Feb 11, 2007

Vibrators and stress relief...

Altimeters installed in jet aircraft are equipped with an item that no propellor-driven aircraft ever needed. It's called an altimeter vibrator. Because jet-engined airplanes are so smooth the altimeters need to be constantly shaken (but presumably not stirred) to overcome mechanical friction so the needles will not stick.

Here's a link to the B747 standby altimeter (click on the altimeter face to see the note):

http://www.meriweather.com/747/center/standbys.html#

I've never seen an altimeter vibrator but after listening to them work for a few thousand hours, I suspect they are comprised of a little 'hammer' that taps away on the casing. When the unit is brand new you barely hear them. After several years the rubber 'isolator' must harden or wear away because the noise can become loud and irritating. I've heard rumors that some pilots resort to pulling the vibrator circuit breakers while parked at the gate to relieve this constant racket. The sudden silence can produce a huge sense of 'aaaaah' -- that feels so good. Or so I've been told.

This is probably not the best solution, but when snags about noisy vibrators are constantly written off as "considered serviceable," and so the "beat goes on and on..." it can become frustrating.

In general, items that are considered to be merely pilot-comfort items such as noisy altimeter vibrators, deficient chart holders, worn seat cushions, inoperative lumbar supports, and so on, get minimal attention from maintenance departments. I've often tried to establish the link between these deficiencies and pilot absentee (sick) days during my annual medical checks with the company docs. If the two items were more directly linked via the budgetary bottom line then perhaps the word would get back to the maintenance department. After all human beings wear out prematurely just like any other aircraft component when their support equipment is not properly maintained.

I'm stepping off my soap box now, my stress somewhat relieved. What did you think I meant?

2 comments:

nec Timide said...

In simple gliders we are reduced to tapping on the panel beside the altimeter to provide the same function. Frequency of tapping is inversely proportional to absolute altitude.

In my Air Cadet days the squadron had a venerable Link Trainer (a very early simulator). It had a number of electric motors with stacks of washers mounted off centre on the shafts to provide the vibration.

Aluwings said...

Ahh, the Link Trainer. I did actually have a short lesson in one many years ago, just before the Edmonton Flying Club updated to a Frasca. Alas, the Frasca had none of those funny bellows etc. to induce motion and air-sickness while 'flying'...