Descent Profile - Fast Airplane, C/S Prop

kyleb

kyleb

Final Approach
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Marietta, GA
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Drake the Outlaw
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I've seen all sorts of debate about climb power, rpm, MP, cruise leaning, LOP, ROP, etc. But one subject I've never seen discussed at length is (are?) appropriate/optimal ways to descend from altitude flying a kinda fast airplane with a constant speed prop. Basically, I'm talking about my IO-540 powered RV-10 here. A couple of scenarios - assume I'm starting at 10K' headed towards a destination airport at 1,000 MSL.

1) Goal is to reduce power, but maintain speed by trading a 500 FPM descent for airspeed. Probably the most efficient approach.
2) Goal is to stay high until late (weather, traffic, whatever), then pull the plug - maybe a 1000 fpm descent to the destination while retaining airspeed.

What are your constraints (primarily engine constraints) for the two scenarios and why? Maintain MP at XX? Maintain RPM at YYYY? Gradual reduction in CHT? I'd think you'd want to lean pretty far at low(er) power, gradually enriching mixture during the descent.
 
Not sure I understand what the actual question is; descent planning for a specific fpm is straightforward.

Where folks tend to get in trouble is on a slam dunk where you’ve got to hit a specific altitude over a short distance. That leads to non-standard fpm descents, which leads to leveling off at the wrong time which results in blowing thru the altitude assignment/restriction and subsequent see-saw getting back to where you should be. Add to that an IMC environment and things can get sporty quickly.

On then engine management side, for a NA airplane, you shouldn’t be too terribly worried about shock cooling. ROP/LOP is an entirely different discussion, but in a slam dunk approach, that’s not what you should be worried about; too rich won’t kill you, but too lean can give you a surprise when you add throttle to level off.
 
Cessna 182 driver here, with the O470. Assuming smooth air, I'm usually coming down about 700-800 ft per min and leave the prop at whatever I've been cruising, say 2,200 rpm.

1,000 FPM+ and passengers ears can have issues, and the airspeed will certainly be well into the yellow. Enrichen a bit on the way down, but no real need to go full rich, unless you need to for a go around.
 
Hmmm.... non-geared engine; dive and drive. Stay outta the yellow arc :)

I pull back to 13", leave prop alone (usually cruised at 2400 RPM) and get 800fpm. If I need more, my planning was poor and I need to do better -- before I roll prop up or drop gear early. I can't explain why, I don't like running my engines under their low green arc on the MP gauge. Probably fear of Conti counterweights doing dumb things, which would not apply to your Lyc.
 
schmookeeg said:
Hmmm.... non-geared engine; dive and drive. Stay outta the yellow arc :)

I pull back to 13", leave prop alone (usually cruised at 2400 RPM) and get 800fpm. If I need more, my planning was poor and I need to do better -- before I roll prop up or drop gear early. I can't explain why, I don't like running my engines under their low green arc on the MP gauge. Probably fear of Conti counterweights doing dumb things, which would not apply to your Lyc.
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Not opposed to that. I've seen plenty of (unsupported) claims about not allowing the engine to drive the prop, keeping MP over 18" (which is a different version of the same thing), etc. It seems silly to use prop braking (AKA high rpm) to eat up all of that potential energy I stored as altitude. My goal would be to pull the MP and RPM back for a low power (but high speed) descent while avoiding rapid cooling. We're probably talking a 15", 1800 RPM, well-leaned engine setting to generate the high speed, 500 FPM descent that seems desirable to conclude an XC trip.
 
Kind of depends on the prop. Some are a little more efficient at lower RPM. With those, I'll pull the prop to bottom of the green arc and adjust MP for the descent profile that I want. People on the ground might appreciate that as well.

But if there are no neighbors and there's no performance gain, I'll just leave the prop at the cruise setting and simply reduce MP.
 
It depends (as do so many things). If no hostile terrain, weather or excessive traffic, I like to descent very slowly (in fpm) at slightly higher airspeed for longer. It's easier on ears, comfort, etc. However, if terrain kinda sucks on the way down, it's bumpy as hell, weather or lots of traffic, and altitude would be helpful in case of an engine out or to not get tossed around by thermals, I stay as high as practical for as long as practical and then descent close to 1,000 fpm close to the destination. This is a very rare scenario in the areas I fly around in.
 
Depends…. If descending at pilot’s discretion, I’ll plan a 600fpm descent for comfort and speed keeping IAS around 140-145 (T210), MP set around 18” and prop isn’t touched until on downwind or on final if a straight in VFR. I will enrich the mixture at level off. If held up high by ATC until late in the game, I’ll slow it down a bit more and put in 10deg of flaps and hit 1000 fpm but my ears aren’t happy. I try to keep in touch with center (or appr) and ask before I have to drop too quick.
 
kyleb said:
I've seen all sorts of debate about climb power, rpm, MP, cruise leaning, LOP, ROP, etc. But one subject I've never seen discussed at length is (are?) appropriate/optimal ways to descend from altitude flying a kinda fast airplane with a constant speed prop. Basically, I'm talking about my IO-540 powered RV-10 here. A couple of scenarios - assume I'm starting at 10K' headed towards a destination airport at 1,000 MSL.

1) Goal is to reduce power, but maintain speed by trading a 500 FPM descent for airspeed. Probably the most efficient approach.
2) Goal is to stay high until late (weather, traffic, whatever), then pull the plug - maybe a 1000 fpm descent to the destination while retaining airspeed.

What are your constraints (primarily engine constraints) for the two scenarios and why? Maintain MP at XX? Maintain RPM at YYYY? Gradual reduction in CHT? I'd think you'd want to lean pretty far at low(er) power, gradually enriching mixture during the descent.
Click to expand...
Why keep the same speed? Nothing wrong with getting a little extra in the descent.

From 10,000 feet (AGL), 500 fpm is 20 minutes. With no wind, I'm probably going to average about 180 GS, so 6 miles per thousand feet. That means starting the descent 60 miles out (I do correct for wind, of course). And at 10K, I'm just gonna set the autopilot for 500fpm descent if it's on, or push the nose over and re-trim for the faster speed if I'm hand flying.

I don't believe in shock cooling, but there's also no downside to pretending it does exist. So, I leave power in during the descent, and enrichen the mixture if necessary. Keep in mind that if you're running LOP, you don't need to worry about the mixture in the descent as much because it's only going to go leaner and thus further from the "red box". If you're running ROP and forget about the mixture you're going to end up in the red box.

Then, 20 miles out from destination (or FAF) I pull back to 22"MP, 15 miles out to 20"MP, 10 miles out to 17"MP, and if I'm VFR, 5 miles out to 14"MP. That's as low as I can go without setting off the gear warning, and a couple minutes after I get there I should be slow enough to get the gear out even if I'm still descending. At 17" and with some level time prior to the FAF, I should also be slow enough to drop the gear at the FAF.

RPM, I leave as is until I'm slow enough to be out of the governing range.
 
kyleb said:
I've seen all sorts of debate about climb power, rpm, MP, cruise leaning, LOP, ROP, etc. But one subject I've never seen discussed at length is (are?) appropriate/optimal ways to descend from altitude flying a kinda fast airplane with a constant speed prop. Basically, I'm talking about my IO-540 powered RV-10 here. A couple of scenarios - assume I'm starting at 10K' headed towards a destination airport at 1,000 MSL.

1) Goal is to reduce power, but maintain speed by trading a 500 FPM descent for airspeed. Probably the most efficient approach.
2) Goal is to stay high until late (weather, traffic, whatever), then pull the plug - maybe a 1000 fpm descent to the destination while retaining airspeed.

What are your constraints (primarily engine constraints) for the two scenarios and why? Maintain MP at XX? Maintain RPM at YYYY? Gradual reduction in CHT? I'd think you'd want to lean pretty far at low(er) power, gradually enriching mixture during the descent.
Click to expand...
With a 540 and10K, you are already ~20” MP, reduce RPM to 2100 is about 60% power and descend.
 
flyingcheesehead said:
From 10,000 feet (AGL), 500 fpm is 20 minutes. With no wind, I'm probably going to average about 180 GS, so 6 miles per thousand feet. That means starting the descent 60 miles out (I do correct for wind, of course). And at 10K, I'm just gonna set the autopilot for 500fpm descent if it's on, or push the nose over and re-trim for the faster speed if I'm hand flying.
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Very similar to what I do in the 201. Set the AP for 500ft/min descent, leave cruise power setting in, which for the 201 is full throttle, 2500rpm, LOP. That gives me ~180kts on the downhill, which makes up for some of the time lost in climb.

We are using a GI275 and an engine monitor, so when engine power reaches 75% I start pulling throttle to maintain 75%, and as I pass thru 3000msl I go full rich.

15mi out, 23”, 10mi out, 20”, 5 miles out, 15”.
 
Turbo Mooney here. I just push the nose over or set desired vertical speed and leave cruise power set. Get back some of the climb fuel in the descent.

And love the speed. :D

If you get slam dunked, about 1000 - 1500 above the desired altitude, reduce the descent rate to a more normal one to not overshoot.
 
In a turboprop, I’m usually planning for about 4 degrees which works out to maybe 2000 fpm.

If ATC wants me down waay earlier than I want to be down, I’ll give them 500fpm.
 
Go as fast as you can till things start glowing, then pull back just a skosh.
 
I haven't read all the others yet, but this is a question I had about my Mooney. There used to be a really good article about this but I have been unable to find it lately. "How to descend and land a Mooney properly" was written by a former Mooney technician (or something like that) and was a very good article.

One of the problems is controlling engine temperature so the engine doesn't cool too rapidly. You can just enter the descent at cruise power but the Mooney speeds up too much in that scenario, so you need to reduce power. If you reduce manifold pressure without reducing RPM, the engine will cool too rapidly. I maintain manifold pressure and reduce the RPM to keep the fire in the engine hot, so the engine cools more slowly and prevents shock cooling.

The next is to consider your speed increase vs descent ratio, I usually descend at 400 FPM because I speed up too much at 500. I have buried the ASI so you have to watch out for that. I try to stay about 180 mph or so during descent. Subtract traffic pattern altitude from cruise. At 1500 TPA and 9500 cruise, that's 8000 feet. At 400 FPM that's 20 minutes, at 180 mph that's 60 miles, or 52 NM from your destination airport. I start by dropping RPM to about 2360, then later down to 2080. My airplane has a red band from 2100 to 2350, so I avoid that. I keep pulling the throttle back to maintain manifold pressure about 20-22 during descent.

One of the problems I have is getting slow enough to drop the landing gear. The easiest way to do that is drop below TPA slightly and zoom climb up to it without adding power. That gets me below 120 MPH to lower the gear. If everything is timed right, I enter downwind at 120 MPH and lower the gear mid field at 2080 RPM and 18-20 manifold pressure. By the end of the runway my first notch of flaps is down and I am in the traffic pattern descent.

I hope this helps.
 
If I don't have any other restrictions on my descent (ATC, obstacles) I will usually shoot for a 250 ft/min descent. I usually just richen slightly and run the trim forward a bit.
250ft/min means I take the 1000's of feet I need to lose and multiply by 4 for how many minutes before getting there I need to start my descent, 180mph = 3 miles/min, So I need to multiply the minutes by 3 to get the number of miles, So I start my descent 12 miles out X the number of 1000's of feet I need to lose. Or I can adjust for 150mph or 120mph or knots.

Going into Tillamook OR in the Mooney descending from 11,000 ft = 10000 ft descent or I start descending 40 minutes out, doing 180mph+, so I start my descent 120miles out.

I don't usually have to pull the power back at all or very much, Very little increased cooling during the descent. Passengers like the gentle descent and i get the increased power and speed of the descent longer than I would with a 500ft/min descent.

Works pretty well with fixed pitch props also as I don't have to pull the power back as much or at all for the descent.

Brian
CFIIG/ASEL
 
William Pete Hodges said:
I haven't read all the others yet, but this is a question I had about my Mooney. There used to be a really good article about this but I have been unable to find it lately. "How to descend and land a Mooney properly" was written by a former Mooney technician (or something like that) and was a very good article.

One of the problems is controlling engine temperature so the engine doesn't cool too rapidly. You can just enter the descent at cruise power but the Mooney speeds up too much in that scenario, so you need to reduce power. If you reduce manifold pressure without reducing RPM, the engine will cool too rapidly. I maintain manifold pressure and reduce the RPM to keep the fire in the engine hot, so the engine cools more slowly and prevents shock cooling.

The next is to consider your speed increase vs descent ratio, I usually descend at 400 FPM because I speed up too much at 500. I have buried the ASI so you have to watch out for that. I try to stay about 180 mph or so during descent. Subtract traffic pattern altitude from cruise. At 1500 TPA and 9500 cruise, that's 8000 feet. At 400 FPM that's 20 minutes, at 180 mph that's 60 miles, or 52 NM from your destination airport. I start by dropping RPM to about 2360, then later down to 2080. My airplane has a red band from 2100 to 2350, so I avoid that. I keep pulling the throttle back to maintain manifold pressure about 20-22 during descent.

One of the problems I have is getting slow enough to drop the landing gear. The easiest way to do that is drop below TPA slightly and zoom climb up to it without adding power. That gets me below 120 MPH to lower the gear. If everything is timed right, I enter downwind at 120 MPH and lower the gear mid field at 2080 RPM and 18-20 manifold pressure. By the end of the runway my first notch of flaps is down and I am in the traffic pattern descent.

I hope this helps.
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When I read the shock-cooling strategies like the above, I always wonder where the procedure is to avoid shock-heating on takeoff?

ie what difference going from 60dF to 380 on takeoff vs. going from 380 to ~250 or so in descent? Surely the takeoff is way more dramatic to the steel.
 
Bill said:
Very similar to what I do in the 201. Set the AP for 500ft/min descent, leave cruise power setting in, which for the 201 is full throttle, 2500rpm, LOP. That gives me ~180kts on the downhill, which makes up for some of the time lost in climb.

We are using a GI275 and an engine monitor, so when engine power reaches 75% I start pulling throttle to maintain 75%, and as I pass thru 3000msl I go full rich.

15mi out, 23”, 10mi out, 20”, 5 miles out, 15”.
Click to expand...
Definitely very similar! In fact, reading your post reminded me that I didn't mention one thing: I never let MP go above 24" in the descent, that keeps me below 75%.

It's almost like we've been hanging around in the same circles for 20 years. ;)

dans2992 said:
In a turboprop, I’m usually planning for about 4 degrees which works out to maybe 2000 fpm.
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I always defaulted to 1500 fpm in the TBM, which the gadgets will calculate as 3 degrees when you're level, but when you speed up in the descent is more like 2.6-2.8.

dans2992 said:
If ATC wants me down waay earlier than I want to be down, I’ll give them 500fpm.
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That works if they needed your altitude. If they need you through a lower altitude you'll get "expedite through ..." But yeah, I've done that exact same thing.

William Pete Hodges said:
One of the problems is controlling engine temperature so the engine doesn't cool too rapidly. You can just enter the descent at cruise power but the Mooney speeds up too much in that scenario, so you need to reduce power. If you reduce manifold pressure without reducing RPM, the engine will cool too rapidly. I maintain manifold pressure and reduce the RPM to keep the fire in the engine hot, so the engine cools more slowly and prevents shock cooling.
Click to expand...
What Mooney do you have? Mine doesn't speed up too much, and it sounds like Bill's doesn't either.

How much does it speed up if you don't reduce power?

William Pete Hodges said:
One of the problems I have is getting slow enough to drop the landing gear. The easiest way to do that is drop below TPA slightly and zoom climb up to it without adding power. That gets me below 120 MPH to lower the gear.
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The most important ingredient to getting a Mooney down to gear speed is patience. It helps to have speed brakes, but I hate using them - What's the point of an efficient airplane if you use speed brakes? So I try to finesse things and not have to use them, just keep them as a tool in the back pocket for if ATC has to slam-dunk me.

Zoom climb from below TPA does not sound like a good idea to me, though. Pax probably wouldn't appreciate it either.
 
flyingcheesehead said:
What Mooney do you have? Mine doesn't speed up too much, and it sounds like Bill's doesn't either.

How much does it speed up if you don't reduce power?


The most important ingredient to getting a Mooney down to gear speed is patience. It helps to have speed brakes, but I hate using them - What's the point of an efficient airplane if you use speed brakes? So I try to finesse things and not have to use them, just keep them as a tool in the back pocket for if ATC has to slam-dunk me.

Zoom climb from below TPA does not sound like a good idea to me, though. Pax probably wouldn't appreciate it either.
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i have a 1964 M20E with arch windows. It has some speed mods, (gear doors and gap seals I think) but original windscreen and cowl. The first time I went to descend at cruise power I buried the ASI. That little bird is really slippery! It also has speed brakes but I prefer not to use them either, while descending in cruise. I don't like wasting my fuel like that. I do use them on final to drop in close to the numbers though, it works as good as a slip!

The last time I went to 2E8, outside Detroit, I did use them to help descend. I was at 10,500 above a broken layer from Toledo around the SW side of Bravo. ATC was really busy so I was keeping clear. All the cloud openings went in gapped lines towards Bravo airspace. I was abreast of ARB when I relayed my situation to ATC. They approved my descent through Bravo, and I dropped the RPM and pulled the speed brake and was dropping 2000 FPM and I could hear Kelsey Grammer in my head saying "let the air out of this thing, Lake!" I was keeping the ASI below 200 and dropping like a stone, then zoomed up a little to slow down, dropped the gear, lined up on a 4 mile final and set down on runway 6 just as pretty as you please.

That is a good airplane! It is really slippery while descending and you have to pay attention, but I love it!
 
flyingcheesehead said:
Zoom climb from below TPA does not sound like a good idea to me, though. Pax probably wouldn't appreciate it eieither.
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Since you brought this up I think I should comment.

You all know this. The typical traffic pattern as you approach an airport is a rectangle starting with downwind, base, final, and landing. If you are on the ground doing touch and goes, you start with takeoff, upwind, crosswind, downwind, base, final and landing. You can enter or leave the pattern from any of these, but the preferred entry is into downwind at about a 45 degree angle away from the pattern, near the end or between the numbers end and midway . If he pattern is busy this is preferred, but I have entered a parallel upwind if coming from the wrong direction so as to better see and merge into traffic without affecting the others flying in the pattern. Culpeper KCJR is often very busy at times and when approaching from the opposite side entering a parallel upwind is a good safe option for me. It let's me see everyone in he pattern and pick my point to merge in seamlessly.

What altitude are you flying in the pattern? Depending on where you are anywhere from ground 0 AGL to 1000 AGL. Larger planes or planes doing short field work might be higher up to 1500 AGL. Now, outside of the traffic pattern what is the minimum safe altitude? Most of the airports I frequent are in rural areas, surrounded by farms, so 500 AGL is what is normally considered minimum altitude. So dropping to 750 and zooming up to 1000 to slow down isn't a problem. If at the end of the zoom I am lined up to enter the pattern at the 45 and below gear speed then I have done well. I don't see why any of you consider that to be a problem.
 
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FWIW, the main problem I see with a fast, slick airplane, is slowing down to approach speed. I haven't flown an RV10, but I do fly an Ovation with 170-180 KT cruise speed. Without even increasing speed for the descent. there is no way (other than speed brakes which I reserve for special occasions) to get down to my 105 KIAS approach speed before the FAF while avoiding unnecessarily fast engine cooling, unless I incrementally reduce power during the descent.

One thing I do is, my approach configurations chart has a step-down speed before "Approach Level." On my chart, I call it "Vectoring," but it's the speed that I choose to be at once in the terminal environment. During the descent from cruise, I incrementally reduce power until I get to the MP setting that will give me my Vectoring speed when I level off in the approach environment.
 
William Pete Hodges said:
Since you brought this up I think I should comment.

You all know this. The typical traffic pattern as you approach an airport is a rectangle starting with downwind, base, final, and landing. If you are on the ground doing touch and goes, you start with takeoff, upwind, crosswind, downwind, base, final and landing. You can enter or leave the pattern from any of these, but the preferred entry is into downwind at about a 45 degree angle away from the pattern, near the end or between the numbers end and midway . If he pattern is busy this is preferred, but I have entered a parallel upwind if coming from the wrong direction so as to better see and merge into traffic without affecting the others flying in the pattern. Culpeper KCJR is often very busy at times and when approaching from the opposite side entering a parallel upwind is a good safe option for me. It let's me see everyone in he pattern and pick my point to merge in seamlessly.

What altitude are you flying in the pattern? Depending on where you are anywhere from ground 0 AGL to 1000 AGL. Larger planes or planes doing short field work might be higher up to 1500 AGL. Now, outside of the traffic pattern what is the minimum safe altitude? Most of the airports I frequent are in rural areas, surrounded by farms, so 500 AGL is what is normally considered minimum altitude. So dropping to 750 and zooming up to 1000 to slow down isn't a problem. If at the end of the zoom I am lined up to enter the pattern at the 45 and below gear speed then I have done well. I don't see why any of you consider that to be a problem.
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A few reasons, in no particular order:
1) Other traffic is going to be looking for you at TPA. Zoom climbing up to pattern altitude increases the chances of a midair - The other aircraft would be above and behind you so you wouldn't see them, and you will be under their cowl so they won't see you. At all. They might see your wing at the last second but there wouldn't be enough time to react.
2) The maneuver isn't "necessary for takeoff or landing" so you may find that some mad airport neighbor reports you to the FAA and given how the FAA interprets the regs, you will almost certainly be given a 91.119 violation, possibly with a bonus 91.13.
3) If a distraction occurs, there is a heightened chance of a stall/spin accident.
4) Pax do not like odd maneuvers like that. If they don't see it on an airliner, don't do it to them in a GA plane if you want them to have a good time and speak positively about the experience (you do).

I would suggest pulling power a little earlier. Find a setting that keeps you level at TPA with the gear up, below gear speed but above approach speed. Then, just get to that power setting early enough that you can put the gear down when you want. Since you have the added luxury of speed brakes, if it doesn't look like you'll be below gear speed when you want, just pop those for a few seconds and you'll be good. :thumbsup: It'll only take a few tries to find a technique that gives you a nice smooth speed transition and gives you a finesse to your flying that people appreciate.
 
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