Peculiar inflight event....common?

[tkatc]

During last night's flight my co-pilot and I were fighting over the cabin temp. I wanted the vents open and she wanted them closed. As we played around with opening and closing I noticed the airplane would pitch up or down depending if the vent was open or closed. Anybody else notice this phenomenom or is it just me?

[AltonD]

Were you on altitude hold? If so, you probably have a leak in the static system.

[L.Adamson]

My auto pilot altitude hold is connected to the rear static ports (Vans style). I use a combination pitot static tube for the instruments. A tube runs from the back static ports to the panel for a future "alternate" static source .............if I ever get around to hooking it up.

Anyway, the tube was pulled off the tee, the auto-pilot static source was now sensing cabin pressure............and it was up & down with ventilation & heat............as temps went from the 70's to 14.

L.Adamson --- RV6A

[tkatc]

Now that you mention it, I was on altitude hold.

If I have a leak, would that cause problems with my AK350 encoder? My mode C has not been functioning correctly.

Ahh, the growing pains of a non-builder. Now how do I locate that leak?

[RV7AV8R]

My alt hold is vented to cabin pressure and it is sensitive to vents exactly as you discribe. Unrelated to this I also had a leaky staic line due to cold flow of nylon fittings at the threaded input of the instruments. They were 5 yrs old at the time and all of them were "finger" loosely threaded in to the instruments. I know they were tight initially. I replaced all threaded input connections with brass and the static port problem went away.

[turbo]

for starters see if the hose has come off the encoder. easy to check and can happen very easy. mine did this year. how is your airspeed reading? mine was off a bit because of this.

[Pat Hatch]

Originally Posted by tkatc

Now that you mention it, I was on altitude hold.

Now how do I locate that leak?

Unfortunately, there is no easy way to find the leak--but, it can and will be found with a little perseverance!

First, you have to have a source for vacuum applied at the static port (tape over the other one). You can use a hypodermic syringe, but the absolute best source IMO is a brake bleeder kit such as the one sold at Sears with the Craftsman label. I believe the criteria used by the FAA is to pump up your altitude to 1,000 feet above current elevation and your altimeter should not lose more than 100 feet in one minute. If you can achieve this, you are good to go.

To locate the leak, begin by unplugging each instrument attached to the static system one at a time and plugging the end of the static tube beginning with the last instrument in the static system schematic. This way you can eliminate each instrument as the source of the problem working backwards from the end of your static system network. After you have checked each instrument, replace its static line. Work backwards or upstream from here back to the static ports, one fitting at a time. Remove the fitting and plug the line until the problem goes away. Eventually you will be all the way back to the T over the two static ports, but you will probably find the offending component way before this.

I recently had a pesky static leak and it turned out to be the RMI µEncoder. There probably aren't that many around any more, but for those of you who built them, the internal tubing will disintegrate with time. I replaced the tubing inside and the problem was solved.

Another good (quick) way to check your static system for integrity inflight is to pull the cabin heat full out with altitude hold engaged and if the aircraft pitches at all, you probably have a static leak. P.S. (Or you can observe the VVI and it should not move at all when pulling the cabin heat out.)

[mrreddick]

Originally Posted by RV7AV8R

My alt hold is vented to cabin pressure and it is sensitive to vents exactly as you discribe.

I have the same setup. When I installed my altitude hold, years after the plane was built, I let it vent to the cabin so every time one of us shuts off a vent there's a slight pitch change. This doesn't necessarily mean you have a leak in your static system since your altitude hold may not be connected, like mine.
When you get your transponder certified the tech should be able to detect a problem, since you didn't specify what the problem is that you're having with it. If ATC keeps telling you you're not maintaining your assigned altitude it's probably your encoder, get it checked! First do what Turbo suggested, check the line on the encoder to make sure it's still hooked up properly.

[tkatc]

My mode C is not being detected by ATC. I know when it is working because the Garmin 327 will display pressure altitude in feet. Lately, it has not been displaying any altitude. It was happening intermittently but the last flight it didn't display at all. I suspect a bad encoder...

[AltonD]

Originally Posted by tkatc

My mode C is not being detected by ATC. I know when it is working because the Garmin 327 will display pressure altitude in feet. Lately, it has not been displaying any altitude. It was happening intermittently but the last flight it didn't display at all. I suspect a bad encoder...

Check the simple stuff first. Is it plugged in? power available? I would bet a loose cable connector.
If it were bad electronics, I doubt you will see intermittent operation.