by Yasmina Platt
Published in Midwest Flyer – December 2017/January 2018 issue
It’s October 15, 2017, and the “end” of Colorado’s best mountain flying season is fast approaching, so I had to take advantage of another beautiful morning to fly before potentially hibernating my mountain flying skills until next year.
I had an idea of the route I wanted to fly; however, the forecast for high winds and turbulence was very present for the early afternoon, so I decided to be flexible and just go “check it out,” with the mindset that we would turn around at the first sight of bad turbulence.
We ended up flying a loop from Front Range (KFTG) to Gunnison and back around. From KFTG, we flew by Devil’s Head Fire Lookout/Tower southwest of Castle Rock, which has an interesting history. Later that afternoon, I hiked for a different perspective and experience, over Cheesman Lake, right over Harriet Alexander Field Airport (KANK) in Salida. Harriet reminded me of Sedona’s Airport (KSEZ), as they are both on mesas, south of Monarch Pass, and around towards Gunnison. From there, we flew over Taylor Park Reservoir, up beautiful Cottonwood Pass (my favorite part!), and down towards Buena Vista before heading back towards KFTG.
This flight was intended more for sightseeing than training, so no landings were made along the route to ensure we got back to lower ground before the winds picked up, causing moderate turbulence. However, we always have room for improvement and learning or experiencing something new, right? This flight was no different.
Two pressure fronts, a cold and a warm, came close to each other at about the mid-point of our flight. In meteorology classes, we learn that cold fronts and warm fronts cannot mix or collide because of the difference in temperatures and densities. Warm air, being lighter, will usually be pushed atop the colder air. The air cools as it rises and the water vapor in it condenses. Precipitation, clouds and storms can be found in these scenarios. Fortunately, the formation of any type of visible moisture was not forecasted for the duration of our flight; however, the next day, on October 9th, almost the entire state of Colorado welcomed snow – the first snow of the season for Denver.
But, what we did experience was quite interesting! Upon reaching the mid-point of our route, our altimeter setting went from 29.77” to 30.21” of Hg in a matter of seconds (ok, maybe a few short minutes) between two nearby weather stations. Do you know what that meant? A 400-plus ft. difference in altitude! Wow!
I had noticed a big altitude discrepancy between my indicated altitude and ForeFlight’s (my iPad’s) altitude, but which one was accurate? I know the source of an altimeter’s information and how it works. An altimeter measures Outside Air Pressure (OAP) from the static source, which gets converted to an altitude and the setting obtained from air traffic control or weather stations corrects it for changing air pressure. After manually adjusting the altimeter with the setting from the next weather station, it read correctly and ForeFlight was dead on, accurate, the entire time. Good to know! I assume ForeFlight must be automatically inserting the latest altimeter setting into the application since I had an ADS-B In unit connected to it.
In the mountains, when you already don’t have too much clearance from the ground as it is, a 400-500 ft. difference in altitude is a huge difference. It did not present a safety concern for us because we were in visual conditions and we judged our altitude based on charts and looking out the window. But, can you imagine if we would have been in instrument conditions? Would we have noticed or picked up (from a weather station) the large change in pressure before it was too late (causing a Controlled Flight Into Terrain or CFIT accident)? I, once again, want to thank the Colorado Department of Transportation and, in particular, the Division of Aeronautics, for installing and maintaining wonderful AWOS stations on top of critical mountain tops/passes throughout the state. They are not only convenient and “nice to have,” but can be lifesaving as well.
One other thing that is quite interesting and that we need to pay attention to when flying at high altitudes is the difference between true airspeed and indicated airspeed. Early on in our flight training, we learn that true airspeed (TAS) is indicated airspeed (IAS), or what the airspeed indicator in the airplane shows us, corrected for altitude and temperature. At sea level on a 15°C day, IAS will be the same as TAS. However, as the temperature or altitude increases, the air density will decrease, causing the IAS to read lower than TAS. A good rule of thumb to approximate the difference between IAS and TAS without looking at specific temperatures, a chart, or a calculator, is to increase IAS by 2% per 1,000 feet of increase in altitude.
At one point in the flight, when going over Cottonwood Pass, we reached 14,000 feet to clear it. Our indicated airspeed was showing 110 kts, while our true airspeed was 28% more or 140 kts, and our groundspeed reached 173 kts.
As for the rest of the flight… it was beautiful as always and surprisingly very smooth. The Cessna 182 did a fantastic job, and climbed without constraints. Those Rocky Mountains sure are beautiful, especially since they had fresh, white snow and the fall colors (in particular the popular Aspens) are still very much at play. We also saw interesting landscapes formed by glaciers, several 14ers (mountain peaks over 14,000 feet in elevation), and the Moon delighted us with its presence throughout the flight and made for some nice pictures, as did sand blowing towards the Great Sand Dunes National Park in the San Luis Valley.
Fly safe and fly often!
Life is short, so we have to enjoy every minute of it!