[FORANE]

I currently have an old Altrak altitude hold in the Lancair. When I initially installed it, I connected it to the static port. After experiencing a frozen static line one time I removed the Altrak's connection to the static port. I figured it was just an altitude hold and didn't matter what altitude that might be; removing the static port increased its safety because if I were in IMC and the static line froze again, at least the altitude hold would keep me level.

I am currently installing a Trio pro pilot in the RV-9A I recently purchased. I am debating if I want to connect it to the static port. My 9A has bilateral static ports which I am told are vans supplied.

On the ground the static line pressure should be the same as inside the cabin. In flight I seem to recall there is a decrease in cabin pressure due to the venturi effect. As such, the pro pilot would level off lower than actual altitude selected due to the error.

Does anyone know how much the error may be?
Has anyone experienced frozen static line in their RV?

 

[BobTurner]

The cabin pressure is usually less than ambient, in flight. But how much depends on things like, "Are the vents open or closed?". 50 or 100' higher might be typical. If you want to have options, why not install an alternate air valve? Stein sells an inexpensive one that looks like a toggle switch.

 

[Snowflake]

Doesn't the alternate air valve just open into the cockpit, in which case, if you have a frozen line you're going to read the cockpit air anyway?

 

[BobTurner]

That's correct. If you can, you really want to use a proper static system to keep the altimeter and the airspeed indicator accurate. Vented to the cabin, the autopilot will go up and down a bit if you open/close the vents, slow down or speed up, etc. But if the static line is really frozen, using cabin pressure is better than having nothing. Note the ASI will read high, use caution to not stall inadvertantly.

 

[FORANE]

The cabin pressure is usually less than ambient, in flight. But how much depends on things like, "Are the vents open or closed?". 50 or 100' higher might be typical. If you want to have options, why not install an alternate air valve? Stein sells an inexpensive one that looks like a toggle switch.

Hey Bob
I have an alternate air in the Lancair that I installed after my frozen static but the RV does not currently have one. My alt air is not as elegant as a toggle switch one. Thinking of installing that.

Doesn't the alternate air valve just open into the cockpit, in which case, if you have a frozen line you're going to read the cockpit air anyway?

Yes, it does unless you have another static port and have ported the alt air line to the other port. Cockpit read alt air is not too far off in my Lancair, maybe 50 feet if I recall last time I used it.

That's correct. If you can, you really want to use a proper static system to keep the altimeter and the airspeed indicator accurate. Vented to the cabin, the autopilot will go up and down a bit if you open/close the vents, slow down or speed up, etc. But if the static line is really frozen, using cabin pressure is better than having nothing. Note the ASI will read high, use caution to not stall inadvertantly.

Hey Bob

I agree cabin pressure is better than nothing but a frozen static, for me in my experience at least, is not immediately identifiable. I certainly would not want it to happen on approach. Also, the ASI may read high or low depending if you climb or descend.

Here is a great pitot / static simulator where one may see what happens with all the failures:
http://www.luizmonteiro.com/Learning_Pitot_Sim.aspx

Is the RV static system such that failure is extremely rare or unheard of? Has nobody experienced frozen static in an RV?

 

[rleffler]

You may want to take a look on how the builder (I am assuming that wasn't the OP) ran the static lines.

Mine are installed and immediately run up the bulkhead and tee and the top of the bulkhead support in my RV-10. The thought was if any moisture gets in the line, gravity will cause the moisture to drain.

Can you locate where the blockage occurs when the line freezes?

Like Bob mentioned, I too have an alternate air switch in the cabin. It's a good "Plan B".

 

[FORANE]

You may want to take a look on how the builder (I am assuming that wasn't the OP) ran the static lines.

Mine are installed and immediately run up the bulkhead and tee and the top of the bulkhead support in my RV-10. The thought was if any moisture gets in the line, gravity will cause the moisture to drain.

Can you locate where the blockage occurs when the line freezes?

Like Bob mentioned, I too have an alternate air switch in the cabin. It's a good "Plan B".

Hey Bob
Correct, I am not the builder. I plan to look at the static lines in the empennage next week when I install the elevator servo for the pro pilot. The builder said the whole plane was stock vans so I am assuming both static ports enter, go up, T at the top, then run forward.

I put one of these (pictured below) in the Lancair but still get water in my static port if the plane is washed or left outside in rain. I believe it freezes right at the static port opening. I may just plumb the pro pilot into the static line, install an alternate air in the panel and see if the RV experiences any freezes.

 

[Snowflake]

It is likely that the point of freezing is right at the surface of the skin, plugging the hole in the pop rivet that most of us use for this purpose. Cabin heat would probably keep the rest of the cabin and tail cone above zero, even if you're not plumbing heated air all the way back there.

I keep wondering if it would be possible to put an electric heater on that... It's such a small hole, it might not take a lot of power to keep the temperature above zero. A small heater resistor epoxied to the skin on the inside, right next to the port, and a regulated power supply with a feedback loop to keep the temp positive...

 

[Finley Atherton]

My recent experience. RV9A with standard Vans static ports installed as per the plans, IFR in IMC and in rain for the first time.

In rain it was apparent that both static ports were blocked with water. All instruments with a static connection were affected - steam gauge ASI and ALT, Skyview ASI, ALT, VSI and attitude and the Trio Pro Pilot.

It seems to me that the Vans static ports have a relatively large hole in them compared to some other ports I have looked at on certified aircraft and perhaps the smaller holes are less likely to let water in

Fin
9A

 

[BobTurner]

Hey Bob

I agree cabin pressure is better than nothing but a frozen static, for me in my experience at least, is not immediately identifiable. I certainly would not want it to happen on approach. Also, the ASI may read high or low depending if you climb or descend.

Blocked static is pretty easy to ID. The airspeed indicator will be way off with altitude changes. Go down, asi will go up; go higher, ASI will drop. With an alternate air switch it's easy to confirm.
But, back to the original question, I don't see anything too wrong with just leaving the autopilot open to the cabin, assuming it works okay. There could be some odd effects, like a climb attitude might increase cabin pressure and fool the autopilot? I don't know other than to try it and see.
At least in my ten, the aft tailcone, where the ports are, can be cold! The cold air seems to be drawn forward, into the cabin, not vice versa.

 

[F1R]

A heated pitot like a Gretz Aero can offer you heated static ports at the same time. Not that icing in a light single is a good plan at all, however a simple pitot cover will prevent the rain water or washing from introducing water to your PS lines.

There are several makes of pitot static tubes where the static ports are coaxial on the same shaft that the pitot is part of.

http://www.aerocontrolex.com/products/pitot_static.asp