No Back Spring Update

[PT20J]

I’ve never had one of these apart to examine the mechanism, but from various documents and published descriptions, it seems clear that the spring performs two functions. First, it acts as a wrap spring brake to keep the wheels in the retracted position. Second, it acts as a wrap spring clutch to engage the motor with the ball screw to extend the gear. This is why the gear won’t extend if the spring breaks; the motor spins, but it is disconnected from the rest of the mechanism.

There are really two preloads on the gear. The first is checked by slowly extending the gear with the emergency system just until the gear down light illuminates and then checking that the preloads are within the specified range. The second test is to cycle the gear electrically and check that a higher maximum preload is not exceeded. This is because momentum will cause the gear to keep moving a bit when the down limit switch cuts off power to the motor, and so the preload will be higher in this case.

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[0TreeLemur]

5 minutes ago, PT20J said:

There are really two preloads on the gear. The first is checked by slowly extending the gear with the emergency system just until the gear down light illuminates and then checking that the preloads are within the specified range. The second test is to cycle the gear electrically and check that a higher maximum preload is not exceeded. This is because momentum will cause the gear to keep moving a bit when the down limit switch cuts off power to the motor, and so the preload will be higher in this case.

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This is the first time I'm learning about this.   Makes sense.  Good to know.  I'd nominate this piece of information for a MS FAQ.  Is it controversial?   Seems not.

[PT20J]

27 minutes ago, 0TreeLemur said:

This is the first time I'm learning about this.   Makes sense.  Good to know.  I'd nominate this piece of information for a MS FAQ.  Is it controversial?   Seems not.

It’s in the service and maintenance manual for the M20J and I presume others as well.

[1980Mooney]

5 hours ago, MikeOH said:

If I'm following this discussion correctly (and I may not be!), I think that once on the ground the weight of the aircraft tries to drive the link open, but since it's over center, it just locks more. The motor is no longer needed once weight is on the wheels.

The part I'm not following is if the no-back spring is causing this, i.e. it has failed, then how come the gear works at all?  I thought a no-back spring failure resulted in the gear refusing to even extend, not just allowing the gear to come off of over center?  What am I missing?

 

14 hours ago, amillet said:

I too questioned why it made any difference with the gear down. This is Greg’s explanation:

The gear actuator is the biggest issue we seem to have.  The reason the gear horn is sounding intermittently is because the gear actuator is turning backwards releasing the gear tension.  The reason for this is the No Back Spring is slipping.  When the tension drops off, the gear actuator kicks on to gear back up against the stop.  Not the best thing for the motor or the actuator.

Like @MikeOH , I am also having a hard time understanding the supposed failure mode on a no-back spring that would let the gears in the gear motor actuator slip backwards slightly when the gear is down and over-center.  Possibly if the no-back spring tang was cracked and flexing it might happen.  If it was broken you would not have any gear operation.

You would be wise to ask for the old no-back spring (#9 in diagram) to be returned to you for inspection. (like @Danb above). See if it is cracked.  If not - this doesn't seem to make sense.  

 

 

 

 

Edited September 2, 2023 by 1980Mooney
Referenced wrong part #

[MikeOH]

@1980Mooney

Isn't the no-back spring item 9, not 10?

[1980Mooney]

18 minutes ago, MikeOH said:

@1980Mooney

Isn't the no-back spring item 9, not 10?

Right

[Lax291]

On 8/30/2023 at 1:05 PM, PT20J said:

I'm firmly in the camp of if it ain't broke, don't mess with it. 

I've looked into this a lot. Eaton never had a recommendation about maintenance on these as far as I can tell until they had a failure of a no-back spring that was probably due to a bad manufacturing lot of springs. That was many years ago. The 1000 hours seems to have come from Mooney and was due to the failure of two springs in Plessey actuators (different design) at a little under 1000 hours each. 

The 1000 hours is really meaningless. It's cycles that are important. Don Maxwell told me that the Eaton actuator is rated for 20,000 cycles. If you cycled the gear once per hour of flight time, that would be 20,000 hours.

So, figuring that any bad springs have worked their way out of the installed base by now, I see no value in messing with it. I think about it this way: The only reason to mess with it is fear of failure. After I had it serviced, I would have to worry about a maintenance induced failure. Same difference -- I'd still have something to worry about and there is no objective means to tell which is better. By the way, when I asked Don about it he said not to mess with it unless it was making an unusual noise.

Skip

I'm having to deal with the NBS for my annual right now.  Since it's a SB from Mooney it's not mandatory to comply with the replacement every 1,000 hrs.  But, I was wondering what implications that has with insurance.  If the spring does break and you have a gear up can insurance deny the claim because you did not comply with a Service Bulletin?  

[PT20J]

36 minutes ago, Lax291 said:

I'm having to deal with the NBS for my annual right now.  Since it's a SB from Mooney it's not mandatory to comply with the replacement every 1,000 hrs.  But, I was wondering what implications that has with insurance.  If the spring does break and you have a gear up can insurance deny the claim because you did not comply with a Service Bulletin?  

Doubt it but we should let @Parker_Woodruff answer. 

[jetdriven]

Those things are not mandatory and not required for airworthiness.

[LANCECASPER]

2 hours ago, Lax291 said:

I'm having to deal with the NBS for my annual right now.  Since it's a SB from Mooney it's not mandatory to comply with the replacement every 1,000 hrs.  But, I was wondering what implications that has with insurance.  If the spring does break and you have a gear up can insurance deny the claim because you did not comply with a Service Bulletin?  

Not at all. 

What if you had a perfectly good no-back-spring and it was replaced with a new one that suffers from infant mortality and snaps in 20 hours?

Not always, but Service Bulletins are sometimes over-reactions - a solution looking for a problem.

[PT20J]

2 hours ago, LANCECASPER said:

Not at all. 

What if you had a perfectly good no-back-spring and it was replaced with a new one that suffers from infant mortality and snaps in 20 hours?

Not always, but Service Bulletins are sometimes over-reactions - a solution looking for a problem.

Actually, there was an accident like that where a noback spring with only a few hundred hours since replacement failed in a Plessey actuator and caused a gear up landing. It was classified as an accident because some structural piece was worn through by contact with the pavement that classified it as substantial damage.

Edited July 18 by PT20J
Updated to include that it was a Plessey actuator

[Fritz1]

I replaced my no back spring about 4 years ago at 1500 because I had the part, knowing what I know now I woul leave it alone. The risk of the spring failing is minuscule and outweighed by the risk of something being damaged in the installation and/or the new spring failing, if you call Brian Kendrick or Don Maxwell they will probably say the same thing, ain't broke ain't fix it

[Parker_Woodruff]

23 hours ago, Lax291 said:

I'm having to deal with the NBS for my annual right now.  Since it's a SB from Mooney it's not mandatory to comply with the replacement every 1,000 hrs.  But, I was wondering what implications that has with insurance.  If the spring does break and you have a gear up can insurance deny the claim because you did not comply with a Service Bulletin?  

Nothing to worry about as this isn't presently a legal requirement to accomplish.