4 ms·

Your point #1 is incorrect. The problem with the larger engines is that their larger nacelles placed further forward produce extra lift at high angles of attac

Your point #1 is incorrect. The problem with the larger engines is that their larger nacelles placed further forward produce extra lift at high angles of attack. This lift is further forward than the centre of mass. The certification requirement is that to produce steadily increasing angles of attack, you need to steadily increase back pressure on the yoke. The problem with the Max is that this is no longer true. Past a certain angle of attack, the back pressure needed to further increase angle of attack reduces. The plane is not actually unstable, but it's closer to being so than the certification requirements allow. And it's certainly behaviour that Boeing couldn't claim was similar enough to older 737s to allow a common type rating. Hence MCAS, which was supposed to detect this condition and make the aircraft fly like an older 737. This allowed a common type rating, and allowed the aircraft to be certified. But fundamentally, the airframe has an undesirable property, and you'd never have designed it this way unless the desire for a common type rating dominated other design decisions.

reacweb 8y ago
It is incredible: the airframe has been designed to reuse the certification of 737. A flaw that could be fixed in a proper way, instead has been worked around using unreliable subterfuges. The aim of the certification process is to ensure the safety (and reliability) of aircraft. The required mindset shall be that the certification process helps to highlight defects in order to build a better aircraft. Here, the mindset was that the certification process is a burden with arbitrary constraints that have to be fulfilled even if this means a worse aircraft. IMHO, the people (managers) with the wrong mindset shall be replaced and the faulty airframe of 737 shall be killed.
- greeneggs 8y ago
  This is unfair. If done appropriately, making a plane behave the way pilots expect should also make it safer. A "worse aircraft" with a better UI can actually be a safer aircraft.
  - rtkwe 8y ago
    Only so far as we can assume the UI never breaks and the sensors are always correct. The issue is pilots need to be trained to understand where the UI hides the underlying performance of the plane because when it breaks it can go wrong incredibly quickly. MCAS altered the way to aircraft reacted and also would restart it's changes unless completely cut out. Pilots didn't receive enough training to recognize the issue as being the MCAS system adjusting the trim fast enough to prevent the crashes during takeoff and ultimately the whole point of the MCAS was to avoid having to retrain and maintain the common type rating.
incongruity 8y ago
So, I must be missing something but all that re-engineering was to put a larger engine on the airframe but still have ground clearance, while on the ground, correct? What was stopping them from increasing clearance by increasing landing gear height? (And thus not impacting flight characteristics)
- mhandley 8y ago
  They did increase the length of the nose landing gear, which helped a bit. But increasing the length of the main landing gear would have required moving the gear further outboard, as there was no space to extend the gear in an inboard direction when it's stowed. You can't move the gear a lot further out, or it ends up behind the engines. See this picture someone else on this thread posted: https://i.stack.imgur.com/GFzcj.jpg https://i.stack.imgur.com/GFzcj.jpg And you really don't want to move the engines further out, because that's going to affect the engine-out behaviour, and require a larger vertical stabilizer to handle asymmetric thrust.
  - tropo 8y ago
    I think that image shows plenty of room. Remember that the tires don't count, because those would be farther away from the wing. There are other height issues with ground support equipment for luggage, fuel, passengers, maid service, sewage, etc.
skykooler 8y ago
Thank you; I've been following this since the second crash and this is the first time I've seen an explanation for why moving the engines forward made the plane unstable at high angles of attack.

reacweb 8y ago

It is incredible: the airframe has been designed to reuse the certification of 737. A flaw that could be fixed in a proper way, instead has been worked around using unreliable subterfuges. The aim of the certification process is to ensure the safety (and reliability) of aircraft. The required mindset shall be that the certification process helps to highlight defects in order to build a better aircraft. Here, the mindset was that the certification process is a burden with arbitrary constraints that have to be fulfilled even if this means a worse aircraft. IMHO, the people (managers) with the wrong mindset shall be replaced and the faulty airframe of 737 shall be killed.

greeneggs 8y ago
This is unfair. If done appropriately, making a plane behave the way pilots expect should also make it safer. A "worse aircraft" with a better UI can actually be a safer aircraft.
- rtkwe 8y ago
  Only so far as we can assume the UI never breaks and the sensors are always correct. The issue is pilots need to be trained to understand where the UI hides the underlying performance of the plane because when it breaks it can go wrong incredibly quickly. MCAS altered the way to aircraft reacted and also would restart it's changes unless completely cut out. Pilots didn't receive enough training to recognize the issue as being the MCAS system adjusting the trim fast enough to prevent the crashes during takeoff and ultimately the whole point of the MCAS was to avoid having to retrain and maintain the common type rating.

incongruity 8y ago

So, I must be missing something but all that re-engineering was to put a larger engine on the airframe but still have ground clearance, while on the ground, correct? What was stopping them from increasing clearance by increasing landing gear height? (And thus not impacting flight characteristics)

mhandley 8y ago
They did increase the length of the nose landing gear, which helped a bit. But increasing the length of the main landing gear would have required moving the gear further outboard, as there was no space to extend the gear in an inboard direction when it's stowed. You can't move the gear a lot further out, or it ends up behind the engines. See this picture someone else on this thread posted: https://i.stack.imgur.com/GFzcj.jpg https://i.stack.imgur.com/GFzcj.jpg And you really don't want to move the engines further out, because that's going to affect the engine-out behaviour, and require a larger vertical stabilizer to handle asymmetric thrust.
- tropo 8y ago
  I think that image shows plenty of room. Remember that the tires don't count, because those would be farther away from the wing. There are other height issues with ground support equipment for luggage, fuel, passengers, maid service, sewage, etc.

skykooler 8y ago

Thank you; I've been following this since the second crash and this is the first time I've seen an explanation for why moving the engines forward made the plane unstable at high angles of attack.