# Noise from non-running jet engine **URL:** \ **Category:** Factual Questions\ **Created:** [February 13, 2021, 11:01pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863 "2021-02-13T23:01:26Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![MikeF](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@MikeF](https://boards.straightdope.com/u/MikeF)\ **Post date:** [February 13, 2021, 11:01pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/1 "2021-02-13T23:01:26Z")
We were boarding an A319 in Key West the other day (you walk out on the tarmac and up a ramp - no jetway) and the the blades of the engine were turning in the breeze. There was metal-on-metal sound that surprised me. What was that? I thought, “Jeez that doesn’t sound right” but then again, I can’t remember when I would have been a position to hear such a thing. I just assumed it would be silent, like a turned off window fan in the wind. ---
**Author:** ![Lord\_Mondegreen](https://avatars.discourse-cdn.com/v4/letter/l/91b2a8/32.png) [@Lord\_Mondegreen](https://boards.straightdope.com/u/Lord_Mondegreen)\ **Post date:** [February 13, 2021, 11:24pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/2 "2021-02-13T23:24:22Z")
Is it possible that what you were hearing was the Auxillary Power unit (APU)? The APU is a small engine located in the tail section which provides electricity for lighting etc. before the main engines are started. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [February 13, 2021, 11:34pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/3 "2021-02-13T23:34:12Z")
We have some airline pilots here who may chime in with their expertise, but what I can suggest here is that the fan blades of turbofans are (intentionally) surprisingly loose within their highly precision-engineered enclosures, so that windmilling in a gentle breeze could cause the blades to wobble around and make that noise. ---
**Author:** ![RollOutTheBarrel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rolloutthebarrel/32/1649_2.png) [@RollOutTheBarrel](https://boards.straightdope.com/u/RollOutTheBarrel)\ **Post date:** [February 14, 2021, 12:17am UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/4 "2021-02-14T00:17:10Z")
I used to be a jet engine mechanic and I know the sound you’re talking about. @wolfpup has it right. The blades are intentionally loose and become tighter when the fan is rotating at high speed because of the centrifugal force on them. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 14, 2021, 5:31pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/5 "2021-02-14T17:31:23Z")
Been well-answered so here’s a bit more nearby info. The area inside the surrounding inlet where the blade tips sit is made of a fiberglass-like material. When the engine is assembled, the blades barely fit in there. As the engine is run in, centripetal forces pull the blades outwards, locking them tightly in their mountings. The now slightly longer blades have the tips dragging on the fiberglass. The very hard blade tips scour a path into the softer fiberglass then everything fits nice and tight. It’s remarkably similar to how piston rings seat into a cylinder during break-in of a newly (re-)built car engine. ---
**Author:** ![MikeF](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@MikeF](https://boards.straightdope.com/u/MikeF)\ **Post date:** [February 14, 2021, 6:18pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/6 "2021-02-14T18:18:16Z")
Ignorance fought. Thanks. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [February 14, 2021, 6:19pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/7 "2021-02-14T18:19:01Z")
I worked on a magnetic bearing test at one time. Among the many things they can do is to actually shift the rotor shaft slightly to the side (we are talking a few thousandths of an inch). Then the magnetic flux is rotated and the gap between the blades and the housing can be made to rotate around the circumference of the turbine. This was said to lessen the chances of a stall. They could also put a “ping” into one end of the rotor and read it on the other end. If the characteristics of the reading changed it could indicate a crack somewhere. The blades fit into the shaft with a “Christmas tree” that tightens up as the rotor gains speed and clunks at low speed: [https://www.ijspeert.com/clamping-of-turbine-blades/](https://www.ijspeert.com/clamping-of-turbine-blades/) ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 16, 2021, 10:51am UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/8 "2021-02-16T10:51:07Z")
[![](https://img.youtube.com/vi/phludDUfakg/maxresdefault.jpg "Jet engine turbine blade noise") ](https://www.youtube.com/watch?v=phludDUfakg) [![](https://img.youtube.com/vi/aKJMUdFhYBs/maxresdefault.jpg "Don't panic! Its normal!") ](https://www.youtube.com/watch?v=aKJMUdFhYBs) ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [February 16, 2021, 12:45pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/9 "2021-02-16T12:45:04Z")
AgentJayZ on youtube (a jet engine mechanic) has discussed this sound many times. The tinging you hear is, like others said, the blades wobbling back and forth in their mounts. There’s also another sound that you can hear as it spins down. IIRC, it’s the starter gear clutch re-engaging with the turbine (don’t quote be on that, it’s been a while since I watched that video). But for any questions on jet turbines, AgentJayZ is your guy. He’ll answer questions you didn’t know you had in so much more detail than you realized you wanted. Seriously. You know when you see a bolt with a piece of wire going through it so it can’t back itself out. He’s got a three part series that must be over an hour altogether, just on installing lock wire…and it’s really good. Maybe it’s the funny Canadian accent, but his videos are borderline hypnotic. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 16, 2021, 12:50pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/10 "2021-02-16T12:50:42Z")
> [@Joey\_P](#): > > AgentJayZ on youtube (a jet engine mechanic) has discussed this sound many times. His is the first video I linked to in the post above yours. ---
**Author:** ![MikeF](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@MikeF](https://boards.straightdope.com/u/MikeF)\ **Post date:** [February 18, 2021, 10:07pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/11 "2021-02-18T22:07:08Z")
The TFE 731-40 example is exactly (or very nearly so) the sound I heard. Looks like an interesting YouTube channel. I’ll have to check it out. ---
**Author:** ![mikecurtis](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mikecurtis/32/3284_2.png) [@mikecurtis](https://boards.straightdope.com/u/mikecurtis)\ **Post date:** [February 18, 2021, 10:29pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/12 "2021-02-18T22:29:20Z")
Why do some of the the fans turn clockwise and some counterclockwise? ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 18, 2021, 10:43pm UTC](https://boards.straightdope.com/t/noise-from-non-running-jet-engine/932863/13 "2021-02-18T22:43:58Z")
I’m not aware of any systematic reason for one or the other direction in turbojets and turbofans. Engines that have 2 or 3 shafts will generally arrange for them to counter-rotate to reduce the net gyroscopic force generated by the total engine package. But as to which shaft turns which way, that seems to be a matter of convention at any given manufacturer. For turboshaft engines I could see another factor. The original version of those engines are frequently designed as a unit with a specific helicopter application in mind. So the rotors, the gearbox = transmission, and the engine are designed all at once to work together. The helo rotor needs to turn a specific direction, with CCW viewed from above being the US standard and CW being the European standard. That direction, as modulated through the particular gearbox forces the output shaft of the engine to turn a specific direction. Which then flows back into the details of which engine shafts turn which way. Many helo engines are later expanded into turboprop, or their core is used in a turbofan. So I’d bet the core keeps turning the way it always has and whatever is novel is designed around that, whether destined to counter-rotate or same-rotate.