5
$\begingroup$

One more question due to the recent and unfortunate events in the case of the second ever B737 Max 8 crash.

As far as I know, the first of the two crashes was caused by the MCAS System, that overwrite the inputs of the pilots as a faulty reading of an angle of attack sensor indicated a stall (that was not actually happening).

Knowing (rather, having an idea of) what a difficult time pilots have in such situations I wonder if it is at all possible to stop the aircraft from executing such maneuvers.

I know that on an Airbus A320, there is different flight laws. While during "Normal Law" all the various protections the A320 has are active (Stall protection, overspeed protection, bank protection with all their proper names and so on), they are not active in "Direct Law". There are two more levels between "Normal" and "Direct" (namely "Alternate" and "Abnormal Alternate"), but let's ignore them for this question.

I have never flown an A320 in real life and lack the training any A320 pilot goes through, but in the Simulators I have flown (not talking FSX here!) you could "exit" out of "Normal Law" by turning off the flight computers on the overhead, like the ELAC, FAC and SEC, and go into alternate or even "Direct Law".

So the question is: is this actually possible on a real A320 as well? And is there a similar method on a B737 (Max 8)?

$\endgroup$
  • 1
    $\begingroup$ Are you asking about disabling flight computer control, or are you asking about disabling the MCAS pitch-down on incorrectly detected high angle of attack specifically? For the latter, see e.g. Nathan Chetram's answer to Boeing 737-MAX stabilizer control. $\endgroup$ – a CVn Mar 13 at 10:03
  • $\begingroup$ @aCVn thanks for the link! Though obviously connected to the incidents, this question was intended to be general in nature ;) $\endgroup$ – Maverick283 Mar 14 at 5:59
9
$\begingroup$

For A320 it is possible also in the real aircraft. The problem with disabling flight control computers is that it also disables associated control surfaces. If you disable all five computers, you disable all surfaces and the aircraft goes in to "Mechanical backup" mode, which leaves pilot only the control of the trimmable horizontal stabilizer (ie. pitch trim) and rudder. Wide bodies are similar, although they have more redundant flight control computer architecture.

The correct procedure to override normal law in Airbuses is to switch two ADRs off, which forces controls to alternate law. This disables most of the protections leaving only load factor protection. All control surfaces would still be operating with automatic pitch trim. This would be used only in undue protection activation.

$\endgroup$
  • $\begingroup$ For those interested, Google found the procedure airlineratings.com/wp-content/uploads/articles/… $\endgroup$ – busdriver Mar 13 at 15:13
  • 2
    $\begingroup$ To add for the mechanical backup, it is only true on A320, because it is the only two things still mechanicaly linked to the surface they control. On all Airbus after A320, it is all fly by wire. But if you kill PRIM and SEC on these plane, you still have control of the surfaces through the stick and rudder. $\endgroup$ – Chris Lau Mar 13 at 20:38
  • 2
    $\begingroup$ @ChrisLau The early A330/A340 have mechanical rudder. The A345, A346 and the A330s built at some point after that switched to electric rudder. There is a completely independent electrical rudder backup unit on those planes. $\endgroup$ – user71659 Mar 14 at 2:00
-3
$\begingroup$

The more correct answer is that it is possible to disable the computer control on the Boeing 737 Max 8, but in order to avoid a crash you may have only 40 seconds in which to do so -- including the crucial first step of diagnosing the problem (which in the first accident was not obvious: a single faulty sensor indicating a stall that was not actually happening).

The pilot can optionally turn off the automatic stabilizer inputs from MCAS, an auto-stabilising feature intended only for a flight state outside the normal flight envelope (because, the B737 being aerodynamically unstable, a software routine was designed so that it would feel and behave in that situation the same as an aircraft that is aerodynamically stable).

But the pilot has first to correctly determin whether the aircraft is outside the normal flight envelope, in circumstances where he cannot rely on what his flight instruments are reporting.

If the aircraft is outside the normal flight envelope, and inherently unstable, disabling the MCAS system would be a bad idea.

$\endgroup$
  • 4
    $\begingroup$ Several elements of this answer don't really reflect the systems in the Max, and the post doesn't really answer the more general question as stated. $\endgroup$ – Ralph J Mar 28 at 18:21
  • $\begingroup$ Also, the 737-8 is not aerodynamically unstable $\endgroup$ – selectstriker2 Mar 28 at 21:24
  • $\begingroup$ Also the statement "you may have 40 seconds to avoid a crash" is way to broad and refers to the recent finding specific to the lion air crash. $\endgroup$ – Maverick283 Mar 29 at 12:40
  • $\begingroup$ @Maverick283 My statement that a pilot has only 40 seconds to decide is entirely correct. You imply that it is hypothetical, because it only happened in half of the cases. Since it did happen, it is a proven fact. $\endgroup$ – Ed999 Mar 30 at 11:41
  • $\begingroup$ The computerised maneuvering characteristics augmentation system was added to the aircraft because shortening the engine mount structure tends to dangerously force the nose up, increasing the possibility of a stall: I characterise this as an aircraft which is aerodynamically unstable. The two crashes tend to prove that this is the case. $\endgroup$ – Ed999 Mar 30 at 11:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.