Menu Sign In Contact FAQ
Banner
Welcome to our forums

KFC225 pitch oscillation, alternate static error, and static tube internal diameter

My plan was to re-run it with series 6000 hose, and this comes in 1/4" as the smallest ID. Or maybe the 306 hose which has a bigger choice.

The Socata stuff is a cloth-covered stuff behind the panel and it looks like ~6mm ID (the static hose is the black one at the top)

but the pipe run from the two static vents to the front, maybe 5m of hose, is a cream coloured plastic, not transparent, and I think it will perish. Anyway I am pretty sure there is a blockage in that pipe run.

Just to re-state something higher up: the oscillation stops immediately if I engage the PIT mode (not ALT mode, which does show it) OR I open the alternate static. I think that pretty well removes the whole autopilot system from the equation.

The vacuum pump related pipework was re-done with the series 306 hose years ago.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

My plan was to re-run it with series 6000 hose, and this comes in 1/4" as the smallest ID

As I can see is no point to talk mm those are imperial as usual.
Why you don’t use 306 Soft Hose since you are not home-build
Are certified hoses for the purpose and you have lots of dimensions to choose from.
It says “Suitable for instrument and vacuum lines only. Sold per foot.”
After this, a check and eventually calibration of the system will be good.
Good luck ! I hate to do work under the panel
Peter wrote:

oscillation stops immediately if I engage the PIT mode

I agree it may be some bug/impurity on the lines.

ES?? - Sweden

Just heard from Socata… it is 1/4".

Administrator
Shoreham EGKA, United Kingdom

The existing hose runs through the centre section and is inaccessible. The only way to get into that cavity is by opening up some covers e.g. the piece of metal with the headset socket on it, and then you can see the pitot and static hoses, but you can’t really get at them. That is the only part of a TB20 which has stuff in it and which is virtually impossible to get to. Same for the pitot hose BTW.

So we ran a whole new one, plugged the old one at the end we could get to, all the way from back to front, and joined it behind the LH instrument cluster. We used the 306-4 hose. Same ID as Socata original – 1/4".

I will find out on the next high altitude flight. The problem doesn’t show below about 5000ft.

BTW there was no blockage in the usual area i.e. near the back of the plane. If this is a blockage, and done by insects, they went a long way forward!

Administrator
Shoreham EGKA, United Kingdom

Test flight done. Very interesting results.

I went to FL074 (the highest practical around here, VFR) and could not get any pitch oscillation at all. Previously it was visible even at 5000ft, just, and more so higher up.

More interesting is the effect of the alternate static: Above I wrote “At 4500ft, qnh 1008, +2C, I get a delta of 10kt and 100ft when pulling the alternate static to the OPEN position.” Today, the numbers are just 3kt and 40ft (QNH 1014, 4500ft -3C). I wonder what that is telling us. It isn’t obviously consistent with just a blockage in the static pipe… especially as the few m of old pipe which we extracted from the plane didn’t have any blockages! I wonder if there was a crack in the pipe? Such a crack would have been exposing the inside to air under the belly panels which is a different pressure environment to the cockpit and different to the outside.

Administrator
Shoreham EGKA, United Kingdom

Unfortunately replacing the static tubing has not eliminated this issue. It merely improved it considerably. I thus suspect there was something wrong with the old pipes; perhaps a split (we did not see any obstructions).

Also there is a little bit of oscillation if the alternate static is used.

I now suspect a subtle fault in the KC225 computer and particularly its internal barometer. I know (from one of the design team) that it doesn’t act on the gray code altitude (from the KEA130A) unless it is seen to change, which is only every 100ft. If the KEA130A was the problem, we ought to be able to eliminate the oscillation by implementing very small changes (a few tens of ft) in altitude, to place the current altitude in between the gray code change points. The KC225 has an adjustment feature like that (10 or 20ft steps). But this doesn’t work.

As before, flying in PIT instead of ALT removes the issue completely, so it can’t be the servos etc. On a calm day one can fly like that for quite a long time (minutes) and it does give another ~1kt speed, which is not surprising.

Administrator
Shoreham EGKA, United Kingdom

The KC225 computer has been swapped (I had two spares which I managed to buy, with fresh Honeywell overhaul docs, for peanuts, about 10 years ago) and the oscillation appears to have disappeared, on a FL080 test flight.

Only more tests, to say FL120, will prove it.

Interesting…

If this is true, the fault must have been a change in the control loop parameters, or the response of the internal barometer. Something very subtle, and most likely a factory refurb won’t fix it – because it will pass all the tests.

Administrator
Shoreham EGKA, United Kingdom

Update: I had the FAR 145 altimeter check done last week and there is no leak. In fact the leakage was found to be just 4fpm; the previous test 2 years previously came out at 15fpm and the limit (for unpressurised aircraft) is 100fpm.

So the mystery continues. The issue is much less than it used to be, after we swapped the main part of the tubing, all the way from the back where the two vents are, to behind the instrument panel.

It looks like this leaves just a blockage/kink in a pipe behind the panel (and these are visually inspectable) or something in the KEA130A encoding altimeter or possibly even the KI256 horizon. But it is visible only in a specific altitude range of about 7000ft to 14000ft.

Administrator
Shoreham EGKA, United Kingdom

This is a known issue with the Jetprop KFC225 Installation.
In flight I often have to switch to the alt static port due to the very same oscillation. Very annoying.

The problem according to a very reliable engineer on Jetprop’s, is the hot air from the exhaust stacks corkscrewing around the airframe under certain wind conditions and affecting the normal static ports.
My alt static ports are under the rear belly at the back and free from this disturbance.
I suffer the same discrepancy, a 5-7 knot difference in airspeed when using the alt ports so my SOP now is to switch back to the normal ports at TOD.

One of those things I have had to learn to live with.
Perhaps something has changed with your exhaust routing recently which may explain it in your case?

E

eal
Lovin' it
VTCY VTCC VTBD

It would amaze me if it was the prop, because that is going around at some 2500rpm while the period of this oscillation is of the order of 10-20 seconds.

I am sure it is the pitch control loop having marginal stability.

The solution (which for me is not completely effective) of opening the alternate static vent is IMHO working because it changes some control loop parameter, incidentally.

It can’t be the wind because a plane is not aware of the wind.

I’ve heard many stories of STEC autopilots doing this, but never the KFC225.

What I might do, on the next Annual, is to replace all the pipework behind the instrument panel. If there is a blockage there, that would explain it.

Administrator
Shoreham EGKA, United Kingdom
Sign in to add your message

Back to Top