ASI discrepancy Aspen vs. steam backup

[Bob_Belville]

I've had an Aspen Pro 1000 since 2012, upgraded to MAX a few months ago. A single Aspen installation requires keeping a BU ASI. 

The 2 ASIs use the same pitot and static air and they have agreed with each other... until very recently. For the last couple of months I note that the Aspen airspeed is significantly higher than the old ASI. About 6 kias.

We've checked the air lines and connections. I wonder what else might explain what's happened. FWIW, I think the Aspen is correct and the old mechanical asi is low.

Thoughts?  

[Marauder]

2 minutes ago, Bob_Belville said:

I've had an Aspen Pro 1000 since 2012, upgraded to MAX a few months ago. A single Aspen installation requires keeping a BU ASI. 

The 2 ASIs use the same pitot and static air and they have agreed with each other... until very recently. For the last couple of months I note that the Aspen airspeed is significantly higher than the old ASI. About 6 kias.

We've checked the air lines and connections. I wonder what else might explain what's happened. FWIW, I think the Aspen is correct and the old mechanical asi is low.

Thoughts?  

Bob - when I had the mechanical ASI, the two Aspens and the mechanical always agreed. I suspect the issue is a calibration issue with the Aspen Max. I know when Aspen introduced their products back in 2008, there were some "burn in" issues that required them to be re-calibrated. I think the same has been true for the altimeter portion. I had my Aspen in for one re-calibration for the altimeter a year or so after mine were installed in 2012. They have been solid since.

[MIm20c]

With as many hours you have in that plane I’d first do a TAS calculation to see which is closest to past performance. Does the aspen TAS agree with previous years?

[Bob_Belville]

16 minutes ago, MIm20c said:

With as many hours you have in that plane I’d first do a TAS calculation to see which is closest to past performance. Does the aspen TAS agree with previous years?

I've slept since then.  

I'd like to think the Aspen is closer to correct but I don't want to prejudice the investigation. 

The discrepancy seems to have appeared coincident with the MAX upgrade but I do not want to jump to conclusions.

Is there a simple way to check ASIs? Perhaps a water column device to hold pressure on the pitot system and a pressure/airspeed formula/chart? 

[Gagarin]

Make sure the alternate air valve is closed (push button all the way).

[Marauder]

I've slept since then.  
I'd like to think the Aspen is closer to correct but I don't want to prejudice the investigation. 
The discrepancy seems to have appeared coincident with the MAX upgrade but I do not want to jump to conclusions.
Is there a simple way to check ASIs? Perhaps a water column device to hold pressure on the pitot system and a pressure/airspeed formula/chart? 

You’re a funny guy Bob. I wish my new ASI made my plane 6 knots faster as well.

You may want to check with some avionics shops and Mike Studley at Aspen if they are seeing some start up issues with the new Max platform. My bet is on the calibration.


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[Bob_Belville]

13 minutes ago, Marauder said:

 

You’re a funny guy Bob. I wish my new ASI made my plane 6 knots faster as well.

You may want to check with some avionics shops and Mike Studley at Aspen if they are seeing some start up issues with the new Max platform. My bet is on the calibration.


Sent from my iPad using Tapatalk Pro

 

You're probably right. I have already talked to Robbie at Twin Lakes Avionics who needs a couple of hours, which he didn't have when he reinstalled the updated Aspen, to tie in the audio panel to the MAX for audio alerts. He will calibrate the ASI then. 

[Bob_Belville]

31 minutes ago, Gagarin said:

Make sure the alternate air valve is closed (push button all the way).

Would that affect one ASI more than another?

In any case my '66E does not have an alternate air valve.

[Gagarin]

1 hour ago, Bob_Belville said:

Would that affect one ASI more than another?

In any case my '66E does not have an alternate air valve.

Check underneath the left panel below the throttle control, it is FAA required for icing on the static port. Depending on the static tube routing it may affect more one than other.

[Bob_Belville]

17 minutes ago, Gagarin said:

Check underneath the left panel below the throttle control, it is FAA required for icing on the static port. Depending on the static tube routing it may affect more one than other.

I do not believe there is any (manual) alternate air valve on older Es. Breaking the VSI glass is sometimes suggested as an emergency action. (The alternate air for the engine to by pass blocked filtered air is a spring loaded disk in the induction plenum.) 

The original Super 21 Owners Manual makes any mention of the static system. Here's one owner's effort to organize and amplify on the shortcomings of the OM: 

Scroll to last page. https://edwilliams.org/m20e/mooneysys.pdf

[DonMuncy]

3 hours ago, Bob_Belville said:

Is there a simple way to check ASIs? Perhaps a water column device to hold pressure on the pitot system and a pressure/airspeed formula/chart? 

All you have to do is come to Dallas and use mine.

[Gagarin]

3 hours ago, Bob_Belville said:

I do not believe there is any (manual) alternate air valve on older Es. Breaking the VSI glass is sometimes suggested as an emergency action. (The alternate air for the engine to by pass blocked filtered air is a spring loaded disk in the induction plenum.) 

The original Super 21 Owners Manual makes any mention of the static system. Here's one owner's effort to organize and amplify on the shortcomings of the OM: 

Scroll to last page. https://edwilliams.org/m20e/mooneysys.pdf

Nice Manual. On my M20J the alternate air is just below the HDG gyro check the placard  and push pull knob on the picture.

Besides opening the valve for icing blockage it is useful for finding a leak like a loose connector. Cheaper than breaking an instrument glass

 

Edited November 6, 2019 by Gagarin

[Bob_Belville]

2 hours ago, Gagarin said:

Nice Manual. On my M20J the alternate air is just below the HDG gyro check the placard  and push pull knob on the picture.

Besides opening the valve for icing blockage it is useful for finding a leak like a loose connector. Cheaper than breaking an instrument glass

 

Yeah, I'm acquainted with Js. But I happen to prefer the Super 21s I've been flying since 1969.

[carusoam]

65C... Alt air was broken VSI glass... static system drain, was a dead leg in the tail... clear plastic tube, easy to inspect... disassemble if filled up... 

Adding an alt static air valve isn’t too complex for a winter project...

Alt air for the engine... important to have as well... but not very helpful for when you get some rain water in the static line...

+1 on performing a pitot static calibration...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Bob_Belville]

So, I built a device to compare ASIs the uses @M20Doc's suggestion to roll up tubing to increase and hold pressure and an ASI from @AGL Aviation to use as reference. The apparatus is a "T" with one branch to the control ASI, one to the plane's pitot, and one to the long 1/4" tubing to roll up to pressurize the system. The end of the line to the pitot is 5/8" id tubing and covers the whole tube sealing the pitot drain hole. (My control asi is not necessarily dead nuts correct, it's just a third reference.)

Turns out the Aspen is slightly lower than the control and the bu asi is lowest:

speed (KIAS)   

control ASI   Aspen   BU ASI  

• 50        50            48
• 60        59            58
• 70        69            68
• 80        78            75
• 90        88            85
• 100      99            95

(The control ASI was for a slower plane or helicopter and it only goes to about 110 kias.)

The diverging relationship between the Aspen and the BU ASI continues at higher speeds. I took a couple of pics yesterday in level flight at 4500'.  The Aspen is showing ~145.5 kias while the BU shows ~139 kias.  

We're suspecting the bu asi is failing and might be leaking. 

Fresh thoughts?

[Guest]

On 11/6/2019 at 9:27 AM, Bob_Belville said:

I've slept since then.  

I'd like to think the Aspen is closer to correct but I don't want to prejudice the investigation. 

The discrepancy seems to have appeared coincident with the MAX upgrade but I do not want to jump to conclusions.

Is there a simple way to check ASIs? Perhaps a water column device to hold pressure on the pitot system and a pressure/airspeed formula/chart? 

You can try this: inches of water to ASI

http://www.tcwtech.com/airspeed conversion.xls

[M016576]

Before you guys go crazy... a really really easy way to see which one is accurate is to take your hand held GPS (iPad w/stratus?) and or panel mount and see if it agrees with your Aspen (my bet is that it does).  If it does... then you know the mechanical is not reporting correctly.  If they do not, then you know it’s the Aspen.

once you figure that part out, then you’ll have a better idea of the way forward.

My bet is that the Aspen derived GS is within a knot of the GPS... then just take a look at the TAS calculations on both to verify.  And note the winds.  You’ll be able to tell if the IAS is accurate based on the TAS calculator and referencing GS&winds.  6 knots is a big enough discrepancy to be able to find the erroneous gauge.

Edit-  I ran the numbers in your pic from above, and guess what- your true airspeed matches perfectly from the Aspen... but not the mechanics ASI.  If your ground speed is indeed correct, and their are no winds (or minimal winds...which is what it looks like based on the Aspen), then your aspen looks to be closure to the “truth data.”

When I plug in 139 based on your mech. ASI..with the same settings that are indicated, the TAS drops to 149... so if your GS is accurate on the Aspen, and aligns with your panel mount gps/hand held GS’s, then it’s definitely your mechanical airspeed that’s reading low.

Edited November 20, 2019 by M016576

[Bob_Belville]

7 minutes ago, M016576 said:

Before you guys go crazy... a really really easy way to see which one is accurate is to take your hand held GPS (iPad w/stratus?) and or panel mount and see if it agrees with your Aspen (my bet is that it does).  If it does... then you know the mechanical is not reporting correctly.  If they do not, then you know it’s the Aspen.

once you figure that part out, then you’ll have a better idea of the way forward.

My bet is that the Aspen derived GS is within a knot of the GPS... then just take a look at the TAS calculations on both to verify.  And note the winds.  You’ll be able to tell if the IAS is accurate based on the TAS calculator and referencing GS&winds.  6 knots is a big enough discrepancy to be able to find the erroneous gauge.

Edit-  I ran the numbers in your pic from above, and guess what- your true airspeed matches perfectly from the Aspen... but not the mechanics ASI.  If your ground speed is indeed correct, and their are no winds (or minimal winds...which is what it looks like based on the Aspen)... then it’s definitely your mechanical airspeed that’s reading low.

 

Where does the Aspen get GPS ground speed? From its own GPS or from the Garmin 750? Or is it vice versa?

I assume TAS as displayed on the Aspen is calculated from the ias adjusted for alt and oat using a built in e6b, right? 

The wind direction and velocity is a geometry calculation, also e6b stuff, from the heading and the gps gs vs the tas. Correct?

[Guest]

Here’s another chart for water to ASI.  And a way to make a chart.

http://www.iflyez.com/Manometer.pdf

[M016576]

1 hour ago, Bob_Belville said:

Where does the Aspen get GPS ground speed? From its own GPS or from the Garmin 750? Or is it vice versa?

I assume TAS as displayed on the Aspen is calculated from the ias adjusted for alt and oat using a built in e6b, right? 

The wind direction and velocity is a geometry calculation, also e6b stuff, from the heading and the gps gs vs the tas. Correct?

Aspen GS comes from its own GPS, the winds do too (none of the speeds displayed on the Aspen come from the panel mount gps, if you have one).  I think the winds are reverse calculated by comparing the calculated TAS to the GS.  Regardless...

2 ground speed calculations in your cockpit- one is on the Aspen, a separate one on the panel mount GPS.  Compare both with the TAS’s you’ve calculated and you’ll find your smoking gun.  Using your screenshot above- it appears to be the mechanical airspeed indicator that’s in error.  If you do the calculations more times, you’ll find a higher confidence as to what instrument is off... but it looks to me based on that one screenshot to be the mechanical.

would probably be best to do the TAS calculations using 3 legs (kind of like a speed course) that should help to factor out winds a bit.

Edited November 20, 2019 by M016576

[DonMuncy]

1 hour ago, M20Doc said:

Here’s another chart for water to ASI.  And a way to make a chart.

http://www.iflyez.com/Manometer.pdf

I used this table in building my airspeed calibration device. Mine has a sliding aluminum strip in the middle to check the difference in water height when attached to the pitot tube.

[Bob_Belville]

 

 

13 hours ago, DonMuncy said:

I used this table in building my airspeed calibration device. Mine has a sliding aluminum strip in the middle to check the difference in water height when attached to the pitot tube.

@DonMuncy Don, very nice. Is that a standard blood pressure machine bulb and relief valve? If so how do you hold the pressure when you get to the desired pressure?

[Bob_Belville]

FWIW, there are 2 Mooney getting annuals at @AGL Aviation at the moment. I put my apparatus on both. One has only a mechanical ASI, the other has 3 ASIs a mechanical ASI, an Aspen, and  what I think is a KI 300. Except for the KI 300 the others were within 2 kias of each other. The KI 300 was about 5 kias lower than the Aspen and the mechanical ASI in the same panel.

I intend to build a water column devise like @DonMuncy and the data provided by @M20Doc.

[DonMuncy]

Bob,

Yes that is a standard blood pressure bulb. It has a knob that you can tighten to hold the pressure or loosen to release it. The only trick to setting up the device is to make the measuring slat moveable. I embedded a steel strip in the wood and epoxied magnets on the back of the slat. Of course, marking the slat is a little tricky, as the scale is not linear.

Also, be careful using it. It doesn't take much air pressure to  do the trick, and it would be easy to blow out your ASI. 

Lest anyone gets picky, I did add a drop or two of food coloring to the water, which undoubtedly changes the density of the water, which will throw off the accuracy.

[Bob_Belville]

1 minute ago, DonMuncy said:

Bob,

Yes that is a standard blood pressure bulb. It has a knob that you can tighten to hold the pressure or loosen to release it. The only trick to setting up the device is to make the measuring slat moveable. I embedded a steel strip in the wood and epoxied magnets on the back of the slat. Of course, marking the slat is a little tricky, as the scale is not linear.

Also, be careful using it. It doesn't take much air pressure to  do the trick, and it would be easy to blow out your ASI. 

Don, I understand re excess pressure. So you move the metal scale, with the magnets holding it in position, to set zero on the lower fluid level and read the delta. Easy enough.

My BP arm band bleeds air, as it has to, and does not hold a pressure. I guess I don't know where the bleed air escapes I don't think it is at the bulb or the relief valve.