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Jet engine question

Pro Member Chief Captain
jarred_01 Chief Captain

Hey guys,

A couple of days ago I brought myself a scanner so I can listen to my local airport frequencies. This afternoons listening proved quite interesting with the pilot announcing to ATC that they would have to let a passenger off who must of been to afraid to fly (it was pretty bad weather).

Anyway, onto my question. Before they started the engines on the A320 I noticed that the fans were rotating round without the engine going. This went on for a few minutes before the fans stopped rotating. 5 or so minutes later the pilot requested permission for the engine start.

Any ideas why the fans were rotating round without the engines going before startup?

Thanks in advance,
Jarred.

Pro Member Captain
atreyu Captain

maybe wind?

Pro Member Chief Captain
jarred_01 Chief Captain

I had a feeling that might be it but it's happened a few times lately, and they were rotating pretty quick, too quick for the wind speed by the look of it.

Pro Member Chief Captain
Jonathan (99jolegg) Chief Captain

I don't think its wind. All commercial jet aircraft do it, I have an idea its something to do with the APU, but couldn't be specific Wink

Pro Member Chief Captain
jarred_01 Chief Captain

My next flying lesson is in a week, so I'll ask the instructor whether they know about it.

Pro Member Chief Captain
Greekman72 Chief Captain

Well a thought that pass through my mind Idea is that maybe the engines are in some kind like slow mode in order to keep warm (metals,lubricants etc).
Of course maybe im totally off but something familiar happens to big ships(super tankers etc.)The engines never stops(only for repairs etc.) when the ship is in port or unload the machines running veryvery slowly.

Let us know what your instructor tells you Wink

Pro Member Chief Captain
Alex (Fire_Emblem_Master) Chief Captain

It's the wind. Those engine fans are lubed up so well, and the bearings are so well made that the smallest of breezes can get them spinning quite quickly.

All the APU is, as far as I know, is a tiny jet type engine in the tail end of the plane, that's why you see that little hole and the exhaust blowing out of there.

Pro Member Chief Captain
Jonathan (99jolegg) Chief Captain

Fire_Emblem_Master wrote:

It's the wind. Those engine fans are lubed up so well, and the bearings are so well made that the smallest of breezes can get them spinning quite quickly.

All the APU is, as far as I know, is a tiny jet type engine in the tail end of the plane, that's why you see that little hole and the exhaust blowing out of there.

Ah ok. How do they stop them spinning when an aircraft is parked Question

Guest

Fire_Emblem_Master wrote:

It's the wind. Those engine fans are lubed up so well, and the bearings are so well made that the smallest of breezes can get them spinning quite quickly.

All the APU is, as far as I know, is a tiny jet type engine in the tail end of the plane, that's why you see that little hole and the exhaust blowing out of there.

Question: What is the APU for?

Pro Member Chief Captain
Tailhook Chief Captain

APU stands for Auxiliary Power Unit. Here is a little description Arrow http://en.wikipedia.org/wiki/Auxiliary_power_unit

Pro Member Chief Captain
Matthew Shope (mypilot) Chief Captain

I've seen the fans spin sometimes too. At that time I thought it was wind, because I saw a bunch of them parked together and they all were spinning at appr. the same speed.

Pro Member Chief Captain
jarred_01 Chief Captain

mypilot wrote:

I've seen the fans spin sometimes too. At that time I thought it was wind, because I saw a bunch of them parked together and they all were spinning at appr. the same speed.

I was thinking the same thing mypilot, the fans always seem to stop spinning a few minutes before startup. Sounds like a question for my instructor I think...

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