Colorado Flying

[denverpilot]

Very surprising to me how much of a difference it actually does make, it is only about a 12 percent increase in power but it feels like much more. Interestingly, different plane so the comparison is not one to one, but I didn't feel like I got the same boost from the 200 hp in the SR20 (but that's for a different thread!)

Nothing to add to the mountain flying discussion, it'a been well-covered.

But I'll explain why a 12% power loss makes such a big difference. We got into this in my multi training but it applies to any airplane.

Your performance for a climb is based upon not total horsepower but EXCESS horsepower available.

Put super simplistically: If it takes 80% of total available horsepower at sea level to maintain level flight, you have 20% left to "go up with".

If you lose 12% of that 20% needed for your sporty sea level climb rate, now you've only got 8% left to climb with.

You lost over half of your climb performance.

Now factor in that the wing isn't as efficient at altitude (Vy falls as you go up), the prop isn't as efficient (it's likely tweaked for best efficiency at cruise at a lower altitude and isn't a "climb prop" anyway), and all of that...

You're probably eating another 2% of your performance up, at least. So we're at 6% now.

All these effects pile on top of each other as you go up. Engine loses power, wing, prop, etc. Which is why that climb curve is a curve and not straight. Performance just tapers off as a curve that flattens all the way up to the highest altitude the airplane can climb to.

Bonus aero-nerd question: What's the actual definition of "service ceiling"? Can the airplane still climb at its "service ceiling"? If so, how much?

Now apply this to a high altitude hot middle of the day takeoff. The runway is around 6000 MSL but the Density Altitude is above 8000. Maybe even pushing or at 9000. Where are you at on that climb curve before you even push the throttle forward on the runway?

A critical item nobody specifically mentioned (unless I missed it) is this: Do your takeoff performance numbers and landing numbers when visiting high altitude airports! You want to know how long that takeoff roll is going to be and how much distance you need to clear that obstacle.

And the book assumes a new airplane, new engine, and a factory test pilot. Add a fudge factor. (In the twin, it's 200%. Twice the book. Not kidding. Why? Because even the most hamfisted pilot can probably make those numbers. (Missed the landing spot by 300'. Didn't rotate until five knots late. Didn't pitch to exactly Vx and hold it perfectly. Etc.)

Come on out. Water's fine. Flying is great. Just run those numbers and know how the airplane will perform. Our ground rolls are longer. Our ground speed is higher. The climb rate is shallower. And the landing distances are longer. But the airplanes fly just fine.

Mountains: Just realize if all you can squeak out is 500 ft/min up, there's downdrafts greater than 2000 ft/min down when the winds flow across rocks. Always have an "out", an escape plan.

 

[Lachlan]

As most guys probably are that wish to play around in the high hills in a small non-turbo aircraft on a hot Rocky Mt. summer day.

I'm not sure you know much about flying on a summer day, but I'm positive you know all about Summer's Eve.

 

[Mtns2Skies]

Bonus aero-nerd question: What's the actual definition of "service ceiling"? Can the airplane still climb at its "service ceiling"? If so, how much?

Service ceilling is where an aircraft at gross weight will climb at 100fpm(50fpm for helicopters). Absolute service ceiling is where it will not climb any more. Interestingly absolute service ceiling is also the point at which Vx and Vy converge.

 

[murphey]

Nothing to add to the mountain flying discussion, it'a been well-covered.

But I'll explain why a 12% power loss makes such a big difference. We got into this in my multi training but it applies to any airplane.

Your performance for a climb is based upon not total horsepower but EXCESS horsepower available.

Put super simplistically: If it takes 80% of total available horsepower at sea level to maintain level flight, you have 20% left to "go up with".

If you lose 12% of that 20% needed for your sporty sea level climb rate, now you've only got 8% left to climb with.

You lost over half of your climb performance.

Now factor in that the wing isn't as efficient at altitude (Vy falls as you go up), the prop isn't as efficient (it's likely tweaked for best efficiency at cruise at a lower altitude and isn't a "climb prop" anyway), and all of that...

You're probably eating another 2% of your performance up, at least. So we're at 6% now.

All these effects pile on top of each other as you go up. Engine loses power, wing, prop, etc. Which is why that climb curve is a curve and not straight. Performance just tapers off as a curve that flattens all the way up to the highest altitude the airplane can climb to.

Bonus aero-nerd question: What's the actual definition of "service ceiling"? Can the airplane still climb at its "service ceiling"? If so, how much?

Now apply this to a high altitude hot middle of the day takeoff. The runway is around 6000 MSL but the Density Altitude is above 8000. Maybe even pushing or at 9000. Where are you at on that climb curve before you even push the throttle forward on the runway?

A critical item nobody specifically mentioned (unless I missed it) is this: Do your takeoff performance numbers and landing numbers when visiting high altitude airports! You want to know how long that takeoff roll is going to be and how much distance you need to clear that obstacle.

And the book assumes a new airplane, new engine, and a factory test pilot. Add a fudge factor. (In the twin, it's 200%. Twice the book. Not kidding. Why? Because even the most hamfisted pilot can probably make those numbers. (Missed the landing spot by 300'. Didn't rotate until five knots late. Didn't pitch to exactly Vx and hold it perfectly. Etc.)

Come on out. Water's fine. Flying is great. Just run those numbers and know how the airplane will perform. Our ground rolls are longer. Our ground speed is higher. The climb rate is shallower. And the landing distances are longer. But the airplanes fly just fine.

Mountains: Just realize if all you can squeak out is 500 ft/min up, there's downdrafts greater than 2000 ft/min down when the winds flow across rocks. Always have an "out", an escape plan.

One more aspect to high altitude flying - for every 1000 ft in altitude, how much HP do you lose? From the ancient Piper documentation, at sea level, I really do have 180 HP and the takeoff roll (according to the book) is 720 ft. But at 5000 msl, 100% 180 hp is really only 165 HP and the takeoff roll (sure, right) is 1300 ft. At 12K, the best I can do is 60% power for 72 HP and TO roll is over 2000 ft.

By the way, my regular route from Denver to Leadville is 12.5K to get over the two ridges. Same for LaVeta Pass where the road is over 9000 ft. At 12.5K there are peak on either side that are considerably higher.

 

[Clark1961]

Mountains: Just realize if all you can squeak out is 500 ft/min up, there's downdrafts greater than 2000 ft/min down when the winds flow across rocks. Always have an "out", an escape plan.

Just a comment 'cause I feel like typing I guess. In the mountains you'll see two kinds of up/downdrafts convective and wave. Both have their own concerns and characteristics.

The convective stuff is usually around from spring until fall and can be found in the winter. Summer from mid morning it starts building. Usually the updrafts are stronger than the downs but that's not a hard observation. I'm comfortable seeing a thousand feet per minute up. If it gets much higher than 1500 fpm I'm thinking I shouldn't be there. I have seen over 2000 fpm down associated with virga. The other convective downdraft to watch out for is a microburst. Like virga these can take you to the ground.

Mountain wave is a different animal and it impacts flights differently. I've always found it smooth and manageable albeit very powerful. The airliners have much more trouble with it. Watch out underneath the wave since rotors live there. In some places those rotors go to the ground. In other places the rotors can destroy the aircraft. Sometimes rotors are marked by clouds, sometimes not. Don't go into those innocent looking puffy little clouds with the torn up edges. I've been near the cloud edges when solo and have little desire to go back.

A final note, strong winds crossing a ridge or a peak generate a lot of turbulence. Sometimes you can see it when clouds form. The Spanish peaks southwest of Pueblo CO have shown it as much as any in my limited experience. The clouds stream for miles downwind of the peaks. Either go over the peaks or go way out east since anything directly downwind is going to be a rough ride. I've encountered the turbulence 30 miles east of the peaks. It's just a little educational.

 

[midlifeflyer]

So I'm definitely not ready to launch the 172N out into an epic cross country journey out to Denver.. at least not yet. Some day would be fun to explore those areas, they seem beautiful

They are beautiful.

 

[midlifeflyer]

Just a comment 'cause I feel like typing I guess. In the mountains you'll see two kinds of up/downdrafts convective and wave. Both have their own concerns and characteristics.

The convective stuff is usually around from spring until fall and can be found in the winter. Summer from mid morning it starts building. Usually the updrafts are stronger than the downs but that's not a hard observation. I'm comfortable seeing a thousand feet per minute up. If it gets much higher than 1500 fpm I'm thinking I shouldn't be there. I have seen over 2000 fpm down associated with virga. The other convective downdraft to watch out for is a microburst. Like virga these can take you to the ground.

Mountain wave is a different animal and it impacts flights differently. I've always found it smooth and manageable albeit very powerful. The airliners have much more trouble with it. Watch out underneath the wave since rotors live there. In some places those rotors go to the ground. In other places the rotors can destroy the aircraft. Sometimes rotors are marked by clouds, sometimes not. Don't go into those innocent looking puffy little clouds with the torn up edges. I've been near the cloud edges when solo and have little desire to go back.

A final note, strong winds crossing a ridge or a peak generate a lot of turbulence. Sometimes you can see it when clouds form. The Spanish peaks southwest of Pueblo CO have shown it as much as any in my limited experience. The clouds stream for miles downwind of the peaks. Either go over the peaks or go way out east since anything directly downwind is going to be a rough ride. I've encountered the turbulence 30 miles east of the peaks. It's just a little educational.

Add a third, although you touched on it in discussing turbulence on the Lee side of a ridge. It's the normal effect of terrain on wind, producing currents and eddies like a rocky stream or, when the winds are stronger, a whitewater river. (I don't think of that as part of mountain wave phenomena, which I always picture as a sine curve).

It's that sudden "waterfall" on the lee side that can produce some pretty nasty downdrafts and turbulence and some fantastic updraft on the windward side. The rocky stream analogy also accounts for the success of "shuttle climbing," in which you fly back and forth on the lee side of a ridge looking for lift.

Speaking of shuttle climbing, the folks who have been there or somewhere similar might recognize this: I flew an Angel Flight bringing a young couple home to Alamosa from Denver. 4 of us and some baggage in a 182. It was a comfortable flight, winds at ridge level in the 15 kts range with no turbulence, giving me the opportunity to fly a little lower and treat my passengers to the view. Approaching Mosca Pass from the east I decided to stay at ridge level and shuttle looking for lee side lift. Sure enough, we hit an eddy and got a 1000' elevator ride to cross the ridge over to Sand Dunes.

 

[Anthony]

As most guys probably are that wish to play around in the high hills in a small non-turbo aircraft on a hot Rocky Mt. summer day.

Well, as others have said that is a fine time to fly depending on the plane, weather, winds, and everything else people have stated. I learned to lean drastically, keep the plane light, use step climbs watching my CHT's, and not fly when it was too windy. Never needed O2 either, so non-turbo, and no O2 is not a deal breaker at all.

 

[Tantalum]

Your performance for a climb is based upon not total horsepower but EXCESS horsepower available.

Put super simplistically: If it takes 80% of total available horsepower at sea level to maintain level flight, you have 20% left to "go up with".

If you lose 12% of that 20% needed for your sporty sea level climb rate, now you've only got 8% left to climb with.

Great way to conceptualize why the jump from 160hp to 180hp makes such a perceived difference. Never thought about it like that before but the approach makes sense, especially since truly it is in the climb that you really see that benefit from the extra 20 horses

 

[coloradobluesky]

I find that the Cessna 172's I have flown will climb higher than their rated service ceiling. I think most planes will. The reason is, at full gross the plane still has 100fpm climb. Also, you can usually catch net lift. Go slow in updrafts (pull up) and go faster in downdrafts is one fundamental concept. The windward side of the mountain almost always has some lift. The lee side tends to have both up and downdrafts. Circle to find them. Its fun coaxing a low powered airplane up. Keep in mind, gliders go up high in these mountains and they have no horsepower whatsoever. Some of these low powered planes like a J3 can get up high on little power, and you could LAND up there at a high altitude airport, but you won't be able to TAKE OFF!! Dont get yourself into that one.

The main thing is to know when its not feasible to proceed. Cancel the flight or turn around when its not working out. You need pretty good weather, fortunately, Colorado HAS pretty good weather.

 

[Mtns2Skies]

Colorado HAS pretty good weather.

And then really terrible weather, and then really great weather and then really terrible weather all in the span of 15 minutes

 

[Tantalum]

Ultimately, what I'm gathering is that if you *live* in CO there are plenty of ways to operate a simple GA plane safely... good planning and vigilant airmanship can open the doors to some beautiful scenery. I bet an early morning 7am flight is one of the most beautiful out there. But it's also a little bit like taking a rowboat out into NY harbor. Sure, in good weather and with some careful planning it can be relatively uneventful and treat you to some great views

But as far planning a trip several weeks into the future from several hundred miles out of state with a schedule to stick to that kind of seems like a no go, at least not with a 172

Ironically enough in last night's CAP meeting they had a whole section on mountain flying, rotors, and waves. One of the folks there have taken a 206 (I believe it was) from the San Jose area out towards Denver and back through the mountains. But a 206 I would put a couple rungs higher than a 172

 

[denverpilot]

But as far planning a trip several weeks into the future from several hundred miles out of state with a schedule to stick to that kind of seems like a no go, at least not with a 172

That's true of any cross county to anywhere really, if the weather goes to crud. ;-)

Usually when I talk to folks who are coming through who want to get a mountain airport or two into their flight plan, they land here in the front range (or somewhere on the west side if coming from over there) the night before and get a good night's sleep and then make a go/no-go decision for the mountains in the early morning, looking at winds and any cloud cover.

If they get no-go criteria they head for one of the "around" routes north or south.

The reason for the morning departure into is to get up there before it gets warm/hot. Usually its also the calmest time of day for winds but DA is your enemy in a 172 or other lower horsepower aircraft. Look up how far off the takeoff performance chart you are in your POH on a Skyhawk if it gets to 70F in Leadville, for example.

(I departed Leadville at 68F in the Skylane once. It was an incredibly long takeoff roll, but we calculated that it would be.)

The reason to land "fairly close" to the mountains is that you can probably find some locals to look at the weather with ya ahead of time and give them a text or ring in the early morning to see if they see anything that would be no-go for them.

I also keep a few of the mountain AWOS phone numbers in my phone on speed dials. They don't tell the whole story, but we have some AWOS sites on some of the passes and peaks.

See page 22... (phone numbers are listed on the list)...

https://www.codot.gov/programs/aeronautics/PDF_Files/AirportDirectory/2015_2016COArptDir.pdf

Or here:

https://www.codot.gov/programs/aeronautics/COMtnAWOSMap

 

[Tantalum]

You can test your skills in San Diego with your Cessna going back and forth over the S Sierras from Chino to Las Vegas.

Thanks for notes, I'll be heading out to Vegas in the next couple weeks and look forward to the trip.