Altimeter bounces

I was flying in hard IMC yesterday in my -6 and noticed that my altimeter needle was bouncing. The ocillation encompassed about 50 feet. It would stop and then restart. It evetually stopped completely. I assumed it probably was caused by moisture in or around the static port. I have never seen this in another aircraft. Anybody have an explanation or theory?

MXpilot1 said:

I was flying in hard IMC yesterday in my -6 and noticed that my altimeter needle was bouncing. The ocillation encompassed about 50 feet. It would stop and then restart. It evetually stopped completely. I assumed it probably was caused by moisture in or around the static port. I have never seen this in another aircraft. Anybody have an explanation or theory?

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I don't have an explanation but it has happened to me, only worse. The original altimeter in my RV would occasionally oscillate, sometimes up to a range of around 500 feet. I finally replaced it and that solved the problem.

The first altimeter I purchased from Vans was made by Falcon. After only a few hours use the needle began to bounce and shortly thereafter stuck on a reading 1500 feet high.

When I removed it from the panel and gave it a gentle shake the needle acted like it was no longer attached internally.

Purchased a new United altimeter from Aircraft Spruce (for three times the price of the Falcon) and have had no problems since.

This is not the first time I have had problems with Falcon brand instruments. You would think I would learn a lesson by now.

Jim Ellis
RV9-A, tipup
flying

How hard?

MXpilot1 said:

I was flying in hard IMC yesterday in my -6 and noticed that my altimeter needle was bouncing.

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Just how hard? With or without an autopilot? Reason for asking is that two weeks ago, a CFI-I friend of mine gave me an 80 minute workout in my -6 and my head was swimming. Turns to headings and GPS approaches with A/S, heading changes and rate of climb requests simultaneously. Whew. sweating bullets but I needed it. Not so sure I can handle "hard" IFR solo, no autopilot.

Regards,

If I'm flying on an IFR flight plan I always ask center to climb/descend into IMC. Forget foggles, there's only one way to stay sharp!

If they ask why I just tell them for groundspeed purposes They usually let me.

My RV4 does that. I asked someone I trust about it and he said that it was probably due to a leak in the static line. I haven't tried to fix it yet but that sounds plausible. It's possible that the leak in one of the instruments that connects to the static port. That could explain how changing the altimeter could fix it though I admit that a problem in the altimeter's mechanism is more likely. Still, on my plane, both the altimeter and vsi bounce together sometimes. That's why I'm going to start by looking for a static leak.

Sounds to me if its only the altimeter that is wonky and not the VSI as well it is probably not a good mating to the static system, allowing static pressure to escape will cause that kind of oscillation.

Just how hard? I flew about 1.3 hours in very thick clouds and moderate rain on that particular day. No auto pilot. I will admit it is a very good workout and I was ready to be done. I fly in IMC a fair amount and have learned a few things that help a lot. 1. Maintain a very good scan. Much better than you did in a Cessna. 2. Hold the stick near your lap not at the end. It gives you less leverage. 3. Maintain an extremely light touch on the stick. In fact, I let go of the stick if I need to retrieve a chart, program the GPS, etc. The RV is such a responsive plane, I can never relax in IMC.

How fast?

How fast were you going? That usually happens to the altimeter in the T-38 right before I break the sound barrier. Maybe that's it?

Altimeter AND VSI bounce. This has been an ongoing issue in the RV that I owned and was wondering if there is any suggestions on what actually is going on.

Hopefully the video is viewable:
https://photos.app.goo.gl/kwooQNGyfHFFZE838

It has done this for the 4+ years that I have owned it. It has not been washed or been outside in the rain or any significant amount of water. I have occasionally given it a quick wax job using just a detailing spray on wipe off wax.

My assumption in that it affects both ALT and VSI that is must be something in the STATIC system. The STATIC system is stock VANS I believe (I didn't build it). There are the 2 ports halfway back on the fuselage that are then connected together and then run forward to under the panel. the fuselage and they are

Photo of them installed:
https://photos.app.goo.gl/TwhnhZjJ1PvinSX88

I have flown lots of transport category aircraft where weird things have happened and they end up finding water in the TRAPS that are designed to catch that water, but this hasn't been out in the water so even if you think it has water in it that you would think that sitting for a few weeks in the hanger it would evaporate out.

Any suggestions?

How about removing the static line where it first joins behind the panel and blow some low pressure air back towards the static ports?

How about doing a PITOT STATIC test where they block off the ports and run the ALTITUDE up and down?

It does have an ALTERNATE static port, or it where they can hook into the STATIC system under the panel where it gets closer to ENCODER. I have removed the cap nut on that port in flight and it doesn't seem to have any effect, they still both bounce.

Thanks in advance!

No, that is not the standard kit supplied static ports, so that could very well be your problem. There is a lot of threads here in the forums discussing the issue and ways to resolve it. Search for static system error....

It is very likely that besides bouncing indications, you also have static system induced errors for airspeed and altitude.

Thanks Scott, I'll do some searching.

Last time I did some stalls, just a week or so ago, it stalled clean and flaps full at book numbers, so that would lead me to believe that the ASI (Air Speed Indicator) was reading correctly.