I understand why passenger jets use software that overrides pilot inputs that might cause the jet to exceed the flight envelope. But why do passenger jet manufacturers design their planes with stall prevention systems? Shouldn't professional pilots be well aware that a stall is possible when the airspeed is too low, or the angle of attack is too high?
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8$\begingroup$ This isn't as simple as it sounds, and AF447 really set this in motion for the industry. The problem is that at differing altitudes your AoA between "flying" and "stall" can be extremely narrow. Couple that in with no visual references and a pilot may not know that the aircraft is stalling... $\endgroup$– Ron BeyerCommented Mar 14, 2019 at 17:51
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$\begingroup$ Be aware, especially on the more advanced, larger aircraft, although the anti-stall systems do pull and act largely on their own against pilot input, 20lb's of force is roughly the industry standard (I believe from personal experience) to override this. $\endgroup$– JihyunCommented Mar 14, 2019 at 19:52
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17$\begingroup$ @RonBeyer AF447 had such a system, but it had been disabled due to erroneous airspeed readings when the pitot tubes iced. If anything, AF447 is something of a cautionary tale of pilots becoming too reliant on such systems instead of knowing how to fly the airplane themselves. $\endgroup$– reirabCommented Mar 14, 2019 at 20:30
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6$\begingroup$ Because stalls are really bad? $\endgroup$– David RicherbyCommented Mar 15, 2019 at 17:57
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5$\begingroup$ I feel "to prevent stalls" might not be the answer you're looking for, but I think it's accurate. $\endgroup$– UKMonkeyCommented Mar 16, 2019 at 9:24
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5 Answers
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Why do car manufacturers install seat belts? Shouldn't licensed drivers be well aware that they should slow down when it's raining or snowing and that they shouldn't run through red lights or stop signs?
A better analogy:
Why do car manufacturers install anti-lock break systems? Shouldn't drivers know that when their brakes lock up they should release brake pressure and/or pump the brakes quickly to slow the car down?*
*To be fair, I don't think this is actually taught in driver's education (at least in the US) anymore - my kids learned this from me, but never reported being taught and/or practicing when they took driver's ed. One of the many reasons flying is safer than driving.
- Because accidents happen.
- Because pilots are human and make mistakes.
- Because when you're flying in the clouds with no visual references, it's easy to get confused.
- Because even with stall warning & prevention systems in place, confused pilots will fight the system. AF 447
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2$\begingroup$ And because even with stall prevention systems, stalls are still a reality. That's how bad it is $\endgroup$ Commented Mar 15, 2019 at 6:16
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5$\begingroup$ To be fair the AF 447's stall prevention were disabled because of incorrect airspeed readings. In contrast, the Lion Air 737 crash was that it didn't get disabled... die if you do, die if you don't... $\endgroup$– NelsonCommented Mar 15, 2019 at 7:14
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1$\begingroup$ AF447’s systems were not disabled. AF447’s envelope protection failed because a needed sensor failed, or to be more precise, 3 redundant sensors all of the same make and model operating in the same conditions. Also contributory: they don't bother fitting actual stall sensors, they just look at airspeed, altitude, flaps, stated weight etc. and compute when stall would be expected. Losing airspeed and altitude sensors breaks that. $\endgroup$ Commented Mar 15, 2019 at 20:23
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2$\begingroup$ @James I am quite sure that pulling all the way back on the stick at all times is not the pilots doing the right thing on AF447. Pulling back all the way on the stick is what you do when you are evil and trying to intentionally stall the airplane into the sea. It's well established that if the pilots had simply let go of the controls, the plane would have righted itself. $\endgroup$ Commented Mar 16, 2019 at 16:23
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To be certifiable, airplanes have to have some kind of cues to warn when you are getting close to a stall, and have decent behaviour during the stall, because nobody is perfect. Airplanes with very strong physical cues prior to stall, like the whole airframe shaking, and good behaviour during a stall, like a good natural pitch over tendency with immediate unstalling of the wing, can get away without stall warning and prevention systems.
Transport aircraft with highly loaded wings and high performance airfoils may have poor behaviour before the stall (no buffeting or shaking), and poor recovery performance after, and need a little help. The airfoils used for airplanes that fly at near trans-sonic speeds tend to suffer from this because they tend to stall from the leading edge, at which point the wing stops lifting all at once, and there is often no prior buffeting or shaking.
The earlier supercritical (higher critical mach#) airfoils developed in the 70s were especially bad for this because they developed a flow separation bubble just aft of the leading edge at high angles of attack, due to the profile that was used to manage the formation of shock waves (the Challenger business jet and CRJ200 Regional Jet is typical). You do not want to experience the natural stall on such an aircraft and some kind of system has to be in place as a backup for mishandling of the airplane by the pilot.
For airplanes with mechanical/hydraulic controls, to provide a tactile warning as a substitute or supplement for the airplane shaking (pre-stall buffet), stick shakers are used, which is just a motor with an eccentric weight on the control column. If the post stall behaviour (not much natural pitch over, or worse, settling into an unrecoverable deep stall) is poor, a stick pusher is installed to give the control column a shove just before the natural stall occurs. The stall protection system calculates when to do all this.
Most high performance aircraft use shakers, and some use stick pushers. With FBW, the FBW computers intervene directly within the control loop to achieve the same end without having to shake or push the controls.
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$\begingroup$ In the Air France 447 accident, the Airbus A330 only had a sound warning, with no physical warning, and the plane was clearly built so that it can't "get away" without a warning. Why is that? $\endgroup$– gparyaniCommented Mar 15, 2019 at 1:08
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2$\begingroup$ The last sentence in my post applies. It's a FBW airplane with side sticks and the computers control the airplane at the margins, only letting the pilot input go so far, with an aural warning when things are getting close, but the computers will simply not allow the pilot to go all the way into an aerodynamic stall. 447 wasn't stalled in the aerodynamic sense on its ride down, it was being held at the maximum attitude the computers would allow, not quite stalled, because the right seater was in a mental fog panic mode holding the right side stick fully aft the entire ride down. $\endgroup$– John KCommented Mar 15, 2019 at 1:28
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2$\begingroup$ And the guy in the left seat was in a similar mentally saturated state and didn't think to hit the left seater's override button to pitch the plane over. He pushed his stick forward a few times but this only gets a 50% input because the two inputs are averaged, unless he overrides, and a 50% input wasn't enough to recover from the low speed mush descent. $\endgroup$– John KCommented Mar 15, 2019 at 1:30
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2$\begingroup$ The cockpit recorded did record the sound of the audible stall warning for the entire decent, so I would agree that the pilots were in a mental fog and completely did not register what they needed to do. $\endgroup$– NelsonCommented Mar 15, 2019 at 7:05
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3$\begingroup$ This is one of the biggest problems in the industry. Pilots go through fast track schools and end up in airliners that are AP engaged 95% of the time and have never really learned how to fly, like the guy who learned on gliders, flew in the bush, flew multi-engine s**boxes hauling crap, for a number or years, to the point where the basic fundamental instincts are burned in. They know what to do, but it's not sufficiently internalized, so when weird stuff happens and the stress level goes to 11, they freeze in confusion. $\endgroup$– John KCommented Mar 15, 2019 at 15:03
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You said you understand systems to prevent the airplane from exceeding the flight envelope. Stall is just another boundary of the flight envelope. The rest of the envelope limitations are listed in the flight manual as well. Shouldn't pilots know not to stall the airplane, just as they know not to over-stress it, or exceed other limitations? Of course.
But humans make mistakes, they can get distracted or disoriented. And just as there's little benefit to allowing a pilot to rip the wings off the plane by pitching too fast, there's little benefit from allowing the plane to stall.
Here is a selection of aircraft that have crashed due to stalls.
Thai Airways International flight 261
If stall protection systems are implemented and functioning properly, they can prevent issues. Here are just a few instances where stall protection worked as intended:
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1$\begingroup$ On the flip size, I'm curious what would have happened to Asiana 214 if it had had a stall prevention system. If I'm remembering correctly, they did stall (or at least very nearly stall) on very short final while trying to make the runway. If a stall prevention system had prevented them from raising the nose, would they have hit the nose on the seawall instead of the tail? That seems like it could have been a bad situation a whole lot worse. $\endgroup$– reirabCommented Mar 14, 2019 at 20:37
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2$\begingroup$ @reirab True. On the other hand, if you allow the aircraft to run into two limits simultaneously (out of room and out of speed), there is not much anyone can do. You could argue the „opposite“ safety system, too, and say an automatic terrain escape manoeuvre would be fantastic, except with Asiana 214, it could have worsened the stall... $\endgroup$ Commented Mar 14, 2019 at 21:17
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3$\begingroup$ @CptReynolds Agreed. The root source of the problem was, of course, lack of energy on very short final, which was totally a result of pilot error. But, given that situation, they pretty much had to choose how they were going to crash rather than whether they were going to crash. In that sort of situation, I'd personally prefer a human pilot who can look out the window and make rapid judgments based on the exact situation in control. It's just not the sort of thing that's easy to account for when you're designing a computer program. $\endgroup$– reirabCommented Mar 14, 2019 at 21:26
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$\begingroup$ I suggest to add Colgan 3407 to the list. $\endgroup$ Commented Mar 15, 2019 at 11:36
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1$\begingroup$ I think there may also be a slight game of numbers played by some airlines. The chances of an aircraft crashing are very slim, and well-trained pilots are more expensive (at least to train). I think the entire industry is seeing the results of a couple of decades of it simply not being worth pursuing an airline career, so there aren't enough young, experienced pilots. Thus, countries with less strict airmen certification or operation standards end up with less experienced pilots operating well beyond their capabilities. $\endgroup$– ShawnCommented Mar 16, 2019 at 3:57
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The relevant certification requirements, set by the FAA/JAA/CAA etc require that a "large aircraft" that is capable of stalling has an automatic stall warning and recovery system. So the simple answer is "because the rule of law says so".
Maybe you could think about re-phrasing the question to ask why the traditional stick shaker and pusher, with a long and satisfactory history, were not used? I expect that Boeing will be having to answer that question to the authorities.
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$\begingroup$ I agree entirely that your re-phrasing would have improved the question, but it's probably too late now :( $\endgroup$ Commented Mar 18, 2019 at 23:58
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Well, stall is a limit to flight envelope, the one exceeding which is most dangerous, so stall prevention system is one of the systems that override pilot input if it would lead to exceeding the flight envelope.
And note that stall is directly related to pilot input, because in stable aircraft¹ the angle of attack is directly controlled by elevator and stabilizer position² and stall occurs when the critical for given configuration is exceeded.
¹ All transport aircraft are longitudinally stable. Only unstable aircraft are some new fighters (and some very early experiments).
² The stability makes the aircraft always pitch so as to assume the “trimmed” angle of attack determined by the control surface position. It is a first order feedback, so no oscillations, and it takes really abrupt control input, or severe turbulence, to create a significant momentary deviation.
answered Mar 16, 2019 at 11:52