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SQ A330/300 nosewheel collapse

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SQ A330/300 nosewheel collapse

Old 11th Oct 2015, 19:59
  #21 (permalink)  
 
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Not the first time this this has happened at Changi. In 1999 Northwest had a B-747-200 taking a bow:

AirDisaster.Com: Accident Photo: Northwest 747 (Singapore)

The narrative in the link above is not exactly what I was told.

Apparently there was a gripe inbound to SIN that the nosegear doors were noisy and possibly not fully closing. There was a maintenance procedure to pin the gear, pressurize one of the hydraulic systems and cycle some of the nosegear doors using the cockpit landing gear lever. A Boeing pin was not readily available in SIN so the mechanics used an Airbus pin. This was a locally modified part with a handle welded on for convenience.

The aft doors cycled, something bumped or caught the gear pin handle and it came out. A mechanic was in the wheel well observing the gear door operation but thankfully saw what was going to happen next and escaped.

A similar nosegear pin snafu on a Boeing occurred with Speedbird ground engineers in LHR working on an EVA plane:

ASN Aircraft incident 23-MAY-2004 Boeing 747-45EM B-16462

Last edited by Airbubba; 11th Oct 2015 at 20:19.
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Old 11th Oct 2015, 21:12
  #22 (permalink)  
 
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I love it when pilots get all technical about aircraft systems. Keep theorising guys!!
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Old 11th Oct 2015, 22:20
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Originally Posted by DaveReidUK View Post
Yes and no, respectively.
Well thatís pretty stupid then! Geometric lock or not.
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Old 11th Oct 2015, 23:18
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Rainman

A QANTAS B707 came to rest on stand at Sydney (i believe.. not many details on t'net; all i can find is gags about a golf course in Bangkok and a 5 iron..)
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Old 11th Oct 2015, 23:43
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A QANTAS B707 came to rest on stand at Sydney
Stretching our memories here, and hopefully not mixing up events, but I recall, prior to pushback, the tug driver had a heart attack, pushed the accelerator pedal with the aircraft brakes still on and the nose gear collapsed. Of course, we are talking here about the nose gear going aft, not forward.

Originally Posted by glad rag
Well that’s pretty stupid then! Geometric lock or not.
So stupid, even Boeing/Airbus don't recommend using a downlock pin
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Old 12th Oct 2015, 03:17
  #26 (permalink)  
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Find it difficult to believe that a flight crew, doing a walk round, having been on the Flt Dk, read the logs, seen a notice on the gear handle, spoken with engineers etc. etc. would then go down and pull the nose pin out. In ten years there I never touched a single gear pin, bound to have seen them but that, to me, is engineers territory and I would talk to them about it.
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Old 12th Oct 2015, 04:21
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I heard it was the guys departing the aircraft who pulled the pins.

Also heard from a reliable source that it was the pilots wallet that caused the nose gear failure.
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Old 12th Oct 2015, 06:51
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Technical question re gear locking

I'm SLF/electrical engineer trying to understand how this could happen. Are there any aircraft engineers out there who can explain how the LG interlocks work ? I've re-read the thread carefully, but can't see any explanation.

From what I understand (forgive my terminology) the gear support struts have some mechanical locking mechanism that prevent the gear folding once it's down and locked, and sufficiently strong to take a good bang on the front of the nosewheel without folding back (Doh!). But I also thought that there was a mechanical fail-safe such that the weight on the LG leg would hold the leg locked even if the hydraulics were trying to raise the gear. Or is that not the case ?

Also, if a mechanical LG locking pin is in place and the hydraulics try to raise the gear who usually wins ? Would the locking pin shear first, or would the hydraulics pop out on overpressure, or shear off the mount, etc ?
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Old 12th Oct 2015, 07:18
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I'll try to clear this 'up'.

If you apply hydraulic pressure to the landing gear system and override the gear lever to select UP and not have gear pins fitted, the gear doors will open and the nose gear will roll forward and up. The only thing stopping the main gear retracting is the weight of the aircraft on the axle/truck.
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Old 12th Oct 2015, 08:31
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Originally Posted by Clipped View Post
I'll try to clear this 'up'.

If you apply hydraulic pressure to the landing gear system and override the gear lever to select UP and not have gear pins fitted, the gear doors will open and the nose gear will roll forward and up. The only thing stopping the main gear retracting is the weight of the aircraft on the axle/truck.
Would the "collapse" have been sudden, or at the speed the gear retracts?

Mickjoebill
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Old 12th Oct 2015, 08:49
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Originally Posted by rob_ginger View Post
Also, if a mechanical LG locking pin is in place and the hydraulics try to raise the gear who usually wins ? Would the locking pin shear first, or would the hydraulics pop out on overpressure, or shear off the mount, etc ?
Pins always win over the retraction jacks. Bear in mind that the pin isn't directly preventing the movement of the jack - that's done by the overcentre geometric lock, the pin merely prevents the lock from disengaging.

The trick is to check the pins are in before operating the lever. And then double-check.
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Old 12th Oct 2015, 09:33
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BRE
 
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Will those engine pylons be tested or replaced so we won't see a repeat of AA191?
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Old 12th Oct 2015, 10:54
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Thanks very much for your replies, Clipped and and DaveReidUK. From them it sounds like there ISN'T a mechanical interlock where weight stops retraction, so it looks like the poor bugger who DIDN'T check that the pins were fitted is in the poo, regardless of whether the gear lever was accidentally activated or the electronic module controlling the retraction misbehaved .

I'm not a mechanical engineer, but such a mechanical interlock seems really easy to design. And if it malfunctioned the gear would be stuck down, which seems like the preferable failure mode. I wonder why it's not done ??
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Old 12th Oct 2015, 11:40
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Main gear is usually designed like that, it would need to move downwards first before it can retract. Nose gear is different, for whatever reason; there you only need to overcome 3000 psi hydraulic pressure (airbus) and the springs that lock the gear for free-fall in case of hydraulic failure. With a conscious effort you may be able to force it to retract on the ground (drain the hydraulic fluid? override the squat switch? push very hard?). I suspect in most cases someone helped it to retract. But what do I know...ask Ngineer? Not judging anyone here, you can invent fool-safe mechanisms, but there's always more ingenious fools. Pilots know nothing about this anyway, and while pilots will be helpful to push any button an engineer asks them to push, they'll usually scramble and leave the flight deck in a hurry if an engineer asks them to touch the gear lever on the ground.

Last edited by deptrai; 12th Oct 2015 at 12:02.
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Old 12th Oct 2015, 12:29
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...they'll usually scramble and leave the flight deck in a hurry if an engineer asks them to touch the gear lever on the ground.
And for good reason.
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Old 12th Oct 2015, 12:37
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Saab 340 will retract all three legs if you override on the ground.
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Old 12th Oct 2015, 12:44
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@ Rob Ginger
On modern aircraft, the nose gear is kept in place by overcentre lock held in place with large springs ("bungees") and, if hydraulics are active, by a small hydraulic piston helping the overcentre lock and a much larger hydraulic piston pushing the nose wheel back down. Also, depending on the aircraft type, there will be either a mechanical restriction on the gear lever in the cockpit to stop you moving it to UP on the ground, or electronics to inhibit the signals from the landing gear lever to the gear.

You can further enhance safety by locking the overcentre lock with a steel pin. These pins are the same pins used during maintenance tows, but, for gear retraction tests, there is usually a requirement for a visual inspection of the pins to make sure they look perfect. There may be a requirement to send the pins away after gear tests have been done.

Regarding weight. Unfortunately, the aircraft weight is not much of an issue with the nose gear. The nose gear has wheels which have no brakes*. Some nose gear struts, however, are sloped backwards slightly (not vertical) or the wheels are castered, so the nose wheels have to scribe an arc "through" the tarmac to move forward (to the retracted position). Unfortunately, the hydraulics are usually strong enough to push the aircraft upwards.

Now, carrying out a test like the engineers did removes some of the safeguards (hydraulics now assist the retraction) and the gear lever inhibit is being removed. Basically, the safety pin, the bungee springs (which will be overcome by hydraulics) and some aircraft weight are the only things stopping the nose gear retracting. The pin is the primary safety device here and perfectly suited to the job, but, as has been rumoured, it was removed by a 2nd party. Note that there needs to be a sequencing of hydraulics. The overcentre lock has to be activated first by hydraulics, then the main retraction hydraulic piston/s can start moving the gear.

I'm just curious to know why door tests on the A330 can't be done without gear lever movement. On a 747-400, for example, you can manually release the door, then pressurise the #1 hydraulics system... The door will go back up.

* chocks may have been an impediment to wheel rotation
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Old 13th Oct 2015, 15:58
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Apparently engineers were changing Landing Gear Control and Interface Unit #2, did a retraction test but hadn't put the pins in . (Aviation Herald)
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Old 13th Oct 2015, 18:10
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Originally Posted by Super VC-10 View Post
did a retraction test but hadn't put the pins in
That's very hard to believe. Other reports suggest that the pin/pins had been put in by Engineering but subsequently removed by the ground handlers.
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Old 13th Oct 2015, 19:15
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From the Aviation Herald -

The landing gear pins were not inserted into any of the gear assemblies during that work.
However, either scenario is plausible. An investigation is under way, final report will be released in due course.
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