The separation affects the ailerons as well, so you lose roll control.
- potato3732842 1y ago
  Comparable physics, different route to the same effect. It has to do with air compression, speed of sound, etc, shielding the control surfaces and the shape of that effect around the plane as you increase speed. IDK if it has a name. Any youtube explanation of supersonic flight should cover it. Most higher speed subsonic aircraft recycle the design principals from supersonic aircraft to alleviate the problem. You could on paper dive a Learjet or 747 or other "designed for the higher end of subsonic" aircraft to Mach 2 and retain control authority if you wanted and if the airframe could take it.

unsnap_biceps 1y ago

Mentour Pilot did a video on the second MCAS crash where they basically crashed due to being unable to pull up. MCAS changed the trim to a level where the pilots were pulling with a recorded 180 lbs pull weight but still couldn't hold the nose up https://www.youtube.com/watch?v=DcB31RgeL50 https://www.youtube.com/watch?v=DcB31RgeL50 is the full video, well worth watching IMHO, but the talk about the fatal trim is at 51:00 to 53:00

WalterBright 1y ago
The thumb switches would have pulled the nose up. But the pilots didn't read the instructions, and turned off the stab trim system with the airplane nose down. They also had the engines at full throttle, and the increased speed made it impossible to hand crank the stabilizer. The instructions were: 1. restore normal trim with the thumb switches 2. turn off the trim system
- throwaway5752 1y ago
  Surely you know this, but MCAS was a badly designed system that failed in a fatal manner in the event of malfunctioning non-redundant sensor (10s of failure events per year). Boeing knew this, but allowed it kill everyone aboard two flights. Documentation in the instructions or not it had no business being on a commercial aircraft, ever.
  - signatoremo 1y ago
    It still exists today after modification that requires input from both AOA sensors. Why do you think it’s badly designed? The implementation was badly done for sure.
    - filleduchaos 1y ago
      Design and implementation are not completely distinct things, though. If e.g. a software company's backups were implemented solely as daily dumps to the same SSD the database was running on, and then they implemented a more sane backup solution after the practically-inevitable data loss, would we just go "well the implementation was badly done" or would people not call it out as bad systems design? Some designs are so poorly thought through (and/or conceived for the wrong reasons to begin with, like MCAS was) that they cannot help but yield poor implementations. (Also, input from one or two sensors was far from the only problem with MCAS' implementation. The fact that it could activate multiple times and that it could trim far harder and faster than human pilots could manually trim were equally contributors to making the saga a tragedy).

Merad 1y ago

Generally speaking these days I think that small planes with manual controls (i.e. your private pilot's Cessna) just aren't capable of going that fast. Most of the planes that will be able to go that fast in a dive will have hydraulically boosted controls if not full fly by wire systems. However this was a real problem during WW2, and high speed dives killed more than a few pilots. By 1942-43 most fighter planes were able to dive fast enough to get into trouble (500+ mph/800 kph) but weren't designed for it at all. The aerodynamics of supersonic flight just weren't understood at the time. In some cases the design of the plane made it essentially impossible to pull out once you went beyond a certain speed, all you could do was attempt to slow down enough to regain control. The P-38 Lightning was notorious for this. I don't have any specific sources handy but I recall reading accounts of things like pilots bracing their feet against the control panel trying to pull the control stick with all their strength, taking extreme measures like lowering the landing gear to slow down (risking it being ripped off and maybe tearing the whole plane apart), or only surviving due to sheer luck - sometimes the thicker air at lower altitudes would slow them down enough before it was too late.

dlcarrier 1y ago

A 737 can be flown by hand, without hydraulics or electricity, despite weighing over 50x more than a Cessna 172, or other small manually controlled airplanes. Airplanes' control surfaces have have smaller control surfaces inside them called tabs. Just as a small movement in a control surface is amplified into the larger movement of the entire airplane, a control tab's even smaller movement is amplified into the movement of the rest of the control surface, which in turn is amplified into the movement of the entire airplane. Here's a good descriptions of different types of control tabs: https://www.boldmethod.com/learn-to-fly/systems/4-types-of-trim-tabs/ https://www.boldmethod.com/learn-to-fly/systems/4-types-of-t... Many small airplanes, and pretty much every large airplane, effectively use tabs as a gross adjustment and the yoke/rudder as a fine adjustment, so under normal operation there shouldn't be large forces on the yoke or rudder. The only time it takes a lot of force to control an airplane is when the tab can't be easily controlled. Flying even a small Cessna 152 out of trim can take tens of pounds of force and be impossible to sustain for long periods of time. I once tried flying a 152 from one airport to another, only a few minutes away, without adjusting the elevator trim, and my arms were sore the next day. Going into too fast of a dive has two much larger dangers, one is that the aircraft can break up from the aerodynamic forces, and the other is that the air becomes turbulent near the trailing of of a control surface, making it no longer able to control the aircraft. When the later happens, the controls don't take much force to move, but they also don't do much.