Southwest 737 grounded for inspection after a "Dutch roll"

[y_p_w]

Can anyone explain that that means? It obviously refers to some sort of instability, but the articles on the incident don't seem to provide much other than it's just kind of rocking. Preliminary reports seem to indicate that the rudder was damaged by a backup power control unit.

The FAA and NTSB are investigating an unusual rolling motion of a Southwest Airlines Boeing 737 Max

Federal officials are investigating an unusual rolling motion during the flight of a Southwest Airlines Boeing 737 Max.

apnews.com

AIN Notices Report

www.asias.faa.gov

 

[Propflux01]

A dutch roll is when the tail of the airplane moves from side to side. Or to put it another way, it's like when the rear wheels of your car slip out and you slide from left to right or right to left. It is usually caused by a rudder malfunction, or severe wind gusts and overcorrrection.

 

[Randol]

The 737 uses a black box called a SMYD (Stall Management Yaw Damper) to counter yaw oscillations.
Dutch Roll or Roll Coupling is primarily when the rudder movement causes one wing to dynamically advance and the other to retreat against the airflow causing asymmetrical lift, hence the roll. Swept wing aircraft are more sensitive, due to the angle of the airflow over the wings. There are other factors too.
The Power Control Unit or PCU is an electrically controlled hydraulic servo valve that moves the rudder.

 

[Just a civilian pilot]

Juan Browne is now talking about it on his YouTube channel

 

[y_p_w]

Juan Browne is now talking about it on his YouTube channel

I thought he was on a long motorcycle trip with his wife.

 

[fdcg27]

I thought that every swept wing aircraft had a yaw damper to prevent Dutch roll, a sort of autopilot that only flies the rudder, and that it was dual, since a complete failure would result in very high crew workload tap dancing the rudder pedals. If I'm wrong about this I can expect a quick rebuttal from a Canadian member.
Early 707 test flying and training involved shutting the yaw damper off and there were instances of aircraft shedding engines during these flights, so violent was the yaw/roll couple.

 

[Just a civilian pilot]

Just back from flying and felt my ears burning.

A320 has 2 yaw dampers that are always on. We cannot turn them off. The only way both could get “ turned off” is if other systems fail.

The engines fell off that B707 because the pilots ( on purpose ) exceeded the bank angle limits when demonstrating Dutch roll characteristics to airline pilots on board.

Had they kept the bank angle within limits, they could have safely demonstrated Dutch roll without causing the engines to fall off, and the plane crashing.

How three engines fell off a brand-new Boeing 707

At 12,000ft above Washington State, the factory-fresh jetliner had three of its engines ripped off and the fourth had to be shut down. Key.Aero reveals how and why this dramatic incident happened

www.key.aero

https://asn.flightsafety.org/asndb/333926

 

[Just a civilian pilot]

Just watched the Juan Browne video.

Interesting point he makes about another highly experienced B737 pilot stating the B737 doesn’t have any problems recovering from Dutch roll with the yaw dampers turned off ( engines also stay attached ).

Very curious to read that preliminary NTSB report.

Not speculating what the cause is but below is an interesting Mayday video on rudder hard overs that Juan Browne talked about.

 

[Just a civilian pilot]

More detailed explanation of that B707 crash.

Boeing 707 jetliner crashes near Oso in Snohomish County, killing fou

On October 19, 1959, four crew members are killed and four passengers are injured when a new Boeing 707 jet transport on a demonstration flight for Braniff International Airways crash-lands in the No

www.historylink.org

 

[john_pifer]

More detailed explanation of that B707 crash.

Boeing 707 jetliner crashes near Oso in Snohomish County, killing fou

On October 19, 1959, four crew members are killed and four passengers are injured when a new Boeing 707 jet transport on a demonstration flight for Braniff International Airways crash-lands in the No

www.historylink.org

Fascinating, but tragic, reading.

Thanks for posting.

 

[mk378]

Interestingly the official report does not mention the existence of a stabilization system on the 707 or that it "had been turned off" as many stories of that crash say.

 

[Just a civilian pilot]

Here is the only article I could find involving a B707 that crashed with an inoperative yaw damper.

If they didn’t have so much smoke in the aircraft, I am sure they would have made a successful landing.

Point is, no commercial jet risks crashing just because the yaw damper isn’t working, unless they are doing other stuff they aren’t supposed to be doing.

I have done 45 degree bank turns ( sim ) in the A320, it’s no big deal.

https://asn.flightsafety.org/asndb/330046