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Airspeed Errors?


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A recent review of airspeed indicator errors (old Rotorcraft Flying Hanbook p.12-3) has me a bit confused!

 

 

1. Ram air inlet clogged, drain hole open = reads zero

 

2. Ram air inlet and drain hole blocked = acts like altimeter (but only if static port remains unobstructed)

 

3. Static port only blocked = works, but incorrectly

 

So, what happens if the ram air inlet, drain hole and the static port are all blocked?

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A recent review of airspeed indicator errors (old Rotorcraft Flying Hanbook p.12-3) has me a bit confused!

 

 

1. Ram air inlet clogged, drain hole open = reads zero

 

2. Ram air inlet and drain hole blocked = acts like altimeter (but only if static port remains unobstructed)

 

3. Static port only blocked = works, but incorrectly

 

So, what happens if the ram air inlet, drain hole and the static port are all blocked?

 

It depends on which ports block first. If you’re at 5,000 feet and 100 knots, and the static and drain blocked first followed by the ram port, the airspeed would hang at 100 knots thereafter. If the ram port blocked first the airspeed drops to zero.

 

It’s not as straightforward as you may think. Remember, the airspeed indicator is a differential pressure-measuring device. It measures the difference between total pressure at the ram port and the airstream static pressure at the static port. The difference is dynamic pressure, that’s what the airspeed indicator actually reads, calibrated on a dial in knots.

 

See for yourself; try the pitot static system simulator and block some ports. The Pitot Static System Simulator allows you visualize the Pitot Static System under varying atmospheric conditions as well as what happens when parts of the system become blocked., link below:

 

Pitot Static System Simulator

Edited by iChris
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See for yourself; try the pitot static system simulator and block some ports. The Pitot Static System Simulator allows you visualize the Pitot Static System under varying atmospheric conditions as well as what happens when parts of the system become blocked., link below:

 

Pitot Static System Simulator

 

Thats a great visual aid. Thanks for sharing!

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A recent review of airspeed indicator errors (old Rotorcraft Flying Hanbook p.12-3) has me a bit confused!

 

 

1. Ram air inlet clogged, drain hole open = reads zero

 

2. Ram air inlet and drain hole blocked = acts like altimeter (but only if static port remains unobstructed)

 

3. Static port only blocked = works, but incorrectly

 

So, what happens if the ram air inlet, drain hole and the static port are all blocked?

 

iChris is exactly right. If all three things were to freeze at the same time (highly unlikely), then it would be as if time stopped and someone just took a picture of the instrument panel. All of the pressures within the system are now locked in and can't change based on the ram or atmospheric pressure. Once you understand how the airspeed indicator works (which that website he provided does superbly), then it's just a matter of figuring how how the relative pressures compare to each other.

 

Edit: And just another fun fact. If your static port becomes blocked and you are in a dire situation with no alternate static source, you can break the glass of the altimeter. Caution should be kept though as it's using the cabin pressure for the measurement. If you are in this situation with an R22 something is very wrong haha.

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Edit: And just another fun fact. If your static port becomes blocked and you are in a dire situation with no alternate static source, you can break the glass of the altimeter. Caution should be kept though as it's using the cabin pressure for the measurement. If you are in this situation with an R22 something is very wrong haha.

 

Or the VSI

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Or the VSI

 

You're right. It is probably more beneficial to break the VSI to get that static air than the altimeter, and then make sure to completely ignore the VSI from that point on.

 

I came across this, which I think is referring to breaking the Altimeter instead of the VSI:

 

"Also you can break the glass of some altimeters so it will then use the cabin pressure to tell altitude. But beware that when you break the glass your vertical speed will read opposite, so it will say you are going up when you are going down etc."

 

Why would the VSI work opposite? Seems like the static air feeding it would originate in the altimeter, just as if it would originate in an alternate static source designed for that purpose.

 

 

EDIT:

 

As I continued to think about this, breaking the VSI didn't seem to be the best course of action. The VSI works off of the calibrated leak, which is what the static system would now be using. This would cause a lag in the Altimeter and ASI, which would probably be more dangerous and unusable then just breaking the Altimeter to begin with. I suppose under the right circumstances or situation, you could also break the ASI instead of the Altimeter.

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You're right. It is probably more beneficial to break the VSI to get that static air than the altimeter, and then make sure to completely ignore the VSI from that point on.

 

I came across this, which I think is referring to breaking the Altimeter instead of the VSI:

 

"Also you can break the glass of some altimeters so it will then use the cabin pressure to tell altitude. But beware that when you break the glass your vertical speed will read opposite, so it will say you are going up when you are going down etc."

 

Why would the VSI work opposite? Seems like the static air feeding it would originate in the altimeter, just as if it would originate in an alternate static source designed for that purpose.

 

 

EDIT:

 

As I continued to think about this, breaking the VSI didn't seem to be the best course of action. The VSI works off of the calibrated leak, which is what the static system would now be using. This would cause a lag in the Altimeter and ASI, which would probably be more dangerous and unusable then just breaking the Altimeter to begin with. I suppose under the right circumstances or situation, you could also break the ASI instead of the Altimeter.

 

Very true. I never thought about the calibrated leak aspect.

 

As for the VSI reading opposite with broken altimeter glass, that doesn't make sense at all.

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What about all these new "glass cockpits",...what type instrument errors can they get?

 

Mostly they get a miscompare issue. One is reading off a bit from the other. I wanna say more than 3% and you'll get an annunciation of some type (dependent on manufacturer), change in color, missing all together or a caution illuminate. Typically you'll refer to a back up instrument for the tie breaker and isolate the faulty system. Beyond that, there is generally an alternate static port you can open inside the cockpit should you suspect a clogged port. That may also be a sign of structural icing forming depending on where that port is located.

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Very true. I never thought about the calibrated leak aspect.

 

As for the VSI reading opposite with broken altimeter glass, that doesn't make sense at all.

 

Try to think of the standard airflow in the system, the calibrated leak is there to equalize the pressure after a change and with it now being the inlet rather than the outlet, or vice versa, then the airflow pattern reverses itself from normal, and consequently the indication does, too.

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Try to think of the standard airflow in the system, the calibrated leak is there to equalize the pressure after a change and with it now being the inlet rather than the outlet, or vice versa, then the airflow pattern reverses itself from normal, and consequently the indication does, too.

 

Well that makes sense if you break the VSI glass. Not so much if you were to break the altimeter glass though, which is what I was referring to.

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Well that makes sense if you break the VSI glass. Not so much if you were to break the altimeter glass though, which is what I was referring to.

 

Sorry, I misread your post. Correct, there shouldn't be a reversal error w the altimeter glass broken, as far as I can tell, but there would be an accuracy error. It would read high, I'd imagine.

Edited by C of G
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Sorry, I misread your post. Correct, there shouldn't be an error w the altimeter glass broken, as far as I can tell.

 

Sorry for the confusion, I believe the source that I was quoting was wrong.

 

If the altimeter is broken then the static air is being referenced off the cabin air pressure, which in most cases will be the current atmospheric pressure at your present altitude. This shouldn't cause any errors with the VSI, although the altimeter may be affected from the sensitivity and blunt force of physically breaking of the glass.

 

If the VSI is broken, the "calibrated leak" is now the wide open gap where the glass once stood. So if you look at the diagram below, the calibrated leak is a very tiny hole in comparison to the static inlet feeding the diaphragm. Now the roles are reversed. The higher pressure is now coming from the gaping hole in the glass, while the calibrated leak is feeding the actual inlet to the diaphragm (as well as the other instruments).

 

 

VSI_expensive_broken-494x331.jpg

 

So breaking the VSI glass will leave the VSI inoperative and give a lag to the airspeed and altimeter, which may make them inaccurate enough to be considered inoperative as well.

 

If you break the altimeter and shatter the glass, you may compromise the sensitivity of the gauge and render that inoperative as well, although the static inlet is much larger and would thus act like a true static source.

 

altimeter.gif

 

On the other hand, you may consider your altitude as more of a necessity than your airspeed, so breaking the ASI may be the best option.

 

Its a tough call, but I would either choose the altimeter or airspeed depending on the aircraft and conditions, or figure out a way to cut the static line all together from under the instrument panel. Lets hope we never get into this situation haha :lol:

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I think I'm just going to start carrying a drill around when I fly. Then I can just drill a hole rather than break the glass! B)

 

I think that's a great idea, bring a drill and a framing hammer, and if anyone asks...."you can never be too safe when it comes to static icing in the R22!"

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