Looking at photos of the damage to Southwest Flight 1830, in particular from here and here, I was thinking about what the forces on an individual fan blade are.
The fan is obviously spinning at umpty thousand times a minute. If a blade parts at the root suddenly, it will want to shoot out radially, but will it tend to blow forward as well? The air pressure behind the fan is higher than that in front of it, right? Plus, the fan blades are angled so it seems like it would want to ‘climb’ forward. OTOH, it would have to jump forward pretty quick to avoid being grabbed by the following blade and pushed back, deeper into the engine. I don’t know how much room there is behind the fan for loose material to go. Maybe it did get caught by the following blades and whanged and bounced around in the intake for a while before getting shaken loose.
The green composite material forward of the fan looks like a saw went through it, but that may just be the way the material fails, and maybe isn’t caused directly by the loose fan blade cutting through it.
Air is being sucked into the engine. Therefore I would think that the blade would tend to go backwards.
A lot of the material inside the cowling which failed is for absorbing sound. Once it was significantly damaged, I’d assume that it fell apart from the airflow around it.
I was an aircraft mechanic for over 20 years, and only once saw a jet engine fail worst that that one.
Air is sucking in, partly because the blade is pulling it in. If you detach the moving blade from the hub it is free to move forward (and radially outward).
That’s not quite correct. Air is being pushed into the engine by the fan blades. Air pressure cannot suck, it can only push. The lady in the plane was not sucked out the window, but pushed out by the air inside the cabin.
For some engines (not sure of this one) - the fan blades are designed to flex for bird strikes. Some engines are tested by hurling whole chickens (dead ones) into a running engine to check for damage.
That maybe a pedantic discussion. Does a household vacuum suck or does the atmosphere push the air (and the dust in the path) into the device ? Same can be said about a person sucking on a drink through a straw.
No, it will depart tangentially. Anything held by force/attachment/rope/etc. and forced to spin around a center will, when the force/attachment/rope/etc. stops, will head off tangentially. Thus, in this sort of situation, the blade will try to go off along a tangent. Naturally, the housing will not let it get all that far in such a direction.
it’s a bit of a false dichotomy to say air can only push, not suck. It’s not pressure pushing or vacuum sucking; it’s a pressure differential.
High pressure can’t “push” without a lower pressure nearby, and low pressure can’t “suck” without a nearby higher pressure. Otherwise, it’s just hydrostatic pressure and that neither pushes nor sucks.
It’s a semantic convention to say “sucked” when one side of the pressure differential is below atmospheric pressure, and “pushed” or “blown” when one side is above atmospheric pressure.
I don’t think it’s wrong to say that poor woman was partially sucked out of the fuselage.
Also, the radial speed of a freshly liberated turbine blade is much, much greater than its axial (fore/aft) speed.
No speculation, here is exactly what happens when a blade breaks off at the hub. this is a video of a blade-off test for an engine from an Airbus A380. The engine is mounted on a test stand, and while running at full power, they set off a small explosive charge to deliberately separate one of the fan blades from the hub. The video is cued to 4:45, during the slow-motion view of the blade separation event. the blade separates at 4:53; it damages a couple of trailing blades before disappearing into the engine.
This is on a static test stand, so ambient/slipstream velocity = 0. If you’re in the air at a few hundred knots, then the air velocity into the engine is higher than what was encountered during that test; it seems very unlikely that a separated blade will come forward out of the engine.
Having said that, I’m a bit puzzled about why the engine cowling forward of the fan came apart on Southwest 1830. Maybe the violent shaking from the imbalanced engine tore it apart? If you watch the normal-speed segment of the A380 video after the blade comes off, you can see what wild gyrations the engine goes through; presumably the Southwest flight experienced similar levels of shaking.
Heard on the news last night that the NTSB is going to inspect ALL of the jets with that engine. It will take a while, but I think it’s a good idea considering the fact that this is actually the second time this has happened with that engine.
It seems that it was indeed the violent shaking and deformation of the engine that wrecked the cowling. The blade never made it outside the containment - but the cowling broke apart, and it was the cowling that smashed the window - not the blade.
This is actually the second such failure Southwest have sustained - the only difference being that the main cowling of the engine remained intact on the first failure, but broke apart in this one.
