"The VSI uses a logarithmic scale display that has a greater sensitivity at small rate of climb or descent (ROC/ROD) values. Zero is usually at the 9 o'clock position with the ROC scale above a ROD scale below"

How come the VSI uses a logarithmic scale display as the VSI goes up in 100 increments and not multiples like a logarithmic scale? and anyone explain why the VSI has a greater sensitivity at a small rate of climb or descent values?

Thanks

Quoting Novice (Thread starter): How come the VSI uses a logarithmic scale display as the VSI goes up in 100 increments and not multiples like a logarithmic scale?

Here's a standard Boeing PFD:


Take a look at the VSI on the right side...the physical distance between 0 and 1 is considerably bigger than the difference between 1 and 2, which is again bigger than the difference between 2 and 6. That's what they're talking about. The same angular movement of the needle corresponds to far more FPM at high deflection than low.

Quoting Novice (Thread starter): and anyone explain why the VSI has a greater sensitivity at a small rate of climb or descent values?

It's not that the instrument is more sensitive, it's that the display *scale* is logarithmic so that a change in rate causes a much bigger deflection at low rates than at high rates. Otherwise, if the scale was linear and full throw, small rates (what you care about when trying to accurately hold altitude) would be impossible to see.

Quoting tdscanuck (Reply 1):

To expand on tdscanuck's answer:

In a rapid descent, it does not matter if you are falling down with 11'000 or 12'000 fpm.

But during the approach, the very same difference (namely, 1000 fpm) between a 2000 fpm and a 3000 fpm descent matters much more.

The scale gives you accuracy - but only where it is important.


David

Thanks for explaining that guys and that diagram really helped cheers Tom