Alaska Airlines 737-900 MAX loses a door in-flight out of PDX
If I were specifying this plug for ease of opening and closing as required for maintenance it would have the following features -
1. The mass of the door would be fully supported by the springs so the slides were on the stops with the plug at any angle.
2. The mass of the plug would pass over the hinge line as the plug was opened and closed giving it two stable positions, one open on the tethers, the other closed but in the fully up position. The plug mass would move "over center" as the plug was pulled towards the fuselage.
3. The hinge slide stops would be set so that the plug naturally fell into the closed but fully raised position, flush with the fuselage, and with the guide rollers engaged in the open section of the guides. There would be no need to "jiggle" the mass of the door to get the rollers to engage in the guides which would be required if the slides were not hard on the stops.
4. The plug would be moved to the closed and fully down position simply by pulling it down until the rollers bottomed out in the guides. In this position something is required to keep it closed and the design requires four retention bolts to be fitted.
Is there any evidence at all that suggests any of these features were not included in the design?
1. The mass of the door would be fully supported by the springs so the slides were on the stops with the plug at any angle.
2. The mass of the plug would pass over the hinge line as the plug was opened and closed giving it two stable positions, one open on the tethers, the other closed but in the fully up position. The plug mass would move "over center" as the plug was pulled towards the fuselage.
3. The hinge slide stops would be set so that the plug naturally fell into the closed but fully raised position, flush with the fuselage, and with the guide rollers engaged in the open section of the guides. There would be no need to "jiggle" the mass of the door to get the rollers to engage in the guides which would be required if the slides were not hard on the stops.
4. The plug would be moved to the closed and fully down position simply by pulling it down until the rollers bottomed out in the guides. In this position something is required to keep it closed and the design requires four retention bolts to be fitted.
Is there any evidence at all that suggests any of these features were not included in the design?
Last edited by EXDAC; 21st Feb 2024 at 03:08.
Psychophysiological entity
No, not really. I can just hear the dulcet tones now. "Ere, just hold this down while I put a tie in. I've got to go and get some more @$^ing bolts."
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What would be the advantage of having the door floating (say) half way up the spring travel range? You might as well have no springs - at least then there would be the additional safety feature of gravity holding the closed door at the bottom of the guides if someone forgot to put the lock bolts in!!
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Exactly that. In fact, I would pose the question: “why wouldn’t the springs hold the plug door fully up”?
What would be the advantage of having the door floating (say) half way up the spring travel range? You might as well have no springs - at least then there would be the additional safety feature of gravity holding the closed door at the bottom of the guides if someone forgot to put the lock bolts in!!
What would be the advantage of having the door floating (say) half way up the spring travel range? You might as well have no springs - at least then there would be the additional safety feature of gravity holding the closed door at the bottom of the guides if someone forgot to put the lock bolts in!!
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What do you want to call them?
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Chris Brady certainly didn't invent the term, may I quote from the AMM (Interestingly this also makes it clear that they indeed are able to hold the 29kg door plug up)
Originally Posted by AMM SDS 52-23-00
Lift Assist Springs
There are two lift assist springs, found in the lower mechanism, near the bottom of the door plug that are attached to the lower guide fittings. The lift assist springs make sure that the door will not fall back to the closed position. The door plug will stay in this position. Apply a downward force to the door plug this forces it down until the rollers clear the guide tracks. At this time the door plug can open outboard. Be careful because the door plug lift assist springs are under compression, but the door is free to move outboard. Move the door plug out until it rests against the straps this releases the compression on the springs. The lift assist springs help support the door plug during the inspection, removal and installation.
There are two lift assist springs, found in the lower mechanism, near the bottom of the door plug that are attached to the lower guide fittings. The lift assist springs make sure that the door will not fall back to the closed position. The door plug will stay in this position. Apply a downward force to the door plug this forces it down until the rollers clear the guide tracks. At this time the door plug can open outboard. Be careful because the door plug lift assist springs are under compression, but the door is free to move outboard. Move the door plug out until it rests against the straps this releases the compression on the springs. The lift assist springs help support the door plug during the inspection, removal and installation.
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I copied the text verbatim... It continues (sic)
Originally Posted by AMM SDS 52-23-00
Operation
After the removal of the fuselage lining and the body insulation. Remove the two vertical movement arrestor bolts from the lower guide hinge fittings and the two upper track bolts from the upper guide track fittings. Push the top of the door plug down to clear the door plug from the guide fittings. When the door plug is clear from the guide fitting, the lift assist spring will lift the door plug approximately 1.5 in. (38.1 mm). The lift assist spring makes sure that the door plug will not fall back to the closed position. The lift assist spring is a part of the hinge assembly. The door plug will stay in this position until a small outboard force is applied to the top of the door plug. When the force is applied, the door plug will fall to the open position. The two strap assemblies will limit the door plug opening to an approximate 15-degree angle.
After the removal of the fuselage lining and the body insulation. Remove the two vertical movement arrestor bolts from the lower guide hinge fittings and the two upper track bolts from the upper guide track fittings. Push the top of the door plug down to clear the door plug from the guide fittings. When the door plug is clear from the guide fitting, the lift assist spring will lift the door plug approximately 1.5 in. (38.1 mm). The lift assist spring makes sure that the door plug will not fall back to the closed position. The lift assist spring is a part of the hinge assembly. The door plug will stay in this position until a small outboard force is applied to the top of the door plug. When the force is applied, the door plug will fall to the open position. The two strap assemblies will limit the door plug opening to an approximate 15-degree angle.
(I have been involved with the preparation of part 25 aircraft support documentation and sometimes found it easier to write sections myself rather than correct the errors made by tech pubs authors).
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Since the guide fitting is open at the bottom, inwards, the plug would indeed have to move up (which the upper guide track bolt as Boeing calls it would prevent when installed). Maybe the writer meant to say that it has to move down to clear the 12 stop fittings? 
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"The door plug will stay in this position until a small outboard force is applied to the top of the door plug. When the force is applied, the door plug will fall to the open position".
As I read that section, it would appear that the door remained in the up and disengaged position until a "Small outboard force...." It finally functioned as it was designed to do. Leaving the question as to why it withstood 150 some flights.
As I read that section, it would appear that the door remained in the up and disengaged position until a "Small outboard force...." It finally functioned as it was designed to do. Leaving the question as to why it withstood 150 some flights.
"The door plug will stay in this position until a small outboard force is applied to the top of the door plug. When the force is applied, the door plug will fall to the open position".
As I read that section, it would appear that the door remained in the up and disengaged position until a "Small outboard force...." It finally functioned as it was designed to do. Leaving the question as to why it withstood 150 some flights.
As I read that section, it would appear that the door remained in the up and disengaged position until a "Small outboard force...." It finally functioned as it was designed to do. Leaving the question as to why it withstood 150 some flights.
The damage photos in the NTSB report make it clear that the plug tore past the stop fittings and the rollers broke out of the guide fittings. That could not possibly have happened if the plug was in the up and "disengaged" position just before it departed.
The plug stayed on the aircraft for some 150 flights because the stop fittings had either complete or partial overlap and were performing intended function.
Acknowledgement here. The AMM SDS appears to be incorrect. You must have to move the door upwards to clear the guide tracks. Maybe it needs a very small downward movement first to free the locking bolts but the manual was not clear.