Strange airspeed indicator behavior in rain

[0TreeLemur]

While on an ILS approach as part of my IFR training this afternoon, something strange happened with the airspeed indicator reading.   I was in a stabilized descent at 90 kts,  flying through rain, when I noticed the airspeed was decreasing.   About the same time my instructor noticed it an he instructed me to increase speed.   All other indications were the same as before- nose low attitude, 500 fpm descent rate, wings level, about 15" MAP.  I hadn't changed anything.  The airspeed indicating kept decreasing towards 70 kts.  I instinctively lowered the nose and the rate of descent indication increased but the airspeed didn't.  This told me that the a/s indicator was incorrect.

Then, the airspeed indicator needle performed a few jerky increases and decreases, then increased back to 95 kts.  It really acted like it was burping some liquid through the system.

The OAT was 48F, and my instructor suggested I turn on pitot heat to perhaps help eliminate water from the pitot tube.   The remainder of the flight was uneventful.

We returned to our home airport.  After shutdown, inspection of the pitot tube revealed nothing obvious.   The drain hole in the back of pitot tube just below where it joins its mount seemed unclogged. 

I cannot tell  you how hard it was raining.  My instructor thinks it was raining pretty hard.

Was the drain plugged and somehow it magically came unplugged?  Was it just raining so hard that it overwhelmed the drain?  Anybody have any similar experience?   I've left a message with my A&P, and thought it would be good to see if anyone here has seen anything like this.

-Fred

 

[carusoam]

A few drops of water in the static system can cause similar reading goofiness...

Do you have push button drains in your pitot static system or just a dead leg?

Either way, See if you can find where the water landed or went to...

The dead leg is between the two ports in the tail if you have it...

PP thoughts only,

-a-

[Browncbr1]

Pitot heat would have been my suggestion as well.  

Edited February 18, 2019 by Browncbr1

[m20kmooney]

1 hour ago, Fred₂O said:

Was it just raining so hard that it overwhelmed the drain?

I think that is exactly what happened. The pitot tube reads ram pressure which is static plus dynamic. Yours became blocked with water which trapped the ram pressure in the system. Since you were descending there came a point when the trapped static pressure component in the system became lower than the actual static pressure and so your airspeed indicator reports low. If you were climbing it would be the opposite.

Edited February 18, 2019 by m20kmooney

[Jim Peace]

1 hour ago, Fred₂O said:

While on an ILS approach as part of my IFR training this afternoon,

My instructor thinks it was raining pretty hard.

oy vey......

[3914N]

Fred- I just posted in the vintage Mooney thread with a similar issue. 

I think you may have had the same issue as me... water in the static system. 

I’m still trying to figure out how to mitigate this problem going forward. It seems like an emergency to me. 

 

[Jim Peace]

Just now, 3914N said:

I’m still trying to figure out how to mitigate this problem going forward. It seems like an emergency to me. 

For starters you can stop flying these lawnmowers with wings in the rain, especially heavy rain like the OP.  IMC in the rain, night time like the last mooney crash, threads about flying in a polar vortex....This is all reserved for multi engine turbines with a purpose to be there in the first place.  Not training.

[3914N]

3 minutes ago, Jim Peace said:

For starters you can stop flying these lawnmowers with wings in the rain, especially heavy rain like the OP.  IMC in the rain, night time like the last mooney crash, threads about flying in a polar vortex....This is all reserved for multi engine turbines with a purpose to be there in the first place.  Not training.

Jim- I found water in my static line on the ground. I don’t like using my IFR for anything other than punching through a marine layer. 

That doesn’t mean I don’t like having margin built in. I’m considering installing an alternate static source for that reason. 

[DXB]

Sounds like intermittently blocked pitot, not water in the static. Agree with the potential drain issue. Could there be some crud accumulated within in the pitot tube that predisposed this to happen? Is it worth servicing?  As an aside, one thing I despise about my Aspen PFD is that this anomaly will wipe out the entire display.  I am upgrading to the Max version that deals with this.  

4 minutes ago, Jim Peace said:

For starters you can stop flying these lawnmowers with wings in the rain, especially heavy rain like the OP.  IMC in the rain, night time like the last mooney crash, threads about flying in a polar vortex....This is all reserved for multi engine turbines with a purpose to be there in the first place.  Not training.

Seems like incredibly valuable experience with an instructor on board to me.  Something the pilots of Air France 447 really could have used in their background. Or were you thinking of aircraft more sophisticated than the A330?

[0TreeLemur]

21 minutes ago, 3914N said:

Fred- I just posted in the vintage Mooney thread with a similar issue. 

I think you may have had the same issue as me... water in the static system. 

I’m still trying to figure out how to mitigate this problem going forward. It seems like an emergency to me. 

 

As @carusoam suggested, I'll open up the tail tomorrow and inspect the static lines for water.   That said, I'm guessing that it was not a static port issue because it didn't affect the altimeter or VSI.

[DonMuncy]

 

What would have happened if dirt daubers had blocked the pitot drain, and after a while, the rain washed it clean?

[carusoam]

There are probably some pictures around here of what can collect in the pitot line....

Sounds like we might be finding out what is inside there now...

After about 50 years... Expect a few inches of bug parts might be hiding in there... soggy bug parts now....

The only thing new between the short body and Long Bodies, for the pitot-static system, is...

• a pair of drains... one under the tail, (static system)... one under the wing, (pitot system)...
• alternate static airsource... no breaking the VSI glass needed...

Yes, flying mower with wings...not really... my mower doesn’t have instruments... 

We still need to fly with imperfect instruments and know how to continue flying after they fail...

Even the C152 student pilot gets to fly using attitude and rpm around the traffic pattern simulating that he has lost the instruments... he does so... so he can become a competent pilot...

If we don’t fly when it is raining... we would all have to move to California.... or Arizona....

I think Jim wants us all to fly Turbine Mooneys!   Hmmmmmm..... turbine....?

Keep on flyin’...

Unless there are thunderstorms and icing.... then a U-turn is an accepted practice... 

PP thoughts only, my lawn tractor would be really cool with a JPI900 keeping track of its one cylinder....

Best regards,

-a-

 

 

 

[0TreeLemur]

47 minutes ago, carusoam said:

... my lawn tractor would be really cool with a JPI900 keeping track of its one cylinder....

Best regards,

-a-

 

 

 

I happen to have a single 2" EGT indicator that I would ship you free of charge. Just say the word.

[0TreeLemur]

1 hour ago, DonMuncy said:

 

What would have happened if dirt daubers had blocked the pitot drain, and after a while, the rain washed it clean?

That is a likely explanation Don.  Last annual I removed significant mud dauber nests from behind the rodent socks in the wheel wells from when the a/c used to be parked outside.

[Jim Peace]

1 hour ago, DXB said:

Seems like incredibly valuable experience with an instructor on board to me.  

Under a hood in VMC yes.....

who is to say the instructor is any good?  because he has a certificate, he is a nice guy, he tells good jokes, he is my neighbor?  If the instructor was really wanting to demonstrate safety the flight would be in VMC.  Going out in heavy rain or continuing into heavy rain during training just proves he needed to buy more pairs of shoes than he needed to show good judgment......fortunately they lived to fly another day.  Unfortunately we read too much about how it goes the other way.  

Maybe the last Mooney crash if the student had an instructor that beat it into him that a student or even a private pilot should not be up in the dark in mountains, that this is where the boogie man lives, he would still be with us. 

[Yetti]

Some mooneys have two drains.  One by the battery compartment and one near the pilot wing root.   Might be good to check and see if water comes out of either.

[DXB]

4 minutes ago, Jim Peace said:

Under a hood in VMC yes.....

who is to say the instructor is any good?  because he has a certificate, he is a nice guy, he tells good jokes, he is my neighbor?  If the instructor was really wanting to demonstrate safety the flight would be in VMC.  Going out in heavy rain or continuing into heavy rain during training just proves he needed to buy more pairs of shoes than he needed to show good judgment......fortunately they lived to fly another day.  Unfortunately we read too much about how it goes the other way.  

Maybe the last Mooney crash if the student had an instructor that beat it into him that a student or even a private pilot should not be up in the dark in mountains, that this is where the boogie man lives, he would still be with us. 

Jim - now you're insulting both an instrument student you don't know and a CFI you don't know, comparing them to someone who took off solo into probable IMC in mountainous terrain at night without bothering to finish a PPL. Your irrational fears and propensity for judgement are the only boogie men I see here. 

[jetdriven]

Ours did this too. After flying the whole way from Dallas to Houston in the rain with 40 knots of airspeed in actual IFR, time to do something.  

We finally took the fitting off the pitot tube inside the wing and back blew it with compressed air. A lot of fuzzy bug debris came out. When that stuff got wet it swelled and clogged the pitot tube. Works perfect since then.  Give it a try.  Nearly free. 

Edited February 18, 2019 by jetdriven

[jaylw314]

2 hours ago, Fred₂O said:

That is a likely explanation Don.  Last annual I removed significant mud dauber nests from behind the rodent socks in the wheel wells from when the a/c used to be parked outside.

FWIW, do you keep a pitot cover on between flights? 

[Jim Peace]

2 hours ago, DXB said:

Jim - now you're insulting both an instrument student you don't know and a CFI you don't know, comparing them to someone who took off solo into probable IMC in mountainous terrain at night without bothering to finish a PPL. Your irrational fears and propensity for judgement are the only boogie men I see here. 

They could be the best pilots in the world but what will that do when the engine quits or the instruments fail, clogged flux capacitor etc. in heavy rain?  Wait I know what will happen, we will all pay more.

[Hank]

7 hours ago, carusoam said:

The only thing new between the short body and Long Bodies, for the pitot-static system, is...

• a pair of drains... one under the tail, (static system)... one under the wing, (pitot system)...
• alternate static airsource... no breaking the VSI glass needed...

Anthony, my C has both of these drains. She left the factory with them in 1970. Some things just aren't so new . . . .

[Hank]

8 hours ago, Jim Peace said:

For starters you can stop flying these lawnmowers with wings in the rain, especially heavy rain like the OP.  IMC in the rain, night time like the last mooney crash, threads about flying in a polar vortex....This is all reserved for multi engine turbines with a purpose to be there in the first place.  Not training.

You're right. Let's all turn in our instrument ratings, swear off night flights and only fly when the sky is clear bkue and winds are less than 10 knots. But then we won't need Mooneys, because we won't ever travel verynfar for fear of getting stuck by clouds and rain (the reason I took instrument training to begin with). 

On second thought, no thanks. I'll never afford my own multiengine turbine, and I'm not a fan of the post-9/11 airline experience.

So I will continue to fly my Mooney where, when and how I see fit, and a little rain or a few clouds won't get in the way. Haven't yet gone much over 2 hours in IMC at a stretch, but night flights are pleasant. My limit is combining night and IMC, don't want to go there yet. Nit because the plane can't do it, but because it increases my workload and reduces my options.

Ya'll flynsafe out there! And don't press your own fears and limits on other people . . . . .

[DXB]

51 minutes ago, Hank said:

Anthony, my C has both of these drains. She left the factory with them in 1970. Some things just aren't so new . . . .

Interesting- my ‘68C has the one under the left wing root - I had assumed it’s for static but what you say makes more sense since the pitot line runs that way.  I am not aware of a static drain under the tail- unless I’ve somehow missed it completely.  My POH says nothing about these drains. I wonder when they first appeared.  

[Chris from PA]

Sounds like classic performance decreasing windshear to me. What were your ground speed indications? What was the reported wind? It sounds to me like you may have flown through a microburst.

Sent from my SM-T350 using Tapatalk

[0TreeLemur]

8 hours ago, Yetti said:

Some mooneys have two drains.  One by the battery compartment and one near the pilot wing root.   Might be good to check and see if water comes out of either.

Our '67C has no drains that I'm aware of.  Definitely nothing near the left wing root.   I like the idea of disconnecting the air fitting to the pitot tube inside the wing, the blowing the pitot tube out with compressed air.