Manifold Pressure/Gear Warning Malfunction

[smccray]

I flew to Houston yesterday and had an interesting problem when during an approach to landing at KTME.  I transitioned from cruise to the approach and the manafold pressure gauge started reading nearly off the gauage low clearly giving a bad indication.  The gear warning beep also didn't work when it got below 12 in.  We confirmed that the engine was still making power and the rest of the gauages all read normal, so we quickly decided it was a MP gauage issue.

When departing KTME, we did a normal runup and we didn't have any issues on the next two flights of the day.  The gear warning also worked on the next flight.

Any ideas what happened?

[Parker_Woodruff]

Scott,

Unless there is something different with your gear warning setup on a 205, your gear horn should be activated by the throttle moving within about 1/4  inch of idle cutoff, not based on a MP readout.

It can also be activated by lower airspeed (around 60 knots, IIRC).

[smccray]

Quote: Parker_Woodruff

Scott,

Unless there is something different with your gear warning setup on a 205, your gear horn should be activated by the throttle moving within about 1/4  inch of idle cutoff, not based on a MP readout.

It can also be activated by an airspeed of about 55 KIAS.

[MooneyMitch]

Hi Scott.

Based on what you are saying, and an experience I've had with an out of control MP gage, it's sounds as though you may have a leak in the MP line between the gage and the engine.  If this is so, chance are your problem will re-occur in the near future.

You might carefully check the integrity of the line from end to end, along the tubing and at each connector.  Gently wiggle and pull the line and at each fitting along the route.  Let us know.

[smccray]

Quote: Mitch

Hi Scott.

Based on what you are saying, and an experience I've had with an out of control MP gage, it's sounds as though you may have a leak in the MP line between the gage and the engine.  If this is so, chance are your problem will re-occur in the near future.

You might carefully check the integrity of the line from end to end, along the tubing and at each connector.  Gently wiggle and pull the line and at each fitting along the route.  Let us know.

[MooneyMitch]

You may be correct about that.  I really don't recall the indications whether they were up or down or a combination thereof.  I just remember seeing the gage behaving irratically. 

I do remember that it felt kind of like when you step on the gas pedal when the traffic signal turns green, and your car doesn't go anywhere. 

[MooneyMitch]

--------

[N601RX]

The throttle shaft will get some wear on it and can be moved from side to side especially when pulled all the way out.  Next time you have it all the way out try pushing it side to side and see it the horn goes off or stops going off.  Sometimes the switch can be adjusted to correct for this.

[sleepingsquirrel]

I found this in the Mooney service notes concerning the gear warning;

http://www.mooney.com/images/pdfs/sb-pdf/sbm20-95.pdf

 

[smccray]

Just thought I'd follow up on this...

Don Maxewell looked under the cowl a couple weeks ago and and addressed his preliminary hypothesis about the MP and found one other problem that was fixed/possibly related.

1. 
   
2. From time to time the hard line on the right side of the engine that leads to the MP gauage gets clogged.  There should be a small hole in the hard line near the number 1 cylinder, but if the like gets clogged (oil/gunk/etc.), or if the small hole is clogged/missing, it's possible that the MP line is blocked causing the problem that I experienced.  The solution was to remove the MP line and clean it after making sure the small hole was there- apparently it's not uncommon for this vent hole to be missing altogether.
   
3. The hard line transitions to a flexible line between the engine and the firewall.  There should be some play in this line, but the length of the line on my plane was too long.  I'm not sure how to describe it other than to say that there was large droop in the hose that Don felt could accumulate gunk (yes that's the technical term) and block the line.  The solution was to shorten the line between the hard line mounted to the engine and the firewall.
   

No solution for the gear warning horn issue as it doesn't seem to occur regularly, but I did get down underneith the panel and it doesn't appear that there's any play in the throttle that would keep the switch from engaging.  I haven't tried the flight test Mike suggested above but I'll put that on my list for my Sunday flight.

[Mooneymite]

Quote: smccray

Just thought I'd follow up on this...

Don Maxewell looked under the cowl a couple weeks ago and and addressed his preliminary hypothesis about the MP and found one other problem that was fixed/possibly related.

From time to time the hard line on the right side of the engine that leads to the MP gauage gets clogged.  There should be a small hole in the hard line near the number 1 cylinder, but if the like gets clogged (oil/gunk/etc.), or if the small hole is clogged/missing, it's possible that the MP line is blocked causing the problem that I experienced.  The solution was to remove the MP line and clean it after making sure the small hole was there- apparently it's not uncommon for this vent hole to be missing altogether.

The hard line transitions to a flexible line between the engine and the firewall.  There should be some play in this line, but the length of the line on my plane was too long.  I'm not sure how to describe it other than to say that there was large droop in the hose that Don felt could accumulate gunk (yes that's the technical term) and block the line.  The solution was to shorten the line between the hard line mounted to the engine and the firewall.

No solution for the gear warning horn issue as it doesn't seem to occur regularly, but I did get down underneith the panel and it doesn't appear that there's any play in the throttle that would keep the switch from engaging.  I haven't tried the flight test Mike suggested above but I'll put that on my list for my Sunday flight.

[carusoam]

Mooneymite,

On my 65 C (all M20Cs?) There were two switches that indicate a gear up situation.

[1] J-Bar Latch, located in the J-Bar receiver under the instrument panel.  Look for switches, bring your contact cleaner.

J-Bar in and locked - switch is satisfied (light=green, no horn),  Switch not satisifed (light=amber, horn ready)

[2] Throttle switch, located behind the instrument panel (throttle in, switch is satisfied) throttle out, switch falls into detent in throttle control, horn ready.

[3] Combination of gear not latched and throttle out activates horn.  This can be annoying if you are trying to lose altitude or slow to VLe by pulling the throttle out.  Most people will temporarily put their finger on the switch in the J-Bar receiver or bump the throttle in slightly to satisfy one of the switches.

There is no MP switch on this simple go/no-go system.

As far as the tiny vent hole in the MP line.  If it is clogged, the MP will be drawn way low off scale when the engine is idling. 

Best regards,

-a-

[Mooneymite]

Quote: carusoam

Mooneymite,

On my 65 C (all M20Cs?) There were two switches that indicate a gear up situation.

[1] J-Bar Latch, located in the J-Bar receiver under the instrument panel.  Look for switches, bring your contact cleaner.

J-Bar in and locked - switch is satisfied (light=green, no horn),  Switch not satisifed (light=amber, horn ready)

[2] Throttle switch, located behind the instrument panel (throttle in, switch is satisfied) throttle out, switch falls into detent in throttle control, horn ready.

[3] Combination of gear not latched and throttle out activates horn.  This can be annoying if you are trying to lose altitude or slow to VLe by pulling the throttle out.  Most people will temporarily put their finger on the switch in the J-Bar receiver or bump the throttle in slightly to satisfy one of the switches.

There is no MP switch on this simple go/no-go system.

As far as the tiny vent hole in the MP line.  If it is clogged, the MP will be drawn way low off scale when the engine is idling. 

Best regards,

-a-

[Hank]

It has always been my understanding that the gear warning horn is activated by a microswitch attached to the throttle quadrant, and is not related to anything on the MP gage itself. The switch is adjustable by referencing the MP gage, but it is either on the cable itself or inside the quadrant and activated by the throttle lever.

[Hank]

Quick Reply--submit--weird blinky screen--error--refresh! Wa-laa, double post!!

[xftrplt]

Quote: Hank

It has always been my understanding that the gear warning horn is activated by a microswitch attached to the throttle quadrant, and is not related to anything on the MP gage itself. The switch is adjustable by referencing the MP gage, but it is either on the cable itself or inside the quadrant and activated by the throttle lever.

[Mooneymite]

Quote: Hank

It has always been my understanding that the gear warning horn is activated by a microswitch attached to the throttle quadrant, and is not related to anything on the MP gage itself. The switch is adjustable by referencing the MP gage, but it is either on the cable itself or inside the quadrant and activated by the throttle lever.

[smccray]

Quote: Mooneymite

Aha!  Hank, it appears that you are correct.  I was led down the wrong path by the verbiage in the POH.

I just went out and disassembled the throttle quadrant covers.  There deep in the cavity behind the quadrant shaft, was a small micro-switch activated by a lobe on the throttle lever.  It appears very difficult to access should it need to be replaced; tomorrow I will see if there is any way to apply contact cleaner without a total disassembly.

With a little luck I can check the switch continuity and see if the switch is the problem, or if the gremlin is elsewhere.

Anyone have any luck with this?

 

[tony]

Quote: Hank

It has always been my understanding that the gear warning horn is activated by a microswitch attached to the throttle quadrant, and is not related to anything on the MP gage itself. The switch is adjustable by referencing the MP gage, but it is either on the cable itself or inside the quadrant and activated by the throttle lever.

[jetdriven]

Nope, in an early J at least, the microswitch in the throttle quadrant triggers the horn.  It activates below about 1/4" of throttle. 

Quote: xftrplt

I agree, at least in my 231.  When I bought 888BC, the horn came on too early for my taste, and, neither knowing nor careing what the specs were, it was an simple mechanical adjustment, unrelated to MP, to "correct" it.  Maybe on NA engines it's different.