Stupid question alert!
 

Hi

I have recently started on my PPL. Had some lessons which were excellent. I was wondering - what is the 'whirring' noise in the cockpit when you switch the master switch on? Also you can hear it spooling down when you switch it off? Is this the vacuum unit or alternator?

Sorry if I sound a bit dim!

If you ask a stupid question, you look stupid for ten minutes. If you don't ask a stupid question, you'll be stupid forever :ok:

The sound you hear is PROBABLY the electric gyro in the turn co-ordinator.

As you probably know, MOST gyro instruments in MOST light aircraft are vacuum driven (the gyros being spun by suction from the engine driven vacuum pump) - but the gyro in the turn co-ordinator is usually electrically driven so that you have 'something' that still works in the event of a vacuum pump failure.

remember the answer for your skills test ;)

Don't be afraid to ask your instructor - that's what you're paying them for!

SD

Entirely reasonable question, though once a PPL, the answer should be fairly clear in your mind. You want to understand what's not going to work any more, when a certain system quits in flight.

As said, it is the electric turn co-ordinator, as you realize with the relationship to the master switch. You will also see the small red flag in the window on the face of the turn co-ordinator, which tells you that it is getting electricity. In very old planes, particularly those which do not have an attitude indicator, or DG, you may have a "Turn and Bank" (upward white bar), as opposed to a "turn Co-ordinator" (little plane). Some turn and banks, are vacuum driven, rather than electric. They will therefore not start with the master.

When you turn the master off, shortly after the engine has stopped turning, you should expect that the electric turn co-ordinator will stop spinning first. The gyros in the attitude indicator, and DG (vacuum) should spin on much longer (many minutes). If they do not, expect that the offender is somewhat defective. The vacuum instruments have much more precise bearings and balance than the electric ones, thus should spin much longer. There is also the electric motor brush drag of the electric gyro to slow it down more quickly.

I hope that helps. (and by the way, you're done your 10 minutes of looking stupid- carry on with pride!)

Pilot DAR

In case of a glass cockpit, could the whirring noise also be caused by cooling fans, as in PCs? Or are these glass cockpits all passively cooled?

Granted, you would not hear the noise when you switch on the master, but rather the avionics then.

I think more or less every plane with any significant avionics does have an avionics cooling fan (behind the dash) and that is quite noisy.

The rest of the noise, as stated, comes from the gyroscopic instruments i.e. the horizon and turn coordinator.

This is why, if you leave the master switch ON and forget about it, even if everything else is turned off, when you get back after a few hours down the pub, the battery is flat :)

IO540
 

I fear that you over estimate the quality of avionics instalations, most installers don't include the avionics fan in the cost of a new avionics fit for fear of being undercut on the price by another installer.

It is only the wise owner who knows that the avionics fan will save £££££££££ over the years and those without the wisdom save £150 on the new avionic fit and pay & pay later!

No-one's mentioned the sound of silence when the master switch is turned on. I fly two different PA28's, one with suction-driven AI and DI plus electric turn & slip; the other with all three instruments suction-driven. It's the latter that is quiet when master goes on of course but the silence always gives me pause for thought until I twig that all is normal. Yet in the two Chipmunks I also fly regularly neither have electrically-driven instruments and the sound of silence never perturbs me.

Interesting conditioning my brain has had:\

The C182 with G1000 that I fly as two fans, one behind the dash and the other at the back where some of the other avionics are located.

Pilot DAR, I'm sure you didn't really mean it that way - presence of the red flag indicates a failure or absence of electricity.

Cheers,

Tom

Indeed, the red flag means a failure of the instrument.

In dealing with the aviation industry and regulator, I'm so used to seeing red flags popping up, I suppose I'm seeing them when they are not really there!

If your cooling fans are that noisy, beware. The bearings may be wearing out. I use hundreds of cooling fans for non-aviation electronics and it is something we watch for.

It could of course be your instructors brain powering down. :}

When I stop the engine in the L4 Cub,there is no noise.
Is something wrong?:}

Only if you've just landed from an IFR filed flight I suppose....

Of course if it were multi crew it would be the co whining. :}

You guys are too funny !

I do agree that the OP's ten minutes are indeed up - the ability to ask "a stupid" question is to be admired, as opposed to "cut and try" methods. :)

Pilot DAR, believe me, I feel your pain. :D

Cheers,

Tom