6 ms·

the airplane did not have stable flight characteristics because of its physical design. that’s a hardware problem. MCAS only exists because of that hardware p

by nopzor 7y ago

the airplane did not have stable flight characteristics because of its physical design. that’s a hardware problem. MCAS only exists because of that hardware problem. the fact that boeing also did not train or tell pilots about MCAS, in order to make the airplane more financially appealing by retaining the 737 type rating, is a separate (also bad) issue.

na85 7y ago
>the airplane did not have stable flight characteristics because of its physical design. This is a gross oversimplification of the problem. In most of the flight envelope the aircraft is stable. At high alpha the aircraft has pitch problems. There are myriad ways to address this, and MCAS was one (bad) choice of many available to Boeing.
- leoc 7y ago
  That's the devilish thing, I think. Making the plane unstable then patching things up with MCAS, or something else, isn't a fault in any straightforward way. But it is taking a risk, and the source of risk isn't simply the risk that eg. MCAS will fail on any given flight, it's also the higher-order risk that eg. the MCAS system will have a faulty design. (A bit like rewriting code in C for performance, or keeping on generally unloved code to satisfy a paying customer, presumably.) I assume that makes the ethical and safety decisions a lot more murky for the engineers than having a specific fault to point to, especially in an organisation afflicted with go fever.
- sokoloff 7y ago
  I believe the airplane is aerodynamically stable even at high alpha. What it fails in certification is the requirement to have continually increasing pitch feedback forces. I believe the pitch feedback forces are still in the stable region, just too low. This is not a flying wing or intentionally unstable airplane (as the F-16).
  - leoc 7y ago
    >> [...] In most of the flight envelope the aircraft is stable. >> At high alpha the aircraft has pitch problems. > I believe the airplane is aerodynamically stable even at high alpha. [...] This is not a flying wing or intentionally unstable airplane (as the F-16). In some ways that's likely worse news than an airplane that's inherently unstable in general, no? A corner case, and one that evidently isn't actually all that uncommon. If you're building something like an F-16 you know that you absolutely have to make the fly-by-wire correct and robust, and the ground crews similarly know that if anything affects its performance the plane isn't fit to fly.
  - inferiorhuman 7y ago
    I believe the airplane is aerodynamically stable even at high alpha. That would put you at odds with Boeing's test pilots.
    - sokoloff 7y ago
      No it wouldn't (or "citation needed" if so). The issue is not that the airplane shows negative or neutral aerodynamic pitch stability, it's that it does not exhibit an increasing stick force gradient, as required by certification rules. https://airfactsjournal.com/2019/03/can-boeing-trust-pilots/ https://airfactsjournal.com/2019/03/can-boeing-trust-pilots/
cmurf 7y ago
Citation needed. FAR 25 applies to all transport category aircraft. The section on stability (§§ 25.171 - 25.181). In exactly what manner is the airplane not stable, with or without MCAS?
- nopzor 7y ago
  i haven’t looked at the fars in quite a few years, but i’m pretty sure there would be stuff in there that references stuff like pitch stability (which is how i’d define the “hardware problem”), is that not the case? i’m afk atm. i still maintain my macro point, either way. making the airframe on a pax airliner aerodynamically stable during normal takeoff and landing operations seems like basic “good engineering” to me.
  - burfog 7y ago
    I'm not so sure that it is good engineering to be inherently stable during normal takeoff and landing operations. Modern fighter planes are inherently unstable because this is required for better maneuverability. Passenger planes can certainly benefit from that too. Consider gusty crosswinds during a landing. With an inherently unstable aircraft, there is greater capability to compensate. You can have a computer stabilize the plane, preventing the tail or wing tips from striking the ground. When wake turbulence threatens to flip the plane or when a microburst threatens to pound the aircraft into the ground, a fast response is possible. Stability would deaden the performance of the needed response. The extreme example is probably wings that are low-mounted anhedral and forward-swept, with the bending controlled by rapidly actuated aerodynamic surfaces near the tips.
    - apcherry 7y ago
      A common fallacy I'm afraid - modern fighter planes are unstable for improved supersonic performance - reduced drag. So relaxed stability may benefit civil aircraft in terms of fuel efficiency. There is an argument that inherently unstable aircraft makes the manoeuvrability worse or harder. What many forget is that issue with manoeuvrability is actually at the end of the manoeuvre. The handling qualities aim is to point the nose in a new direction and an unstable aircraft makes the design challenge harder to stop the aircraft at the position required.
WalterBright 7y ago
All jetliners are unstable at altitude in cruise, and require augmentation. https://en.wikipedia.org/wiki/Yaw_damper https://en.wikipedia.org/wiki/Yaw_damper Aircraft design is a giant bag of compromises between desirable characteristics, most which are in conflict with each other.
- jngreenlee 7y ago
  Best-stated comment so far. We seem to forget that this whole problem exists in an edge domain only.
- ulfw 7y ago
  an INOP yaw damper doesn’t cause a hull loss crash
  - WalterBright 7y ago
    It has: https://en.wikipedia.org/wiki/Dutch_roll#Accidents https://en.wikipedia.org/wiki/Dutch_roll#Accidents http://code7700.com/accident_kc-135r_63-8877.htm http://code7700.com/accident_kc-135r_63-8877.htm http://www.failure-interactions.com/boeing---19-oct-1959.html http://www.failure-interactions.com/boeing---19-oct-1959.htm...
    - VBprogrammer 7y ago
      Fun fact. The original 737 actually had two yaw dampeners because they expected it to have diverging yaw stability. Just by chance the airframe is actually positively stable so can operate safely with no yaw dampeners so one was removed.
mannykannot 7y ago
Even the 737 Max variants are statically stable in pitch, at least up to the stall. The issue that MCAS was intended to address was the handling characteristics, again in the flight regime prior to the stall. I do not know whether the airplane could have been certified, as a separate type, without some sort of augmentation, but it seems that Boeing did not think it could be certified under the common type rating that covers prior variants.

by nopzor 7y ago

na85 7y ago

>the airplane did not have stable flight characteristics because of its physical design. This is a gross oversimplification of the problem. In most of the flight envelope the aircraft is stable. At high alpha the aircraft has pitch problems. There are myriad ways to address this, and MCAS was one (bad) choice of many available to Boeing.

leoc 7y ago
That's the devilish thing, I think. Making the plane unstable then patching things up with MCAS, or something else, isn't a fault in any straightforward way. But it is taking a risk, and the source of risk isn't simply the risk that eg. MCAS will fail on any given flight, it's also the higher-order risk that eg. the MCAS system will have a faulty design. (A bit like rewriting code in C for performance, or keeping on generally unloved code to satisfy a paying customer, presumably.) I assume that makes the ethical and safety decisions a lot more murky for the engineers than having a specific fault to point to, especially in an organisation afflicted with go fever.
sokoloff 7y ago
I believe the airplane is aerodynamically stable even at high alpha. What it fails in certification is the requirement to have continually increasing pitch feedback forces. I believe the pitch feedback forces are still in the stable region, just too low. This is not a flying wing or intentionally unstable airplane (as the F-16).
- leoc 7y ago
  >> [...] In most of the flight envelope the aircraft is stable. >> At high alpha the aircraft has pitch problems. > I believe the airplane is aerodynamically stable even at high alpha. [...] This is not a flying wing or intentionally unstable airplane (as the F-16). In some ways that's likely worse news than an airplane that's inherently unstable in general, no? A corner case, and one that evidently isn't actually all that uncommon. If you're building something like an F-16 you know that you absolutely have to make the fly-by-wire correct and robust, and the ground crews similarly know that if anything affects its performance the plane isn't fit to fly.
- inferiorhuman 7y ago
  I believe the airplane is aerodynamically stable even at high alpha. That would put you at odds with Boeing's test pilots.
  - sokoloff 7y ago
    No it wouldn't (or "citation needed" if so). The issue is not that the airplane shows negative or neutral aerodynamic pitch stability, it's that it does not exhibit an increasing stick force gradient, as required by certification rules. https://airfactsjournal.com/2019/03/can-boeing-trust-pilots/ https://airfactsjournal.com/2019/03/can-boeing-trust-pilots/

cmurf 7y ago

Citation needed. FAR 25 applies to all transport category aircraft. The section on stability (§§ 25.171 - 25.181). In exactly what manner is the airplane not stable, with or without MCAS?

nopzor 7y ago
i haven’t looked at the fars in quite a few years, but i’m pretty sure there would be stuff in there that references stuff like pitch stability (which is how i’d define the “hardware problem”), is that not the case? i’m afk atm. i still maintain my macro point, either way. making the airframe on a pax airliner aerodynamically stable during normal takeoff and landing operations seems like basic “good engineering” to me.
- burfog 7y ago
  I'm not so sure that it is good engineering to be inherently stable during normal takeoff and landing operations. Modern fighter planes are inherently unstable because this is required for better maneuverability. Passenger planes can certainly benefit from that too. Consider gusty crosswinds during a landing. With an inherently unstable aircraft, there is greater capability to compensate. You can have a computer stabilize the plane, preventing the tail or wing tips from striking the ground. When wake turbulence threatens to flip the plane or when a microburst threatens to pound the aircraft into the ground, a fast response is possible. Stability would deaden the performance of the needed response. The extreme example is probably wings that are low-mounted anhedral and forward-swept, with the bending controlled by rapidly actuated aerodynamic surfaces near the tips.
  - apcherry 7y ago
    A common fallacy I'm afraid - modern fighter planes are unstable for improved supersonic performance - reduced drag. So relaxed stability may benefit civil aircraft in terms of fuel efficiency. There is an argument that inherently unstable aircraft makes the manoeuvrability worse or harder. What many forget is that issue with manoeuvrability is actually at the end of the manoeuvre. The handling qualities aim is to point the nose in a new direction and an unstable aircraft makes the design challenge harder to stop the aircraft at the position required.

WalterBright 7y ago

All jetliners are unstable at altitude in cruise, and require augmentation. https://en.wikipedia.org/wiki/Yaw_damper https://en.wikipedia.org/wiki/Yaw_damper Aircraft design is a giant bag of compromises between desirable characteristics, most which are in conflict with each other.

jngreenlee 7y ago
Best-stated comment so far. We seem to forget that this whole problem exists in an edge domain only.
ulfw 7y ago
an INOP yaw damper doesn’t cause a hull loss crash
- WalterBright 7y ago
  It has: https://en.wikipedia.org/wiki/Dutch_roll#Accidents https://en.wikipedia.org/wiki/Dutch_roll#Accidents http://code7700.com/accident_kc-135r_63-8877.htm http://code7700.com/accident_kc-135r_63-8877.htm http://www.failure-interactions.com/boeing---19-oct-1959.html http://www.failure-interactions.com/boeing---19-oct-1959.htm...
  - VBprogrammer 7y ago
    Fun fact. The original 737 actually had two yaw dampeners because they expected it to have diverging yaw stability. Just by chance the airframe is actually positively stable so can operate safely with no yaw dampeners so one was removed.

mannykannot 7y ago

Even the 737 Max variants are statically stable in pitch, at least up to the stall. The issue that MCAS was intended to address was the handling characteristics, again in the flight regime prior to the stall. I do not know whether the airplane could have been certified, as a separate type, without some sort of augmentation, but it seems that Boeing did not think it could be certified under the common type rating that covers prior variants.