confidence shaker

[witrakw]

I had a peculiar episode the other day while under the hood doing some ifr currency training with CFII safety pilot around KDLH where I live.

Scenario-- multiple approach shots, VORS, VOR/dme, Ils. Half way through vectors to ILS Approach says "mode C" not working. We recycled several times and for the next hour, it never came back. Finally, was shooting gps approach to my home airport, on FAF fix, trying to descend to FAF crossing height, VSI 500fps down, altimeter not budging. Informed safety pilot of discrepancy. Opened static port with no effect. Increased VSI to 1000 fps(we were visual at the time), altimeter very very slowly bleeding down(maybe 10 ft/min). We visually levelled off. Landed, taxied quickly, shut down and inspected both static ports--looked OK. Altimeter on the ground was 1000 feet high and kept bleeding down extremely slowly.

Weather during approaches was 38 deg. We were flying through light intermittent rain. Pirep 2000 feet above had some ice.

Me thinks that I could have easily been in the trees had I been in real IMC and without a safety pilot

Flew VFR next day. Mode C and altimeter A OK

Any thoughts to help this rookie pilot?

[kwmoore]

Very odd that the VSI was working but not the altimeter. A bit of ice in the line just proximal to the altimeter...? :confused:

In this situation the altitude readout on your MX20 and/or 430 or handheld GPS could save your bacon. Not good enough for a precision approach, but it should keep you out of the trees.

Yet another alternative: a wristwatch with an altimeter. Tina gave me one for my birthday a year ago. I regarded it initially as a frivolity, but it is amazingly accurate, almost always within 100 feet or less of N812KT's altimeter reading. I always wear it when I fly.

[kwmoore]

This happened at the beginning of an Angel Flight two weeks ago, in an STEC55X-equipped SR22.

We departed San Carlos (SQL) rwy 30 on an IFR-to-VFR-on-top clearance. Conditions were 1100 broken-to-overcast, no tops report. In compliance with instructions I turned right crosswind and then downwind towards a heading of 120 and altitude restriction of 1100 feet. Midway through the downwind turn I engaged HDG on the 55X and the autopilot obediently continued turning towards the bug at 120. Approaching 1100 feet I put in some nose-down trim to slow the ascent and pressed ALT. Sure enough, the autopilot began to pitch toward level, and I thought all was right with the world, so I diverted my attention to make a power reduction. Silly me! The 55X continued to pitch down past level into a descent. The turn to 120 was not quite complete so we were in a banked descent. My right-seater--also a pilot--called out “altitude” as I was about to return my gaze front-and-center. We had lost some altitude, we were between 900 and 1000 feet and the altimeter pointer was trending downward but not dramatically so. However the VSI was showing more than 1000 fpm down! A quick (and worried) cross-check of the AI and airspeed didn’t confirm any dramatic nose-down attitude or increasing airspeed. I disengaged the 55X and manually corrected with both roll and pitch trim adjustments—a chore as I was a bit rattled and in IMC.

While this was going on we were issued a turn to 050 and climb to 2000. I completed the turn and began to climb. This time the VSI suddenly scooted up to >1500 fpm. I realize the SR22 is a strong climber but not at 130-140 KIAS and only a ~5 degree pitch up indication on the AI! I was able to level out at ~2000 feet manually, then engaged HDG (no problem). In retrospect I shouldn’t have done so but I again pressed ALT. Once again we started a descent which had to be corrected manually after disengaging the autopilot. No more ALT in IMC! We were shortly issued a turn to 100 and climb to 3000. The VSI was quite erratic and of little help in leveling off: power, pitch, and altimeter accomplished the task, albeit in a more labor-intensive manner. I was yet more rattled and had some difficulty holding heading to, shall we say, PTS standards. We weren’t yet on top so they gave us 3500, where we were finally in the clear. The whole episode couldn’t have lasted more than 3-4 minutes, but it seemed much longer!

Once stabilized in VFR conditions, I tried out the 55X again. HDG and NAV/GPSS were fine, but ALT was a different story: porpoising up and down 400-500 fpm. At least that was my guess from the altimeter; the VSI was bouncing around. It seemed to behave reasonably much of the time in the 0-400 fpm range, but would then jump suddenly to larger numbers on either end. In level cruise it now indicated ~100 fpm up.

Fortunately the rest of the trip—4.1 hr with several stops--was VFR (some of it marginal). I did not again tempt fate by pressing ALT. However near the end of the flight the VSI seemed to improve its demeanor. I did elect to fly the GPS 30 approach into SQL because of mist and reduced visibility—all manually, using only power adjustments to change altitude!

This was a bit more adventure than I had planned for that day. I did not feel that we were in any imminent peril, but had I not known we would be on top in short order out of SQL, I might well have declared an emergency and requested help to get us back on the ground. In retrospect I should have at least tried the alternate static source. However I was quite busy ensuring control of the plane and the alternate static-activating lever is way down by the pilot's feet in the SR20/SR22.

You can bet I wrote up a few "squawks" on that plane.

[n2099x]

One of the main reasons I went with the Chelton Autopilot is the on going reports of porpoising, a few runaways and altitude hold problems. Although the same could be true with other autopilots I don’t like what appears to be an arrogant attitude from S-tec itself on these important issues.

Glen

[azb5gh]

N735GH had a similar problem two years ago. When engaging ALT, you would get an erratic pitch up or down depending on which way the S-Tec 55 wanted you to go. We initially thought it was a trim servo problem but they checked out fine and it ended up being a software problem in the S-Tec. Doesn’t give you a good warm fuzzy when that kind of thing happens.

[witrakw]

Is simpler better? My stec 30 with alt has had nary a problem, although I do far less ifr than you all in these posts and maybe I just haven't noticed any. How much does one actually use the stec55 altitude preselect or vertical rate(wasn't that a factor in that sr22 spinning from 12000 over california somewhere and popping the chute,,,,the cfii stalled the plane because of too high a vrate preset?)
I've never flown with the stec 55. The 30 is rock solid on all nav and approaches, doesn't capture the glideslope, but staying on the slope is pretty easy with the localizer locked on even for us old guys; in fact, one time I was using the autopilot on the localizer, forgot it was on, and I kept trying to inappropriately overcorrect for heading, but the a/p wouldn't let me budge the yoke; it was right and I was wrong.

[kwmoore]

I had an STEC30 in 812KT and it was rock-solid. I would highly recommend this autopilot.

[Todd Peterson]

Sorry to be away for so long but once again things have been busy here.

If memory serves me correct, and it may not, it seems like the static line goes to the altimeter first then down to the VSI. If that were the case it could be that a little water got in the line then lodged in the altimeter causing the lag.

Overall I think the Stec 55 is a very good autopilot but offers more than most will ever need. You just about have to have electric/auto trim to really get the full benefit from the system which puts it up near $20,000. I can't say that I have ever shot an ILS with the autopilot as I prefer to hand fly the approach. Without question I think the Stec 30/50 is a great autopilot for a reasonable amount.

We are currently building up a 260SE that will be bound for England and it is getting a Chelton autopilot. Intially the new system looks promising.