Comment by sillysaurusx | Hacker News Reader

Comment by sillysaurusx | Hacker News Reader

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Wait, what?

Anyone have any more info on this? It seems completely insane for flight control inputs to invert themselves, but only in certain temperatures. I'd love to see the incident report on that...

EDIT: Ah: https://en.wikipedia.org/wiki/USAir_Flight_427

I'm so sorry that happened to your wife's father.

The accident report is here: https://www.ntsb.gov/investigations/AccidentReports/Reports/...

Section 2.7.2 is interesting:

2.7.2 Unusual Attitude Training for Boeing 737 Pilots

At the time of the USAir flight 427 accident, no air carrier training programs were specifically aimed at training 737 pilots to recognize and address a rudder jam or reversal. The guidance available at that time from Boeing advised pilots, as a first consideration, to maintain or regain full control of the airplane. Specifically, the guidance advised pilots to counter unwanted roll tendencies from a malfunctioning rudder with the application of up to full aileron control inputs. However, the guidance did not advise pilots that, at some airspeeds, an uncommanded full rudder input could not be successfully opposed by full wheel (aileron and spoiler) inputs and that a reduction in the airplane’s angle-of-attack could improve the effectiveness of the roll controls relative to the effectiveness of the rudder. Boeing’s guidance for relieving a jammed rudder informed pilots only that they should use maximum force to overpower the jam and specifically warned pilots against turning off flight control switches “unless the faulty control was positively identified.” No additional guidance was provided about the effects of flight control switch selections on rudder jam conditions.

The Mayday/Air Crash Investigation episode titled "Hidden Danger" has good info on this and covers several crashes iirc.

If I start reading I go for hours haha. Am I correct that it inverted/reversed?

Yes, but I'm not sure why it inverted. If you open the accident report (https://www.ntsb.gov/investigations/AccidentReports/Reports/...) and ctrl-F "rudder", you get 2,112 matches. So "reading for hours" is a distinct possibility in this case...

(Oh, by the way, I didn't mean to imply you were mistaken, or anything like that. I was just so shocked that a plane's controls could invert themselves in any conceivable circumstance. Currently digging through the accident report to try to figure out what happened.)

"the probable cause of the USAir flight 427 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and overtravel of the primary slide"

Unfortunately I don't know what most of these terms mean. It sounds like it might be saying "they moved the rudder to its limit, and then it jammed at the limit; so when they tried to command the rudder to go the other way, the rudder was pointed in the opposite direction of the control inputs." But I'm really not sure.

No offense taken. I meant that if I start researching it again I’ll be up for 5 hours because of the personal ties to it.

My recollection in researching this a decade ago was that the cold weather made the behavior of the rudder change (my term was invert, maybe that’s not accurate), and pilots were trying to pull up, but in fact putting the plane deeper into a nosedive. Some 25 seconds later they hit the ground at some 300-400 MPH and vaporized.

The only way to have saved the plane would have been to dump the hydraulics and/or switch to a manual fly by wire mode, neither of which were a logici thing to do in the 15 seconds they had to save the flight.

I’ve only flown for 20 hours in a Cessna so sorry if my terminology isn’t perfect!

> ... behavior of the rudder change (my term was invert, maybe that’s not accurate), and pilots were trying to pull up, but in fact putting the plane deeper into a nosedive.

The rudder controls yaw, not pitch. The rudder controls did invert: the pilot tries to turn slightly left, the plane turns right, so the pilot naturally tries turning harder left until the rudder is in the full right position, and they're in an uncoordinated turn. The wing on the inside of the turn starts to drop significantly or even stalls. The plane is then sideways and in a dive, and on top of that, the rudder is all the way over in the direction that makes the dive steeper. The planes quickly rolled over on their sides and went into steep dives.

My understanding is that a big break in the case came when a pilot figured out what was going on and successfully landed a plane with reversed rudder controls.

Yes. This took a long time to work out with initial finger pointing between Boeing and the Pilot unions.

"Testing revealed that under certain circumstances, the Power Control Unit (PCU)'s dual servo valve could jam and deflect the rudder in the opposite direction of the pilots' input. Thermal shock testing revealed that the uncommanded rudder movement could be replicated by injecting a cold PCU with hot hydraulic fluid. Thermal shock resulted in the servo's secondary slide becoming jammed against the servo housing, and that when the secondary slide was jammed the primary slide could move to a position that resulted in rudder movement opposite of the pilot's commands"

https://en.wikipedia.org/wiki/Boeing_737_rudder_issues

resulted in rudder movement opposite of the pilot's commands

Did the rudder actually move in opposition to the commands, or was it stuck at some limit value?

i.e. in that situation, if you command the rudder to go left, does it move even more in the wrong direction? Or is it simply frozen at whatever position it happened to get stuck in?

Thanks for the info!

I'm not sure. I'm not a pilot and only have memory of this from prior reading. As I look at it now it reads more like it was uncommanded movement to the limit of travel and getting stuck there. I've also heard this problem referred to as the "rudder hard-over" problem. But there are also statements about rudder movement opposite of that commanded.

In theory the problem was recoverable but the pilots had about 10 seconds to realize what was going on and react appropriately. Sounds familiar to the 737-MAX uncommanded trim issues.

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