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4 Answers

Does high altitude operations effect the VSI/FPM ?

Asked by: 632 views , , , ,
Aerodynamics, Aircraft Systems, Weather

Will the VSI be effected ? and show a greater or lesser then actual vertical speed at high altitude operations ?

Similar to when you come into land at a high altitude airport. Your true airspeed/Groundspeed is actually going to be faster, but your airspeed indicator will read the same.

Will the aircraft actually have a higher vertical FPM also ? and perhaps the VSI reads the same indications ? like the airspeed scenario above ?

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4 Answers

  1. KDS on Dec 03, 2017

    I love working with students because they ask questions I never thought about before.

    The best answer I can give to your question is “not that I am aware of”. However, I’ve been scratching my head thinking about the inner mechanism of a VSI and trying to come up with a better answer than that.

    I also have to caveat my answer with the statement that there are a great many things of which I’m not aware and I look forward to further and better answers to your question.

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  2. RickS on Dec 03, 2017

    I cannot say with absolute certainty, but I do not believe so. I say this based on the fact the the VSI operates off static pressure, not ram air pressure as the pitot tube (Airspeed indicator) does. Since both parts of the VSI (the bellows and the calibrated leak) are connected to the same static source, the measurement is simply a differential. This article gives a good, but easy to understand explanation.


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  3. Steve Pomroy on Dec 04, 2017

    The VSI measures the rate of change of pressure. That rate of change is then presented to us as a vertical speed. However, this conversion assumes that there is a fixed relationship between pressure changes and altitude changes. This assumption is not correct.

    The rate of change of pressure with height is proportional to density. At higher altitudes, the air is less dense, and so the rate of change of pressure is also lower. As a result, the VSI will understate your vertical speed.

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  4. Skyfox on Jan 03, 2018

    The pressure lapse rate with altitude is not a straight line. It’s not sharply curved either, but there is a difference between a thousand foot change in pressure down near sea level and a thousand foot change in pressure up where the airliners fly. The pressure lapse rate at FL300 is less than at sea level, meaning that for a given fixed vertical speed (let’s call it true vertical speed), the VSI will see a smaller change in pressure and therefore show a slower indicated vertical speed than actual. Or put another way, for a given indicated vertical speed up in the flight levels, the true vertical speed will be greater. There are also variations based on latitude, which affects the thickness of the atmosphere, and temperature which affects the density of the atmosphere.

    This webpage has a nice chart showing the atmospheric pressure across the altitudes:

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