Pilot error and maintenance likey cause in 757 event

[Hopeful]

http://online.wsj.com/article/SB10001424052748703889204576078531893643542.html?ru=yahoo&mod=yahoo_hs

 

[eolesen]

That's not how I read the preliminary report....

There was a 16-18 second lag in the thrust reverser deployment, which I thought was an automatic deploy when the squat switch was closed. The missing bushing is a concern, but a 767 pilot at another airline whom I know didn't think the two were related.

Here's the actual report verbage, since reporters don't always know how to read...:

The NTSB systems investigator conducted testing on the incident airplane from December 31, 2010, through January 6, 2011. Operational testing and examination was accomplished on the spoiler/auto speed brake, air/ground, autobrake, and thrust reverser systems. No discrepancies were found in the air/ground, autobrake, and thrust reverser systems.

Examination of the auto speed brake mechanism in the cockpit pedestal found that the linear actuator aft attachment was improperly installed and was missing a bushing. This loose connection allowed the cam to be rotated slightly relative to the switch, which could cause the switch roller and the notch in the cam to not always align. System operation with this condition present is being investigated.

* The recorded speed brake handle position indicates that the speed brakes were manually extended by the flight crew during the approach after which the handle was left in the armed position until landing. The FDR records only the speed brake handle position and not the individual speed brake (spoiler) panel positions.

* The FDR data indicate that the aircraft touched down at approximately 132 knots.

* At touchdown, the air/ground parameter changes to "ground" for approximately one second and then switches to "air" for approximately ½ second before changing back to "ground" for the remainder of the recording.

* During the time period when the air/ground parameter switched back to "air," the speed brake handle position momentarily moved toward the down position and then returned to the armed position where it stayed for the remainder of the recording.

* Thrust Reverser (T/R) discrete parameters indicate that the T/Rs moved into the in-transit position during the ½ second that the air/ground logic parameter indicated "air."

* The T/Rs remained in the in-transit position for approximately 10 seconds before transitioning to the stowed position for one second. The T/Rs then moved back to the in-transit position for an additional 6 seconds before becoming deployed.

* The T/R discrete parameters indicate that approximately 18 seconds elapsed from the time the T/Rs began moving until they were fully deployed.

 

[Hopeful]

That's not how I read the preliminary report....

There was a 16-18 second lag in the thrust reverser deployment, which I thought was an automatic deploy when the squat switch was closed. The missing bushing is a concern, but a 767 pilot at another airline whom I know didn't think the two were related.

Here's the actual report verbage, since reporters don't always know how to read...:

They may not be related. T/R systems are independent of each other in their operation. But if both reversers experience the same delay, then a switch could be the culprit. I tend to agree with the pedestal mechanism/switching configuration/bushing issue as the probable cause. That is where both reversers have commonality.
Also, preliminary reports are just that...preliminary. They probably did a little research into other 757 similar events and found common causes of that problem.

 

[glasspilot]

There was a 16-18 second lag in the thrust reverser deployment, which I thought was an automatic deploy when the squat switch was closed.

There is an automatic deploy of the ground spoilers with the air/ground switch, IF they are armed before landing. The thrust reversers do not deploy without pilot movement of the reverse levers.

 

[eolesen]

There is an automatic deploy of the ground spoilers with the air/ground switch, IF they are armed before landing. The thrust reversers do not deploy without pilot movement of the reverse levers.

Thanks for the clarification... I had spoilers on the brain...

I'm still wondering how they're spinning this as pilot error. The prelim says spoilers were armed, and yes, the switch might have had something to do with that, but if the switch wasn't in contact, why & how did it show up on the FDFR as armed?

Hopeful, how does the spoiler switch affect the T/R deploying?

I recall there being two interlockers at play here -- one preventing the T/R from moving until the air/ground is showing in ground mode, and a second which prevents thrust changes when they're in transit... but I don't recall any dependency on the spoilers.

 

[Hopeful]

Thanks for the clarification... I had spoilers on the brain...

I'm still wondering how they're spinning this as pilot error. The prelim says spoilers were armed, and yes, the switch might have had something to do with that, but if the switch wasn't in contact, why & how did it show up on the FDFR as armed?

Hopeful, how does the spoiler switch affect the T/R deploying?

I recall there being two interlockers at play here -- one preventing the T/R from moving until the air/ground is showing in ground mode, and a second which prevents thrust changes when they're in transit... but I don't recall any dependency on the spoilers.

In my earlier post i wasn't referring to a spoiler switch, but rather the switch that activates from t/r levers sending the signal for t/r's to deploy. A couple of things are in play here. Throttles to idle, reverser lever moved, reversers deployed.
You're right there is no dependency on the spoliers. But, like glass pilot said, once the auto speed brake lever is armed, and once the t/r levers are moved to reverse, the speed brakes will deploy. But there is no AUTO deploy of the reversers.
I was basically trying to say that if BOTH reversers were slow to deploy, it would have had to be caused by a "common" factor such as a switch.

 

[eolesen]

Got it. I thought the switch you were commenting on was the one mentioned for the speed brake lever mentioned in the first section of the NTSB quote in my first reply...

 

[Hopeful]

Got it. I thought the switch you were commenting on was the one mentioned for the speed brake lever mentioned in the first section of the NTSB quote in my first reply...

It's funny how the very first paragraph states "no discrepancies" were found in those systems. But further examination discovered a missing bushing in the auto speedbrake mechanism. To me that just sounds like a "by the way" finding.

This problem could have very well been caused by the air/gnd sensing system. Affecting both reversers, that is one of those common systems.