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First SAS Q400 Accident Final Report Out  
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6424 posts, RR: 54
Posted (5 years 1 month 1 week 3 days 6 hours ago) and read 5380 times:

The final report of the fist SAS Q400 MLG accident has now been published and can be read at
http://www.hcl.dk/graphics/Synkron-L...0433%20Final%20report%20LN-RDK.pdf

There are no big surprises. The only thing, which is really new to me, is that the corrosion was a galvanic corrosion.

The actuator piston rod ane the rod end with ball joint were connected with male treads on the rod end into female treads in the piston rod. And secured with a jam nut.

The piston rod and the rod end were made of two different and galvanically incompatible sorts of steel. The rod end was made of "steinless steel" with 12% crome and 8% nickle while the piston rod was made of plain type 4340 steel.

The female tread bore was significantly deeper than the male treads on the rod end leaving a pretty large empty room behind the rod end treads. Air would blow in and out of this room when the plane climbed and descended to/from altitude. When descending the steel would be cold from cooler air at cruising altitude and dew would often form and be blown into the treads and the empty space.

The moisture started the galvanic corrosion eating the less noble of the metal couple which was the 4340 steel piston rod's female treads.

Then the joint became lose. The jam nut was often tightened at C checks, but then the load was transferred mainly to the extreme tips of the male treads on the rod end, which consequently suffered fatigue cracks.

The joint was supposed to be inspected first time after 22,400 cycles when the rod end should be replaced due to wear on its ball joint. The accident plane had just over 15,000 cycles in 7 years.

The report includes some very high quality pictures of the failed components, some of them with extreme magnification.

When the hydraulic rod end failed and separated, then the landing gear fell freely due to gravity and air drag. When it was fully extended, then the kinetic energy from the free fall broke the main hinges of the forward strut. That left the landing gear hanging as a pendulum on the main hinges on the back only.

Crew acted less than briliantly

The crew fast discovered that they had no checklist which fitted exactly to their failure indication. They got somehow mentally stuck on the checklist for manual gear operation. Even if the indication was clearly "right MLG in transit", then they seem to have been convinced after visual inspection trough the cabin windows that the gear was down and operational. They decided that the FO would land the plane, brake using both engines with reverse pitch, while the captain would leave the plane as soon as possible and secure the gear with securing pins.

But as we all know the gear collapsed two seconds after touch down.

An FA proposed to the captain that pax along the propeller disk would be removed from that area. Since there was only 7 vacant seats, then the captain decided that pax on row 6, 7 and 8 on the right hand side only should be reseated.

As if a propeller blade, which made it trough the fuselage side at half rifle bullet speed, wouldn't make it over the aisle also. The consequense was one pax seated at left hand side aisle on row 8 got cuts on a hand. It could have been fatal for a few pax in less lucky circumstances.

At the last two similar accidents the crews acted much more cleverly. They shut down the engine on the failing side and feathered the propeller. They did not engage spoilers, and they kept balance with ailerons as long as possible as speed was bleeding off. One of those planes was even repairable.

Why did the crew of the first accident do such an inferior job? My guess: Since introduction into service they had experienced so many false fault indications on the Q400 that they had named the main display "The Slot Machine". Indicating that there was always irrelevant blinking indicators on that screen. Therefore they dared not diviate from the book and take own decisions. And this fault, although correctly indicated on the screen, was not covered by the QRH (Quick Reference Handbook).

If only they had made it to the emergency landing checklist (which they never did), then they would have been directed to do the things required.

SAS MX acted less than briliantly

When a jam nut comes lose over and over again practically fleetwide, what do you do then? Over and over again you...
- issue a compalint card (nut lose)
- correct the fault in accordance with the book (nut tightened)
- sign off the complaint card.

I mean, had this same thing happened on a car or a lawn mover, then a courious car or lawn mover mechanic would have said to himself: "I want to know why this nut is constantly coming lose". And the design fault would have been discovered years before any catastrophic failure.

Recommendations

Even if the report is long - 142 pages - then the recommendations are short and clear, and there are only two of them:

1. It is recommended to review the design, the certification and the maintenance program of the MLG retraction/extension actuator and rod end.

2. It is recommended to review the landing gear abnormal and emergency procedures in the manufacturer's Airplane Flight Manual and Quick Reference Handbook.

Short but tough words, which to my knowledge already have been taken into account by Bombardier and various Q400 operators around the world long time ago.

Some 64k$ questions

How could an old and well respected landing gear manufacturer make such a fundamental design fault?

How could SAS tell the press that this was a "one off" failure, when any lawn mover mechanic needed five minutes to see on the accident plane that design was fundamentally at fault? And wait 60 hours and one more idetical accident to ground and rebuild the fleet.

Note: The third accident, which prompted SAS management to retire the fleet for PR reasons, was totally different - foreign object in the hydraulic system. Very likely caused by hasty rebuilding of the fleet with new actuators after the first two accidents. But the exact origin of the object may never be proved, and in any case the final report isn't out yet.


Always keep your number of landings equal to your number of take-offs, Preben Norholm
4 replies: All unread, jump to last
 
User currently offlineOyKIE From Norway, joined Jan 2006, 2732 posts, RR: 4
Reply 1, posted (5 years 1 month 1 week 2 days 23 hours ago) and read 5106 times:



Quoting Prebennorholm (Thread starter):
Why did the crew of the first accident do such an inferior job? My guess: Since introduction into service they had experienced so many false fault indications on the Q400 that they had named the main display "The Slot Machine".

I believe that your theory is correct. But there is one thing I want to know. Would the indicatorlamp in on the Q400 light up if there was some inconsistency in the force required to lower the MLG? IIRC most emergency landing with the Q400 was because of MLG warning lights.

Quoting Prebennorholm (Thread starter):
I mean, had this same thing happened on a car or a lawn mover, then a courious car or lawn mover mechanic would have said to himself: "I want to know why this nut is constantly coming lose". And the design fault would have been discovered years before any catastrophic failure.

I would have thought that the final report would look at the working culture at the maintenance personel. I know that have been done in other accidents.

Quoting Prebennorholm (Thread starter):
How could SAS tell the press that this was a "one off" failure, when any lawn mover mechanic needed five minutes to see on the accident plane that design was fundamentally at fault? And wait 60 hours and one more idetical accident to ground and rebuild the fleet.

That tells us that SAS management looks a bit like Boeing management. They talk about things they do not know to much about....  Wink

[Edited 2009-07-14 23:42:07]


Dream no small dream; it lacks magic. Dream large, then go make that dream real - Donald Douglas
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6424 posts, RR: 54
Reply 2, posted (5 years 1 month 1 week 2 days 8 hours ago) and read 4671 times:



Quoting OyKIE (Reply 1):
But there is one thing I want to know. Would the indicatorlamp in on the Q400 light up if there was some inconsistency in the force required to lower the MLG?

Huh, the whole normal operation of the landing gear is described in the report section 1.6.2. on pages 11 to 14. I think you should study that.

Quoting OyKIE (Reply 1):
I would have thought that the final report would look at the working culture at the maintenance personel. I know that have been done in other accidents.

This report is purely technical. Practically no company names or maintenance locations are mentioned.

It does not mention which maintenance tasks were done in-house and what was outsourced, and which sourcing partners were involved. It doesn't even mention the name of the company which produced the landing gear. In section 2.21 it talks about the MLG manufacturer's test of the landing gear, but without mentioning the company name.

Only in the heading you find the complete type number and manufacturer of the plane, the engines and the propellers.

The report doesn't even mention that SAS was operating the plane. I don't think I have ever seen such a large report before, which is so totally clean when it comes to "pointing fingers".

That cannot be accidental. It must be on purpose. Probably because too much finger pointing has already been going on since it started at full speed the day after the accident. Problem is that there are two things to be learned:

1. How could the design fault pass?
2. How come that is wasn't discovered by maintenance before it became catastrophic?

Both issues involve a whole string of companies which are only interrelated by complicated business contracts.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineOyKIE From Norway, joined Jan 2006, 2732 posts, RR: 4
Reply 3, posted (5 years 1 month 1 week 2 days ago) and read 4471 times:



Quoting Prebennorholm (Reply 2):
Huh, the whole normal operation of the landing gear is described in the report section 1.6.2. on pages 11 to 14. I think you should study that.

I must confess that I did not read the whole report. I just looked at the picture, and that was some scary interior shots. From what I understand after reading page 11-14 is that the MLG lights would not detect if a nutbolt came loose. Correct?

Quoting Prebennorholm (Reply 2):
The report doesn't even mention that SAS was operating the plane. I don't think I have ever seen such a large report before, which is so totally clean when it comes to "pointing fingers".

That cannot be accidental. It must be on purpose. Probably because too much finger pointing has already been going on since it started at full speed the day after the accident.

Indeed. Could it be that the Scandinavian aviation authorities might follow SAS airplanes maintenance more closely from now on?



Dream no small dream; it lacks magic. Dream large, then go make that dream real - Donald Douglas
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6424 posts, RR: 54
Reply 4, posted (5 years 1 month 1 week 1 day 6 hours ago) and read 4110 times:

Sure each and every bolt on an airplane doesn't have a "loose bolt sensor" attached. If that was the case, then I'm afraid that planes would be so heavily loaded with wires and cables that they couldn't lift payload off the ground.

Bolts are tightened with a certain torque, then often locked with a thin steel wire, which is closed in a loop with a lead seal which carries the "signature" of the shop which last time tightened that bolt.

Quoting OyKIE (Reply 3):
Indeed. Could it be that the Scandinavian aviation authorities might follow SAS airplanes maintenance more closely from now on?

That we will never know. If this caused special attention, then it wouldn't be now, but rather 10th September 2007.

But I do remember that the Danish CAA was quite open to the press back in 2001 and 2002 about special attention to the Q400 after the unusually high number of minor incidents right after introduction of the type. False fire alarms, smoke in the cabin after engine gearbox seal failure, and a rather bad tail strike after faulty air speed indication, etc. And attention was renewed after the propeller problems in 2004 to 2006 (six of them IIRC), especially the so called Kalmar incident in Sweden, which was the closest they ever came to a fatal catastrophe.
(The Kalmar incident was, however, a crystal clear pilot error. Those propeller problems were 100% manageable as long as they happened on one engine only, and just dreaming about it during a take-off would certainly turn into a nightmare).

The MLG accidents were bad enough, but not really dangerous. After the first one they almost seemed to become routine. The only reason why the first one caused injuries was in fact sort of pilot error. They didn't believe the failure indication they had, and they had no checklist for it.

The crews of accidents #2 and #3 did a perfect job. And nobody got hurt. #2 at least had the advantage of experience from #1. #3 was different. They could see out of the window that the right MLG never extended, so they could never be in doubt what they were up against.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
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