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instruments

Asked by: 1899 views
Aircraft Systems

if an aircraft is in level flight, what will the vertical speed indicator indicate?

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5 Answers



  1. Mark Kolber on Apr 28, 2019

    What would you expect the vertical speed to be if you are going neither up or down?

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  2. KDS on Apr 28, 2019

    O METLAK

    The answer is “it depends”.

    In theory, it should read zero (the number between the rate up scale and the rate down scale).

    However, in reality, it gets a little more complex.

    To begin with, the vertical speed indicator (VSI) doesn’t tell what the aircraft is doing. It tells what the aircraft did recently. It’s a history instrument. To convince yourself of that fact, just try holding altitude with the VSI as your only reference and reacting to what you see at that instant. You’ll be in for an ever increasing roller coaster ride. (Phugoid if you want a fancy word for it.)

    We would assume that it would read zero after a period of level flight, and it would if there wasn’t any instrument error in it. However, if you look at the cockpits of aircraft sitting on the ground, you’ll notice that the VSI’s do not all read zero. That is an error in the instrument.

    Think of it like this. How fast is your car going when the speedometer reads 60 MPH? If you compare the speed shown on many speedometers to the speed show in a GPS, you’ll see they aren’t the same. The VSI error is sort of the same thing.

    I don’t know if it is a question on the Instrument knowledge (written) test today, but there used to be a question on that very thing on those tests. The situation that was presented is that when you sat on the ground, you noted a 100 FPM climb on the VSI and asked what you should do about that. The book answer was to fly allowing for the 100 FPM error by mentally adjusting for the error on the instrument.

    I used to enjoy testing instrument students on that question. They would study well and know all the questions they could possibly see on the test and the correct answer. So, I’d ask them what if they saw a 100 FPM error while sitting on the ground. They would get a little smile on their face confident in their ability to answer the question. After they answered I would give them a big smile right back and tell them it was a great answer. Then I’d get a serious look on my face and lean towards them and ask “But what would you do if it was a 50 FPM error?” Amazingly, about half wouldn’t be able to answer the question. It was a classic case of people being prepared with the answer, but not with the knowledge behind the answer.

    KDS

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  3. KDS on Apr 28, 2019

    O METLAK

    The answer is it depends.

    In theory, it should read zero (the number between the rate up scale and the rate down scale).

    However, in reality, it gets a little more complex.

    To begin with, the vertical speed indicator (VSI) does not tell what the aircraft is doing. It tells what the aircraft did recently. It is a history instrument. To convince yourself of that fact, just try holding altitude with the VSI as your only reference and reacting to what you see at that instant. You\’ll be in for an ever increasing roller coaster ride. (Phugoid if you want a fancy word for it.)

    We would assume that it would read zero after a period of level flight, and it would if there was not any instrument error in it. However, if you look at the cockpits of aircraft sitting on the ground, you\’ll notice that the VSIs do not all read zero. That is an error in the instrument.

    Think of it like this. How fast is your car going when the speedometer reads 60 MPH? If you compare the speed shown on many speedometers to the speed show in a GPS, you\’ll see they are not the same. The VSI error is sort of the same thing.

    I don\’t know if it is a question on the Instrument knowledge (written) test today, but there used to be a question on that very thing on those tests. The situation that was presented is that when you sat on the ground, you noted a 100 FPM climb on the VSI and asked what you should do about that. The book answer was to fly allowing for the 100 FPM error by mentally adjusting for the error on the instrument.

    I used to enjoy testing instrument students on that question. They would study well and know all the questions they could possibly see on the test and the correct answer. So, I\’d ask them what if they saw a 100 FPM error while sitting on the ground. They would get a little smile on their face confident in their ability to answer the question. After they answered I would give them a big smile right back and tell them it was a great answer. Then I\’d get a serious look on my face and lean towards them and ask, cut what would you do if it was a 50 FPM error? Amazingly, about half would not be able to answer the question. It was a classic case of people being prepared with the answer, but not with the knowledge behind the answer.

    KDS

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  4. KDS on Apr 28, 2019

    P.S.

    The reasons I answered twice was I can’t stand when the software sticks a slash in next to every apostrophe that I write and I didn’t know the first time had been posted.

    I’ll also add that Mark’s answer is similar to the way Socrates taught, by asking questions instead of answering them.It is really more effective. I just couldn’t resist the urge to be long-winded.

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  5. Mark Kolber on May 02, 2019

    LOL! I’m not sure my question was particularly Socratic.

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