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Mellivora capensis
11-13-2007, 02:58 AM
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.

But this makes me curious. According to this article http://www.mg.co.za/articlePage.aspx?articleid=324387&area=/breaking_news/breaking_news__national/, an unidentified object was ingested into the engine, and

The engine-to-wing support structure is designed to release the engine when extreme forces are applied, to prevent any structural damage to the wing.

Is this true?

wolfstu
11-13-2007, 03:09 AM
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.

But this makes me curious. According to this article http://www.mg.co.za/articlePage.aspx?articleid=324387&area=/breaking_news/breaking_news__national/, an unidentified object was ingested into the engine, and



Is this true?
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.

Sam Stone
11-13-2007, 03:32 AM
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.

Ale
11-13-2007, 04:03 AM
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.

But this makes me curious. According to this article http://www.mg.co.za/articlePage.aspx?articleid=324387&area=/breaking_news/breaking_news__national/, an unidentified object was ingested into the engine, and



Is this true?

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 donīt 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.

si_blakely
11-13-2007, 05:00 AM
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.

Si

Broomstick
11-13-2007, 06:23 AM
Is this true?
Yes.

An airliner can continue to fly if an engine departs the structure, but the wing itself is not optional.

ElvisL1ves
11-13-2007, 07:02 AM
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.

Xema
11-13-2007, 09:53 AM
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.
Are you referring to aerodynamic drag? I seriously doubt that would be anywhere near what's required to sever the shear pins.

fortytwo
11-13-2007, 04:25 PM
Are you referring to aerodynamic drag? I seriously doubt that would be anywhere near what's required to sever the shear pins.

I'm inclined to agree with you since some aircraft can be modified to ferry an engine externally.

There has to be some drag penalty but here's a non running engine still attached to the wing (http://www.airliners.net/open.file/635769/M/)

ElvisL1ves
11-13-2007, 09:27 PM
Are you referring to aerodynamic drag? I seriously doubt that would be anywhere near what's required to sever the shear pins.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.

Xema
11-13-2007, 09:42 PM
asymmetric drag can make the airplane difficult to control, or even impossible on a twin.
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.

ElvisL1ves
11-13-2007, 09:45 PM
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.

Mellivora capensis
11-14-2007, 01:52 AM
Thanks all for your posts. Once the final report is out on the exact cause I'll update.

Xema
11-14-2007, 09:54 AM
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 (http://www.goshen.edu/physics/PropellerDrag/thesis.htm). 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.

Zambini57
11-15-2007, 01:25 PM
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.

ElvisL1ves
11-17-2007, 09:29 AM
I had understood that conventional wisdom said the opposite - a stopped prop produces less drag. Interesting paper, thanks.

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.

ElvisL1ves
11-17-2007, 09:31 AM
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.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.

LSLGuy
11-17-2007, 09:42 AM
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.That's classic GenAv mechanic blather.

There was a DC-10 accident (http://en.wikipedia.org/wiki/DC-10#American_Airlines_Flight_191) 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.

Martiju
11-17-2007, 09:54 AM
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)?

Broomstick
11-17-2007, 01:20 PM
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.

Stranger On A Train
11-17-2007, 02:30 PM
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.It's less likely that any unbalance rotations would create criticla resonance in the wing structure; the fan will be turning (and thus the unbalance will be at the frequency of) tens of thousands of RPM (1000's of Hz), while the resonance modes of the wing structure are going to be on the order of single digit frequencies. It's more likely the actual unbalance reaction force that'll put loads on the wings, and the shear pins are designed to shear free before the wing structure tears apart. In other words, the shear pins holding in the engine are strong, but are designed to have marginally less strength than the wing itself.

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)?I can't believe this thread has gotten this far without making an obigatory Donnie Darko reference. Okay, I'll do it: "28 days... 6 hours... 42 minutes... 12 seconds. That... is when the world... will end."

That taken care of, sure, there's a danger that a falling engine might pose some hazard to someone on the ground. But despite the fact that it's a highly critical failure, the likelyhood of occurance is very, very low, and most likely to occur (like most airliner failures) on take-off or landing. Designing an engine to fragment after detacting from the airframe would be extremely difficult (I'd venture to say virtually impossible) and would then just create an expanded fly-down hazard of marginally less dangerous debris over a wider area. Better to kill just one kid in his bedroom than a schoolyard full of children, non? Other schemes like putting parachutes or the like on an engine are functionally problematic (how would you attach the lines and assure that the chute isn't dragged into the intake) and would increase weight and maintenance. So instead we live with the extremely unlikley hazard of jet engines falling out of the sky, which is less likely to terminate your life than the drunk driver in the lane next to you or the improperly wired GFCI outlet in your bathroom.

"I'm voting for Dukakis."

Stranger

ElvisL1ves
11-17-2007, 06:25 PM
does anyone know if this is mitigated against (perhaps through further breakup upon descent or something)?No, it isn't. No practical way exists to do so. Any "mitigation" is, bluntly, along the lines of the maximum damage a single falling (and nonburning) engine can do to people or property on the ground, compared to the damage an entire crashing and flaming airplane can do.

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.Sorry, missed this. Yes, it is. Twins are certified with, among all the other critical airspeeds, one called Vmc - minimum control airspeed. Normally one does not take off until reaching Vmc, in case of an engine failure at the most critical time, takeoff.


Pay attention to Stranger - he speaks truth.

Just FTR, though, a rotor has not only its own speed but a "whirl' speed that has to be taken into account. That's where its CG rotates about the nominal centerline, at some speed not related to the rotor's own rotating speed, as a result of its own mass and bending stiffness. As is tragically the usual case, that was discovered the hard way, after several Lockheed Electra turboprops crashed due to propeller whirl (with worn engine mounts) happened to excite a wing bending mode with positive feedback - i.e. the wings tore off. Turbofan as well as turboprop installations are now certified to have no such synchronous modes in the operating range.

GusNSpot
11-18-2007, 02:30 AM
Piston twin engine pilot here.

First, to be clear, a 'flat' pitch is used for takeoff and is usually called 'fine' pitch. But either way, it is the position used to allow the engine to spin the prop the fastest, so to speak.

High pitch or a 'coarse' pitch is a 'cruise' pitch and is used as is stated.

When 'fixed' pitch props are used, they are in between some place so as to make the best use of the engine on that airframe in the best way. Some people actually switch props depending on what they are doing. Like water or land use in Alaska.

Anyway, if an engine with a constant speed propeller actually sizes up without tearing off the airframe when it stops turning, that is where the pitch stays or they are of the design that between the slip stream and the springs inside, they 'might' push the blades towards the 'coarse' or 'high' pitch or the 'feathered' position. Do not count on it.

Lets take the most usual reason for engine stoppage on a piston engine general aviation aircraft. "Lack of fuel."

The engine will 'windmill' ( The apparent wind passing over the propeller keeps it spinning at some RPM depending on glide speed the aircraft is at and engine compression and friction.) If there is no glide speed, the pilot won't care about the rest of this.

The speed of the propeller is below that set by the propeller controller ( Average lowest setting is about 2000 RPM and so the propeller hub flattens the pitch as power falls so that the engine can easier spin the prop. Of course it can't because it has no fuel.

If you angle the blades into the wind more than the flat pitch, the engine will; 1) spin slower, 2) the drag on the airframe will be less and you will glide farther. 3) best thing is to 'feather' the propeller, ie, align it with the slip stream and that will be the least drag, other than removing it completely, so as to glide the farthest. Most single engine planes do not have a feather position. (weight, cost, complexity {cost benefit ratios})

If you really must; and have sufficient time and altitude and know that the engine can not be restarted, (You know you are out of fuel and there is no way it is going to run again without a gas truck.) in say a Cessna -180, you can go to full flaps and the 'highest', 'coarsest,' pitch and do a really excessive stall so that you get below say 40 MPH before the nose drops, and if the engine has good compression, you can get the propeller to stop and it will remain stopped unless you then exceed about 80-140 MPH depending on engine compression and a few other engine drag parameters. Of course this is all dependent on the fact that you are high enough to make doing this sufficiently worthwhile to possibly make the only dry land anywhere around, or some such. If other emergency landing spots are within reach that you have a reasonable chance of living over, do that instead.
The point being, if the engine is no longer producing power but is on the airframe, propeller 'removed' is best, 'feathered' is next best, 'stopped' is next and, 'high' pitch (coarse) is next and then the rest..... ;)

The differences do not need fine instruments to detect. They are large and obvious.

Been there, done that.....

DSYoungEsq
11-18-2007, 09:12 AM
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.
Do you have a sample of that that can be viewed? I'm getting wide eyes just thinking about it... :eek:

ElvisL1ves
11-18-2007, 10:30 AM
The Rolls-Royce Trent 1000 fan blade-off test (http://www.youtube.com/watch?v=j973645y5AA) is public info, perhaps because it's pretty mild-looking compared to others. That's the engine on the A380, the world's largest airliner.

Not sure what this one (http://www.youtube.com/watch?v=Ek6adm4iV4Y) is, maybe the Trent 900, but it looks cooler.

Bird-ingestion test videos are cool in the way snuff flicks are cool. Like this one (http://www.youtube.com/watch?v=Wi8qVTFCTVI&feature=related).

DSYoungEsq
11-18-2007, 03:47 PM
The Rolls-Royce Trent 1000 fan blade-off test (http://www.youtube.com/watch?v=j973645y5AA) is public info, perhaps because it's pretty mild-looking compared to others. That's the engine on the A380, the world's largest airliner.

Not sure what this one (http://www.youtube.com/watch?v=Ek6adm4iV4Y) is, maybe the Trent 900, but it looks cooler.

Bird-ingestion test videos are cool in the way snuff flicks are cool. Like this one (http://www.youtube.com/watch?v=Wi8qVTFCTVI&feature=related).
Wow, that second one is pretty amazing. I find it quite amazint that the blade right behind the one that is released manages to deform as much as it does, yet stays attached.

It's a good thing when I fly, I don't think of these things happening. It's also good to know that others have already thought of it and designed with it in mind. :)