ASI and Alternate Static

[Bartman]

My ASI reads about 10 knots low, confirmed multiple flights as well as 3 and 4 direction GPS averages too. My VSI also reads low and to descend at 500fpm I have to do 800 on the gauge. It has been that way for a long time and I just got used to it. It has passed numerous IFR Pito Static check and most recently in May this year. I push the static drain valves from time to time, but have never seen a drop of fluid come out  

On yesterday's flight I decided to pull the Alt Static and I gained my 10 knots back, but did not try to confirm the VSI. 

Whats up with the this? Can I just leave the Alt Static valve pulled ? Is there something I need to address?

[Yetti]

clean all tubing and static ports in the back of the plane.  Check for wax in the static ports.  Tubing come off at the drain port via the battery access cover.  You can blow BACK to the static ports.  Do NOT blow forward to the guages or you will toast them.  Not all water comes out via the drains. simple mouth pressure works best.  below is what I found in mine.   tape up one side, then the other.

[carusoam]

To answer the question of does it need to be addressed...

Most people use their ASI to control their speed in the traffic pattern.  The VSI is used to set power in the pattern.  If the ASI is showing a number that is different when the alt static valve is open.  All it wil take is to forget if the valve is open or closed while turning to final.

Going faster than you expect... you don't land, or you set yourself up for a prop strike...

Going slower than you expect... Stall on the turn to final...

The alt static source is a back-up system that is not as good as the external dual static source.  So using the alt as the primary static source comes with caveats that are in your POH regarding the error. 

People won't appreciate that they didn't tell you...  You know something is wrong, no matter what test it passed.  Get it fixed.

What is the upside to leaving the system as it is?

What if somebody flies your plane for maintenance? Will they know to adjust the readings properly?

 

while your at it, update your avatar to include the plane you fly.  The static system is different on the Cs, vs. Js......

Friends don't let friends skimp on important maintenance...

Just trying to put some perspective on what I think I read.  As a PP, I could have missed something....

Best regards,

-a-

[Bartman]

Thanks guys,

It might be trapped water. I remember a couple of times when the Altimeter would jump quickly when climbing out, but it does not do that consistently. I could see how a couple of drops of water could do that. I will have it looked at.

I obviously have exposed the fact that I need to brush up on how the Pitot Static system works, and failure modes.

[mooniac15u]

1 hour ago, Yetti said:

clean all tubing and static ports in the back of the plane.  Check for wax in the static ports.  Tubing come off at the drain port via the battery access cover.  You can blow BACK to the static ports.  Do NOT blow forward to the guages or you will toast them.  Not all water comes out via the drains. simple mouth pressure works best.  below is what I found in mine.   tape up one side, then the other.

Just remember that if you open the static system to blow out water it must be retested per 91.411.

[Yetti]

5 hours ago, carusoam said:

To answer the question of does it need to be addressed...

Most people use their ASI to control their speed in the traffic pattern.

 

I use my throttle and the big speed brakes with big black round donuts to control speed in the pattern...  But to each his own  :-)

 

[carusoam]

But how do you know if you need more throttle or less donuts?  

-a-

[Piloto]

On m 1982 M20J there is a drain button on the left bottom side of the fuselage behind the wing to drain the static line. On the front bottom side next to wing leading edge there is another drain plug for the pitot line. During heavy rain the static line can collect a considerable amount of water. Look for these drain ports.

José  

[Andy95W]

8 hours ago, Piloto said:

On m 1982 M20J there is a drain button on the left bottom side of the fuselage behind the wing to drain the static line. On the front bottom side next to wing leading edge there is another drain plug for the pitot line. During heavy rain the static line can collect a considerable amount of water. Look for these drain ports.

José  

If that drain had a leak, the OP could have a slightly positive pressure in the static system which could cause exactly the resultant errors he's seeing.

[Yetti]

8 hours ago, carusoam said:

But how do you know if you need more throttle or less donuts?  

-a-

Unless there is someone in front of me, do I really need to slow down until the runway?

[Browncbr1]

I have a related question.  I have a 67 F and a friend said I should have an alternate static air somewhere, but I can't find it...   Isn't that required for IFR cert?  

[kpaul]

26 minutes ago, Browncbr1 said:

I have a related question.  I have a 67 F and a friend said I should have an alternate static air somewhere, but I can't find it...   Isn't that required for IFR cert?  

In my 75 F there is a valve to turn under the panel near my left knee. You can see the label for it just below the master switch.

[Browncbr1]

I saw in my POH that it was an option and I also read that they are often hidden and not labeled.  The plane was certified for IFR...  I do have pitot heat, but am unclear if alternate air is just required if flying for hire.

In any case, if it's not there,  it seems adding a T and valve to the static line would be very cheap and easy to accomplish.

 

Edited August 31, 2016 by Browncbr1

[Yetti]

two adel clamps to the bar just left of the yoke behind the panel.   75 F mine was not labeled

[carusoam]

Older planes used a glass panel...  In case of emergency, break glass....

The glass panel was also the front of the VSI...

How would a pilot know he had a static line problem without breaking the glass?

If you fly in IMC...get an alt static valve...

If your plane doesn't have static drains or an alt static valve, it was probably built in the 60's and hasn't flown a lot of hard IFR.

Valves and drains are not very expensive compared to the value they provide.

PP thoughts, not a CFI...

Best regards,

-a-

 

[Browncbr1]

i know mine has a drain in the tail.. i'll be out there this week and look under the yoke  

[Marauder]

26 minutes ago, Browncbr1 said: I have a related question.  I have a 67 F and a friend said I should have an alternate static air somewhere, but I can't find it...   Isn't that required for IFR cert?  

In my 75 F there is a valve to turn under the panel near my left knee. You can see the label for it just below the master switch.

Interesting to see the changes in the later 75 models. It looks like they did away with the annunciator strip on the glare shield and switched the flap lever to the early 201 style.

Got pictures of the rest of the interior. Wondering what else is different. I know on the 76 Fs they went with a single rear window.

Sent from my iPad using Tapatalk

[kpaul]

 

1 hour ago, Marauder said:

Got pictures of the rest of the interior

Why yes, I do have more pictures, thanks for asking.  Mine has the single rear window. Also the single bench in the back, wish it had the independent reclining seats, and headrests.  

[Marauder]

 

1 hour ago, Marauder said: Got pictures of the rest of the interior

Why yes, I do have more pictures, thanks for asking.  Mine has the single rear window. Also the single bench in the back, wish it had the independent reclining seats, and headrests.  

It is interesting to see the F transformation from the 74 to 77 model years. The 74 models had a different panel and yokes. The 75 through 77 models evolved until the Js rolled out. I also noticed you are missing the machine gun landing light button on the yoke.

Thanks for sharing!

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[Robert C.]

Returning to the original topic....

Bartman, you may want to have a chat with your avionics shop. How in the world did the aircraft pass the pitot/static check with that kind of issue?

[Bartman]

1 hour ago, Robert C. said:

Returning to the original topic....

Bartman, you may want to have a chat with your avionics shop. How in the world did the aircraft pass the pitot/static check with that kind of issue?

I have owned the aircraft since 2006 and for various reasons each and every IFR Pitot Static certification has been done by a different shop.  Based on the information given I do not think you can rule out the ASI itself.  Nor does the fact I gained 10 KTS on the ASI using alternate static does it prove that the Pitot Static has an issue. In fact, since 5 different shops found no issue with Pitot Static checks, it may suggest that the ASI is the problem. 

[N201MKTurbo]

When I had my 67 F I could reach behind the panel and unscrew the fitting off of the VSI, voila alt static.

[Mooneymite]

On 8/31/2016 at 8:45 AM, kpaul said:

In my 75 F there is a valve to turn under the panel near my left knee. You can see the label for it just below the master switch.

This is the alternate static source selector in my '74 C.  Like Maurauder says, just above the left knee.

Edited September 1, 2016 by Mooneymite

[Robert C.]

On August 31, 2016 at 10:40 PM, Bartman said:

I have owned the aircraft since 2006 and for various reasons each and every IFR Pitot Static certification has been done by a different shop.  Based on the information given I do not think you can rule out the ASI itself.  Nor does the fact I gained 10 KTS on the ASI using alternate static does it prove that the Pitot Static has an issue. In fact, since 5 different shops found no issue with Pitot Static checks, it may suggest that the ASI is the problem. 

Good point, I responded a bit hastily.