THS Jackscrew design

[David36]

Reading Alaska Airlines flight 261 reports , I wonder why NTSB points out only to DC-9, MD-80/90 and Boeing 717 fleet regarding the jackscrew design considering that (as far as I know) the designs used on the 727,737,747 etc. are similar to the one used on those cited.

Could anyone with technical knowledge on the issue explain how do they differ justifying this differentiation between aircraft types in reports?

[TheRobe]

It's not the design that is the problem, it's lazy mechanics that didn't want to pull over a latter and a can of grease, just signing it off as inspected.

Same thing with the C130s cracking up doing Fires...guys weren't doing the wing spar inspections, just signing them off.

[David36]

I know that and it is not the point of this thread (even if the reports conclusion was that it is not appropriate to rely solely on maintenance and inspection intervention to prevent the failure from occurring).

[TheRobe]

David - I get your point. but if you want to prove that this or that design is inherently flawed then you need criteria to determine 'safe' vs 'flawed' and by what universal standard.

Simply put, all sorts of products are put into service that haven't had the benefit of thousands of hours of testing. They have a general idea on how it will work, how it will hold up, and they put maintenance procedures in place to mitigate the design flaws.

Some times you can't look after and upkeep something so much, and thus get an STC or in the case of cars, a recall.

At issue is whether they purposefully put out a crappy unsafe product and try to cover it up. Sure, it happens.

I've noticed a number of aircraft that had what I would call an Achilles heel, that thing that I need to really watch...an item, flying characteristic, whatever...

If you are asking whether the jack screw is a problem, I think hundreds of thousands of hours of safe flying would say no, as long as it's looked at. Are there better systems, I don't know. Go research it.

If you want a windmill to joust about safety, push for hiring better pilots. The Alaska flight went down because the pilots hired flew a worsening flight control problem but defaulted to SOPS, calling dispatch as it got worse. They did what the company trained them to do and it got everyone killed.

Simply put you can fly any plane and for whatever reason it could have a problem, how you handle the problem is key....and it flies in the face of current theory that you design all the problems out of an aircraft so you can hire incompetents to fly that will never be faced with making a decision if something goes wrong outside of the check list.

[David36]

As I said, it is pretty clear for me that it's not a safety flaw considering that as far as I know it was the only one crash related to that jackscrew and the cause was gross improper maintenance.

However, maybe I'm too dumb for this, but I still don't get it. Why did they asked changes for certain aircraft while almost all other use the same system? Are there some particular differences related to design on DC-9, MD-80/90 and Boeing 717 fleet over others?

[TheRobe]

Most of these things come down to money...how to make it, how to save it, how not to lose it.

[Hand Solo]

I seem to remember the DC-9 series was the only model to operate a single jack screw/drive mechanism. Basically the others had greater redundancy designed in so the failure mode was less likely to apply. It's been a while so I could be wrong, but I think that's the right direction.

[David36]

Yeah, I figure out that would be the only logical reason to make the recommandation for those aircrafts, however, as far as I know, it is not true. Most of big jets use single jackscrew design to actuate the horizontal stabilizer, so it's not the case. In fact the only one that I know they use different systems are DC-10,MD-11 (I hope my memory is not at fault) which use double jackscrew and L-1011 which has hyd actuated. The others, including Airbus, I know they have just a single jackscrew.

Maybe there are particular design features which make it different even being just one jackscrew. That's why I'm asking, I hope someone here has some detailed knowledge on these designs to clarify.

[Volume]

The others, including Airbus, I know they have just a single jackscrew.While indeed havin just one threaded portion (hence a failure of the nut will result in total failure), the jackscrew is dual loadpath design with an inner pin in the screw, so fracture of the screw will not result in total failure.

[David36]

And that's different? I thought it's the same thing on MD types.
Edit: What I found is that Boeing uses a ballscrew design, that is on series 727,737,757,767 and maybe others.

[DozyWannabe]

In the wake of Alaska 261 there were cross-type and cross-manufacturer inspections and ADs mandated. No implementation (of which each manufacturer's was slightly different) was entirely free of problems in terms of maintenance or design:

Airbus : FAA mandates near-term A320 stabiliser inspections (http://www.flightglobal.com/news/articles/faa-mandates-near-term-a320-stabiliser-inspections-223556/)

Boeing : FAA calls for heightened 737 trim actuator vigilance (http://www.flightglobal.com/news/articles/faa-calls-for-heightened-737-trim-actuator-vigilance-223347/)

Here's a contemporary article on the subject:

Aviation Today :: Jackscrew Jugular Can Be Fixed (http://www.aviationtoday.com/regions/austnz/2877.html#.UPQyCCfZZf0)

[Fargoo]

It's not the design that is the problem, it's lazy mechanics that didn't want to pull over a latter and a can of grease, just signing it off as inspected.

Ever so slightly simplistic view of the incident. There were just a few more contributing factors to that accident.

[Lyman]

What if an aircraft lost good data from sensors, and cycled elevator and THS automatically, to twelve degrees pitch down, at the same time disallowing manual override?

Uncontrollable. Loss of Control. Program Error.

Is that similar to Alaskan? As I recall, the MD80 had metal shards in the threads of the jackscrew, remnants of gross negligence.

What's a crew to do?

Innovate? I would say, yes.

[DozyWannabe]

As others have pointed out, there's not an airliner type in the world that cannot have its THS angle manually overridden - at least to some extent.

[TheRobe]

So in the end you just don't want to hear that lazy mechanics and robots that won't get the plane down right away is the problem?

What are you digging for? Some inherent flaw that if fixed will allow the mechanics to go home and the pilots not have to think?

[Typhoon650]

I thought the problem with the Alaska airlines aircraft was that the jackscrew assembly failed and due to either inadequate or non existent limit stops, the horizontal tail went well past limits, and well past what the elevators could overcome. I suspect the horizontal tail would've been stalled if it had full elevator one way and full trim the other.
Hardly bad piloting.

[DozyWannabe]

Inadequate maintenance meant that the stab jackscrew assembly had not been properly lubricated, eventually stripping the thread completely. The acme nut that was supposed to provide the stop eventually came away as there was no further resistance to it doing so. By that point the aircraft was unrecoverable, but the crew fought to recover it all the way to impact.

[TheRobe]

Keep in mind they were up in the air for about an hour screwing with the trim, got harder and harder to keep the plane level.....the problem wasn't the jackscrew...any more then a vibration that got worse and worse from a bearing failure, or that oil temp that gets hotter and hotter, or that little whistle in the window that gets louder and louder, or that nut job in row 17 that keeps going to the bathroom and brings a package with him.

Things break, for whatever reason and the solution isn't always in the checklist or a call to dispatch.

The passengers had the wrong pilots that day and they paid for it with their lives.

[sevenstrokeroll]

well, we really need to wait a second.

I flew the DC930 for 10 years. It is a great plane...our fleet was checked for the ''jackscrew'' thing and NONE, NONE of our planes had any problems...DC930 and MD80...NONE...

We were following the DOUGLAS manual and did everything it said.

One or two times I had problems with the trim. There is a backup trim (alt/aux) that remained in service just fine and there was a thermal cutout on the main trim motor, and when allowed to cool returned to service. But once we were on the ground, and wrote it up, it was taken into MX and checked out.

AS to the pilots...well I sure don't want to blame them on this situation. But I will say this...a sure thing is the way to go when ever there is a control problem...esp when approaches to a number of airport runways could be made while over the ocean, eliminating the hazard to innocents on the ground.

There has been a terrible philsophy promoted out there of always calling someone on the ground for advice. I've seen some pretty STUPID advice coming up from the ground.

One piece of advice was sent to a brand new 737-400 that couldn't get one of its main landing gears down. The brilliant advice was to take the crash axe, go back in the cabin, CHOP through the floor, cut the hydraulic line (under pressure) and this was SURE to get the gear down.

The pilots, smarter than average said: NUTS (paraphrasing...and offering tribute to the 82nd at Bastogne) They landed with the offending gear up. Upon investigation the wheel chocks were found in the wheel well making the gear STUCK. Mechanics left the wheel chocks in the gear well during pushback.

Call for advice any time...only take stupid advice once safely on the ground.

The screw is one of the simple machines of man...but everything needs maintenance.

in true retrospect, the pilots should have landed at any of the coastal airports along the way. Shuddering or any difficulty controling the plane is an immediate emergency, priority number one get everyone out of the way I'm coming in any runway any taxiway get the hell out of the way.

Flying the DC9 series by hand is a joy for an experienced pilot...anything that is unpleaseant in ''feel'' should be warning enough for an otherwise great flying plane.

[DozyWannabe]

Keep in mind they were up in the air for about an hour screwing with the trim, got harder and harder to keep the plane level.....

It wasn't a gradual thing, it was stuck - literally held in place by a sliver of metal. Once that went there was nothing they could do. They kept trying the trim because experience suggested that they had a bad motor or electrical connection.

We were following the DOUGLAS manual and did everything it said.

So did Alaska, just at twice the required intervals between checks.

This was primarily an MX issue.

[sevenstrokeroll]

i thought there were some other issues with alaska mx...a ''home made tool to judge tolerance'', a different lubrication than mfr rec'd.

just a lesson for all pilots...its up to you if the flight is safe or not. not some guy in maintenance control or ''dispatch''.

dispatch...ha...I got a message during landing rollout on a snowy runway at KSYR...it came on ACARS after we landed...message follows: DO NOT LAND AT KSYR.

our message in response, once we had stopped was...DO YOU WANT US TO TAKEOFF AGAIN?

dispatch.!

[DozyWannabe]

i thought there were some other issues with alaska mx...a ''home made tool to judge tolerance'', a different lubrication than mfr rec'd.

In general yes, but no evidence of that on the accident airframe. For bringing the general lapses in maintenance practice to light, the whistleblower concerned never worked in aviation again.

[TheRobe]

No Dozy, your wrong. Read the report. The threads were becoming more and more stripped. So when the pilots trimmed for an altitude, it would hold, then give...then they needed to retrim for the next place that held...this went on and on and on, until such time trim wouldn't cover the problem, then slop in the tail became so great and where it would hold at a weakened thread, then give, then hold, then give, until there wasn't any threads in which to hold, thus loosing control. So ponder this as the pilots, Alaska's finest, top notch guys, hired from the top of resume pile, went through all the checklists (ooos no tail control failure)...SOPS(ooops no jack screw failure)..Dispatch (gosh we don't know...just keep flying to LA) and lastly the pilots yanking back and forth on the yoke trying to keep the plan upright, flip it over into the ocean. You can call this mechanical failure but it's actually top down from the Chief Pilot and HR, people I have met. If you hire mechanics that don't do inspections and robots that can't make a decision these problems will happen from time to time. Another great example of CRM at work by the way.

[sevenstrokeroll]

the robe

I tend to agree with your views...but the crew was told to continue past LA...I think either to San Francisco or Seatle...memory fails me now.

Each accident, if remembered by another crew may save another flight.

And, like I said...call dispatch after you land...they really don't know too much. IF THEY DID< THEY WOULD FLY THE PLANE and YOU WOULD CLEAN IT

[TheRobe]

Seven - Thanks for correcting me, your right, they lost it around SFO on the way up North.

[David36]

No Dozy, your wrong. Read the report. The threads were becoming more and more stripped. So when the pilots trimmed for an altitude, it would hold, then give...then they needed to retrim for the next place that held...this went on and on and on, until such time trim wouldn't cover the problem, then slop in the tail became so great and where it would hold at a weakened thread, then give, then hold, then give, until there wasn't any threads in which to hold, thus loosing control.I'm sorry, where did you find such detailed reports to prove that pilots have made such exageratted attempts until all the threads wore out?

In all reports I found there are no detailed actions to prove this. I think everyone agrees that they would have to land soon, but it makes me think that the system gave it because then no longer have what to keep it in place, not as they would have spent time on it to totally wear it out. As someone here pointed, It wasn't a gradual thing, it was stuck - literally held in place by a sliver of metal. Once that went there was nothing they could do.


What if an aircraft lost good data from sensors, and cycled elevator and THS automatically, to twelve degrees pitch down, at the same time disallowing manual override?
Uncontrollable. Loss of Control. Program Error.Those computers do not have a shutdown button?:E


the jackscrew assembly failed and due to either inadequate or non existent limit stops, the horizontal tail went well past limits,
In my opinion, this is one of the most important aspects in this case.


FAA mandates near-term A320 stabiliser inspections (http://www.flightglobal.com/news/articles/faa-mandates-near-term-a320-stabiliser-inspections-223556/)
I agree it is such a stupid idea to contact MX inflight, what I think is more important is how it comes to trust MX regarding installation and correct rigging?

[barit1]

The issue of "design fault" was raised a few posts back. I might agree in terms of the maintainability of the jackscrew. Pulling a ladder or scaffold to reach the stab isn't exactly the easiest sometimes.

OTOH one failure over the decades doesn't exactly shout "bad design" when dozens of other operators had minimal issues with the jackscrew. The finger definitely seems to point to the operator in this case.

BTW, the accident occurred off Santa Barbara, past LAX on the way to SFO.

[DozyWannabe]

Those computers do not have a shutdown button?:E

No, but with Airbus FBW the pilot can override automatic trim by setting the trim manually with the trim wheel and holding it there. The hard-alpha protections can be disabled by shutting off 2 of the ADR computer modules.

Obviously this is outside of recommended procedure, but on the flip side it's a potential solution for an extremely rare occurrence. It's also way off the topic for this thread, but given the source of the query that's no surprise...

@barit1 : The design met the certification requirements for the time. I believe they've since been tightened...

[boofhead]

Years ago I had the same problem with a jammed brake on one side (policy to test the brakes prior to landing and one stuck fully on). Pedal was hard and could not release it.
Called Maintenance and they said take the crash axe and break the servo hydraulic line (described to me) to release the pressure. It took me 45 minutes due to the confined space but it worked.
Sometimes they give good advice. Just saying.

[Brian Abraham]

No Dozy, your wrong. Read the report. So when the pilots trimmed for an altitude, it would hold, then give...then they needed to retrim for the next place that held...this went on and on and on, until such time trim wouldn't cover the problem,Wrong. Read the report.FDR data indicated that the horizontal stabilizer’s last movement during the climbout was to 0.4º airplane nose down at 1349:51 as the airplane was climbing through 23,400 feet at 331 knots indicated airspeed (KIAS). After this, no horizontal stabilizer movement was recorded on the FDR until the airplane’s initial dive 2 hours and 20 minutes later.The passengers had the wrong pilots that dayAbsolute nonsense. You can sit in your lounge chair as a non pilot and Monday morning quarter back the decisions made by the crew with 20/20 hindsight, but I venture any typical line crew probably would have made the same decisions.
2.2.5 Flight Crew Decision-Making
2.2.5.1 Decision to Continue Flying Rather than Return to PVR
Safety Board investigators considered several reasons that might explain the captain’s decision not to return immediately to PVR after he experienced problems with the horizontal stabilizer trim system during the climbout from PVR.

Neither the Alaska Airlines MD-80 Quick Reference Handbook (QRH) Stabilizer
Inoperative checklist nor the company’s QRH Runaway Stabilizer emergency checklist required landing at the nearest suitable airport if corrective actions were not successful. These checklist procedures were the only stabilizer-related checklist procedures contained in the QRH, and the flight crew most likely followed these checklist procedures in their initial attempts to correct the airplane’s jammed stabilizer

The airplane’s takeoff weight of 136,513 pounds was well below the takeoff and climb limits for the departure runway, but it exceeded the airplane’s maximum landing weight of 130,000 pounds. Because the airplane did not have an in-flight fuel dumping system, the airplane would have had to remain in flight for about 45 minutes after takeoff until enough fuel had burned to reduce the airplane’s weight by the 6,500 pounds needed to reach the airplane’s maximum landing weight. A return to PVR to execute an overweight landing would have required higher than normal approach speeds for landing and would have created additional workload and risk. An overweight landing at PVR would have been appropriate if the flight crew had realized the potentially catastrophic nature of the trim anomaly. However, in light of the airplane’s handling characteristics from the time of the initial detection of a problem to the initial dive, the flight crew would not have been aware that they were experiencing a progressive, and ultimately catastrophic, failure of the horizontal stabilizer trim system.

The flight crew would have been aware that Alaska Airlines’ dispatch and
maintenance control in Seattle-Tacoma International Airport (SEA), Seattle, Washington, and LAX could be contacted by radio (via ground-based repeater stations) when the airplane neared the United States. However, even though the last horizontal stabilizer trimming movement was recorded by the FDR about 1349:51, the flight crew did not contact Alaska Airlines’ maintenance until shortly before the beginning of the CVR transcript about 1549,225 which suggests that control problems caused by the jammed horizontal stabilizer remained manageable for some time. Further, as previously mentioned, the positive aerodynamic effects of the higher cruise airspeed and fuel burn would have reduced the necessary flight control pressures to roughly 10 pounds and made the airplane easier to control. Therefore, the Safety Board concludes that, in light of the absence of a checklist requirement to land as soon as possible and the circumstances confronting the flight crew, the flight crew’s decision not to return to PVR immediately after recognizing the horizontal stabilizer trim system malfunction was understandable.

Although they elected not to return to PVR, later in the flight the flight crew
decided to divert to LAX, rather than continue to San Francisco International Airport (SFO), San Francisco, California, where the flight was originally scheduled to make an intermediate stop before continuing to SEA. Comments recorded by the CVR indicated that the flight crew may have felt pressure from Alaska Airlines dispatch personnel to land in SFO.227 However, after discussing the malfunctioning trim system and current and expected weather conditions at SFO and LAX with Alaska Airlines dispatch and maintenance personnel, the captain decided to land at LAX rather than continue to SFO. The decision to divert to LAX was apparently based on several factors, including more favorable wind conditions at LAX (compared to a direct crosswind at SFO) that would reduce the airplane’s ground speed on approach and landing228 and the captain’s concern, expressed to Alaska Airlines dispatch personnel, about “overflying suitable airports.” The Safety Board concludes that the flight crew’s decision to divert the flight to LAX rather than continue to SFO as originally planned was prudent and appropriate. Further, the Safety Board concludes that Alaska Airlines dispatch personnel appear to have attempted to influence the flight crew to continue to SFO instead of diverting to LAX.

[TheRobe]

Brian - Maybe you should read you own report. Simply put it's not like they were flying along and all of a sudden the tail fell off. They were having trim and flight control problems for an hour, hence the call to dispatch.

Apologize all you want for the pilots...but it was a top down culture at Alaska that caused this issue...from mechanics that didn't do inspections to pilots who couldn't make a decision and would fly into a mountain if dispatch told them to.

[Brian Abraham]

pilots who couldn't make a decision and would fly into a mountain if dispatch told them toSo do tell what request/order from dispatch did they comply with?

[TheRobe]

The fact that they screwed with their trim for an hour then called dispatch for a 'what do we do?' is the point Brian.

[Brian Abraham]

They did not make a request for "'what do we do". They elicited information as would any crew and made their own decisions with respect to the information received.

According to Alaska Airlines documents, ATC and CVR information, and postaccident interviews with Alaska Airlines dispatch and maintenance personnel, the flight crew contacted the airline’s dispatch and maintenance control facilities in SEA some time before the beginning of the CVR transcript at 1549:4912 to discuss a jammed horizontal stabilizer and a possible diversion to Los Angeles International Airport (LAX), Los Angeles, California. These discussions were conducted on a shared company radio frequency between Alaska Airlines’ dispatch and maintenance facilities at SEA and its operations and maintenance facilities at LAX.

At 1549:56, the autopilot was disengaged; it was re-engaged at 1550:15.
According to the CVR transcript, at 1550:44, SEA maintenance asked the flight crew, “understand you’re requesting… diversion to LA …is there a specific reason you prefer LA over San Francisco?” The captain replied, “well a lotta times its windy and rainy and wet in San Francisco and uh, it seemed to me that a dry runway…where the wind is usually right down the runway seemed a little more reasonable.
”
At 1552:02, an SEA dispatcher provided the flight crew with the current SFO
weather (wind was 180° at 6 knots; visibility was 9 miles). The SEA dispatcher added, “if uh you want to land at LA of course for safety reasons we will do that…we’ll …tell you though that if we land in LA… we’ll be looking at probably an hour to an hour and a half we have a major flow program going right now.” At 1552:41, the captain replied, “I really didn’t want to hear about the flow being the reason you’re calling us cause I’m concerned about overflying suitable airports.” At 1553:28, the captain discussed with the first officer potential landing runways at SFO, stating, “one eight zero at six…so that’s runway one six what we need is runway one nine, and they’re not landing runway one nine.” The first officer replied, “I don’t think so.” At 1553:46, the captain asked SEA dispatch if they could “get some support” or “any ideas” from an instructor to troubleshoot the problem; he received no response. At 1555:00, the captain commented, “it just blows me away they think we’re gonna land, they’re gonna fix it, now they’re worried about the flow, I’m sorry this airplane’s [not] gonna go anywhere for a while …so you know.” A flight attendant replied, “so they’re trying to put the pressure on you,” the captain stated, “well, no, yea.”

At 1556:08, the SEA dispatcher informed the flight crew that, according to the SFO automatic terminal information service, the landing runways in use at SFO were 28R and 28L and that “it hasn’t rained there in hours so I’m looking at…probably a dry runway.” At 1556:26, the captain stated that he was waiting for a requested center of gravity (CG) update (for landing), and then he requested information on wind conditions at LAX. At 1556:50, the SEA dispatcher replied that the wind at LAX was 260° at 9 knots.

Nine seconds later, the captain, comparing SFO and LAX wind conditions, told the SEA dispatcher, “versus a direct crosswind which is effectively no change in groundspeed…I gotta tell you, when I look at it from a safety point I think that something that lowers my groundspeed makes sense.”16 The SEA dispatcher replied, “that’ll mean LAX then for you.” He then asked the captain to provide LAX operations with the information needed to recompute the airplane’s CG because “they can probably whip out that CG for you real quick.” At 1558:15, the captain told the SEA dispatcher, “we’re goin to LAX we’re gonna stay up here and burn a little more gas get all our ducks in a row,
and then we’ll uh be talking to LAX when we start down to go in there.” At 1558:45, the captain asked LAX operations if it could “compute [the airplane’s] current CG based on the information we had at takeoff.”

At 1602:33, the captain asked LAX operations for wind information at SFO. LAX operations replied that the winds at SFO were 170° at 6 knots. The captain replied, “that’s what I needed. We are comin in to see you.” At 1603:56, the first officer began giving LAX operations the information it needed to recompute the airplane’s CG for landing.

At 1607:54, a mechanic at Alaska Airlines’ LAX maintenance facility contacted
the flight crew on the company radio frequency and asked, “are you [the] guys with the uh, horizontal [stabilizer] situation?” The captain replied, “affirmative,” and the mechanic, referring to the stabilizer’s primary trim system, asked, “did you try the suitcase handles and the pickle switches?” At 1608:03, the captain replied, “yea we tried everything together.” At 1608:08, the captain added, “we’ve run just about everything if you’ve got any hidden circuit breakers we’d love to know about ‘em.” The mechanic stated that he would “look at the uh circuit breaker uh guide just as a double check.” The LAX mechanic then asked the flight crew about the status of the alternate trim system, and, at 1608:35, the captain replied that “it appears to be jammed…the whole thing, it [the AC load meter] spikes out when we use the primary, we get AC [electrical] load that tells me the motor’s tryin to run but the brake won’t move it. when we use the alternate, nothing happens.”

At 1608:50, the LAX mechanic asked, “you say you get a spike…on the meter up there in the cockpit when you uh try to move it with the …primary right?” According to the CVR transcript, at 1608:59, the captain addressed the first officer before responding to the mechanic, stating, “I’m gonna click it off you got it.” One second later, the first officer replied, “ok.” At 1609:01, the captain reiterated to the LAX mechanic that the spike occurred “when we do the primary trim but there’s no appreciable uh change in the uh electrical uh when we do the alternate.” The LAX mechanic replied that he would see them when they arrived at the LAX maintenance facility.

[TheRobe]

No your right Brian, when the flight controls are failing, a crew calls up dispatch to talk to let the chief pilot know to have catering ready when they land.

[Brian Abraham]

No your right Brian, when the flight controls are failing, a crew calls up dispatch to talk to let the chief pilot know to have catering ready when they land.You're a jerk (first class), but almost forgivable in a non pilot such as yourself. They did no such thing, as the dialogue of discussion with dispatch I posted above proves.

Lets have a look at the reports discussion re failing controls. I absolutely hate introducing facts into the discussion.
The Safety Board recognizes that, from an operational perspective, the flight crew could not have known the extent of airplane damage. Although flight crews are trained in jammed stabilizer and runaway stabilizer scenarios, the loss of acme nut and screw engagement exceeded any events anticipated in emergency training scenarios, and the flight crew was not trained to devise or execute appropriate configurations and procedures to minimize further damage to the airplane or to prevent the accident. However, the flight crew’s earlier attempts to activate the trim motor and configuration changes may have worsened the situation. As previously discussed, the captain’s activation of the primary trim motor at 1609:16 precipitated the release of the jam and the initiation of the initial dive. However, it was not clear how many times previous to that the flight crew activated the primary trim motor nor was it clear whether or to what extent the prior activations hastened the release of the jam. Therefore, the Board could not determine the extent to which the activation of the primary trim motor played a role in causing or contributing to the accident.
2.2.5.5 Adequacy of Current Guidance
The Safety Board notes that after the flight 261 accident, Boeing issued a flight operations bulletin outlining procedures to be followed in the event of an inoperative or malfunctioning horizontal stabilizer trim system. The bulletin advised flight crews to

complete the flight crew operating manual (FCOM) checklist(s). Do not attempt additional actions beyond that contained in the checklist(s). If completing the checklist procedures does not result in operable trim system, consider landing at the nearest suitable airport.

The Safety Board agrees that this advice is generally appropriate. However, the Board does not agree that the flight crew should merely “consider” landing at the nearest suitable airport if accomplishing the checklist items does not result in an operational trim system. In such a case, the flight crew should always land at the nearest suitable airport as expeditiously and safely as possible. Further, the bulletin provides additional information regarding the possibility that repeated or continuous use of the trim motors may result in thermal cutoff and states that the motor may reset after a cooling period. The Board is concerned that this additional information addressing repeated or continuous use of the trim motors may weaken or confuse the initial guidance to refrain from attempting troubleshooting measures beyond those specified in the checklist procedures.

The Safety Board concludes that, without clearer guidance to flight crews
regarding which actions are appropriate and which are inappropriate in the event of an inoperative or malfunctioning flight control system, pilots may experiment with improvised troubleshooting measures that could inadvertently worsen the condition of a controllable airplane. Accordingly, the Safety Board believes that the FAA should issue a flight standards information bulletin directing air carriers to instruct pilots that in the event of an inoperative or malfunctioning flight control system, if the airplane is controllable they should complete only the applicable checklist procedures and should not attempt any corrective actions beyond those specified. In particular, in the event of an inoperative or malfunctioning horizontal stabilizer trim system, after a final determination has been made in accordance with the applicable checklist that both the primary and alternate trim systems are inoperative, neither the primary nor the alternate trim motor should be activated, either by engaging the autopilot or using any other trim control switch or handle. Pilots should further be instructed that if checklist procedures are not effective, they should land at the nearest suitable airport.The Safety Board also believes that the FAA should direct all Certificate Management Offices (CMO) to instruct inspectors to conduct surveillance of airline dispatch and maintenance control personnel to ensure that their training and operations directives provide appropriate dispatch support to pilots who are experiencing a malfunction threatening safety of flight and instruct them to refrain from suggesting continued flight in the interest of airline flight scheduling.

[TheRobe]

Brian: So what your saying is that if you were flying an MD80, you would just keep flying along, trimming, retrimming, getting worse and worse, until the plane became uncontrollable and then crash. Yes, Brian, I believe you.

[Brian Abraham]

you would just keep flying along, trimming, retrimming, getting worse and worseYou really as thick as a brick son. That is not what happened, and is self evident in what I've posted from the report, if indeed you ever passed english comprehension.

[Machinbird]

Let's not have a mud slinging contest gentlemen.:=

Being a bit of a gear head, I've always wondered whether I would have been able to figure out the likely cause for the jamming trim while inflight.

The proper recovery action for that Air Alaska aircraft, before the nut let go completely, was probably to do a no flap approach into some place with a very long runway-something like Edwards AFB-and to trim the aircraft by moving passengers as necessary. Let the airline figure out how to get it out of there afterwards.

I learned my lesson about malfunctioning flight controls a long while ago. Don't even bother going flying with them if they are not fully functional. They are either completely right, or they are not right at all for flight.

Sea story:
I was assigned a combat air patrol in my F-4 and the pilot who had just flown it told me that there was a little "catch" in the stick in the last bit of nose up stabilator travel and to check it out.
When I did my control checks, by golly, he was right! The last inch of stick travel had an abnormal feel to it, but it moved the full travel. Being gung-ho, I went flying off the ship and returned after an uneventful flight. I downed the aircraft because the stick was supposed to be free for the whole travel. Maintenance needed an aircraft badly for the next launch, so they Upped the aircraft and gave it to the next pilot who also downed it when he felt the slight bind, this time before flying it.

What they found when they investigated the problem made me realize how lucky we had been. An electrical control box for the ARI was mounted on a platform just aft of the stabilizer control rod. The bolts attaching the box to the platform had all fallen out and the box was dangling from its wire bundle on the edge of the platform and contacting the linkage which pushed it out of the way with aft stick causing the "catch". If the box had gone over the edge of the platform, it would have limited how much aft stick was available. Not good when you have to slow down to land aboard an aircraft carrier. :eek:

As soon as you are aware of a flight control malfunction that FCOM does not properly address, that is the time to become very defensive and very conservative in your approach. Get it on the ground as safely as possible and let the geniuses figure it out.:}

[TheRobe]

Brian - So you want to rewrite history? So the pilots didn't have any problems, they called dispatch to just talk about the weather and all of a sudden they flopped over into the ocean. Is that about it?

---------------------

Machine - After the death spiral in the ocean, the response by the other Alaska pilots told the tale. You had guys running their trims all the way up...all the way down, full control movements instead of the cursory half ass 'good enough'....

Half the fleet wouldn't take off the next day after a number of pilots found 'discrepancies' in their trim and flight controls. The fleet was grounded and they admit to finding atleast 3 more jackscrews on their way out.

I don't know how many guys took off then landed abruptly after having problems, or thinking they had problems in flight.

Whatever the case, other jackscrew problems existed and once on the radar pilots started taking notice, and hence, no more crashes.

[Brian Abraham]

So you want to rewrite historyI'm afraid you are the one attempting to rewrite history.The fleet was grounded and they admit to finding atleast 3 more jackscrews on their way outThe fleet was never grounded. Alaska commenced voluntary inspections of the jackscrew on the 9th Feb, following advice from the NTSB that the jackscrew had been recovered from the ocean in a damaged condition. The inspections were complete by the following day, with two other aircraft found to have discrepancies. These two were the only ones grounded.pilots started taking notice, and hence, no more crashesNo more crashes had nothing to do with the pilots, rather the maintenance deficiencies were rectified.

A word of advice. You are welcomed by one and all to make contributions. Just cease making out to be something that you are not. You are not a pilot, so please stop pretending you are one.

And cease telling professional aviators that they know not what they are talking about - especially in the derisive, language you are so fond of using. I refer to your contributions on subjects such as derate, flex, runway performance, implications of V1 etc etc in particular.

If you are out to log the record as the poster with the most aliases and bannings, please continue on your merry way.

[TheRobe]

Here is the wiki to Flight 261 may I suggest that you are sailing close to the wind, my friend.. take care

Alaska Airlines Flight 261 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Alaska_Airlines_Flight_261)

1- They called dispatch for help, a solution...

'and on a shared company radio with operations and maintenance facilities at Los Angeles International Airport (http://en.wikipedia.org/wiki/Los_Angeles_International_Airport) (LAX) discussed a jammed horizontal stabilizer and a possible diversion to LAX'

2- Dispatch told to continue...

During this time the flight crew had several discussions with the company dispatcher (http://en.wikipedia.org/wiki/Flight_dispatcher) about whether to divert to Los Angeles, or continue on as planned to San Francisco. Ultimately the pilots chose to divert.[8] (http://en.wikipedia.org/wiki/Alaska_Airlines_Flight_261#cite_note-NTSB_Final-8) Later the NTSB found that while "the flight crew's decision to divert the flight to Los Angeles...was prudent and appropriate", nonetheless "Alaska Airlines dispatch personnel appear to have attempted to influence the flight crew to continue to San Francisco

3- (CVR) transcripts indicate that the dispatcher was concerned about the effect on the schedule ("flow") should the flight divert.[9] (http://en.wikipedia.org/wiki/Alaska_Airlines_Flight_261#cite_note-CVR_transcript-9)

[Brian Abraham]

My post #30
Further, the Safety Board concludes that Alaska Airlines dispatch personnel appear to have attempted to influence the flight crew to continue to SFO instead of diverting to LAX. So you add absolutely nothing with your dispatch personnel appear to have attempted to influence the flight crew to continue to San FranciscoI'm intrigued that you take Wiki (as good as it is most cases) should have precedence over and above the official NTSB report.

[thermostat]

My 2 cents worth. First I like the post by Machinbird. When it comes to controls, if it doesn't feel right, land the plane. Had I been flying that DC-9, it would have been put on the ground much earlier. I wouldn't give a heck what the company wanted, I'm flying, not the company or anyone in the company.
About the design, if the hinge was placed at the leading edge of the stab and the screwjack at the trailing edge, this accident would not have happened. Why they designed it the other way around, I don't know.
Having said that, there is NO excuse for bad maintenance. People's lives are at stake. If I remember correctly, the FAA inspector did warn of shoddy work and was told to "shut up" or be transferred. People should have gone to jail over this incident. So what happened? Today, 4 people are going to jail over the 737 crash. There has to be accountability. The same for the Nation Air DC-8 crash. There should have been jail terms. Too many managers getting away with crap.

[DozyWannabe]

My 2 cents worth. First I like the post by Machinbird. When it comes to controls, if it doesn't feel right, land the plane. Had I been flying that DC-9, it would have been put on the ground much earlier. I wouldn't give a heck what the company wanted, I'm flying, not the company or anyone in the company.

For what it's worth, they were planning to divert to LAX once they realised the problem was more serious than a simple electrical malfunction - unfortunately by then the damage was too severe and they ran out of time.

While the jackscrew/stab design was criticised in the NTSB report for not being "fail-safe", the fact is that because the DC-9's certification ("grandfathered" to the MD-80 series) was performed during an era where such designs could be permitted as long as maintenence schedules were strictly adhered to, the regulators had to allow it.

If I remember correctly, the FAA inspector did warn of shoddy work and was told to "shut up" or be transferred.

I'm pretty sure that the FAA inspectors just trusted the paperwork - the whistleblower in the Alaska case was one of their own senior engineers. The accident occurred after he had raised objections, but before his warning was heeded by the FAA. Despite his being completely in the right, he was (through inter-airline management collusion) effectively blacklisted from working in aviation ever again.

[thermostat]

Thanks for your 2 cents worth Dozy. As you say they were planning to divert but ran out of time. That is why I said they should have landed earlier, long before LAX. LAX was too late as we now know.
Just because something is "certified" doesn't make it foolproof. there are a number of cases in aviation history where bad, but certified systems have killed people. The design is stupid. The screwjack should have placed at the trailing edge, not the leading edge, just as the rudder of a plane or boat is aft of the rudder post.
As far as the whistleblower is concerned, the mechanic unions should have made a stink and threatened to shut down the entire industry.
That's how I feel about it. Aviation is not the place to fool around. There is too much at stake.
Thanks.

[DozyWannabe]

That is why I said they should have landed earlier, long before LAX. LAX was too late as we now know.

Where else could they have gone? Lindbergh in San Diego is a challenging approach at the best of times from what I've been told, and certainly not ideal with a potential control issue.

Just because something is "certified" doesn't make it foolproof. there are a number of cases in aviation history where bad, but certified systems have killed people.

Agreed.

The screwjack should have placed at the trailing edge, not the leading edge, just as the rudder of a plane or boat is aft of the rudder post.

You can't fit it aft of the rudder post because there's no room. I'm having a little trouble understanding this theory of yours - could you humour me and go into a little more detail?

As far as I can tell, mounting it aft would cause the same failure mode to force the stab to nose-up and stall the aircraft rather than nose-down into a dive.

As far as the whistleblower is concerned, the mechanic unions should have made a stink and threatened to shut down the entire industry.

In an ideal world yes - but we're talking about the middle of a recession, and I don't know if the engineer concerned was affiliated.

Aviation is not the place to fool around. There is too much at stake.

Agreed in principle, but real-world issues tend to cloud things.

[Lonewolf_50]

Lindbergh in San Diego is a challenging approach at the best of times from what I've been told, and certainly not ideal with a potential control issue.
Agreed

As to the whistleblower getting the axe ... so much for whistleblower protection laws. :mad:

I note The Robe is banned.

The problem with trying to trouble shoot a problem in an aircraft is that if you don't actually know what is broken, or what the failure mode is that you are trying to mitigate, it is easy to make the wrong decision and potentially make a wrong move.

I've seen it in real life when an engine went wrong in a twin engined helicopter, and the pilot flying identified incorrectly the engine that was low power, and tried to fly on that engine before I corrected him and he got the right emergency throttle selected.

We could have torched an engine.

The gents in Alaska 261 were, in terms of trouble shooting, in the blind insofar as what their real problem was. That they had a flight control problem was not as accurate a problem statement as they had a flight control linkage problem ... a lot different than what they seemed to have begun trouble shooting for.

[DozyWannabe]

As to the whistleblower getting the axe ... so much for whistleblower protection laws. :mad:

I think the latest laws were written as a result of what happened there - too late to help the individual involved, sadly.

The problem with trying to trouble shoot a problem in an aircraft is that if you don't actually know what is broken, or what the failure mode is that you are trying to mitigate, it is easy to make the wrong decision and potentially make a wrong move.

Quite. As I said on another thread, Rule 1 is "Don't make it worse!".

I've seen it in real life when an engine went wrong in a twin engined helicopter, and the pilot flying identified incorrectly the engine that was low power, and tried to fly on that engine before I corrected him and he got the right emergency throttle selected.

The same problem reared its head on the Kegworth B734 with a much nastier outcome. Used to the B733 where AC bleed air came from the right engine only, the crew assumed that it was the right engine that was failing and shut it down. It was in fact the left engine that had lost and ingested blade debris. The B734 used bleed air from both engines, but this design change was not mentioned during conversion training.

The gents in Alaska 261 were, in terms of trouble shooting, in the blind insofar as what their real problem was. That they had a flight control problem was not as accurate a problem statement as they had a flight control linkage problem ... a lot different than what they seemed to have begun trouble shooting for.

Indeed - the problem first manifested as unresponsive trim, but everything else seemed to be OK. It'd be interesting to ask members who are or were Mad Dog crew how often trim problems cropped up on the line.

[noske]

The screwjack should have placed at the trailing edge, not the leading edge, just as the rudder of a plane or boat is aft of the rudder post.
You can't fit it aft of the rudder post because there's no room. I'm having a little trouble understanding this theory of yours - could you humour me and go into a little more detail?

As far as I can tell, mounting it aft would cause the same failure mode to force the stab to nose-up and stall the aircraft rather than nose-down into a dive.

I think I know what he means. After the screwjack was forcefully disconnected, had the hinge point not been so far aft on the stabilizer, it would not have been pushed against the next available mechanical stop, but would have swung freely, like a weather vane.

I don't know if I like that scenario better, btw., because it seems to leave the crew with no pitch control at all.

[PJ2]

Dozy, noske;

Concerning the MD83 Stabilizer accident there is a decent NTSB animation of the THS installation here (http://web.archive.org/web/20090319073228/http://www.ntsb.gov/Events/2000/Aka261/animations/jackscrew_261.wmv).

I think a reversal of the hinge-point and the jackscrew mechanism arrangements, (hinge point forward, jackscrew aft),would permit a certain weather-cocking as per noske's comments, (or possibly flutter), but I think Dozy is correct in observing that making room for such an arrangement would be challenging, (though not impossible, given some of the variations of airplane designs extant!).

The design, (single jackscrew, aft hinge), is industry-proven as are the outcomes of not following established maintenance schedules and procedures. Frankly I don't think there is much to learn here except the obvious.

Regarding the flight crew there is no direct instruction to attempt to move a jammed stabilizer but nor in the manuals I have for the DC9 & DC8 are there procedures for handling a jammed stabilizer in cruise. Perhaps those flying the MD80 can comment.

David36 mentioned in post #8 the DC10/MD11 as having two jackscrews, I assume as an example of redundancy and mentioned the L1011 as having a "hydraulic" THS. There would be no jackscrew for the THS on the L1011 as the entire stabilizer moved, serving as both "elevators" and a "THS".

[3holelover]

Did anyone ever answer the OP's question? ....in a T-tail access is more difficult.

[cockney steve]

As an aside- many vehicle-maintenance-lifts have used a jackscrew,sometimes on one leg only, using cables or chains to send the otherside/corners up.

When you're stood under 1 1/2 tons of metal,unprotected, it concentrates the mind :hmm:

Invariably, a brass or Phosphor-Bronze nut winds up/down the Acme-thread with the attached carriage conttaining arms/bed.
Floating in that connection is a SECOND nut(often steel) which carries no weight. This can be arranged a short distance from the load-carrier, which, as it's threads wear, closes-up on the secondary, safety nut thus giving an instant visual indication. The safety-nut would not be fit to use regularly but would safely lower the vehicle if the mechanic should witness "jolt" "clonk" as the main nut threads stripped.

I.m amazed that, even back then, an aircraft was certified with no backup safety-nut. must have had more concientious maintenance mechanics back then!