By David Jack Kenny
By its nature, density altitude is a warm-weather problem, and this is one context in which what’s “warm” isn’t a question of personal taste. It simply means “warmer than standard,” which is to say 15 degrees Celsius minus two for every thousand feet of elevation.
Close to sea level, density altitude is most often a problem getting in and out of short, obstructed strips. Once clear of the immediate hazards, there’s usually room to climb to a safe altitude before traversing higher ground. At high-elevation fields, on the other hand, terrain clearance can’t always be taken for granted, and density altitude only makes things worse.
Shortly before 9 a.m. on June 30, 2014, three witnesses near the Loveland ski area west of Denver saw a low-wing, single-engine airplane flying westbound at very low altitude, perhaps 200 to 300 feet above the ground—which is to say about 11,500 feet mean sea level. One described it as “going pretty fast ... [but] pretty flat with very little gain in elevation.” As it became clear that the airplane would not be able to clear the mountaintop, it began a left turn in an apparent attempt to return downslope before its nose dropped and it descended out of sight behind a ridge. Another witness, an Embry-Riddle student with single-engine, multiengine, and instrument ratings, described it as having broken into a spin with a steep left bank and a “deep” nose-down attitude. A plume of smoke quickly followed, and rescuers found the wreckage almost entirely consumed by fire.
The airplane proved to be a 1964 Piper PA-28-235 Cherokee. Its owner was a 42-year-old, 278-hour private pilot from Ohio travelling west with his wife and 6-year-old son. They’d arrived at the Rocky Mountain Metropolitan Airport in Broomfield, Colorado, the previous Friday en route to Moab, Utah, the Grand Canyon, and eventually Los Angeles. The morning of the accident, the pilot had asked the customer service manager at the FBO for advice on crossing the mountains. Not being a pilot, she was unable to help, and no instructors were present to offer any guidance. The manager and a lineman both heard him say that he’d decided to follow Interstate 70 across the mountains.
He took on just shy of 58 gallons from the self-service fuel pump and was airborne at 8:10 a.m. Skies were clear with good visibility and 15-knot west winds, but it was already warm—26 degrees Celsius at 7:55 a.m., more than 20 degrees above standard temperature for the field elevation of 5,673 feet. Updated reports from stations closer to the accident scene suggest that at the time of the crash, the density altitude there was some 12,850 feet. The performance chart in the PA-28-235 owner’s handbook indicates that at that altitude, the maximum possible rate of climb would be about 150 feet per minute—in a brand-new airplane flown with perfect technique. Investigators concluded that while trying to outclimb the terrain, the pilot increased his angle of attack until the additional load factor imposed by the turn was enough to precipitate a stall. By that time, there was too little ground clearance left to lower the nose as he banked.
Recent accidents in Angel Fire and Los Alamos, New Mexico demonstrated the value of seeking local expertise in situations where the aircraft won’t perform in the familiar way. The decision to follow the highway across the mountains might seem reasonable on the expectation that the road would follow the lowest available route. In this case, though, that “lowest route” entered a tunnel through a ridge some 13,000 feet high—above the Cherokee’s service ceiling in that day’s weather, even without the updrafts and downdrafts of 300 to 500 fpm caused by west winds blowing across the ridgeline. A pilot familiar with local conditions might have suggested a safer route or convinced the Cherokee’s owner to postpone until temperatures cooled. Either piece of advice would have been worth the wait required to have that discussion.
Every fatal accident is a tragedy, but some are especially poignant. The lineman who’d greeted the family on their arrival in Broomfield recalled that the father carried his 6-year-old son to their rental car. News reports published after the crash revealed that the boy suffered from an extremely rare genetic disease. Because he was unlikely to live past his teens and was expected to be incapacitated by age 10, his father had learned to fly expressly so his parents could show him as much of the world as possible while he was still well enough to travel. We’re left to wish that some combination of better luck and better planning had allowed them to complete the trip.
Mountain Flying online course
Aging Aircraft online course
Density Altitude Safety Quiz
Essential Aerodynamics: Stalls, Spins, and Safety online course