Taking the High Road

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It's amazing how many piston pilots shy away from using the high-altitude capabilities of their airplanes. That's a shame, says AVweb's Mike Busch, because whether you fly a pressurized twin or a humble straight-leg Skylane, up high is almost always the best way to fly eastbound...and often westbound, too! Using actual performance and winds aloft data, Busch makes a compelling case for high-altitude flight, and offers some handy rules-of-thumb for picking an optimum cruise altitude.

AirmanshipI recently returned from a marvelous trip from California to the Cayman Islands and return in my Cessna T310R, some 5,000 nautical miles and 27 flight hours in all. The outbound half of the trip was flown entirely at FL190, and the first third of the return trip was done at FL180. Although it was summertime and the winds aloft were relatively mild, they did provide 20 to 30 knots of "help" all the way from Santa Maria, Calif., to Key West, Fla. On the return flight a week later, the winds aloft were essentially calm between Key West and Shreveport, La., at FL180. Heading west from Shreveport, the headwinds became rather stiff, so the last two legs were flown down low (12,000').

Trip mapMy good friend Chris served as my copilot for this trip, and we swapped seats on each successive leg of the trip. Although Chris is an experienced instrument pilot and aircraft owner based on the west coast, I was surprised to discover that this was the first time he'd ever flown at the Flight Levels (other than as a passenger), or even at oxygen altitudes. He found it to be quite a revelation, and by the time we got to Grand Cayman, Chris was making discreet inquiries about the possibility of turbocharging his Bonanza.

When I made the same trip to the Caymans and back last year, my copilot was another friend, Joe, who is also an experienced instrument pilot and aircraft owner. Turned out that Joe had never done any high-altitude flying before, either.

From what I've been able to tell, the great majority of piston pilots shy away from using the high-altitude capabilities of their airplanes. Most pilots of normally aspirated airplanes seem to confine most of their flying to altitudes of 10,000' and below, and most pilots of turbocharged but unpressurized airplanes have never flown them at the Flight Levels. In fact, a surprising number of pilots of pressurized birds seem averse to flying much above the low teens.

I suppose it shouldn't be all that surprising. When we go through primary flight training, our instructors tend to take us just high enough to make sure we don't hit anything. Even our student cross-countries tend to be short enough that a lengthy climb doesn't make much sense. The same holds true when we train for our instrument rating.

After that, our instructors turn us loose to commit aviation on our own recognizance. Few of us ever receive formal instruction on how best to fly an 800-mile trip (much less a 5,000-mile one). So we tend to stay in our comfort zone, which for most of us means four-figure altitudes.

That's a shame, because it's at the high end of the altitude spectrum that most of our airplanes achieve their best efficiency — and in many cases, their best speed as well.

High isn't just for turbos...

I'm not just talking about turbocharged airplanes, either. Most normally-aspirated birds are perfectly capable of cruise altitudes well into the teens, and even a fixed-gear Skylane can claw its way to FL180 when appropriate — for instance, to top the weather or catch a big tailwind.

182 Performance ChartIn fact, let's take a look at a plain-vanilla, fixed-gear, normally-aspirated Skylane. Here's the "Range Profile" page out of the 182Q Pilots Operating Handbook.

At a low altitude like 4,000', maximum cruise speed is 139 KTAS at 75% power. Continue climbing until the airplane "runs out of throttle" at 8,000' and max cruise climbs to 144 KTAS. That extra 5 knots will save you 9 minutes on an 800 NM trip when you take the extra climb into account. (5:38 instead of 5:47, big deal!).

Continue climbing to 12,000' and max cruise drops back to 139 KTAS (same as at 4,000'), but at a much more fuel-efficient 64% power (which is all you can get at that altitude with wide-open throttle). The same 800 NM trip will take 6 more minutes at 12,000' than at 4,000' (5:53 to be exact) because of the longer climb, but you'll burn a whopping 12 gallons less fuel in the process, and increase your IFR range by a full hour and 130 NM!

How far can we take this? Stick a cannula in your nose and climb all the way up to 16,000' — high enough to fly right over the highest part of the Rocky Mountains IFR — and max cruise drops to a still-respectable 130 KTAS at a miserly 53% power. Because it takes a Skylane nearly 40 minutes to climb from sea level to 16,000' at max gross, the 800 NM trip will take a half-hour longer than at 12,000' (6:23), but you'll save 20 gallons and increase IFR range by a full two hours compared to our 4,000' benchmark.

Cruise
Altitude
Max
Cruise
IFR
Range
To fly an
800 NM Trip
4,000 139 K 820 NM 5:47 78 gal
8,000 144 K 840 NM 5:38 79 gal
12,000 139 K 950 NM 5:53 67 gal
16,000 130 K 1,040 NM 6:23 59 gal

Normally-aspirated, fixed-gear 182Q
(maximum gross weight, standard day,
no wind, 88 gallons, 45 min reserve)

Unless you just happen to like low-and-slow, there's no logical reason to cruise a Skylane lower than 8,000' because doing so makes all the numbers worse: cruise speed, trip time, and range. On the other hand, climbing to 10,000' or 12,000' will cost you a negligible amount of time, and reward you with substantially lower fuel burn and increased range.

These calculations are all based on zero-wind, but in real life the winds aloft are often a decisive factor in determining the best altitude to choose. If you're headed eastbound, odds are you'll have a tailwind — and the higher you fly, the better it'll be.

In wintertime, climbing up high to catch favorable winds can pay off spectacularly. In the low-to-mid teens, 50 knot tailwinds are commonplace and a 70 or 80 knot tailwind is a real possibility. Even in summer, when the winds tend to be relatively light, going high can pay off. Here are some typical summer winds I just pulled off the weather wire:

      6000    9000   12000   18000
STL 2410+18 2809+12 3110+07 2917-04
SPI 2510+18 3010+12 3211+07 2919-05
JOT 2511+17 3012+12 3116+06 2926-07
EVV 2509+17 3012+11 3216+07 3018-05
IND 2411+16 3011+11 3114+07 2922-06
FWA 2312+15 2812+10 2916+06 2926-07
CVG 2210+15 2809+11 3012+07 3021-05
CMH 2210+14 2710+10 2914+06 3026-07
CRW 2108+15 2509+10 2908+06 3225-05
AGC 2010+12 2510+09 2813+05 2930-09
EKN 1907+13 2608+09 2810+06 3028-07
PSB 1911+11 2509+08 2813+04 2930-11
EMI 9900+11 2905+09 2811+05 2927-10

Even in these docile summertime conditions, we can expect 10 to 15 knots more tailwind component at 16,000' than at 8,000', which almost exactly offsets the TAS advantage of the lower altitude (144K vs. 130K). By climbing up high on an eastbound trip, we'll go just as fast, burn considerably less fuel, and increase our IFR range nearly 400 NM!

During the winter, when the winds tend to be stronger, going high on eastbound trips tends to be an even better deal, saving both time and fuel.

T310R Performance Chart...but for turbos, it's even better

If you've got a turbocharger, the argument for flying high becomes much more compelling, because the higher you fly in a turbo, the higher your speed, range and efficiency — at least up to the low Flight Levels in most turbocharged airplanes. These birds really shine up in the high teens and low twenties, and pilots who don't take advantage of this capability don't know what they're missing.

For example, take a look at the "Range Profile" page for my T310R.

Starting with 180 KTAS at sea level — which is nothing to write home about for a 570 hp airplane — max cruise speed at 73.6% power steadily increases with altitude to a relatively blistering 221 KTAS at FL200. (Above that altitude, available power starts dropping off fairly rapidly.)

Cruise
Altitude
Max
Cruise
IFR
Range
To fly an
800 NM Trip
5,000 190 K 860 NM 4:14 143 gal
10,000 199 K 890 NM 4:04 137 gal
15,000 209 K 930 NM 3:55 131 gal
20,000 221 K 970 NM 3:45 125 gal

Turbocharged, twin-engine T310R
(73.6% cruise,maximum gross weight
standard day, no wind,
163 gallons, 45 min reserve)

At the same time, range with IFR reserves climbs from 820 NM to 970 NM. Naturally, trip time and fuel burn for the proverbial 800 NM trip both drop accordingly — from 4:14 and 143 gallons at 5,000 to 3:45 and 125 gallons at FL200.

Any of you who've ever heard me talk about powerplant management know that I don't believe in pushing my engines this hard, particularly in view of the fact that they're not intercooled. I almost always throttle back to between 60% and 65% power and settle for around 205 KTAS at FL200 at a miserly fuel burn of 26 gallons/hour, giving me a range of well over 1,000 NM with IFR reserves (or 1,200 NM if I fill my 20-gallon wing locker tank).

Once again, these figures assume no-wind conditions. Add in the wind on an eastbound trip and the results can get downright exciting. In the winter, I've seen my groundspeed edge above 300 knots from time to time. That's fun! During the summer, on the other hand, I'm happy with 230 or 240 on the GPS readout.

Needless to say, you pay the piper going westbound. But if the winds aren't too strong, it may still pay to go high rather than low. In my airplane, I gain 22 knots of true airspeed by climbing from 10,000' to FL200. So if the headwind at FL200 is only 10 or 15 knots stronger than at 10,000' (which is usually the case in summertime), higher is still better.

In wintertime, of course, westbound aircraft are all in the same boat, turbo or non-turbo. We bounce along at the MEA, try not to look at the groundspeed readout, hope the fillings in our teeth don't fall out, and think about how much fun the eastbound part of the trip was (or will be).

Picking your altitude

Choosing what altitude to file can be done with scientific precision or by rule-of-thumb. If you do your flight planning by computer (as I generally do), the precise method comes with the territory.

One of my favorite flight planning tools is the one on CompuServe operated by EMI Aerocorp, Inc. This sophisticated online system can automatically choose your route, prepare your weather briefing, and file your flight plan. It's not free, but it costs only a few bucks per trip and I think it does a far better job than DUATS.

One nice feature of the EMI system is its ability to help you choose an optimum altitude. You can give it a bunch of different altitudes to try, and in seconds it'll spit back a nice little matrix showing what your trip time and fuel burn will be at each one. Here's what EMI just gave me for a theoretical 950 NM flight from Wichita, Kansas, to Frederick, Maryland:

ALT/FL

TTE

FUEL

5000

5:01

843 LB

9000

4:54

804 LB

13000

4:47

796 LB

FL190

4:32

763 LB

No surprises here. Going eastbound, higher is invariably better. The only real question here is whether to go at 13,000' without oxygen (admittedly stretching the regs a bit), or to stick a tube up my nose and go at FL190, thereby saving 15 minutes and 5.5 gallons of gas. Which I choose probably depends whether my wife is going along or not.

Okay, how about the return trip westbound from Frederick to Wichita? Let's see what EMI says this time:

ALT/FL

TTE

FUEL

6000

5:46

941 LB

10000

5:32

906 LB

14000

5:28

906 LB

FL200

5:20

885 LB

Here's a perfect example of what I was talking about earlier. With mild summertime winds aloft, higher is quicker even going westbound. By strapping on oxygen and climbing from 12,000' to FL200, I gain 10 minutes and 3.5 gallons of gas. Is it worth it? Tough call. I'd probably say yes if the forecast calls for bumps at 12,000' (I hate bumps!), no if it's expected to be smooth.

In a pressurized aircraft, of course, this is a no-brainer. Higher is quicker, cheaper, and smoother. No tubes up the nose required. End of discussion.

Of course, if the trip is a short one, climbing way up to the Flight Levels may not make sense because by the time you get there, it'll be time to start down. Consider, for example, the flight I plan to make tomorrow: Santa Maria, California, to Las Vegas, Nevada — a distance of only 280 NM. Let's see what the EMI system says:

ALT/FL

TTE

FUEL

9000

1:41

299 LB

13000

1:41

304 LB

15000

1:41

309 LB

FL190

1:42

315 LB

Sure enough, the trip's so short that climbing doesn't help, either in time or fuel burn. So unless it's bumpy, I'll go low.

No computer? Use your thumb!

If you'd rather do flight planning the old fashioned way, or if you're caught on-the-road without your laptop, how can you tell whether it's worth going high or not? Actually, with a few simple rules of thumb, you can usually get close enough. The following hack works pretty well for me:

"It doesn't usually pay to climb for
more than one-third of the trip."

For instance, my POH says that it takes 30 minutes for my T310R to cruise-climb from sea level to FL200. Using the "one-third rule," it wouldn't be worth climbing that high unless the total trip time is at least an hour and a half. (Given the extra hassle of using oxygen, I'd probably raise that minimum to two hours.) On the other hand, it only takes me 15 minutes to cruise-climb to 12,000', so that's a reasonable cruise altitude for trips as short as 45 minutes.

Suppose we're talking about a long trip that easily meets the requirements of the "one-third rule." If our aircraft is turbocharged, we know that it flies faster and more efficiently the higher we climb. If tailwinds are forecast, then we want to go up high for sure — if we're not pressurized, the only question is whether we are willing to go on oxygen (and that's probably a function of whether we're carrying passengers, and how many). On the other hand, if the forecast calls for headwinds, then the question becomes:

"Will the higher airspeed help us
more than the headwind hurts us?"

That's easy enough to answer. For example, my airplane gains 22 knots of cruise speed climbing from 10,000' to FL200. If the headwinds at FL200 are less than 22 knots stronger than the winds at 10,000', chances are good that the higher altitude will be better. The longer the trip and the less the additional headwind, the better it will be.

What about a normally-aspirated airplane? Up to the highest altitude where you can achieve the desired cruise power — for example, 11,500' if we're flying a 182Q and like to cruise at 65% power — the rules are exactly the same as for a turbo — the higher you climb, the faster you'll go. Above that altitude, however, the airplane slows down as we continue to climb. Continuing to climb beyond this altitude seldom makes sense if we have a headwind which increases with altitude. But if a tailwind is forecast, the question now becomes:

"Will the lower airspeed hurt us
more than the tailwind helps us?"

For example, the Skylane loses about 5 knots of cruise speed climbing from 11,000' to 15,000'. If we gain more than 5 knots of tailwind by climbing to 15,000', then chances are good that the higher altitude will be better. Once again, the longer the trip and the stronger the additional tailwind, the better it will be.

Repeat after me: high is good!

If you're one of those pilots who comes from the "I won't climb higher than I'm willing to fall" school, you've got nothing to be embarrassed about. Believe me you've got plenty of company. But you're also missing something good.

Author's Cessna T310RI'm typing this on my notebook computer from a hotel room in the Wichita Airport Hilton. I flew here yesterday from my home base in California — a distance of 1,150 NM — in 5 hours flat at FL190, nonstop. I've flown this route a lot, and I usually plan on 6.5 hours of flying at 13,000' plus an hour-long fuel-and-burrito stop at Farmington, New Mexico. But this time, my preflight briefing indicated that the tailwinds in the low Flight Levels would be 40 to 50 knots all the way, making a nonstop trip feasible with comfortable IFR reserves. I couldn't pass up a deal like that! The view of the Grand Canyon from three miles up was simply spectacular, the Rockies were afire with yellow and orange foliage as I looked down on them, and I touched down at ICT with an hour and fifteen minutes of fuel in the tanks.

Do yourself a favor: give high a try. It's cooler and smoother up there. Your airplane flies faster and more efficiently up high. ATC will usually give you direct to just about anywhere. You're above terrain, obstructions, and often the weather and the ice. The view is usually terrific. So are the tailwinds, if you're lucky enough to be going in the right direction. Try it...you just might like it!

NOTE: Other articles by the author related to high-altitude flight that you might like to read: