December 2012 –
Remember that an aircraft wing always stalls at one angle of attack, but can stall at any airspeed.
Q: Dear Steve,
I’m in the fourth quarter of the game, age-wise, and have decided that it’s now or never! I’m planning a once-in-a-lifetime flying trip around the western United States and up into Canada next year. All of my flying—I have logged 880 hours over the last eight years—has been east of the Rockies. I’ve never flown in mountainous terrain.
My airplane is a Cherokee 180D and I’m going to pack it with a tent, sleeping bag and some emergency gear. I’m an experienced camper. I’ve been dreaming about this trip; I’m looking forward to seeing the sun’s rays crawl down across the western mountains.
What I am unsure of is this: Does my Cherokee perform well enough to safely fly in the western mountains, and does it have enough performance to get in and out of mountain strips?
A pretty experienced pilot here says I should install an angle of attack gauge before I go. He says it’s the best instrument I can have for mountain flying. What do you think?
And can you give me some advice on things to do before I go that will help me fly safely?
—Big Tripper
A: Dear Big,
I commend you on your plan. About 95 percent of my flying has been in the western U. S. and I still appreciate every opportunity to fly a small airplane in the mountain west.
Your airport neighbor is correct. An angle of attack (AOA) indicator is an unbeatable safety aid, especially when flying into short or high altitude airports. Remember that an aircraft wing always stalls at one angle of attack, but can stall at any airspeed. An AOA indicator gives a visual indication of where the airplane wing is in relation to its absolute amount of lift. Unlike airspeed meters, AOA indications are not affected by air density.
AOA systems consist of the display, a probe that is mounted on the bottom of one of the wings and flexible tubing to connect the probe to the gauge in the case of an analog display, or to connect to a small computer in the case of the light array display.
Alpha Systems AOA of Minnesota has developed and sells nine different analog and light array systems. The company recently introduced new displays for the Ultra and the Legacy AOA systems. The Legacy display is very small—2.5 inches long by 0.75 inches wide— and features an easy-to-interpret display.
Lift Reserve of Ohio, Advanced Flight Systems, Inc. of Oregon and Rite Angle of Washington also offer AOA systems. (All of these manufacturers are listed in Resources at the end of this column. —Ed.)
Prices for the systems start at around $500 for a simple analog system; light array systems range from $1,100 to $1,500. Installation usually takes from six to eight hours.
Although none of these systems is approved through the STC process, Alpha Systems has obtained a letter from the FAA Small Airplane Directorate stating that the installation of its AOA system is a minor alteration.
Make sure you obtain FAA approval for the installation of the AOA system you’re interested in before you purchase it. Interpretation of what constitutes a minor alternation varies widely from one FAA region to the next. All of these vendors should be able to help with supporting documentation.
Next, you’ll need to read up. If you’re an AOPA member, you should take a good look at “The Pilot’s Guide to Mountain Flying” in the Pilot Information Center of the AOPA website. You can find plenty of information on remote strips through the Recreational Aviation Foundation and the Idaho Aviation Foundation websites. Information on remote airstrips in Montana, Colorado, Washington, Oregon and other western states is also available on the Internet.
Let’s take a look at the performance numbers of your Cherokee 180 in order to determine whether it’s safe to fly in the backcountry. For this we’re going to pretend that you’re planning a flight into Johnson Creek, Idaho (3U2), a strip that is listed as “developed” on the five classifications on the Idaho Aviation website. (Other classifications include Hazardous, Wild, Primitive and Community. The majority of Idaho strips are in the Primitive and Developed categories.)
Johnson Creek’s runway is 3,400 feet long by 150 feet wide; it’s located near the village of Yellow Pine and is 4,933 feet above sea level. Johnson Creek is only 26 miles from McCall Municipal (KMYL), home of McCall Mountain/Canyon Flying, a well-known mountain flying school.
I strongly recommend that you hire an experienced mountain flight instructor to check you out on mountain flying procedures, or sign up for one of the McCall Mountain/Canyon courses.
Every pilot should own a copy of “Mountain Flying Bible and Flight Operations Handbook” by Sparky Imeson. In it you’ll find tidbits such as “Sparky’s 10 Commandments of Mountain Flying,” and a very good section on airplane performance.
The following is the Sparky Rule of Thumb for determining density altitude takeoff distance (for fixed pitch propeller-equipped airplanes):
To the standard, sea level takeoff distance, add 12 percent for each 1,000 feet of density altitude up to 8,000 feet. Add an additional 20 percent for each 1,000 feet density altitude above 8,000 feet.
I pulled information from the performance section of the Piper Archer III Information Manual. It’s not quite the same as your 180D, but it’s close. If I’m correct in working my way through the “Flaps 25° Takeoff Ground Roll” page, a zero-wind takeoff roll at sea level on a standard day at MTOW will take about 1,100 feet.
When the air temperature at Johnson Creek is standard (44 degrees F.) for the airstrip altitude the performance charts in the Piper manual predicts a 1,500 feet ground roll at maximum takeoff weight (MTOW) in zero-wind conditions.
However, the takeoff ground roll chart is for a paved, level, dry runway. Johnson Creek is a grass runway with a slight three-degree slope, so the book numbers should be viewed with some skepticism.
Let’s apply Sparky’s rule and see what his system predicts as far as the takeoff distance for your Cherokee 180 from the Johnson Creek runway on a standard day.
The rule of thumb calculation looks like this: 0.12 x 4.9 = 58.8 percent increase in takeoff distance. Sparky’s rule yields a takeoff run of 1,749 feet. (Starting with 1,100 feet at sea level, apply the additional 59 percent—or 649 feet—and the result is 1,749 feet.) The runway length is sufficient for your Cherokee.
What if you’re there on a summer day and the air temperature is 65 degrees F? Provided the atmospheric pressure is constant, the temperature increase will yield a density altitude of 6,416 feet. Sparky’s ROT equation (0.12 x 6.4 = 77 percent or 847 feet) yields a total ground run of 1,947 feet. The point is pay close attention to air temperature and practice your density altitude calculations.
Other Sparky Rules of Thumb are for Sloped Runways (a 2 percent down slope reduces takeoff run distance by 11 percent) and Runway Surfaces (firm turf, add 7 percent; grass up to four inches long, add 10 percent).
In addition to learning how to cipher in the effects of runway condition, runway slope, winds and density altitude to arrive at true performance numbers, you’ll need to learn how to lean the fuel/air mixture for best performance during high density altitude conditions.
It’s simple; if you don’t have an EGT gauge, lean until the engine begins to feel slightly rough, and then richen the mixture until it feels smooth. If you have an EGT gauge, lean before takeoff until the EGT number on any one cylinder matches the EGT number achieved for that same cylinder during a sea level takeoff.
I’d recommend you remove the wheel pants before you head up into the hills. FBOs in the western states can do this for you once you transit the flatlands.
You will also need to get used to the decrease in climb performance that results from high density altitude operations. You can simulate high altitude performance by practicing climbs at your best rate airspeed at altitude. Do this a few times to become accustomed to the lessened performance.
In closing, the key to safe flying in the mountains is preparation, a realistic evaluation (and upgrading, if necessary) of your pilot skills, adapting your plans to fit the prevailing atmospheric conditions—and a commitment to be patient when conditions aren’t auspicious for a safe and sane flying adventure.
But do press on. Flying into and out of the mountain strips in the western U.S. is awesome.
Happy flying.
Know your FAR/AIM and check with your mechanic before starting any work.
Steve Ells has been an A&P/IA for 39 years and is a commercial pilot with Instrument and Multi-Engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as Associate Editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to editor@www.piperflyer.com.
RESOURCES
Angle of Attack Indicators
Alpha Systems AOA, a PFA Supporter
alphasystemsaoa.com
Lift Reserve Indicator by
InAir Instruments LLC
liftreserve.com
Rite Angle by EM Aviation LLC
riteangle.com
Pilot Resources
Recreational Aviation Foundation
theraf.org
Idaho Aviation Foundation
idahoaviation.com
“The Pilot’s Guide to Mountain Flying”
in the members-only Pilot Information Center at AOPA.org
McCall Mountain/Canyon Flying Seminars LLC
mountaincanyonflying.com
“Mountain Flying Bible and Flight Operations Handbook” by Sparky Imeson
Available at mountainflying.com
