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When thinking about fly-by-wire, I always think about civilian aircraft (Dassault Falcon 7X, Airbus A350...) for which protections are implemented to ensure the aircraft stays in its designed flight envelope (load factor limits to avoid overstressing the airframe, AoA limits to avoid stall, bank angle limits to avoid inverted flight...). All these restrictions help maintaining the aircraft in a manageable situation while in flight. Some (e.g. pitch or bank angle limits) are OK for airliners but counterproductive for fighters. Other are clearly applicable to both (e.g. load factor. You don't want to overstress your airframe or risk a G-LOC).

My question concerns AoA protection. For fighters, I imagine this protection can avoid loss of roll control while stalling. Yet, some fighters are able to fly at AoA obviously beyond stall angle (e.g. during a cobra), suggesting this protection is not present.

Do fly-by-wire fighters have an AoA protection? If so does it prevent stall? (I can imagine a protection triggered only if a limit is reached for at least a certain amount of time, or a limit beyond stall angle but not far beyond.) Does it have to be deactivated for manoeuvre such as the cobra (I expect every system to be able to be deactivated on such aircraft)?

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Brief answer: Yes, but the protection does not always work.

Most modern fighter aircraft have an AoA limiter. That is different from a stall warning – in case of the McDonnell Douglas F-4 the limit was only 23 degree, below the stall limit. The reason for the limit was the loss of directional stability above 23 degree AoA.

In case of the JAS-39 Gripen the limiter worked in a straight pitch-up but when the pilot repeatedly pitched up during a flight display over Stockholm, the dynamic response overpowered the AoA limiter and the aircraft stalled. It fell like a stone into a public park, miraculously without loss of life.

In another accident the JAS-39 FCS software got confused after the controls could not follow the change in angle of attack quickly enough when one aircraft flew through the wake of another, its adversary in an air combat exercise. This caused the safe limit of 20 degree to be exceeded and the aircraft became uncontrollable.

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Let's start from the fact that nearly all airplanes have 'AoA protection' in the form of stall warning.

For automatic stall prevention, strictly speaking, one doesn't need FBW: this can be achieved with traditional automatic control means (the infamous MCAS is, in essence, an example of such a system). On the other hand, FBW does not have to have stall protection: it just allows it to be introduced (typically) without adding extra equipment. So, it's hard to answer the question without considering each particular aircraft.

Of those I know, for example, the original MiG-29 didn't have FBW, but had AoA limiter which pushed the stick forward with considerable force depending on the AoA and its rate. Su-27, with its FBW (originally only in the pitch channel), has a somewhat more comprehensive AoA/load limiter, but essentially works the same way, by forcing the stick forward.

Normally, such system would prevent stall; in fact, the limiters are often set with a considerable margin, allowing an 'ordinary' pilot easy safe control. For 'supermanoeuvres' like Cobra, these limiters have to be disabled.

I'm not completely sure how it works on the latest integrated control systems, but I expect the same thing: normally, stall would be prevented, and only in special circumstances higher AoA would be allowed. In a typical fight, the primary danger is to stall (or overload) during a tight turn or loop, and you want protection for such cases.

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  • $\begingroup$ When asking the question, I was thinking about FBW aircraft in which computer have full authority over primary flight control surfaces. By "protection", I was thinking of computer refusing pilot's input that can lead to AoA greater than a threshold (such as what is implemented in airbuses with alpha prot and alpha max) $\endgroup$ – Manu H Dec 6 '19 at 11:12
  • $\begingroup$ Well, here you are, Su-27 is an example of full-authority FBW. It kind of 'refuses' to follow pilot's input, but this is implemented by resisting the pilot. Call it 'Boeing style' if you like, but I guess this is the 'normal' way for fighters (that have force feedback on the stick at all), which expect the pilot to be in control. $\endgroup$ – Zeus Dec 9 '19 at 2:40
  • $\begingroup$ I was expecting the aircraft to authoritatively limit the AoA because in tight turns with the adrenaline of a dog fight, a pilot may give input that can overstress the airframe, risk a G-loc, and a stall. $\endgroup$ – Manu H Dec 9 '19 at 7:29
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    $\begingroup$ @ManuH, yes, apparently pilots of the early F-16s, in the heat of a (simulated) dog fight regularly gave input that bent the side-stick, but the aircraft would not stall, so it clearly has AoA limiter. In those early F-16s, the side-stick did not move at all, only registered force, so the pilot didn't know whether they are at the backstop already. That's why a bit of movement was added to provide that information. $\endgroup$ – Jan Hudec Dec 28 '19 at 20:28
  • $\begingroup$ @Zeus, an Airbus-style FBW (first appearing in F-16) does have force feedback, because the force on the side-stick corresponds to the desired pitching or rolling moment, which is proportional to the aerodynamic force produced by the control surface. What it does not have is position feedback (except in direct law). In contrast hydraulic control always have position feedback, but depending on the type of power-drive the force may be just crudely approximated to increase with speed (e.g. 737). $\endgroup$ – Jan Hudec Dec 28 '19 at 20:37
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Every fly-by-wire type has protective measures to keep it within the designated flight envelope. In the case of agile fighters capable of the Cobra and suchlike, they are designed to maintain predictable airflow at extreme angles of attack, long after the conventional flow over the wing has broken away. Typically this will involve vortex lift at intermediate angles, while at extremely steep angles the forward airspeed (i.e. in the direction of the pilot's feet) will be severely limited by the drag of the wing planform and various techniques may be employed to maintain controllability. Their flight envelope is thus vastly expanded and needs not so much AoA protection as AoA facilitation.

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