Alaska Airlines 737-900 MAX loses a door in-flight out of PDX

[SRMman]

Originally Posted by ozaub

NTSB’s Preliminary Report is so disappointing. We still don’t know which way’s up. Or more specifically do “lift assist springs” exert a force on “MED plug” less or greater than weight of plug?

Or do springs exert force greater than weight of MED plug? Then you’d have to push plug down to align guides with rollers. Push in and continue pushing down until plug is secured with “vertical movement arrestor bolts” or cable tie. Let’s call this the ejector configuration. What’s the purpose?

Yes, I believe the springs do exert a force greater than the plug weight. See Post #1671, where I suggested this as a reason:

“With the door held fully up by the springs there is a certainty that as the door is pulled closed the 12 stop fittings will pass over the top of their mating stop pads. This surely is an important feature, because if during closing the stop fittings impacted their stop pads (because the springs only held the door partly up its travel) there is obviously a possibility of damage to one or the other or both. Should this happen, but went unnoticed, unreported or uninspected there is a chance of such damage to critical primary structure causing door problems in the future.”

[MarineEngineer]

I'm sure that cable tie is strong enough to hold the door down for many flights, if not indefinitely. there are no sharp edges on the stop fitting brackets.

[MZoVrsmg]

Originally Posted by SRMman

as the door is pulled closed the 12 stop fittings will pass over the top of their mating stop pads. This surely is an important feature, because if during closing the stop fittings impacted their stop pads (because the springs only held the door partly up its travel) there is obviously a possibility of damage

Solid point. I'd been thinking that the roller track solved that problem, but that can't be the case: If the hinge springs weren't at full extension as the door closed, the lowest stop pads/fittings would come into wrong-way-around contact long before the misaligned track encountered the roller. The door is a pretty long (and heavy) lever too. The potential for damage seems very real.

[SRMman]

Originally Posted by Europa01

I agree about spring force but I’d like to see the action of the guide rollers and guide tracks included in the description of how the door closes. My understanding is that the door is pulled inwards to engage the rollers in the horizontal section of the guide tracks and then pulled down so that the guides pass down over the rollers. I think the first part of this motion is what positions the pins in the vertical plane behind the support pads and the second part of the motion is what positions them in the horizontal plane but am happy to be corrected.

I agree, Post#1211 & 1671.

[MechEngr]

Originally Posted by 639

Dont forget, springs used as in this context can only exactly balance the Door-plug weight at one precise position. I notice some making calculations as to the spring rate used in earlier posts, As I havn't seen anyone come up with the spring's free length and compressed length with the door-plug closed and hence its preload, these calculations mean nothing.

Spring rate is largely independent of compression - it changes only a few percent with the change in geometry. We know the spring is near solid when fully compressed and that the maximum load the spring can carry is when it is fully compressed, regardless of free length. The number of coils is countable. This information would allow calculating the free length, though that isn't of any interest in this problem.

The question being answered was - is there a reasonable range of springs that could support most or more than the weight of the door and, specifically, can a reasonable spring support the weight door against the hinge guide stop washers?The answer is clearly yes.

The calculation can run the other way - start with a free length, but that's not how spring design is done. Free length is a manufacturing input, not a design input.

[DTA]

Originally Posted by MZoVrsmg

Solid point. I'd been thinking that the roller track solved that problem, but that can't be the case: If the hinge springs weren't at full extension as the door closed, the lowest stop pads/fittings would come into wrong-way-around contact long before the misaligned track encountered the roller. The door is a pretty long (and heavy) lever too. The potential for damage seems very real.

Yes, that is a compelling argument. But it does bring it back full circle - what held the door/plug in place when the photo (with missing bolts) was taken? A solitary cable tie would not be anywhere near enough. NTSB have said there was no evidence of any lock bolts. One can imagine the trim and insulation helping to hold it in place later on. So some other thing inserted into the upper aft guide which is not visible? A plastic dowel for example?

[MechEngr]

Originally Posted by MZoVrsmg

Ah, thank you for pointing that out. I'd also been wondering how this safety wire arrangement works. Is it just a loose tail shoved down the "trough" in the pin?

I've never seen a setup like that before. Tried looking for pictures, but my google skills fell short.

Regarding the placement of the zip tie, I guess the white spot in the photo at 12" on the threaded pin is a reflection, and not the zip tie disappearing into the fastener as I'd assumed.

Maybe the zip tie is in a "figure 8" arrangement around the two lugs?

From the wire diameter and the depth of the slot in the pin I think there is a matching slot in the door stop fitting, allowing 1/4th turn increments on adjusting the stop. That would be enough considering there are no turning loads and the gap on the other end is too small to work on. I have seen similar uses for split pins in axially aligned slots, but they require access to both sides, one to install the pin and the other to bend the legs. This wire allows making the adjustment and then, without moving the door, to lock the adjustment. At the 3 o'clock position the clearance at the thread is far smaller than the wire diameter, hence my belief in the mating groove at the wire install position.

[DaveReidUK]

Originally Posted by EXDAC

I think the idea that the tie supported a tag and/or parts bag is more likely. The argument that it's not an ideal place for a tag are equally applicable to the position if used to hold the plug closed.

Where else are you suggesting would be a better place to use a tie in order to keep the door closed ?

[Europa01]

Door Closing

Originally Posted by SRMman

I agree, Post#1211 & 1671.

Thinking about this a bit more, whilst pulling the door down, the guide tracks will position the stop pins / pads correctly relative to each other at the top of the door but it is probably necessary to keep pulling inwards and downwards to ensure that the lower stop pins / pads are positioned correctly. This is because the hinge posts have substantial free inboard-outboard movement available at the floor brackets (see Chris Brady’s last video) so need to be pulled in the inboard direction to ensure the pins clear the pads as they move down.

[EXDAC]

Originally Posted by DaveReidUK

Where else are you suggesting would be a better place to use a tie in order to keep the door closed ?

Imagine one person closing and securing the plug. Right hand pulls the plug in and then down. A boot at the bottom frees both hands. The cable tie would most easily be fitted round a stop pad pair near chest height. Bending down while keeping a foot in the plug would be quite awkward.

My preferred scenario doesn't need a foot on the plug. The left hand is free to insert the temporary pin in the upper aft guide fitting.

All speculation of course. Maybe one day the NTSB will populate the docket with interviews of those that actually did (or didn't do) the work.

[DaveReidUK]

Originally Posted by EXDAC

Imagine one person closing and securing the plug. Right hand pulls the plug in and then down. A boot at the bottom frees both hands. The cable tie would most easily be fitted round a stop pad pair near chest height. Bending down while keeping a foot in the plug would be quite awkward.

My preferred scenario doesn't need a foot on the plug. The left hand is free to insert the temporary pin in the upper aft guide fitting.

My preferred scenario (which I think is more likely) is that closing and securing the door plug would be a two-person job not requiring any acrobatics. Also speculation, of course ...

[MZoVrsmg]

Originally Posted by MechEngr

From the wire diameter and the depth of the slot in the pin I think there is a matching slot in the door stop fitting, allowing 1/4th turn increments on adjusting the stop. That would be enough considering there are no turning loads and the gap on the other end is too small to work on. I have seen similar uses for split pins in axially aligned slots, but they require access to both sides, one to install the pin and the other to bend the legs. This wire allows making the adjustment and then, without moving the door, to lock the adjustment. At the 3 o'clock position the clearance at the thread is far smaller than the wire diameter, hence my belief in the mating groove at the wire install position.

I get how the wire stops the pin from turning, that it allows 1/4 turn increments, and that there's probably a matching groove in the female thread ... possibly involving a broaching operation? wild stuff.

I meant that I'd never seen any beauty shots in a manufacturer catalog, installer training info, details about inspection (how long is the tail?) and so forth.

Is it merely a length of safety wire, or a special purpose clip with some remanufactured bends?

... that kind of thing.

[EXDAC]

Originally Posted by MZoVrsmg

Is it merely a length of safety wire, or a special purpose clip with some remanufactured bends?

Almost certainly a special clip designed for that application. Much the same idea as the clips used to lock control cable turnbuckles.

[jdaley]

Originally Posted by DTA

Yes, that is a compelling argument. But it does bring it back full circle - what held the door/plug in place when the photo (with missing bolts) was taken? A solitary cable tie would not be anywhere near enough. NTSB have said there was no evidence of any lock bolts. One can imagine the trim and insulation helping to hold it in place later on. So some other thing inserted into the upper aft guide which is not visible? A plastic dowel for example?

Doesn't the way the aft bracket fractured indicate that the aft roller was almost in the correct position - otherwise the 'tear' wouldn't have gone as far as it did? Given the forward bracket was not fractured then presumably it had almost completely disengaged and the plug was skewed anti-clockwise.



Figure 10. Fractured aft upper guide fitting. Arrows point to the fractured and deformed piece of the guide fitting’s inboard wall. The rectangle indicates where the upper guide track bolt would be installed

A scenario being a single bolt in the aft upper guide, failure of the upper forward seal, depressurisation rotating the plug and tearing the forward bracket as the plug exits the frame. Something had to keep the upper guide in place for the tear to happen. The single bolt and an intact seal (plus a tie?) being sufficient to keep the plug in place until it wasn't.

[EXDAC]

Originally Posted by jdaley

Given the forward bracket was not fractured then presumably it had almost completely disengaged and the plug was skewed anti-clockwise.

Why would you assume that when NTSB report states:

"Both upper guide fittings installed on the forward and aft sides of the MED plug, respectively,
were fractured vertically through the inboard wall of the track. Examination revealed features
consistent with overstress fracture and no evidence of pre-existing cracks or damage. The
outboard upper guide track bolt holes were intact and exhibited no deformation. Figure 10
shows the aft upper guide fitting; the forward upper guide fitting was similar."

[DTA]

The door plug only needs to lift half an inch or so for the 12 stops to disengage. The damage to the upper guides appears consistent with that to me. That there was (effectively) no bolt in any of the four positions is most plausible at the time of the incident.

[SRMman]

Ref #1737, Interesting point. But one assumes for the plug to move outwards the stop pads would have had to move passed and cleared their stop fittings, I’m not sure exactly where the roller pin would be in its track at that point . . .

[MechEngr]

Originally Posted by jdaley

Doesn't the way the aft bracket fractured indicate that the aft roller was almost in the correct position - otherwise the 'tear' wouldn't have gone as far as it did? Given the forward bracket was not fractured then presumably it had almost completely disengaged and the plug was skewed anti-clockwise.



Figure 10. Fractured aft upper guide fitting. Arrows point to the fractured and deformed piece of the guide fitting’s inboard wall. The rectangle indicates where the upper guide track bolt would be installed

A scenario being a single bolt in the aft upper guide, failure of the upper forward seal, depressurisation rotating the plug and tearing the forward bracket as the plug exits the frame. Something had to keep the upper guide in place for the tear to happen. The single bolt and an intact seal (plus a tie?) being sufficient to keep the plug in place until it wasn't.

There is evidence of where the roller was when the fracture happened - the torn off tab has a reverse curvature to it that is below the bolt hole, where the middle arrow is pointing. This is where the roller was pushing when the tab sheared off. The roller starts in the pocket entirely above the blue rectangle when the door is fully seated.

[DaveReidUK]

Originally Posted by SRMman

Ref #1737, Interesting point. But one assumes for the plug to move outwards the stop pads would have had to move passed and cleared their stop fittings, I’m not sure exactly where the roller pin would be in its track at that point . . .

At the point where the stops have just cleared, the roller is still engaged in the guide. The damage noted to the guides would suggest that was the point where the door plug departed, hence the damage noted to both the roller guides.

[zueriflyer]

Originally Posted by MechEngr

Looking at the photos and estimating the spring parameters:
10 coils
.125 inch diameter music wire
1.6 inch OD

Gives a fully compressed load of over 50 lbf per spring and a rate of about 10 pounds per inch; that would leave 70-80 pounds of lift at the time the fingers cleared.

17-7 stainless is a few percent decrease in force and rate.

If the wire diameter is 0.09 inch, then it drops to 40 pounds fully compressed for the pair and 2.5 pounds per inch, so 35 pounds to clear the fingers.

C'mon NTSB, these are day 1, did the parts conform questions/answers.

Maybe the door seals are just that sticky?

They have the previous 16 flights. Was the last one with a big slam on the landing? Something popped that door up and if it wasn't the springs, what was it?

I would like to draw your attention to something in your spring calculation that I haven't fully understood yet.

If we only change the wire dia from 0,125 inch down to 0,09 inch, the spring force should decrease by factor 3,7 ( 0,125^4 / 0,09^4 =3,7 ) The spring load should be then 50/3,7 = 13,5 pounds compared to the 20 pounds you suggest.
t could be beneficial to examine whether your program has adjusted other parameters during the dia change without you being aware of it.

Irrespective of that, please allow me to highlight a general issue when estimating spring rates
The wire dia is a fourth-order term in the calculation of both the spring force and the spring rate and the spring diameter is a third-order term,
If we are only 10 % off with our estimation of both the wire dia and the coil dia, we are wrong by about factor 2 in the worst case : 1,1^4 / 0,9^3 = 2 (normalized)

I would consider ony 10 % off a remarkable good guess based on the blurr pics we have at hand.