Cubsrule
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Strange DC-9 Problem (NW 1749 10/26)

Sun Oct 26, 2008 10:57 pm

My wife asked me to try to get to the bottom of what happened on her NW flight, so I'm going to give it a shot...

She said they were descending normally when the f/as secured the cabin very quickly and then briefed the pax in the exit row. Despite the winds being relatively calm, she reported a lot of lateral motion on approach (like what happens on the ERJ, but worse) and then a very hard landing. They then taxied normally to the gate and deplaned; she didn't see any emergency equipment. She asked me what had happened and, based on what she reported, I didn't have a clue. I figured I'd at least take a stab at it here.
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liedetectors
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RE: Strange DC-9 Problem (NW 1749 10/26)

Mon Oct 27, 2008 4:43 am

Yaw damper failure on approach perhaps?
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Cubsrule
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RE: Strange DC-9 Problem (NW 1749 10/26)

Mon Oct 27, 2008 11:26 am



Quoting Liedetectors (Reply 1):
Yaw damper failure on approach perhaps?

That was the first thing that popped into my mind too; I wonder if that is serious enough to brief the exit row passengers.
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CosmicCruiser
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RE: Strange DC-9 Problem (NW 1749 10/26)

Mon Oct 27, 2008 12:35 pm

Maybe someone who has flown a DC-9 can comment but in the 727 I don't remember it being an issue at low altitudes. I had only one yaw damp failure and at altitude there was some dutch roll but descending to a slightly lower alt toook care of that and we completed the flight.
 
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CrimsonNL
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RE: Strange DC-9 Problem (NW 1749 10/26)

Mon Oct 27, 2008 2:21 pm

Pardon for asking, but what effect does it have once the Yaw-Dampers fail?
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EMBQA
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RE: Strange DC-9 Problem (NW 1749 10/26)

Mon Oct 27, 2008 5:30 pm



Quoting Cubsrule (Thread starter):
Despite the winds being relatively calm,

According to who...?Sounds like gusty conditions to me. Often you will get layers of winds. At 20,000ft it's calm, at 15,000ft it's gusty
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EMBQA
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RE: Strange DC-9 Problem (NW 1749 10/26)

Mon Oct 27, 2008 5:36 pm



Quoting Cubsrule (Reply 2):
I wonder if that is serious enough to brief the exit row passengers

Briefing the exit row passengers is very common on many airlines before landing. It's just to remind them of their conditions for sitting in that row and what they might be called on to do.
"It's not the size of the dog in the fight, but the size of the fight in the dog"
 
Cubsrule
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RE: Strange DC-9 Problem (NW 1749 10/26)

Mon Oct 27, 2008 5:39 pm



Quoting EMBQA (Reply 5):
According to who...?Sounds like gusty conditions to me. Often you will get layers of winds. At 20,000ft it's calm, at 15,000ft it's gusty

She said the yaw issue occurred on final, so the METAR should be pretty close to what the conditions were.

KMEM 262053Z 31010KT 10SM FEW050 24/12 A3002 RMK AO2 SLP164 T02440117 55007
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Starlionblue
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RE: Strange DC-9 Problem (NW 1749 10/26)

Mon Oct 27, 2008 11:35 pm



Quoting CrimsonNL (Reply 4):
Pardon for asking, but what effect does it have once the Yaw-Dampers fail?

As I understand it: The pilot flying has to coordinate turns himself (work the rudder) and in a swept wing jet this is not a trivial endeavor. You can thus get some fishtailing.
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CosmicCruiser
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RE: Strange DC-9 Problem (NW 1749 10/26)

Tue Oct 28, 2008 2:09 am

DUTCH ROLL DESCRIPTION
This section describes the procedures and techniques
used to familiarize the pilot with dutch roll recovery.
At high altitudes and high Mach number, dutch
roll may occur with both yaw dampers inoperative. At
low altitudes and low airspeed, the roll is dampened
sufficiently so that a no-yaw-damper approach and
landing presents no problem.
Dutch roll, as experienced in the B-727, differs from
that in other swept wing airplanes not only in speed of
occurrence, but in characteristics. The B-727 has
more side-slip per degree of bank angle and the bank
angle should not be allowed to exceed plus or minus
8° from the initial bank angle. Oscillations exceeding
the 8° limits impose excessive stress on the empennage
and could possibly cause structural damage.
 
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RE: Strange DC-9 Problem (NW 1749 10/26)

Tue Oct 28, 2008 6:36 am



Quoting Starlionblue (Reply 8):
As I understand it: The pilot flying has to coordinate turns himself (work the rudder) and in a swept wing jet this is not a trivial endeavor. You can thus get some fishtailing.

This can get VERY ugly in a hurry. Especially for anyone seated aft of the wing- make sure those barf bags are in place!!!!! Lol.

It's hard for a pilot to coordinate his turns properly in this class of aircraft. I mean, granted, they're just pilots, but still- I'll give them some credit with this one!!
 
CosmicCruiser
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RE: Strange DC-9 Problem (NW 1749 10/26)

Tue Oct 28, 2008 12:37 pm

As I posted IF you did have yaw damp fail you certainly would not be pushing on the rudders, at least not in the 727. The slightest input both aileron or rudder starts a yawing that is stabilized only thru opposite inputs of the aileron. As the description says above at high mach and alt you would stabilize the yaw and descend; at low alt it's not a factor. As I posted it happened to me only once CLT-MEM and the recovery was much easier than what we always saw in the sim. We went down a few flight levels and continued on. The recovery requires an aileron input opposite the direction of the bank/yaw followed by a quick small input in the other direction. Done right the yaw stops very quickly, done wrong and you will lose control.
 
PhilSquares
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RE: Strange DC-9 Problem (NW 1749 10/26)

Tue Oct 28, 2008 12:44 pm



Quoting Starlionblue (Reply 8):
As I understand it: The pilot flying has to coordinate turns himself (work the rudder) and in a swept wing jet this is not a trivial endeavor. You can thus get some fishtailing.



Quoting AA737-823 (Reply 10):
This can get VERY ugly in a hurry. Especially for anyone seated aft of the wing- make sure those barf bags are in place!!!!! Lol.

It's hard for a pilot to coordinate his turns properly in this class of aircraft. I mean, granted, they're just pilots, but still- I'll give them some credit with this one!!

I have to jump in here and back up what CosmicCruiser is saying. I have never flown the DC-9 series, but like CC have flown the 727. Down at lower altitudes failure of a yaw damper is a non event. The airspeeds are too slow to have any adverse yaw be strong enough to start the Dutch Roll process. I can't imagine the DC-9 being any worse than the 727.

As to what it was, who knows, perhaps an over zealous FA or some minor malfunction that the cockpit crew didn't have a chance to advise the passengers about. Either way, at the end of the day, the takeoffs and landings were the same and that's what counts.
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Cubsrule
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RE: Strange DC-9 Problem (NW 1749 10/26)

Tue Oct 28, 2008 3:06 pm



Quoting PhilSquares (Reply 12):
As to what it was, who knows, perhaps an over zealous FA or some minor malfunction that the cockpit crew didn't have a chance to advise the passengers about.

Based on the lack of emergency equipment (and the normal taxi), I'd say an overzealous f/a might be a good guess... still, I've probably flown close to 100 NW flights, and I've never seen them say anything to exit row pax prior to landing.
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kimberlyRJ
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RE: Strange DC-9 Problem (NW 1749 10/26)

Tue Oct 28, 2008 5:32 pm



Quoting EMBQA (Reply 6):
Briefing the exit row passengers is very common on many airlines before landing. It's just to remind them of their conditions for sitting in that row and what they might be called on to do.

Its something we are starting to do at British Airways on short haul flights and on long haul flights an announcement is made prior to landing asking passengers to remind themselves where their nearest exit is located.

On flights that are 11 hours plus the crew will point out the nearest exits to were passengers are seated.

Not sure what other airlines do though…

Kimberly.
 
PGNCS
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RE: Strange DC-9 Problem (NW 1749 10/26)

Sun Nov 02, 2008 9:32 pm



Quoting AA737-823 (Reply 10):
Quoting Starlionblue (Reply 8):
As I understand it: The pilot flying has to coordinate turns himself (work the rudder) and in a swept wing jet this is not a trivial endeavor. You can thus get some fishtailing.

This can get VERY ugly in a hurry. Especially for anyone seated aft of the wing- make sure those barf bags are in place!!!!! Lol.

It's hard for a pilot to coordinate his turns properly in this class of aircraft. I mean, granted, they're just pilots, but still- I'll give them some credit with this one!!

It is not difficult in the DC-9.

Quoting PhilSquares (Reply 12):
I have to jump in here and back up what CosmicCruiser is saying. I have never flown the DC-9 series, but like CC have flown the 727. Down at lower altitudes failure of a yaw damper is a non event. The airspeeds are too slow to have any adverse yaw be strong enough to start the Dutch Roll process. I can't imagine the DC-9 being any worse than the 727.

You are right, Phil. The DC-9 series is much tamer than the 727 in this regard. Unlike the 727, it is a nonevent at any speed and altitude.

Quoting PhilSquares (Reply 12):
As to what it was, who knows, perhaps an over zealous FA or some minor malfunction that the cockpit crew didn't have a chance to advise the passengers about. Either way, at the end of the day, the takeoffs and landings were the same and that's what counts.

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