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Stick and Rudder is the best read out there if you want an intuitive pilots understanding of what happened here. It unpacks angle of attack from a pilots perspe

Stick and Rudder is the best read out there if you want an intuitive pilots understanding of what happened here. It unpacks angle of attack from a pilots perspective rather than using engineer speak. At that speed the control surfaces of the Blackbird would have had very little effect which is incredibly scary as you’re approaching a stall low and slow. They’d need big deflections to keep her straight and it would have felt very mushy. It’s also worth noting that airflow from the engines don’t flow over control surfaces like some light aircraft which means they’d actually have had to gain airspeed before they’d stop mushing around, further delaying recovery. Although I’m guessing that happened pretty damn fast.

FearlessNebula 5y ago
Wouldn’t a well trained pilot (like somebody learning on the SR71) be very familiar with what a slow airplane feels like? I’m shocked that a skilled pilot would have allowed that plane to get so dangerously slow.
- GekkePrutser 5y ago
  Yeah it sounds like a tall tale. "Falling at a slight bank" sounds like ideal conditions for onset of a spin. Which a pilot would be very wary of when close to the ground because it means certain death. Also, would it be possible to fly level at such a speed in an SR71? And isn't there a HUD and vertical speed warner? On the other hand their expectation of being grounded sounds pretty accurate :) Oh well I never flew anything better than a Cessna 172 so don't take my word for it.
  - imoverclocked 5y ago
    > isn't there a HUD and vertical speed warner? On a 1960's SR-71? Probably not [1]. [1] https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#/media/File:Lockheed_SR-71A_Blackbird,_National_Museum_of_the_United_States_Air_Force,_Wright-Patterson_Air_Force_Base,_near_Dayton,_Ohio,_USA,_cockpit,_forward_view.png https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#/medi...
  - jshier 5y ago
    The human mind, even one highly trained, is capable of ignoring all sorts of inputs when conditions don't match expectations. For an SR-71 pilot I can easily imagine flying low and slow could create such an issue. It's scary in general just how many airline crashes are "controlled flight into terrain". In 2009, Air France flight 447 crashed off the coast of Brazil after the pilots flew it at a stall from 35,000 feet. https://admiralcloudberg.medium.com/?p=8a7678c37982 https://admiralcloudberg.medium.com/?p=8a7678c37982
    - loonster 5y ago
      Thanks, this was a good read. I decided to search HN, and this [0] is the most popular thread on "Air France 447". [0] https://news.ycombinator.com/item?id=3327202 https://news.ycombinator.com/item?id=3327202
- imoverclocked 5y ago
  We are all human and this was probably an unusual exercise. Given their distraction of trying to find a field in this aircraft, I can easily imagine how quickly the plane got away from them. 152 kts is roughly 174 mph... imagine trying to find an address when going any slower than 200 mph is potentially fatal.
- teeray 5y ago
  Yeah, probably. But in aeronautical decision making training, you learn that other stuff going on in the cockpit can cause “simple” things to get overlooked. They were looking for the field in low visibility which took the pilot’s attention off the aeronautical health of the aircraft. This is why the stall horn and things like GPWS callouts are a thing. His skill did come into play when he had an atavistic sense that something was wrong, confirmed it was so, and firewalled the throttles.
- Diesel555 5y ago
  Some aircraft don't buffet much when they get slow, while other aircraft may buffet a lot (slow as a simple term for high angle of attack (AOA), but high AOA can occur from G-forces - back stick pressure - as well). Those aircraft also use a digital flight control system that commands G instead of direct deflection of the surfaces. Therefore the aircraft feels the same at the speeds in which it is capable of delivering the G it is asked to deliver. The same back stick pressure delivers the same G. This is much different than a mechanical system in which you get more G if you are faster. The F-16 is one such aircraft. It feels very similar at 250 knots and 650 knots. Of course an experienced pilot immediately recognizes when the aircraft hits the G-limit due to speed. For example, at 200 knots it can't pull 5 Gs. But that only occurs when you are pulling on the stick and not in straight and level flight. The angle of attack of being slow (just being more leaned back in the jet) is more subtle and and doesn't usually present itself until you are already really slow. The automatic flaperons and leading edge flaps, which give the wing more camber automatically, are partially a reason for this. Point is, the slowness maybe not be as obvious as you think.
- throwaway0a5e 5y ago
  You don't get a lot of practice at the limit of the flight envelope in an SR71 for the same reasons crane operators don't do a ton of heavy picks in high winds. It's not worth the risk to the equipment.
  - lostlogin 5y ago
    > It's not worth the risk to the equipment. … or life.
userbinator 5y ago
A similar thing happens with boats --- the rudder only has an effect when water is moving past it, so it's more difficult to maneuver when moving slowly.
- mmaunder 5y ago
  That’s right. The significance for pilots is that, with low airflow over control surfaces which gives a mushy feel, you also have a high angle of attack which means you are approaching stall as flow over the wing separates. So the plane is harder to control but you’re also about to fall out of the sky. Stalls can also occur at higher speed in high g maneuvers, so mushiness isn’t the only indicator of approaching a stall. But it’s pretty reliable in straight and level flight.
- jacquesm 5y ago
  Maneuvering a regular (stern) driven boat without a bow screw at low speeds is super hard. There is this 10 year old kid at a marina near where I live that handles all of the rental boats, I've seen them do this in absolutely impossible situations with a grace and skill that really blows my mind, it's like the boat is on rails and does exactly what they want it to do. Even the box shaped 'party boats', and those are really next to impossible to move around in predictable ways.
  - javajosh 5y ago
    A testament to the plasticity of the human mind. Like getting (and losing) "sea legs". You walk your whole life on solid land; now you don't for a few days and your freaky brain figures it out. You're a kid and you don't know anything about boats, but you take the job parking the rentals. By the end of the summer, they move to your whim. But steerage at slow speed is a tricky subject. I had a trawler I was terrified of at slow speed, because it was absolutely massive, and the rudder did nothing at low speed. Plus the transmission was iffy so putting it in reverse would take from 5-50 seconds. Plus being single screw her rear-end would pull to port in reverse. Never again! (A sail-boat with a big keel and big rudder, and much less beam and mass? No problem!)
mannykannot 5y ago
Plus if the engines do not respond at the same rate, you have asymmetric thrust leading to a yaw if not corrected - and I assume the rudders are not particularly effective at low speed, especially if the engine nacelles, at high angle of attack, are partially blanking them with turbulence. Stall + yaw = spin as a general rule.
shostack 5y ago
In the middle of this book now. So excellent. Is what you're referring to here called coffin corner? Or is that something else?
- mnw21cam 5y ago
  No, the coffin corner is something else, and it doesn't apply to the SR71 (or any aircraft capable of supersonic flight). The coffin corner is where an aircraft is flying as slow as it possibly can, but that slow speed is also very close to its maximum mach speed. So the aircraft is both in severe danger of stalling and losing lift because the speed is too low, and in severe danger of getting supersonic airflow over the wings and losing lift because the speed is too high. This occurs at high altitudes where the air is thinner and the stall speed increases. The problem with the coffin corner is that you need to maintain the exact air speed very accurately. If you let it increase or decrease by just a couple of knots, then one of the two failures happens, and you can pitch down, which will further increase your speed above supersonic, potentially making the pilot lose control or causing damage to the airframe. See https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics) https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics)
  - dan_hawkins 5y ago
    Like Air France Flight 447?
    - mnw21cam 5y ago
      No. Commercial airlines generally don't fly in the coffin corner. That's just asking for trouble. AF447 had nothing to do with the coffin corner - that accident was caused by the aircraft flying through icing conditions, the speed-sensing equipment icing over, the computer detecting that and switching the autopilot off and handing control over to the pilots, who then flew the aircraft into a stall and couldn't work out what they were doing wrong.

FearlessNebula 5y ago

Wouldn’t a well trained pilot (like somebody learning on the SR71) be very familiar with what a slow airplane feels like? I’m shocked that a skilled pilot would have allowed that plane to get so dangerously slow.

GekkePrutser 5y ago
Yeah it sounds like a tall tale. "Falling at a slight bank" sounds like ideal conditions for onset of a spin. Which a pilot would be very wary of when close to the ground because it means certain death. Also, would it be possible to fly level at such a speed in an SR71? And isn't there a HUD and vertical speed warner? On the other hand their expectation of being grounded sounds pretty accurate :) Oh well I never flew anything better than a Cessna 172 so don't take my word for it.
- imoverclocked 5y ago
  > isn't there a HUD and vertical speed warner? On a 1960's SR-71? Probably not [1]. [1] https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#/media/File:Lockheed_SR-71A_Blackbird,_National_Museum_of_the_United_States_Air_Force,_Wright-Patterson_Air_Force_Base,_near_Dayton,_Ohio,_USA,_cockpit,_forward_view.png https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#/medi...
- jshier 5y ago
  The human mind, even one highly trained, is capable of ignoring all sorts of inputs when conditions don't match expectations. For an SR-71 pilot I can easily imagine flying low and slow could create such an issue. It's scary in general just how many airline crashes are "controlled flight into terrain". In 2009, Air France flight 447 crashed off the coast of Brazil after the pilots flew it at a stall from 35,000 feet. https://admiralcloudberg.medium.com/?p=8a7678c37982 https://admiralcloudberg.medium.com/?p=8a7678c37982
  - loonster 5y ago
    Thanks, this was a good read. I decided to search HN, and this [0] is the most popular thread on "Air France 447". [0] https://news.ycombinator.com/item?id=3327202 https://news.ycombinator.com/item?id=3327202
imoverclocked 5y ago
We are all human and this was probably an unusual exercise. Given their distraction of trying to find a field in this aircraft, I can easily imagine how quickly the plane got away from them. 152 kts is roughly 174 mph... imagine trying to find an address when going any slower than 200 mph is potentially fatal.
teeray 5y ago
Yeah, probably. But in aeronautical decision making training, you learn that other stuff going on in the cockpit can cause “simple” things to get overlooked. They were looking for the field in low visibility which took the pilot’s attention off the aeronautical health of the aircraft. This is why the stall horn and things like GPWS callouts are a thing. His skill did come into play when he had an atavistic sense that something was wrong, confirmed it was so, and firewalled the throttles.
Diesel555 5y ago
Some aircraft don't buffet much when they get slow, while other aircraft may buffet a lot (slow as a simple term for high angle of attack (AOA), but high AOA can occur from G-forces - back stick pressure - as well). Those aircraft also use a digital flight control system that commands G instead of direct deflection of the surfaces. Therefore the aircraft feels the same at the speeds in which it is capable of delivering the G it is asked to deliver. The same back stick pressure delivers the same G. This is much different than a mechanical system in which you get more G if you are faster. The F-16 is one such aircraft. It feels very similar at 250 knots and 650 knots. Of course an experienced pilot immediately recognizes when the aircraft hits the G-limit due to speed. For example, at 200 knots it can't pull 5 Gs. But that only occurs when you are pulling on the stick and not in straight and level flight. The angle of attack of being slow (just being more leaned back in the jet) is more subtle and and doesn't usually present itself until you are already really slow. The automatic flaperons and leading edge flaps, which give the wing more camber automatically, are partially a reason for this. Point is, the slowness maybe not be as obvious as you think.
throwaway0a5e 5y ago
You don't get a lot of practice at the limit of the flight envelope in an SR71 for the same reasons crane operators don't do a ton of heavy picks in high winds. It's not worth the risk to the equipment.
- lostlogin 5y ago
  > It's not worth the risk to the equipment. … or life.

userbinator 5y ago

A similar thing happens with boats --- the rudder only has an effect when water is moving past it, so it's more difficult to maneuver when moving slowly.

mmaunder 5y ago
That’s right. The significance for pilots is that, with low airflow over control surfaces which gives a mushy feel, you also have a high angle of attack which means you are approaching stall as flow over the wing separates. So the plane is harder to control but you’re also about to fall out of the sky. Stalls can also occur at higher speed in high g maneuvers, so mushiness isn’t the only indicator of approaching a stall. But it’s pretty reliable in straight and level flight.
jacquesm 5y ago
Maneuvering a regular (stern) driven boat without a bow screw at low speeds is super hard. There is this 10 year old kid at a marina near where I live that handles all of the rental boats, I've seen them do this in absolutely impossible situations with a grace and skill that really blows my mind, it's like the boat is on rails and does exactly what they want it to do. Even the box shaped 'party boats', and those are really next to impossible to move around in predictable ways.
- javajosh 5y ago
  A testament to the plasticity of the human mind. Like getting (and losing) "sea legs". You walk your whole life on solid land; now you don't for a few days and your freaky brain figures it out. You're a kid and you don't know anything about boats, but you take the job parking the rentals. By the end of the summer, they move to your whim. But steerage at slow speed is a tricky subject. I had a trawler I was terrified of at slow speed, because it was absolutely massive, and the rudder did nothing at low speed. Plus the transmission was iffy so putting it in reverse would take from 5-50 seconds. Plus being single screw her rear-end would pull to port in reverse. Never again! (A sail-boat with a big keel and big rudder, and much less beam and mass? No problem!)

mannykannot 5y ago

Plus if the engines do not respond at the same rate, you have asymmetric thrust leading to a yaw if not corrected - and I assume the rudders are not particularly effective at low speed, especially if the engine nacelles, at high angle of attack, are partially blanking them with turbulence. Stall + yaw = spin as a general rule.

shostack 5y ago

In the middle of this book now. So excellent. Is what you're referring to here called coffin corner? Or is that something else?

mnw21cam 5y ago
No, the coffin corner is something else, and it doesn't apply to the SR71 (or any aircraft capable of supersonic flight). The coffin corner is where an aircraft is flying as slow as it possibly can, but that slow speed is also very close to its maximum mach speed. So the aircraft is both in severe danger of stalling and losing lift because the speed is too low, and in severe danger of getting supersonic airflow over the wings and losing lift because the speed is too high. This occurs at high altitudes where the air is thinner and the stall speed increases. The problem with the coffin corner is that you need to maintain the exact air speed very accurately. If you let it increase or decrease by just a couple of knots, then one of the two failures happens, and you can pitch down, which will further increase your speed above supersonic, potentially making the pilot lose control or causing damage to the airframe. See https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics) https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics)
- dan_hawkins 5y ago
  Like Air France Flight 447?
  - mnw21cam 5y ago
    No. Commercial airlines generally don't fly in the coffin corner. That's just asking for trouble. AF447 had nothing to do with the coffin corner - that accident was caused by the aircraft flying through icing conditions, the speed-sensing equipment icing over, the computer detecting that and switching the autopilot off and handing control over to the pilots, who then flew the aircraft into a stall and couldn't work out what they were doing wrong.