Top news last week, a Boeing “loses” its engine on takeoff, plane lands half an hour later, pilot and crew are heroes, passengers are a tad shaken, most take the next flight, all’s well that ends well, and another bear shits in the woods in good ole SA. Okay, we don’t have bears in SA, so sue me.
I’ve never designed an airliner, but I am an engineer and a pilot, and I heard this line at both engineering school and flying school.
Most airliners these days have the engines mounted on pods below the wings. This makes maintenance easier than if they were integrated inside the wing structure or elsewhere, and also makes them, in an extreme case, removable without great structural damage. Similarly, if there’s an engine fire, it’ll probably be confined to the engine pod.
As well, since they’re under the wings, they’re fairly close to the (longitudinal) centre of gravity, so they aircraft won’t be upset if one falls off.
So: this is what I hear, but I can’t speak with the authority of an aircraft engineer.
I’m not an aircraft engineer, but this is my understanding:
Many pod-mounted engines have shear pins that are engineered to fail when certain loads are reached. There are typically 2-4 shear pins on the engine mount, or the mount itself is designed to shear away without pins. I don’t know which aircraft in particular have these designs.
A modern high-bypass ducted fan can create a huge amount of drag if it seizes up due to a catastrophic failure. It may be better to shed the engine in such cases, as the airplane may be unflyable otherwise. In addition, if the engine seizes, all that rotational energy has to go somewhere, and if the engine doesn’t break away, that energy is being transmitted to the rest of the structure.
Finally, in the event of a gear-up landing or crash, if the engines snag on something you want them to rip away from the airframe, rather than ripping open the wing and spilling jet fuel everywhere.
Yep, true; if something is gonna break you want it to break in a manner that will case the least collateral damage. For example airliners wheels have temperature and pressure fuses so that they deflate instead of blowing up if conditions exeed certain limits.
Of course things sometimes don´t go as planned and calamity strikes. Like when an El-Al cargo 747 lost an engine over Amsterdam in the 90s, it had defective fuse pins on the mounts so one engine detached, flew up and to the side and smashed the neighboring engine; the plane eventually lost control and demolished an apartment building.
I suspect the vibrations caused by an unbalanced but still rotating turbofan are a big factor, too. Those sorts of shakes could create a resonance in the wing that breaks a spar or rips the wing off.
So shear pins that tear the engine off under certain loads are probably a good thing.
Jet engine design engineer here: The design regulation for aircraft engines, FAR 33 if you’re interested, does require the mounting structure to stay intact in the event of the loss of a single fan blade, or the ingestion of a large bird or several smaller ones. A new engine design actually has to have its blade-loss capability demonstrated in a ground test, and those, my friend, are spectacular events, even on high-speed film, let me tell ya.
But it’s possible for more than 1 fan blade to be lost in an ingestion incident, and then the mounts are indeed designed to fail before they can break the wing, just to permit an emergency landing. This event demonstrated it. There has been no public information yet on the exact cause of the failure, though.
It isn’t. But the asymmetric drag can make the airplane difficult to control, or even impossible on a twin. The ferry engine in the linked photo does not create enough asymmetry to be more than a nuisance, and there are still 2 working engines on that side to produce extra thrust to compensate.
FWIW, engines have been separated from airplanes by the shear pins loosening from improper maintenance, not actual shear loads. This incident just smells like that, somehow.
Would it not be a certification requirement that a twin-engine aircraft be controllable with an engine out? Otherwise, the failure of either engine is fatal.
It is, but the requirement normally takes the form of making sure the fan can still turn (“windmill”) in case of “normal” malfunctions. That induces a lot less drag than a seized fan.
I had understood that conventional wisdom said the opposite - a stopped prop produces less drag. I did some googling and found this paper. Its introduction states that all but one of the sources consulted by the authors ageed - a stopped prop produces less drag. The paper describes wind tunnel experiments and concludes the answer depends - a stopped fine-pitch prop will have lower drag than a windmilling prop; coarsening the pitch will increase the drag, which will eventually exceed that of the windmilling prop.
But in this experiment, the prop is freewheeling, whereas in the real world it’s nearly always turning the shaft of an engine. Various sources indicate that this significantly increases the drag, but they can’t be called conclusive. However, there seems to be little to no support that a windmilling prop has much less drag than a stopped prop - at best (i.e. freewheeling) it could be slightly better.
Some of the mechanics at the hangar where I work are always claiming that Boeing engines will drop straight down off of the pylon whereas McDonnell-Douglas wing-mounted engines are designed to flip forward and pass over the top of the wing.
I don’t know if this can be true, and I have never been able to find any info confirming this.
Jet engines have large fans up front, with chord/spacing ratios so large you can’t see through them from the front. A stopped one might as well be a barn door dragwise. A windmilling fan produces less resistance by not forcing the air to make such tight turns to get through it.
Conventional wisdom for stretching the glide of a piston airplane with a dead engine and a constant-speed prop is to put the prop control in low-RPM mode (coarse pitch) to reduce drag. I’ve never been in that situation, but pilots who have say it makes a noticeable difference. But the prop isn’t turning at all in such a case, since it’s direct-driven by a (dead) engine.
The engine will only go over the top of the wing if it’s still producing thrust when the aft mount lets go, or maybe if there’s a very high angle of attack. If the engine’s dead, or the forward mount goes first, it’ll drop. I know of no difference between Boeing and Douglas that would make any difference.
There was a DC-10 accident (McDonnell Douglas DC-10 - Wikipedia) where a failed engine did just that, rotate forward & over the wing. Tore out the leading edge slats/flaps, created a partial hydraulic failure (not total like UAL 232 in Sioux City), and the airplane became uncontrollable & crashed.
Wiki doesn’t say so, but the NTSB found that the up & over the front engine separation was a failure of the design, not a deliberate part of the design.
The only underwing pylon jets McD-D ever built are the DC-8, DC-10, and C-17. The latter two have been out of production for 35 & 19 years respectively. There are fairly few of either type still flying.
The C-17 is a McD-D design still being built by Boeing.
It’s becoming increasingly difficult to talk about anything McD-D-related in the present tense, particularly design.
I understand the logic of this perfectly, as Broomstick explained. As a corollary, isn’t there a certain amount of danger implicit on the ground if an engine separates and tumbles towards the earth and does anyone know if this is mitigated against (perhaps through further breakup upon descent or something)?
Well, yeah, there’s always a chance a dropped engine will hit something on the ground, but most of the time there’s nothing really critical (from a human viewpoint) underneath the airplane. If it drops off and kills a few squirrels, knocks down a tree, scares some fish in a lake, etc., not such a big deal. Where it could become an issue is over urban areas where the percentage of land covered with Valuable Objects or people is significant. Out in the country, the odds of scoring a direct hit on, say, a farmhouse is pretty low.