4 ms·
While accurate, it omits that part of the certification Boeing were chasing was a common type rating with previous 737s on the basis of it having similar handli
by Gwypaas 6y ago
While accurate, it omits that part of the certification Boeing were chasing was a common type rating with previous 737s on the basis of it having similar handling characteristics. MCAS did not exist because the 737 Max was unsafe, just different.
This is a common miss conception, planes can behave quite differently, they already do depending on loading. Then you have common type ratings, e.g. A330, A340 and A350 or 777 and 787 which only requires a short difference training to move between types.
The real reason is that the aircraft does not comply with stick force regulations when approaching stalls.
The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.
http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/mcas.htm
- deleted 6y ago[deleted]
- atoav 6y agoThis is the aerospace equivalent of trying to correct boomy room acoustics with an equalizer: Yeah you can do that, but like painting over a cracked wall it wont fix the underlying issue. And like the painted cracked wall, it might be okay in 99 out of 100 cases, but that one case where it isn't might kill a hundred people.
- foldr 6y agoThere are lots of commercial aircraft that don’t meet certification requirements without some form of artificial stability enhancement (yaw dampers being the classic example). There’s nothing inherently wrong with this kind of thing - you just have to get it right.
- dataflow 6y agoOut of curiosity, which airliners are like this?
- foldr 6y agoThe Boeing 727 is one example where the yaw damping systems are an important feature. Yaw dampers in general are very common. Most (all?) airliners have them in one form or another. The reason for this is that swept wings inherently tend to increase susceptibility to Dutch roll. And yet we don't conclude from this that it's inherently irresponsible of aircraft manufacturers to make airliners with swept wings.
- dataflow 6y agoInteresting, thanks!
- tschwimmer 6y agoPerhaps this is overly simplistic, but I can't imagine it's preferable from a safety perspective to have stability enhancement vs just having inherent stability.
- foldr 6y agoIt's not. The point is that virtually no transport category aircraft have inherent stability in all dimensions of the flight envelope, as this would require too many compromises in other areas. There's nothing unusual about the extent to which the 737-MAX relies on stability enhancement. There are existing commercial airliners that are far more dependent on it for safety. For example, the Boeing 727 has two redundant yaw dampers because failure of both yaw dampers would result in the aircraft becoming irrecoverably uncontrollable at certain altitudes. In contrast, if your removed MCAS from the 737-MAX it would still be completely flyable.
- tiahura 6y agoHundreds (thousands?) of commercial pilots flew the Max and none of them even noticed MCAS until the crashes. It seems to have served its purpose remarkably well. The unforgivable sin was the brain dead engineering decisions around not sanity checking the AOA sensor.
- labawi 6y agoI seem to recall reports of pilots disabling MCAS because it was acting up. They would disable autotrim or something, not sure it they even heard of MCAS.