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

Also looks like you had strong headwinds…30kts? Were you downwind of any mountains? Opening the alternate static may create some damping of the pressure changes caused by mountain waves…

Last Edited by AnthonyQ at 08 Sep 15:27
YPJT, United Arab Emirates

This is not related to the location at all.

Administrator
Shoreham EGKA, United Kingdom

I have now made a better video


If you watch it carefully, you can see that when the EHSI shows a pitch UP, the VSI and the altimeter both move downwards.

This is obviously wrong, and given the simplicity of the static system it probably does point at a static pipe blockage.

There is something like a 2-3 second lag in the altimeter and VSI response, and the two track each other quite well.

However, there must always be some lag in that pipework. In fluid flow terms, the airframe manufacturer needs to find a compromise between

  • using a too-thin pipe and suffering a propagation delay due to friction / viscosity effects
  • using a too-thick pipe and suffering a propagation delay due to the mass of gas which has to be shifted

I wonder if anyone can go up for a flight and try very small but quick pitch up/down movements, and check whether the altimeter and the VSI track the pitch change immediately. I bet they won’t; the Q is whether any lag is visible like it is in my case.

An even better video is here


Administrator
Shoreham EGKA, United Kingdom

Avionik Straubing replaced the “pitch tach” in my pitch servo in 90 min for €550 plus two hours’ labour on my way home from LGKJ. Completely fixed. I’ll go there again.

Spending too long online
EGTF Fairoaks, EGLL Heathrow, United Kingdom

I am now 100% sure it isn’t the servo, because

  • in PIT mode it doesn’t happen
  • there is the antiphase behaviour seen in the videos

I wonder if anyone going for a flight can confirm whether they see a similar antiphase behaviour, for small changes in pitch, made at about 0.3Hz?

The tacho in King servos is a Faulhaber tacho which costs about £30; I wish it was that simple. If that tacho wears out (and it does) you get really big pitch oscillation, in all autopilot modes.

If that antiphase behaviour is normal (I can’t say it isn’t – never looked at it that closely) then it could be something specific to the ALT mode: KEA130A altimeter, or something in the KC225 computer. I have spares for both.

Administrator
Shoreham EGKA, United Kingdom

The wizard to contact in this case is Mr. Martin Scheifl, at Avionik Straubing.

Safe landings !
EDLN, Germany

At FL100, I see a 10kt speed difference between normal static and alternate static.

Is that normal?

There is about a 200ft altimeter difference, which I recall as being normal.

I have yet to try to blow the static line out with compressed air… will try to do that at the next 50hr service.

Administrator
Shoreham EGKA, United Kingdom

FWIW my a/c has a placard(per poh) which says:
Close canopy when using alternate static air. subtract 6 kts from IAS above 87 kts IAS. Subtract 80 ft from alt above 86 kts.

So not unreasonable?

EGNS, Other

200ft altitude delta is roughly equal to the delta in dynamic pressure from 120 to 138 kias. Hence the IAS error due to static pressure error should be a bit higher than 10 kias for 200 ft altitude error

Both my mechanical altimeter and VSI lag one or more seconds behind my AI.

I find your out-of-phase AI vs ALT and VSI behaviour perfectly normal. VSI has a restrictor within itself anyway so a restriction outside it wont make a difference unless it is of similar size (ie one order of magnitude smaller in area than the sum of your static ports)

Be very careful when you blow static lines that there is nothing connected to them. Mine has autopilot altitude sensor, alt pressure switch as well as all instrumentation at different places in the static line…so you have to make sure everything is disconnected or you will surely damage it.

Antonio

Last Edited by Antonio at 05 Oct 21:28
Antonio
LESB, Spain

I have done a proper test. At 4500ft, qnh 1008, +2C, I get a delta of 10kt and 100ft when pulling the alternate static to the OPEN position.

At higher altitudes it seems to be a lot more but I haven’t written down the numbers.

I am going to buy new static tubing, because the only proper maintenance opportunity I have is at the Annual when we rent a hangar for it where work is permitted, and time will be limited, so I may as well change the whole run front to back.

Is there a commonly used tubing size? From inspection it looks like 3-4mm ID.

It is obvious that if one goes too thin, there will be a heavy lag due to the viscosity of the air slowing down the progress of the pressure change. And if one goes too large a diameter there is will be a heavy lag due to the volume which has to be shifted.

I have been doing googling and there are reports of some AoA sensors using ~1mm ID tubing and this must be for a reason, too. In the TB20 IPC I see

and Y UNION YS6 does google, but only to another proprietary Socata P/N. I can climb in the back and measure the tube…

Administrator
Shoreham EGKA, United Kingdom
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