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As technology becomes more advance airplanes are becoming less and less stable. In flight school I learned that it's dangerous to load a Cessna out of it's CG range, and now in college (for aerospace engineering) I'm learning about modern fighters and their fundamentally unstable designs.

A lot of these unstable aircraft are supermaneuverable but rely on computers to fly them more than the human pilot in the cockpit.

I'm sure that these airplanes are thoroughly checked before every flight to make sure the computer systems will perform accordingly, but in the event of a failure what kind of side affects might a pilot experience?

The two conditions I can think of are

  1. No computer aid, but the control surfaces are still "responsive"
  2. The pilot loses all control of the aircraft and none of the inputs work.

X-29

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    $\begingroup$ Military aircraft don't care about safety. They have ejection seats. $\endgroup$
    – Jan Hudec
    Commented Feb 19, 2014 at 5:48
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    $\begingroup$ @JanHudec not true. Survivability is important as pilots (and aircraft) are expensive and slow to replace, repairing them is much prefered. $\endgroup$
    – jwenting
    Commented Feb 19, 2014 at 12:46
  • $\begingroup$ Actually, the X-29's (shown above) wiki page has an excellent paragraph discussing the 6 (!) computers used to control the plane via FBW. $\endgroup$
    – CGCampbell
    Commented Mar 18, 2021 at 11:26
  • $\begingroup$ In short, they do not. Many modern aircraft (usually top-end fighters) would shred themselves within fractions of a second without constant, active corrective (computer) control. Even the Stabile ones would die, due to uncontrolled turbine behaviour, incorrect computer-applied trim, etc. $\endgroup$
    – PcMan
    Commented Jun 16, 2021 at 9:45

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"fundamentally unstable" but über-responsive aircraft have multiple computers that take a vote on everything the pilot wants to do. If one computer fails the others will take over. There is often no direct connection between the control column and the flight surfaces so the "no computer aid" situation would be the same as #2.

If everything fails, the checklist is rather short: eject immediately.

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Unfortunately you seem to be conflating a two distinct issues in one question.

  1. Computer control of aircraft.
  2. Unstable aircraft design.

First dealing with computer control, or flight control systems. Many modern designs military and civil are using digital fly by wire (FBW) systems. This has little or nothing to do with aircraft stability. These systems often have no mechanical backup, so in reality the pilot is merely instructing the computer how to fly the plane. Modern FBW systems use a quadraplex system, i.e. there are 4 duplicate computers. 3 are active and one is hibernating. Then if one system goes duff the other 2 can switch the duff one off and bring the 4th out of hibernation. If you only had 2 computers how do you know which one is wrong? - hence 3 are used. Prior to flight clearance these systems are tested with thousands of hours of rig testing, safety checks etc. Whole volumes have been written on the subject.

Second issue - unstable aircraft design. Again a couple of comments, first aircraft instability levels are not driven by manoeuvrability requirements - modern military aircraft are aerodynamically unstable for supersonic performance not manoeuvrability. Modern FBW systems allow designers to explore unstable designs but there is a limit. Originally the FBW systems were looking at pitch instability but now it is being extended to lateral instability for some of the more novel stealth designs.

So in terms of your two proposed conditions - if there is a mechanical backup then pilots can adapt and are capable of controlling slightly unstable configurations in the event of a failure, although they may whinge a little about the handling qualities. For a fully digital FBW system then for your proposed conditions then as the systems failed there would be a gradual degradation in flying qualities but there would still be a backup system allowing a return to base capability. To have a total loss in control then there would have to occurred a series of multiple failures. Again risk and safety assessments are another whole volume of work.

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    $\begingroup$ "aircraft instability has nothing to do with manoeuvrability" Really?? $\endgroup$
    – Philippe Leybaert
    Commented Feb 19, 2014 at 23:43
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    $\begingroup$ Yes - aerodynamic instability is used to reduce the trim induced drag supersonically. As you go supersonic the the aerodynamic centre moves aft. If you start from a stable design subsonically this requires large angles to trim in the supersonic region and a performance penalty. If you are unstable subsonically then in the supersonic region you don't need the same flap angle to trim hence better performance. There is an argument that an unstable design is detrimental to manoeuvrability. All the pilot wants is to point the nose and unstable design is harder to stop in the right place. $\endgroup$
    – Adrian
    Commented Feb 20, 2014 at 0:27
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    $\begingroup$ I was just questioning your general statement that aircraft instability has nothing to do with maneuverability, which is not true. In general, the more maneuverable an aircraft is, the less stable it is. By definition $\endgroup$
    – Philippe Leybaert
    Commented Feb 20, 2014 at 1:29
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    $\begingroup$ Using FBW you can make an naturally unstable design fly like a very stable aircraft, but the opposite is not true. You can modify a Typhoon to act like a Cessna Citation, the Citation is never going to roll and pitch like a Typhoon. +1 For decoupling FBW from fundamental stability though. You can fly unstable aircraft without FBW and stable aircraft are often produced with FBW. $\endgroup$
    – DeltaLima ♦
    Commented Feb 21, 2014 at 12:25
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    $\begingroup$ Most modern fighter aircraft are not mechanically linked. A massive failure in all computer systems (probably as a result of battle damage) would leave the pilot no choice but to eject. $\endgroup$
    – Rhino Driver
    Commented Feb 22, 2015 at 18:02
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In the event of a computer failure it boils down to two factors,

  1. Manual control - For a naturally unstable aircraft, manually controlling it depends on the pilots skill, which I am sure they are trined for. The safest method would be to manually take control and emergency land.
  2. Eject - If th aircraft is completely uncontrollable despite pilots efforts, the pilot has to eject.
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  • $\begingroup$ This is not a bad answer, and in fact would be the decision process the pilot must make. "Unstable" planes can be made more stable by: 1. reducing speed, to where it is more stable (transsonic to subsonic). 2. adding drag to the tail with speedbrakes, flaps, rear gear, or even a parachute. $\endgroup$
    – Robert DiGiovanni
    Commented Mar 18, 2021 at 13:36
  • $\begingroup$ It also, effectively, repeats the 2 points made in the original question, therefore doesn't really provide an answer. $\endgroup$
    – FreeMan
    Commented Mar 18, 2021 at 16:12
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    $\begingroup$ @Robert, yes, this happens, but rather as an exception. Watch how a test pilot lands an unstable MiG-AT with a failed control system and damaged tail. $\endgroup$
    – Zeus
    Commented Mar 18, 2021 at 23:53

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