It seems that modern fly by wire systems should / could limit pilot inputs to prevent structural damage.

For example, American Airlines 587:

From the NTSB report of the accident:

The National Transportation Safety Board determines that the probable cause of this accident was the in-flight separation of the vertical stabilizer as a result of the loads beyond ultimate design that were created by the first officer’s unnecessary and excessive rudder pedal inputs. Contributing to these rudder pedal inputs were characteristics of the Airbus A300-600 rudder system design and elements of the American Airlines Advanced Aircraft Maneuvering Program.

  • $\begingroup$ relevant: aviation.stackexchange.com/q/149/1467 $\endgroup$
    – Federico
    Jul 19, 2016 at 21:34
  • 2
    $\begingroup$ Welcome to the site! I don't really understand what your question is: the title asks if loads can be exceeded, and the text gives an example where they were. Can you edit your question to make it clearer what you want to know? $\endgroup$
    – Pondlife
    Jul 19, 2016 at 23:49
  • $\begingroup$ No the question is clear, the concerned aircraft is old fashioned compared to modern aircrafts such as B 787 , A350 etc $\endgroup$
    – user40476
    Jun 10, 2019 at 11:01

3 Answers 3


Airbus fly-by-wire aircraft have what is called envelope protection which is to prevent pilot inputs from exceeding structural limits as well as keep them from getting the aircraft into an upset flight regime (stall, overbank, etc.). The philosophy behind it is that, if the pilots need to perform an extreme maneuver they can be as aggressive with the controls as they want, confident that they won't break the plane.

Boeing generally disagrees with this approach saying that, in an emergency, the pilot needs to have all possible force at his disposal, even if it breaks the plane. See the question Federico linked to in comments for more on this.

Of course, the Airbus system can only provide these protections when systems are all functioning properly. If part of the system is not working the protections are removed. Examples: AF447, XL Airways 888T, Indonesia AirAsia 8501,

As far as AA587, the A-300 is not a fly-by-wire aircraft and doesn't have envelope protections. It's debatable, though, if envelope protection would have helped in that situation. None of the rudder inputs by itself was outside of the structural limitations of the aircraft so the computer would probably not have prevented them. The failure was due to the pilot making alternating inputs in quick succession which caused the sideslip angle to build up to twice the certified design strength of the vertical stabilizer. The A300 system was designed to limit the rudder travel available as speed increased, which is probably fairly similar to what the fly-by-wire computer is programed to do in other Airbus planes. But it would be difficult to determine what complex combinations of inputs might exceed the limits and prevent them.

  • $\begingroup$ The repeated rudder doublets didn't rapidly fatigue the vertical stabiliser; they drove the aircraft to increasingly-large sideslip angles that eventually became great enough that the last doublet produced enough aerodynamic loading to snap the stabiliser off the plane. The stabiliser didn't separate until it was subjected to loads sufficient to remove even a brand-new vertical stabiliser. Source: the official NTSB report. $\endgroup$
    – Vikki
    Jun 6, 2019 at 2:28
  • 1
    $\begingroup$ @Sean Thanks. Edited answer to reflect that. $\endgroup$
    – TomMcW
    Jun 9, 2019 at 3:02
  • $\begingroup$ No Boeing does not disagree with this approach, with the 737 max accidents the pilots had no force at their disposal, the faulty software killed the passengers and the crews. Boeing and airbus are in phase in their approaches. The difference is that Airbus introduced FBW ahead of Boeing. $\endgroup$
    – user40476
    Jun 10, 2019 at 10:10
  • $\begingroup$ Same thing with rudder doublets happened with this crash, baltimoresun.com/news/bs-xpm-1993-04-18-1993108262-story.html $\endgroup$
    – MikeY
    Jun 10, 2019 at 13:33
  • $\begingroup$ @user40476 This answer was written before the 737max. Boeing has changed their approach over time. They even make fully FBW aircraft such as the 777 and 787 although I'm not certain as to what type of envelope protections they have. $\endgroup$
    – TomMcW
    Jun 10, 2019 at 19:39

Doublets effect can’t be avoided with cabled pedals, so their effects couldn’t be avoided even with early FBW aircraft, because on these the rudder was partially cabled with corrective FBW actions.

Today, AIRBUS and BOEING have full FBW authority on the rudder, doublets are dampened by the software and the structure is protected.

These protections do apply to all surfaces with modern aircrafts within certain human factors limits.

Any apparent authority of the pilots over the system is via the software, in Airbus technique this authority is active in case of degraded conditions, I.e. alternate or direct laws instead of normal laws.

With Boeing modern aircrafts pilots full authority is activated too when the system is degraded or on manual selection, but all this via the software, no more conventional mechanical cables are used on flight controls


A fly-by-wire system is going to be controlled by software. All software has bugs and conditions that it is not designed to handle. Thus, even in a fly by wire system it is possible for the control inputs to exceed structural loads.

  • $\begingroup$ Is this an answer to the question or a comment? $\endgroup$
    – user40476
    Jun 10, 2019 at 10:11
  • $\begingroup$ Can you not read? $\endgroup$ Jun 10, 2019 at 17:59
  • $\begingroup$ « All software has bugs and conditions that it is not designed to handle ». Any general proof? $\endgroup$
    – user40476
    Jun 10, 2019 at 20:57

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