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Comment by silverquiet | Hacker News Reader

I read "Airframe" by Michael Crichton probably twenty years ago, and it was around ten years old at that time. I remember that that book talked about how the planes were unstable by design and required software to maintain proper flight characteristics, and that this was so because it was more efficient. The book is fiction, but I doubt it was far off the mark at the time. I suspect that there is no going back from this state of things, and so if there must be software, it should be good software.

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The 737-MAX is not "unstable by design". It has the exact same positive stability as other planes in 99% of it's flight envelope.

The remaining exception is that, at a certain AoA, speed, thrust, etc, there is a case where adding thrust pushes the nose up more than it does for the normal 737

The nose push is not abnormal, it is not unsafe, it is not unexpected. All planes with engines below the inertial "center" of the plane have this, including every 737 and every A320.

The problem was, this meant that it's flight characteristics were "different" from the older 737s. The entire point of any plane that is even a little bit 737 is to sell to airlines as "This is still a 737 and you don't need to train anyone in anything extra".

MCAS was built to change how the plane acts in this very specific regime, to act more like older 737s and counteract this nose push.

MCAS was entirely unnecessary except for business and policy goals. MCAS killed people because properly training aircrews for it would have gone against the entire point of the 737 MAX.

This is not correct and a gross simplification of the true issue.

All aircraft with underslung engines have similar pitch up tendencies to varying degrees and different handling characteristics between models are fine.

As seen with 757/67, 777/787 and A330/40/50 sharing type ratings despite being massively different aircraft.

> 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

The feeling of the aircraft is not the problem. The stall characteristics not complying with the regulation is.

If every aircraft has such pitch up tendencies, why did only the 737MAX violate regulation?

That reg says:

>No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall.

(how do airbus planes pass that I wonder....)

I guess most planes are "not abnormal" but the behavior of the MAX crossed into "abnormal"

This is no different than modern traction control, and in no way is "wrong" from a design perspective. If I recall, the more fundamental flaw here is the degraded behavior of MCAS with dual-sensor AoA system was not they adequately trained pilots for, which was clearly part of the business case for Boeing, negligent or not.

And boeing's whole philosophy is that they should make a passenger jet feel like Cessna, which is an increasingly tenuous fiction.

> boeing's whole philosophy is that they should make a passenger jet feel like Cessna

Where did you get this?

War planes dropped natural stability a while ago, IIRC the F16 was the first relaxed stability production aircraft (it's naturally stable at supersonic speeds but not subsonic).

In fact there was a flying airliner with relaxed stability (though only neutral not negative) when Airframe was published: the MD-11. Though I don't know that there have been others since.

I think you're right about the F-16 being the first. My money would have been on the F-117, but that apparently started development three years later – the F-16 is surprisingly old!

Isn't the high landing speed one of the unstable traits of this plane?

Fighter planes are unstable by design, and require computer control to stay in the air, because they're expected to do some pretty insane things in the air. A passenger plane has a somewhat different performance envelope, and while they are by-and-large fly-by-wire these days, they aren't designed to be aerodynamically very unstable.

Starting with the F-16, many fighter jets are designed to be intentionally unstable to improve maneuverability.

https://en.wikipedia.org/wiki/Relaxed_stability#Intentional_...

When that book was published, the F-117 Nighthawk was already retired after 10 years in service. The thing doesn't even look like it could fly on account of the rudimentary stealth features.

Passenger aircraft are stable by design. Fighter jets are unstable as this reduces drag and increases maneuverability.

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