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A Vertical Speed Indicator indicates rate of altitude change. Traditional VSIs use a calibrated leak to measure the rate of pressure change, and therefore the rate of altitude change.

However, pressure does not change uniformly per altitude: in a standard atmosphere, air pressure at 0 MSL is 29.921 inHg, and at 1000 MSL is 28.856 inHg: a 1.065 inHg difference. But 10,000 MSL is 20.577 inHg and 11,000 is 19.791 inHg: a .789 inHg difference. A calculation suggests that a VSI calibrated for sea level and climbing at 1,000 ft/min would indicate 739 ft/min at 10,000 feet.

The airspeed indicator has a knob to calculate true airspeed based on altitude and temperature, but the VSI has no pilot-accessible controls. Glass panel instruments would presumably handle this automatically, but is there any correction for altitude in a steam-gauge VSI?

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  • $\begingroup$ Interesting question! While this doesn't answer what you asked, I'll mention that on most, maybe all, large jet aircraft these days, the VSI indication actually comes from the INS, and doesn't use static pressure at all. The advantages to doing it this way are that the indication is instantaneous and and not subject to any of the usual, or unusual, static system errors. $\endgroup$
    – Ralph J
    Apr 4 '15 at 21:05
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    $\begingroup$ Research suggests that the current TSO for VSI's is TSO-C8e, which just references SAE (AS)8016A - though the standard costs almost an hour of flight time. $\endgroup$
    – NathanG
    Apr 10 '15 at 22:53
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Yes. It appears to be the case for "steam gauges". At least for a traditional vertical speed indicator. It's a great question and not one I had considered before. At first glance, I suspected you may be right, though I don't ever remember actually learning that.

To prove the theory, one pilot apparently took his airplane with a traditional steam gauge altimeter/VSI up to FL220 and found the following:

=>Your altimeter is perfectly accurate at the airport if you enter your barometric pressure correctly.

=>At the cruise altitude of about 8,500 feet or so, your altimeter is off by about FIVE-HUNDRED FEET.

=>At that altitude, it does not matter much that your altimeter is off, because everyone else’s is off by the same amount, and you are far from the airport, and thus landing.

=>The error always seems to be off in the safe direction: You are always HIGHER than you think. Clearly, when they designed the system in 50’s, they designed the altimeters to follow the most conservative lapse-rate we could encounter, so in real-world flying, we are almost-always much HIGHER than we thought, which must be the safest error we can make.

=>The vertical speed indicator perfectly tracks the PRESSURE-based altimeter, not the true altitude, so the vertical speed indicator can be perfectly used to see how much the altimeter will change over time, the ACTUAL climb or descent rate of the plane is actually considerably DIFFERENT… at high altitudes, by 1,290 parts in 24,000, or just OVER FIVE PERCENT.

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  • $\begingroup$ Hello, and welcome to the Aviation Stack Exchange! Note that the post mentions the aircraft having a G1000, so while this confirms my theory about glass panels, it unfortunately doesn't help with the core of the question about steam-gauge VSI's. $\endgroup$
    – NathanG
    Apr 30 '15 at 3:29
  • $\begingroup$ I added references to steam gauges, those were what the answer was for. The pilot who tested it, used a tradition, steam gauge VSI and altimeter. The glass cockpit gauges work differently. I moved that reference to clarify it. $\endgroup$
    – SOCPilot
    Apr 30 '15 at 10:44
  • $\begingroup$ One small addition to Ralph's comment - that's not entirely true for all large airplanes. Some may use INS. My airplane uses the air data system but the net effect is the same because the VSI displays a calculated value that the system has adjusted for things like temperature, altitude, sideslip (which has a bigger effect on a small airplane), etc. $\endgroup$
    – SOCPilot
    Apr 30 '15 at 10:45
  • $\begingroup$ Can you please quote or point out where he mentions a traditional VSI? The aircraft in question is a Corvalis TT, which does not usually have a steam-gauge VSI. It'd have a backup altimeter, but that's no help here. $\endgroup$
    – NathanG
    May 1 '15 at 0:40

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