Altimeter and VSI jumped when going through rain shower

[Mark89114]

Flying back home and came through a small line of weather, light to maybe moderate rain and the Altimeter and VSI jumped immediately, Altimeter about 100+ feet and the VSI to 500 foot climb.  AP tries to correct, but I hurry up and hit the disconnect and hand fly and the "variation" goes away very quickly.  Obviously moisture causing it, but how and why?  This happened once several years ago, very similar rain conditions.  I have also spent a lot of time flying in constant rain in Florida and the Southeast and no odd effects.  I doubt if this has anything to do with it, but the other time it occurred was similar geographical conditions, what I would consider high desert.  First time was remnants of a hurricane and I was going into Santa Fe NM and this time was in Eastern Wyoming.       

[N201MKTurbo]

I was flying through a rain storm in Ohio once when the airspeed started climbing. I pitched up to reduce the airspeed, but it kept going higher. A look at the attitude indicator convinced me it was lying. I turned on the pitot heater and the needle danced around for a while and then came back to normal. It was well above freezing, so I don't think it was ice. I'm not sure the pitot heat did anything, but what else are you going to do?

[PT20J]

Rain can enter the static ports. I’ve had it happen once in a PA28 (easy to cure as pitot and static drains are in the cockpit), and once in a Beaver (had to suck water out the static port after landing since there was no static drain).

That’s why there is an alternate static source. 

I flew the Cherokee through a heavy rain after which the instruments started jumping around and a few drops of water came out the static drain when opened which solved the problem. The Beaver floatplane was at the dock with a wind driven rain hitting from the side. When I flew it the airspeed stuck low and the altimeter didn’t work. I was amazed at how much water I sucked out of the static port; it was completely blocked.

Water in the pitot (unless it freezes) is possible but less common because pitot tubes have a drain hole built it. 

Skip

[carusoam]

Great opportunity to pull the Alt stat lever right there!  

Water doesn’t get collected in the Mooney pitot / static system very often…

Start with draining the two drains for the pitot/static system… see if anything comes out…  sometimes these drains stop sealing… that can cause some oddities as well…

 

If the problem persists… the static system is probably open to the atmosphere in the cabin…. There are a few rubber hoses in some Mooneys that are attached to the alt hold system in the AP….  This causes difficulty holding altitude as well….

 

If flying in rain caused it… see if the rain has drained out… check the pitot tube drain while you are looking…

PP thoughts only, not a mechanic…

Best regards,

-a-

[GeeBee]

It is not unusual when entering gust fronts or rain fronts to have a pressure change. Think about it. You drop a large mass of water through the air. It is going to have a bow wave just as dropping a rock in water.  During wind shear recoveries, VSIs are often useless unless they are IVSI units.

[Hank]

4 hours ago, GeeBee said:

It is not unusual when entering gust fronts or rain fronts to have a pressure change. Think about it. You drop a large mass of water through the air. It is going to have a bow wave just as dropping a rock in water.  During wind shear recoveries, VSIs are often useless unless they are IVSI units.

Just one more reason to love my IVSI. Can't go back!

[GeeBee]

Airline crews are taught to only call out trends from the altimeter in wind shear, i.e. "Climbing" or "descending" because of rapid pressure changes.

 

[carusoam]

Interesting responses…. Let’s see if @Mark89114 has been back this way.

 

Seems like it is normal to see pressure variations when flying into weather systems….

Stuff we have read about… and now see first hand… 

Go MS!

Best regards,

-a-