1976 M20F ram air creeping open uncommanded while in flight

[Vance Harral]

1976 M20F with factory ram air.  Prior to last year's annual, we had an issue where the ram air warning light was illuminating, because the cable sheath that is supposed to trip the associated microswitch only just barely engaged when the control knob was pushed full forward to close the door.  The door itself was staying sealed, but after just a few minutes of flying and the tiniest amount of vibration, the cable sheath would move a hundredth of an inch or so, slide off the microswitch, and set off the warning.  Those of you who've fiddled with this system know the position of the microswitch is adjustable.  But it had already been adjusted as far aft as possible, so fixing the problem required something other than re-positioning the microswitch.

During debug at the annual, we noticed that the ram air knob was a good 1/2" or so aft of the panel when pushed forward to the limit of travel.  Seeing an obvious fix, we disconnected the other end of the cable in the engine compartment where it's attached to the ram air door actuator, slid it forward a little less than 1/2", and re-attached.  Voila!  plenty of travel of the cable sheath to actuate the ram air warning microswitch when closed, and the ram air knob actually lined up with the cowl flap knob when both were closed to boot.  Awesome!

"While we were in there", we cleaned and lubricated the ram air door actuator mechanism (it's quite the Rube Goldberg apparatus).  Opening the ram air door had required significant force and felt "gummy", but with appropriate cleaning and lubrication, everything worked the way it seemed like it was supposed to.

No good deed goes unpunished, I guess.  Over the last couple of months, we're having an increasingly occurring problem where the ram air door eases open in flight.  This time, it's definitely the door opening that's the issue - landing without trying to fix the warning and inspecting the cowl reveals the ram air door to be slightly open, with the gap closing when the ram air control knob is pushed forward.  It's gotten bad enough now that at max cruise speed, you can push the knob full forward, and watch it slide back about 1/2 inch all by itself over a period of just a couple of seconds.

It's hard to believe the cable adjustment we made would cause this, though I guess anything's possible.  Rather than that, my guess is that the ram air actuator mechanism has some sort of over-center apparatus which is supposed to hold the ram air door closed against air pressure, and ours is mal-adjusted; but that adjustment problem was obscured by the entire mechanism being gummy prior to working on it.  Guessing that now that everything is lubricated, it's easier for the force of air hitting the door to push it open.

Anyone been through this and care to offer tips and/or pictures?

[Ragsf15e]

3 hours ago, Vance Harral said:

1976 M20F with factory ram air.  Prior to last year's annual, we had an issue where the ram air warning light was illuminating, because the cable sheath that is supposed to trip the associated microswitch only just barely engaged when the control knob was pushed full forward to close the door.  The door itself was staying sealed, but after just a few minutes of flying and the tiniest amount of vibration, the cable sheath would move a hundredth of an inch or so, slide off the microswitch, and set off the warning.  Those of you who've fiddled with this system know the position of the microswitch is adjustable.  But it had already been adjusted as far aft as possible, so fixing the problem required something other than re-positioning the microswitch.

During debug at the annual, we noticed that the ram air knob was a good 1/2" or so aft of the panel when pushed forward to the limit of travel.  Seeing an obvious fix, we disconnected the other end of the cable in the engine compartment where it's attached to the ram air door actuator, slid it forward a little less than 1/2", and re-attached.  Voila!  plenty of travel of the cable sheath to actuate the ram air warning microswitch when closed, and the ram air knob actually lined up with the cowl flap knob when both were closed to boot.  Awesome!

"While we were in there", we cleaned and lubricated the ram air door actuator mechanism (it's quite the Rube Goldberg apparatus).  Opening the ram air door had required significant force and felt "gummy", but with appropriate cleaning and lubrication, everything worked the way it seemed like it was supposed to.

No good deed goes unpunished, I guess.  Over the last couple of months, we're having an increasingly occurring problem where the ram air door eases open in flight.  This time, it's definitely the door opening that's the issue - landing without trying to fix the warning and inspecting the cowl reveals the ram air door to be slightly open, with the gap closing when the ram air control knob is pushed forward.  It's gotten bad enough now that at max cruise speed, you can push the knob full forward, and watch it slide back about 1/2 inch all by itself over a period of just a couple of seconds.

It's hard to believe the cable adjustment we made would cause this, though I guess anything's possible.  Rather than that, my guess is that the ram air actuator mechanism has some sort of over-center apparatus which is supposed to hold the ram air door closed against air pressure, and ours is mal-adjusted; but that adjustment problem was obscured by the entire mechanism being gummy prior to working on it.  Guessing that now that everything is lubricated, it's easier for the force of air hitting the door to push it open.

Anyone been through this and care to offer tips and/or pictures?

Jeez that sucks.  I swear mine (‘68F) has a “squeeze” mechanism in the handle that you have to pull to make it move either direction.  Ill look next time.

[ArtVandelay]

I would consider finding the intake hardware from a J and then removing the ram air, which has little effect on the J and was removed in later years.

[Vance Harral]

6 hours ago, ArtVandelay said:

I would consider finding the intake hardware from a J and then removing the ram air, which has little effect on the J and was removed in later years.

That's a $10,000 solution to a $10 problem.  Requires a new cowl, new intake plumbing, etc.  I'd rather just silicone the door shut, legalities aside.

I'm aware the J model had essentially zero ram-air benefit, but that's not true of older models like ours.  Opening ram air not only bypasses the air filter, but also achieves a straight shot into the fuel servo vs. the windy turns through the normal induction path through the filter.  Good for somewhere between 1/2" and 1" of MP, and a few knots, depending on who you believe.  I don't particularly care about it, as it doesn't produce a meaningful difference in cruise speed or climb rate IMO.  But I'm the kinda guy that wants everything that's installed on the airplane to actually work as designed.

[carusoam]

Some control knobs have a twist part that increases friction…

Cleaned and lubed removed all of the friction…

See what it takes to add a controlled amount back in… 

Most push pull controls must have a way to adjust friction, or else they would all do this mis-behavior… (?)

PP thinking out loud… not a mechanic… 

Best regards,

-a-

[ArtVandelay]

That's a $10,000 solution to a $10 problem.  Requires a new cowl, new intake plumbing, etc.  I'd rather just silicone the door shut, legalities aside.
I'm aware the J model had essentially zero ram-air benefit…

Not sure about the cowl, on my J we just patch the hole. Why not do that on the F?
I figure the intake hardware can be found by one of the salvage sites or members on MS, so not super expensive.
The improved intake hardware on the J is why there’s no benefit, should work on the F as well unless there’s more to it.

[N201MKTurbo]

Does your cable knob have a button? If it does, it should Hold the cable in place when the button is released. If it doesn’t, then the ball fell out of it. It is an 1/8 inch ball bearing.

[Ragsf15e]

2 hours ago, N201MKTurbo said:

Does your cable knob have a button? If it does, it should Hold the cable in place when the button is released. If it doesn’t, then the ball fell out of it. It is an 1/8 inch ball bearing.

Yeah that’s the part I was talking about! You sure it’s 1/8” because mine fell out once and I eventually found it with a damn microscope.  It’s a really tiny ball!

[N201MKTurbo]

6 minutes ago, Ragsf15e said:

Yeah that’s the part I was talking about! You sure it’s 1/8” because mine fell out once and I eventually found it with a damn microscope.  It’s a really tiny ball!

I’m pretty sure it’s 1/8 inch. I have 99 of them at the hangar. I needed one once and had to buy 100 from McMaster.

[skydvrboy]

I have this exact same problem in my F model. I bought the balls only to have the mechanic tell me the ball’s still there. After tinkering with this during two annuals, the problem remains.  The door just blows open anytime my IAS gets over about 140.  It’s not really a problem on takeoff or climb, but opens in cruise and descent.  If anyone gets this fixed, I’d like to know what finally solved the problem.

[Yetti]

Mine has a lock on the knob.   Or what the PM calls a Catch.

Right angle post on knob Turns into a right angle catch (like spring steel) that is part of the cable assembly

Item 91

 

 

Edited January 10, 2023 by Yetti

[Vance Harral]

No catch/button/friction lock on our airplane.  Per the IPC, later models had a different knob, which is simple and smooth, see attached photo.  This was taken before the adjustment, and you can see about 1/2" of exposed cable sheath with the knob pushed as far forward as it will go.

 

 

 

[Yetti]

Well the Parts manual 205 section that I got the catch number from was on page 206 and was listed for 1975 and up.   Mine was built in dec 1975 and is a 1975.   The catch seems to be working as designed.

[Vance Harral]

Yeah, at this point I wish we had the catch.  The IPC page you posted lists two entries for item #85, which is the knob.  The first is P/N 640282-007 for effectivity note "X" (see below), which is presumably the knob with the catch.  The second is P/N #278 for effectivity note "XI", which is the smooth knob on our airplane.

 

[M Terry]

In My F the ram air has no button or friction on the control lever. The door is held closed by pressure from the outside air by the lever going "over center"

I think if you adjust the cable at its attachment point that should solve the problem.

 

[RobertGary1]

The 76 F had a different ram air system that earlier Mooneys. The door has an over center lock on it.

You are lucky though. Usually the ram air is so stiff its almost impossible to use. 

[Vance Harral]

Thanks guys.  Sounds like the over-center mechanism isn't actually going over center when the knob is all the way forward.  I'm hoping the fix is as simple as adjusting the cable in such a way as to find a sweet spot between the over-center mechanism engaging as designed, but not getting false ram air warnings.

[RobertGary1]

3 hours ago, Vance Harral said:

Thanks guys.  Sounds like the over-center mechanism isn't actually going over center when the knob is all the way forward.  I'm hoping the fix is as simple as adjusting the cable in such a way as to find a sweet spot between the over-center mechanism engaging as designed, but not getting false ram air warnings.

Yes I recall some fiddling too to get that spot.