Pitot/Static Failure in Rain/Mist?

I recently was flying my RV-6A on a low visability day, and stumbled into some very heavy mist, clouds and light rain. I had flown about 10 minutes into these conditions when I deemed it prudent to land at a nearby airport.

The OAT was about 40 deg F aloft, and I had no reason to suspect icing conditions were present, however, when I made my approach to land, I had a sense that something was not right with the airspeed, as it was indicating 100 mph on what seemed like a normal approach (75-80 mph). Luckily I ignored the airspeed, and landed without using it. (Looking back on it, I was amazed how easy this was...this was partly due to the long runway I was landing on.)
My EFIS, my mechanical backup ASI, and my EIS, which all use the same pitot/static port, showed the same indications.

In an event, as I touched down, in my normal landing attitude, and what seemed like my normal touchdown speed (about 60 mph), I glaced down and saw the airspeed indicator reading over 90 mph...clearly wrong. As I taxied off the runway (10 mph), the airspeed was slowly decaying down through 50 mph, and by the time I parked, it was down to zero.

I checked the pitot and static system for water by blowing it out. Only a fraction of a drop of water came out of the pitot tube...nothing from the static ports as near as we could tell.

No problems have occurred since then.

Has anyone had any experience like this?

I am going to test a second pitot/static port system that one of our customers suggested. This is to use the air pressure on top of the engine (before it has gone through the baffling) as the pitot source, and the cabin air as the static source. Our customer said it worked good for him as a backup, and it probably offers some icing protection.

Still, I would like to know why happend that day. In 300 hours, including flying in snow, rain, freezing and hot conditions, it only occurred that one time. Has anyone else had such an experience?

Thanks
Greg Toman
RV-6A 269DB
Building RV-10

If your airspeed reads too high, there might be suction from some source affecting your static side. I had a static line in the back of the A/C crack, and the low pressure in the aft fuselage caused the A/S to read high. Airspeed being the difference between static and ram, causing the static to be lower than normal would fool the ASI into reading high... Any chance of an air leak somewhere in your system?
Good luck, please post what you find.
Dennis

Hi Greg,

That sounds just a LITTLE like an open static system problem I had - I was getting generally high airspeeds, although it was more of a linear scale factor effect (more error at higher speeds, less at lower speeds). It was due to a static line that had popped off behind the panel. You might get a more varying effect like you described depending on how well (or not) your cockpit is sealed. But that doesn't explain why it came and then went away.

I think I have to vote for temporary moisture in the system - somehow. Maybe you collected a little dam of moisture near the static ports - are your ports stock Van's pop rivets, or one of the fancier sets?

Welcome to the VAF forums BTW!

Paul

Where is your OAT mounted Greg?

Did you have cabin heat ON during the flight.

I was flying along (cabin heat on) under an overcast with what my OAT said was 45 degrees or so OAT (I knew it was a bit high due to less than optimum mounting place) and I got ice/freezing rain/freezing mist or something that took away forward visibility like RIGHT NOW! Did a 180 and cancelled the trip.

So my take is a different one. Maybe, just maybe you were right on the edge temperature-wise and did not know it. Some of the moisture collected and froze in your pitot, sealing the pressure. Then as you came down and landed, the small amount melted (slight bleed off on final and more during taxi.

Just something else to consider.

By the way, I think my OAT reads 5-8 degrees HOT (maybe more at times).

James

Greg - A too high IAS indication means either the pitot pressure was too high, perhaps because there was some ice blocking it, and not allowing the pressure inside the pitot lines to come out as you slowed down. Or, it means the static pressure was too low, possibly because there was some sort of blockage, not allowing the higher ambient pressure to come into the static system as you descended. We need more information in order to figure out whether your problem was on the static side, or the pitot side.

Was your altimeter indicating normally? I.e, did you notice whether it indicated the correct altitude on landing? Did the altimeter stay steady after landing, or did it decrease for awhile? If the problem was due to blockage in the static system, a 50 mph ASI indication at zero speed equates to an altimeter error of around 85 ft. So the altimeter would have decreased about 85 ft as the pressure in the static system equalized after landing.

How about the VSI? Did it appear to indicate normally during descent, and after landing?

If the altimeter and VSI indicated normally, then this must have been a pitot problem. Are you sure your OAT indication is accurate? As indicated above, I have seen OAT indicators that read too high, as they were affected by cockpit heat on the back side of the OAT probe.

To rule out other issues, do a leak check on the pitot and static systems. A static system leak in the cockpit area could give symptoms a bit like you describe, but I would have expected less of an error.

My vote is for ice

From the symptoms I think it was ice in the pitot. Remember that it is the "pointy things" that ice first. Not much to reference in an RV, but in airliners the edges of the windshield wipers were a good indicator and would show ice before other areas. We were required to fly with the pitot heat on all the time and to take additional precautions at +10 degrees C, (50 degrees F.) With the right set of aerodynamic conditions and moisture present it could have iced just enough to cause the airspeed problem.

John Clark, ATP CFI
RV8 N18U "Sunshine"
KSBA

Ill bet Ice on the pointy thing. Thawed on decent to landing.
Try to reproduce and report back.
Only kidding.

Ice on the pitot head caused your problems. If ice had formed on the static ports the VSI and Altimeter would have been stuck till it melted. A plugged pitot will only effect the airspeed instrument and anything else tied to the pitot system such as AOA or EFIS or Auto-pilot.

Ice may well have been the culprit, but I don't think that is what got me leaving OSH last summer! My airspeed stayed low (I noticed it as I rotated but elected not to abort) for about ten minutes, then suddenly came up. I declared it had to be a bug (the real, flying kind) that pushed itself on through the opening. Never really found anything, however. Bugs can plug a pitot in flight as well as given a few days with an uncovered tube.

Bob Kelly

pitot/bug mid-air

I had a mid-air with a bug one time in a Cessna. It happened in the pattern near midfield downwind. By the time I was on final the airspeed had decayed to 40 kts and eventually to 0. Landed fine and the maintenance people blew out the tube and found the bug.

I too think it was ice on the pitot that melted or "evanesced" as you decended.

Given I know what your day job is, I won't explain the basic physics behind static and total pressure.

Pitot Ice!

I had not considered until I read one of these posts that if it had been a blocked static port, I may have noticed unusual altimeter and vsi behaviour. I do not recall seeing a non-zero VSI or a changing altimeter while I was on the ground and the airspeed was slowly decaying, and I think I would have noticed the VSI espeically.

I don't recall if the altimeter was accurate after landing...I'm going to go back and double check to see if I had recorded that flight.

(My RV has the pop-rivet type static ports.)

I'm beginning to agree with the speculation that it may have been ice on the pitot probe. Assuming that was the case, it is interesting that I did not see any ice accumulation on the windscreen. Very likely as others had pointed out, the ice may have been forming on the pitot probe before it ever formed on the windscreen.

I have two encounters with ice in other airplanes in the past, and in those cases I saw accumulation on the windscreen, and the airspeed was unaffected (I got out of the ice as quickly as I could). In one case I flew into freezing rain (very fast accumulation, airspeed unaffected) and anther time at the base of clouds (slower accumulation).

I had not considered that ice might be forming before I see it on the windscreen.

I think I learned something!

Thanks
Greg