Planning descents

[Hank]

Quote: Mooneymite

Okay, this thread has been way too peaceful with way too much agreement.

I just have to ask:  What MAP/RPM/mixture do you set on descent and what IAS do you use?

[Mooneymite]

 

Obvously not all descents are equal depending on traffic, clearance, etc.

On my O-360, normally aspirated "C" model, when I am allowed to descend as I like from cruise altitude (usually 8,000, or above):

MAP: 19'

RPM: 1900

IAS: increasing to maintain cruise TAS

Carb heat:  on.

Mixture: as lean as it will go and still run.

This combination normally yields about 500'/min average ROD.

 

.

 

[jetdriven]

You run the carburetor hear from cruise all the way to landing?  In clear air as well?

[scottfromiowa]

Quote: AustinPynes

I was out about 21 miles this week when Greensboro Approach said I could start my descent from 4500 to pattern atitude 1900. I was shocked and hesitated before replying. That never happened before! I guess he saw the M20F coming in blazing and figured I would need help with reentry. At least that is what I told myself, even as I was surrounded by airliners. Tomorrow I will be descending to real sea level as we fly to North Myrtle Beach for a Steve Miller Band concert. I love my Mooney!

[Ksaunders]

Like the idea about the tail number whisper to get their attention.  They forgot to turn me base to final in Savannah last Thursday and it seems in Atlanta they are always holding me up high when I need to get my bird LOW and SLOW when coming home to the roost.  

Try this simple formula for getting back down to earth:

Altitude = Drop the Zero's and multiply by 3 (Gives you when to begin descent).

1/2 ground speed and add a zero.  (Gives you your necessary rate of descent).  

Example.  9,000 feet and 160 kts.

9 x 3 = 27.  Begin your descent 27 miles out.

160 KTS ground speed = 160/2 = 80 then add a zero = 800 fpm descent.  

Not absolutely perfect but works out pretty good.  And....most will probably not need to remove their shoes or a wiz-wheel to calculate this.  

 

 

 

 

 

[jetdriven]

Many people here start down from 9000' at 60 miles out and 400 FPM.

[Ksaunders]

know them well.   

[WardHolbrook]

Quote: Ksaunders

Like the idea about the tail number whisper to get their attention.  They forgot to turn me base to final in Savannah last Thursday and it seems in Atlanta they are always holding me up high when I need to get my bird LOW and SLOW when coming home to the roost.  

Try this simple formula for getting back down to earth:

Altitude = Drop the Zero's and multiply by 3 (Gives you when to begin descent).

1/2 ground speed and add a zero.  (Gives you your necessary rate of descent).  

Example.  9,000 feet and 160 kts.

9 x 3 = 27.  Begin your descent 27 miles out.

160 KTS ground speed = 160/2 = 80 then add a zero = 800 fpm descent.  

Not absolutely perfect but works out pretty good.  And....most will probably not need to remove their shoes or a wiz-wheel to calculate this.  

[Mooneymite]

Quote: jetdriven

You run the carburetor hear from cruise all the way to landing?  In clear air as well?

[fantom]

Quote: jetdriven

Many people here start down from 9000' at 60 miles out and 400 FPM.

[jlunseth]

And some start from 19,000 and 120 miles out, 500 fpm.  Actually, from FL190 about 100 miles would do the trick unless there is a ferocious tailwind.  Doesn't mean we are bad people either.

[Hank]

Quote: Mooneymite

Sure do.  Here in the SE, humidity is part of life.

On a normally aspirated engine, carb icing can take place in clear air at low power settings any time there's humidity...the greater the humidity, the greater the danger.  There's a really good discussion of it on page 47 of this Lycoming publication:  http://www.lycoming.com/support/tips-advice/key-reprints/pdfs/Key%20Operations.pdf

[Mooneymite]

Quote: Hank

[Hank]

I've yet to hit yellow, much less redline, with my cruise-power-on descents. Sometimes I flirt with 170 MPH, but yellow starts at 175. If it's not smooth, of course I slow down as required. My target is 500 fpm from cruise to pattern; to pass into yellow I need to hold >700 fpm.

[jetdriven]

I havent seen carburetor ice in any airplane as long as power is at ~50% or more, unless flying through rain with an O-470 or a Lycoming O-540.  That is unfiltered air you are breathing, and the inside of that cowl is dusty from your grass strip.

[Hank]

Quote: jetdriven

I havent seen carburetor ice in any airplane as long as power is at ~50% or more, unless flying through rain with an O-470 or a Lycoming O-540.  That is unfiltered air you are breathing, and the inside of that cowl is dusty from your grass strip.

[jetdriven]

I was more referring to Mooneymite.  I use carb heat too, but only when there is a risk of carb ice.

[Hank]

Your J doesn't have a carb . . .

I was also surprised that Mooneymite uses carb heat dor his entire descent from cruise to the field. I've never heard anyone advocate that type of carb heat usage. With the carb temp gage, I have a much better idea of when carb heat is necessary, aside from power-off situations. The tight Mooney cowling does a good job of minimizing carb ice opportunities.

[rob]

I encountered carb ice on takeoff in my M20C a few years back. In southern Missouri on a July night. The temp was around 75 and there was a thunderstorm bearing down on us (extremely high humidity). We made a quick fuel stop and departed in front of the storm. At about 1000' AGL the engine became very unhappy. After a moment of jockeying with the mixture and throttle to see what in the devil was going on, a pull on the carb heat cleared the issue up rather quickly.

So while it's not common, apparently we hit the perfect conditions for carb ice to occur at full throttle.

[Mooneymite]

Quote: jetdriven

I was more referring to Mooneymite.  I use carb heat too, but only when there is a risk of carb ice.

[jetdriven]

Our J has no carb hear or ram air.  That was a poorly worded sentence on my part but I meant when I fly in aircraft that have carb heat.  Other than a Comanche 250 or 182, which can make ice faster than a Hoshizaki, I usually descend with a minimum of 50%-65% power, and every few minutes, check the power with a gentle bump of throttle or a 5 second pull of carb heat.  Now, when i reduce power in the pattern for landing, better believe it is on.  I just doubt the need for carb heat 30 miles out.  The J is WOT from takeoff roll until the 45 on downwind.

 

Gosh, did I just admit to doing a 45 pattern entry?? 

[Mooneymite]

Quote: jetdriven

I just doubt the need for carb heat 30 miles out.

[jetdriven]

The engine breathes filtered air. It is a 20k$ unit out there.  Since you fly from a grass strip, the inside of your cowling is covered in dust, and your engine is eating it up. Lycoming says 1 TBSP of dirt can cause need for an overhaul.

the engine is more efficient.  A carbuteted engine using carb heat is running a less dense atmosphere. Run less MP, less FF, and a lower intake temp for the same crankshaft horsepower without carb heat.

Powewr on descents are more efficient, IE less fuel burn per trip.  Using a higher power setting in descent eliminates need for carb heat.  If you get ice,  you can detect it by a slow reduction in MP.

The engine runs richer with carb heat on. This adds more lead and carbon fouling into the cyinder heads.  Nobody believes this is good for an engine. Air cooled engines are designed to run a minimim of 250-300 CHT due to their tapered cylinder barrels.  let downs at low power ruin cylinders.

 

 

[orangemtl]

All of the standard equations are good, unless you are landing in 'interesting' terrain: e.g. airstrip-in-a-bowl locations out west, or an airstrip at 790 above sea level in similar, widespread terrain, except for that ONE range of foothills at 4000 between you and the numbers...

Personally, I hang onto an extra 1500 feet of altitude on the way home, then shed it in an extended downwind leg, so as to get past the mountains w/o sweating it.  They look fairly solid from above, and I'm betting it would hurt to hit one.

[Mooneymite]

Quote: jetdriven

The engine breathes filtered air. It is a 20k$ unit out there.  Since you fly from a grass strip, the inside of your cowling is covered in dust, and your engine is eating it up. Lycoming says 1 TBSP of dirt can cause need for an overhaul.

the engine is more efficient.  A carbuteted engine using carb heat is running a less dense atmosphere. Run less MP, less FF, and a lower intake temp for the same crankshaft horsepower without carb heat.

Powewr on descents are more efficient, IE less fuel burn per trip.  Using a higher power setting in descent eliminates need for carb heat.  If you get ice,  you can detect it by a slow reduction in MP.

The engine runs richer with carb heat on. This adds more lead and carbon fouling into the cyinder heads.  Nobody believes this is good for an engine. Air cooled engines are designed to run a minimim of 250-300 CHT due to their tapered cylinder barrels.  let downs at low power ruin cylinders.