The title pretty much says it all. I have a very specific research question in relation to a writing project I'm working on.

The main character is in an Su-15 performing low flying military training, and has a total hydraulic flight system failure (which I understand is extremely rare) and crashes into the side of a mountain. I envision moments before, the pilot pulling back on the stick as hard as he can, with no authority, or give in the stick, resulting in the plane smashing into the mountains ahead. However, I'm not entirely sure this would have been the case and need to be as accurate as possible for this project.


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    $\begingroup$ The pilot would most likely eject before crashing into the side of a mountain. $\endgroup$ – GdD Sep 24 at 15:47
  • $\begingroup$ You need to specify an actual aircraft @JohnP.Good. Some soviet fighters of the era (Mig-15, Mig-17) didn't have hydraulic controls to fail in the first place. Those with hydraulic controls will vary significantly in characteristics after a hydraulic failure. $\endgroup$ – GdD Sep 24 at 15:50
  • $\begingroup$ @GdD, thanks for the suggestion. The aircraft is an Su-15. $\endgroup$ – John P. Good Sep 24 at 16:19
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    $\begingroup$ The SU-15 was an interceptor, not a fighter, it's not realistic it would be flying low level. $\endgroup$ – GdD Sep 24 at 16:28
  • $\begingroup$ Regardless of the role of the jet, low level flight would be trained. $\endgroup$ – Jpe61 Sep 25 at 9:29

Su-15 (as well as all Sukhoi interceptors since Su-7) had a completely irreversible control system with no manual override. This means, 100% of torque was produced by hydraulic boosters, and the force on the control stick was simulated with a special variable spring loading mechanism.

So, a theoretical total hydraulic failure would result in loss of control, while the stick would move 'normally' - just without any effect.

Su-15 had 4 hydraulic systems, with 2 of them used for flight control. The most anticipated reason for dual failure would be loss of the engines; for that, there was an electric pump in one of the systems. In case of control problems, the pilot would check and ensure that the emergency pump was on. This pump provided only a limited amount of control (and for a limited time).

Now I'll have to challenge the premise a bit. First, Su-15 was an interceptor, and low-altitude flight (even training) would be quite unusual for it (though not impossible). Its lowest altitude of intercept was 2 km, and the main emphasis was on intercepting higher altitude targets. Its radar was unable to track targets on the earth surface background. It was not meant for terrain following, so even if it was flying at a low altitude, it would be fairly clear of immediate obstacles. Which gives the pilot at least a few seconds to react.

Second, the pilot would, of course, eject. Even though in emergency situations (esp. unanticipated emergencies) people do all sorts of things, pilots, and esp. pilots of aircraft with irreversible control, know that loss of control = eject. It's pointless to 'fight' with the stick. However, as I said, in all likelihood there would be a few seconds to think. In most situations (like the engines failure) you won't have an immediate loss of control: the failure is fairly gradual. This would see the pilot trying to ascertain what was happening. Difficulties with control at low altitudes often require reducing the airspeed.1 But a catastrophic sudden loss of control is a clear signal to eject.

1 By the way, early Su-15 had pitch control problems at transonic speeds at low altitude due to insufficient booster torque. It felt like no response (or poor response) at higher G manoeuvres. This things could be resolved by reducing speed.

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In an aeroplane with hydraulically powered, irreversible flight controls, the controls are very hard to move with no hydraulic pressure. The controls have a bit of movement around the servo valve travel, a couple of centimeters at the stick. The unpowered hydraulic actuator acts as a damper, and the pilot must exert quite a bit of force to deflect the stick past the servo valve travel.

On ground, the elevator/ailerons will deflect when in the servo valve stops, at great effort. In the air, the aerodynamic hinge moments resist stick deflection, and the pilot may pull what they want, without this effort resulting in any surface deflection.

In flight, the effects of gradual pressure loss are not much noticeable at first. Flight control deflections (and associated aerodynamic hinge moments) in normal flight are relatively small, and the remaining pressure in the last hydraulic system will be able to cope with these until the underpowerd actuator runs out of authority. Which also happens when the pilot attempts a drastic pull-up manoeuvre when near the ground.

Total hydraulic pressure loss in an irreversible hydraulic aeroplane is a catastrophic event, and there will always be redundancy built into the aeroplane. Multiple hydraulic systems, each with multiple pumps, with urgent warning signals tothe pilot when one of the system parameters is out of the normal range. So the pilot won’t be taken by complete surprise, but aeroplane response can suddenly run out when all pressure is lost. A suspenseful scenario.

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