201 not stable in pitch ?

[N177MC]

A friend loaned me his '76 201 for a 300NM flight last week.  I had to hand fly since there is no AP onboard. On the leg back, crusiing @ 8000' to 9000', on top and clear of clouds, I found it very difficult to keep the plane stable and was getting +/- 50' oscillations @ 500fpm .

 

Here's a link to the video :

 

https://drive.google.com/file/d/0B3YQ9-lYBiW7eEJuZnhvajJ1NUU/edit?usp=sharing

 

I have'nt flow a Mooney in some time but I always felt they were rock solid in cruise.

 

Any ideas ?

[Super Dave]

With oscillations like that on the VSI and altimiter I would expect to see pitch oscillations visible on the attitude indicator, but this is hard to see on the video. Was the nose pitching up and down? If not, I'd guess static line moisture/blockage.

[carusoam]

How was the weather below you?

Were you in a mountainous area?

Mooneys are as stable as the air surrounding them...

That flight depicted in that video would be disheartening without knowing what was causing it.

Power sounded stable...

Best regards,

-a-

[1964-M20E]

I suspect it is in the instruments

[Mike A]

With oscillations like that on the VSI and altimiter I would expect to see pitch oscillations visible on the attitude indicator, but this is hard to see on the video. Was the nose pitching up and down? If not, I'd guess static line moisture/blockage.

 

+1.  You would be able to feel those types of movements as well.

[John Pleisse]

Even riding sharp leeward mountain waves won't oscillate that quickly. If it were updrafts or mountain waves, you'd see the VSI and altimeter change, but not pitch changes or airspeed changes. The airspeed concurred fluctuating 7 knots with the oscillations and pitch stability was seen on both the Aspen and the back up AI.  I say it's a p/s issue.

[carusoam]

With a 1,000 fpm descent, and power stable in cruise, air speed would be screaming towards red line...

I didn't hear any screams of discomfort?

What did the GPS say? Did the Pitot / static data match?

Best regards,

-a-

[N177MC]

WoW, that's fast feedback !

 

Me thinking p/s as well ...

 

The plane slept outdoors under a full day of rain the day before, perhaps water ingestion in the static line ? 

[carusoam]

There are probably two static drains that you can check...

One on the pilot side wing. The other, on the fuselage, same side behind the wing. I don't fly a J, I could be incorrect...

If you have the opportunity to fly that bird often, it would be greatly helpful to get Mooney specific training.

We have several European Mooney pilots here that can help point you in the right direction...

Welcome aboard friend,

-a-

[John Pleisse]

On the J...... there is a finger pump on the port forward wing drain. Oh....was your pitot heat on climbing up?

 

 

 

With a 1,000 fpm descent, and power stable in cruise, air speed would be screaming towards red line...
 

 

His airspeed fluctuated equally with the altimeter and VSI.

[N177MC]

carusoam 

 

Thanks for the kind words.  I DO have training in the Mooney, it's just been a few years since I last flew PIC in one.

 

I'm also an A&P/IA and will be anualing this particilar 201 next month.  The flights served to work-up a detailed squawk list as well.

 

Last year when I had the bi-annual xponder p/s checks done, I found one of the drains fouled with rust. That would suggest that it is prone to water ingestion.

 

Anyone else see this phenomena ?

[ArtVandelay]

 there is a AD 85-24-03 about the fuel caps that addresses water getting into the tanks, it should be done with every annual.

Last year when I had the bi-annual xponder p/s checks done, I found one of the drains fouled with rust. That would suggest that it is prone to water ingestion.

 

Anyone else see this phenomena ?

[jlunseth]

The drain on the port underbelly behind the wing is the static drain.  The one just in front of the port wing is the pitot drain.

 

I would guess the static line also, because of the way the airspeed is fluctuating.  First, it is fluctuating in rhythm with the VSI and Alt. so it would not seem to be the result of the pilot (you) responding to the Alt. and VSI, it is too fast and too well timed.  Second, the airspeed is responding counter to what would be expected if the aircraft were actually pitching up and down.  In other words, when the VSI and Alt. show an ascent, one would expect the airspeed to fall if the aircraft were actually ascending, and vice versa if the VSI and Alt. show a descent, the airspeed would increase.  But the airspeed is going the opposite direction from what is expected.  So I thought of it this way.  If the pressure in the static line drops then the VSI would show an ascent and the Alt. would show a higher altitude, because they think they have entered into an area of lower pressure.  The airspeed, on the other hand, would show an increase in speed despite the apparent ascent, because it is comparing the pitot pressure, which has not changed, to the static pressure, which has dropped, therefore showing a greater difference and a higher airspeed. 

 

I would guess the pitot line is good and the static line is plugged so hit the button on the belly behind the wing.

 

One other thought is that the Alt. Static switch/knob/system could be opening and closing a small amount, causing the instruments to read a fluctuation in the static line pressure.  The pressure would typically fall if the Alt. Static opened a little because it reads cabin air which is at a slightly lower pressure than ambient air.  I don't know how you would find the issue if that is it.  Good luck.

 

That did give me a thought though.  Could you not diagnose the problem by taking a flight, observing the faulty behaviour, and then opening the Alt. Static system.  One would expect that the altitude would indicate slightly higher and the ASI as well, but they would stablize since the pressure source is one that is not fluctuating.  That would not tell you what is causing it, but would tell you that the static line is where the problem is.  

[N201MKTurbo]

Did you try pulling the alt static knob?

[M20S Driver]

My pitch stability improved after replacing the engine mounts and correcting the drooping prop on my M20S.

M20S Driver

[cleatus99]

Looks like static water/clog unless you felt ups and downs

[Bob - S50]

After checking all elevator and trim connections for excessive play, I'd try flying it again.  I've run into some 'mountain wave' in eastern Canada where there were no mountains.  I was level at about FL330 in a DC9 on autopilot and noticed some variation in altitude.  I then noticed the elevator trim cycling back and forth.  I though I had a trim problem so I disconnected the autopilot.  The trim movement stopped but I had a hard time maintaining altitude while hand flying.  I put the autopilot back on and in about 10 minutes the problem went away after we flew through the area.

 

Bob

[N177MC]

I would guess the static line also, because of the way the airspeed is fluctuating.  First, it is fluctuating in rhythm with the VSI and Alt. so it would not seem to be the result of the pilot (you) responding to the Alt. and VSI, it is too fast and too well timed.  Second, the airspeed is responding counter to what would be expected if the aircraft were actually pitching up and down.  In other words, when the VSI and Alt. show an ascent, one would expect the airspeed to fall if the aircraft were actually ascending, and vice versa if the VSI and Alt. show a descent, the airspeed would increase.  But the airspeed is going the opposite direction from what is expected.  So I thought of it this way.  If the pressure in the static line drops then the VSI would show an ascent and the Alt. would show a higher altitude, because they think they have entered into an area of lower pressure.  The airspeed, on the other hand, would show an increase in speed despite the apparent ascent, because it is comparing the pitot pressure, which has not changed, to the static pressure, which has dropped, therefore showing a greater difference and a higher airspeed. 

 

I would guess the pitot line is good and the static line is plugged so hit the button on the belly behind the wing.

 

 

 

 

I believe Jlunseth nailed it above.

 

I did pull the alternate air in flight and was a bit surprised by the rather large jump on the gauges, much more than usual. This would suggest a static problem.

 

I was referencing the static drain that was rusty last year (not the fuel drains!) which would suggest that this plane is prone to water ingestion in the static circuit.

 

I will be at the hangar today and will purge the static line and report back.

 

Thanks for the suggestions !  

[carusoam]

When you get a chance...

Describe a little more of what you see in and around the static drain.

I have experience with storing Mooneys indoors and outdoors. Fortunately, I have not drained water out of the P/S system...

Yes they have drains for a reason, but how often are people getting anything to drain out of them?

Be cautious because rust can cause difficulty with the drain's ability to seal.

As usual, I am a PP not a mechanic...

Best regards,

-a-

[jlunseth]

 

"I did pull the alternate air in flight and was a bit surprised by the rather large jump on the gauges, much more than usual. This would suggest a static problem."

 

 

Probably just terminology, but in my 231 you would need to pull the Alt. Static not the Alt. Air.  The Alt. Air opens and closes a door that normally allows outside air into the induction intake.  Closing it reduces the aircraft's ability to make manifold pressure to some degree because the engine is using heated intake air, but eliminates problems with the air intake filter plugging usually due to ice crystals in the atmosphere, or possibly also snow.  The Alt. Static is the knob that would switch the static source from the opening in the exterior skin of the back of the fuselage, to a source inside the cabin.  The Alt. Air, in other words, does nothing to or for the static source, that would be the Alt. Static.  Maybe the J does not have an Alt. Air, the problem it addresses is mostly a high altitude problem. 

 

At any rate, it looks like we collectively got lucky and figured out where your problem is, hope you can get it addressed easily.