woody71
Posts: 17
Joined: Fri Jun 27, 2008 7:27 pm

Does VSI Fluctuate In Level Flight?

Wed Oct 01, 2008 5:56 pm

I hope any pilots out there can help me. I need to know does a VSI whether analog or on a digital screen, fluctuate in level flight? I mean when flying level is it always at 0 or does the needle sometimes wiggle around the 0? Thanks in advance.
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: Does VSI Fluctuate In Level Flight?

Wed Oct 01, 2008 6:16 pm



Quoting Woody71 (Thread starter):
I hope any pilots out there can help me. I need to know does a VSI whether analog or on a digital screen, fluctuate in level flight? I mean when flying level is it always at 0 or does the needle sometimes wiggle around the 0? Thanks in advance.

Not usually, no...the VSI has some lag in it. Remember, an analog VSI is just an altimeter with a calibrated leak in it's aneroid wafers...  Wink I can't speak for it's digital counterparts.
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
PhilSquares
Posts: 3371
Joined: Sun Mar 28, 2004 6:06 pm

RE: Does VSI Fluctuate In Level Flight?

Wed Oct 01, 2008 6:22 pm



Quoting Woody71 (Thread starter):
I hope any pilots out there can help me. I need to know does a VSI whether analog or on a digital screen, fluctuate in level flight? I mean when flying level is it always at 0 or does the needle sometimes wiggle around the 0? Thanks in advance.

It's really an IVSI on most current generation aircraft, meaning it's instantaneous. But, to answer your question, it depends.

In pure ALT (altitude hold) mode the IVSI is pretty much fixed at 0. There will be momentary deviations but they are minimum.

In VNAV, both Airbus and Boeing have a "soft" hold function. That provides a better ride for the passengers, minimises throttle movements. The range is of the soft ride is generally less than about 75'.
Fly fast, live slow
 
pilotpip
Posts: 2821
Joined: Fri Sep 19, 2003 3:26 pm

RE: Does VSI Fluctuate In Level Flight?

Wed Oct 01, 2008 6:23 pm

Glass aircraft usually fluxuate. They're very precise and will move with every little change. This can lead to some overcorrections for 1 or 2 knot deviations, and leads to lots of chasing of needles. The VSI will do the same.

It's also a different presentation and a lot of times people have a little trouble interpreting them at first. I had a lot of trouble with students new to the G-1000 back in the day and when I first started flying a jet I found myself constantly jockeying the thrust levers back and forth with every little change in the airspeed indicator.
DMI
 
luv2cattlecall
Posts: 649
Joined: Fri Sep 28, 2007 6:25 am

RE: Does VSI Fluctuate In Level Flight?

Wed Oct 01, 2008 6:28 pm



Quoting Woody71 (Thread starter):
I hope any pilots out there can help me. I need to know does a VSI whether analog or on a digital screen, fluctuate in level flight? I mean when flying level is it always at 0 or does the needle sometimes wiggle around the 0? Thanks in advance.

In the old 6 pack C172s, a little bit - the lag is terrible though, to the point where I just pretend the VSI isn't there, and use the altitude Feet per minute +/- indicator in a $60 Garmin Geko hiking GPS instead  shhh  !

The G1000 is much much better, and it doesn't flucuate at leval altitude, since it's mathematically computing the vertical speed. Hope that helps!
.
 
Pihero
Posts: 4235
Joined: Mon Jan 31, 2005 5:11 am

RE: Does VSI Fluctuate In Level Flight?

Wed Oct 01, 2008 6:37 pm

Yes, the VSI fluctuates very slowly.
A lot can disturb the equilibrium of a cruising aircraft : winds, turbulence, CoG shifts due to people / objects moving inside and the airplane's own stability...
Modern autopilots have a mode called "soft cruise" that allows the airplane to oscillate on a very long period around the cruise level. That's for passengers' comfort - and airframe life !.
Furthermore : The flying accuracy required is quite high : a VSI of 100 ft/min comes from an initial error of some 1/10th of a degree !

So, to answer your question, yes the VSI needle doesn't rest during cruise.
Contrail designer
 
woody71
Posts: 17
Joined: Fri Jun 27, 2008 7:27 pm

RE: Does VSI Fluctuate In Level Flight?

Wed Oct 01, 2008 7:08 pm

Thanks everyone.

Pihero,

I figured that was the case seeing as how most aircraft do not fly perfectly straight and I was curious as to how the tolerances of a VSI would pick up on those fluctuations. My question was answered and then some. Thanks again.

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