Originally Posted by
Machinbird
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.
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.