Negative readout airspeed indicator

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digits_
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Negative readout airspeed indicator

Post by digits_ »

Hi all,

Can a standard small airplane airspeed indicator show a negative/below scale value?

My ASI showed a big overspeed during flat spin training today. For a moment I thought I entered a spiral dive by accident, but that wouldn't have made sense. As I recovered, the airspeed increased through the lowest value of the scale and kept increasing up to normal cruise speed values.

I was flying flat spins, so the rate of rotation in the yaw axis was quite high. Somehow I thought these indicators were pegged on the lower end.

If I look at the scale, which starts at 30 kts, the default position is roughly where the 20 kts position would be, not where 0 would be. This seems to indicate something is preventing indications below 20 kts.

So, is this normal/expected behavior? Or is my ASI starting to break down?
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ahramin
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Re: Negative readout airspeed indicator

Post by ahramin »

Airspeed indicators don't actually measure the speed of the aircraft. They measure the difference between the pressure at the pitot tube and the pressure at the static port. If you have more pressure at the static port than at the pitot tube, the airspeed will be negative unless there is a peg on the dial to prevent that.
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digits_
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Re: Negative readout airspeed indicator

Post by digits_ »

ahramin wrote: Tue Nov 03, 2020 12:42 pm Airspeed indicators don't actually measure the speed of the aircraft. They measure the difference between the pressure at the pitot tube and the pressure at the static port. If you have more pressure at the static port than at the pitot tube, the airspeed will be negative unless there is a peg on the dial to prevent that.
Is there a "peg system" that would peg the default, no-movement value at 20 kts, yet would allow a value even lower than that when there is a big enough pressure difference?
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Re: Negative readout airspeed indicator

Post by photofly »

Bendy peg? Shake it hard and listen for loose bits.
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Re: Negative readout airspeed indicator

Post by PilotDAR »

The ASI should not read below zero. Some have pegs at zero, though it might not. If the ASI pointer is mechanically capable of moving less than zero, and then returning to zero, it is probably okay. Otherwise if the pointer does not come to rest gently at zero, or against the zero pointer, it'll need calibration. Before sending it for calibration, disconnect the pitot static likes from it, and check for obstructions and leaks. If the lines are both fine, the instrument will need work.
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digits_
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Re: Negative readout airspeed indicator

Post by digits_ »

It does return to the 12 o'clock position after flight and it behaved normally afterwards. Maybe it has done it before, not sure. I'll keep an eye on it. Thanks for the info/advice!
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