Alternate air use

[VetRepp]

For the IFR experts,

When do you use Alternate Air and / or Alternate Static on IFR flights?

In any visible moisture at +/-10 C Freezing?

In heavy rain conditions? (in all rain conditions?)

What determines your decision of when to use or not?

Thanks

 Roger

 

[201er]

I’m no expert but I never use it unless there’s a suspicion of a misreading, then I might give it a tug and check what I get and then switch back.

[kortopates]

Almost never. But the main exception is to verify that the alternate air door knobs is working correctly on the ground during the run-up by pulling it and seeing the Alt Air annunciator come on and then see it go out when you push the knob. (be sure to do this on a clean ramp, and never on a dirt runway).

Otherwise the alternate air door is reserved for emergency procedures dealing with power loss; either full or partial power loss suspected due to manifold pressure loss. But the alternate air door will/should open automatically before you need to manually open it if there really is induction blockage (ice or snow in the inlet and any blockage in the air filter).

Its not a good idea to open it prophylactically because the warmer induction air will cause a small drop of 1-2" of manifold pressure just by nature of losing the ram air effect and the warmer air. Also of course its unfiltered air bypassing the air filter. Heavy rain in itself doesn't cause an induction issue. But if you really believe it is, the long induction hose (if its the original Mooney spec'd part #) has a water drain at it lowest point to allow water to drain out before going into the planes induction system past the filter.  (but beware many though have replaced their worn Mooney alternate induction tube with cheaper Scat tubing that doesn't have this drain installed in it) 

Alternate Static is a similar in that it should only be used when required per the POH. But its use may not be limited to icing conditions but also if moisture has gotten into the static port and make its way into static lines. This isn't limited to rain but includes washing the aircraft and not being careful to not allow water into the static port. Opening the static port in the cabin will help unstick a sticky altimeter/airspeed  etc. till you can get it drained. Back on the ground, pushing the static drain alone may not release the water till someone else "gently" blows into the static port to move the water to the drain. If you do use the alternate static port in the cabin, remember there is a correction table in the POH to consult.

Edited April 11, 2019 by kortopates

[buddy]

59 minutes ago, kortopates said:

Almost never. But the main exception is to verify that the alternate air door knobs is working correctly on the ground during the run-up by pulling it and seeing the Alt Air annunciator come on and then see it go out when you push the knob. (be sure to do this on a clean ramp, and never on a dirt runway).

Otherwise the alternate air door is reserved for emergency procedures dealing with power loss; either full or partial power loss suspected due to manifold pressure loss. But the alternate air door will/should open automatically before you need to manually open it if there really is induction blockage (ice or snow in the inlet and any blockage in the air filter).

Its not a good idea to open it prophylactically because the warmer induction air will cause a small drop of 1-2" of manifold pressure just by nature of losing the ram air effect and the warmer air. Also of course its unfiltered air bypassing the air filter. Heavy rain in itself doesn't cause an induction issue. But if you really believe it is, the long induction hose (if its the original Mooney spec'd part #) has a water drain at it lowest point to allow water to drain out before going into the planes induction system past the filter.  (but beware many though have replaced their worn Mooney alternate induction tube with cheaper Scat tubing that doesn't have this drain installed in it) 

Alternate Static is a similar in that it should only be used when required per the POH. But its use may not be limited to icing conditions but also if moisture has gotten into the static port and make its way into static lines. This isn't limited to rain but includes washing the aircraft and not being careful to not allow water into the static port. Opening the static port in the cabin will help unstick a sticky altimeter/airspeed  etc. till you can get it drained. Back on the ground, pushing the static drain alone may not release the water till someone else "gently" blows into the static port to move the water to the drain. If you do use the alternate static port in the cabin, remember there is a correction table in the POH to consult.

I agree totally with Paul, great advice. The only thing I would add is if you do see the alternate air annunciator light come on in flight and your in icing conditions make sure the alternate air knob is pulled all the way out, not just partially open.

Buddy

[elimansour]

I have a ‘98 J without the Alt Air knob - only the automatic opening valve.  On a recent VFR flight in warm in warm weather, the amber Alt Air annunciator light was flickering.  Any idea what would cause that?

[Ragsf15e]

8 minutes ago, elimansour said:

I have a ‘98 J without the Alt Air knob - only the automatic opening valve.  On a recent VFR flight in warm in warm weather, the amber Alt Air annunciator light was flickering.  Any idea what would cause that?

Faulty sensor or loose door spring.  Shouldn’t open unless you get significant induction ice.

[chrisk]

Alternate air would be used if there was something that obstructed the air filter.   Ice/snow comes to mind.

And the alternate air door should open on its own if necessary.   At least on a 1981 M20K.  I believe the very first K models had an issue with ice obstruction.

 

[jaylw314]

2 hours ago, elimansour said:

I have a ‘98 J without the Alt Air knob - only the automatic opening valve.  On a recent VFR flight in warm in warm weather, the amber Alt Air annunciator light was flickering.  Any idea what would cause that?

I have an '86 J, and there is no Alt Air knob or annunciator?

[bradp]

I have just the automatic spring loaded gate  for alternate air.  One thing I do is check the action of the spring loaded door when the lower cowling is off. Make sure it moves freely.  

For alternate static only in an emergency.  It’s checked with my instrument flow/checklist to make sure the valve works.  That’s it.  

Dont forget to occasionally check the alternate static and pilot tube drains.  Common cause of failed IFR checks. 

[moosebreath]

Be prepared for an exciting ride if you open the alternate static source while the autopilot altitude hold is engaged.  Try it in VFR to see what happens.

[DXB]

On 4/11/2019 at 1:07 PM, kortopates said:

Heavy rain in itself doesn't cause an induction issue. But if you really believe it is, the long induction hose (if its the original Mooney spec'd part #) has a water drain at it lowest point to allow water to drain out before going into the planes induction system past the filter.  (but beware many though have replaced their worn Mooney alternate induction tube with cheaper Scat tubing that doesn't have this drain installed in it) 

In my  carb'd plane (C model), the alternate air is the carb heat.  I generally use partial carb heat to keep my carb temp in cruise above 40F in IMC, rain, or a humid atmosphere, and I use full carb heat on descent.  Though I doubt excess water ingestion into the engine is a common cause of failure, this story seems to creep into my mind whenever I'm flying in rain:  https://www.airspacemag.com/daily-planet/it-was-a-dark-and-stormy-night-1-66741228/

Does anyone know if the C is supposed to have this H2O drain as part of the carb heat system?  Mine just has the scat hose as far as I can tell.

[Andy95W]

There is a small drain fitting on the lower rear side of the carburetor air box.  It's mainly there to drain excess fuel out of the airbox in the event of overpriming or carburetor leak.  I suppose it would work as a water drain, but my gut feeling is that any carburetor ice that is removed by using carb heat will probably sublimate to a vapor and be sucked into the engine.  

Flying through a rain storm would be a different matter, of course.

[chrisk]

I knew I remembered the initial 231 had in induction ice issue that resulted in an auto alternate door being retrofit.  I finally found the info at  http://www.mooneypilots.com/mapalog/M20K231 Eval Files/M20K231_Eval.htm It also gives a good example of when to use the alternate air and what happens if you don't. 

 

Here is the section

Other Hints Operating the 231

Here are a few other hints taken from my engineering flight test days with the M20K 231.  The 231 is an airplane that has a primary engine air induction system that is prone to forming ice on the face of the induction air filter. We found this problem after I flew icing tests in northern Canada with our prototype and test M20K in 1984. We found that the original manually activated alternate air door mounted underneath the turbocharger was not an adequate solution to providing the engine a secondary source of induction air.  The alternate air design was okay, it was the pilots who didn't recognize induction system icing quickly enough and didn't pull open the alternate air door in time to keep the engine running. The solution was a new design for the alternate air system that mounted a box on the firewall with a door that opens automatically for alternate air.  The factory made the decision to offer this redesigned alternate air system free to all 231 owners in 1984.  If your airplane has not been upgraded over the years to the firewall mounted alternate air door, get one now. This system is a huge improvement in safety and engine performance.   

 

One other hint from that trip to Canada with the instrumented 231 test airplane - we got lots of airframe ice while looking for engine induction system icing.  We learned more about the handling characteristics of the 231 with airframe ice during that trip than ever before.  We found that the 231 lost airspeed in cruise at the rate of about 10-15 KIAS every 1/2 inch accumulation.  The airplane would handle about 1 inch of airframe ice on the leading edges, but after that the indicated airspeed was down low enough that the airplane wasn't pleasant to fly.                         

 

But the big thing we learned concerned landing with any ice accumulation on the airframe.  We found that landing with ice would cause a significant tail buffet condition if the flaps were used.  There was no buffeting present during he approach and landing with up to 1 inch of airframe ice if the flaps were kept retracted.  The only precaution was to keep the speed up - I held 90KIAS to the landing flare and it seemed to work out okay with 1 inch on the airplane. 

 

But lower those flaps for landing with ice on the leading edges of the wings and horizontal tail and there would be a considerable airframe or tail buffet beginning around 90 KIAS and lower.  So we recommended that any approaches and landings in the M20K with airframe ice accumulation be done with the flaps retracted. 

[RobertGary1]

When did Mooney introduce an alternate air door you can control yourself? All the Mooney's I"ve flown have had spring loaded air doors.

 

-Robert

[carusoam]

Probably before ‘94...

As the O got both the manual alt air door, with a magnetic door holder...

This way the pilot can open the same door manually, or the door can open itself by breaking the power of the magnet that is holding it closed...

Clear as mud? 

-a-