Altimeter Bounce

[Mark Anderson]

On a flight last night my altimeter and VSI were both reading eradicaly. The needles were bouncing around + or - 50 feet or so. I was in and out of IMC, freezing rain and light icing. Temperatures were -3 to + 5. I am thinking maybe my static port was partially blocked by ice. I have never experienced this problem before. I had the altimeter replaced a few weeks ago. This was the second flight out of annual inspection, don't think they messed with anything behind the panel. Has anyone experienced this before ?

Thanks

Mark

[Jay]

I've had that happen when water finds it's way into the pitot static system. For me at least a few days in a dry hangar cured the problem.

Jay

[Zach Grant L1011jock]

Mark,

Very typical. If you do not have an alternate static port plumbed into the system, I highly suggest you contact your local shop and have it done. The Comanche static system, especially the pre 65 birds with the flush ports, has been known to become totally blocked with water to the point of freezing all of the static needles where they are, which is very dangerous when shooting any approach that does not have hard verticle guidance such as an ILS or LPV. It will also cause your airspeed to become unreliable. As you decend the bubble of water stuck near the port usually gets sucked into the system, creating a hydraulic lock, and usually results in the ASI still indicating significant airspeed when the plane is parked on the ramp. Opening the alternate static will equalize the pressure with cabin air and all indications will return to somewhat normal, if somewhat less accurate than normal, but you will not fly into the ground thinking you are still at 5000 feet! The encoder will be fooled also in this scenario, so ATC will think you are still at your assigned altitude (this brings in all sorts of traffic conflict issues as well). I know you can fly pitch and power, but trust me, when this happens, it takes a while for you to not trust the instruments and do something else. I speak from experience. Just before breaking the glass on my VSI, I figured out I could use my GPS derived altitude and groundspeed to succesfully get vectored to an ILS, and flew it to minimums, disregarding the pitot/static information presented, and relying on known power settings and configurations, while crossreferencing ground speed. The next time I flew I had an alternate static source, and have had to employ it on several occasions since in heavy rain.

-Zach

[N3322G]

Mark,

I defer to the folks who have had this experience that you described for advice - however - the removal and installation of the altimeter does open the static system to contamination and as I understand it, the static system is supposed to be checked any time that happens.

FWIW - Had the air speed gauge worked on at annual at Clifton and Clifton explained that to me. They checked it and it was fine but that concept was new news to me.

[Mark Anderson]

Zach and Jay, thanks for the great responses. I will definitely get the alternate static source plumbed. It would probably be a little upsetting to the wife breaking the glass on the VSI with the crash axe. I imagine it would be a little more difficult than it sounds. I was wondering what I was going to do when the needles stopped. These are great ideas you mentioned with the GPS. Interesting on the transponder and ATC, I didn't think about that. As it turned out I did a localilizer approach into Shreeveport Downtown close to minimums. Good idea on the glide slope.

Zach, Also I have gotten carb ice several times now. Are the early Comanches more susceptible to carb ice or the Lycoming O-540 than other engines or planes? Another problem I have had in cold wet weather flying is my manifold pressure going to 30 inches at like 10,0000 feet, with all other engine indications normal. Only thing I can think could be causing it is the drain hole in the pick up line freezing over. Can anything be done about this?

Pat, thanks, Just saw your post, I'll look into that as well.



Thanks

Mark

[Mark Anderson]

I think I figured out why my pitot/ static instruments where bouncing around. One of the static ports was completely blocked with wax. I waxed my plane a few weeks ago and carelessly rubbed wax in it. I cleaned it out before I flew today and everything is normal.

Thanks

Mark

[md11flyer]

Question:
Is it not a requirement for all aircraft that fly IFR to have an alternate static source?

Happy New Year to all,
Gary

[Zach Grant L1011jock]

No. That is why everyone is taught in initial instrument training about plugged pitot/static ports, and the associated issues, as well as what to do if you really need to get static pressure to the system (break the glass on the vsi). Having a resettable valve is much more desirable, but I flew lots of IFR in Cherokees, 172s, etc., that never thought of having alternate static. I also flew lots of IFR with airplanes with one nav/com with only a vor obs(no GS). As long as the TXP and static system has a 24 month check, then you can fly IFR with only equipment appropriate to the navaids that you plan to use during the flight.

Zach

[md11flyer]

Hmmm:
This is one time I agree with Transport Canada's more restrictive rules than the FAA's. In Canada you are required to have
an alternate static source. Yes breaking the glass is a very good alternate to this for VFR aircraft that accidently find themselves
in trouble in IFR conditions and blocked static ports. (and also if you find you pull on the alternate static knob and it breaks in your hand as you pull...)
But if you fly hard IFR, I believe an alternate static source is a must.
Part of the IFR preflight is to check the alternate static by watching the bounce on the needle as you pull it on. You can't check the alternate source if you don't have one.

In Canada you can't even book an IFR ride in VFR conditions if your aircraft doesn't have the minimum IFR requirements met including an alternate static source.
In any case I believe, and I might be wrong,... but I think if you fly a foreign registered aircraft in Canadian airspace, in IFR conditions, it must also have an
alternate static source.


Gary