Good Day,

I’m new to this website, therefore apologise in advance if my question has been posted in the wrong area. Please feel free to move it, or simply remove it if this is not the place for such amateur enquiries.

I have a query regarding the engines on, what I think was a 747-300, (or possibly a -200), on a journey I made from Brize Norton to Mount Pleasant in 2007.

During a scheduled stop in Ascension, all passengers dis-embarked, and I happened to notice the engines as I was getting off, or at least one of them. The engine continued to rotate at very slow speed, despite being shut down, and a strange rattling noise was quite apparent. The noise was a very metally/ tinny sort of scraping noise, and in my mind sounded like the tips of the fan blades scraping around the inside of the engine cowling.

This seemed odd to me at the time as other engines I’ve noticed haven’t done this, though I don’t fly that often so perhaps it is more common than I think.

My questions are:

Why were the engines rotating at slow speed despite being shut off, and what was driving them to do so (air/electric motor etc.)? There was little wind so I don’t believe external forces were causing them to do so. What is the benefit of rotating them like this during a stop-over, or indeed at any time?

What was causing the noise I heard? In my mind it was almost like the blades were not fixed, yet sort of floating within their attachments. I guessed at the time that, with the engine started, centrifugal force would force them all out to their respective fixed positions, yet would this not push them out further and endanger the ends coming into contact with the the insides of the engine housing?

Apologies again if this seems like a stupid/futile question, it’s just something that has bugged me over the years and I thought perhaps this was the place to get an answer. Don’t get me wrong, I’m not losing any sleep over it or anything!

Thank you

It was the wind. It doesn't take much breeze if it is at the correct angle to spin the fan. What you heard were the mid span shrouds on the back side of the blades hitting each other. The fan blades are actually a little loose when the engine is not running. You can wiggle them with your hand very easily.

Most engines make a similar noise when windmilling at slow speed.

If you see it in a very light breeze, with the fan spinning very slowly. You can see each blade "falling" into place at top of the engine. You can hear each individual "clink" that makes your "rattling" noise.

Thank you both for your replies. So is this a feature of a lot of aircraft engines then, or specific to the engines on that plane? I don’t even know what the engines were on that plane, but I’m guessing they could have been a number of different types, depending on what was ordered at the build stage? If wind is the cause, then I guess part of the reason I’ve not noticed it on other aircraft is that a lot of them park directly in front of terminal buildings, sheltered from any head winds. This aircraft was parked right in the middle of a large empty area, so perhaps that’s why I spotted it on this occasion.

Well, I'm talking about the CFM-56 on the A320. So, yes it's a feature of fan engines in general.

I suppose you also wouldn't notice this if you board via a jet bridge (because you're inside). But I'm sure you'd have used steps on the occasion in question. I can't imagine Wide Awake has many jet bridges.

greg85 wrote:

Well, I'm talking about the CFM-56 on the A320. So, yes it's a feature of fan engines in general.

I suppose you also wouldn't notice this if you board via a jet bridge (because you're inside). But I'm sure you'd have used steps on the occasion in question. I can't imagine Wide Awake has many jet bridges.

This will of course become less common as blisk fans mature and become more common. Blisk fans used to be mostly on smaller business jet engines, but as far as I know the GE Passport is the largest blisk fan out there at 52 inches. CFM-56 is 61 inches (although LEAP is of course bigger at 69 or 78 inches), so we are getting close to 737/A320 scale blisks.

Is this the noise you heard?

https://youtu.be/6ZVaeP1fE6w

Mark

Yes that’s the noise! Thank you all for clearing it up for me.

Do any modern engines have this quirk?

CCGPV wrote:

Do any modern engines have this quirk?

They still spin in the wind but you don't get the clinking. The blade attachment is still slightly loose when the engine is not running, but the blades are far larger and don't have mid-span shrouds, so they don't move around noticeably.

Compare an early turbofan on the 747-100 with a modern one on a 350-900.

AA737-823 wrote:

Just to add to the good information that others have already provided you, different engines are more or less noticeable with this noise.
The Pratt and Whitney JT9D, which your 747 most likely had (though, if you could remember the airline, that would narrow it down; BA used a lot of Rolls Royce engines on their 747 fleet, starting with the -200s I believe) are VERY loud and clanky as they "windmill."
The GE CF6 (also powered some 747-200s, and many 747-400s) also makes the noise, but it's less noticeable.
The CFM56, mentioned above, is actually one of the least clanky engines I've worked around. The -7 engines on the 737NG fleet have fan blades that barely move at all. The older versions, the -5 on the Airbus and the -3 on 737 classics, were a bit floppier.
Anyway. That's way more information than anyone would ever want about fan blade rattle.
THOUGH, I just remembered, on transit checks on the eldest JT9's in our fleet, which would be the -A and -Q series, I believe, we'd actually have to do visual checks before departure to verify that "shingling" had not occurred. This is what happens when one blade becomes a bit too free, and flops over just far enough that the mid-span shroud gets caught behind the mid-span shroud of its next door neighbor. In that case, it cannot free itself during startup.
Later revisions of the JT9D (the -7R and others) had an additional mid-span shroud added, to improve fan blade performance under these conditions.
At least, that's the best of my recollection!

Thank you for the interesting information. The aircraft in question was TF-ARU. I did find a picture of it online (at MPN as well!) but I’m not sure about the rules regarding posting other people’s photos here so I haven’t included it.

I remember taking a JAL 747-200 from BNE-NRT in 2000, we had to disembark at a remote stand, down stairs, and the windmilling engines were making the same noise as you describe. It made me think the engines were faulty when I heard it!

Fjm1982 wrote:

Yes that’s the noise! Thank you all for clearing it up for me.

Were they spinning pretty fast like this ?

https://www.youtube.com/watch?v=fvEb0H2w-pk

CALTECH wrote:

Fjm1982 wrote:

Yes that’s the noise! Thank you all for clearing it up for me.

Were they spinning pretty fast like this ?

https://www.youtube.com/watch?v=fvEb0H2w-pk

Yes they were. I guess the speed is just dictated by the head wind experienced at the time?

Fjm1982 wrote:

CALTECH wrote:

Fjm1982 wrote:

Yes that’s the noise! Thank you all for clearing it up for me.

Were they spinning pretty fast like this ?

https://www.youtube.com/watch?v=fvEb0H2w-pk

Yes they were. I guess the speed is just dictated by the head wind experienced at the time?

Or tail wind. Fans will spin "backwards" as well.

That noise was (is) the bane of mechanics everywhere. Especially the JT9. I remember changing a generator on a JT9 with the wind coming straight down the front.

CLACK-CLACK-CLACK-CLACK-CLACK

It's the sound of nightmares. The damn thing was turning so fast, I couldn't get in there to stop and chock it.

CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK-CLACK

By the way, the acoustics are awesome right behind the fan.

A time came where we had to put Molykote G on the part span abutments on return to home base, it was a battle to stop the fan especially in stiff breezes..

Balerit wrote:

A time came where we had to put Molykote G on the part span abutments on return to home base, it was a battle to stop the fan especially in stiff breezes..

Ya, the wind can spin a fan fairly fast. We would just get up there and just kinda choke the spinner until the fan came to a stop. You only tried it once without gloves because you could get a serious burn from the friction. When we were doing NAMS (Nautical Air Miles) validation testing, we would fly several flights and had to do a bore scope inspection before the first flight and between subsequent flights so we got pretty good at bringing the fans to a stop. (Boeing Commercial Airplanes)

Balerit wrote:

A time came where we had to put Molykote G on the part span abutments on return to home base, it was a battle to stop the fan especially in stiff breezes..

Don't know why, but I really disliked doing that task. As for stopping the fan, put on a pair of good gloves and "hug" the spinner until it stopped. If you were a little adventurous and the fan was spinning the backwards, you could lay your hand on the blade and let the blades slap your hand until the friction finally brought it to a stop, but that tended to get a little painful.

fr8mech wrote:

Balerit wrote:

A time came where we had to put Molykote G on the part span abutments on return to home base, it was a battle to stop the fan especially in stiff breezes..

Don't know why, but I really disliked doing that task. As for stopping the fan, put on a pair of good gloves and "hug" the spinner until it stopped. If you were a little adventurous and the fan was spinning the backwards, you could lay your hand on the blade and let the blades slap your hand until the friction finally brought it to a stop, but that tended to get a little painful.

I couldn't have put it in better words myself and brings back many memories

Fjm1982 wrote:

Thank you both for your replies. So is this a feature of a lot of aircraft engines then, or specific to the engines on that plane?

It was a feature of a great many turbofan engines of that era. The RB-211 did it, the CF-6 did it, the PW2000 and PW4000 did it.

Each fan blade had a certain amount of "give" at the hub. They also all had a "peg" sticking out of each blade about 3/4 of the way from the hub that would help space the blades from their neighbors. This "peg" is called a "clapper."

Modern engines have blades that are much larger and broader (the GE9X will have a grand total of 18 blades, IIRC). They are fixed much more firmly into the hub and have no clappers.

Here's a video that answers your question: https://www.youtube.com/watch?v=6ZVaeP1fE6w

DocLightning wrote:

Fjm1982 wrote:

Thank you both for your replies. So is this a feature of a lot of aircraft engines then, or specific to the engines on that plane?

It was a feature of a great many turbofan engines of that era. The RB-211 did it, the CF-6 did it, the PW2000 and PW4000 did it.

Each fan blade had a certain amount of "give" at the hub. They also all had a "peg" sticking out of each blade about 3/4 of the way from the hub that would help space the blades from their neighbors. This "peg" is called a "clapper."

Modern engines have blades that are much larger and broader (the GE9X will have a grand total of 18 blades, IIRC). They are fixed much more firmly into the hub and have no clappers.

Here's a video that answers your question: https://www.youtube.com/watch?v=6ZVaeP1fE6w

The correct term is: 'part span abutment', not clapper - that video has a lot of incorrect info in it.

greg85 wrote:

Well, I'm talking about the CFM-56 on the A320. So, yes it's a feature of fan engines in general.

I suppose you also wouldn't notice this if you board via a jet bridge (because you're inside). But I'm sure you'd have used steps on the occasion in question. I can't imagine Wide Awake has many jet bridges.

Thank you for this - I thought the fans of an Easyjet A320 were lightly rubbing the nacelle as I walked past a CFM 56 making this rattly noise in a light breeze a few months ago, on my way to the back door boarding of shame (I didn't say anything though )

tigamilla wrote:

greg85 wrote:

Well, I'm talking about the CFM-56 on the A320. So, yes it's a feature of fan engines in general.

I suppose you also wouldn't notice this if you board via a jet bridge (because you're inside). But I'm sure you'd have used steps on the occasion in question. I can't imagine Wide Awake has many jet bridges.

Thank you for this - I thought the fans of an Easyjet A320 were lightly rubbing the nacelle as I walked past a CFM 56 making this rattly noise in a light breeze a few months ago, on my way to the back door boarding of shame (I didn't say anything though )

That’s exactly what I thought. But being British, I thought it best in my mind to let 300 people potentially plunge to their death because of a faulty engine rather than actually say anything! Glad I didn’t now anyway!

DocLightning wrote:

Fjm1982 wrote:

Thank you both for your replies. So is this a feature of a lot of aircraft engines then, or specific to the engines on that plane?

It was a feature of a great many turbofan engines of that era. The RB-211 did it, the CF-6 did it, the PW2000 and PW4000 did it.

Each fan blade had a certain amount of "give" at the hub. They also all had a "peg" sticking out of each blade about 3/4 of the way from the hub that would help space the blades from their neighbors. This "peg" is called a "clapper."

Modern engines have blades that are much larger and broader (the GE9X will have a grand total of 18 blades, IIRC). They are fixed much more firmly into the hub and have no clappers.

Here's a video that answers your question: https://www.youtube.com/watch?v=6ZVaeP1fE6w

The role of snubbers (or mid-span shrouds, different companies and groups have different names for them) is less about spacing and more about flutter prevention. The high aspect ratio blades of yore were much less stiff than their modern, wide-chord counterparts. The snubbers were typically around 2/3 of the span to add bending and torsional stiffness to increase the flutter mode frequencies and bring them away from any common excitation frequencies (typically multiples of shaft frequency). The interfaces between the snubbers also provided damping via friction, further reducing vibratory and flutter issues. Snubbers reduce fan efficiency (more wetted area and associated wake/entropy generation) so were only used out of necessity.

Balerit wrote:

DocLightning wrote:

Fjm1982 wrote:

Thank you both for your replies. So is this a feature of a lot of aircraft engines then, or specific to the engines on that plane?

It was a feature of a great many turbofan engines of that era. The RB-211 did it, the CF-6 did it, the PW2000 and PW4000 did it.

Each fan blade had a certain amount of "give" at the hub. They also all had a "peg" sticking out of each blade about 3/4 of the way from the hub that would help space the blades from their neighbors. This "peg" is called a "clapper."

Modern engines have blades that are much larger and broader (the GE9X will have a grand total of 18 blades, IIRC). They are fixed much more firmly into the hub and have no clappers.

Here's a video that answers your question: https://www.youtube.com/watch?v=6ZVaeP1fE6w

The correct term is: 'part span abutment', not clapper - that video has a lot of incorrect info in it.

I have seen them referred to as snubbers, clappers (typically in a more casual context), part-span abutments, mid-span shrouds and more. It doesn't seem that there is a single name for them, it just depends on the group or company (just like how GE refers to a low-pressure compressor as a "booster" vs P&W which just calls it the LPC).