From high to low, look out below

Many years ago, I got a call from a retired friend of mine, Chuck (“Chug”); he wanted to brag a little bit. He and an equally intrepid buddy of his, Ray (sadly, both have left this world now), from South Carolina had recently purchased an early 1980s straight-tail Bonanza. Chug said it was a great airplane bought at a great price. So, naturally, after several trial and proving flights, they both wanted to stretch their wings, so-to-speak, and see the country. I wished them well and hoped they would enjoy the journey.

Being the history buffs they were, they loaded up their new bird a couple of weeks before Christmas and took off to explore first the winter wonderland that is New England. They then headed west to experience the Great Lakes region – hoping to see the water of Lake Superior iced over. Next, they continued their trek west to see the Rockies and the storied lands of the Sioux tribes before turning back to the south and east for home.

The trip took several days longer than they expected because – neither of them possessing an instrument rating – they had to watch from the bench through several days of IMC weather. Chug, having spent 41 years as a railroad locomotive engineer (that’s why we called him “Chug”) before retiring, was accustomed to operating by stringent rules and regulations, and I know he transferred that same SOP-adherence attitude to all his flying endeavors; he was always level-headed, and not the type to take foolish chances.

Altimeter
It’s not always telling the whole truth.

One anecdote about their cross-country exploit made me very glad he wasn’t. While motoring into Great Falls, Montana, on a crystal-clear morning, both of the guys came face-to-face with the shocking realities of non-standard temperature effects on barometric altimeters.

Both Chug and Ray had grown up in the southeastern U.S., and they both had done almost all of their flying in that region. Except for a few reconnoiter flights in slow-moving Cessnas and Pipers on pleasant spring and summer days over the Blue Ridge into enemy territory to see what the Yanks in Pennsylvania were up to, neither of them had any experience with mountain flying or extreme cold conditions. They learned that experience is, if not the best, then at least the most effective teacher.

For a solid week during their travels, they had maintained the practice of flying many short flights each day instead of a few extended ones so they could explore each region of every state they landed in. It is the best way to actually learn of the nation. And on this particular leg they departed Sidney, Montana early on a sunny, very cold morning with the intention of flying up the Yellowstone River to its confluence with the Missouri River. They then turned west to follow it to Great Falls. The area had recently received quite a bit of snow, so in order to ensure accurate navigation they decided to fly lower to the ground so they could maintain eye contact with the river; over the stark plains of the Dakotas and Montana that decision involved little risk.

East of Great Falls, Montana, at the northern tip of the Lewis and Clark National Forest, there is an island in the sky protruding almost obtrusively from the plains called the Highwood Mountains. The loftiest peak in the formation is Highwood Baldy, at around 7,760 feet in elevation. Chug had told me years earlier of a poignant, romantically tragic story about two white wolves – Ghost Wolf and his mate, Snowdrift – that, in the early 1900s, were wily enough to evade the ranchers’ and hunters’ guns for several years. He knew that the legendary wolves lived and died in those mountains many decades ago and he wanted to see where the story took place. So at a point close to Fort Benton, they turned south and climbed for the high rocks on the plains.

Ray was at the controls that morning as the pair closed on the Highwoods. Ray tuned in to and received the ATIS information for the Great Falls airport. He then set the altimeter to match the broadcasted pressure setting. Thinking they would fly low over Baldy Peak – clearing the summit by a few hundred feet while Chug captured the event on his film camera – they chose 8,200 feet as their target altitude for the first track over the area. But what they didn’t pay attention to was the broadcasted temperature at the airport – it was a -16 degrees C.

The air was smooth because the wind was very light, so they elected to make the first pass quite dramatic by keeping their speed high as they approached from the north. Ray leveled out at exactly 8,200 feet and aimed straight at the peak; Chug’s camera was rolling. Until their last days, neither of them could be exactly sure of the sequence of events, but they both said the air got choppy as they closed to within several hundred feet of the summit. Ray said the airplane lost some altitude due to the light turbulence, but not very much.

Highwood Baldy
Those mountains can spell trouble for a flatlander.

In what they both said was a very sudden, terrifying moment, the airplane kicked to the left in a yaw condition then hit some moderate turbulence, and then they were looking only yards ahead at the radio tower on the peak of Highwood Baldy, well above their altitude.

Ray said he thought they were at 8,100 feet, but when he banked left and pulled hard on the elevators to avoid the crash – he knew they were no higher than the top of the mountain. Immediately forgetting about any more passes, they zoomed up and away from Baldy Peak and headed for Great Falls – still sweating a little as they entered the traffic pattern and landed.

To his credit though, Chug maintained his composure and captured the entire event on film, which he laughed uneasily at for years. Later, after refueling and relaxing in the Holman Aviation lounge at Great Falls a little bit, they discussed the event with a couple of local pilots who were there. It didn’t take Chuck and Ray long to understand what had happened.

The forensic analysis of the near-miss with disaster would list the following discoveries. I say forensic because the actions they undertook were obviously illegal.

  1. They chose a much-too-low altitude to fly past the mountain to begin with – even under standard conditions.
  2. They either forgot about, or didn’t understand, how wind moves around mountains and mountain ridges – windward and leeward sides.
  3. They were not familiar with extremely low temperature effects on barometric altimeters.

The charted temperature at the time of the incident was -16 degrees C at Great Falls. The airport there is at an elevation of 3,680 feet. Ray stated that his altitude was 8,200 feet before they hit the turbulence. That would have put them at an altitude of around 4,500 feet above the airport elevation on a standard day as they approached to within only yards of Baldy Peak. But conditions were not standard.

The ICAO Cold Temperature Error Table in section 7-2-4 of the Aeronautical Information Manual shows clearly that a standard barometric altimeter will indicate an error of at least 500 feet under those conditions (-16 degrees) – possibly as much as 600 feet.

In other words, Chuck and Ray were probably already below the mountain top when they leveled out at an indicated 8,200 feet of altitude. Perhaps their minds could not believe what their eyes were telling them until it was almost too late…

If you’re going to fly in mountainous regions in very cold air, get familiar with the Cold Temperature Error Table in the AIM and apply its corrections accordingly.

2 Comments

  • Thanks for a very informative article. I do most of my flying in Mexico and my home airport is 8,000′ high. The airport has no AWOS or other method to get an altimeter reading. I often fly up from sea level and the altimeter reading is often way off. I have a Stratus ADS-B unit which supposedly has very accurate altitude reporting, and I usually adjust my altimeter to the Stratus reading when I get close to the airport. It always seems to work, and my altimeter always reads 8,000′ upon landing.
    So what do other readers think? Should we be using GPS altitude? Should the altimeter be consigned to the dustbin of history?

  • Michael, thanks for the comments. As I understand it, your GPS geometric altitude readout is much more accurate than your altimeter’s pressure altitude indication. However, I wouldn’t expect to see any official switch over to GPS altitude cruise levels anytime soon. It could happen someday though. At the age I am now, almost 62, I wouldn’t expect to see it in the days I have left… I’m sure there are many out there who know much more about it than I do.

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