only while it is contained in its reservoir, not after it leaks

Quote:
remember the pressure is approx 3000psi
When a fluid is pressurized in a hydraulic system, it has potential energy. At the point where said fluid is released from the pressurized system, that potential energy is converted into kinetic energy (high velocity).
When the high velocity stream is directed at structure, the momentum of the fluid is converted to a force on the impacted structure.

With enough fluid flowing under pressure from a leak, things can get torn up.

Despite my comment above, it still looks like some sort of maintenance process got interrupted, no matter what type fasteners were used to anchor the panel.

A few clues:
At the fwd inboard corner the flange is not damaged or displaced, therefore the screws were not in those holes.

None of the landings are damaged: if the screws were fitted you would expect the departing panel to rip off or damage the landings, or leave parts of itself behind.

The black bits trailing over the wing are micros witch harnesses for u/c indication/sequencing, which were attached to the inside face of the panel for support.

The fluid is probably fuel from a vent pipe or pressure sensing pipe also attached to a bracket on the inside face of the panel.

So the most likely explanation is that the panel was incorrectly / partially fitted

I would hope the reservoir had a LOT less than 3000psi in it!!

Most airliners are now using 4000 or even higher, to save weight.

The hydraulic system will not have enough flow to cause the door to pop off, but a compressed air system might, but I don't think the 757 uses one, in that area at least.

A catastophic failure of an outflow valve might give enough over pressure!

I'm pretty sure older gen Boeings don't have (over) 4000psi hydraulic systems.

What kind of an outflow valve are you referring to? That wing area is certainly not pressurised.

For info, reservoir pressures are probably in the range of 30~50psi. And, according to Avherald.com, the aircraft was nearing top of descent when the panel came adrift. With the landing gear lever in OFF, one would think that the gear actuators would be unpressurised (except, perhaps, bogey tilt actuators).

Had a panel fly off a 747 wing in a similar position during the climb - long story, ultimately proved to be a fibreglass fillet panel but one didn't know, nor what had punched it off, gear or flap retraction etc. so I dumped fuel and landed back at the departure airport in fairly short order.

At the engineering enquiry Boeing asked why I had returned, as the 747 could fly with a panel 6' x 9' ( forget the exact dimensions ) missing in that area (Oh, really, news to me ).

The company engineer said " What's the Captain supposed to do, go out and measure it ? "

Too right, I was back on the ground, fix it.

Well, in Boeing's defense, it apparently did fly, no?

Most airliners are now using 4000 or even higher, to save weight.

The hydraulic system will not have enough flow to cause the door to pop off, but a compressed air system might, but I don't think the 757 uses one, in that area at least.

A catastophic failure of an outflow valve might give enough over pressure

I appreciate there isn't really a lot to do in your location, at night, but even by R n N's "fly sim analysts speculation " standards, the above is a an outstanding effort.

The "door" being a wing panel, for a start, and then, the little matter of the outflow valve.

If you could provide a schematic of the, lets call it the cabin pressurisation system, that shows how the outflow valves now affect the wing structure and upper surface in particular in this case, this may well be of interest to many for whom this development is a revelation.

That is correct.
However, the kinetic energy of a thin stream of Hydraulic fluid will not be very high. Since Hydraulic fluid is incompressible, the pressure will immediately drop once a leak develops. There will be a sharp stream of fluid, able to cut through soft material but the Overall force it would excert on a plate will be negligeable. If we saw a small cut in the Panel I would agree. But blowing of a Panel with liquid fluid? Rather not.
If you had a leak of 5 mm Diameter at 3000PSi (if it is much bigger than that the pressure will drop immediately to much, much lower values) the force excerted on the plated would be 40kg. That won't blow off a Panel.
Even 10mm Diameter (which would cause drastic drop of pressure) would theoretically only deliver 160kg max of force.

Aerodynamic (Lift) Forces on the Panel won't be very high, either, but significantly higher than that:
At 60% chord length and only the upper surface being affected, lift contribution would be only a fraction (probably ~20%, depending on exact pressure distribution along the chord at that Re Number and AoA) of its area compared to the wing area. (anyone knows how big this Panel is on the 757? Looks like 3-4 m^2. Based on that we would be talking about the Order of Magnitude of 300 - 400kg of lift force on the panel.
That is, unless it starts to lift on the front edge first. In that case, it would catch air underneath and be blown off instantly, no mater how the rear end is fastened.
Conclusion:
Somehow the fasteners (at least at the leading edge) must have come loose/missing, for this hatch do depart the aircraft. Air pressure underneath could achieve the same but I would expect it to blow the gear doors open, first, since they will not be designed for that load direction at all - and where should it have come from?. Hydraulic pressure could cut a hole into the Panel but probably not blow it off.

Very fortunate it didn't wipe out the tail feathers on this bird. Might have ended badly.
Chances are, someone was sleeping badly after that one.

Just an example of something that COULD cause a door to pop off.

For the ignorami DOOR is used in many more modern /US aircraft instead of PANEL.

Buttrick - the outflow valve is associated with the pressurisation system and has nothing to do with it! The wing is not pressurised.

I think some people are getting confused between reservoir and accumulator.

Yes, but I wasn't aware of that prior to the incident, not exactly laid out in the MEL, and as the panel apparently blew off as either the gear or flaps were being retracted, and vibration became obvious i.e. what other damage might have occurred, would you have continued for 7 hours over shark infested ocean ? I'm on the ground mate, sort it.

Next time - I might, might, be tempted to take a different viewpoint, but as it happened not long after a 747 had experienced a bit of fuselage skin peeling off taking a row of passenger seats with it, and we couldn't see how far in towards the fuselage the opening was .... tough, did it.

Hindsight is great.

" For the ignorami DOOR is used in many more modern /US aircraft instead of PANEL"

Thank you. I am always grateful to have my education expanded.....despite the minor detail of having worked on 757's for, say, 12 years....give or take....with a Hyd system px of 3000psi. And yourself, other than as pax ?

Compressed air ?.....erm, no.

Compressed N2...yes....I wonder why an inert gas would be preferred.....and please, don't try and quote bleed air as an example of compressed air.

Emergency u/c lowering.....blow down on the FJ world, for obvious reasons, but something called.....gravity...... does the job just as well.

I must though, have missed something rather unique with regard to the location of the outflow valve(s).

Are you thinking of making engineering a career....or just a passing hobby ?

Quite why the panel took off with the a/c, and departed to do it's own thing thereafter, has not yet been answered however. As yet.

It's the "inspection Panel" !
Boeing are becoming like Omega.

Krystal:

At what point did I write
"The 757 has....."

I just pointed out inconsistencies in other peoples posts!

Overpressure from whatever source is a widely known cause of DOOR pop off syndrome.

some small doors/panels are designed to blow off under an overpressure. I wasn't aware that this large one was designed so with all those fasteners. Seems like there may have been other escapes if there was an overpressure in that area.

Wing/Strut Blow out panels/doors are generally in the area of pneumatic ducts which run along the wing leading edges. They are usually spring-loaded and often hinged.

I have to agree with the 15 year guy. In my 40 year experience in the aircraft engineering world, riveted or screwed down panels have never been called doors.

Neither did I until recently, it's a modern yank thing.